Cardiomyopathy, Familial Hypertrophic, 4

A number sign (#) is used with this entry because familial hypertrophic cardiomyopathy-4 (CMH4) is caused by heterozygous, homozygous, or compound heterozygous mutation in the gene encoding cardiac myosin-binding protein C (MYBPC3; 600958) on chromosome 11p11.

For a phenotypic description and a discussion of genetic heterogeneity of familial hypertrophic cardiomyopathy, see CMH1 (192600).

Clinical Features

Xin et al. (2007) studied 23 Old Order Amish infants with severe neonatal hypertrophic cardiomyopathy, 20 from the Geauga County settlement in Ohio, 1 from the Holmes County settlement in Ohio, and 2 from a settlement in New York. All of the infants presented with signs and symptoms of congestive heart failure during the first 3 weeks of life and had hypertrophic nonobstructive cardiomyopathy on echocardiography (ECG); life span averaged 3 to 4 months, and all died before 1 year of age except for 2 children who underwent cardiac transplantation.

Kimura et al. (1997) stated that they identified a 2-bp deletion at codon 945 in the MYBPC3 gene in a patient with hypertrophic cardiomyopathy who also displayed Wolff-Parkinson-White ventricular preexcitation (WPW; 194200); they also detected the same mutation in 3 additional CMH patients without WPW. Kimura et al. (1997) noted that although a locus for 'CMH with WPW' had been mapped to chromosome 7q3 (CMH6; 600858), their findings indicated that more than 1 form of CMH is associated with WPW syndrome.

Wang et al. (2013) studied a consanguineous Chinese family in which the 21-year-old proband was referred for cardiac evaluation after the sudden cardiac death of his 23-year-old brother, who had been diagnosed with CMH but was not offered an implantable cardioverter-defibrillator due to the lack of clinical symptoms. The proband had a 2-year history of mild chest pain after intense physical exertion, and diffuse repolarization changes with inverted T waves on ECG. Echocardiography showed mid to distal interventricular septal hypertrophy, and cardiac magnetic resonance imaging (CMR) revealed hypertrophy of the mid to distal interventricular septum and the inferior ventricular wall. The proband's younger brother, who was asymptomatic, had similar findings on ECG and echocardiography, with isolated hypertrophic septum and inferior ventricular wall on CMR. Both brothers had preserved cardiac function with left ventricular ejection fractions of 66% and 71%, respectively, normal atrial and ventricular chamber dimensions, no left ventricular outflow tract obstruction at rest or after exercise, and negative late gadolinium enhancement.

Mapping

Carrier et al. (1993) found evidence of a locus on chromosome 11 responsible for familial hypertrophic cardiomyopathy. In a French pedigree in which the disease was not linked to the MYH7 gene (160760), they found linkage to several microsatellite (CA)n repeats located on chromosome 11. They concluded that the gene could be localized to a 17-cM region in 11p13-q13. Kullmann et al. (1993) reported the case of a patient with Holt-Oram syndrome (142900) who had atrial septal defect and developed hypertrophic cardiomyopathy during the first year of life. A reciprocal translocation was found in this patient between 1p13 and 11q13.

Ko et al. (1996) reported results of linkage analysis in a Chinese family with apical hypertrophic cardiomyopathy. Apical hypertrophic cardiomyopathy (Japanese type) appears to be a distinct subtype of hypertrophic cardiomyopathy. It is characterized by giant negative T waves on EKG and left ventricular hypertrophy localized to the apex. The authors reported a maximal lod score of 3.38 at theta = 0.00 between the disease gene and the microsatellite markers D11S905, D11S987, and D11S913, which had been mapped to 11p13-q13.

Bonne et al. (1995) concluded that the COX8 gene (123870) that encodes cytochrome c oxidase subunit XIII is probably not the site of the mutation in CMH4, since in affected members of a family with chromosome 11-linked CMH, no deletions or insertions were found in COX8 cDNA or mRNA and no abnormality was detected in the COX8 sequence.

