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J Am Coll Cardiol, 2001; 38:322-330 © 2001 by the American College of Cardiology Foundation |

* Departments of Internal Medicine and Cardiology, Charité, Campus Virchow-Klinikum, Humboldt University and Deutsches Herzzentrum, Berlin, Germany
Department of Medical Genetics, Child Health Institute, University of Istanbul, Istanbul, Turkey
Manuscript received February 23, 2000; revised manuscript received April 10, 2001, accepted April 12, 2001.
Reprint requests and correspondence: Dr. V. Regitz-Zagrosek, Kardiologie, Charité, Campus Virchow-Klinikum und Deutsches Herzzentrum Berlin, Augustenburger Platz 1, D-13353 Berlin, Germany
zagrosek{at}dhzb.de
| Abstract |
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We studied the clinical and genetic features of hypertrophic cardiomyopathy (HCM) caused by mutations in the myosin-binding protein C gene (MYBPC3) in 110 consecutive, unrelated patients and family members of European descent.
BACKGROUND
Mutations in the MYBPC3 gene represent the cause of HCM in
15% of familial cases. MYBPC3 mutations were reported to include mainly nonsense versus missense mutations and to be characterized by a delayed onset and benign clinical course of the disease in Japanese and French families. We investigated the features that characterize MYBPC3 variants in a large, unrelated cohort of consecutive patients.
METHODS
The MYBPC3 gene was screened by single-strand conformational polymorphism analysis and sequencing. The clinical phenotypes were analyzed using rest and 24-h electrocardiography, electrophysiology, two-dimensional and Doppler echocardiography and angiography.
RESULTS
We identified 13 mutations in the MYBPC3 gene: one nonsense, four missense and three splicing mutations and five small deletions and insertions. Of these, 11 were novel, and two were probably founder mutations. Patients with MYBPC3 mutations presented a broad range of phenotypes. In general, the 16 carriers of protein truncations had a tendency toward earlier disease manifestations (33 ± 13 vs. 48 ± 9 years; p = 0.06) and more frequently needed invasive procedures (septal ablation or cardioverter-defibrillator implantation) compared with the 9 carriers of missense mutations or in-frame deletions (12/16 vs. 1/9 patients; p < 0.01).
CONCLUSIONS
Multiple mutations, which include missense, nonsense and splicing mutations, as well as small deletions and insertions, occur in the MYBPC3 gene. Protein truncation mutations seem to cause a more severe disease phenotype than missense mutations or in-frame deletions.
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trast, the clinical features of patients with mutations in the beta-myosin heavy chain (beta-MHC) gene are more heterogeneous, with some point mutations being associated with a relatively benign outcome (V606M) and others (R403Q) with a high incidence of sudden death (12).
Since Bonne et al. (7) and Watkins et al. (8) reported the first mutations in MYBPC3 responsible for HCM, a total of 33 mutations in this gene have been identified (7,8,1316), which accounts for
15% of cases of familial HCM. Most of the mutations led to premature stop codons, resulting in proteins lacking myosin- or titin-binding sites, or both (13). Two cross-sectional family studies (14,16) suggest that patients with MYBPC3 mutations have a favorable clinical profile that is characterized by late-onset HCM and a good prognosis.
Considering the spectrum of phenotypes described in family studies, we investigated how patients with MYBPC3 mutations were represented in a large cohort of unrelated, consecutive patients presenting with HCM at a tertiary-care referral center and which type of mutation occurred in these patients. We now present the broad spectrum of genetic variability and clinical phenotypes in 15 unrelated patients and their family members with 13 MYBPC3 mutations, including the first founder mutation in a European cohort.
