Case#: Family B, Case IV.2; 8-year-old male; diagnosed at age 6

DiseaseAssertion: Dilated cardiomyopathy (DCM)

FamilyInfo: Positive family history of sudden cardiac death (SCD). The proband's brother (IV.1) died at age 2 of SCD and renal failure. Both parents (III.1 and III.2) were asymptomatic heterozygous carriers of MYL3 c.106G>T. Two unaffected siblings were tested: IV.3 was wild-type and IV.4 was heterozygous (Fig. 1b; Supplementary Fig. S2b).

ParentalTesting: Both parents (III.1 and III.2) were confirmed heterozygous carriers of MYL3 c.106G>T by PCR and bidirectional Sanger sequencing, supporting autosomal recessive inheritance (Supplementary Genetic Findings; Fig. S2b).

CasePresentingHPOs: HP:0001644 (Dilated cardiomyopathy); HP:0000085 (Horseshoe kidney); HP:0005162 (Reduced left ventricular ejection fraction).

CaseHPOFreeText: DCM diagnosed at age 6; heart transplantation performed shortly after diagnosis. Clinical evaluation revealed horseshoe kidney, severe left ventricular dysfunction (LVEF 15–20%), moderate right ventricular dysfunction (TAPSE 9 mm), chamber cardiomegaly on coronary CT angiography, and rapid ventricular tachycardia on ECG (Main text, Family B; Supplementary Results, p. 6).

CaseNotHPOs: NR

CaseNotHPOFreeText: NR

CasePreviousTesting: NR (no prior diagnostic genetic testing reported before the study's exome sequencing)

Genotyping Method: Exome sequencing identified the homozygous MYL3 c.106G>T (p.Glu36Ter) variant in the proband (IV.2). PCR and bidirectional Sanger sequencing validated the variant and confirmed segregation in the parents and two unaffected siblings (Supplementary Methods; Supplementary Genetic Findings; Fig. S2b).

FunctionalAnalysis: Zebrafish cmlc1 knockdown and rescue experiments were performed using human MYL3 RNA containing c.106G>T (p.Glu36Ter). Unlike wild-type MYL3 RNA, the mutant RNA failed to rescue the cardiac phenotype, with persistent cardiac edema, ventricular dysfunction, and atrial dilation (Fig. 2; Supplementary Methods; Supplementary Movie S7).

Variant: MYL3 NM_000258.2:c.106G>T (p. Glu36Ter); homozygous nonsense variant

ClinVar: NR

CAID: CA352499542

gnomAD: NR

AdditionalInfo: The authors classified the homozygous MYL3 c.106G>T (p.Glu36Ter) nonsense variant as pathogenic. The variant introduces a premature termination codon at amino acid 36. The proband was born to asymptomatic, consanguineous (first-cousin) parents of Bakhtiyari ancestry. Family segregation and zebrafish functional studies provide supporting evidence (Supplementary Results; Supplementary Genetic Findings; Supplementary Table S2).

Case#: Family C, Case VI.1; female; diagnosed at age 2.5 years; died at age 8

DiseaseAssertion: Unclassified cardiomyopathy

FamilyInfo: Family C has a positive family history of childhood-onset cardiomyopathy and sudden cardiac death. The affected individual (VI.1) was diagnosed with cardiomyopathy at age 2.5 years and died at age 8. Three maternal relatives (V.5, V.6, and V.8) are depicted as affected and deceased in the pedigree (Fig. 1C). Both parents (V.1 and V.2) were clinically unaffected heterozygous carriers of the MYL3 c.482-1G>A splice-acceptor variant. The genotype of VI.1 could not be confirmed because DNA was unavailable.

ParentalTesting: PCR and bidirectional Sanger sequencing confirmed that both parents (V.1 and V.2) were heterozygous carriers of MYL3 c.482-1G>A. Direct testing of the affected daughter (VI.1) was not possible because DNA was unavailable. Her homozygous genotype was therefore suspected but not confirmed (Fig. 1C; Supplementary Genetic Findings).

CasePresentingHPOs: HP:0001638 (Cardiomyopathy); HP:0001645 (Sudden cardiac death).