Xin et al. (2007) performed genomewide mapping analysis in 3 Old Order Amish infants from 3 different consanguineous families with severe neonatal hypertrophic cardiomyopathy and identified a 4.6-Mb block of homozygosity on chromosome 11p11.2-p11.12, encompassing the MYBPC3 gene (600958).

Inheritance

The transmission pattern of CMH4 was autosomal dominant in the families reported by Watkins et al. (1995) and autosomal recessive in the family reported by Wang et al. (2013).

Molecular Genetics

Both Watkins et al. (1995) and Bonne et al. (1995) demonstrated heterozygous mutations in the MYBPC gene that cause CMH4 (600958.0001, 600958.0002, 600958.0003). Incomplete penetrance was observed.

Niimura et al. (1998) identified 12 novel mutations in the MYBPC3 gene in probands from 16 families with CMH. The clinical expression of these mutations was similar to that observed for other genetic causes of hypertrophic cardiomyopathy, but the age at onset of the disease differed markedly. Only 58% of adults under the age of 50 years who had a mutation in the MYBPC3 gene (68 of 117 patients) had cardiac hypertrophy; disease penetrance remained incomplete through the age of 60 years. Survival was generally better than that observed among patients with hypertrophic cardiomyopathy caused by mutations in other genes for sarcomere proteins. Most deaths due to cardiac causes in these families occurred suddenly. Niimura et al. (1998) pointed out that delayed expression of cardiac hypertrophy and a favorable clinical course may hinder recognition of the heritable nature of mutations in the MYBPC3 gene. Clinical screening in adult life may be warranted for members of families characterized by hypertrophic cardiomyopathy.

Hengstenberg et al. (1993, 1994) studied a family with familial hypertrophic cardiomyopathy in which preliminary haplotype analyses excluded linkage to previously identified CMH loci at 14q1, 1q3, 11p13-q13, and 15q2, suggesting the existence of another locus, designated CMH5, for this disorder. Further studies in this family by Richard et al. (1999) demonstrated that of 8 affected family members, 4 had a mutation in the MYH7 gene (160760.0033), 2 had a mutation in the MYBPC3 gene (600958.0014), and 2 were doubly heterozygous for the 2 mutations. The doubly heterozygous patients exhibited marked left ventricular hypertrophy, which was significantly greater than that in the other affected individuals.

In a 28-year-old Australian man with CMH who had previously been studied by Ingles et al. (2005) and found to be compound heterozygous for missense mutations in the MYBPC3 gene (600958.0021 and 600958.0022), Chiu et al. (2007) also identified a heterozygous R73Q substitution in the CALR3 gene (611414). The proband was diagnosed at 18 years of age and had severe asymmetric septal hypertrophy on echocardiography (ECG). His father and 1 brother also had CMH, but declined to participate in the study. Chiu et al. (2007) suggested that calreticulin may be involved in both disease pathogenesis and modification.

Lekanne Deprez et al. (2006) reported 2 unrelated Dutch infants with severe hypertrophic cardiomyopathy in whom they identified compound heterozygosity for truncating mutations in the MYBPC3 gene (see, e.g., 600958.0023). The infants died at 5 and 6 weeks of age. The nonconsanguineous asymptomatic parents were heterozygous carriers of 1 of the mutations in each case; 1 of the fathers was found to have mild hypertrophic cardiomyopathy on cardiac MRI.

In 23 Old Order Amish infants with severe neonatal hypertrophic cardiomyopathy, 20 of whom were from the Geauga County settlement in Ohio, Xin et al. (2007) identified homozygosity for a splice site mutation in the MYBPC3 gene (3330+2T-G; 600958.0020). In addition, DNA analysis of a Mennonite couple with a child who died of CMH revealed that both parents were heterozygous for the 3330+2T-C mutation. The authors calculated the heterozygous carrier frequency in the Geauga County settlement to be approximately 10%. Noting the many reports of cardiac symptoms, including sudden death, among these probands' parents and relatives, and the close similarity between this mutation and the 3330+5G-C mutation (600958.0001) previously documented by Watkins et al. (1995) as the cause of CMH in heterozygous carriers, Xin et al. (2007) suggested that heterozygotes for the 3330+2T-G mutation may also be at risk for CMH.

