Functional effects of KCNJ11 mutations causing neonatal diabetes: Enhanced activation by MgATP

University Laboratory of Physiology, Oxford University, Parks Road, Oxford OX1 3PT, UK.
Human Molecular Genetics (Impact Factor: 6.39). 10/2005; 14(18):2717-26. DOI: 10.1093/hmg/ddi305
Source: PubMed


Recent studies have shown that heterozygous mutations in KCNJ11, which encodes Kir6.2, the pore-forming subunit of the ATP-sensitive potassium (K(ATP)) channel, cause permanent neonatal diabetes either alone (R201C, R201H) or in association with developmental delay, muscle weakness and epilepsy (V59G,V59M). Functional analysis in the absence of Mg2+, to isolate the inhibitory effects of ATP on Kir6.2, showed that both types of mutation reduce channel inhibition by ATP. However, in pancreatic beta-cells, K(ATP) channel activity is governed by the balance between ATP inhibition via Kir6.2 and Mg-nucleotide stimulation mediated by an auxiliary subunit, the sulphonylurea receptor SUR1. We therefore studied the MgATP sensitivity of KCNJ11 mutant K(ATP) channels expressed in Xenopus oocytes. In contrast to wild-type channels, Mg2+ dramatically reduced the ATP sensitivity of heterozygous R201C, R201H, V59M and V59G channels. This effect was predominantly mediated via the nucleotide-binding domains of SUR1 and resulted from an enhanced stimulatory action of MgATP. Our results therefore demonstrate that KCNJ11 mutations increase the current magnitude of heterozygous K(ATP) channels in two ways: by increasing MgATP activation and by decreasing ATP inhibition. They further show that the fraction of unblocked K(ATP) current at physiological MgATP concentrations correlates with the severity of the clinical phenotype.

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Available from: Peter Proks, Oct 07, 2015
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    • "The majority of the mutations identified in this study had previously been reported. Examples include: the most common mutation in the KCNJ11 gene c.602GNA (p.Arg201His), which causes oral sulfonylurea-responsive NDM [26] [27] [28], in a 41 year old female (Table 2, patient 20) diagnosed with NDM at 6 months of age; the INS c.94GNA (p.Gly32Ser) mutation [29] in a 60 year old female (Table 2, patient 21) diagnosed at 9 months of age; and the c.188-31GNA sequence change in the INS gene [30] previously reported to affect proper splicing of exon 2 in the INS gene identified in a patient with NDM diagnosed at 10 months of age (Table 2, patient 22). A novel, likely pathogenic c.204GNC (p.Trp68Cys) change in the KCNJ11 gene was identified in a patient with NDM (Table 2, patient 23) in the de-novo state. "
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    ABSTRACT: Single gene mutations that primarily affect pancreatic β-cell function account for approximately 1-2% of all cases of diabetes. Overlapping clinical features with common forms of diabetes makes diagnosis of monogenic diabetes challenging. A genetic diagnosis often leads to significant alterations in treatment, allows better prediction of disease prognosis and progression, and has implications for family members. Currently, genetic testing for monogenic diabetes relies on selection of appropriate individual genes for analysis based on the availability of often-limited phenotypic information, decreasing the likelihood of making a genetic diagnosis. We thus developed a targeted next-generation sequencing (NGS) assay for the detection of mutations in 36 genes known to cause monogenic forms of diabetes, including transient or permanent neonatal diabetes mellitus (TNDM or PNDM), maturity-onset diabetes of the young (MODY) and rare syndromic forms of diabetes. A total of 95 patient samples were analyzed: 19 with known causal mutations and 76 with a clinically suggestive phenotype but lacking a genetic diagnosis. All previously identified mutations were detected, validating our assay. Pathogenic sequence changes were identified in 19 out of 76 (25%) patients: 7 of 32 (22%) NDM cases, and 12 of 44 (27%) MODY cases. In 2 NDM patients the causal mutation was not expected as consanguinity was not reported and there were no clinical features aside from diabetes. A 3 year old patient with NDM at 3 months of age who previously tested negative for INS, KCNJ11 and ABCC8 mutations was found to carry a novel homozygous mutation in EIF2AK3 (associated with Wolcott-Rallison syndrome), a gene not previously suspected because consanguinity, delayed growth, abnormal bone development and hepatic complications had not been reported. Similarly, another infant without a history of consanguinity was found to have a homozygous GCK mutation causing PNDM at birth. This study demonstrates the effectiveness of multi-gene panel analysis in uncovering molecular diagnoses in patients with monogenic forms of diabetes.
    Molecular Genetics and Metabolism 09/2014; 113(4). DOI:10.1016/j.ymgme.2014.09.007 · 2.63 Impact Factor
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    • ") Exon 1 i-DEND PNDM DEND Missense c.175G>A V59M p.Val59Met Yes (Gloyn, et al., 2004b; Sagen, et al., 2004; Vaxillaire, et al., 2004; Massa, et al., 2005; Flanagan, et al., 2006; (Koster, et al., 2005; Proks, et al., 2005a; Lin, et al., 2006a "
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    • "The third cluster of mutations is located at the interface between the subunits such as the interface between adjacent Kir6.2 subunits (eg, F35 and E322) and the interface between Kir6.2 and SUR1 subunits (eg, Q52 and G53). These mutations likely alter channel activity by affecting the interactions between adjacent Kir6.2 and SUR1 subunits that are important for correct channel gating.78–82 "
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    ABSTRACT: Neonatal diabetes mellitus (NDM) is a monogenic disorder caused by mutations in genes involved in regulation of insulin secretion from pancreatic β-cells. Mutations in the KCNJ11 and ABCC8 genes, encoding the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel Kir6.2 and SUR1 subunits, respectively, are found in ∼50% of NDM patients. In the pancreatic β-cell, K(ATP) channel activity couples glucose metabolism to insulin secretion via cellular excitability and mutations in either KCNJ11 or ABCC8 genes alter K(ATP) channel activity, leading to faulty insulin secretion. Inactivation mutations decrease K(ATP) channel activity and stimulate excessive insulin secretion, leading to hyperinsulinism of infancy. In direct contrast, activation mutations increase K(ATP) channel activity, resulting in impaired insulin secretion, NDM, and in severe cases, developmental delay and epilepsy. Many NDM patients with KCNJ11 and ABCC8 mutations can be successfully treated with sulfonylureas (SUs) that inhibit the K(ATP) channel, thus replacing the need for daily insulin injections. There is also strong evidence indicating that SU therapy ameliorates some of the neurological defects observed in patients with more severe forms of NDM. This review focuses on the molecular and cellular mechanisms of mutations in the K(ATP) channel that underlie NDM. SU pharmacogenomics is also discussed with respect to evaluating whether patients with certain K(ATP) channel activation mutations can be successfully switched to SU therapy.
    Pharmacogenomics and Personalized Medicine 11/2010; 3(1):145-61. DOI:10.2147/PGPM.S6969
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