MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement.

MSTO1 encodes a cytosolic mitochondrial fusion protein, misato homolog 1 or MSTO1. While the full genotype-phenotype spectrum remains to be explored, pathogenic variants in MSTO1 have recently been reported in a small number of patients presenting with a phenotype of cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic and pigmentary retinopathy. The proposed underlying pathogenic mechanism of MSTO1-related disease is suggestive of impaired mitochondrial fusion secondary to a loss of function of MSTO1. Disorders of mitochondrial fusion and fission have been shown to also lead to mitochondrial DNA (mtDNA) depletion, linking them to the mtDNA depletion syndromes, a clinically and genetically diverse class of mitochondrial diseases characterized by a reduction of cellular mtDNA content. However, the consequences of pathogenic variants in MSTO1 on mtDNA maintenance remain poorly understood. We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants, and we provide functional characterization from seven MSTO1-related disease patient fibroblasts. Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy. MSTO1 protein was not detectable in the cultured fibroblasts of all seven patients evaluated, suggesting that pathogenic variants result in a loss of protein expression and/or affect protein stability. Consistent with impaired mitochondrial fusion, mitochondrial networks in fibroblasts were found to be fragmented. Furthermore, all fibroblasts were found to have depletion of mtDNA ranging from 30 to 70% along with alterations to mtDNA nucleoids. Our data corroborate the role of MSTO1 as a mitochondrial fusion protein and highlight a previously unrecognized link to mtDNA regulation. As impaired mitochondrial fusion is a recognized cause of mtDNA depletion syndromes, this novel link to mtDNA depletion in patient fibroblasts suggests that MSTO1-deficiency should also be considered a mtDNA depletion syndrome. Thus, we provide mechanistic insight into the disease pathogenesis associated with MSTO1 mutations and further define the clinical spectrum and the natural history of MSTO1-related disease.

Importance and challenge of making an early diagnosis in LMNA-related muscular dystrophy.

OBJECTIVE: To identify the most useful clinical and histologic markers that facilitate early diagnosis in LMNA-related muscular dystrophy and to assess the usefulness of Western blotting (WB) for lamin A/C. METHODS: We analyzed the clinical and histologic features and WB results of all patients with laminopathies diagnosed in a research-based diagnostic service over 8 years. RESULTS: Although patients with congenital muscular dystrophy (MDCL) (n = 5) and Emery-Dreifuss muscular dystrophy (EDMD) (n = 5) had distinctive early clinical features, the lack of a suggestive clinical phenotype significantly delayed diagnosis in 2 of 3 patients with limb-girdle muscular dystrophy (LGMD) (n = 3). In addition, 6 of 20 muscle biopsy samples were considered nondystrophic, which contributed to delays in diagnosis in some patients. Neck extensor involvement (weakness or contractures) was the most consistent clinical sign, present in all patients. Reduced lamin A/C levels on WB were seen in 5 of 9 patients with laminopathies. CONCLUSION: Clinical features provide the best clues for diagnosing MDCL and EDMD early in the disease, and we urge clinicians to become familiar with those phenotypes. WB for lamin A/C may contribute to diagnosis but requires technical expertise, and results are normal in many individuals with LMNA mutations. Because of the survival benefit of early diagnosis and treatment, we recommend that LMNA gene sequencing be performed in all patients with undiagnosed congenital muscular dystrophy and neck extensor weakness, all patients with genetically undiagnosed LGMD, and those with suggestive clinical signs and nonspecific histologic abnormalities.

Eight new mutations and the expanding phenotype variability in muscular dystrophy caused by ANO5.

OBJECTIVE: Description of 8 new ANO5 mutations and significant expansion of the clinical phenotype spectrum associated with previously known and unknown mutations to improve diagnostic accuracy. METHODS: DNA samples of 101 patients in 95 kindreds at our quaternary referral center in Finland, who had undetermined limb-girdle muscular dystrophy (LGMD), calf distal myopathy, or creatine kinase (CK) elevations of more than 2,000 IU/L, were selected for ANO5 genetic evaluation, and the clinical findings of patients with mutations were retrospectively analyzed. RESULTS: A total of 25 patients with muscular dystrophy caused by 11 different recessive mutations in the ANO5 gene were identified. The vast majority of mutations, 8 of 11, proved to be previously unknown new mutations. The most frequent mutation, c.2272C>T (p.R758C), was present in 20 patients. The phenotypes associated with this and the common European mutation, c.191dupA, varied from nearly asymptomatic high hyperCKemia to severe LGMD with consistently milder phenotypes in female patients. CONCLUSIONS: Mutations in ANO5 are a frequent cause of undetermined muscular dystrophy, with both distal and proximal presentation. Other types include high hyperCKemia, myalgia, or calf hypertrophy over decades without significant weakness, especially in female patients. Mutations are distributed all over the gene, indicating that muscular dystrophy caused by ANO5 can be expected to occur in all populations.