Departments of Pediatrics and of Human Genetics, University of Michigan, Ann Arbor, Michigan
Address correspondence to: Dr. Friedhelm Hildebrandt, Departments of Pediatrics and of Human Genetics, University of Michigan Health System, 8220C MSRB III, 1150 West Medical Center Drive, Ann Arbor, MI 48109-0646. Phone: 734-615-7285 (office), 734-615-7895 or 7896 (laboratories); Fax: 734-615-1386 or 7770; E-mail: fhilde{at}umich.edu
Nephronophthisis (NPHP), an autosomal recessive cystic kidneydisease, represents the most frequent genetic cause of end-stagekidney disease in the first three decades of life. Contraryto polycystic kidney disease, NPHP shows normal or diminishedkidney size, cysts are concentrated at the corticomedullaryjunction, and tubulointerstitial fibrosis is dominant. NPHPcan be associated with retinitis pigmentosa (Senior-Løkensyndrome), liver fibrosis, and cerebellar vermis aplasia (Joubertsyndrome) in approximately 10% of patients. Positional cloningof six novel genes (NPHP1 through 6) as mutated in NPHP andfunctional characterization of their encoded proteins have contributedto the concept of "ciliopathies." It has helped advance a newunifying theory of cystic kidney diseases. This theory statesthat the products of all genes that are mutated in cystic kidneydiseases in humans, mice, or zebrafish are expressed in primarycilia or centrosomes of renal epithelial cells. Primary ciliaare sensory organelles that connect mechanosensory, visual,osmotic, and other stimuli to mechanisms of cell-cycle controland epithelial cell polarity. The ciliary theory explains themultiple organ involvement in NPHP regarding retinitis pigmentosa,liver fibrosis, ataxia, situs inversus, and mental retardation.Mutations in NPHP genes cause defects in signaling mechanisms,including the noncanonical Wnt signaling pathway. The "ciliopathy"NPHP thereby is caused by defects in tissue differentiationand maintenance as a result of impaired processing of extracellularcues. Nephrocystins, the proteins that are encoded by NPHP genes,are highly conserved in evolution. Positional cloning of additionalcausative genes of NPHP will elucidate further signaling mechanismsthat are involved, thereby establishing therapeutic approachesusing animal models in mouse, zebrafish, and Caenorhabditiselegans.
Nephronophthisis: A Frequent Genetic Cause of Kidney Failure in Children
Renal Involvement
Nephronophthisis (NPHP) is an autosomal recessive cystic kidneydisease that constitutes the most frequent genetic cause forend-stage kidney disease (ESKD) in the first three decades oflife (1–4). Three clinical forms of NPHP have been distinguishedby onset of ESKD: Infantile (5), juvenile (6), and adolescentNPHP (7), which manifest with ESKD at the median ages of 1,13, and 19 yr, respectively. Initial symptoms are relativelymild (except in infantile NPHP type 2) and consist of polyuria,polydipsia with regular fluid intake at nighttime, secondaryenuresis, and anemia (8). At an average age of 9 yr, a slightlyraised serum creatinine is noted, before ESKD invariably developswithin a few years. Renal ultrasound reveals increased echogenicity.Later, cysts appear at the corticomedullary junction withinkidneys of normal or slightly reduced size (Figure 1, A andB) (9). Renal histology reveals a characteristic triad of tubularbasement membrane disruption, tubulointerstitial nephropathy,and corticomedullary cysts (Figure 1C) (10,11). In NPHP, cystsarise from the corticomedullary junction of the kidneys (Figure 1,A and B). Because kidneys size is normal or slightly reduced(except in infantile NPHP type 2, in which there is moderaterenal enlargement), cysts seem to develop e vacuo by loss ofnormal tissue. This is in contrast to polycystic kidney disease(PKD), in which cysts are evenly spread out over the entireorgan and lead to gross enlargement of the kidneys (4).
Figure 1. Morphology of nephronophthisis (NPHP). (A) Macroscopic pathology reveals cysts that arise from the corticomedullary junction of normal-sized kidneys. (B) Renal ultrasound demonstrates increased echogenicity, loss of corticomedullary differentiation, and the presence of cysts. In contrast to polycystic kidney disease (PKD), in NPHP, cysts are concentrated at the corticomedullary border and kidneys are not enlarged. (C) Renal histology in NPHP shows the characteristic triad of renal tubular (and glomerular) cysts, tubular membrane disruption, and tubulointerstitial cell infiltrates with interstitial fibrosis and periglomerular fibrosis. Adapted from Hildebrandt et al. (9a); and courtesy of D. Bockenhauer.
NPHP is inherited in an autosomal recessive mode. This includesNPHP variants with extrarenal manifestations (4). NPHP has previouslybeen grouped together with the clinical entity of medullarycystic kidney disease (MCKD) (6,10) because of similaritiesof clinical and pathologic features (12). Both NPHP and MCKDfeature corticomedullary cysts in kidneys of normal or slightlyreduced size. However, MCKD is clearly distinct from NPHP regardingmultiple aspects: (1) MCKD follows autosomal dominant inheritance,(2) ESKD occurs in the fourth decade and later, and (3) in MCKDthere is no extrarenal involvement other than hyperuricemiaand gout.
NPHP was first described by Smith and Graham in 1945 (2) andby Fanconi et al. (3), who introduced the term "familial juvenilenephronophthisis." Since then, >300 cases have been publishedin the literature (10). In NPHP, the earliest presenting symptomsare polyuria, polydipsia, decreased urinary concentrating ability,and secondary enuresis. They occur in >80% of cases (13)and start at approximately 6 yr of age. Anemia and growth retardationdevelop later in the course of the disease (8). Regular fluidintake at nighttime is a characteristic feature of the patient’shistory and starts at approximately age 6 yr. Because of themild nature of symptoms and the lack of edema, hypertension,or urinary tract infections, there is often a delay in the diagnosisof NPHP. This causes a risk for sudden death from fluid andelectrolyte imbalance. Disease recurrence has never been reportedin kidneys that were transplanted to patients with NPHP (14).By positional cloning, we and others have identified recessivemutations in six different novel genes as causing NPHP: NPHP1(15,16), NPHP2/inversin (17), NPHP3 (18), NPHP4 (19), NPHP5(20), and NPHP6 (21), defining NPHP types 1, 2, 3, 4, 5, and6, respectively. This has made definite molecular genetic diagnosticspossible (www.renalgenes.org). Homozygous deletions in the NPHP1gene account for approximately 25% of all NPHP cases, whereasthe other genes contribute <2% each. As expected in a recessivedisease, penetrance of the renal phenotype seems to be 100%.
In NPHP, chronic renal failure develops within the first threedecades of life (7,22,23). Infantile NPHP, which is characterizedby mutations in NPHP2/inversin, leads to ESKD between birthand age 3 yr (17,23). In a study conducted in 46 children whohad juvenile NPHP type 1 caused by mutations in the NPHP1 gene,a serum creatinine of 6 mg/dl was reached at a median age of13 yr (range 4 to 20 yr) (6,22). Similarly, the median age ofESKD in patients with mutations in the NPHP5 gene was 13 yr(20). The median time lapse between a serum creatinine of 2and 4 mg/dl was 32 mo and between 4 and 6 mg/dl was 10 mo (24).In patients with adolescent NPHP as a result of mutations inthe NPHP3 gene, ESKD develops by 19 yr of age (18). If renalfailure has not developed by the age of 25 yr, then the diagnosisof recessive NPHP should be questioned and autosomal dominantMCKD considered as a differential diagnosis. In MCKD, whichfollows autosomal dominant inheritance, ESKD occurs later inlife. MCKD types 1 and 2 show a median onset of ESKD at 62 (25)and 32 yr (26), respectively. MCKD type 2 can be positivelydiagnosed by detection of mutations in the UMOD gene that encodesuromodulin/Tamm-Horsfall protein.
