Common variants of the glial cell-derived neurotrophic factor gene do not influence kidney size of the healthy newborn

Association Between RET (rs1800860) and GFRA1 (rs45568534, rs8192663, rs181595401, rs7090693, and rs2694770) Variants and Kidney Size in Healthy Newborns

BACKGROUND

Abnormal congenital nephron number has been implicated in the pathogenesis of hypertension and renal disease. The RET receptor complex propagates signals essential for nephrogenesis and the RET c.1296G>A polymorphism, leading to aberrant splicing of exon 7, is associated with reduced kidney volume, a surrogate for nephron endowment. The glial cell-derived neurotrophic factor (GDNF) family receptor alpha 1 (GFRA1) is a component of the RET receptor complex, and three alternatively spliced GFRA1 transcripts (with or without exon 5) have been identified. In rats, exclusion of exon 5 results in stronger GDNF binding affinity and RET activation. The aims of this study were to investigate further the relationship between RET c.1296G>A and kidney volume, and also to investigate the association between the GFRA1 polymorphisms near and within the alternatively spliced exon 5, as well as the functional 5'-UTR c.-193C>G with kidney volume.

MATERIALS AND METHODS

The study included 188 healthy full-term newborns. Genotyping of the RET (NM_020975.4:c.1296G>A, rs1800860) and GFRA1 (NM_005264.5:c.-193C>G, rs45568534; c.419-87A>G, rs8192663; c.429G>A, rs181595401; c.433+127A>G, rs7090693; c.433+245A>G, rs2694770) polymorphisms was performed using polymerase chain reaction-restriction fragment length polymorphism, minisequencing, or sequencing. Total kidney volume (TKV) was determined by ultrasound and normalized to body surface area (TKV/BSA). Both marker-by-marker and haplotype-based methods were used to test for associations between polymorphisms and TKV/BSA.

RESULTS

TKV/BSA in RET c.1296A allele carriers was significantly lower compared with GG homozygotes (103 ± 23 vs. 110 ± 19 mL/m², p = 0.034). c.429G>A was invariant in our sample. There was no association between any of the GFRA1 polymorphisms and renal volume.

CONCLUSIONS

RET c.1296A may be a common susceptibility allele for nephron underdosing-related diseases. The 5'-UTR and intronic variants near exon 5 of GFRA1 are not associated with nephron endowment.

Factors associated with a vicious cycle involving a low nephron number, hypertension and chronic kidney disease

It has been reported that there is substantial variation in the nephron number between individuals. Previous studies using autopsy kidneys have demonstrated that a low nephron number, in relation to a low birth weight, may result in hypertension (HTN) and/or chronic kidney disease (CKD). However, recent studies have revealed that the association between a low nephron number and HTN is not a universal finding. This observation indicates that a low nephron number is unlikely to be the sole factor contributing to an elevated blood pressure. In addition to the nephron number, various genetic and congenital factors may contribute to increased susceptibility to HTN and/or CKD in a complex manner. Acquired factors, including aging, obesity and related metabolic abnormalities, and various causes of renal injury, may additionally promote further nephron loss. Such a vicious cycle may induce HTN and/or CKD via the common mechanisms of renal hemodynamic maladaptation.

Different genetic impact in the development of renal length and width: a twin study

BACKGROUND

Ultrasound measurements of renal dimensions are conventionally limited to renal length, shape and cortical thickness. These are regarded as adequate for normal therapeutic decision-making and volume measurements are reserved for a few clinical trials. However, there is no evidence concerning the degree to which renal length or volume is independently susceptible to heritable and environmental influences.

AIM

We aimed to determine whether renal length or width (as a surrogate of volume) was more influenced by heritability.

METHODS

A single operator measured renal length and width in 114 adult monozygotic and same-sex dizygotic Hungarian twin pairs (mean age 43.6 ± 16.3 years), using an Esaote MyLab 70X ultrasound machine with curved array transducer (1-8 MHz, CA431).

