Inferring disease course from differential exon usage in the wide titinopathy spectrum

Abstract

Exon; Titinopathy spectrum

Exón; Espectro de la titinopatía

Exó; Espectre de la titinopatia

Objective Biallelic titin truncating variants (TTNtv) have been associated with a wide phenotypic spectrum, ranging from complex prenatal muscle diseases with dysmorphic features to adult-onset limb-girdle muscular dystrophy, with or without cardiac involvement. Given the size and complexity of TTN, reaching an unequivocal molecular diagnosis and precise disease prognosis remains challenging. Methods In this case series, 12 unpublished cases and one already published case with biallelic TTNtv were collected from multiple international medical centers between November 2022 and September 2023. TTN mutations were detected through exome or genome sequencing. Information about familial and personal clinical history was collected in a standardized form. RNA-sequencing and analysis of TTN exon usage were performed on an internal sample cohort including postnatal skeletal muscles, fetal skeletal muscles, postnatal heart muscles, and fetal heart muscles. In addition, publicly available RNA-sequencing data was retrieved from ENCODE. Results We generated new RNA-seq data on TTN exons and identified genotype–phenotype correlations with prognostic implications for each titinopathy patient (whether worsening or improving in prenatal and postnatal life) using percentage spliced in (PSI) data for the involved exons. Interestingly, thanks to exon usage, we were also able to rule out a titinopathy diagnosis in one prenatal case. Interpretation This study demonstrates that exon usage provides valuable insights for a more exhaustive clinical interpretation of TTNtv; additionally, it may serve as a model for implementing personalized medicine in many other genetic diseases, since most genes undergo alternative splicing.

M.S. received support from the Academy of Finland (grant 339437), Association Française contre les Myopathies (grant 23281), Sydäntutkimussäätiö, and Samfundet Folkhälsan i Svenska, Finland. A.O. received supported by Magnus Ehrnrooth Foundation. B.U. received support from the European Joint Program on Rare Diseases (project IDOLS-G), Academy of Finland, Juselius Foundation, and Samfundet Folkhälsan i Svenska Finland. F.M. received support from the European Joint Program on Rare Diseases (project IDOLS-G) and Instituto de Salud Carlos III, Spain (project number AC19/00048). P.H. received support from the Jane and Aatos Erkko foundation.