Truncated titin is integrated into the human dilated cardiomyopathic sarcomere

Heterozygous (HET) truncating mutations in the TTN gene (TTNtv) encoding the giant titin protein are the most common genetic cause of dilated cardiomyopathy (DCM). However, the molecular mechanisms by which TTNtv mutations induce DCM are controversial. Here we investigated 127 clinically identified DCM human cardiac samples with next-generation sequencing (NGS), high-resolution gel electrophoresis, Western blot analysis and super-resolution microscopy in order to dissect the structural and functional consequences of TTNtv mutations. The occurrence of TTNtv was found to be 15% in the DCM cohort. Truncated titin proteins matching, by molecular weight, the gene-sequence predictions were detected in the majority of the TTNtv samples. The total amount of expressed titin, which includes the truncated fragments, was comparable in the TTNtv+ and TTNtv-samples, indicating that titin haploinsufficiency is not the leading cause of the molecular pathogenesis. Proteomic analysis of washed cardiac myofibrils and Stimulated Emission Depletion (STED) super-resolution microscopy of myocardial sarcomeres labeled with sequence-specific anti-titin antibodies revealed that truncated titin is structurally integrated in the sarcomere. Sarcomere lengthdependent anti-titin epitope position, shape and intensity analysis pointed at structural defects in the I/A junction and the M-band of TTNtv+ sarcomeres, which may contribute, via faulty mechanosensor function, to the development of manifest DCM.