Ryanodine receptor type 1 gene variants in the malignant hyperthermia-susceptible population of the United States.

BACKGROUND: Mutations in the ryanodine receptor type 1 gene (RYR1) that encodes the skeletal muscle-specific intracellular calcium (Ca(2+)) release channel are a cause of malignant hyperthermia (MH). In this study, we examined RYR1 mutations in a large number of North American MH-susceptible (MHS) subjects without prior genetic diagnosis. METHODS: RYR1 was examined in 120 unrelated MHS subjects from the United States in a tiered manner. The α-1 subunit of the dihydropyridine receptor gene (CACNA1S) was screened for 4 variants in subjects in whom no abnormality was found in ≥ 100 exons of RYR1. RESULTS: Ten known causative MH mutations were found in 26 subjects. Variants of uncertain significance in RYR1 were found in 36 subjects, 16 of which are novel. Novel variants in both RYR1 and CACNA1S were found in the 1 subject who died of MH. Two RYR1 variants were found in 4 subjects. Variants of uncertain significance were found outside and inside the hotspots of RYR1. Maximal contractures in the caffeine-halothane contracture test were greater in those who had a known MH mutation or variant of uncertain significance in RYR1 than in those who did not. CONCLUSIONS: The identification of novel RYR1 variants and previously observed RYR1 variants of uncertain significance in independent MHS families is necessary for demonstrating the significance of these variants for MH susceptibility and supports the need for functional studies of these variants. Continued reporting of the clinical phenotypes of MH is necessary for interpretation of genetic findings, especially because the pathogenicity of most of these genetic variants associated with MHS remains to be elucidated.

Exome sequencing reveals SCO2 mutations in a family presented with fatal infantile hyperthermia.

We applied whole-exome sequencing (WES) for identification of an underlying genetic cause of a disease in a family presented with fatal infantile hyperthermia. Analysis of WES results revealed novel, deleterious compound missense mutations, Val160Ala and Pro233Thr, in the synthesis of cytochrome C oxidase 2 gene (SCO2) encoding a mitochondrial protein, Sco2, which is important for cytochrome C oxidase (COX) synthesis. Autosomal recessive mutations in SCO2 are known to be associated with COX deficiency recognized as fatal infantile cardio-encephalomyopathy (604272, OMIM). The Val160Ala and Pro233Thr mutations occurred in the conserved thioredoxin domain of Sco2 and predicted to disrupt protein folding and interaction of Sco2 with other proteins. Our results show applicability of WES in identification of disease-causing mutations and in establishing molecular diagnosis of severe, infantile onset disorder with a challenging diagnosis.

KBTBD13 interacts with Cullin 3 to form a functional ubiquitin ligase.

Autosomal dominant mutations in BTB and Kelch domain containing 13 protein (KBTBD13) are associated with a new type of Nemaline Myopathy (NEM). NEM is a genetically heterogeneous group of muscle disorders. Mutations causing phenotypically distinct NEM variants have previously been identified in components of muscle thin filament. KBTBD13 is a muscle specific protein composed of an N terminal BTB domain and a C terminal Kelch-repeat domain. The function of this newly identified protein in muscle remained unknown. In this study, we show that KBTBD13 interacts with Cullin 3 (Cul3) and the BTB domain mediates this interaction. Using ubiquitination assays, we determined that KBTBD13 participates in the formation of a Cul3 based RING ubiquitin ligase (Cul3-RL) capable of ubiquitin conjugation. Confocal microscopy of transiently expressed KBTBD13 revealed its co-localization with ubiquitin. Taken together, our results demonstrate that KBTBD13 is a putative substrate adaptor for Cul3-RL that functions as a muscle specific ubiquitin ligase, and thereby implicate the ubiquitin proteasome pathway in the pathogenesis of KBTBD13-associated NEM.