Epidemiology
NPHP has been reported from virtually all regions of the world(13). The incidence of the disease has been given as nine patientsper 8.3 million (27) in the United States or one in 50,000 livebirths in Canada (10,28). Although a rare disorder, it representsthe most frequent genetic cause of ESKD in the first three decadesof life and is a major cause of ESKD in children, accountingfor 10 to 25% of these patients in Europe (13,29–31).In the North American pediatric ESKD population, pooled dataindicate a prevalence of approximately 5% of all children withESKD (32,33).
Extrarenal Manifestations of Eye, Brain, and Liver
NPHP may be associated with tapetoretinal degeneration (Senior-Løkensyndrome [SLSN] [34,35]), cerebellar vermis aplasia (Joubertsyndrome [JBTS] [36,37]), ocular motor apraxia type Cogan (38),mental retardation (21,39), liver fibrosis (40), or cone-shapedepiphyses of the phalanges (Mainzer-Saldino syndrome [41]).Infantile NPHP type 2 (17) can be associated with situs inversus(17), retinitis pigmentosa (42), or cardiac ventricular septaldefect (17). The ciliary theory of NPHP elucidates the pathogenicbasis of extrarenal organ involvement regarding retinitis pigmentosa,liver fibrosis, ataxia, situs inversus, and mental retardation.
Retinal Involvement (SLSN)
SLSN, represented by the concomitant occurrence of NPHP withretinitis pigmentosa, was first described by Contreras (43),Senior (34), and Løken (35). Three different terms havebeen used in the literature to describe the retinal findingsof SLSN: Retinitis pigmentosa, tapetoretinal degeneration, andretinal-renal dysplasia. This most likely reflects a spectrumwithin the pathogenesis that includes developmental defects(dysplasia) as well as defects of tissue maintenance (degeneration)(44). In children with recessive mutations in the NPHP1, 2,3, and 4 genes, retinitis pigmentosa occurs in approximately10% of all affected families, without any obvious genotype/phenotypecorrelation. Whether patients with retinal involvement carryan additional mutation in an unknown modifier gene is an openquestion. Early-onset and late-onset types of SLSN have beendistinguished. The early-onset type seems to represent a formof Leber’s congenital amaurosis, because children exhibitcoarse nystagmus and/or blindness at birth or develop thesesymptoms within the first 2 yr of life (45). It is interestingthat patients with mutations in the NPHP5 or NPHP6 genes exhibitearly-onset retinitis pigmentosa in all known cases (20,21).Fundoscopic alterations are present in all patients with late-onsetSLSN by the age of 10 yr. The late-onset form manifests firstwith night blindness, followed by development of blindness duringschool age. Retinal degeneration is characterized by a constantand complete extinction of the electroretinogram, which precedesthe development of visual and fundoscopic signs of retinitispigmentosa (46). The kidney involvement in SLSN is identicalclinically to what is known from patients with NPHP withoutocular involvement regarding age of onset, symptoms, and histologyof renal disease.
Cerebellar Vermis Aplasia (JBTS)
In JBTS, a developmental disorder with multiple organ involvement,NPHP or cystic dysplasia occurs in association with colobomaof the eye (or retinal degeneration); with aplasia/hypoplasiaof the cerebellar vermis causing ataxia; and with the facultativesymptoms of psychomotor retardation, polydactyly, occipitalencephalocele, and episodic neonatal tachypnea/dyspnea (36,37,47–49).A pathognomonic diagnostic feature of JBTS on axial magneticresonance imaging of the brain is the presence of prominentsuperior cerebellar peduncles, termed the "molar tooth sign"of the midbrain-hindbrain junction (49,50). In patients withthe association of JBTS and NPHP, mutations have been describedin three different genes, NPHP1 (37,49,50), AHI (51,52) (JBTStype 3), and NPHP6 (21,53). Patients with JBTS have abnormalaxonal decussation (crossing in the brain) that affects thecorticospinal tract and superior cerebellar peduncles, therebyexplaining the motor and behavioral abnormalities (54). Somepatients also have abnormal cerebral structure with corticalpolymicrogyria (55). Ocular motor apraxia type Cogan, definedas the transient inability of horizontal eye movements in thefirst few years of life, may be associated with JBTS. This symptomhas been described in patients with mutations in the NPHP1 (38,56)("JBTS4") and NPHP4 (57) genes. It may be due to defects inthe nuclei of the abducens nerve, which contain both ipsilaterallyprojecting motor neurons and contralaterally projecting interneurons,or supranuclear control regions such as the pontine paramedianreticular formation that projects to the abducens and oculomotornuclei, which has been postulated for other forms of horizontalgaze palsy (58). Defects in axon guidance may be related todefects in renal tubule development by shared signaling pathways(see Signal Mechanisms Relevant for NPHP). Two additional locifor JBTS have been identified: JBTS1 on chromosome 9q34.3 (59)and JBTS2/CORS2 on chromosome 11p12-q13.3 (60).
Liver Fibrosis and Skeletal Changes
NPHP may be associated with liver fibrosis (40,61–63).Patients develop hepatomegaly and moderate portal fibrosis withmild bile duct proliferation. This pattern differs from thatof classical congenital hepatic fibrosis, whereby biliary dysgenesisis prominent. A recessive mutation in the NPHP3 gene was recentlydescribed in a patient with NPHP and liver fibrosis (18). Hepaticinvolvement in NPHP type 2 (infantile NPHP) seems to involveonly transient elevation of transaminases (23). The associationof NPHP with cone-shaped epiphyses of the phalanges (type 28and 28A), known as Mainzer-Saldino syndrome, was first publishedby Mainzer et al. (41) in patients who also had retinal degenerationand cerebellar ataxia.
Situs Inversus
The presence of situs inversus was shown in a patient with infantileNPHP and mutations in the NPHP2/inversin gene (17). Therefore,the role of inversin in left–right axis specificationthat had been described in mice was confirmed in humans (64,65).The patient with situs inversus also had a cardiac ventricularseptal defect as a heterotaxy phenotype. This finding was analogousto the randomization of heart looping that was seen in nphp2/inversinknockdown experiments in zebrafish. Recently, it has becomeapparent that products of other genes that are associated withrenal cystic disease (in addition to inversin) are importantfor left–right axis determination of the body plan (66,67).The gene PKD2, mutations in which cause autosomal dominant PKDand that encodes the calcium release channel polycystin-2, hadbeen shown in a Pkd2–/– mouse model to representa gene that regulates left–right axis determination, actingupstream of Nodal, Ebaf, Leftb, and Pitx2 (68,69).
Other Syndromes with NPHP
Bardet-Biedl syndrome (BBS) exhibits renal histology that issimilar to NPHP (70,71). Positional cloning of recessive genesthat are mutated in BBS has revealed that the molecular relationbetween NPHP and BBS may lie in coexpression of the respectivegene products in primary cilia, basal bodies, and centrosomesof renal epithelial cells (72). For BBS, an oligogenic inheritancepattern has been described. This refers to the finding thatmutations in more than one BBS gene may be required for fullpenetrance of some aspects of organ involvement (73).
