RyR1 is a skeletal muscle calcium release channel associated with excitation-contraction coupling . The RYR1 gene is composed of 106 exons and encodes 5,038 amino acids, making it one of the largest genes in the human genome . Mutations in RYR1 are the most common cause of congenital myopathies . Both dominant and recessive mutations have been reported in RYR1. Dominant mutations have traditionally been associated with central core disease (CCD) and/or a susceptibility to malignant hyperthermia (MHS) , while recessive mutations predominate in patients with multiminicore disease (MmD), centronuclear myopathy (CNM), and congenital fiber type disproportion (CFTD) [4–6]. At this time, no specific treatments are available for any RYR1-related myopathy, though modifying oxidative stress may be one therapeutic avenue .
Until recently, the majority of research on RYR1-related myopathies has focused on dominant mutations in RYR1 that lead to CCD and MHS phenotypes. Dominant mutations are enriched in three hotspots, with mutations in the N-terminus and central regions most commonly associated with MHS and mutations in the C-terminus associated with CCD . Previous literature may be biased due to the fact that analysis was limited to the hotspot regions. Comprehensive studies of selected dominant mutations have led to the hypothesis that MHS associated mutations cause RyR1 hyper-excitability, while CCD associated mutations result in chronic channel dysfunction, either through excitation-contraction uncoupling or by persistent channel leakiness .
Much less is known about recessive mutations and their mechanism(s) of disease. Several case series have been published reporting patients with recessive mutations, though overall they have lacked sufficient patient number and power needed for more broad conclusions. The largest existing study was performed by Klein and colleagues (2012), which included 36 families with recessive inheritance. They found, as compared to patients with dominant mutations, that patients with recessive RYR1 mutations had (1) more severe presentations with earlier onset, (2) more significant widespread weakness, and (3) more involvement of the extraocular and bulbar musculature. A smaller study from Zhou and colleagues (2007) observed that recessive RYR1 mutations are located throughout the gene and are associated with variable histological patterns and symptoms. An additional finding, from this and from other existing studies, is that many recessive RYR1 mutations are hypomorphic sequence changes that lead to markedly reduced or absent protein expression [1, 10].
Given the growing number of cases reported with recessive RYR1 mutations, a larger study combining and comparing these many reports is required in order to understand how various recessive mutations influence clinical phenotype, disease severity, and long term prognosis. The current study seeks to address this need by examining genotype-phenotype correlations in a cohort of 106 patients with recessive RYR1 mutations. This cohort includes 14 previously unreported cases together with published cases from the medical literature (n = 92). We specifically analyzed whether associations exist between mutation type and location, histopathologic diagnosis, and severity of clinical features. In addition, we analyzed the distribution of recessive mutations in relation to specific domains throughout the RyR1 protein. Overall, several associations were identified, including an association between the presence of a hypomorphic allele and increased clinical severity, association of the diagnosis of CNM and/or hypomorphic alleles with ophthalmoparesis, and an enrichment of missense mutations in the MH/CCD hotspots and the pore selectivity filter. In all, this study provides a comprehensive analysis of genotype-phenotype relationships for recessive RYR1 mutations.