Homozygous truncating mutation in prenatally expressed skeletal isoform of TTN gene results in arthrogryposis multiplex congenita and myopathy without cardiac involvement

Abstract

We report the case of a newborn with arthrogryposis multiplex congenita and severe axial hypotonia without cardiac involvement in which, using a customized targeted next-generation sequencing assay for 64 myopathy-associated genes, we detected a novel homozygous truncating mutation, c.38661_38665del, in exon 197 of the TTN gene that is expressed only in the fetal skeletal isoform. Its pathogenicity is supported by evidence of maternal isodisomy for chromosome 2. Muscle pathology showed fibers with core-like areas devoid of oxidative staining and cytoplasmic bodies. Electron microscopy showed the replacement of the sarcomeric structure with filamentous material. Identification of this mutation expands the phenotypic spectrum of the TTN gene and shows for the first time that a mutation not found in adult TTN isoforms is involved in the development of a neuromuscular disorder. TTN mutations should be considered in all severe congenital myopathies with arthrogryposis without cardiac involvement.

Keywords: Arthrogryposis multiplex congenita; Core myopathy; Isodisomy; Prenatally expressed isoform; TTN.