De novo mutations of GCK,HNF1A and HNF4A may be more frequent in MODY than previously assumed

Aims/hypothesis

MODY is mainly characterised by an early onset of diabetes and a positive family history of diabetes with an autosomal dominant mode of inheritance. However, de novo mutations have been reported anecdotally. The aim of this study was to systematically revisit a large collection of MODY patients to determine the minimum prevalence of de novo mutations in the most prevalent MODY genes (i.e. GCK, HNF1A, HNF4A).

Methods

Analysis of 922 patients from two national MODY centres (Slovakia and the Czech Republic) identified 150 probands (16%) who came from pedigrees that did not fulfil the criterion of two generations with diabetes but did fulfil the remaining criteria. The GCK, HNF1A and HNF4A genes were analysed by direct sequencing.

Results

Mutations in GCK, HNF1A or HNF4A genes were detected in 58 of 150 individuals. Parents of 28 probands were unavailable for further analysis, and in 19 probands the mutation was inherited from an asymptomatic parent. In 11 probands the mutations arose de novo.

Conclusions/interpretation

In our cohort of MODY patients from two national centres the de novo mutations in GCK, HNF1A and HNF4A were present in 7.3% of the 150 families without a history of diabetes and 1.2% of all of the referrals for MODY testing. This is the largest collection of de novo MODY mutations to date, and our findings indicate a much higher frequency of de novo mutations than previously assumed. Therefore, genetic testing of MODY could be considered for carefully selected individuals without a family history of diabetes.     

Genetic variation in the Sorbs of eastern Germany in the context of broader European genetic diversity

Population isolates have long been of interest to genetic epidemiologists because of their potential to increase power to detect disease-causing genetic variants. The Sorbs of Germany are considered as cultural and linguistic isolates and have recently been the focus of disease association mapping efforts. They are thought to have settled in their present location in eastern Germany after a westward migration from a largely Slavic-speaking territory during the Middle Ages. To examine Sorbian genetic diversity within the context of other European populations, we analyzed genotype data for over 30 000 autosomal single-nucleotide polymorphisms from over 200 Sorbs individuals. We compare the Sorbs with other European individuals, including samples from population isolates. Despite their geographical proximity to German speakers, the Sorbs showed greatest genetic similarity to Polish and Czech individuals, consistent with the linguistic proximity of Sorbian to other West Slavic languages. The Sorbs also showed evidence of subtle levels of genetic isolation in comparison with samples from non-isolated European populations. The level of genetic isolation was less than that observed for the Sardinians and French Basque, who were clear outliers on multiple measures of isolation. The finding of the Sorbs as only a minor genetic isolate demonstrates the need to genetically characterize putative population isolates, as they possess a wide range of levels of isolation because of their different demographic histories.     

Severe Insulin Resistance and Intrauterine Growth Deficiency Associated With Haploinsufficiency for INSR and CHN2: New Insights Into Synergistic Pathways Involved in Growth and Metabolism

OBJECTIVE

Digenic causes of human disease are rarely reported. Insulin via its receptor, which is encoded by INSR, plays a key role in both metabolic and growth signaling pathways. Heterozygous INSR mutations are the most common cause of monogenic insulin resistance. However, growth retardation is only reported with homozygous or compound heterozygous mutations. We describe a novel translocation [t(7,19)(p15.2;p13.2)] cosegregating with insulin resistance and pre- and postnatal growth deficiency. Chromosome translocations present a unique opportunity to identify modifying loci; therefore, our objective was to determine the mutational mechanism resulting in this complex phenotype.

RESEARCH DESIGN AND METHODS

Breakpoint mapping was performed by fluorescence in situ hybridization (FISH) on patient chromosomes. Sequencing and gene expression studies of disrupted and adjacent genes were performed on patient-derived tissues.

