Abstract
In 2001, we described an autosomal dominant myopathy characterized by neuromuscular ventilatory failure in ambulant patients. Here we describe the underlying genetic basis for the disorder, and we define the neuromuscular, respiratory and radiological phenotype in a study of 31 mutation carriers followed for up to 31 years. A combination of genome-wide linkage and whole exome sequencing revealed the likely causal genetic variant in the titin (TTN) gene (g.274375T>C; p.Cys30071Arg) within a shared haplotype of 2.93 Mbp on chromosome 2. This segregated with the phenotype in 21 individuals from the original family, nine subjects in a second family with the same highly selective pattern of muscle involvement on magnetic resonance imaging and a third familial case with a similar phenotype. Comparing the mutation carriers revealed novel features not apparent in our original report. The clinical presentation included predominant distal, proximal or respiratory muscle weakness. The age of onset was highly variable, from early adulthood, and including a mild phenotype in advanced age. Muscle weakness was earlier onset and more severe in the lower extremities in nearly all patients. Seven patients also had axial muscle weakness. Respiratory function studies demonstrated a gradual deterioration over time, reflecting the progressive nature of this condition. Cardiomyopathy was not present in any of our patients despite up to 31 years of follow-up. Magnetic resonance muscle imaging was performed in 21 affected patients and revealed characteristic abnormalities with semitendinosus involvement in 20 of 21 patients studied, including 3 patients who were presymptomatic. Diagnostic muscle histopathology most frequently revealed eosinophilic inclusions (inclusion bodies) and rimmed vacuoles, but was non-specific in a minority of patients. These findings have important clinical implications. This disease should be considered in patients with adult-onset proximal or distal myopathy and early respiratory failure, even in the presence of non-specific muscle pathology. Muscle magnetic resonance imaging findings are characteristic and should be considered as an initial investigation, and if positive should prompt screening for mutations in TTN. With 363 exons, screening TTN presented a major challenge until recently. However, whole exome sequencing provides a reliable cost-effective approach, providing the gene of interest is adequately captured.