P18 N-of-1 ASO therapeutic development for ultra-rare deep intronic mutations in X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM) is a rare, life-threatening congenital neuromuscular disorder caused by mutations in the MTM1 gene. Affected infants typically present with severe hypotonia and respiratory failure, with up to 50% mortality in the first year of life. No approved disease-modifying therapies are currently available. At Great Ormond Street Hospital, through the Highly Specialised Congenital Myopathy Service, we have identified two ultra-rare deep intronic MTM1 mutations: one previously reported (c.1468-577A>G) and one novel (c.342+575C>G). Both mutations cause pseudo-exon inclusion and introduce premature stop codons, resulting in loss of MTM1 protein. We propose to develop two N-of-1 exon-skipping antisense oligonucleotide (ASO) therapies tailored to these mutations. By inducing skipping of the pseudo-exons, the ASOs aim to restore normal splicing and functional MTM1 protein expression. ASOs targeting both variants have already demonstrated effective pseudo-exon skipping and restoration of MTM1 protein in patient-derived fibroblasts. Building on these results, we will further optimise the ASO sequences and assess their efficacy and safety in MyoD-reprogrammed muscle cells and hepatocytes differentiated from patient-specific iPSCs, with the goal of demonstrating therapeutic benefit in both muscle and liver tissues.