RESULTS

Affected individuals had increased insulin, C-peptide, insulin–to–C-peptide ratio, and adiponectin levels consistent with an insulin receptoropathy. FISH mapping established that the translocation breakpoints disrupt INSR on chromosome 19p15.2 and CHN2 on chromosome 7p13.2. Sequencing demonstrated INSR haploinsufficiency accounting for elevated insulin levels and dysglycemia. CHN2 encoding β-2 chimerin was shown to be expressed in insulin-sensitive tissues, and its disruption was shown to result in decreased gene expression in patient-derived adipose tissue.

CONCLUSIONS

We present a likely digenic cause of insulin resistance and growth deficiency resulting from the combined heterozygous disruption of INSR and CHN2, implicating CHN2 for the first time as a key element of proximal insulin signaling in vivo.The genetic susceptibility to insulin resistance can involve the disruption of a single gene (e.g., INSR) or may involve the interplay of many genetic loci (including PPARG, FTO, HNF1B, etc.). However, there is currently only one known digenic disorder of insulin resistance resulting from mutations in peroxisome proliferator–activated receptor gamma (PPARG) and protein phosphatase 1 regulatory subunit 3 (PPPTR3A) (1). Compound heterozygous mutations in these genes, which are primarily involved in carbohydrate or lipid metabolism, respectively, can combine to produce a phenotype of extreme insulin resistance and lipodystrophy (1). Interestingly, individuals who possess only one mutation have normal insulin levels demonstrating that disruption of both genes and therefore pathways is necessary to result in disease (1).INSR encodes the insulin receptor with a key role in both major arms of the insulin signaling pathways, specifically the metabolic pathway mainly via IRS-1/Akt2/AS160 signaling and the growth pathway mainly via IRS-2/extracellular signal–related kinase (ERK) signaling (2). INSR mutations are the most common cause of monogenic insulin resistance and cause a clinical spectrum of disease ranging from type A insulin resistance to the most severe form of insulin receptoropathy, leprechaunism (also known as Donohue''s syndrome) (3–6).Growth is a complex biological process with multiple interacting pathways. The key pathways involved in growth include the insulin signaling pathway and the growth hormone/insulin-like growth factor pathway (7–9). Intrauterine growth is also regulated by multiple fetal and maternal factors including genetic and epigenetic factors and various environmental factors (10).We describe a family with a reciprocal translocation [t(7,19)(p15.2;p13.2)] cosegregating with insulin resistance and pre- and postnatal growth deficiency. We demonstrate that the breakpoint on chromosome 19 disrupts INSR, causing monoallelic expression. Haploinsufficent INSR individuals have type A insulin resistance with no apparent severe growth deficiency (5). Given the short stature and intrauterine growth retardation also seen in this family, we hypothesized this could be due to either complete loss of the functional INSR protein (due to the second INSR allele harboring a mutation or due to a dominant negative effect of a mutant INSR protein) or a digenic syndrome due to disruption of a second gene involved in growth by the chromosome 7 breakpoint. The first two hypotheses were excluded by the demonstration of a normal INSR DNA sequence and monoallelic INSR expression, while further cytogenetic analysis established that the second breakpoint on chromosome 7p15.2 disrupted CHN2 encoding β-2 chimerin. The demonstration that disruption of CHN2 affects both pre- and postnatal growth suggests that chimerins may play an important role in early growth and development.     

Molecular and hereditary mechanisms of sensorineural hearing loss with focus on selected endocrinopathies

