Background: Myocarditis is one of the most common causes leading to heart failure in children and a possible genetic background has been postulated. We sought to characterize the clinical and genetic characteristics in patients with myocarditis ≤18 years of age to predict outcome. Methods: A cohort of 42 patients (Genetics in Pediatric Myocarditis) with biopsy-proven myocarditis underwent genetic testing with targeted panel sequencing of cardiomyopathy-associated genes. Genetics in Pediatric Myocarditis patients were divided into subgroups according to the phenotype of dilated cardiomyopathy (DCM) at presentation, resulting in 22 patients without DCM (myocarditis without phenotype of DCM) and 20 patients with DCM (myocarditis with phenotype of DCM). Results: Myocarditis with phenotype of DCM patients (median age 1.4 years) were younger than myocarditis without phenotype of DCM patients (median age 16.1 years; P <0.001) and were corresponding to heart failure–like and coronary syndrome–like phenotypes, respectively. At least one likely pathogenic/pathogenic variant was identified in 9 out of 42 patients (22%), 8 of them were heterozygous, and 7 out of 9 were in myocarditis with phenotype of DCM. Likely pathogenic/pathogenic variants were found in genes validated for primary DCM ( BAG3, DSP, LMNA, MYH7, TNNI3, TNNT2 , and TTN ). Rare variant enrichment analysis revealed significant accumulation of high-impact disease variants in myocarditis with phenotype of DCM versus healthy individuals ( P =0.0003). Event-free survival was lower ( P =0.008) in myocarditis with phenotype of DCM patients compared with myocarditis without phenotype of DCM and primary DCM. Conclusions: We report heterozygous likely pathogenic/pathogenic variants in biopsy-proven pediatric myocarditis. Myocarditis patients with DCM phenotype were characterized by early-onset heart failure, significant enrichment of likely pathogenic/pathogenic variants, and poor outcome. These phenotype-specific and age group–specific findings will be useful for personalized management of these patients. Genetic evaluation in children newly diagnosed with myocarditis and DCM phenotype is warranted.

Dominant mutations in the MYH7 and MYBPC3 genes are common causes of inherited cardiomyopathies, which often demonstrate variable phenotypic expression and incomplete penetrance across family members. Biallelic inheritance is rare, but allows gaining insights into the genetic mode of action of single variants. Here, we present three cases carrying a loss‐of‐function variant in a compound heterozygous state with a missense variant in either MYH7 or MYBPC3 leading to severe cardiomyopathy with left ventricular non‐compaction. Most likely, MYH7 haploinsufficiency due to one loss‐of‐function allele results in a clinical phenotype only in compound heterozygous form with a missense variant. In contrast, haploinsufficiency in MYBPC3 results in a severe early onset ventricular non‐compaction phenotype requiring heart transplantation when combined with a de novo missense variant on the second allele. In addition, the missense variant may lead to an unstable protein, as overall only 20% of the MYBPC3 protein remain detectable in affected cardiac tissue compared to control tissue. In conclusion, in patients with early disease onset and atypical clinical course, biallelic inheritance or more complex variants including CNVs and de novo mutations should be considered. Additionally, the pathogenic consequence of variants may differ in heterozygous versus compound heterozygous state.

Mutation of the PRDM16 gene causes human dilated and non-compaction cardiomyopathy. The PRDM16 protein is a transcriptional regulator that affects cardiac development via Tbx5 and Hand1, thus regulating myocardial structure. The biallelic inactivation of Prdm16 induces severe cardiac dysfunction with post-natal lethality and hypertrophy in mice. The early pathological events that occur upon Prdm16 inactivation have not been explored. This study performed in-depth pathophysiological and molecular analyses of male and female Prdm16^csp1/wt mice that carry systemic, monoallelic Prdm16 gene inactivation. We systematically assessed early molecular changes through transcriptomics, proteomics, and metabolomics. Kinetic modelling of cardiac metabolism was performed in silico with CARDIOKIN. Prdm16^csp1/wt mice are viable up to 8 months, develop hypoplastic hearts, and diminished systolic performance that is more pronounced in female mice. Prdm16^csp1/wt cardiac tissue of both sexes showed reductions in metabolites associated with amino acid as well as glycerol metabolism, glycolysis, and the tricarboxylic acid cycle. Prdm16^csp1/wt cardiac tissue revealed diminished glutathione (GSH) and increased inosine monophosphate (IMP) levels indicating oxidative stress and a dysregulated energetics, respectively. An accumulation of triacylglycerides exclusively in male Prdm16^csp1/wt hearts suggests a sex-specific metabolic adaptation. Metabolic modelling using CARDIOKIN identified a reduction in fatty acid utilization in males as well as lower glucose utilization in female Prdm16^csp1/wt cardiac tissue. On the level of transcripts and protein expression, Prdm16^csp1/wt hearts demonstrate an up-regulation of pyridine nucleotide-disulphide oxidoreductase domain 2 (Pyroxd2) and the transcriptional regulator pre-B-cell leukaemia transcription factor interacting protein 1 (Pbxip1). The strongest concordant transcriptional up-regulation was detected for Prdm16 itself, probably through an autoregulatory mechanism. Monoallelic, global Prdm16 mutation diminishes cardiac performance in Prdm16^csp1/wt mice. Metabolic alterations and transcriptional dysregulation in Prdm16^csp1/wt affect cardiac tissue. Female Prdm16^csp1/wt mice develop a more pronounced phenotype, indicating sexual dimorphism at this early pathological window. This study suggests that metabolic dysregulation is an early event in the PRDM16 associated cardiac pathology.

Myocarditis is an inflammatory disease of the heart. Pediatric myocarditis with the dilated cardiomyopathy (DCM) phenotype may be caused by likely pathogenic or pathogenic genetic variants [(L)P] in cardiomyopathy (CMP) genes. Systematic analysis of immune disorder gene defects has not been performed so far. We analyzed 12 patients with biopsy-proven myocarditis and the DCM phenotype together with their parents using whole-exome sequencing (WES). The WES data were filtered for rare pathogenic variants in CMP (n = 89) and immune disorder genes (n = 631). Twelve children with a median age of 2.9 (1.0–6.8) years had a mean left ventricular ejection fraction of 28% (22–32%) and myocarditis was confirmed by endomyocardial biopsy. Patients with primary immunodeficiency were excluded from the study. Four patients underwent implantation of a ventricular assist device and subsequent heart transplantation. Genetic analysis of the 12 families revealed an (L)P variant in the CMP gene in 8/12 index patients explaining DCM. Screening of recessive immune disorder genes identified a heterozygous (L)P variant in 3/12 index patients. This study supports the genetic impact of CMP genes for pediatric myocarditis with the DCM phenotype. Piloting the idea that additional immune-related genetic defects promote myocarditis suggests that the presence of heterozygous variants in these genes needs further investigation. Altered cilium function might play an additional role in inducing inflammation in the context of CMP.