A Genome-Wide DNA Microsatellite Association Screen to Identify Chromosomal Regions Harboring Candidate Genes in Diabetic Nephropathy
Amy Jayne McKnight*,
A. Peter Maxwell*,
Stephen Sawcer,
Alastair Compston,
Efrosini Setakis,
Chris C. Patterson,
Hugh R. Brady|| and
David A. Savage*
* Nephrology Research Group; Department of Epidemiology & Public Health, Queens University Belfast, Belfast, Northern Ireland; Department of Neurology, University of Cambridge, Cambridge, United Kingdom; Department of Epidemiology and Public Health, Imperial College, London, United Kingdom; and || Conway Institute, University College Dublin, Dublin, Ireland
Address correspondence to: Dr. David A. Savage, Nephrology Research Group, Queens University Belfast, c/o Medical Genetics, Floor A, Tower Block, Belfast City Hospital, Lisburn Road, Belfast BT9 7AB, Northern Ireland. Phone: +44-0-28-90329241 ext. 2149; Fax: +44-0-28-90236911; d.savage{at}qub.ac.uk
Received for publication May 12, 2005.
Accepted for publication January 3, 2006.
In an effort to accelerate the identification of susceptibilitygenes in diabetic nephropathy, the first genome-wide fluorescence-basedDNA microsatellite (n = 6000) association screen was performed,using pools of genomic DNA derived from Irish patients with(cases; n = 200) and without (controls; n = 200) type 1 diabeticnephropathy. Allele image profiles were generated for 5353 (89.2%)microsatellite markers for both case and control pools. Allelecounts (estimated from allele image profiles) were comparedin case versus control groups, and empirical P values were generated.Markers then were ranked on the basis of their empirical P values(lowest to highest). Repeat PCR amplification and electrophoresisof pooled samples were performed systematically on ranked markersuntil the 50 most associated markers with consistent resultswere identified. DNA samples that composed the pools then weregenotyped individually for these markers. Two markers on chromosome10, D10S558 (Pcorrected = 0.005) and D10S1435 (Pcorrected =0.016), revealed statistically significant associations withdiabetic nephropathy. An additional four markers (D6S281, D4S2937,D2S291, and D17S515) also are worthy of further investigation.Relevant functional candidate genes have been identified inthe vicinity of these markers, demonstrating the feasibilityof low-resolution genome-wide microsatellite association screeningto identify possible candidate genes for diabetic nephropathy.
Diabetic nephropathy (DN) is an extremely serious clinical complicationof diabetes in which patients exhibit persistent proteinuria,hypertension, declining renal function, and an increased prematuremortality largely as a result of cardiovascular disease. Thepathophysiology of DN involves glomerular capillary hypertension,glomerular hyperfiltration, mesangial matrix expansion, andglomerulosclerosis (13). In Western populations, DN hasemerged as the leading cause of ESRD (4,5), resulting in patientsrequiring dialysis and transplantation.
The risk for developing nephropathy in patients with diabetesbecomes greater with increasing duration of diabetes and poorregulation of blood glucose and BP. Studies have shown, however,that irrespective of good glycemic control, some patients withdiabetes still develop nephropathy (6,7). In addition, althoughadministration of antihypertensive therapy to microalbuminuricpatients may retard progression toward ESRD (8,9), irreversiblestructural changes in the kidney have already occurred by thetime microalbuminuria is identified. Disease prevention thereforeis an important goal.
Evidence from epidemiologic and family-based studies stronglysuggest a role for both susceptibility and protective genesin the development of DN in type 1 diabetes. This is illustratedby the observation that the disease generally develops within20 yr after diagnosis of type 1 diabetes, whereas the risk fordeveloping nephropathy decreases in patients who have diabeteswithout nephropathy after 20 yr of duration of diabetes (10).Also, there is strong concordance for nephropathy status amongsiblings with diabetes (11,12), and there is evidence to supportthe involvement of genes with major and minor effects in thepathogenesis of this disease (12,13).
