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.

Mitochondrial dynamics: Biological roles, molecular machinery, and related diseases.

Mitochondria are dynamic organelles that undergo fusion, fission, movement, and mitophagy. These processes are essential to maintain the normal mitochondrial morphology, distribution, and function. Mitochondrial fusion allows the exchange of intramitochondrial material, whereas the fission process is required to replicate the mitochondria during cell division, facilitate the transport and distribution of mitochondria, and allow the isolation of damaged organelles. Mitochondrial mobility is essential for mitochondrial distribution depending on the cellular metabolic demands. Mitophagy is needed for the elimination of dysfunctional and damaged mitochondria to maintain a healthy mitochondrial population. The mitochondrial dynamic processes are mediated by a number of nuclear-encoded proteins that function in mitochondrial transport, fusion, fission, and mitophagy. Disorders of mitochondrial dynamics are caused by pathogenic variants in the genes encoding these proteins. These diseases have a high clinical variability, and range in severity from isolated optic atrophy to lethal encephalopathy. These disorders include defects in mitochondrial fusion (caused by pathogenic variants in MFN2, OPA1, YME1L1, MSTO1, and FBXL4), mitochondrial fission (caused by pathogenic variants in DNM1L and MFF), and mitochondrial autophagy (caused by pathogenic variants in PINK1 and PRKN). In this review, the molecular machinery and biological roles of mitochondrial dynamic processes are discussed. Subsequently, the currently known diseases related to mitochondrial dynamic defects are presented.

Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia.

We report here the first families carrying recessive variants in the MSTO1 gene: compound heterozygous mutations were identified in two sisters and in an unrelated singleton case, who presented a multisystem complex phenotype mainly characterized by myopathy and cerebellar ataxia. Human MSTO1 is a poorly studied protein, suggested to have mitochondrial localization and to regulate morphology and distribution of mitochondria. As for other mutations affecting genes involved in mitochondrial dynamics, no biochemical defects typical of mitochondrial disorders were reported. Studies in patients' fibroblasts revealed that MSTO1 protein levels were strongly reduced, the mitochondrial network was fragmented, and the fusion events among mitochondria were decreased, confirming the deleterious effect of the identified variants and the role of MSTO1 in modulating mitochondrial dynamics. We also found that MSTO1 is mainly a cytosolic protein. These findings indicate recessive mutations in MSTO1 as a new cause for inherited neuromuscular disorders with multisystem features.

Case Report: Evidences of myasthenia and cerebellar atrophy in a chinese patient with novel compound heterozygous MSTO1 variants.

Misato Mitochondrial Distribution and Morphology Regulator 1 (MSTO1) is a soluble cytoplasmic protein that regulates mitochondrial dynamics by promoting mitochondrial fusion. Variants in the MSTO1 gene cause a rare disease characterized by early-onset myopathy and cerebellar ataxia, with almost 30 cases reported worldwide. Here we report a case of a 3-year-old boy with novel heterozygous variants of the MSTO1 gene (c.1A>G (p.M1?) and c.727G>C(p.Ala243Pro)). Sequencing data and subsequent validation show that the two variants were inherited from the mother and father of the patient (both were heterozygous). The clinical features are infancy-onset mental and motor retardation, language disorder, dysarthria, scoliosis, cerebellar atrophy, tremor, lower-extremity muscle weakness, elevated muscle enzymes, extensive myopathy with chronic atrophy, hyperventilation lungs, and previously unreported hairy back and enlarged gastrocnemius. Finally, novel heterozygous MSTO1 variants were discovered in this case, which expands the gene spectrum and clinical phenotype of this type of disease, and provides a new direction for future treatment and research. Then we summarize the mutational spectrum, pathological, clinical features and imaging of MSTO1 variants in a cohort of reported 31 patients and discuss the pathogenesis of MSTO1 in humans.

Indentification of novel MSTO1 compound heterozygous mutations in a Chinese family with recessive cerebellar atrophy and ataxia.

Misato mitochondrial distribution and morphology regulator 1 (MSTO1) is a nuclear-encoded cytoplasmic protein involved in mitochondrial fusion and distribution. Its disruption causes an extremely rare mitochondrial disorder characterized by early-onset myopathy and cerebellar ataxia. The genotype-phenotype correlation in the MSTO1 gene is rarely studied before 2017, and only 25 mutations have been described in the patients. Here, we reported two siblings with progressive cerebellar atrophy and ataxia in a Chinese family. Two compound heterozygous mutations in the MSTO1 gene, a novel missense mutation c.571C>T (p.Arg191Trp), and a reported frameshift mutation c.1259delG (p.Gly420ValfsTer2) were identified in the patients by whole exome sequencing. in vitro experiments found both of the mutations lead to reduced protein abundance and link to decreased mtDNA content. Except for ataxia and delayed motor, both of the siblings also have low birth weights, learning difficulties, and dysarthria. Our report enriched the genotype and phenotype spectrums of the MSTO1-related disorder and supported the recessive inheritance of the disease.

