<i>FLNC</i> Complex Structural Variant Causing Distal Myopathy Identified by Family‐Based Genome Sequencing

Distal myopathies (DM) are clinically and genetically heterogeneous neuromuscular disorders, and identifying a molecular genetic cause may remain challenging in a subset of cases. Moreover, DM may be misdiagnosed as hereditary neuropathies due to overlapping clinical features. Here, we report a novel structural variant in FLNC associated with DM identified through genome sequencing (GS). Two affected relatives initially presented independently with referral diagnoses of Charcot-Marie-Tooth disease and amyotrophic lateral sclerosis. Clinical re-evaluation led to a change of the diagnosis to DM. Muscle MRI revealed a consistent pattern of selective muscle involvement characteristic of DM, enabling identification of six affected individuals within the family. GS was performed in seven family members, including six affected individuals and one unaffected relative. The analysis identified an insertion of two inverted fragments derived from the adjacent intron 2 into exon 3 of the FLNC gene. This complex rearrangement was accompanied by short non-templated nucleotide insertions at the junctions and a 3-bp exonic deletion at the insertion site, ultimately resulting in a frameshift. The structural variant was segregated with disease and was confirmed by Sanger sequencing and one Oxford nanopore long-read sequencing. Our findings expand the mutational spectrum of FLNC-associated disorders and highlight the importance of GS combined with a detailed clinical examination for the diagnosis of DM.