Putative malignant hyperthermia mutation CaV1.1-R174W is insufficient to trigger a fulminant response to halothane or confer heat stress intolerance

Abstract

Malignant hyperthermia susceptibility (MHS) is an autosomal dominant pharmacogenetic disorder that manifests as a hypermetabolic state when carriers are exposed to halogenated volatile anesthetics or depolarizing muscle relaxants. In animals, heat stress intolerance is also observed. MHS is linked to over 40 variants in RYR1 that are classified as pathogenic for diagnostic purposes. More recently, a few rare variants linked to the MHS phenotype have been reported in CACNA1S, which encodes the voltage-activated Ca²⁺ channel Ca_V1.1 that conformationally couples to RyR1 in skeletal muscle. Here, we describe a knock-in mouse line that expresses one of these putative variants, Ca_V1.1-R174W. Heterozygous (HET) and homozygous (HOM) Ca_V1.1-R174W mice survive to adulthood without overt phenotype but fail to trigger with fulminant malignant hyperthermia when exposed to halothane or moderate heat stress. All three genotypes (WT, HET, and HOM) express similar levels of Ca_V1.1 by quantitative PCR, Western blot, [³H]PN200-110 receptor binding and immobilization-resistant charge movement densities in flexor digitorum brevis fibers. Although HOM fibers have negligible Ca_V1.1 current amplitudes, HET fibers have similar amplitudes to WT, suggesting a preferential accumulation of the Ca_V1.1-WT protein at triad junctions in HET animals. Never-the-less both HET and HOM have slightly elevated resting free Ca²⁺ and Na⁺ measured with double barreled microelectrode in vastus lateralis that is disproportional to upregulation of transient receptor potential canonical (TRPC) 3 and TRPC6 in skeletal muscle. Ca_V1.1-R174W and upregulation of TRPC3/6 alone are insufficient to trigger fulminant malignant hyperthermia response to halothane and/or heat stress in HET and HOM mice.

Keywords: TRPC3/6; heat stress intolerance; knock-in mouse; malignant hyperthermia susceptibility; muscle diseases; ryanodine receptor; voltage-activated Ca(2+) channels.