RESULTS

Analysis of within-pair co-twin correlations adjusted for age and gender showed that the age- and sex-adjusted heritability of average renal length was 51% (95% confidence interval, 29-72%). Renal width showed negligible genetic influence. Common environmental effects had no influence, and unshared environments were responsible for 49-80% of the variance, mainly renal width.

CONCLUSIONS

This study is the first to demonstrate the moderate heritability and limited environmental influence on renal length, and the contrasting lack of heritability of renal width, which is mainly influenced by unshared environmental components, that is lifestyle habits. Renal width therefore better represents the influence of modifiable environmental factors than renal length. The results suggest that renal width not length should be reported to facilitate early detection and monitoring of renal disease.

Traditional and targeted exome sequencing reveals common, rare and novel functional deleterious variants in RET-signaling complex in a cohort of living US patients with urinary tract malformations

Signaling by the glial cell line-derived neurotrophic factor (GDNF)-RET receptor tyrosine kinase and SPRY1, a RET repressor, is essential for early urinary tract development. Individual or a combination of GDNF, RET and SPRY1 mutant alleles in mice cause renal malformations reminiscent of congenital anomalies of the kidney or urinary tract (CAKUT) in humans and distinct from renal agenesis phenotype in complete GDNF or RET-null mice. We sequenced GDNF, SPRY1 and RET in 122 unrelated living CAKUT patients to discover deleterious mutations that cause CAKUT. Novel or rare deleterious mutations in GDNF or RET were found in six unrelated patients. A family with duplicated collecting system had a novel mutation, RET-R831Q, which showed markedly decreased GDNF-dependent MAPK activity. Two patients with RET-G691S polymorphism harbored additional rare non-synonymous variants GDNF-R93W and RET-R982C. The patient with double RET-G691S/R982C genotype had multiple defects including renal dysplasia, megaureters and cryptorchidism. Presence of both mutations was necessary to affect RET activity. Targeted whole-exome and next-generation sequencing revealed a novel deleterious mutation G443D in GFRα1, the co-receptor for RET, in this patient. Pedigree analysis indicated that the GFRα1 mutation was inherited from the unaffected mother and the RET mutations from the unaffected father. Our studies indicate that 5% of living CAKUT patients harbor deleterious rare variants or novel mutations in GDNF-GFRα1-RET pathway. We provide evidence for the coexistence of deleterious rare and common variants in genes in the same pathway as a cause of CAKUT and discovered novel phenotypes associated with the RET pathway.

Genetic variants associated with adult blood pressure and kidney function do not affect fetal kidney volume. The Generation R Study

BACKGROUND

Smaller kidneys with reduced number of nephrons in early life lead to impaired kidney function and risk for hypertension and chronic kidney disease. These associations might be partly explained by common genetic variation.

AIMS

To assess the associations between common genetic variants, which have recently shown to be associated with blood pressure or kidney function, with fetal kidney volume.

STUDY DESIGN

A prospective population based cohort study in Rotterdam, The Netherlands.

SUBJECTS

855 children, followed from early fetal life onwards (born 2003-2005).

PREDICTOR

Common genetic variants previously associated with blood pressure or kidney function.

OUTCOME MEASURES

Combined third trimester fetal kidney volume.

RESULTS

After taking into account multiple testing, only rs12940887 (near ZNF652) was significantly associated with fetal kidney volume (β: 0.88 (95% CI: 0.40; 1.37) cm(3) per minor allele, P-value<0.001), but the effect showed the opposite direction as expected. The remaining common genetic variants were not associated with fetal kidney volume. We also did not find associations of genetic variants previously shown to affect newborn kidney volume, with third trimester fetal kidney volume.

CONCLUSIONS

Our results suggest that common genetic variants, associated with kidney function or disease and blood pressure, do not affect the third trimester fetal kidney volume. Further studies are needed to elucidate the mechanisms underlying the associations between small kidney size and increased risks of hypertension and impaired kidney function in adulthood.