Further disease variants have been described in associationwith NPHP, including Jeune syndrome (asphyxiating thoracic dysplasia)(74–77), Ellis van Creveld syndrome (78), RHYNS syndrome(retinitis pigmentosa, hypopituitarism, NPHP, and skeletal dysplasia)(79), Alstrom syndrome (retinitis pigmentosa, deafness, obesity,and diabetes without mental defect, polydactyly, or hypogonadism)(80), and Meckel-Gruber syndrome (81,82), which in the caseof MKS3 mutations can be allelic with JBTS (83). In Alstromsyndrome, the single underlying gene, ALMS1, encodes a novelprotein that contains coiled-coil domains and a putative nuclearlocalization signal, as well as serine-rich and histidine-richregions (84,85). ALMS1 forms a part of the centrosome (86,87).This, together with the finding that BBS proteins localize tocentrosomes, confirms the role of centrosomal proteins in cystickidney diseases that are associated with diabetes, obesity,and retinitis pigmentosa (88,89). Additional NPHP-associateddisorders are Sensenbrenner syndrome (cranioectodermal dysplasia)(90,91) and Arima syndrome (cerebro-oculo-hepato-renal syndrome)(92–94). NPHP has also been described in association withulcerative colitis (95).
Positional Cloning Reveals Seven Causative Genes for NPHP
Since its first description in 1945 (2,3), the pathogenesisof NPHP had been elusive. Positional cloning revealed novelgenes that cause NPHP when mutated. These are monogenic recessivegenes, suggesting that mutations in each single one of thesegenes is sufficient to cause NPHP in a patient who bears thesemutations, indicating that their products are necessary fornormal kidney function. Positional cloning thereby generatednew insights into disease mechanisms of NPHP and demonstratedthat they are related to signaling mechanisms of sensory cilia,centrosomes, and planar cell polarity (1,72,96). Seven NPHP-associatedgenes have been identified so far: NPHP1 through 6 and AHI1(15–21,54,55,57) (Table 1).
Table 1. Genetics and frequency of extrarenal associations in NPHP
NPHP1
We identified mutations in NPHP1 as causing juvenile NPHP type1 (15,16). NPHP1 encodes nephrocystin-1, a protein that interactswith components of cell–cell and cell–matrix signaling,including p130Cas (97), focal adhesion kinase 2 (98), tensin,and filamin A and B (99,100). It also interacts with the productsof other NPHP genes, such as nephrocystin-2/inversin (17), nephrocystin-3(18), and nephrocystin-4 (57,101). Nephrocystin-1 localizesto adherens junctions and focal adhesions of renal epithelialcells (99,100), which are involved in cell–cell and cell–basementmembrane communications, respectively (101,102) (Figure 2).
Figure 2. Cystoproteins are proteins of genes that are mutated in cystic kidney diseases of humans, mice, or zebrafish. They share the common feature of expression in primary cilia, basal bodies, or centrosomes. Depending on cell-cycle stage, some cystoproteins localize to adherens junctions or focal adhesions. Many cystoproteins have been localized to more than one intracellular domain. The speckled arrow in the primary cilium indicates the direction of anterograde transport along the microtubule system mediated by kinesin 2, a heterotrimeric protein that is composed of two motor units (Kif3a and Kif3b) and one nonmotor unit (KAP3). AJ, adherens junction; BB, basal body; Cen, centriole; ER, endoplasmic reticulum; FAP, focal adhesion plaque; TJ, tight junction; PC-1, polycystin-1; PC-2, polycystin-2. Modified from Watnick and Germino (72), with permission.
NPHP2
The renal cystic changes of infantile NPHP (NPHP type 2) combineclinical features of NPHP and of PKD (5). Guided by mappingof a locus for infantile NPHP to chromosome 9q21-q22 (23) andby the observation that a deletion in the inversin (Invs) genecauses renal cystic phenotype in the inv/inv mouse model (64,65),we identified mutations in human inversin (INVS) as the causeof infantile NPHP (type 2) with and without situs inversus (17).Inversin interacts with nephrocystin-1 and with -tubulin, whichconstitutes the microtubule axoneme of primary cilia. We demonstratedthat nephrocystin-1 and inversin localize to primary cilia ofrenal tubular cells (17)—the same subcellular compartmentthat was identified as central to the pathogenesis of PKD (1,66,103).This was one of the first findings to support a unifying theoryof renal cystogenesis (1,72), which states that proteins that,when mutated, cause renal cystic disease in humans, mice, orzebrafish ("cystoproteins") are expressed in primary cilia,basal bodies, or centrosomes (66,72). The interaction and co-localizationto cilia of nephrocystin-1, inversin, and -tubulin provideda functional link between the pathogenesis of NPHP, the pathogenesisof PKD, primary cilia function, and left–right axis determination(17). The functional relationship between ciliary expressionof these so-called "cystoproteins" (proteins mutated in cystickidney disease) and the renal cystic phenotype, however, isstill somewhat unclear. One of the first concepts for this relationshipproposes that cilia may act as mechanosensors to sense fluidmovement in the kidney tubule, where polycystin-1 transmitsthe signal to polycystin-2, which is a TRP type calcium channel.This would produce sufficient Ca2+ influx to induce Ca2+ releasefrom intracellular storage, which then regulates numerous intracellularsignaling activities that are linked to the regulation of cellcycle and planar cell polarity (103). In particular, inversin/NPHP2function has been implicated in signaling mechanisms of planarcell polarity (see The Wnt Pathway) (104). Okada et al. (105)previously demonstrated that inversin is needed to positionthe cilia in cells of the ventral node.
NPHP3
By positional cloning in a large Venezuelan kindred (7), weidentified mutations in NPHP3 as responsible for adolescentNPHP (18). We demonstrated that mutations in the murine orthologNphp3 cause the renal cystic mouse mutant pcy (18), which wasrecently shown to be responsive to treatment with a vasopressinreceptor antagonist (106).
NPHP4
Mutations in NPHP4 were identified by homozygosity mapping andtotal genome search for linkage (19,57,107). The encoded protein,nephrocystin-4/nephroretinin, is in a complex with other proteinsthat are involved in cell adhesion and actin cytoskeleton organization,such as nephrocystin-1, p130Cas, Pyk2, tensin, filamin, and-tubulin. In polarized epithelial cells, nephrocystin-4 localizesto primary cilia, basal bodies, and the cortical actin cytoskeleton,whereas in dividing cells, it localizes to centrosomes (101).Nephrocystin-4 is conserved in Caenorhabditis elegans and expressedin ciliated head and tail neurons of the nematode (108). Uponknockdown, it exhibits a male mating phenotype, similar to orthologsof other genes that are mutated in cystic kidney disease (108).
NPHP5
Recently, we identified another novel gene (NPHP5) as beingmutated in NPHP type 5 (20). It is interesting that all mutationsfound were truncations of the encoded protein nephrocystin-5,and all patients had early-onset retinitis pigmentosa (SLSN).Nephrocystin-5 contains two IQ domains, which directly interactwith calmodulin (20) and is in a complex with the retinitispigmentosa GTPase regulator, which when defective causes X-linkedretinitis pigmentosa. Both nephrocystin-5 and retinitis pigmentosaGTPase regulator localize to connecting cilia of photoreceptorsand in primary cilia of renal epithelial cells (20). The factthat connecting cilia of photoreceptors are the structural equivalentsof primary cilia of renal epithelial cells may explain retinalinvolvement in the retinal-renal syndrome SLSN.