Identification of validated genetic markers for high and lowrisk for nephropathy will facilitate prediction of nephropathyin patients with diabetes; this information also could leadto the development of pharmacogenetic treatments to preventthis disease. Genome-wide linkage analysis, however, has beencomplicated largely by difficulties in obtaining sufficientnumbers of sibling pairs who have diabetes and are concordantor discordant for nephropathy. Consequently, case-control associationstudies, using a candidate gene approach, have become the methodof choice for identifying causal gene variants that contributeto DN. A number of such studies have been performed, but noconsistent positive associations have been identified (1418).Conflicting findings may be due to several factors, includinggenetic heterogeneity, inadequate sample sizes, and use of inappropriateselection criteria for patients (19).
In an attempt to identify chromosomal regions that harbor possiblecandidate genes for DN, we performed a low-resolution genome-widemicrosatellite [i.e., anonymous nucleotide repeat units, e.g.,(CA)n] association screen, using the same method and markersused in the Genetic Analysis of Multiple sclerosis in EuropeanS(GAMES) collaborative project (2023; http://www-gene.cimr.cam.ac.uk/MSgenetics/GAMES).Separate pools of DNA, derived from Irish patients with type1 diabetes and with (cases) and without (controls) nephropathy,were screened for association using 6000 fluorescently labeledmicrosatellite markers (the GAMES marker set). A comparisonof allele image profiles (AIP; a series of peaks of varyingheights reflecting the allele frequency distribution withineach analyzed DNA pool) from cases and controls permitted rankingof markers according to empirical P values; individual genotypingthen was performed on the 50 most associated markers. In thisreport, we describe the major findings of this genome-wide associationscreen and demonstrate the feasibility of this approach foridentifying possible candidate genes in DN.
Patient Groups and Selection Criteria
Patients were recruited from Northern Ireland (n = 105 cases;n = 105 controls) and the Republic of Ireland (n = 95 cases,n = 95 controls). Parents and grandparents of patients wereborn in Ireland. All patients were white and had type 1 diabetes,defined as diabetes that was diagnosed before the age of 31yr and required insulin from diagnosis. Cases with nephropathywere defined by development of persistent proteinuria (>0.5g protein/24 h) at least 10 yr after diagnosis of diabetes,hypertension (BP >135/85 mmHg and/or treatment with antihypertensiveagents), and presence of diabetic retinopathy. In contrast,controls were patients who had diabetes duration of at least15 yr and urinary albumin in the normal range and were not receivingantihypertensive treatment. Patients with microalbuminuria wereexcluded from both groups. Ethical approval was obtained fromthe appropriate research ethics committees, and written informedconsent was obtained from patients before this study was conducted.
Construction and Assessment of DNA Pools
Genomic DNA samples from cases and controls were quantifiedusing the Picogreen method (Molecular Probes, Eugene, OR) andnormalized to 10 ng/µl in 10 mM Tris/0.1 mM EDTA (pH 7.6).Equal volumes (200 µl) of individual DNA samples (10 ng/µl)from cases and controls were combined to generate the DNA pools.Before use in the genome-wide screen, the pools were validatedby comparing estimated allele frequencies that were generatedfrom pools (five replicate AIP in each case) with allele frequenciesobtained by individual genotyping of samples that composed thepools, for two randomly selected microsatellite markers, D19S49(dinucleotide) and D7S1821 (tetranucleotide), as follows. PCRreactions (15 µl in total) that contained 25 ng DNA, 5pmol of each primer (forward primer for both markers was labeledwith FAM), and 9 µl of True Allele PCR Premix (AppliedBiosystems, Warrington, UK) were set up. All PCR reactions wereperformed using an MJ Tetrad thermal cycler using the followingcycling conditions; 95°C for 12 min, 10 cycles at 94°Cfor 15 s, 55°C for 15 s, and 72°C for 30 s followedby 20 cycles of 89°C for 15 s, 55°C for 15 s, 72°Cfor 30 s, and finally 72°C for 20 min. Each PCR productwas diluted 10-fold in ddH2O; 1 µl of diluted PCR productthen was combined with 12 µl of ROX/HiDi (prepared bycombining 0.3 µl of GS-400HD [ROX] and 11.7 µl ofHiDi formamide, both supplied by Applied Biosystems), and themixture was subjected to capillary electrophoresis on a 3100genetic analyser (Applied Biosystems). The raw data were exportedto ABI Genescan software to confirm size labeling. Files thenwere imported into ABI Genotyper software, where alleles wereidentified appropriately.
