Renal cysts and diabetes syndrome resulting from mutations in hepatocyte nuclear factor-1
AbstractMutations in the gene encoding the transcription factor hepatocyte nuclear factor (HNF)-1β have recently been described in association with a variety of abnormalities of renal development. The most consistent clinical feature is the presence of renal cysts and most affected subjects also have early-onset diabetes. The association of renal cysts and diabetes with an HNF-1β mutation is termed the renal cysts and diabetes (RCAD) syndrome [1–3]. As more families have been described with HNF-1β mutations it has become apparent that there are additional phenotypic features in some subjects, including genital tract malformations [4–6], hyperuricaemia, young-onset gout [7], deranged liver function tests [8] and pancreatic atrophy [9]. Mutations in the HNF-1β gene may, therefore, be considered to cause a multisystem disorder. This article reviews the background to the discovery of HNF-1β mutations as a cause of renal disease, describes the variable phenotype and considers some of the possible mechanisms whereby HNF-1β mutations lead to disease. The description of HNF-1β mutations as a cause of developmental disorders of the kidney arose from unexpected observations in the study of monogenic diabetes. Maturity-onset diabetes of the young (MODY) is a monogenic form of young-onset (usually diagnosed before 25 years), non-insulin-dependent diabetes resulting from pancreatic beta-cell dysfunction inherited as an autosomal dominant trait. It represents 1–2% of cases of diabetes in European populations [10]. Heterozygous mutations in the gene encoding the transcription factor HNF-1α on chromosome 12q are the commonest cause of MODY [11]. The HNF-1α and HNF-1β proteins show >80% sequence homology, bind to the same DNA sequence and can exist as both homodimers and heterodimers [12]. HNF-1β (located on chromosome 17q) was an excellent candidate gene for MODY and a mutation was found associated with early-onset diabetes in a Japanese family in 1997 [13]. HNF-1β has been shown to be a rare cause of MODY, accounting for 1% of cases in the UK [14]. In the initial family, all three diabetic mutation carriers had renal disease, which was attributed to diabetic nephropathy, although no renal histology was available, and, subsequently, renal cysts were detected in the subject with proteinuria [15]. Presentation with renal disease is much more common than presentation as familial diabetes. The identification of further families with HNF-1β mutations has shown that severe non-diabetic renal disease, in particular, renal cystic disease, is present in all the families and is the major feature of the phenotype (Table 1) [1–3,6,7,16,17]. In many cases, renal cysts are seen on ultrasound and biopsies are not performed. When biopsies are performed there is considerable variation in the histological diagnosis, as shown in Table 1. The cysts are frequently detected antenatally, with the earliest reports being on fetal scans performed at 17 weeks gestation [16]. The wide prevalence of cysts has led to us proposing that the clinical syndrome associated with HNF-1β mutations is the ‘renal cysts and diabetes’ syndrome [1–3]. This is a clinically useful classification, although it is recognized that these patients have a multisystem disorder and a small minority of patients with HNF-1β mutations do not have either renal cysts or diabetes, although their family members have had at least one of these features, to date. Renal phenotype summary for all reported subjects with HNF-1β mutations Renal phenotype summary for all reported subjects with HNF-1β mutations Despite having a single genetic aetiology, considerable variation is seen in the renal phenotype. Kidneys are of variable size, although mostly enlarged kidneys are seen in the fetus and children and small hypoplastic kidneys are seen in some adults [18]. Longitudinal studies have suggested that the early enlarged kidneys may fail to grow and, thus, the kidneys become relatively smaller as the subject ages [18]. Both symmetrical and asymmetrical differences in growth have been seen, including single and horseshoe kidneys [6,19]. Renal function is usually impaired, but ranges from normal to end-stage renal failure. Some 15% of subjects with mutations are either on dialysis or transplanted. There is considerable variation in the histology and it includes cystic