NPHP6
Very recently, we identified by positional cloning of recessivetruncating mutations in a novel gene NPHP6/CEP290, which encodesa centrosomal protein, as the cause of NPHP type 6 and JBTStype 5 (21). We demonstrated that abrogation of NPHP6 functionin zebrafish causes planar cell polarity (convergent extension)defects and recapitulates the human phenotype of NPHP type 6,including renal cysts, retinitis pigmentosa, and cerebellardefects (21). In addition, a defect in cell size control andmorphogenesis was found upon nphp6 knockdown in the nonvertebrateCiona intestinalis. Nephrocystin-6 modulates the activity ofATF4/CREB2, a transcription factor that is implicated in cAMP-dependentrenal cyst formation (106). Nephrocystin-6 is expressed in centrosomesand the mitotic spindle in a cell cycle–dependent manner.Its identification establishes a link between centrosome functionand tissue architecture in the pathogenesis of cystic kidneydisease, retinitis pigmentosa, and central nervous system development.Mutations in NPHP6/CEP290 have been confirmed as causing JBTSwith and without renal involvement (21,53). It is interestingthat a 300–amino acid in-frame deletion of NPHP6/CEP290caused retinal degeneration only, without renal or cerebellarinvolvement in the rds16 mouse model (109). This is in accordancewith the recent finding that a hypomorphic mutation of NPHP6/CEP290represents the most frequent cause of Leber’s congenitalamaurosis (110).
AHI1
Finally, mutations in AHI1 have been detected in patients withJBTS with and without renal involvement (51,52,54,55). Takentogether, these findings indicate that the nephrocystin proteinsare involved in functions of sensory cilia, cell polarity, andcell division (101).
Cilia: A Unifying Theory for Cystic Kidney Disease
The demonstration that nephrocystin-1 and inversin/NPHP-2 localizeto primary cilia of renal tubular cells (17) was among the firstfindings to support a new unifying theory of renal cystogenesis(72). This theory states that proteins that, when mutated, causerenal cystic disease in humans, mice, or zebrafish ("cystoproteins")are expressed in primary cilia, basal bodies, or centrosomes(66,72) (Figure 3).
Figure 3. Cilia structure and intraflagellar transport (IFT). (A) A typical cilium consists of an axoneme of nine doublet microtubules. Each doublet arises from the inner two microtubules of the basal body microtubule triplets. The axoneme is surrounded by a specialized ciliary membrane that is separated from the cell membrane by a zone of transition fibers. (B) A cross-section of 9 + 2 and 9 + 0 cilium. Cilia are broadly divided into two types on the basis of the presence or absence of a central pair of microtubule singlets in the axoneme (9 + 2 or 9 + 0 structure, respectively). Inner and outer dynein arms, which are usually associated with 9 + 2 cilia, can be present in either type of cilium and are important for ciliary motility. Ciliary assembly and maintenance is accomplished by IFT, which relies on the microtubule motor proteins kinesin 2 and cytoplasmic dynein to transport IFT protein complexes and their associated cargo up and down the length of the cilium (depicted in A). Eb1, end-binding protein 1. Adapted from Bisgrove and Yost (110a).
Cilia Structure and Function
The cilium is a hair-like structure that extends from the cellsurface into the extracellular space. Virtually all vertebratecell types have cilia in developing or mature tissue. Ciliaconsist of a microtubule-based axoneme covered by a specializedplasma membrane. The axoneme has nine peripheral microtubuledoublets arranged around a central core. There may be two centralmicrotubules (9 + 2 or 9 + 0 axoneme; Figure 3). 9 + 2 ciliausually have dynein arms that link the microtubule doubletsand are motile, whereas most 9 + 0 cilia lack dynein arms andare nonmotile ("primary cilia"). The ciliary axoneme is anchoredin the basal body, a microtubule-organizing center derived fromthe older of the two centrioles. The transition zone at thejunction of the basal body acts as a filter for the moleculesthat can pass into or out of the cilium. Nephrocystin-1 is localizedat the transition zone of epithelial cells (111). During generationof cilia (ciliogenesis), cilia elongate from the basal bodyby the addition of new axonemal subunits to the distal tip,the plus end of the microtubules. Axonemal and membrane componentsare transported in raft macromolecular particles (complex Aand B) by so-called intraflagellar transport (IFT) along theaxonemal doublet microtubules (Figure 3) (112). Anterogradetransport toward the tip is driven by heterotrimeric kinesin2, which contains motor subunits Kif3a and Kif3b and a nonmotorsubunit. Mutations of Kif3a cause renal cysts in mice (113).Kinesins also help form signaling complexes within the ciliarymembrane. Retrograde transport back to the cell body occursvia the motor protein cytoplasmic dynein 1B (114).
The unexpected convergence in primary cilia of proteins thatunderlie cystic kidney diseases is supported by the followingfindings: (1) there is high evolutionary conservation of theproteins involved (Figure 4, A through D), (2) most of theseproteins interact with each other (Figure 4C), and (3) clinicalphenotypes are related to tissue-specific expression in thesensory cilia (Figure 4, E through G).
Figure 4. The cilia/basal body hypothesis of renal cystic disease and related disorders. This illustration summarizes the new unifying pathogenic theory of cystic kidney disease, which states that virtually all proteins that are mutated in cystic kidney disease of humans, mice, or zebrafish ("cystoproteins") show expression of their encoded proteins in primary cilia, basal bodies, or centrosomes. The horizontal axis symbolizes the flow of genetic information from genes over proteins to disease phenotypes. The vertical axis represents evolutionary time. (A) On the evolutionary scale, genes that are responsible for renal cystic disease in vertebrates are shown, including the unicellular organism Chlamydomonas reinhardtii. (B) Many of the orthologs of human cystic kidney disease genes are expressed in ciliated neurons of head and tail of the nematode Caenorhabditis elegans, where they lead to a male mating phenotype when knocked down (e.g., lov-1 or nph-4) (see E). Most proteins that are mutated in Bardet-Biedl syndrome (BBS) are conserved as basal body components of motile cilia (flagella) C. reinhardtii, where mutations in these genes lead to a phenotype of defective IFT or propulsion (see E). (C) Many cystoproteins directly interact with each other (e.g., NPHP1 and NPHP2/inversin). (D) Recently, it was discovered that the products of all genes that are mutated in cystic kidney diseases of humans, mice, or zebrafish show expression of their encoded proteins in primary cilia, basal bodies, or centrosomes. These findings placed primary cilia and centrosomes at the center of these disease processes (D has no vertical dimension). (E through G) The convergence of the pathogenesis at sensory cilia that serve distinct functions in different tissues may explain the broad organ involvement (pleiotropy) of NPHP and BBS that includes many different organ systems through defects of sensory cilia. Pleiotropic phenotypes in NPHP or BBS include cystic kidney disease, retinitis pigmentosa, infertility, diabetes, and other diseases of premature aging of organs.
Cilia: Conserved Modules to Sense Cell External Signals
It is becoming apparent that primary cilia are highly conservedstructures for sensing of a wide variety of extracellular cuesby a broad variety of specialized tissues. A common "theme"conserved through evolution seems to suggest that many timeswhen cells are to receive cues from the outside of the cell,they use a primary cilium. A broad range of cues can be receivedby specific ciliary receptors. These include photosensation(rhodopsin), mechanosensation (polycystin-1 and -2), osmosensation,and olfactory sensation (seven-membrane spanning olfactory receptors).The decision rules by which cells place a specific receptormolecule into the cilium are unknown. In general, it seems thatthe pathogenesis of ciliopathies is based on an inability ofepithelial cells to sense or process extracellular cues (115).
