Case#: 29 year old female; diagnosed with DCM 8 years before death; died suddenly at home on 22 February 2017; forensic autopsy case

DiseaseAssertion: Dilated cardiomyopathy (DCM) with sudden cardiac death; MYBPC3-related

FamilyInfo: Family from Central-South China with a history of SCD. Father (I:1) reportedly died of a heart attack at 37, not genetically tested. Mother (I:2) and sister (II:2) are unaffected and do not carry the variant. No other affected relatives reported.

ParentalTesting: Not performed, both parents were not genotyped for the variant; the father was deceased before the study. Maternal non-carriage confirmed by Sanger, so the variant is either paternal or de novo; phase not established.

CasePresentingHPOs: HP:0001644; HP:0001645

CaseHPOFreeText: Diagnosed with DCM 8 years before death, with no other abnormalities. Found unresponsive at home at noon and expired; no prodromal symptoms documented. Autopsy: enlarged heart with valve thickening, myocardial fibrosis, and multi-organ congestion; no other fatal injury or disease. HE staining of left ventricle showed no hypertrophy or other anomalies. Toxicological screening for ethanol, benzodiazepines, barbiturates, amphetamine, methamphetamine, ketamine, morphine, MDMA, and THC-acid was negative; serum IgE normal (27 IU/ml); eosinophils 4.7% with no mast-cell degranulation.

CaseNotHPOs: HP:0001639; HP:0001634; HP:0003124

CaseNotHPOFreeText: No cardiomyocyte hypertrophy on histology, distinguishing this from HCM. No coronary, valvular, or congenital structural disease identified. No toxicological cause of death.

CasePreviousTesting: Whole-exome sequencing on postmortem blood, retaining synonymous variants; filtering against gnomAD and CMDB; in silico prediction with MutationTaster, PolyPhen-2, and SIFT; NetGene2-2.42 used to predict splice effects. Five variants in cardiomyopathy-related genes were found; only the MYBPC3 variant was classified likely pathogenic.

Genotyping Method: WES with Sanger confirmation on ABI 3100; family members genotyped by Sanger.

FunctionalAnalysis: Minigene assay pcDNA3.1-MYBPC3 construct spanning exon 1-intron 1-exon 2, transfected into HEK293 cells: wild-type and the c.10G>A negative control both produced a ~1,500 bp full-length band and a ~300 bp spliceosome band; the c.24A>C construct produced no spliceosome band, indicating abnormal splicing with intron 1 retention and frameshift. Immunohistochemistry showed decreased MYBPC3 protein in the proband's left ventricle versus a control heart.

Variant: MYBPC3 NM_000256.3:c.24A>C (p.Pro8Pro); heterozygous synonymous variant at the penultimate base of exon 1; disrupts the splice donor site; novel

ClinVar: 164098

CAID: CA11138

gnomAD: 0.00001541

AdditionalInfo: Classified likely pathogenic with PS3, PM2, PP1, PP3; note the paper's Discussion relabels the prediction code as PP4, an internal inconsistency. The authors describe it as the first reported deleterious synonymous MYBPC3 variant, acting by intron retention and reduced protein. The same splice site is affected by the known HCM variants c.24+1G>A and c.24+3A>C. Four other variants were found and classified likely benign or VUS: LDB3 c.2131T>C (VUS), BBS2 c.422A>G, EVC2 c.2643G>C, and BBS4 c.1548_1549del.