Genome-Wide Microsatellite Genotyping
The 6000 microsatellite markers that were used in the screeningphase consist of 811 that make up the Applied Biosystems high-densitylinkage mapping set (LMS-HD5) together with an additional 5189identified from the Co-operative for Human Linkage Centre database(http://gai.nci.nih.gov/CHLC), the Genethon database (http://www.genethon.fr),and the Genome database (http://www.gdb.org). The total numberof markers from each chromosome was chosen to reflect the numberof genes that map to that chromosome (according to the NationalCenter for Biotechnology Information web site in December 1999,http://www.ncbi.nlm.nih.gov). The density of markers thereforeis biased toward gene-rich and against gene-poor chromosomes.However, markers were not selected to be necessarily intragenic.Within each chromosome, markers were selected so as to makethe distribution of markers as uniform as possible accordingto their location scores on the genetic location database (http://cedar.genetics.soton.ac.uk/public_html).The 6000 microsatellite markers that were used in this studyresulted in an average intermarker spacing of 0.6 cM (or 0.5Mb; Appendix 1); gender-averaged cM measurements were derivedfrom the Marshfield Master Map (http://www.ncbi.nlm.nih.gov/mapview).There are 329 gaps >2 cM in gene-poor regions, 11 of whichare >10 cM. These gaps correspond mainly to pericentromericregions, particularly on acrocentric chromosomes, and sectionsof the genome that generally contain very few genes. In total,4723 (79%) of the markers are dinucleotides, 1019 (17%) aretetranucleotides, and 258 (4%) are trinucleotides. The estimatedaverage heterozygosity (according to data in the originatingdatabases) of the 6000 markers is 70%. Details of the markers,including primer sequences, are available on the GAMES web site(http://www-gene.cimr.cam.ac.uk/MSgenetics/GAMES).
Appendix 1. Marker densities (cM and Mb) in each chromosome for 6000 microsatellite markersa
Aliquots of DNA (2.5 µl; 25 ng) from each pool were PCRamplified by individual primer pairs that flanked the 6000 microsatellitemarkers as described above for pool assessment. Amplificationproducts were subjected to capillary electrophoresis on a 3700genetic analyser (Applied Biosystems), either as a uniplex ora triplex essentially as described above. For PCR products withAIP peak heights above 10,000 (i.e., saturated), the productswere diluted and run again. To minimize electrophoretic artifacts,we performed repeat electrophoresis on all PCR products; whencontradictory results were observed, PCR products were run ona third occasion to assist in resolving ambiguities. RepeatPCR amplification and electrophoresis were performed systematicallyon ranked markers with the smallest empirical P values as describedin the Statistical Analyses section to facilitate the identificationof the 50 most associated markers.
Statistical Analyses
Clinical characteristics of the patients were compared by theindependent samples t test. The electrophoretic output for agiven microsatellite marker using pooled DNA comprises a seriesof peaks, an AIP (Figure 1), where the height of each peak reflectsthe frequency of the corresponding microsatellite allele. Allelecounts were estimated by normalizing the observed peak heightsaccording to the number of alleles in the respective pool. Allelecounts from cases and controls then were compared using a 2test. Because pooling introduces additional sources of varianceabove and beyond the expected sampling variance, exact P valuescannot be determined for these statistics. We therefore usedthe observed distribution of results to calculate empiricalP values for each marker and therefore enable their ranking(23), i.e., markers that were ranked according to lowest empiricalP value showed the greatest difference between case and controlAIP. To refine this ranking and reduce the effects of the additionalsources of error introduced by pooling, we generated replicateAIP from both the case and control pools for each of the 63most highly ranked markers; the total data then were reanalyzed,and the markers were reranked.