Evolutionary Conservation of Cystoproteins
For many "cystoproteins," the renal cystic phenotype is conservedamong vertebrates. For example, mutations in inversin lead toNPHP type 2 in humans, mice, and zebrafish (Figure 4) (17).At least two nephrocystins are conserved even in the nematodeC. elegans by amino acid sequence, functional features, andtheir expression patterns and knockdown phenotypes: Nphp-1 andnphp-4 are expressed in head (amphid) and tail (phasmid) neurons,which are ciliated osmosensor neurons of C. elegans (Figure 4A)(108,116). Localization of nephrocystin-1 and -4 to some ofthese ciliated neurons overlaps with localization of the cystoproteinorthologs polycystin-1 (lov-1) and polycystin-2 (pkd-2) andwith many orthologs of BBS proteins (117). Knockdown of nphp-1and nphp-4 leads to impaired male mating behavior (108), similarto what has been described for lov-1 and pkd-2 mutants (118).These data have been confirmed and refined for specific neuronalcell type (119,120). In addition, a role for nphp-4 in the lifespanof the worm has been demonstrated (121). In C. elegans, bbs-7and bbs-8 are required for the correct localization/motilityof the IFT proteins osm-5/polaris and che-11 (122). A bioinformaticsapproach based on the ciliary/basal body hypothesis of BBS pathogenesisalso helped to identify the cystoprotein BBS5 (123). Using bioinformaticscreens for ciliary genes in combination with data from positionalcloning, mutations in ARL6 were identified as being responsiblefor BBS type 3 (124,125). ARL6, a small GTPase, is specificallyexpressed in ciliated cells and undergoes IFT.
Evolutionary conservation of cystoproteins goes even further:Some cystoproteins have been conserved over >1.5 billionyears of evolution from the unicellular organism Chlamydomonasreinhardtii to vertebrates (Figure 4, A and B). C. reinhardtiiuses two motor cilia (flagella) for locomotion. Strikingly,nephrocystin-4 and at least six proteins that are mutated inBBS are conserved in C. reinhardtii where they are part of thebasal body proteome (117,126). Defects of cystoprotein orthologsin C. reinhardtii have deficient IFT and flagellar propulsion(127).
Nephrocystin Complexes
Many cystoproteins participate in protein–protein interactioncomplexes (Figure 4C). These interactions may partially explainwhy mutations in different NPHP genes lead to similar phenotypes.Some of the domains that occur in cystoproteins are shared betweendifferent proteins. Coiled-coil domains, for example, occurin six of eight proteins that are defective in NPHP-like diseases(nephrocystin-1, -2, -3, -5, and -6 and ALMS1). Nephrocystin-1is targeted to the transition zone of motile and primary cilia(111), and casein kinase 2–mediated phosphorylation ofthree critical serine residues within a cluster of acidic aminoacids in nephrocystin mediates phosphofurin acidic cluster sortingprotein (PACS)-1 binding and is essential for co-localizationof nephrocystin with PACS-1 at the base of cilia (128). Thebasal body/centrosomal expression of proteins that are involvedin NPHP has led to identification of mutations in NPHP6/CEP290as the cause of NPHP type 6 (JBTS type 5). Its gene productnephrocystin-6/Cep290 (21) is part of the centrosomal proteome(86).
Extrarenal Organ Involvement by Ciliary Dysfunction
A prominent feature of NPHP is that in a certain percentageof cases, there can be involvement of multiple organs (pleiotropy)other than the kidney. In some instances, there seems to bea genotype/phenotype correlation regarding pleiotropy. For instance,there is involvement of the retina in all known cases with mutationsof NPHP5 or NPHP6. In other instances, such as NPHP1 mutations,the molecular basis of eye involvement is unknown. The pleiotropyof NPHP has now found a potential explanation in the ciliaryhypothesis of cystic kidney diseases (Figure 4, E through G).The extrarenal organ involvement in NPHP by organ system isdiscussed as follows.
Retinal Degeneration.
The renal-retinal involvement in SLSN can be explained by thefact that the primary cilium of renal epithelial cells is astructural equivalent of the connecting cilium of photoreceptorcells in the retina. We have shown that nephrocystin-5 and nephrocystin-6are expressed in the connecting cilia of photoreceptors (20,21).In analogy to motor transport along the axoneme of primary ciliain kidney epithelial cells, in the connecting cilia of photoreceptors,cargo is trafficked along microtubule tracks from the photoreceptorinner segment to the outer segment via a motor protein complexthat contains kinesin 2 and back to the cell body via a cytoplasmicdynein (Figure 5) (129). In this way, 10 billion molecules ofthe visual pigment rhodopsin are trafficked up and down theconnecting cilia per human retina per day.
Figure 5. Primary (nonmotile) cilia of renal epithelial cells and connecting cilia of retinal photoreceptors are analogous structures. In the primary cilium (A) of renal epithelial cells (B), "cargo" proteins are trafficked along the microtubule tracks from the region of the Golgi stack to the tip of the cilia via the motorprotein kinesin 2 and back down via cytoplasmic dynein 1b. (C) In an analogous manner, approximately 109 molecules of the visual pigment rhodopsin are transferred up and down the connecting cilia per human retina per day. Adapted from references (66,129).
Liver Fibrosis.
Both NPHP and BBS can be associated with liver fibrosis (40),whereas autosomal recessive PKD is associated with bile ductectasia. Bile duct involvement in these cystic kidney diseasesmay be explained by the ciliary theory, because the epithelialcells lining bile ducts (cholangiocytes) possess primary cilia(Figure 4, F and G).
Central Nervous System.
Recent findings suggest that oculomotor apraxia type Cogan (associatedwith NPHP1 and NPHP4 mutations), cerebellar vermis hypoplasia(in JBTS), and mental retardation (in NPHP type 6) may be dueto defects in microtubule-associated functions during neuriteoutgrowth and axonal guidance. Mechanotransport along microtubulesplays a role not only in intraciliary but also in axonal transport(130). An example is the motor protein KIF3A, which is mutatedin a renal cystic mouse model and also plays a role in axonaltransport (113). Because the malformations of the cerebellumthat occur in JBTS (37) consist of abnormal "wiring" of decussating(crossing) neurons, impaired axonal outgrowth and axon guidancemay be central to the neurologic defects in JBTS, in analogyto the lissencephaly phenotype that is caused by the centrosomalproteins LIS1 and doublecortin (131).
Congenital Cardiac Malformations.
In a patient with infantile NPHP, we observed a ventricularseptal defect as a congenital cardiac malformation (17). Thisdevelopmental defect may be viewed as a "heterotaxy" (left–rightorientation) phenotype that is caused by the same mechanism(68) that leads to situs inversus in this patient. The phenotypiccombination of NPHP, situs inversus, and cardiac septal defecton the basis of inversin mutations is observed in humans, mice,and zebrafish (17).
Obesity.
Obesity is part of the clinical spectrum of the ciliopathy BBS,and excessive obesity has been described in children with NPHP6mutations after renal transplantation (B. Hoppe, Universityof Cologne, Germany, personal communication, 2006). It is interestingthat in the Bbs6 knockout mouse model, obesity was associatedwith hyperphagia and decreased activity of the mice (132).
Nephrocystins and other cystoproteins are expressed in differentsubcellular compartments in a cell cycle–dependent manner(21,133). These subcellular compartments include focal adhesions,adherens junctions, cilia, basal bodies, centrosomes, and themitotic spindle (Figure 6). It is still mostly unclear whichof these subcellular localizations are most proximal to thepathogenesis of NPHP or other cystic kidney diseases. In relationto these subcellular localizations, many hypotheses on mechanismsof cystogenesis have been put forward. Among them are (1) themechanosensory hypothesis of renal cilia function, (2) participationin signaling at focal adhesions and adherens junctions, (3)a role in the maintenance of planar cell polarity within thenoncanonical Wnt signaling pathway, and (4) a role in centrosome-relatedfunctions of cell-cycle regulation. These hypotheses are discussednext (Figure 6).