| Methods |
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15 mm in the absence of confounding disease on two repeated echocardiograms (as defined in the Coronary Artery Risk Development In young Adults [CARDIA] study [17]) or on the presence of borderline hypertrophy (wall thickness of 13 or 14 mm), together with other characteristic features of HCM (i.e., outflow tract gradient). Patients with earlier myectomy were accepted independently on the echocardiographic criteria. In all patients, the clinical investigation included rest and 24-h electrocardiography, prospective evaluation using two-dimensional and Doppler echocardiography, cardiac catheterization and angiography. Patients in whom symptomatic or sustained ventricular tachycardia was suspected were analyzed by electrophysiologic studies. Only life-threatening, symptomatic ventricular tachyarrhythmia was treated with an implantable cardioverter-defibrillator (ICD). In patients with outflow tract obstruction, functional deterioration toward New York Heart Association functional class III, despite long-term medication, was accepted as an indication for subvalvular myectomy or transcoronary ablation of septal hypertrophy (TASH). Family members were invited to undergo genetic analysis and clinical investigation. Nine of 110 patients were from Turkey; all other patients were of German descent. The study was approved by the Ethical Committee of the Charité, Campus Virchow-Clinic, Humboldt University of Berlin, and written, informed consent was obtained from all subjects. Mutation detection. Isolation of deoxyribonucleic acid (DNA), polymerase chain reaction (PCR) and single-strand conformational polymorphism analysis were performed as reported previously (18). All primer sequences used in this study are available on request (from the authors). Mutations in the MYBPC3 gene were independently confirmed in DNA samples from probands by restriction-enzyme digestion or, if not available, by repeated sequencing of independent PCR products (Table 1).
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Genotyping markers at the MYBPC3 locus and haplotype construction.
Family members of patients 1331 and 169 (IVS7+1G>A) and patients 39 and 315 (Q1233X) were genotyped for IVS7+1G>A and Q1233X mutations, respectively. In addition, an intragenic marker in intron 20 and two MYBPC3-flanking markers (D11S1385 and D11S1313) covering a distance of
0.6 cM were analyzed. Most likely, haplotypes were constructed with reference to gene and marker order on the respective chromosome, in a family context. In the case of a proband without a traced family (DNA 315), evidence of identity by descent was accepted if the alleles present allowed construction of a haplotype (designated as the "assumed haplotype") identical to the disease-associated haplotype.
Statistics. Data on clinical variables are expressed as the mean value ± SD. Student t and chi-square tests were used to compare groups.
| Results |
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In 11 beta-MHC mutation carriers identified by our group in a separate study, the age of onset of patients with MYBPC3 mutations seemed comparable (37 ± 10 vs. 41 ± 15 years, p = 0.3). The degree of hypertrophy (20 ± 4 vs. 17 ± 4 mm, p = 0.08) and the necessity for myectomy or TASH (6/15 vs. 2/11 patients) were also similar.
Founder mutation.
The mutation IVS7+1G>A was identified in two apparently unrelated patients of German descent. Using three highly polymorphic markers, we constructed the most likely haplotype carrying the disease-causing mutation in both families. A common haplotype encompassing the MYBPC3 locus was segregated with the disease-causing mutation, suggesting descent from a shared ancestor (Fig. 4). The mutation Q1233X was identified in two apparently unrelated patients of Turkish descent. Using the previously described highly polymorphic markers, we constructed the most likely haplotype in the family of patient 39 and an assumed disease-associated haplotype for patient 315. According to these haplotype constructions, the Q1233X mutation was embedded in one haplotype segregated with the disease-causing mutation (data not shown). These data suggest that the mutation Q1233X is identical by descent. Within the analyzed families, the shared haplotype extended
0.6 cM across the region between D11S1385 and D11S1313.
