Three novel missense variants in two families with JAG2-associated limb-girdle muscular dystrophy.

Limb-girdle muscular dystrophy recessive 27 is associated with biallelic variants in JAG2, encoding the JAG2 notch ligand. Twenty-four affected individuals from multiple families have been described in two reports. We present two Australian families with three novel JAG2 missense variants: (c.1021G>T, p.(Gly341Cys)) homozygous in two siblings of Pakistani origin, and compound heterozygous variants (c.703T>C, p.(Trp235Arg); c.2350C>T, p.(Arg784Cys)) in a proband of European ancestry. Patients presented with childhood-onset limb-girdle-like myopathy with difficulty or inability walking. MRI revealed widespread torso and limb muscle involvement. Muscle pathology showed myopathic changes with fatty infiltration. Muscle RNA sequencing revealed significant downregulation of myogenesis genes PAX7, MYF5, and MEGF10 similar to previous JAG2-related muscular dystrophy cases or Jag2-knockdown cells. In absence of functional assays to characterise JAG2 variants, clinical, MRI and transcriptomic profiling collectively may help discern JAG2-related muscular dystrophy, diagnosis of which is essential for patients and families given the severity of disease and reoccurrence risk.

Loss-of-function variants in JPH1 cause congenital myopathy with prominent facial involvement.

Background: Weakness of facial, ocular, and axial muscles is a common clinical presentation in congenital myopathies caused by pathogenic variants in genes encoding triad proteins. Abnormalities in triad structure and function resulting in disturbed excitation-contraction coupling and Ca 2+ homeostasis can contribute to disease pathology. Methods: We analysed exome and genome sequencing data from three unrelated individuals with congenital myopathy characterised by striking facial, ocular, and bulbar involvement. We collected deep phenotypic data from the affected individuals. We analysed the RNA-seq data of one proband and performed gene expression outlier analysis in 129 samples. Results: The three probands had remarkably similar clinical presentation with prominent facial, ocular, and bulbar features. Disease onset was in the neonatal period with hypotonia, poor feeding, cleft palate and talipes. Muscle weakness was generalised but most prominent in the lower limbs with facial weakness also present. All patients had myopathic facies, bilateral ptosis, ophthalmoplegia and fatiguability. While muscle biopsy on light microscopy did not show any obvious morphological abnormalities, ultrastructural analysis showed slightly reduced triads, and structurally abnormal sarcoplasmic reticulum. DNA sequencing identified three unique homozygous loss of function variants in JPH1 , encoding junctophilin-1 in the three families; a stop-gain (c.354C>A; p.Tyr118*) and two frameshift (c.373del p.Asp125Thrfs*30 and c.1738del; p.Leu580Trpfs*16) variants. Muscle RNA-seq showed strong downregulation of JPH1 in the F3 proband. Conclusions: Junctophilin-1 is critical to the formation of skeletal muscle triad junctions by connecting the sarcoplasmic reticulum and T-tubules. Our findings suggest that loss of JPH1 results in a congenital myopathy with prominent facial, bulbar and ocular involvement. Key message: This study identified novel homozygous loss-of-function variants in the JPH1 gene, linking them to a unique form of congenital myopathy characterised by severe facial and ocular symptoms. Our research sheds light on the critical impact on junctophilin-1 function in skeletal muscle triad junction formation and the consequences of its disruption resulting in a myopathic phenotype. What is already known on this topic: Previous studies have shown that pathogenic variants in genes encoding triad proteins lead to various myopathic phenotypes, with clinical presentations often involving muscle weakness and myopathic facies. The triad structure is essential for excitation-contraction (EC) coupling and calcium homeostasis and is a key element in muscle physiology. What this study adds and how this study might affect research practice or policy: This study establishes that homozygous loss-of-function mutations in JPH1 cause a congenital myopathy predominantly affecting facial and ocular muscles. This study also provides clinical insights that may aid the clinicians in diagnosing similar genetically unresolved cases.

A CCG expansion in ABCD3 causes oculopharyngodistal myopathy in individuals of European ancestry.

Oculopharyngodistal myopathy (OPDM) is an inherited myopathy manifesting with ptosis, dysphagia and distal weakness. Pathologically it is characterised by rimmed vacuoles and intranuclear inclusions on muscle biopsy. In recent years CGG • CCG repeat expansion in four different genes were identified in OPDM individuals in Asian populations. None of these have been found in affected individuals of non-Asian ancestry. In this study we describe the identification of CCG expansions in ABCD3, ranging from 118 to 694 repeats, in 35 affected individuals across eight unrelated OPDM families of European ancestry. ABCD3 transcript appears upregulated in fibroblasts and skeletal muscle from OPDM individuals, suggesting a potential role of over-expression of CCG repeat containing ABCD3 transcript in progressive skeletal muscle degeneration. The study provides further evidence of the role of non-coding repeat expansions in unsolved neuromuscular diseases and strengthens the association between the CGG • CCG repeat motif and a specific pattern of muscle weakness.