Figure 6. Nephrocystins localize to different subcellular compartments in a cell cycle–dependent manner and participate in multiple signaling pathways together with other "cystoproteins" (proteins that are mutated in cystic kidney disease). Functional complexes that play a role in the planar cell polarity (PCP) pathway are highlighted for focal adhesion (A), adherens junction (B), cilium (C), basal body (D), centrosome (E), nucleus (F), mitotic spindle (G), and the Wnt pathways (H). Cystoproteins are shown on colored background and in bold type using blue for nephrocystins (NPHP), orange for BBS proteins, green for polycystins (PKD) and fibrocystin (PKHD1), and yellow for cystoproteins of renal cystic mouse models. Proteins that are not bold have been described in the context of the pathogenesis of cystic kidney disease (e.g., as a binding partner to a bona fide cystoprotein). Associated proteins with no known role in cystic kidney diseases are shown on gray background. Black dots connect proteins that directly interact. (A) Focal adhesions. (1) Nephrocystin-1 directly interacts with the focal adhesion adapter protein p130Cas ("crk-associated substrate") (99,100,102), which is a major mediator of focal adhesion assembly, binds to focal adhesion kinase, and mediates stress fiber formation (149). Nephrocystin-1 competes for binding to p130Cas with the proto-oncogene products Src and Fyn (99). (2) Nephrocystin-1 is in a protein complex with the focal adhesion proteins Pyk2/Fak2 (focal adhesion kinase 2), tensin (98), and filamin A and B. Its overexpression leads to activation of extracellular signal–regulated kinases 1 and 2 (ERK1 and ERK2) (98). (3) In children with NPHP, overexpression of 51 integrin was described in proximal tubules, which most likely results from defective 6-integrin expression (150). (4) The knockout mouse models for tensin (151) and for the Rho GDI gene (152) both exhibit an NPHP-like phenotype, thereby implicating further proteins of the focal adhesion signaling cascade in the pathogenesis of NPHP-like diseases. (B) Adherens junctions. (1) Nephrocystin-1 co-localizes with E-cadherin and p130Cas to adherens junctions. (2) The C-terminal half of nephrocystin, the "nephrocystin homology domain," is able to promote nephrocystin self-association and epithelial adherens junctional targeting (100). (3) Disruption of this targeting leads to reduced transepithelial resistance (100). (4) Nephrocystin-4 is in a protein complex with nephrocystin-1, NPHP2/inversin, p130Cas, and Pyk2/Fak2 and has been localized to adherens junctions in confluent MDCK cells (101). (C) Primary cilia. Recently, the development of a unifying hypothesis of renal cystogenesis was established (72). This hypothesis states that proteins that, when mutated, cause renal cystic disease in humans, mice, or zebrafish are part of a functional module, as defined by their subcellular localization to primary cilia, basal bodies, or centrosomes. This applies to polycystin-1 and -2; fibrocystin/polyductin; nephrocystin-1, -2 (inversin), -3, -4, and -5; BBS-associated proteins; cystin; polaris; ALMS1; and many others. Because nephrocystins interact, they are represented here as the "nephrocystin complex." On the basis of positional cloning, mutations in the inversin gene were identified as causing infantile NPHP (type 2) (17). This established a link between the pathogenesis of NPHP and disease mechanisms of PKD (17), in which nephrocystin-1 interacts with both inversin and -tubulin. Because -tubulin is a major component of primary cilia, this led to demonstration of co-localization of all three proteins in the primary renal cilia of epithelial cells (17). The complex also contains NPHP3, which is mutated in adolescent NPHP (type 3) and in the renal cystic mouse model pcy. The ciliary hypothesis of cystic kidney disease was confirmed by revealing that Nphp3 mRNA was expressed in kidney, retinal connecting cilia, ciliated bile ducts, and the node, which regulates left–right body axis in mice (18), and by identifying mutations in nephrocystin-5 (IQCB1), which co-localizes with calmodulin to primary cilia of renal epithelial cells and retinal connecting cilia (20). (D) Basal bodies. Basal bodies are short cylindrical arrays of microtubules and other proteins that are found at the base of cilia and that organize the assembly of the ciliary axoneme. They are analogous to centrosomes. Nephrocystins localize to the transition zone of basal bodies (111). Proteins that are mutated in BBS are components of the basal body transitional zone and are highly conserved in evolution. (E) Centrosomes. For a protein that is mutated in the related disease BBS4, it was shown that BBS4 is instrumental in recruiting proteins (e.g., PCM1) to the pericentrosomal matrix, confirming the role of centrosomal function in the pathogenesis of BBS (116). (F) Transcriptional programs. The transcription factor HNF1 regulates transcription of multiple genes that are mutated in cystic kidney disease–related genes (153). (G) Cell-cycle regulation. The hypotheses of functional involvement of the nephrocystin complex at focal adhesions and adherens junction on the one hand and the ciliary/centrosomal hypotheses on the other hand may be integrated by demonstration that different locations of the complex may predominate in a cell cycle–dependent manner. This is evidenced by the findings that inversin/nephrocystin-2 (17), nephrocystin-4 (101), and nephrocystin-6 (21) expression occurs in a cell cycle–dependent manner. Inversin exhibits a dynamic expression pattern in MDCK cells that show expression at centrosomes in early prophase, at spindle poles in metaphase and anaphase, and at the midbody in cells that undergo cytokinesis (133,154). Nephrocystin-4 was detected in MDCK cells at centrosomes of dividing cells and in polarized cells close at the cytoskeleton and in the vicinity of the cortical actin cytoskeleton, with co-localization of p130Cas, Pyk2, and -catenin (101). (H) Wnt pathways. Recent data demonstrated that in renal tubules, inversin/NPHP2 may induce switching from the canonical to the noncanonical Wnt signaling pathway in response to flow sensing by primary cilia of renal tubular cells (104). It is thought that this function of inversin is important to maintain renal epithelial cell polarity (96). The hypothesis that the renal cystic disease phenotype is due to defects in the maintenance of planar cell polarity seems plausible for multiple reasons: (1) It would reconcile previous functional hypotheses, because focal adhesions, adherens junctions, cilia, centrosomes, basal bodies, and regulation of the cell cycle all play a pivotal role in the regulation and maintenance of planar cell polarity (155); (2) because planar cell polarity plays an important role in developmental morphogenesis and also in the regeneration of differentiated tissue, a defect in planar cell polarity may explain both the occurrence of cysts during organogenesis and degenerative cystogenesis as it occurs in NPHP; and (3) the mechanism of convergent extension, which may be central to renal tubular morphology, was shown to be disturbed in many renal cystic diseases (156). APC, adenomatous polyposis coli protein; APC2, anaphase promoting complex subunit 2; CBP, CREB-binding protein; CREB, cAMP response element–binding protein; CK1, casein kinase ; DKK, Dickkopf-related protein; CK2, casein kinase II; GAP, RhoGTPase activating protein; GEF, guanine nucleotide exchange factor; GSK3, glycogen synthase kinase 3; JUNK, JUN kinase; PCM1, pericentriolar material 1; p150, p150glued/dynactin-1; PKC, protein kinase C; ROCK, Rho-associated protein kinase.
The Mechanosensory Hypothesis
On the basis of the initial finding that bending the primarycilium elicits Ca2+ influx (134–136), it was shown thatcilia can act as mechanosensors to sense fluid movement in thekidney tubule, in cooperation with polycystins. In this model,polycystin-1 transmits the signal to polycystin-2, which isa TRP calcium channel (103). This produces sufficient Ca2+ influxto induce Ca2+ release from intracellular stores, which thenregulates numerous signaling activities inside the cell thatare linked to cell-cycle regulation. It is thought that defectsin cell-cycle regulation may be ultimately responsible for thedevelopment of kidney cysts (103). In support of the ciliaryhypothesis of cystic kidney diseases, a motor protein of thekinesin II family, KIF3A, which is involved in intraciliarytransport, was shown to cause a murine renal cystic diseasewhen mutated (113). Kramer-Zucker et al. (137) recently showedthat cilia of larval zebrafish kidney tubules have a 9 + 2 configurationand are motile. Disruption of cilia structure or motility resultedin pronephric cyst formations, with left–right asymmetrydefects. Despite many data in support of the ciliary hypothesis,some data are still hard to reconcile with this model; for example,the autosomal dominant variant of NPHP, MCKD2, is caused bymutations in uromodulin, which has so far not been detectedin cilia, basal bodies, or centrosomes.