CaseHPOFreeText: Female diagnosed with unclassified cardiomyopathy at age 2.5 years; died of sudden cardiac death at age 8 (Family C, Fig. 1C).

CaseNotHPOs: NR (no patient-specific negative HPO findings confirmed for VI.1).

CaseNotHPOFreeText: The authors reported no evidence of muscle weakness by neurological examination or history among affected individuals from the three families; however, it is unclear whether this assessment specifically included VI.1.

CasePreviousTesting: NR (no prior genetic testing reported for VI.1).

Genotyping Method: Genetic analysis of the affected individual's mother (V.2) identified MYL3 c.482-1G>A. PCR and bidirectional Sanger sequencing validated the variant and confirmed both parents (V.1 and V.2) as heterozygous carriers. VI.1 was not directly genotyped because DNA was unavailable (Supplementary Genetic Findings; Fig. 1C and S2C).

FunctionalAnalysis: A minigene splicing assay in human HEK293FT cells demonstrated that MYL3 c.482-1G>A causes abnormal RNA splicing. The predominant outcome (~75%) was skipping of the 78-bp exon 5, producing an in-frame deletion (p.Gly161_Glu186del). A less frequent outcome (~25%) was deletion of the first nucleotide of exon 5, causing a frameshift (p.Gly161Valfs*4). Both outcomes are predicted to disrupt EF-hand domains. The authors note that the minigene assay may not fully reflect in vivo splicing frequencies (Supplementary Results, pp. 7–8; Fig. 2E–I).

Variant: MYL3 c.482-1G>A (rs727503300)

ClinVar: 164486

CAID: CA013932

gnomAD: NR MYL3 c.482-1G>A was not found in the gnomAD v4 (GRCh38) region results at chr3:46,860,813. Allele frequency not reported for this variant in the searched dataset.

AdditionalInfo: The affected individual (VI.1) died at age 8, and DNA was unavailable for direct genetic testing. Both clinically unaffected parents (V.1 and V.2) were confirmed heterozygous carriers of MYL3 c.482-1G>A, suggesting that VI.1 may have inherited the variant from both parents; however, her genotype remains unconfirmed. A minigene assay demonstrated that the variant disrupts RNA splicing, predominantly causing exon 5 skipping, with a less frequent frameshift-producing splicing event. These functional findings support a damaging effect of the variant but do not establish VI.1's genotype (Fig. 1C; Fig. 2E–I; Supplementary Genetic Findings and Minigene Assay Results).

Case#: Family A, Case III.7; 38-year-old female; diagnosed at age 26

DiseaseAssertion: Hypertrophic cardiomyopathy (HCM)

FamilyInfo: Positive family history of HCM and SCD. Mother (II.4), brother (III.8), uncle (II.7), and cousin (III.11) died of SCD at ages 55, 26, 57, and 24, respectively. The proband's three children (IV.3, IV.4, IV.5) were homozygous for the MYL3 variant but had normal echocardiograms at ages 18, 13, and 6.

ParentalTesting: Supplementary Table S2 reports parental testing supportive of PM3 (variants in trans). However, DNA was unavailable for the proband's deceased mother (II.4), and individual parental genotypes were not specified in the supplementary genetic findings.

CasePresentingHPOs: HP:0001639 (Hypertrophic cardiomyopathy); HP:0002094 (Dyspnea); HP:0001653 (Mitral regurgitation); HP:0001712 (Left ventricular hypertrophy)

CaseHPOFreeText: HCM diagnosed at age 26 following dyspnea. At age 38, echocardiography showed reverse-curvature HCM with asymmetric left ventricular hypertrophy involving anterior, anterolateral, and lateral segments; maximum wall thickness 22 mm; severe mitral regurgitation; LVEF 55%; E/e' ratio 16.4; resting LVOT gradient 9 and post-maneuver gradient 30. Holter monitoring showed no arrhythmias (Supplementary Results, pp. 5–6; Table S1; Fig. S1).

CaseNotHPOs: HP:0001324 (Muscle weakness)

CaseNotHPOFreeText: Neurological evaluation revealed normal muscle tone, strength 5/5 in upper and lower limbs, no distal or proximal muscle atrophy or fasciculations, and normal creatine phosphokinase levels.

