Loss of protein function resulting in severe phenotypes of X-linked myotubular myopathy caused by novel synonymous and splice-site variants in the MTM1 gene.

BACKGROUND: X-linked myotubular myopathy (XLMTM) is a severe, often neonatally lethal congenital neuromuscular disorder caused by pathogenic variants in the MTM1 gene. Affected neonates typically present with profound hypotonia, generalized muscle weakness, and respiratory insufficiency requiring immediate ventilatory support at birth. Splice-site variants in MTM1 account for approximately 20% of molecularly confirmed cases of XLMTM.

METHODS: Two male neonates with molecularly confirmed severe XLMTM, born at distinct gestational ages, were enrolled. Next-generation sequencing followed by validation with Sanger sequencing identified a de novo synonymous variant (c.183A > G) in MTM1 in Patient 1 and a canonical splice-site variant (c.444+1G > A) in Patient 2. The functional impact of these variants was assessed via longitudinal clinical assessments, skeletal muscle biopsy with histopathological and immunohistochemical analyses, and minigene splicing assays.

RESULTS: Minigene assays confirmed pathogenic exon skipping for both variants, with the c.183A > G variant causing complete skipping of exon 4 in Patient 1 and the c.444+1G > A variant causing complete skipping of exon 6 in Patient 2. Both events resulted in frameshifts and the subsequent loss of myotubularin protein expression. Despite sharing severe, life-threatening clinical and histopathological features, including neonatal-onset respiratory failure, centrally located myonuclei, reduced myofiber diameter, and rapid disease progression, the two patients displayed markedly divergent respiratory trajectories.

CONCLUSIONS: These findings indicate that the novel synonymous variant c.183A > G and the splice-site variant c.444+1G > A in MTM1 can cause comparable lethal phenotypes via pathogenic mechanisms mediated by exon skipping.