Genetics of congenital anomalies of the kidney and urinary tract : towards elucidation of genetic factors in the etiology of vesico-ureteral reflux

Abstract

Chronic renal failure and end stage renal disease (ESRD) can be life-threatening conditions. In a significant number of cases with ESRD the primary cause l is a congenital anomaly of the kidney and/or urinary tract (CAKUT). Vesico-ureteral reflux (VUR) is the most common of the CAKUT spectrum. The studies in this thesis focus on isolated VUR. Although most children grow out of VUR without serious morbidity, a subset has associated complications. Together these account for 15% of ESRD in Dutch children. An autosomal dominant inheritance pattern with reduced penetrance is seen in some VUR families. Other segregation patterns have also been described, but for most cases a multifactorial etiology is most likely. Disruption of ureter budding in mouse embryos has been shown to lead to CAKUT. Genes involved in this budding process are therefore considered to be candidate genes for VUR susceptibility. The studies in this thesis were aimed at identifying genetic risk factors for VUR and/or CAKUT. Chapter 2 presents a literature study regarding the relationship between prenatally detected hydronephrosis (PNH) and postnatal VUR. In 15% of the patients with PNH primary VUR was detected, approximately 35% had other urogenital anomalies and in 50% postnatal examinations were normal. To explore whether we could provide clinical evidence for a new hypothesis on a contributory constitutional factor to VUR predisposition, and therefore on VUR genetics, we evaluated joint hypermobility in 50 VUR patients (chapter 3). The results of this study suggest that patients with hypermobile joints may have underlying systemic laxity that might in turn contribute to VUR. In chapters 4, 5 and 6 we investigated candidate genes in the ureter budding process for their role in VUR development with varied genetic approaches. We studied a syndromal patient with severe VUR who had a complex chromosomal aberration disturbing the ROBO2 gene. ROBO2 is a protein known to have a role in the ureter budding pathway. This led us to search for mutations in this gene in 124 VUR patients. Indeed mutations were detected in two familial cases. In four families we performed a linkage study, that focused on a subset of genes in the ureter budding process and other candidate regions. We could significantly exclude a role for the majority of genes under investigation. We performed an association study of common SNPs in 44 genes in the ureter budding pathway in > 400 Dutch VUR cases and > 1400 controls. None of the SNPs were significantly associated to VUR. Common SNPs in four ureter budding genes (GREM1, EYA1, ROBO2 and UPK3A) did show a trend towards association. In a subset of VUR patients with duplex collecting systems we identified 3 patients with mutations in UPK3A that possibly contribute to their phenotype. In chapter 7 we investigated patients with renal adysplasia, another part of the CAKUT spectrum. We investigated a gene known to be involved in renal adysplasia in other populations (UPK3A) and a new candidate gene (FGF7). We studied 19 patients and identified the first known stop-mutation in UPK3A.