Correlation of Phenotype–Genotype and Protein Structure in RYR1-Related Myopathy

In situ structural insights into the excitation-contraction coupling mechanism of skeletal muscle

Excitation-contraction coupling (ECC) is a fundamental mechanism in control of skeletal muscle contraction and occurs at triad junctions, where dihydropyridine receptors (DHPRs) on transverse tubules sense excitation signals and then cause calcium release from the sarcoplasmic reticulum via coupling to type 1 ryanodine receptors (RyR1s), inducing the subsequent contraction of muscle filaments. However, the molecular mechanism remains unclear due to the lack of structural details. Here, we explored the architecture of triad junction by cryo-electron tomography, solved the in situ structure of RyR1 in complex with FKBP12 and calmodulin with the resolution of 16.7 Angstrom, and found the intact RyR1-DHPR supercomplex. RyR1s arrange into two rows on the terminal cisternae membrane by forming right-hand corner-to-corner contacts, and tetrads of DHPRs bind to RyR1s in an alternating manner, forming another two rows on the transverse tubule membrane. This unique arrangement is important for synergistic calcium release and provides direct evidence of physical coupling in ECC.

Functional characterization of RYR1 variants identified in malignant hyperthermia susceptible individuals

Malignant hyperthermia is a pharmacogenetic disorder triggered by halogenated anesthetic agents in genetically predisposed individuals. Approximately 70 % of these individuals carry mutations in RYR1, the gene encoding the ryanodine receptor calcium channel of skeletal muscle. In this study, we performed functional analysis of 5 RYR1 variants identified in members from 8 families who had been diagnosed by the IVCT. Of the 68 individuals enrolled in the study, 43 were diagnosed as MHS, 23 as MHN, and 2 individuals were not tested. Here we demonstrate that the 5 RyR1 variants cause hypersensitivity to RyR1 agonist-mediated calcium release. According to the EMHG scoring matrix these five genetic variants can be classified as follows: c.8638G>A (p.E2880K) and c.11314C>T (p.R3772W) likely pathogenic, c.11416G>A (p.G3806R), c.14627A>G (p.K4876R) and c.14813T>C (p.I4938T), pathogenic (RefSeq NM_000540.3). We propose that the newly functionally characterized RYR1 variants, be included in the panel of variants to be used for the molecular diagnosis of MHS.

The Spectrum of the Heterozygous Effect in Biallelic Mendelian Diseases-The Symptomatic Heterozygote Issue

Heterozygous carriers of pathogenic/likely pathogenic variants in autosomal recessive disorders seem to be asymptomatic. However, in recent years, an increasing number of case reports have suggested that mild and unspecific symptoms can occur in some heterozygotes, as symptomatic heterozygotes have been identified across different disease types, including neurological, neuromuscular, hematological, and pulmonary diseases. The symptoms are usually milder in heterozygotes than in biallelic variants and occur "later in life". The status of symptomatic heterozygotes as separate entities is often disputed, and alternative diagnoses are considered. Indeed, often only a thin line exists between dual, dominant, and recessive modes of inheritance and symptomatic heterozygosity. Interestingly, recent population studies have found global disease effects in heterozygous carriers of some genetic variants. What makes the few heterozygotes symptomatic, while the majority show no symptoms? The molecular basis of this phenomenon is still unknown. Possible explanations include undiscovered deep-splicing variants, genetic and environmental modifiers, digenic/oligogenic inheritance, skewed methylation patterns, and mutational burden. Symptomatic heterozygotes are rarely reported in the literature, mainly because most did not undergo the complete diagnostic procedure, so alternative diagnoses could not be conclusively excluded. However, despite the increasing accessibility to high-throughput technologies, there still seems to be a small group of patients with mild symptoms and just one variant of autosomes in biallelic diseases. Here, we present some examples, the current state of knowledge, and possible explanations for this phenomenon, and thus argue against the existing dominant/recessive classification.

Prenatal diagnosis identifies compound heterozygous variants in RYR1 that causes ultrasound abnormalities in a fetus

Objective

We presented a non-consanguineous healthy Chinese couple with five pregnancies, three early miscarriages, the fetus II-2 and II-5 with similar abnormal phenotypes of fetal hydrops, scoliosis, fetal akinesia and polyhydramnios. This study aimed to uncover the molecular etiology of this family with a history of multiple adverse pregnancies.

Materials and methods

DNA extracted from the fifth fetal umbilical cord and parents’ peripheral blood were subjected to SNP-array and whole exome sequencing. The result was verified by Sanger sequencing. Functional characterization of the c.2682G > C (p.Ile860_Pro894del) variant was completed by minigene splicing assay.

Results

Trio whole-exome sequencing has identified compound heterozygous variants in RYR1 (c.2682G > C; p.Ile860_Pro894del and c.12572G > A; p.Arg4191His) in fetus II-5. The variant c.2682G > C (p.Ile860_Pro894del) comes from the father and the c.12572G > A (p.Arg4191His) comes from the mother. The c.2682G > C (p.Ile860_Pro894del) affects the splice site resulting in exon 21 skipping, therefore is classified as likely pathogenic. The c.12572G > A (p.Arg4191His) locates in the C-terminal hot spots region of the RYR1, classified as of uncertain significance.

Conclusions

We report the first prenatal case of RYR1-related disorders in Chinese population, expanding the variant spectrum of RYR1 in fetuses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01358-x.