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Background Pediatric cardiomyopathy is a genetically heterogeneous disease with substantial morbidity and mortality. Current guidelines recommend genetic testing in children with hypertrophic, dilated, or restrictive cardiomyopathy, but practice variations exist. Robust data on clinical testing practices and diagnostic yield in children are lacking. This study aimed to identify the genetic causes of cardiomyopathy in children and to investigate clinical genetic testing practices. Methods and Results Children with familial or idiopathic cardiomyopathy were enrolled from 14 institutions in North America. Probands underwent exome sequencing. Rare sequence variants in 37 known cardiomyopathy genes were assessed for pathogenicity using consensus clinical interpretation guidelines. Of the 152 enrolled probands, 41% had a family history of cardiomyopathy. Of 81 (53%) who had undergone clinical genetic testing for cardiomyopathy before enrollment, 39 (48%) had a positive result. Genetic testing rates varied from 0% to 97% between sites. A positive family history and hypertrophic cardiomyopathy subtype were associated with increased likelihood of genetic testing ( P =0.005 and P =0.03, respectively). A molecular cause was identified in an additional 21% of the 63 children who did not undergo clinical testing, with positive results identified in both familial and idiopathic cases and across all phenotypic subtypes. Conclusions A definitive molecular genetic diagnosis can be made in a substantial proportion of children for whom the cause and heritable nature of their cardiomyopathy was previously unknown. Practice variations in genetic testing are great and should be reduced. Improvements can be made in comprehensive cardiac screening and predictive genetic testing in first‐degree relatives. Overall, our results support use of routine genetic testing in cases of both familial and idiopathic cardiomyopathy. Registration URL: https://www.clinicaltrials.gov ; Unique identifier: NCT01873963.

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Background: Identifying genetic variants which impact the level of cell cycle reentry and establishing the degree of cell cycle progression in those variants could help guide development of therapeutic interventions aimed at effecting cardiac regeneration. We observed that C57Bl6/NCR (B6N) mice have a marked increase in cardiomyocyte S-phase activity following permanent coronary artery ligation as compared to infarcted DBA/2J (D2J) mice. Methods: Cardiomyocyte cell cycle activity post-infarction was monitored in D2J, (D2J x B6N)-F1 and [(D2J x B6N)-F1 x D2J] backcross mice via bromodeoxyuridine or 5-ethynyl-2' -deoxyuridine incorporation, using a nuclear-localized transgenic reporter to identify cardiomyocyte nuclei. Genome-wide quantitative trait locus (QTL) analysis, fine scale genetic mapping, whole exome sequencing and RNA-seq analyses of the backcross mice were performed to identify the gene responsible for the elevated cardiomyocyte S-phase phenotype. Results: (D2J x B6N)-F1 mice exhibited a 14-fold increase in cardiomyocyte S-phase activity in ventricular regions remote from infarct scar as compared to D2J mice (0.798 ± 0.09% vs. 0.056 ± 0.004%; p < 0.001). QTL analysis of [(D2J x B6N)-F1 x D2J] backcross mice revealed that the gene responsible for differential S-phase activity was located on the distal arm of Chromosome 3 (LOD score = 6.38; p < 0.001). Additional genetic and molecular analyses identified 3 potential candidates. Of these, troponin I-interacting kinase ( Tnni3k ) is expressed in B6N hearts but not in D2J hearts. Transgenic expression of Tnni3k in a D2J genetic background results in elevated cardiomyocyte S-phase activity post-injury. Cardiomyocyte S-phase activity in both TNNI3K-expressing and TNNI3K-nonexpressing mice results in the formation of polyploid nuclei. Conclusions: These data indicate that TNNI3K expression increases the level of cardiomyocyte S-phase activity following injury.