A homozygous variant in cardiac troponin I3, TNNI3, causes severe pediatric restrictive cardiomyopathy.

Dilated cardiomyopathy (DCM) results from systolic dysfunction, while restrictive cardiomyopathy (RCM) is due to diastolic dysfunction. The diverse pathophysiology of primary DCM and RCM suggests distinct underlying genetic mechanisms. A well-established disease gene for DCM and RCM is cardiac troponin I3 (TNNI3), which causes dominant and recessively inherited forms. In children, bi-allelic truncating TNNI3 variants have typically been associated with DCM, and heterozygous missense TNNI3 variants are associated with RCM. We report a 2-year-old female with severe RCM that is genetically caused by a homozygous TNNI3 nonsense variant, c.406C>T (p.Arg136∗), which results in a more distal (C-terminal) truncation than most previously reported disease-associated nonsense variants. In myocardial biopsies of the patient, TNNI3 protein abundance was diminished, suggesting that residual TNNI3 function may underlie RCM, while TNNI3 absence causes DCM. The RCM in this patient was treatment refractory and resulted in a heart transplant at the age of 28 months. Overall, recessive TNNI3 protein truncation causes severe pediatric RCM, suggesting that the allelic status, type of genetic alteration, and length of TNNI3 protein truncation determine cardiomyopathy onset and subtype manifestation.