Gene discovery and mechanism of disease in the myopathies

Congenital myopathy and muscular dystrophy are two groups of inherited muscle diseases characterised by muscle weakness, and sub-classified by hallmark pathological features within a skeletal muscle biopsy. In order to understand the pathogenesis of inherited muscle disorders, and develop or apply therapies based on mechanistic insight, one must elucidate deep knowledge of the associated gene, genetic variant and the function of the encoded protein. This thesis focuses on three aspects of gene discovery in the inherited myopathies: (1) Identification of a novel variant and phenotype for a known disease gene; (2) understanding the functional role of a recently identified disease gene in skeletal muscle biology and disease; and (3) discovering a novel disease gene for congenital myopathy. We identified the first recessive variant within ACTA1 (encoding α-skeletal actin) as the genetic cause of congenital muscular dystrophy with rigid spine. This case uniquely describes recessive ACTA1 variants where α-skeletal actin protein is expressed. The unique clinical and histological presentation expands the spectrum of ACTA1 disease, and will help guide clinical care and future genetic diagnoses. Our team identified LMOD3 (leiomodin-3) as a novel disease gene for severe nemaline myopathy (NM). KLHL40 (encoding kelch-like family member 40) is another disease gene for severe NM. A recent study suggests mouse Klhl40 protects mouse Lmod3 protein from proteasome-mediated degradation, with the mechanistic basis of KLHL40-NM resulting from secondary loss of LMOD3. We investigated the regulation of human LMODs by human KLHL40, and unexpectedly found evidence that disputes the central paradigm that KLHL40 protects LMOD3 from proteasome-mediated degradation. We identified PYROXD1 as a new genetic cause of early-onset congenital myopathy. We provide the first characterisation of PYROXD1 as a nuclear-cytoplasmic oxidoreductase and our discovery highlights oxidative distress as a core mechanistic pathway in the myopathies. We derived a mouse model of Pyroxd1 deficiency, determining that global loss of mouse Pyroxd1 is embryonic lethal. We subsequently developed a mouse model with skeletal muscle knock-out of Pyroxd1 – as a means to elucidate the role of PYROXD1 in biology and disease