CasePreviousTesting: Exome sequencing was performed; no rare, likely pathogenic variants in other cardiomyopathy-associated genes identified after filtering.

Genotyping Method: Exome sequencing identified the MYL3 variant; bidirectional Sanger sequencing validated the variant. Familial genotyping supported recessive inheritance (Fig. 1a).

FunctionalAnalysis: Zebrafish cmlc1 knockdown and rescue experiments were performed using human MYL3 RNA carrying c.170C>A (p.Ala57Asp). Mutant RNA showed partial phenotypic rescue but no statistically significant improvement in ventricular contractility compared with knockdown alone (Fig. 2a-d).

Variant: MYL3 NM_000258.2:c.170C>A (p.Ala57Asp); homozygous missense variant; rs139794067

ClinVar: 43121

CAID: CA013575

gnomAD: Allele frequency 0.000163 (0 homozygotes), as reported in Supplementary Table S2.

AdditionalInfo: Family A, Case III.7. Clinical characteristics and genetic findings described in the Results section (Fig. 1a and 1d-e). Functional experiments described in Fig. 2. The authors classified c.170C>A as a variant of uncertain significance (VUS).

Case#: Patient 1, individual II.1 (Fig. 2A); 22-year-old Salvadoran female

DiseaseAssertion: Restrictive cardiomyopathy (RCM) with advanced heart failure

FamilyInfo: Patient denied a family history of cardiomyopathy, although supporting records were unavailable. Her mother (I.2) was clinically unaffected at age 45 and heterozygous for both MYL3 p.Glu143Lys and MYL2 p.Gly57Glu. Other family members were unavailable for evaluation (Fig. 2A; Table I).

ParentalTesting: Mother (I.2) was confirmed heterozygous for MYL3 c.427G>A (p.Glu143Lys) and MYL2 c.170G>A (p.Gly57Glu). Paternal testing was not reported.

CasePresentingHPOs: HP:0012764 (Orthopnea); HP:0004749 (Atrial flutter); HP:0031295 (Left atrial enlargement); HP:0030718 (right atrial enlargement)

CaseHPOFreeText: Worsening fatigue and exertional dyspnea over 12 months, paroxysmal nocturnal dyspnea, palpitations, NYHA Class III–IV heart failure, and ascites suggestive of congestive hepatopathy. Echocardiography showed severe biatrial enlargement and advanced left ventricular diastolic dysfunction with preserved systolic function. Additional findings included myocardial fibrosis and myofiber disarray on biopsy.

CaseNotHPOs: NR

CaseNotHPOFreeText: At initial presentation, patient denied dizziness, syncope, and lower-extremity edema. No S4 heart sound was detected. Recurrent syncope developed later.

CasePreviousTesting: Targeted sequencing of eight sarcomere genes (ACTC, MYBPC3, MYH7, MYL2, MYL3, TNNI3, TNNT2, TPM1). Genome-wide analysis was not reported. An additional heterozygous MYL2 variant, c.170G>A (p.Gly57Glu), was identified (ClinVar Variation ID: 43458; CAID: CA009910).

Genotyping Method: Sequencing of coding exons and intron–exon boundaries of MYL3 and seven additional sarcomere genes, performed by a commercial CLIA-certified laboratory (Correlagen Diagnostics, Inc.). The specific sequencing platform was not reported.

FunctionalAnalysis: NR

Variant: MYL3 NM_000258.2:c.427G>A (p.Glu143Lys); homozygous

ClinVar: 14063

CAID: CA013774

gnomAD: MYL3 c.427G>A (p.Glu143Lys). Highest reported ancestry-specific allele frequency: 0.0002333 (0.02333%) in the Admixed American population (AC = 14). Overall AF (exomes): 0.000008209; AF (genomes): 0.00001314; AF (total): 0.000008674. https://gnomad.broadinstitute.org/variant/3-46859529-C-T?dataset=gnomad_r4.

AdditionalInfo: Patient developed recurrent syncope and received an implantable cardioverter-defibrillator. She underwent orthotopic heart transplantation six months after RCM diagnosis. Authors considered the homozygous MYL3 and heterozygous MYL2 variants likely contributors to disease, although their individual contributions were not established. Clinical and family findings are reported in Patient 1, Figure 2A, and Table I.