Figure 1. Allele image profiles for D10S1435 marker in case and control pools.
Individual genotyping then was performed in the 50 most highlyranked markers to distinguish definitively those that showedgenuine allele frequency differences from those that rankedhighly as a result of pooling-induced variance. Results fromthe individual typing were tested for significance using a 2test after amalgamation of rare alleles. When any cell in thecontingency table had an expected value <5, the associatedallele was pooled with the next rarest allele and this processwas repeated until no cell had an expected value <5. Thisanalysis was performed using the T2 statistic in the CLUMP program(24). P values for individual genotyping were corrected forthe total number of comparisons made in the initial analysisby DNA pooling (5353). Markers that attained significance wereexamined using the Clump T4 statistic; the alleles were pooledinto two groups in such a way as to maximize the 2 test statistic.This pooling enabled odds ratios (OR) and 95% confidence intervals(CI) to be obtained.
Clinical Characteristics of Patients Table 1 lists the clinical characteristics of the case and controlgroups. There were a total of 84 and 124 women in the case andcontrol groups, respectively. The age at diagnosis of diabeteswas not significantly different between the groups. The meanduration of diabetes was at least 25 yr for both groups. Asexpected, the average BP was higher in cases versus controls,despite the use of antihypertensive drug treatment by patientswith DN.
Table 1. Clinical characteristics of cases (n = 200) and controls (n = 200) who composed DNA poolsa
Assessment of DNA Pools and Genome-Wide Screen
Excellent concordance was found between allele frequencies thatwere determined by pooling versus individual genotyping methodsfor D19S49 and D7S1821, in both case and control pools (datanot shown). Interpretable AIP were generated for both case andcontrol pools in 5353 (89.2%) markers. A total of 647 markerswere excluded because of PCR failures in case and/or controlpools or difficulties in data interpretation (e.g., weak ordiffuse peaks in AIP). Markers were ranked according to empiricalP value (lowest to highest). To refine the marker ranking, wesystematically performed repeat PCR amplification and electrophoresisof pooled samples on the 63 most highly ranked markers until50 markers that showed consistent results were identified. EmpiricalP values that were obtained for these markers were not corrected.These markers then were genotyped individually in the same samplesthat composed the pools (Appendix 2). Of note, two markers onchromosome 10 (D10S558 and D10S1435) revealed significant differencesin allele distribution between groups after correction by thestringent Bonferroni method (Table 2). The allele distributionsfor these two markers are presented in Table 3 and indicatethat larger D10S558 alleles were associated with increased riskfor nephropathy (OR 2.50; 95% CI 1.83 to 3.41), whereas theD10S1435 272 allele was associated with decreased risk (OR 0.44;95% CI 0.32 to 0.61). Furthermore, four additional markers (D2S291,D4S2937, D6S281, and D17S515) gave small uncorrected P values,and although they failed to achieve statistical significanceafter Bonferroni correction, they warrant further consideration(Table 2).
