Benjamin Glaser

Pendred syndrome is caused by mutations in a putative sulphate transporter gene (PDS)

Pendred syndrome is a recessively inherited disorder with the hallmark features of congenital deafness and thyroid goitre. By some estimates, the disorder may account for upwards of 10% of hereditary deafness. Previous genetic linkage studies localized the gene to a broad interval on human chromosome 7q22–31.1. Using a positional cloning strategy, we have identified the gene (PDS) mutated in Pendred syndrome and found three apparently deleterious mutations, each segregating with the disease in the respective families in which they occur. PDS produces a transcript of approximately 5 kb that was found to be expressed at significant levels only in the thyroid. The predicted protein, pendrin, is closely related to a number of known sulphate transporters. These studies provide compelling evidence that defects in pendrin cause Pendred syndrome thereby launching a new area of investigation into thyroid physiology, the pathogenesis of congenital deafness and the role of altered sulphate transport in human disease.

Adenosine diphosphate as an intracellular regulator of insulin secretion

Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels couple the cellular metabolic state to electrical activity and are a critical link between blood glucose concentration and pancreatic insulin secretion. A mutation in the second nucleotide-binding fold (NBF2) of the sulfonylurea receptor (SUR) of an individual diagnosed with persistent hyperinsulinemic hypoglycemia of infancy generated KATP channels that could be opened by diazoxide but not in response to metabolic inhibition. The hamster SUR, containing the analogous mutation, had normal ATP sensitivity, but unlike wild-type channels, inhibition by ATP was not antagonized by adenosine diphosphate (ADP). Additional mutations in NBF2 resulted in the same phenotype, whereas an equivalent mutation in NBF1 showed normal sensitivity to MgADP. Thus, by binding to SUR NBF2 and antagonizing ATP inhibition of KATP channels, intracellular MgADP may regulate insulin secretion.

Common variants in WFS1 confer risk of type 2 diabetes

We studied genes involved in pancreatic β cell function and survival, identifying associations between SNPs in WFS1 and diabetes risk in UK populations that we replicated in an Ashkenazi population and in additional UK studies. In a pooled analysis comprising 9,533 cases and 11,389 controls, SNPs in WFS1 were strongly associated with diabetes risk. Rare mutations in WFS1 cause Wolfram syndrome; using a gene-centric approach, we show that variation in WFS1 also predisposes to common type 2 diabetes.

A recessive contiguous gene deletion causing infantile hyperinsulinism, enteropathy and deafness identifies the Usher type 1C gene

Usher syndrome type 1 describes the association of profound, congenital sensorineural deafness, vestibular hypofunction and childhood onset retinitis pigmentosa. It is an autosomal recessive condition and is subdivided on the basis of linkage analysis into types 1A through 1E (refs 2–6). Usher type 1C maps to the region containing the genes ABCC8 and KCNJ11 (encoding components of ATP-sensitive K + (KATP) channels), which may be mutated in patients with hyperinsulinism. We identified three individuals from two consanguineous families with severe hyperinsulinism, profound congenital sensorineural deafness, enteropathy and renal tubular dysfunction. The molecular basis of the disorder is a homozygous 122-kb deletion of 11p14–15, which includes part of ABCC8 and overlaps with the locus for Usher syndrome type 1C and DFNB18 (ref. 11). The centromeric boundary of this deletion includes part of a gene shown to be mutated in families with type 1C Usher syndrome, and is hence assigned the name USH1C. The pattern of expression of the USH1C protein is consistent with the clinical features exhibited by individuals with the contiguous gene deletion and with isolated Usher type 1C.