renal dysplasia, glomerulocystic kidney disease and oligomeganephronia (Table 1) [1,4,6,16,20]. The most specific phenotype for HNF-1β mutations is familial hypoplastic glomerulocystic kidney disease [1,20]. HNF-1β mutations have been found in all cases examined to date with familial hypoplastic glomerulocystic kidney disease (OMIM 137920). This is characterized by autosomal dominant inheritance, small kidneys with abnormal calyces and papillae, renal impairment and glomerulocystic histology [1,21]. The renal histology is cortical glomerular cysts with dilatation of the Bowman spaces and primitive glomerular tufts in ≥5% of the cysts [1,21]. Diabetes is now clearly a feature of familial hypoplastic glomerulocystic kidney disease, as the affected adult subjects in all the families have developed early-onset (mean: 31 years; range: 22–39 years) diabetes or impaired glucose tolerance [1,20]. Diabetes typically presents after the renal disease. Diabetes has been described in 58% of reported HNF-1β mutation carriers and impaired glucose tolerance in a further 4% of the cases (Table 2). The mean age of diagnosis of diabetes is 26 years, with a range of 10–61 years. Over half of the 38% who do not have diabetes are aged <25 years and may still develop diabetes. There is a considerable range of severity of glycaemia, from normal glucose tolerance at age 35 years to insulin-treated diabetes presenting with ketoacidosis [6,22]. Diabetes has been described in a 14-year-old who developed transitory diabetes only when treated with corticosteroids following renal transplantation [18]. Non-renal phenotype summary for all reported subjects with HNF-1β mutations Non-renal phenotype summary for all reported subjects with HNF-1β mutations The main pathophysiology is reduced insulin secretion due to beta-cell dysfunction and this may be related to pancreatic atrophy [9,16]. The pancreatic histology is unknown. In all cases investigated there has been some endogenous insulin production, so they are not insulin-dependent, although glycaemic control may be poor, and the majority of patients are insulin-treated [4,16]. Despite having diabetes, the renal dysfunction found in these patients is thought to be caused by developmental abnormalities of the kidney and not diabetic renal disease; although in the majority of cases renal histology has not been examined. It is increasingly being realized that HNF-1β mutations result in a multisystem disorder with features in addition to diabetes and renal disease. These features are shown in Table 2. Genital tract malformations are found in some kindred, but the penetrance is incomplete. Malformations resulting from Müllerian duct aplasia (vaginal aplasia and rudimentary uterus) [4] and failure of fusion of the Müllerian ducts (bicornuate uterus, uterus didelphys and double vagina) have been described [5,6]. The Müllerian (paramesonephric) ducts develop into the main genital duct in the female embryo. The caudal, vertical parts of the two Müllerian ducts fuse to form the corpus and the cervix of the uterus and the upper third of the vagina. One male subject with an HNF-1β mutation has hypospadias; however, this is a common malformation present in one in 300 male children, so the association with an HNF-1β mutation may be coincidental [6]. Young-onset gout can be a prominent clinical feature in patients with HNF-1β mutations. Moderate hyperuricaemia appears to be a feature of most patients, even if they do not have gout [7]. The aetiology of this hyperuricaemia is uncertain and it probably reflects altered urate transport by the kidney as well as being an early reflection of renal impairment, as it is seen even when creatinine levels are <130 µmol/l. One family with an HNF-1β mutation with hyperuricaemia, young-onset gout and renal disease fitted conventional criteria for familial juvenile hyperuricaemic nephropathy (FJHN) [7], but this is not a common cause of FJHN, which in 50% of affected families results from mutations in the gene encoding uromodulin [23,24]. Elevated liver enzymes are frequent in patients with HNF-1β mutations [8] (unpublished data). This has been characterized by elevation of alanine aminotransferase and γ-glutamyl transpeptidase without jaundice or liver insufficiency. There have been no liver cysts or other clear aetiological markers seen on ultrasound examination, although in one subject the ultrasound examination suggested liver