Refined Mapping of the Renal Failure Rf-3 Quantitative Trait Locus
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BASIC RESEARCH www.jasn.org Refined Mapping of the Renal Failure Rf-3 Quantitative Trait Locus Caitlin C. O’Meara,*† Jozef Lazar,*‡ Matthew Hoffman,*† Carol Moreno,*† and Howard J. Jacob*†§ *Human and Molecular Genetics Center, †Department of Physiology, ‡Department of Dermatology, and §Department of Pediatrics, Medical College of Wisconsin, Milwaukee, Wisconsin ABSTRACT Rf-3, a quantitative trait locus (QTL) on rat chromosome 3, affects the development of CKD in Fawn- Hooded Hypertensive (FHH) rats. This QTL spans 110 Mb and approximately 1400 genes; therefore, narrowing the position of this locus is necessary to elucidate potential candidate genes. Here, we used congenic models and comparative genomics to refine the Rf-3 candidate region. We generated congenic lines carrying smaller intervals (subcongenics) of the Rf-3 region and used these lines to reduce the Rf-3 candidate region by 94% (to 7.1 Mb). We used comparative genomics to identify QTL for both nephropathy and albuminuria in the syntenic region of this interval for both human and mouse. We also used the overlapping homologous regions to reduce the number of likely positional candidate genes to 13 known or predicted genes. By combining congenic models and cross-species studies, we narrowed the list of candidate genes to a level that we could sequence the whole interval to further identify the causative gene in future studies. J Am Soc Nephrol 22: 518–525, 2011. doi: 10.1681/ASN.2010060661 Chronic kidney disease (CKD) is a growing health sive (FHH) rat is a well-established model for hy- risk in the United States and worldwide, with the pertension-associated kidney disease.16–20 This incidence continuing to rise at an alarming rate.1 particular strain spontaneously develops systolic Epidemiologic studies have shown that familial and and glomerular hypertension, and consequently, ethnic components contribute to an individual’s renal complications, as indicated by proteinuria, al- risk of developing renal complications as a result of buminuria, and glomerular sclerosis.16,18–23 Be- hypertension and/or diabetes.1–5 Human associa- cause of its robust phenotype, we crossed this strain tion and linkage studies have identified specific re- with the normotensive, renal failure–resistant Au- gions of the genome that significantly contribute to gust Copenhagen Irish (ACI) rat and performed F2 renal disease susceptibility6–12; however, the degree linkage analyses to identify regions of the rat ge- of genetic heritability accounted for by these genes nome that cause kidney disease susceptibility in the is only a small percentage of the total heritability.13 FHH rat.20,21 These linkage analyses showed the Consequently, there is a need to pursue other strat- presence of five renal failure quantitative trait loci egies for identifying genes and their associated (QTLs) called Renal failure 1 through 5 (Rf-1 pathways that are driving CKD. The rat model of- fers numerous advantages, such as an abundance of Received June 23, 2010. Accepted October 16, 2010. physiologic data on many well-characterized dis- Published online ahead of print. Publication date available at ease models and available consomic and congenic www.jasn.org. strains that can be used to study the genetic basis of Correspondence: Dr. Howard J. Jacob, HRC5200, 8701 Water- kidney disease.14,15 town Plank Road, Milwaukee, WI 53226. Phone: 414-456-4887; As in humans, genes play a role in the develop- Fax: 414-456-6516; E-mail: [email protected] ment of CKD in rats. The Fawn-Hooded Hyperten- Copyright © 2011 by the American Society of Nephrology 518 ISSN : 1046-6673/2203-518 J Am Soc Nephrol 22: 518–525, 2011 www.jasn.org BASIC RESEARCH through Rf-5). Subsequent phenotypic analysis of single and RESULTS double congenic animals has shown a synergistic relationship between the various Rf QTLs. Specifically, an interaction was Assessment of Albumin Excretion and BP in Rf-1 ؉ identified between Rf-1 and Rf-3, and Rf-1 and Rf-4, whereas, 3؉4 Congenic Strains the Rf-3 and Rf-4 loci had little to no apparent effect on renal To physically narrow the candidate region of the Rf-3 QTL, we gen- function alone.24,25 erated and phenotyped a panel of subcongenic lines targeting the Rf-3 To elucidate the specific gene variant(s) causing the ob- QTL for UAV at 9 weeks of age after UNX. We found that congenic served phenotype, it is necessary to narrow the candidate lines containing the FHH genotype between