RYR1 mutations as a cause of ophthalmoplegia, facial weakness, and malignant hyperthermia

Abstract

Importance: Total ophthalmoplegia can result from ryanodine receptor 1 (RYR1) mutations without overt associated skeletal myopathy. Patients carrying RYR1 mutations are at high risk of developing malignant hyperthermia. Ophthalmologists should be familiar with these important clinical associations.

Objective: To determine the genetic cause of congenital ptosis, ophthalmoplegia, facial paralysis, and mild hypotonia segregating in 2 pedigrees diagnosed with atypical Moebius syndrome or congenital fibrosis of the extraocular muscles.

Design, setting, and participants: Clinical data including medical and family histories were collected at research laboratories at Boston Children's Hospital and Jules Stein Eye Institute (Engle and Demer labs) for affected and unaffected family members from 2 pedigrees in which patients presented with total ophthalmoplegia, facial weakness, and myopathy.

Intervention: Homozygosity mapping and whole-exome sequencing were conducted to identify causative mutations in affected family members. Histories, physical examinations, and clinical data were reviewed.

Main outcome and measure: Mutations in RYR1.

Results: Missense mutations resulting in 2 homozygous RYR1 amino acid substitutions (E989G and R3772W) and 2 compound heterozygous RYR1 substitutions (H283R and R3772W) were identified in a consanguineous and a nonconsanguineous pedigree, respectively. Orbital magnetic resonance imaging revealed marked hypoplasia of extraocular muscles and intraorbital cranial nerves. Skeletal muscle biopsy specimens revealed nonspecific myopathic changes. Clinically, the patients' ophthalmoplegia and facial weakness were far more significant than their hypotonia and limb weakness and were accompanied by an unrecognized susceptibility to malignant hyperthermia.

Conclusions and relevance: Affected children presenting with severe congenital ophthalmoplegia and facial weakness in the setting of only mild skeletal myopathy harbored recessive mutations in RYR1, encoding the ryanodine receptor 1, and were susceptible to malignant hyperthermia. While ophthalmoplegia occurs rarely in RYR1-related myopathies, these children were atypical because they lacked significant weakness, respiratory insufficiency, or scoliosis. RYR1-associated myopathies should be included in the differential diagnosis of congenital ophthalmoplegia and facial weakness, even without clinical skeletal myopathy. These patients should also be considered susceptible to malignant hyperthermia, a life-threatening anesthetic complication avoidable if anticipated presurgically.

Figures

Figure 1. Pedigree structures of OH and DR

Schematic of pedigrees OH (A) and DR (B). Genotypes of RYR1 variants c.2966A>G and c.11314C>T in pedigree OH, and of variants c.848A>G and c.11314C>T in pedigree DR are shown under genotyped family members; black schematic haplotype bars denote wildtype sequence, while red schematic haplotype bars denote mutant sequence. Note that the clinically unaffected parents in pedigree OH each harbor the same two RYR1 mutations on one allele (red), and have one wild-type allele (black). The clinically unaffected parents in pedigree DR each harbor a single, different RYR1 mutation on one allele (half red and half black) and have one wild-type allele (black). DR I:1 harbors the identical c.11314C>T mutation also harbored as one of the two mutations carried by OH II:4, III:1, and III:2. Squares, males; circles, females; filled symbol, affected; asterisk, enrolled in the study.

Figure 2. Homozygosity mapping and mutation analysis

(A) Schematic showing regions of shared homozygosity on chromosome 19 in pedigree OH created by genotypes from individuals III:1, III:2, II:4, IV:1, III: 4 and III:6 by dChip software. Red or blue denotes homozygous AA or BB; yellow denotes heterozygous AB, and white denotes absent call. The homozygous region shared by the 3 affected children and no parent is bordered by single nucleotide markers rs725985 and rs886466. (B) Sanger sequencing chromatograms from an unrelated control individual (top), and from an unaffected parent (middle) and affected child (bottom) of pedigree OH. Note that the parent is heterozygous and the affected children are homozygous for RYR1 c.2966A>G (left) and RYR1 c.11314C>T (right) nucleotide substitutions. The wild-type and the predicted amino acid substitutions are provided below each sequence. (C) Sanger sequencing chromatograms from unaffected father (top), unaffected mother (middle) and an affected child (bottom) of pedigree DR. The father has wild-type sequence at RYR1 c.848 and a heterozygous RYR1 11314C>T nucleotide substitution, the mother has a heterozygous RYR1 848A>G nucleotide substitution and is wild-type at RYR1 c.11314. The affected child is heterozygous at both nucleotides. The wild-type and the predicted amino acid substitutions provided below each sequence. (D) Evolutionary conservation of RYR1 glutamic acid 989, histidine 283 and arginine 3772 residues in 8 species.

Figure 3. Quasi-coronal MRI of the right orbit of individual DRII:2

Note severe hypoplasia of the lateral and medial rectus muscles, moderate hypoplasia of the superior and inferior obliques, and apparent sparing of the inferior rectus. There is central high-intensity material seen within muscles suggestive of fat deposition (red arrowheads). Nerves to the extraocular muscles appear hypoplastic, while the optic nerve, superior orbital vein and intracoronal fat appear normal. Med, medial; Lat, lateral; Sup, Superior; Orb, orbital; Obl, oblique; LLA, lateral levator aponeurosis; n., nerve; IO, inferior oblique; SO, superior oblique; Tndn, tendon.

Figure 4. Morphological findings of quadriceps muscle biopsy sections from affected individual OH IV:1

(A) Hematoxylin and eosin stain showing variability of fiber sizes with increased endomysial connective tissue and some internalized nuclei (20X). (B) Myosin adenosine triphosphatase (ATPase) 9.4 stain demonstrating fibers of variable sizes with small type I (light) and II (dark) fibers (10X). Electron micrographs show focal accumulation of mitochondria accompanied by glycogen and lipid droplets (C; 4000X); with some fibers showing an internalized nucleus (D; 2500X).