fibrosis [8]. There have been no reports of liver histology, other than in a 7-month-old child where the histology was normal [18]. There are other developmental abnormalities reported in patients with HNF-1β mutations (Table 2). At present these have usually only occurred in single patients and it is not possible to establish if they are related findings. The high prevalence of developmental anomalies and their distribution in tissues where HNF-1β is expressed (Figure 1), like gut (pyloric stenosis), suggest that, at least for some, the HNF-1β mutation is aetiological. Developmental abnormalities in the lungs have not been described yet in any subjects, but the expression of HNF-1β in lung tissue would suggest that this may occur. The sites of expression and disease manifestations of HNF-1β mutations. Animal models have been used to investigate the expression of HNF-1β. During mouse embryonic development HNF-1β is expressed earlier than HNF-1α, which is activated only during organogenesis [25]. HNF-1β is required for visceral endoderm specification. HNF-1β homozygous null mice embryos die at day 6.5–7.0 after conception without developing visceral or parietal endoderm [26]. In adult animals, HNF-1α and HNF-1β are expressed in the liver, kidney, intestine, stomach and pancreatic islets; all these organs have specialized polarized epithelia. HNF-1α is predominantly expressed in the liver and HNF-1β in the kidney. HNF-1β alone is expressed in the lungs, thymus and gonads [27]. Liver-specific inactivation of the HNF-1β gene in mice using a Cre/loxP system produced animals with severe jaundice caused by abnormalities of the gallbladder and intrahepatic bile ducts [28]. Renal-specific inactivation of the HNF-1β gene has been achieved in mice using the same technique and the mice developed polycystic kidneys. The renal cyst formation is associated with reduced transcriptional activation of the Umod, Pkhd1 and Pkd2 genes. Chromatin immunoprecipitation experiments showed that HNF-1β binds to the murine Umod, Pkhd1 and Pkd2 genes [29]. Autosomal recessive polycystic kidney disease is caused by mutations of PKHD1. The proximal promoter of the mouse Pkhd1 promoter contains an evolutionarily conserved HNF-1 binding site. HNF-1β and HNF-1α bind to this Pkhd1 promoter and stimulate gene transcription. Expression of a dominant-negative HNF-1β mutant inhibits Pkhd1 promoter activity in transfected cells. Transgenic mice expressing this dominant-negative HNF-1β mutant under the control of a kidney-specific promoter develop cysts in the renal tubules and glomeruli. Cyst epithelial cells that highly express the dominant-negative HNF-1β mutant have no detectable expression of Pkhd1. This suggests that there is a link between the HNF-1β and Pkhd1 genes and their effects on kidney development [30]. The renal and uterus expressions in rodents clearly fit with the clinical manifestations of HNF-1β mutations in man. In situ hybridization studies in the rat have shown that HNF-1β transcripts are present from the earliest inductory phases of kidney development, when the ureteric bud bulges from the Wolffian duct and invades the metanephrogenic mesenchyme. In the neonatal rat kidney, HNF-1β is expressed in the proximal and distal convoluted tubules and loop of Henle, which are derived from the metanephrogenic mesenchyme, and in the collecting ducts, which are derived from the ureteric bud [27]. In female mice, HNF-1β expression has been demonstrated in the oviduct and the uterus, which are derived from the Müllerian ducts [31]. In males, HNF-1β is expressed in the epididymis, vas deferens and seminal vesicles, which are derived from the Wolffian duct, and in the prostate and testes [31]. The studies in man are more limited, but confirm the results in animals. Human metanephroi express HNF-1β at pre-glomerular stages (54–56 days gestation) through to 91 days gestation [2]. In situ hybridization studies have shown that HNF-1β transcripts are prominently expressed in fetal collecting duct branches, with lower levels of expression in the metanephric mesenchyme. In addition, HNF-1β transcripts were detected in fetal stomach and lung epithelia [2]. All reported HNF-1β mutations are shown in Figure 2. Mutations predominantly cluster in the first five exons. Nonsense, frameshift, splice site and missense mutations have all been described. The intron 2 splice site appears to be a mutational hotspot [5,7,19]. Despite heterozygous mutations resulting in the phenotype, not all cases are familial; availability of parental DNA has allowed a spontaneous mutation to be established in one case [16]. The distribution of HNF-1β mutations. There is not a clear correlation of either the type or position of the mutations with the clinical phenotype. The phenotypic variation between family members and between families with the same mutation would point to other genetic or environmental modifiers of the phenotype. A candidate modifier is exposure to maternal hyperglycaemia in utero, which alters age of diagnosis of diabetes in patients with HNF-1α mutations [32]. Functional studies have been reported on nine HNF-1β mutants. These mutants include one missense mutation, S151P with a change from serine to proline within the DNA-binding region, and two nonsense and five frameshift mutations, all of which are truncating mutations. Seven of these mutants occur within exons 1–3 and lack all or part of the domains involved in DNA binding and all fail to bind DNA. Two of the mutants in exons 4 and 5 have an intact DNA-binding domain and they bind DNA as efficiently as wild-type protein. The presence of intact DNA binding correlates with the ability to form dimers and transactivate a reporter gene. These mutants have been introduced into Xenopus embryos and all interfere with the development of the pronephros, which is the first kidney in the amphibian. Six of the mutants led to an enlargement of the pronephros. All these mutants lack DNA binding and have no transactivation potential. Three mutants led to a partial or complete agenesis of the pronephros. These included the two mutants with intact DNA binding but also the mutant R137-K161del, which has an in-frame deletion of 24 amino acids within the DNA-binding domain and, thus, fails to bind DNA, although the transactivation domain is intact. The functional studies in Xenopus may define features of the HNF-1β protein which are not detected by in vitro studies [33,34]. Zebrafish have been used to study organogenesis, since conservation of organogenesis between zebrafish and mammals has been demonstrated for a number of organs. An insertional mutagenesis screen in zebrafish identified three mutant alleles of the zebrafish homologue of HNF-1β. The mutants had underdevelopment of the liver and pancreas and cyst formation in the pronephros, the functioning kidney of the zebrafish embryo and larva [35]. In human subjects, the three renal histologies of cystic renal dysplasia, glomerulocystic disease and oligomeganephronia are all the result of abnormal renal development. In human metanephroi, HNF-1β transcripts are predominantly expressed in the collecting ducts with lower levels over the glomeruli [2]. The formation of glomerular cysts might be secondary to another abnormal process occurring during renal development, such as transient microscopic obstruction of immature nephrons. Calyceal abnormalities have been described in HNF-1β mutation carriers with hypoplastic glomerulocystic kidney disease and in a subject with a solitary functioning kidney [1,6]. Pelviureteric junction obstruction has also been described and one subject required a pyeloplasty [7]. The renal pelvis and calyces are derived from the ureteric bud in the embryo, which is a site of HNF-1β expression [2]. The RCAD syndrome has a diverse phenotype that varies both within and between families. All the families described to date have disorders of renal development, with renal cysts the most consistent clinical feature. Renal abnormalities have been detected in utero in a number of subjects, supporting a major role for HNF-1β in human nephrogenesis. The non-renal manifestations include early-onset diabetes, genital tract malformations, hyperuricaemia, gout and mild derangement of liver function tests. Recognition of the variable features that form the HNF-1β phenotype is important to both adult and paediatric nephrologists. We thank the National Kidney Research Fund (grant TF13/2000), Exeter Kidney Unit Development Fund, Diabetes UK and the British Medical Association, who all supported this work. Conflict of interest statement. None declared. [See related article by Fischer et al. (this issue, pp. 2700–2702)] 1Exeter Kidney Unit, Royal Devon and Exeter Hospital (Wonford) and 2Diabetes and Vascular Medicine, Peninsula Medical School, Exeter, UK
https://doi.org/10.1093/ndt/gfh348