In 250 unrelated patients with CMH, Frank-Hansen et al. (2008) used SSCP analysis and sequence confirmation of the MYBPC3 gene to determine whether intronic variation flanking the 3 MYBPC3 microexons is disease-causing. Functional studies and segregation analysis indicated that 4 of the 7 mutations they identified are associated with CMH (see, e.g., 600958.0016 and 600958.0017): all 4 mutations result in premature termination codons, suggesting that haploinsufficiency is a pathogenic mechanism of this type of mutation. In 1 family, a second mutation in the MYBPC3 gene was also identified (V1125M; 600958.0018). None of the mutations were found in DNA samples from 192 Caucasian controls.

Waldmuller et al. (2003) identified a 25-bp deletion in intron 32 of the MYBPC3 gene (600958.0019) in 2 CMH families, 1 of which was also known to carry a mutation in the MYH7 gene (160760). The authors stated that the relationship to disease was 'not unequivocal' and suggested that the deletion may represent a modifier polymorphism that may enhance the phenotypes of mutations responsible for disease. Dhandapany et al. (2009) analyzed the 25-bp deletion in the MYBPC3 gene in Indian patients with hypertrophic, dilated, and restrictive cardiomyopathies found an association with familial cardiomyopathy and an increased risk of heart failure (overall odds ratio, 6.99; p = 4 x 10(-11)). Analysis of RNA and protein from endomyocardial biopsies of 2 heterozygous individuals revealed 2 transcript structures, a normal transcript and a mutated allele with skipping of the associated exon, but the altered protein was not detected in tissue samples. Expression of mutant and wildtype protein in neonatal rat cardiomyocytes demonstrated a highly disorganized and diffuse pattern of sarcomeric architecture as a result of aberrant incorporation of the mutant protein. The authors concluded that the 25-bp MYBPC3 deletion is associated with a lifelong increased risk of heart failure. Dhandapany et al. (2009) tested 63 world population samples, comprising 2,085 individuals from 26 countries, for the 25-bp deletion, and they identified samples heterozygous for the deletion from Pakistan, Sri Lanka, Indonesia, and Malaysia but not in other samples. Haplotype analysis determined that the common 25-bp deletion likely arose approximately 33,000 years ago on the Indian subcontinent.

Ehlermann et al. (2008) screened the MYBPC3 gene in 87 patients with hypertrophic cardiomyopathy and 71 patients with CMD and identified heterozygous mutations in 16 (18.4%) of the CMH patients and in 2 (2.8%) of the CMD patients. However, in the first CMD family, 3 additional carriers of the MYBPC3 missense mutation had no certain pathologic findings, and the authors noted that in the index patient, hypertensive heart disease could not be ruled out as the cause of his CMD phenotype. In the second CMD family, the 2 oldest carriers of the splice site mutation displayed CMD, whereas 4 younger mutation carriers showed CMH; the authors stated that it was mostly likely that the 2 older patients suffered from end-stage CMH with progression to a CMD phenotype. Screening the cohort for variation in 5 additional cardiomyopathy-associated genes (MYH7, 160760; TNNT2, 191045; TNNI3, 191044; ACTC1, 102540; and TPM1, 191010) revealed no further mutations. Of a total of 45 affected individuals, from 12 families and 6 sporadic patients, 23 (51%) suffered an adverse event such as progression to severe heart failure, transient ischemic attack, stroke, or sudden death.

Tajsharghi et al. (2010) reported a female infant with fatal cardiomyopathy and skeletal myopathy who was homozygous for a nonsense mutation in the MYBPC3 gene (R943X; 600958.0023). Skeletal muscle biopsy at 2 months of age showed pronounced myopathic changes with numerous small fibers, which all expressed slow/beta-cardiac myosin heavy chain protein (MYH7; 160760). Electron microscopy revealed disorganization of the sarcomeres and partial depletion of thick filaments in the small fibers; immunohistochemical staining showed the presence of cardiac MYBPC in the small abnormal fibers. RT-PCR and sequencing demonstrated the R943X mutation in transcripts of skeletal muscle. Tajsharghi et al. (2010) noted that cardiac MYBPC is not normally expressed in skeletal muscle and stated that the reason for the ectopic expression of cardiac MYBPC remained unknown. The R943X mutation had previously been reported in compound heterozygosity with other truncating MYBPC3 mutations in 2 unrelated Dutch infants with fatal hypertrophic cardiomyopathy (Lekanne Deprez et al., 2006); skeletal myopathy was not mentioned in that report.