Induction of Long-Term Glycemic Control in Newly Diagnosed Type 2 Diabetic Patients by Transient Intensive Insulin Treatment

OBJECTIVE Type 2 diabetes is a slowly progressive disease, in which the gradual deterioration of glucose tolerance is associated with the progressive decrease in β-cell function. Hyperglycemia per se has deleterious effects on both beta-cell function and insulin action, which are partially reversible by the short-term control of blood glucose levels. We hypothesized that the induction of euglycemia, using intensive insulin therapy at the time of clinical diagnosis, could lead to a significant improvement in insulin secretion and action and thus alter the clinical course of the disease. RESEARCH DESIGN AND METHODS Thirteen newly diagnosed diet-unresponsive type 2 diabetic patients were treated with continuous subcutaneous insulin infusion (CSII) for 2 weeks and followed longitudinally while being treated with diet alone. RESULTS Four patients were considered therapeutic failures since CSII failed to induce euglycemia (n = 1) or glucose control deteriorated within 6 months after CSII (n = 3). The remaining nine patients were maintained on diet alone with adequate control from 9 to > 50 months (median ± SE, 26 ± 4.8 months). In five patients, glycemic control deteriorated after 9–36 months, but a repeat 2-week CSII treatment reestablished control in four patients. One of these patients underwent a third CSII treatment 13 months later. At the time this article was written, six patients of the initial group were still controlled without medication 16–59 months (median ± SE, 45.5 ± 6.6 months) after the initiation of treatment. Body weight remained unchanged in all patients. CONCLUSIONS These findings suggest that in a significant proportion of type 2 diabetic patients who fail to respond to dietary measures, short-term intensive insulin treatment can effectively establish responsiveness, allowing long-term glycemic control without medication. Further studies are required to establish whether simpler treatment regimens could be equally effective. If the hypothesis offered here finds support, present approaches to the management of newly diagnosed type 2 diabetes may need to be revised.

Loss-of-function mutations in SLC30A8 protect against type 2 diabetes

Loss-of-function mutations protective against human disease provide in vivo validation of therapeutic targets, but none have yet been described for type 2 diabetes (T2D). Through sequencing or genotyping of ∼150,000 individuals across 5 ancestry groups, we identified 12 rare protein-truncating variants in SLC30A8, which encodes an islet zinc transporter (ZnT8) and harbors a common variant (p.Trp325Arg) associated with T2D risk and glucose and proinsulin levels. Collectively, carriers of protein-truncating variants had 65% reduced T2D risk (P = 1.7 × 10−6), and non-diabetic Icelandic carriers of a frameshift variant (p.Lys34Serfs*50) demonstrated reduced glucose levels (−0.17 s.d., P = 4.6 × 10−4). The two most common protein-truncating variants (p.Arg138* and p.Lys34Serfs*50) individually associate with T2D protection and encode unstable ZnT8 proteins. Previous functional study of SLC30A8 suggested that reduced zinc transport increases T2D risk, and phenotypic heterogeneity was observed in mouse Slc30a8 knockouts. In contrast, loss-of-function mutations in humans provide strong evidence that SLC30A8 haploinsufficiency protects against T2D, suggesting ZnT8 inhibition as a therapeutic strategy in T2D prevention.

Genome-wide survey reveals predisposing diabetes type 2-related DNA methylation variations in human peripheral blood

Inter-individual DNA methylation variations were frequently hypothesized to alter individual susceptibility to Type 2 Diabetes Mellitus (T2DM). Sequence-influenced methylations were described in T2DM-associated genomic regions, but evidence for direct, sequence-independent association with disease risk is missing. Here, we explore disease-contributing DNA methylation through a stepwise study design: first, a pool-based, genome-scale screen among 1169 case and control individuals revealed an excess of differentially methylated sites in genomic regions that were previously associated with T2DM through genetic studies. Next, in-depth analyses were performed at selected top-ranking regions. A CpG site in the first intron of the FTO gene showed small (3.35%) but significant (P = 0.000021) hypomethylation of cases relative to controls. The effect was independent of the sequence polymorphism in the region and persists among individuals carrying the sequence-risk alleles. The odds of belonging to the T2DM group increased by 6.1% for every 1% decrease in methylation (OR = 1.061, 95% CI: 1.032-1.090), the odds ratio for decrease of 1 standard deviation of methylation (adjusted to gender) was 1.5856 (95% CI: 1.2824-1.9606) and the sensitivity (area under the curve = 0.638, 95% CI: 0.586-0.690; males = 0.675, females = 0.609) was better than that of the strongest known sequence variant. Furthermore, a prospective study in an independent population cohort revealed significant hypomethylation of young individuals that later progressed to T2DM, relative to the individuals who stayed healthy. Further genomic analysis revealed co-localization with gene enhancers and with binding sites for methylation-sensitive transcriptional regulators. The data showed that low methylation level at the analyzed sites is an early marker of T2DM and suggests a novel mechanism by which early-onset, inter-individual methylation variation at isolated non-promoter genomic sites predisposes to T2DM.

