Clinical and Genetic Analysis of Digenic Muscular Dystrophy due to SRPK3 and TTN Variants in Two Siblings

We present a family with two male siblings diagnosed with a newly described digenic myopathy, involving likely pathogenic loss-of-function variants in the SRPK3 and TTN genes: hemizygous p.(Pro68ArgfsTer55) and heterozygous p.(Trp14174Ter), respectively. Both siblings experienced prenatal disease onset, characterized by weak fetal movements, but showed significant clinical improvement over two last years of our follow-up. Key features included early onset, delayed motor development, and prominent axial and proximal weakness, while adult variants' carriers remained asymptomatic, without any myopathic or cardiac manifestations. Lower limb MRI revealed distinctive abnormalities, with different patterns between the siblings: the older brother showed more pronounced involvement of the thigh muscles, while the younger brother exhibited greater changes in the lower leg muscles. Given the early stage of the disease in our patients and the initial changes observed on MRI, we suggest that the semitendinosus and vastus lateralis muscles are primarily involved at the thigh level in SRPK3/TTN-myopathy. This case highlights the importance of considering digenic inheritance in neuromuscular disorders and underscores the necessity of comprehensive genetic analysis in similar cases.