A Titin Truncating Variant Causing a Dominant Myopathy with Cardiac Involvement in a Large Family: The Exception That Proves the Rule

Abstract

<p>Titin truncating variants (TTNtvs) have been repeatedly reported as causative of recessive but not dominant skeletal muscle disorders.ObjectiveTo determine whether a single heterozygous nonsense variant in TTN can be responsible for the observed dominant myopathy in a large family.MethodsIn this case series, all available family members (8 affected and 6 healthy) belonging to a single family showing autosomal dominant inheritance were thoroughly examined clinically and genetically.ResultsAll affected family members showed a similar clinical phenotype with a combination of cardiac and skeletal muscle involvement. Muscle imaging data revealed titin-compatible hallmarks. Genetic analysis revealed in all affected patients a nonsense TTN variant c.70051C>T p.(Arg23351*), in exon 327. RNA sequencing confirmed the lack of complete nonsense-mediated decay, and protein studies convincingly revealed expression of a shortened titin fragment of the expected size.DiscussionWe conclude that a single heterozygous nonsense variant in titin occasionally can cause a dominant myopathy as shown in this large family. Therefore, monoallelic titin truncating variants should be considered as possible disease-causing variants in unsolved patients with a dominant myopathy. However, large segregation studies, muscle imaging, and RNA and protein assays are needed to support the clinical and genetic interpretation.</p>