In a 21-year-old man from a consanguineous Chinese family with hypertrophic cardiomyopathy, Wang et al. (2013) screened 26 CMH-related genes and identified a homozygous missense mutation in the MYBPC3 gene (G490V; 600958.0029). His affected younger brother was also homozygous for the mutation; 6 other relatives, including their unaffected parents, were heterozygous for the mutation. None of the heterozygous carriers had any of the typical clinical manifestations of CMH, including the 2 oldest carriers at ages 62 years and 71 years, and none showed abnormalities on electrocardiography or left ventricular hypertrophy on echocardiography. CMR of 3 heterozygous individuals showed no structural abnormalities or cardiac fibrosis. Family members who did not carry the mutation all had normal electrocardiograms (ECGs) and echocardiograms except for the maternal grandfather, who had a more than 20-year history of uncontrolled hypertension and showed concentric hypertrophy on echocardiography without abnormal T or Q waves or arrhythmia on ECG.

Genotype/Phenotype Correlations

Calore et al. (2015) screened 97 Italian probands with CMH for mutations in the MYBPC3 gene and identified 16 different mutations in 39 (39.8%); among the MYBPC3 mutation carriers, none had additional mutations in the MYH7, TNNI3, or TNNT2 genes. The same 2-bp deletion (600958.0030) was detected in 19 probands; haplotype analysis revealed a shared 1.29-Mb haplotype, indicating a common founder in these families, which all came from the Veneto region of northeastern Italy. Overall, disease penetrance was incomplete (64.4%), age-related, and greater in men than women (85% vs 48%; p = 0.009). Probands carrying the founder mutation exhibited significantly higher prevalence of nonsustained ventricular tachycardia and implantable cardioverter-defibrillator placement compared to patients without MYBPC3 mutations or with other MYBPC3 mutations. Reduced survival due to sudden cardiac death (SCD) or aborted SCD also occurred more frequently after the fourth decade of life in probands carrying the founder mutation than in those without MYBPC3 mutations. Calore et al. (2015) noted that the overall annual mortality rate of 2% among the 48 affected founder-mutation carriers was higher than that previously described in MYBPC3 carriers and in the general population of CMH patients.

Animal Model

Meurs et al. (2005) identified a reduction in Mybpc3 protein in myocardium from Maine Coon cats with hypertrophic cardiomyopathy in comparison to control cats (P less than 0.001). In affected cats, the authors identified a G-C transversion in exon 3 of the feline Mybpc3 gene, resulting in an ala31-to-pro (A31P) substitution in the linker region between the C0 and C1 domains. The mutation was predicted to alter protein conformation and result in sarcomeric disorganization. Affected cats had some variability of phenotype from mildly affected to severe hypertrophy. Some cats developed congestive heart failure, and others died suddenly.

Pohlmann et al. (2007) found that cardiac myocytes from 6-week-old Mybpc3-null mice exhibited mild hypertrophy that became more pronounced by 30 weeks of age. Isolated Mybpc3-null myocytes showed markedly lower diastolic sarcomere length without change in diastolic Ca(2+). This reduced sarcomere length was partially abolished by inhibition of actin-myosin ATPase, indicating residual actin-myosin interaction in diastole. Mybpc3-null myocytes started to contract at lower Ca(2+) concentration, and both sarcomere shortening and Ca(2+) transients were prolonged in Mybpc3-null cells. Isolated Mybpc3-null left atria exhibited a marked increase in sensitivity to external Ca(2+) and, in contrast to wildtype, continued to develop twitch force at low micromolar Ca(2+) concentration. Pohlmann et al. (2007) concluded that MYBPC3 functions as a restraint on myosin-actin interaction at low Ca(2+) concentrations and short sarcomere length to allow complete relaxation during diastole.