A Nonsense Mutation in the Inward Rectifier Potassium Channel Gene, Kir6.2, Is Associated With Familial Hyperinsulinism

ATP-sensitive potassium (KATP) channels are an essential component of glucose-dependent insulin secretion in pancreatic islet β-cells. These channels comprise the sulfonylurea receptor (SUR1) and Kir6.2, a member of the inward rectifier K+ channel family. Mutations in the SUR1 subunit are associated with familial hyperinsulinism (HI) (MIM:256450), an inherited disorder characterized by hyperinsulinism in the neonate. Since the Kir6.2 gene maps to human chromosome 11p15.1 (1,2), which also encompasses a locus for HI, we screened the Kir6.2 gene for the presence of mutations in 78 HI probands by single-strand conformation polymorphism (SSCP) and nucleotide sequence analyses. A nonsense mutation, Tyr→Stop at codon 12 (designated Y12X) was observed in the homozygous state in a single proband. 86Rb+ efflux measurements and single-channel recordings of COS-1 cells co-expressing SUR1 and either wild-type or Y12X mutant Kir6.2 proteins confirmed that KATP channel activity was abolished by this nonsense mutation. The identification of an HI patient homozygous for the Kir6.2/Y12X allele affords an opportunity to observe clinical features associated with mutations resulting in an absence of Kir6.2. These data provide evidence that mutations in the Kir6.2 sub-unit of the islet β-cell KATP channel are associated with the HI phenotype and also suggest that the majority of HI cases are not attributable to mutations in the coding region of the Kir6.2 gene.

Control of pancreatic β cell regeneration by glucose metabolism.

Recent studies revealed a surprising regenerative capacity of insulin-producing β cells in mice, suggesting that regenerative therapy for human diabetes could in principle be achieved. Physiologic β cell regeneration under stressed conditions relies on accelerated proliferation of surviving β cells, but the factors that trigger and control this response remain unclear. Using islet transplantation experiments, we show that β cell mass is controlled systemically rather than by local factors such as tissue damage. Chronic changes in β cell glucose metabolism, rather than blood glucose levels per se, are the main positive regulator of basal and compensatory β cell proliferation in vivo. Intracellularly, genetic and pharmacologic manipulations reveal that glucose induces β cell replication via metabolism by glucokinase, the first step of glycolysis, followed by closure of K(ATP) channels and membrane depolarization. Our data provide a molecular mechanism for homeostatic control of β cell mass by metabolic demand.

Genetics of neonatal hyperinsulinism

Congenital hyperinsulinism (HI) is a clinically and genetically heterogeneous entity. The clinical heterogeneity is manifested by severity ranging from extremely severe, life threatening disease to very mild clinical symptoms, which may even be difficult to identify. Furthermore, clinical responsiveness to medical and surgical management is extremely variable. Recent discoveries have begun to clarify the molecular aetiology of this disease and thus the mechanisms responsible for this clinical heterogeneity are becoming more clear. Mutations in 4 different genes have been identified in patients with this clinical syndrome. Most cases are caused by mutations in either of the 2 subunits of the β cell ATP sensitive K+ channel (K ATP ), whereas others are caused by mutations in the β cell enzymes glucokinase and glutamate dehydrogenase. However, for as many as 50% of the cases, no genetic aetiology has yet been determined. The study of the genetics of this disease has provided important new information about β cell physiology. Although the clinical ramifications of these findings are still limited, in some situations genetic studies might greatly aid in patient management.