A milder phenotype of megaconial congenital muscular dystrophy due to a novelCHKBmutation

Immune-Mediated Megaconial Myopathy: A Novel Subtype of Autoimmune Myopathy

OBJECTIVES

To describe a novel subtype of autoimmune myopathy, immune-mediated megaconial myopathy (IMMM), myopathologically characterized by giant mitochondria (megaconia).

METHODS

In this case series, we reviewed the Mayo Clinic Muscle Pathology database, between 2018 and 2023, to identify patients with megaconial pathology, subacute progressive weakness, and hyperCKemia, clinically resembling myositis. We recruited 1 patient from another institute, who had similar clinicopathologic features.

RESULTS

Five patients were identified. Age at onset of weakness ranged from 19 to 45.5 years. All patients had proximal weakness, elevated creatine kinase levels (1,214 to 5,920 U/L), negative myositis antibodies, necrotizing myopathology, and nonnecrotic myofibers harboring giant mitochondria. Immunohistochemical studies conducted in 4 patients showed sarcolemmal MHC-1 and C5b9 immunoreactivities. Megaconial pathology was considered pathognomonic of congenital muscular dystrophy due to biallelic pathogenic variants in CHKB. Sequencing of CHKB in 4/5 patients was unrevealing. Immunomodulatory therapy improved weakness and hyperCKemia in 4 treated patients. Of interest, all patients had coexisting pancreatic diseases (3 cystic fibrosis-related exocrine pancreatic insufficiency, 1 pancreatic cancer, and 1 pancreatitis).

DISCUSSION

In addition to incurable CHKB-congenital muscular dystrophy, giant mitochondria can also occur in this new subtype of treatable autoimmune myopathy, IMMM. The association between IMMM and pancreatic disorders remains to be elucidated.

Reduced mitochondrial fission and impaired energy metabolism in human primary skeletal muscle cells of Megaconial Congenital Muscular Dystrophy

Megaconial Congenital Muscular Dystrophy (CMD) is a rare autosomal recessive disorder characterized by enlarged mitochondria located mainly at the periphery of muscle fibers and caused by mutations in the Choline Kinase Beta (CHKB) gene. Although the pathogenesis of this disease is not well understood, there is accumulating evidence for the presence of mitochondrial dysfunction. In this study, we aimed to investigate whether imbalanced mitochondrial dynamics affects mitochondrial function and bioenergetic efficiency in skeletal muscle cells of Megaconial CMD. Immunofluorescence, confocal and transmission electron microscopy studies revealed impaired mitochondrial network, morphology, and localization in primary skeletal muscle cells of Megaconial CMD. The organelle disruption was specific only to skeletal muscle cells grown in culture. The expression levels of mitochondrial fission proteins (DRP1, MFF, FIS1) were found to be decreased significantly in both primary skeletal muscle cells and tissue sections of Megaconial CMD by Western blotting and/or immunofluorescence analysis. The metabolomic and fluxomic analysis, which were performed in Megaconial CMD for the first time, revealed decreased levels of phosphonucleotides, Krebs cycle intermediates, ATP, and altered energy metabolism pathways. Our results indicate that reduced mitochondrial fission and altered mitochondrial energy metabolism contribute to mitochondrial dysmorphology and dysfunction in the pathogenesis of Megaconial CMD.

Megaconial congenital muscular dystrophy: Same novel homozygous mutation in CHKB gene in two unrelated Chinese patients

Megaconial congenital muscular dystrophy (CMD) is a rare form of congenital muscular dystrophy attributed to an autosomal recessive CHKB mutation. We report two unrelated Chinese girls with Megaconial CMD who harbored the same novel homozygous CHKB mutation but exhibited different phenotypes. Patient 1, who is now 8 years old, has autism, intellectual disabilities, mild girdle weakness, and characteristic muscle biopsy with COX-negative fibers. Patient 2, now 12 years old, has limited intelligence and marked weakness, with scoliosis, hip subluxation and early loss of ambulation. Both exhibited mildly elevated creatine kinase levels, have relative sparing of adductor longus and extensor digitorum longus on MRI leg muscles, and a c.598del (p.Gln200Argfs*11) homozygous CHKB loss-of-function mutation. Their parents are heterozygous carriers. This is the first report of Megaconial CMD in Chinese patients demonstrating the pathogenicity of the identified homozygous CHKB variant. A case review of all previously reported patients of different ethnicities is also included.