Congenital anomalies of the kidney and urinary tract: a genetic disorder?

Congenital anomalies of the kidney and urinary tract (CAKUTs) occur in 3–6 per 1000 live births, account for the most cases of pediatric end-stage kidney disease (ESKD), and predispose an individual to hypertension and cardiovascular disease throughout life. Although CAKUTs are a part of many known syndromes, only few single-candidate causative genes have been implicated so far in nonsyndromic cases of human CAKUT. Evidence from mouse models supports the hypothesis that non-syndromic human CAKUT may be caused by single-gene defects. Because increasing numbers of children with CAKUT are surviving to adulthood, better understanding of the molecular pathogenesis of CAKUT, development of new strategies aiming at prevention of CAKUT, preservation of renal function, and avoidance of associated cardiovascular morbidity are needed. In this paper, we will focus on the knowledge derived from the study of syndromic and non-syndromic forms of CAKUT in humans and mouse mutants to discuss the role of genetic, epigenetic, and in utero environmental factors in the pathogenesis of non-syndromic forms of CAKUT in children with particular emphasis on the genetic contributions to CAKUT.

Glomerular number and size variability and risk for kidney disease

PURPOSE OF REVIEW

This review discusses current understandings of variability in glomerular number and size, and the implications for renal health.

RECENT FINDINGS

The quantitative microanatomy of the normal human kidney varies widely. Of greatest significance, total nephron number varies at least 13-fold, and several genes and environmental factors that regulate human nephron endowment have been identified. Full or partial deletion of more than 25 genes in mice has been shown to result in renal hypoplasia and, when measured, reduced nephron endowment. Many more will likely be identified. As would be expected, some gene abnormalities increase nephron endowment above that found in control mice. Glomerular volume also varies widely, both between and within kidneys, and increased heterogeneity of glomerular volume within kidneys is associated with risk factors for kidney disease, including birth weight, age, race, body size and hypertension.

SUMMARY

Data from several human populations indicate that the quantitative microanatomy of the human kidney varies considerably: total glomerular number varies at least 13-fold, mean glomerular volume varies up to seven-fold and the volumes of individual glomeruli within single kidneys can vary as much as eight-fold. Human glomerular number, size and size distribution are being found to correlate with risk factors for kidney disease. The genetic and fetal environmental regulators of nephrogenesis, and thereby nephron endowment, are being rapidly identified and will provide the bases for future clinical interventions. In contrast, the molecular regulation of glomerular size remains unclear.

Genetics of congenital anomalies of the kidney and urinary tract

Congenital anomalies of the kidney and urinary tract (CAKUT) occur in 1 in 500 births and are a major cause of morbidity in children. Notably, CAKUT account for the most cases of pediatric end-stage renal disease and predispose the individual to hypertension and cardiovascular disease throughout life. Although some forms of CAKUT are a part of a syndrome or are associated with a positive family history, most cases of renal system anomalies are sporadic and isolated to the urinary tract. Broad phenotypic spectrum of CAKUT and variability in genotype-phenotype correlation indicate that pathogenesis of CAKUT is a complex process that depends on interplay of many factors. This review focuses on the genetic mechanisms (single-gene mutations, modifier genes) leading to renal system anomalies in humans and discusses emerging insights into the role of epigenetics, in utero environmental factors, and micro-RNAs (miRNAs) in the pathogenesis of CAKUT. Common gene networks that function in defined temporospatial fashion to orchestrate renal system morphogenesis are highlighted. Derangements in cellular, molecular, and morphogenetic mechanisms that direct normal renal system development are emphasized as a major cause of CAKUT. Integrated understanding of how morphogenetic process disruptions are linked to CAKUT will enable improved diagnosis, treatment, and prevention of congenital renal system anomalies and their consequences.