Appendix 2.P values for individual genotyping of case and control DNA samples that composed pools for the 50 highest ranked markers derived from DNA pooling and listed by chromosome
Table 2.P values for individual genotyping of case and control DNA samples that composed pools for markers that revealed statistically significant differences between groups and those that warrant further investigation
Table 3. Comparison of allele distributions between cases (n = 200) and controls (n = 200) from individual genotyping for two markers that remained significant after Bonferroni correction
Genetic association studies that have used a candidate geneapproach have been unsuccessful to date in identifying single-nucleotidepolymorphisms (SNP) that show a consistent association withDN in type 1 diabetes. In addition, high-resolution whole-genomeSNP screening in DN currently is not feasible, partly becauseof the extremely high total cost of SNP genotyping and uncertaintyas to the composition of an appropriate panel of SNP for a comprehensivescan. However, with recent advances in novel, low-cost, high-throughputgenotyping technologies (25,26) and improvements in statisticalmethods for data analysis (27), as well as projects such asthe International HapMap (28), such screening certainly willbe feasible in the future. It is interesting that a recent Japanesestudy assessed >80,000 SNP for association with DN in type2 diabetes and a significant SNP association was reported betweenthe engulfment and cell motility (ELMO 1) gene and DN (29).
In this report, we applied a highly efficient but low-resolutionapproach for whole-genome association screening in DN that uses6000 microsatellite markers and a DNA pooling strategy; thisstrategy originally was used in the GAMES collaborative project(2023). Importantly, the GAMES project succeeded in identifyingthe well-established association of multiple sclerosis withthe MHC region, confirming that the approach is able to identifymicrosatellites in linkage disequilibrium with genuine but modestsusceptibility factors.
Strict phenotyping criteria were used in the selection of casesand controls in our study to reduce the possibility of falseoutcomes related to phenotype misclassification. In addition,all participants were white and of Irish descent, thereforereducing potential problems associated with population stratification.We identified the 50 most associated markers that showed consistentresults on repeat analysis using DNA pools. Individual genotypingof D10S558 and D10S1435 markers on chromosome 10 revealed statisticallysignificant differences in the distribution of marker allelesin cases compared with controls (Table 2). It is interestingthat suggestive linkage of D10S1435 in early-onset nondiabeticESRD was reported recently in black patients (30). Also, a recentstudy reported linkage of a locus on chromosome 6q (D6S281)to essential hypertension in UK patients (31). Relevant functionalcandidate genes are located within the vicinity of D10S558 andD10S1435, as well as for four other markers (D2S291, D4S2937,D6S281, and D17S515) that are worthy of further investigation(Table 2).
The discrepancies that were observed in allele frequency datathat were obtained for DNA pooling and individual genotypingare not unexpected. However, this was minimized by careful constructionand assessment of the pools and applying checks for false-positiveoutcomes that resulted from electrophoretic and/or PCR artifacts(e.g., stutter bands, length-dependent amplification). Recently,it was shown that multiple disjoined pools are preferable tosingle pools (32), although there are additional cost considerationsassociated with this strategy.
The power of our study is limited by the sample size used, theconfounding produced by our use of the pooling approach, andthe low density of markers used. We recognize that genes thatexert small effects or are not in linkage disequilibrium withthe markers used cannot be identified by this method. Nevertheless,we have shown that this genome-wide microsatellite screen, usingpools of DNA, is a cost-effective strategy for identifying positiveassociations between microsatellite markers and DN. Furthermore,we have demonstrated the utility of the GAMES method for theidentification of possible functional candidate genes for thisdisease. We intend to assess haplotype tag SNP (33) within thesecandidate genes for association with DN using relatively largecase-control studies such as the Juvenile Diabetes ResearchFoundationfunded Genetics of Kidneys in Diabetes (UnitedKingdom) case-control collection and Diabetes UK Warren 3 familycollection; replication studies will be performed in case andcontrol collections derived from other populations, such theUS Genetics of Kidneys in Diabetes collection.
Acknowledgments
Some of these data were presented at the American Society ofNephrology, San Diego, CA, November 12 to 17, 2003.
We thank the Northern Ireland Research and Development Officeand the Northern Ireland Kidney Research Fund for supportingthis project. We also thank Julia Gray and Tai Wai Yeo (NeurologyUnit, University of Cambridge) and Nick Leaves (HGMP ResourceCentre, Cambridge) for technical assistance.
Footnotes
Published online ahead of print. Publication date availableat www.jasn.org.
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