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| Discussion |
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Four amino acid exchanges that were not present in 100 chromosomes of healthy control subjects were found, all of which affect amino acids that are conserved at least among the human, mouse and chicken cardiac MYBPC3 protein and between human fast- and slow-type skeletal myosin-binding proteins. All of them concern functionally relevant regions: R282W lies within the LAGGGRRIS loop, and G507R and C566R are clustered in a highly conserved segment that spans the phosphorylation domain of the molecule (19) and are located in proximity of a total of four missense mutations, which have already been reported as HCM-causing variants (16). The V1115I substitution is located in a stretch of highly conserved amino acids in the fibronectin type 3-like motif of module C9, which is responsible for titin binding (20). The position of these mutations in functionally important regions of the protein, the loss or incorporation of charged amino acids leading to conformational changes of the protein and the nonappearance of the mutations in 100 chromosomes of healthy control subjects are strong hints for a disease-causing action of these mutations.
Three splicing variants occur in splice consensus sequences of the MYBPC3 gene. The IVS7+1G>A mutation led to two alternative transcripts. Both aberrant transcripts were due to skipping of either exon 7 alone or exons 7 and 8, which in both cases led to a frame shift and the incorporation of either 26 or 16 aberrant residues before a premature stop codon occurred. The IVS20-2A>G mutation altered the consensus AG of the splice-acceptor site of exon 21 and led to use of a cryptic splice-acceptor site and subsequently to the deletion of 11 base pairs, which introduces a premature stop codon at residue 661. The splice variant IVS27+1G>A led to in-frame skipping of exon 27 and loss of 56 highly conserved amino acid residues that are important for the incorporation of MyBPC into the A band (21).
Eight of our 13 mutations (two splice variants, one nonsense mutation and five small deletions and insertions) led to premature protein truncation and subsequently to the lack of carboxyl-terminal amino acids, which are required for the incorporation of cardiac MyBPC into the A band, titin interaction and myosin binding (20,2224). It is not yet clear whether these defects will result in stable, truncated proteins, which than act as "poison polypeptides" through a dominant negative effect (25), or whether these defects act as null alleles and cause HCM by haplo-insufficiency. Recent studies have published conflicting results: Gilbert et al. (21) and Yang et al. (26) demonstrated the incorporation of stable, truncated proteins encoding truncation mutants of MYBPC3 in transfected skeletal muscle myoblasts (
C910) and in a transgenic mouse model, respectively. These truncated proteins did not correctly incorporate into the sarcomere and were only weakly associated with the rest of the sarcomeric proteins. In contrast, Rottbauer et al. (27) and Moolman et al. (28) were unable to detect the truncated protein in human myocardial biopsies.
Nine of the 13 HCM-causing mutations detected in our study group are novel and thus far represent private mutations, as do most of the HCM-associated mutations. This is in accordance with the fact that mutations associated with significant mortality confer a reproductive disadvantage. Consequently, within populations, recurring identical disease-causing mutations are more likely to have arisen independently, rather than reflecting a common ancestor (29). Two of our mutations (IVS7+1G>A and Q1233X) were identified in two apparently unrelated patients. Haplotype analysis across the MYBPC3 locus in the relevant families revealed that all of the individuals who carried the same mutation also shared either a disease-associated haplotype or a haplotype that was assumed to be disease-associated. The haplotypes were conserved over a region of
0.6 cM. To the best of our knowledge, this report is the first to show the existence of a founder effect as a cause of HCM in a German and Turkish population. Until now, the report by Moolman-Smook et al. (30) was the first to describe strong founder effects as a cause of HCM in a South-African population.
According to our findings, MYBPC3 mutations may cause a broad spectrum of phenotypes. The most severe manifestations of the disease, including life-threatening arrhythmia and the necessity for myectomy or TASH, accumulated in the group with protein truncations. In contrast, the patients and family members with missense mutations and in-frame deletions generally exhibited less pronounced signs and symptoms of the disease. In comparison to 11 beta-MHC mutation carriers, the age of onset, degree of hypertrophy and necessity for ICD, myectomy or TASH were comparable.
Study limitations. We clearly realize that phenotype-genotype relations require large families with identical mutations, which cannot be established in our patient cohort. Therefore, we did not aim to study disease penetrance and limited ourselves to describing the characteristic clinical features of the index patients and available family members.
| Acknowledgments |
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| Footnotes |
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| References |
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