Focal Adhesion Hypothesis
When NPHP1 was first identified (15), we proposed a pathogenichypothesis that tied in nephrocystin-1 with defects of cell–celland cell–matrix signaling (102,138). This was based onthe finding that nephrocystin-1 contains an SH3 domain, localizesto adherens junctions and focal adhesions of renal epithelialcells, and interacts with integral components of these structures,such as p130CAS (99,100). This "adherens junction/focal adhesionhypothesis" of NPHP pathogenesis (102,138) recently was partiallyreconciled with the "cilia/centrosome" hypothesis in an integrativehypothesis by showing that nephrocystin-4/nephroretinin in polarizedepithelial cells co-localizes with -catenin at cell–cellcontact sites and to primary cilia, whereas in dividing cells,it localizes to centrosomes (101) (Figure 6, A and B).
The Wnt Pathway
Recent results on inversin/NPHP2 shed light on the mechanosensoryhypothesis of bending of primary cilia by tubular flow. Theyhave provided data on downstream signaling events that are necessaryto maintain normal tubular development and morphology (104):In this model (Figure 6,C, F, and H) the canonical Wnt signalingoccurs primarily through -catenin–dependent pathways inthe absence of tubular flow. Stimulation of the primary ciliumby flow, however, increases expression of inversin, which thenreduces levels of cytoplasmic disheveled through proteasomaldegradation and subsequently switches off the canonical pathwayby allowing activation of the -catenin destruction complex (96).When inversin is defective (as in NPHP type 2), the canonicalWnt pathway will prevail and disrupt apical-basolateral polarityof the renal epithelium (96). Because planar cell polarity signalingis important for oriented cell division, it seems logical thatFisher et al. (139) recently were able to demonstrate abnormalorientation of the mitotic spindle in two different rodent modelsof cystic kidney disease.
Centrosomes
Nephrocystin-6, which is mutated in JBTS, is a component ofthe centrosomal proteome (86). In addition, BBS4 is instrumentalin recruiting proteins to the pericentrosomal matrix, implicatingthe centrosome and its relation to cell-cycle control in thepathogenesis of BBS (117,140). The products of the genes mutatedin the ciliopathy Meckel-Gruber syndrome MKS1 (81) and MKS3(82) recently were shown to localize to basal bodies and centrosomes(141) (Figure 6D).
Cell Cycle
A balance between hyperproliferation and apoptosis may playan important role in the pathogenesis of cystic kidney diseases(Figure 6, F and G). For example, whereas in PKD, kidneys aregrossly enlarged, in NPHP and BBS, kidney size remains normaland cysts grow at the expense of normal tissue (e vacuo). Itseems that hyperproliferation may be the predominant mechanismin PKD-like diseases (142), whereas apoptosis is predominantin diseases of the NPHP and BBS group. A role of apoptosis wasin fact confirmed in the Bbs2–/– and Bbs4–/–mouse models (143,144). Polycystin-1 and -2 signaling and therenal cystic phenotype may be linked by a function of theseproteins in cell growth regulation. Polycystin-1 expressionactivates the JAK-STAT pathway, thereby upregulating p21(waf1)and inducing cell-cycle arrest in G0/G1 (145). The cell-cyclearrest requires polycystin-2. Involvement of polycystin-1/2signaling in the JAK/STAT pathway might explain how mutationsof either gene can result in dysregulated growth (145). Veryrecently, this hypothesis was confirmed by demonstration thattwo mouse models of PKD (jck and cpk) can be efficiently treatedwith the cyclin-dependent kinase inhibitor (R) roscovitine (146).
NPHP: Defects of Tissue Differentiation and Maintenance
It is striking that in renal cystic diseases (and in the associatedextrarenal organ involvement), mutations of monogenic diseasegenes may lead to developmental defects (dysplasia), in whicha structural organ defect is present at birth, but also to degenerativedefects (degeneration), in which organ structure and functionare normal at birth but deteriorate over time. Examples of thisphenomenon from the side of cystic kidney disease are autosomalrecessive PKD, in which there is a structural defect of thekidney present at birth, as opposed to NPHP, in which kidneysare normal at birth (with the exception of NPHP2) but degenerationleads to loss of renal function over the course of years. Regardingretinal involvement, NPHP can be associated with the developmentaldefect of retinal coloboma (lack of retinal tissue) in JBTSbut also with the degenerative processes of tapetoretinal degeneration/retinitispigmentosa (21,44,109). The joint occurrence of developmentaland degenerative defect may be explained by the fact that manydevelopmental transcriptional programs are reinitiated for mechanismsof tissue maintenance and repair. Because planar cell polarityplays an important role in developmental morphogenesis and alsoin the regeneration of differentiated tissue, a defect in planarcell polarity may explain both occurrence of cysts during organogenesisand degenerative cystogenesis as it occurs in NPHP.
No effective prophylaxis or treatment is available for NPHP.The only therapeutic options are supportive treatment once chronicrenal failure has developed and dialysis and transplantationfor terminal renal failure. An important future challenge willbe the development of therapies that capitalize on what we havelearned about the biology of NPHP and other cystic diseasesof the kidney. Gattone et al. (106) recently showed that therenal cystic phenotype of pcy mice, which is the equivalentof human NPHP type 3, can be strongly mitigated or even reversedby treatment with the vasopressin V2 receptor antagonist OPC31260.Similar results were obtained using a pkd2 mouse model (147).This effect is thought to be mediated by a reduction in intracellularcAMP levels, a finding that awaits reconciliation with the ciliary/centrosomehypothesis of NPHP. In this context, it is interesting thatnephrocystin-6 directly interacts with the transcription factorATF4/CREB2, which plays a role in the regulation of intracellularcAMP levels (21). Additional therapeutic approaches are beingconsidered for PKD (148).
Omran H, Fernandez C, Jung M, Haffner K, Fargier B, Villaquiran A, Waldherr R, Gretz N, Brandis M, Ruschendorf F, Reis A, Hildebrandt F: Identification of a new gene locus for adolescent nephronophthisis, on chromosome 3q22 in a large Venezuelan pedigree.
Am J Hum Genet 66
: 118
–127, 2000[CrossRef][Medline]
Ala-Mello S, Kivivuori SM, Ronnholm KA, Koskimies O, Siimes MA: Mechanism underlying early anaemia in children with familial juvenile nephronophthisis.
Pediatr Nephrol 10
: 578
–581, 1996[CrossRef][Medline]
Hildebrandt F, Sayer JA, Jungers P, Grünfield J-P: Nephronophthisis—medullary cystic and medullary sponge kidney disease. In:
Diseases of the Kidney and Urinary Tract, edited by Schrier WB, Philadelphia, Lippincott, Williams & Wilkins, 2007
, pp 478
–501
Waldherr R, Lennert T, Weber HP, Fodisch HJ, Scharer K: The nephronophthisis complex. A clinicopathologic study in children.
Virchows Arch A Pathol Anat Histol 394
: 235
–254, 1982[CrossRef][Medline]
Zollinger HU, Gaboardi F, Imbasciati E, Lennert T: Nephronophthisis (medullary cystic disease of the kidney). A study using electron microscopy, immunofluorescence, and a review of the morphological findings.
Helv Paediatr Acta 35
: 509
–530, 1980[Medline]
Kleinknecht C: The inheritance of nephronophthisis. In:
Inheritance of Kidney and Urinary Tract Diseases, Vol 9, edited by Avner ED, Boston, Kluwer Academic Publishers, 1989
, pp 299
–305
Steel BT, Lirenman DS, Battie CW: Nephronophthisis.