Regulation of kidney development by Shp2: an unbiased stereological analysis

Genes that regulate renal branching morphogenesis are likely to indirectly regulate nephron endowment, but few have been validated to do so in vivo. PTPN11, which encodes the nonreceptor protein tyrosine phosphatase Shp2, acts downstream of receptor tyrosine kinases to modulate the Ras-MAPK pathway and has been implicated in branching morphogenesis in vitro and in invertebrates, and is therefore a candidate in vivo regulator of nephron number. In this work, heterozygous null mutant Shp2(+/-) mice at postnatal days 30-35 were compared with their wild-type (WT) littermates using unbiased stereology to determine if, indeed, the former had decreased nephron number due to their 50% decrease in gene/protein dosage. Although there was a trend toward decreases in total glomerular (nephron) number and kidney volume in Shp2(+/-) mice compared with WT, neither difference was statistically significant (11310 vs. 12198 glomeruli, P = 0.22; 62.8 mm(3) vs. 66.0 mm(3) renal volume; P = 0.40). We conclude that loss of 50% gene/protein dosage of PTPN11/Shp2 is insufficient to affect glomerular (and thereby nephron) number in mouse kidneys in vivo.

Genetics of renal hypoplasia: insights into the mechanisms controlling nephron endowment

Renal hypoplasia, defined as abnormally small kidneys with normal morphology and reduced nephron number, is a common cause of pediatric renal failure and adult-onset disease. Genetic studies performed in humans and mutant mice have implicated a number of critical genes, in utero environmental factors and molecular mechanisms that regulate nephron endowment and kidney size. Here, we review current knowledge regarding the genetic contributions to renal hypoplasia with particular emphasis on the mechanisms that control nephron endowment in humans and mice.

Identification of GDNF gene sequence variations in patients with medullary sponge kidney disease

BACKGROUND AND OBJECTIVES

Medullary sponge kidney (MSK) is a rare nephropathy characterized by cystic anomalies of precalyceal ducts, nephrocalcinosis, renal stones, and tubule dysfunctions. Its association with various malformations and cases of familial aggregation supports the conviction that genetic factors are involved, but no genetic studies have been conducted to date. It is hypothesized that MSK is due to a disruption at the "ureteric bud/metanephric blastema" interface caused by critical developmental genes functioning abnormally.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Fifty-five apparently sporadic MSK patients were analyzed by direct DNA sequencing of all exons and exon-intron boundaries of glial cell-derived neurotrophic factor (GDNF) gene and rearranged during transfection (RET) gene, which have a leading role in renal development.

RESULTS

Two novel variants were found in heterozygosity in the MSK case population: GDNF{ENST00000344622}:c.-45G>C and c.-27+18G>A in a putative binding domain for paired-box 2 transcription factor. As a whole, eight patients showed these variations: four patients carried the c.[-45G>C; -27+18G>A] complex allele, and the others had the c.-27+18G>A alone. A case-control study revealed that these two alleles were significantly associated with MSK. Five of the eight cases were found to be familial, and the allele variants cosegregated with the disease in a seemingly dominant pattern of inheritance. Patients revealed no mutations in the RET gene.

CONCLUSIONS

This is the first report identifying GDNF gene sequence variations in patients with MSK and suggesting a role for this gene in the pathogenesis of some cases of the disease.

The implications of fetal programming of glomerular number and renal function

Large epidemiological studies suggest a clear relation between low birth weight and adverse renal outcomes evident as early as during childhood. Such adverse outcomes may include glomerular disease, hypertension, and renal failure. Data from autopsy material and from experimental models suggest that reduction in nephron number via diminished nephrogenesis may be a major mechanism, and factors that lead to this reduction are incompletely elucidated. Other mechanisms appear to be renal (e.g., via the intrarenal renin-angiotensin-aldosterone system) and nonrenal (e.g. changes in endothelial function). It also appears likely that the outcomes of fetal programming may be influenced postnatally, for example, by the amount of nutrients given at critical times.