Novel Mutations in Titin Exon 363 With Different Phenotypes Including a Founder Mutation in Eastern Europe

Background: Titin, the largest human protein, is essential for sarcomere structure and function. The TTN gene, spanning 364 exons, undergoes extensive alternative splicing thus producing multiple isoforms. The M-band region, encoded by exons 359–364, plays a critical role in sarcomere integrity and mechanical stability. Exon 363 is of interest due to its involvement in titinopathies. Pathogenic truncating variants in this exon have been linked to recessive myopathies, including and mainly young-onset recessive distal titinopathy. Methods: A multicenter study was conducted on six patients from five unrelated families with confirmed recessive titinopathy and truncating variants in exon 363. Clinical evaluations were performed. Genetic testing and segregation analysis confirmed the phase of the variants. Results: A novel truncating variant c.107578C>T, p.(Gln35860Ter) was identified in four unrelated patients of Eastern European ancestry, all carrying a second pathogenic variant in a canonical TTN exon. These patients exhibited juvenile/young-adult onset recessive distal titinopathy with progressive lower limb weakness, frequently asymmetric muscle involvement, and no cardiac or respiratory complications. A Belgian family presented with a congenital myopathy caused by a novel frameshift deletion c.107430delA, p.(Ser35811AlafsTer32) in exon 363, in compound heterozygosity with a truncating variant in exon 208. These patients showed a more severe phenotype. Conclusions: This study expands the spectrum of TTN-related myopathies, emphasizing exon 363's pathogenic significance. Truncating exon 363 variants contribute to young onset recessive distal and sometimes early onset titinopathy with contractures, and the phenotype severity is influenced by the second variant's location and exon usage.