genetic markers region to a manageable number of genes, because the Rf-3 D3Got102 and D3Got121 had higher UAV compared with other sub- region is 110 Mbp in size and contains Ͼ1400 genes. Our congenic lines (data not shown). To further study the contribution group has previously used this strategy to narrow the Rf-2 of this region to renal impairment, we selected two subcongenic QTL region and identify Rab38 as a candidate gene.22 Be- lines—Rf-1 ϩ 3ϩ4_a (ACI.FHH [D1Mit18-D1Rat90]/[D3Rat6- cause gene–gene interactions have been identified between D3Got149]/[D14Mit11-D14Rat33/D14Rat65-D14Rat90]) and the various Rf QTLs and because of the polygenic nature of Rf-1 ϩ 3ϩ4_b (ACI.FHH [D1Mit18-D1Rat90]/[D3Got102- renal disease, we received triple congenic animals from Dr. D3Got149]/[D14Mit11-D14Rat33/D14Rat65-D14Rat90])—for Abraham Provoost (Erasmus MC, Rotterdam, The Nether- phenotypic analysis. These overlapping congenic lines are geneti- lands) that have an ACI disease-resistant background and cally identical to Rf-1 ϩ 4 except for the Rf-3 region, where FHH-sensitive locus introgressed onto Rf-1 (D1Mit18- Rf-1 ϩ 3ϩ4_a is FHH from D3Rat6 to D3Got149 and Rf-1 ϩ D1Rat90), Rf-3 (D3Rat84-D3Rat59), and Rf-4 (D14Mit11- 3ϩ4_b is FHH from D3Got102 to D3Got149 (Figure 1). At 9 D14Rat33/D14Rat65-D14Rat90), called Rf-1 ϩ 3ϩ4, be- weeks of age, Rf-1 ϩ 3ϩ4_b had significantly higher levels of cause the presence of FHH alleles on the Rf-1 locus, and UAV than Rf-1 ϩ 3ϩ4_a (40.06 Ϯ 7.60 versus 15.40 Ϯ 2.40 possibly the Rf-4 locus, is necessary for Rf-3 to have a mea- mg/d, respectively, P ϭ 0.024) and also compared with surable effect on renal disease.24,25 In preliminary studies, Rf-1 ϩ 4 (11.18 Ϯ 3.18 mg/d, P ϭ 0.002; Figure 2), indicat- we observed that, after unilateral nephrectomy (UNX), albumin excretion (UAV) of Rf-1 ϩ 3ϩ4 is almost three times higher than that of the Rf-1 ϩ 4 double congenics (ACI.FHH [D1Mit18-D1Rat90]/[D14Mit11-D14Rat33/ D14Rat65-D14Rat90]), showing the utility of the triple congenic model for mapping disease- causing variant(s) in Rf-3 on a resistant genome background. In this study, we crossed Rf-1 ϩ 3ϩ4 animals to Rf-1 ϩ 4 animals to generate subcongenic lines targeting the Rf-3 QTL. Phenotypic analysis of these sub- congenic lines showed a 7.1-Mb region of rat chromosome 3 that significantly con- tributes to the early development of renal disease. Interestingly, three separate link- age analysis studies in human and mouse have mapped kidney disease loci con- cordant to this 7.1-Mb candidate re- gion,12,26,27 suggesting that the same ge- netic elements may play a role in renal disease susceptibility across species. By comparing the breakpoints of our candi- date region to the boundaries of mouse and human renal function QTLs, we were able to further narrow down the list of candi- Figure 1. Schematic representation of the flanking markers on rat chromosome 3 for Rf-1 ϩ 4, Rf-1 ϩ 3ϩ4_a, and Rf-1 ϩ 3ϩ4_b congenic lines and the homologous regions date genes from 1400 to just 13 known and in human and mouse. The flanking markers for the FHH rat chromosome 3 (RNO3) are predicted genes. In addition to cross-species D3Got102-D3Got149 for Rf-1 ϩ 3ϩ4_b and for Rf-1 ϩ 3ϩ4_a are D3Rat6-D3Got149. analysis, we also sequenced the entire 7.1-Mb The region differentiating lines a and b (Rf-3_b), from D3Got102 to D3Got121,is region to identify variants between ACI and homologous to two regions on human chromosome 20, from the p end of the FHH potentially responsible for the Rf-3 re- chromosome to approximately 1.3 Mb and from 29 to 37 Mb. Rf-3_b is homologous to nal phenotype. mouse chromosome 2 from approximately 151.4 to 158.1 Mb. J Am Soc Nephrol 22: 518–525, 2011 Fine Mapping the Rf-3 QTL 519 BASIC RESEARCH www.jasn.org Figure 3. Mean arterial pressure (MAP) is elevated in Rf- 1ϩ3ϩ4_b animals. Number of animals was 6, 7, and 6 for Rf-1 ϩ ϩ Figure 2. Rf-1 3 4_b animals excrete higher levels of albumin ϩ 4, Rf-1 ϩ 3ϩ4_a, and Rf-1 ϩ 3ϩ4_b, respectively. **P Ͻ 0.01. ϩ ϩ ϩ than Rf-1 4 and Rf-1 3 4_a animals at 9 weeks of age. Number Statistical comparison was done using a one-way ANOVA fol- ϩ ϩ ϩ of animals was 9, 13, and 16 for Rf-1 4, Rf-1 3 4_a, and lowed by a Holm-Sidak post hoc test. Rf-1 ϩ 3ϩ4_b, respectively. *P Ͻ 0.05; **P Ͻ 0.01. Datasets were not normally distributed, so each data point was log ϩ Ϯ Ͻ ϩ ϩ transformed before statistical analysis. Statistical comparison with Rf-1 4 (0.34 0.10, P 0.001) and Rf-1 3 4_a Ϯ ϭ was done using a one-way ANOVA followed by a Holm-Sidak (0.52 0.05, P 0.004) glomeruli (Figure 4C).