Whole-exome sequencing identifies rare compound heterozygous mutations in the MSTO1 gene associated with cerebellar ataxia and myopathy.

Human MSTO1 is involved in the regulation of mitochondrial distribution and morphology and its unregulated expression leads to mitochondrial disorder. Despite its significance for mitochondrial functions, human MSTO1 gene is rarely studied before 2017. As of late, MSTO1 mutations have been reported to cause clinical manifestations such as myopathy, cerebellar atrophy and ataxia, motor developmental delay, and pigmentary retinopathy. Here we have performed a whole-exome sequencing in a family which includes two brothers showing cerebellar atrophy and ataxia, intellectual disability, and myopathy. As a result, two mutations were identified. One of these mutations has been identified as a missense mutation, c.836G > A; p. (Arg279His) and a novel frameshift variant, c.1259delG; p. (Gly420ValfsTer2). So, the two brothers both had compound heterozygous mutations with a combination of protein-truncation mutation and missense mutation. These findings suggested an association of MSTO1 mutations with the early onset of symptoms and revealed the genotype-phenotype correlation between different mutation cases. In this case, the two brothers both have pes planus which is not reported in other cases. This might suggest that the novel mutation is responsible for dysmorphia. Thus, the recessive and novel MSTO1 mutations enriches genetic information on the pathogenicity of MSTO1 in humans.

A novel case of MSTO1 gene related congenital muscular dystrophy with progressive neurological involvement.

Recessive mutations in the MSTO1 gene, encoding for a mitochondrial distribution and morphology regulator, have been recently described in a very limited number of patients with multisystem involvement, mostly characterized by myopathy or dystrophy, cerebellar ataxia, pigmentary retinopathy and raised creatine kinase levels. Here we report an additional patient with recessive MSTO1-related muscular dystrophy (MSTO1-RD), and clinical and radiological evidence of progressive cerebellar involvement. Whole-exome sequencing identified two novel MSTO1 missense variants, c.766C > T (p. (Arg256Trp) and c.1435C > T (p. (Pro479Ser), predicted as damaging by in silico tools. We also report a distinct pattern of selective involvement on muscle MRI in MSTO1-RD. This case confirms a consistent MSTO1-related neuromuscular phenotype and in addition suggests a progressive neurological component at least in some patients, in keeping with the mitochondrial role of the defective protein.

Mitochondrial myopathy with ataxia caused by MSTO1 gene mutation: a family case report and literature review / 中华实用儿科临床杂志

The clinical characteristics and gene variations of a family with mitochondrial myopathy and ataxia caused by MSTO1 gene mutation who visited Xiangya Hospital of Central South University in October 2019 were retrospectively analyzed.The proband was an 11-year-old female, who was found to have delayed motor and language development and dysarthria at the age of 1 year and 6 months.The 9-year-old younger brother of the proband had similar symptoms at the age of 1 year and 3 months.Both the proband and her younger brother had muscle weakness and ataxia.Their head magnetic resonance imaging showed cerebellar atrophy, and their electromyography showed neuroge-nic changes.Genetic testing revealed compound heterozygous mutations in MSTO1: c.1259delG; p.G420VfsX2 and c.571 C > T; p.R191X, which were inherited from their parents, respectively.The same site mutations were found in the younger brother.After 2 weeks of " cocktail therapy" , the symptoms of the children were alleviated, and their language and movement improved.

MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans.

The protein MSTO1 has been localized to mitochondria and linked to mitochondrial morphology, but its specific role has remained unclear. We identified a c.22G > A (p.Val8Met) mutation of MSTO1 in patients with minor physical abnormalities, myopathy, ataxia, and neurodevelopmental impairments. Lactate stress test and myopathological results suggest mitochondrial dysfunction. In patient fibroblasts, MSTO1 mRNA and protein abundance are decreased, mitochondria display fragmentation, aggregation, and decreased network continuity and fusion activity. These characteristics can be reversed by genetic rescue. Short-term silencing of MSTO1 in HeLa cells reproduced the impairment of mitochondrial morphology and dynamics observed in the fibroblasts without damaging bioenergetics. At variance with a previous report, we find MSTO1 to be localized in the cytoplasmic area with limited colocalization with mitochondria. MSTO1 interacts with the fusion machinery as a soluble factor at the cytoplasm-mitochondrial outer membrane interface. After plasma membrane permeabilization, MSTO1 is released from the cells. Thus, an MSTO1 loss-of-function mutation is associated with a human disorder showing mitochondrial involvement. MSTO1 likely has a physiologically relevant role in mitochondrial morphogenesis by supporting mitochondrial fusion.