Interpreting Genetic Variants in Titin in Patients With Muscle Disorders

Abstract

IMPORTANCE Mutations in the titin gene (TTN) cause a wide spectrum of genetic diseases. The interpretation of the numerous rare variants identified in TTN is a difficult challenge given its large size. OBJECTIVE To identify genetic variants in titin in a cohort of patients with muscle disorders. DESIGN, SETTING, AND PARTICIPANTS In this case series, 9 patients with titinopathy and 4 other patients with possibly disease-causing variants in TTN were identified. Titin mutations were detected through targeted resequencing performed on DNA from 504 patients with muscular dystrophy, congenital myopathy, or other skeletal muscle disorders. Patients were enrolled from 10 clinical centers in April 2012 to December 2013. All of them had not received a diagnosis after undergoing an extensive investigation, including Sanger sequencing of candidate genes. The data analysis was performed between September 2013 and January 2017. Sequencing data were analyzed using an internal custom bioinformatics pipeline. MAIN OUTCOMES AND MEASURES The identification of novel mutations in the TTN gene and novel patients with titinopathy. We performed an evaluation of putative causative variants in the TTN gene, combining genetic, clinical, and imaging data with messenger RNA and/or protein studies. RESULTS Of the 9 novel patients with titinopathy, 5 (55.5%) were men and the mean (SD) age at onset was 25 (15.8) years (range, 0-46 years). Of the 4 other patients (3 men and 1 woman) with possibly disease-causing TTN variants, 2 (50%) had a congenital myopathy and 2 (50%) had a slowly progressive distal myopathy with onset in the second decade. Most of the identified mutations were previously unreported. However, all the variants, even the already described mutations, require careful clinical and molecular evaluation of probands and relatives. Heterozygous truncating variants or unique missense changes are not sufficient to make a diagnosis of titinopathy. CONCLUSIONS AND RELEVANCE The interpretation of TTN variants often requires further analyses, including a comprehensive evaluation of the clinical phenotype (deep phenotyping) as well as messenger RNA and protein studies. We propose a specific workflow for the clinical interpretation of genetic findings in titin.

Novel TTN mutations and muscle imaging characteristics in congenital titinopathy

“…Clinically, four patients younger than five years old presented with delayed motor development since early infancy. Except for the 1‐year‐old infant who had basically normal muscle strengths, neck flexion weakness was prominent in the three other patients, which was in keeping with previous reports . Furthermore, rigid spine, contractures of Achilles, and high‐arched palate were all observed in these three patients, showing high clinical consistency in clinical phenotypes with a higher prevalence than previous reports .…”

A new congenital multicore titinopathy associated with fast myosin heavy chain deficiency

“…In the study by Savarese et al, 10 homology structural modeling analyses suggested deleterious consequence of missense variants 10 . The substitution p.(Thr6324Pro) is located on the external surface of a β strand in an Ig‐domain of the I‐band region, probably affecting the protein stability 10 . The missense variant p.(Thr31339Ala), located in an Fn3 domain within the A‐band, could affect hydrogen bonding with other molecules and may influence the interaction with titin ligands 10 .…”

“…In silico analyses suggested that the two missense mutations had deleterious consequences, based on their absence in the general population and the TITINdb predictions. In the study by Savarese et al, 10 homology structural modeling analyses suggested deleterious consequence of missense variants 10 . The substitution p.(Thr6324Pro) is located on the external surface of a β strand in an Ig‐domain of the I‐band region, probably affecting the protein stability 10 .…”

“…The substitution p.(Thr6324Pro) is located on the external surface of a β strand in an Ig‐domain of the I‐band region, probably affecting the protein stability 10 . The missense variant p.(Thr31339Ala), located in an Fn3 domain within the A‐band, could affect hydrogen bonding with other molecules and may influence the interaction with titin ligands 10 . Moreover, Thr31339 is in a myosin‐binding domain, 17 and the missense variant could alter titin‐myosin interactions.…”

“…Here, we describe three TTN variants, one frameshift mutation (c.79683dupA; p.(Arg26562Thrfs*12)) and two missense mutations (p.(Thr31339Ala) and p.(Thr6324Pro)), in two siblings with congenital multicore myopathy. This family was previously included in the study by Savarese et al 10 who analyzed 93 neuromuscular genes and provided a workflow for interpreting TTN variants. In this report, we present additional phenotypical and molecular data that establish the pathogenicity of these TTN variants.…”