Research in the Division of Nephrology includes clinic trials and work by nephrology faculty in the areas of cardiovascular disease, hypertension, and chronic kidney disease as well as basic science research lead by Dr. Guy Perry and his team.
Identifying the Genetic Basis of Kidney Stones and Urinary Calcium
Kidney stones are a serious health and welfare issue for millions of Americans, affecting up to 12% of male Americans and 5% of females. Most stones are formed of calcium salts, and urinary hypercalciuria is the single greatest risk factor of this disorder. Earlier work suggested that nephrolithiatic disease was an autosomal dominant disorder, but later findings indicate a quantitative, polygenic construction of this disease.
Rodent models have been used for the investigation of this disorder: we are using F2 intercrosses Genetic Hypercalciuric Stone-forming (GHS) rats (a strain developed by our collaborator Dr. David Bushinsky via selection for increased calcium excretion, and which resembles all major features of human idiopathic hypercalciuria) with normocalciuric Wistar rats to identify quantitative trait loci (QTL) for increased calcium excretion. We have detected two QTL for urinary calcium: hc1 and hc2, located on rat chromosomes 1 (RNO1; 229 MB) and 4 (RNO4; 51 MB). Both are additive and their expression is partially limited to males, which is evocative of differences in kidneystone rate in male and female humans. Our next objective is to identify gene candidates for these loci using congenic line development and gene expression analysis. Initial analysis using pedigreed QTL mapping confirms the presence of the HC1 QTL in our congenic lines.
Examining the Genetic Basis of Residual Deviations in Phenotype
Nearly the entirety of genetic analysis based on quantitative genetic assays and techniques implicitly assumes the constancy of genetic effects. However, increasing empirical evidence exists that this assumption may be incorrect: model systems using Drosophila, fish and rodents suggest the presence of genetic effects on residual variance in morphology, physiology and behavior. We are investigating this phenomenon using a fortuitous rodent model of urinary calcium excretion (GHS rats) and of Attention-Deficit/Hyperactivity Disorder (ADHD), the Spontaneous Hypertensive Rat (SHR); GHS females have higher variance in urinary calcium excretion than male GHS rats, and WKY males higher variance than WKY females. Using our F2 GHS×WKY mapping cohort, we have identified two significant QTL (hcpd1, hcpd2) and several suggestive QTL for individual phenotypic dispersion (PDi) in urinary calcium. Genes with high coefficients of variance (CV) near these QTL include those associated with methlyation (Mmadhc, Suv420h2), spliceosome formation (Lsm5, Lsm8, Prpf38b, Tnpo3, Sprk1, Sprk2), serine physiology (serine-rich proteins being critically involved with spliceosome formation: Cdk5, Dkk2, Dmpk, Egf, Galnt11, Riok2, Serpine2, Srpk1, Srpk2, Tnpo3). In particular, Mmadhc and Tra2a were linked to hcpd1, and Lsm5 and two genes in the Fk gene family—Fkp506, Fk506—to hcpd2; Mmadhc has extreme CVs for expression in both kidney and duodenal tissue (>P3). These results strongly indicate the presence of genes responsible for residual variance, and indicate a number of gene candidates in expected pathways: spliceosome formation, methlyation and the regulation of RNA transcription. Several other genes with high CVs linked to our QTL for PDi in urinary calcium were associated with metal ion binding: as a result, we do not know whether the dispersion of phenotype is a general process, ubiquitous to all genes (via general variability in spliceosome induction/mRNA modification/serine protein proportion) or a specific effect on metal ion genes, including those that manipulate Ca2+ physiology and that of associated ions. Sex appears to be a critical element of this process: Tra2a, a key gene in of sexual differentiation linked to hcpd2, also had high CVs in expression; our QTL for PDi are only significantly expressed in female F2 GHS×WKY rats, and previous work in these strains illustrates a marked difference in residual variance according to sex.
The Genetics of Craniosynostosis and Associated Disorders
Craniosynostosis, a relatively common congenital anomaly (1 in 2000 live births), has been linked to fibroblast growth factors (FGF) and FGF receptor mutations, but there is considerable heterogeneity among the various syndromes of craniosynostosis. We have recently discovered a putative spontaneous rat model of craniosynostosis, the spontaneous pug rat (SPR). SPR rats present hypoplasia of the mandible, premaxilla and maxilla, along with fusion of both the coronal and sagittal sutures. Analysis by our lab indicates that this phenotype is recessive, and has highly negative genetic correlation with body size and highly positive genetic correlation with calcium excretion. We are attempting to isolate the genetic basis of this phenotype and its connection with calcium excretion physiology.