Introduction: Structural variants (SVs) of the nebulin gene (NEB), including intragenic duplications, deletions, and copy number variation of the triplicate region, are an established cause of recessively inherited nemaline myopathies and related neuromuscular disorders. Large deletions have been shown to cause dominantly inherited distal myopathies. Here we provide an overview of 35 families with muscle disorders caused by such SVs in NEB. Methods: Using custom Comparative Genomic Hybridization arrays, exome sequencing, short-read genome sequencing, custom Droplet Digital PCR, or Sanger sequencing, we identified pathogenic SVs in 35 families with NEB-related myopathies. Results: In 23 families, recessive intragenic deletions and duplications or pathogenic gains of the triplicate region segregating with the disease in compound heterozygous form, together with a small variant in trans, were identified. In two families the SV was, however, homozygous. Eight families have not been described previously. In 12 families with a distal myopathy phenotype, eight unique, large deletions encompassing 52 to 97 exons in either heterozygous (n = 10) or mosaic (n = 2) state were identified. In the families where inheritance was recessive, no correlation could be made between the types of variants and the severity of the disease. In contrast, all patients with large dominant deletions in NEB had milder, predominantly distal muscle weakness. Discussion: For the first time, we establish a clear and statistically significant association between large NEB deletions and a form of distal myopathy. In addition, we provide the hitherto largest overview of the spectrum of SVs in NEB.

The authors have declared no competing interest.

This study was supported by the Folkhalsan Research Foundation, the Sigrid Juselius Foundation, the Medicinska understodsforeningen Liv och Halsa r.f., Finska Lakaresallskapet, Stiftelsen Perklens Minne, the Jane and Aatos Erkko Foundation, Muscular Dystrophy UK, and the Waldemar von Frenckells stiftelse. L.S. received additional fellowship support from the Sigrid Juselius Foundation (220540). F.H. and K.N. were supported by an MRC strategic award to establish an International Centre for Genomic Medicine in Neuromuscular Diseases (ICGNMD) MR/S005021/1. K.N. received additional Fellowship support from the Guarantors of Brain (UK Charity 1197319). M.S. was supported by the Academy of Finland. The work in C.G.B.'s laboratory is supported by intramural funds from the NIH National Institute of Neurological Disorders and Stroke. Analysis of families F22, F23 and F33 was supported in part by A Foundation Building Strength for Nemaline Myopathy, Boston Children's Hospital IDDRC Molecular Genetics Core Facility funded by P50HD105351 from the National Institutes of Health of USA, and the Boston Children's Hospital CRDC Initiative.

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The study was performed according to the stipulations of the Declaration of Helsinki of 1975. Written informed consent was obtained from participants and data was de-identified. The study has been approved by the Ethics Committee of the Children's Hospital, University of Helsinki, Finland (approval number 6/E7/05). The approval was renewed by the Ethics Review Board of Helsinki University Hospital in 2021. Where participants were recruited as part of a research project, local ethical approval was obtained: Research Ethics Board of the Hospital for Sick Children (approval number 1000009004), the Human Research Ethics Committee of Stellenbosch University (approval number S22/01/004), the Institutional Review Board of the National Institutes of Health (protocol 12-N-0095), and the Boston Children's Hospital Institutional Review Board (protocol number 03-08-0128R).

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All data produced in the present study are available upon reasonable request to the authors.