Am J Med 68
: 531
–538, 1980[CrossRef][Medline]
Hildebrandt F, Otto E, Rensing C, Nothwang HG, Vollmer M, Adolphs J, Hanusch H, Brandis M: A novel gene encoding an SH3 domain protein is mutated in nephronophthisis type 1.
Nat Genet 17
: 149
–153, 1997[CrossRef][Medline]
Saunier S, Calado J, Heilig R, Silbermann F, Benessy F, Morin G, Konrad M, Broyer M, Gubler MC, Weissenbach J, Antignac C: A novel gene that encodes a protein with a putative src homology 3 domain is a candidate gene for familial juvenile nephronophthisis.
Hum Mol Genet 6
: 2317
–2323, 1997[Abstract/Free Full Text]
Otto EA, Schermer B, Obara T, O’Toole JF, Hiller KS, Mueller AM, Ruf RG, Hoefele J, Beekmann F, Landau D, Foreman JW, Goodship JA, Strachan T, Kispert A, Wolf MT, Gagnadoux MF, Nivet H, Antignac C, Walz G, Drummond IA, Benzing T, Hildebrandt F: Mutations in INVS encoding inversin cause nephronophthisis type 2, linking renal cystic disease to the function of primary cilia and left-right axis determination.
Nat Genet 34
: 413
–420, 2003[CrossRef][Medline]
Olbrich H, Fliegauf M, Hoefele J, Kispert A, Otto E, Volz A, Wolf MT, Sasmaz G, Trauer U, Reinhardt R, Sudbrak R, Antignac C, Gretz N, Walz G, Schermer B, Benzing T, Hildebrandt F, Omran H: Mutations in a novel gene, NPHP3, cause adolescent nephronophthisis, tapeto-retinal degeneration and hepatic fibrosis
Nat Genet 34
: 455
–459, 2003[CrossRef][Medline]
Otto E, Hoefele J, Ruf R, Mueller AM, Hiller KS, Wolf MT, Schuermann MJ, Becker A, Birkenhager R, Sudbrak R, Hennies HC, Nurnberg P, Hildebrandt F: A gene mutated in nephronophthisis and retinitis pigmentosa encodes a novel protein, nephroretinin, conserved in evolution.
Am J Hum Genet 71
: 1167
–1171, 2002
Otto EA, Loeys B, Khanna H, Hellemans J, Sudbrak R, Fan S, Muerb U, O’Toole JF, Helou J, Attanasio M, Utsch B, Sayer JA, Lillo C, Jimeno D, Coucke P, De Paepe A, Reinhardt R, Klages S, Tsuda M, Kawakami I, Kusakabe T, Omran H, Imm A, Tippens M, Raymond PA, Hill J, Beales P, He S, Kispert A, Margolis B, Williams DS, Swaroop A, Hildebrandt F: Nephrocystin-5, a ciliary IQ domain protein, is mutated in Senior-Loken syndrome and interacts with RPGR and calmodulin.
Nat Genet 37
: 282
–288, 2005[CrossRef][Medline]
Sayer JA, Otto EA, O’Toole JF, Nurnberg G, Kennedy MA, Becker C, Hennies HC, Helou J, Attanasio M, Fausett BV, Utsch B, Khanna H, Liu Y, Drummond I, Kawakami I, Kusakabe T, Tsuda M, Ma L, Lee H, Larson RG, Allen SJ, Wilkinson CJ, Nigg EA, Shou C, Lillo C, Williams DS, Hoppe B, Kemper MJ, Neuhaus T, Parisi MA, Glass IA, Petry M, Kispert A, Gloy J, Ganner A, Walz G, Zhu X, Goldman D, Nurnberg P, Swaroop A, Leroux MR, Hildebrandt F: The centrosomal protein nephrocystin-6 is mutated in Joubert syndrome and activates transcription factor ATF4.
Nat Genet 38
: 674
–681, 2006[CrossRef][Medline]
Hildebrandt F, Strahm B, Nothwang HG, Gretz N, Schnieders B, Singh-Sawhney I, Kutt R, Vollmer M, Brandis M: Molecular genetic identification of families with juvenile nephronophthisis type 1: Rate of progression to renal failure. APN Study Group. Arbeitsgemeinschaft fur Padiatrische Nephrologie.
Kidney Int 51
: 261
–269, 1997[Medline]
Haider NB, Carmi R, Shalev H, Sheffield VC, Landau D: A Bedouin kindred with infantile nephronophthisis demonstrates linkage to chromosome 9 by homozygosity mapping.
Am J Hum Genet 63
: 1404
–1410, 1998[CrossRef][Medline]
Gretz N: Rate of deterioration of renal function in juvenile nephronophthisis.
Pediatr Nephrol 3
: 56
–60, 1989[CrossRef][Medline]
Christodoulou K, Tsingis M, Stavrou C, Eleftheriou A, Papapavlou P, Patsalis PC, Ioannou P, Pierides A, Constantinou Deltas C: Chromosome 1 localization of a gene for autosomal dominant medullary cystic kidney disease.
Hum Mol Genet 7
: 905
–911, 1998[Abstract/Free Full Text]
Scolari F, Ghiggeri GM, Amoroso A, Caridi GL, Aridon P: Genetic heterogeneity for autosomal dominant medullary cystic kidney disease (ADMCKD) [Abstract].
J Am Soc Nephrol 9
: 393A
, 1998
Potter DE, Holliday MA, Piel CF, Feduska NJ, Belzer FO, Salvatierra O Jr: Treatment of end-stage renal disease in children: A 15-year experience.
Kidney Int 18
: 103
–109, 1980[Medline]
Pistor K, Scharer K, Olbing H, Tamminen-Mobius T: Children with chronic renal failure in the Federal Republic of Germany: II. Primary renal diseases, age and intervals from early renal failure to renal death. Arbeitsgemeinschaft fur Padiatrische Nephrologie.
Clin Nephrol 23
: 278
–284, 1985[Medline]
Cantani A, Bamonte G, Ceccoli D: Familial juvenile nephronophthisis.
Clin Pediatr 25
: 90
–95, 1986[Abstract/Free Full Text]
Green A, Allos M, Donohoe J, Carmody M, Walshe J: Prevalence of hereditary renal disease.
Ir Med J 83
: 11
–13, 1990[Medline]
Warady BA, Hebert D, Sullivan EK, Alexander SR, Tejani A: Renal transplantation, chronic dialysis, and chronic renal insufficiency in children and adolescents. The 19
-tubulin, which constitutes the microtubule axoneme of primary cilia. We demonstrated that nephrocystin-1 and inversin localize to primary cilia of renal tubular cells (17)—the same subcellular compartment that was identified as central to the pathogenesis of PKD (1,66,103). This was one of the first findings to support a unifying theory of renal cystogenesis (1,72), which states that proteins that, when mutated, cause renal cystic disease in humans, mice, or zebrafish ("cystoproteins") are expressed in primary cilia, basal bodies, or centrosomes (66,72). The interaction and co-localization to cilia of nephrocystin-1, inversin, and 
-tubulin. In polarized epithelial cells, nephrocystin-4 localizes to primary cilia, basal bodies, and the cortical actin cytoskeleton, whereas in dividing cells, it localizes to centrosomes (101). Nephrocystin-4 is conserved in Caenorhabditis elegans and expressed in ciliated head and tail neurons of the nematode (108). Upon knockdown, it exhibits a male mating phenotype, similar to orthologs of other genes that are mutated in cystic kidney disease (108). 
Top
Abstract
Nephronophthisis: A Frequent...
, casein kinase