Hearing loss is one of the most widespread sensory disorders. The incidence of deafness in general population is 1:1000 newborns. About one half of the cases of the congenital sensorineural hearing loss (SNHL) is inherited. Recessive mutations in the gap junction beta 2 (GJB2) gene are the most common genetic causes of the nonsyndromic SNHL. The GJB2 encodes a?protein connexin 26 which forms a?subunit of gap junction essential for the correct function of the inner ear. The syndromic SNHL is associated with a?wide range of other symptoms, which encompass also dysfunctions of endocrine organs. The Pendred syndrome associated with the hearing impairment is characterized by a?prelingual, bilateral sever to profound SNHL, goiter, and iodine organification defect. It is an autosomal recessive disorder, which develops due to mutations in pendrin, an anion channel encoded by SLC26A4 gene. Another important type of syndromic hearing loss is the Maternally Inherited Diabetes and Deafness syndrome, which is caused by several mitochondrial DNA mutations. These mutations are clinically manifested by a?hearing impairment with development of the diabetes in the adult age. Hearing impairment occurs during puberty when sensation of high frequency tones is affected following with further progress to profound bilateral sensorineural hearing impairment in the whole frequency range. This review deals with the molecular mechanisms of common genetic causes of the hereditary SNHL along with the selected endocrinopathies emphasizing that the DNA analyses along with the functional studies significantly contribute to the early SNHL diagnosis followed by personalized therapy and genetic counseling. Keywords: hearing impairment, inner ear, connexin, Pendred syndrome, diabetes.     

Prevalence of permanent neonatal diabetes in Slovakia and successful replacement of insulin with sulfonylurea therapy in KCNJ11 and ABCC8 mutation carriers

CONTEXT: Mutations in the KCNJ11 and ABCC8 genes encoding the pancreatic beta-cell K(ATP) channel have recently been shown to be the most common cause of permanent neonatal diabetes mellitus (PNDM). Information regarding the frequency of PNDM has been based mainly on nonpopulation or short-term collections only. Thus, the aim of this study was to identify the incidence of PNDM in Slovakia and to switch patients to sulfonylurea (SU) where applicable. DESIGN: We searched for PNDM patients in the Slovak Children Diabetes Registry. In insulin-treated patients who matched the clinical criteria for PNDM, the KCNJ11 or ABCC8 genes were sequenced, and mutation carriers were invited for replacement of insulin with SU. RESULTS: Eight patients with diabetes onset before the sixth month of life without remission were identified since 1981, which corresponds to the PNDM incidence in Slovakia of one case in 215,417 live births. In four patients, three different KCNJ11 mutations were found (R201H, H46Y, and L164P). Three patients with the KCNJ11 mutations (R201H and H46Y) were switched from insulin to SU, decreasing their glycosylated hemoglobin from 9.3-11.0% on insulin to 5.7-6.6% on SU treatment. One patient has a novel V86A mutation in the ABCC8 gene and was also substituted with SU. CONCLUSIONS: PNDM frequency in Slovakia is much higher (one in 215,417 live births) than previously suggested from international estimates (about one in 800,000). We identified one ABCC8 and four KCNJ11 mutation carriers, of whom four were successfully transferred to SU, dramatically improving their diabetes control and quality of life.     

EIF2S3 Mutations Associated with Severe X‐Linked Intellectual Disability Syndrome MEHMO

Impairment of translation initiation and its regulation within the integrated stress response (ISR) and related unfolded‐protein response has been identified as a cause of several multisystemic syndromes. Here, we link MEHMO syndrome, whose genetic etiology was unknown, to this group of disorders. MEHMO is a rare X‐linked syndrome characterized by profound intellectual disability, epilepsy, hypogonadism and hypogenitalism, microcephaly, and obesity. We have identified a C‐terminal frameshift mutation (Ile465Serfs) in the EIF2S3 gene in three families with MEHMO syndrome and a novel maternally inherited missense EIF2S3 variant (c.324T>A; p.Ser108Arg) in another male patient with less severe clinical symptoms. The EIF2S3 gene encodes the γ subunit of eukaryotic translation initiation factor 2 (eIF2), crucial for initiation of protein synthesis and regulation of the ISR. Studies in patient fibroblasts confirm increased ISR activation due to the Ile465Serfs mutation and functional assays in yeast demonstrate that the Ile465Serfs mutation impairs eIF2γ function to a greater extent than tested missense mutations, consistent with the more severe clinical phenotype of the Ile465Serfs male mutation carriers. Thus, we propose that more severe EIF2S3 mutations cause the full MEHMO phenotype, while less deleterious mutations cause a milder form of the syndrome with only a subset of the symptoms.