Short Communication: The Cardiac Myosin Binding Protein C Arg502Trp Mutation
A Common Cause of Hypertrophic Cardiomyopathy
Rationale: The myosin-binding protein C isoform 3 (MYBPC3) variant Arg502Trp has been identified in multiple hypertrophic cardiomyopathy (HCM) cases, but compelling evidence to support or refute the pathogenicity of this variant is lacking.
Objective: To determine the prevalence, origin and clinical significance of the MYBPC3 Arg502Trp variant.
Methods and Results: The prevalence of MYBPC3 Arg502Trp was ascertained in 1414 sequential HCM patients of primarily European descent. MYBPC3 Arg502Trp was identified in 34 of these 1414 unrelated HCM patients. Segregation of MYBPC3 Arg502Trp with clinical status was assessed in family members. Disease haplotypes were examined in 17 families using two loci flanking MYBPC3. Family studies identified an additional 43 variant carriers, many with manifest disease, yielding a calculated odds ratio of 11 000:1 for segregation of MYBPC3 Arg502Trp with HCM. Analyses in 17 families showed at least 4 independent haplotypes flanked MYBPC3 Arg502Trp. Eight individuals (4 probands and 4 family members) also had another sarcomere protein gene mutation. Major adverse clinical events occurred in approximately 30% of MYBPC3 Arg502Trp carriers by age 50; these were significantly more likely (P<0.0001) when another sarcomere mutation was present.
Conclusions: MYBPC3 Arg502Trp is the most common and recurrent pathogenic mutation in a diverse primarily European descent HCM cohort, occurring in 2.4% of patients. MYBPC3 Arg502Trp conveys a 340-fold increased risk for HCM by 45 years of age, when more than 50% of carriers have overt disease. HCM prognosis worsens when MYBPC3 Arg502Trp occurs in the setting of another sarcomere protein gene mutation.
Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disorder characterized by unexplained LVH in a nondilated ventricle. HCM displays marked variability in penetrance and expressivity. Some patients exhibit massive hypertrophy with heart failure, arrhythmias, and sudden cardiac death, whereas others have mild hypertrophy, late onset disease, and minimal symptoms.1–3 To date, more than 800 different mutations in 10 sarcomere protein genes responsible for HCM have been identified.4,5 The prevalence of sarcomere protein gene mutations is estimated to be 1 in 300, making HCM a very common monogenic cardiovascular disorder.6 Disease-causing genotype influences clinical course,7 as is illustrated by the particularly severe manifestations of HCM in individuals with compound mutations.1,8 Despite recognized correlations between clinical phenotypes and HCM mutations (eg, delayed expression of myosin-binding protein C isoform 3 [MYBPC3] mutations and early-onset severe disease of specific MYH7 mutations),7,9 defining clinical consequences of most mutations has been thwarted by the large number of unique pathogenic mutations and the intrinsic genetic and lifestyle diversity even among related patients who share an identical HCM mutation.
A variant (1504C>T) in exon 17 of MYBPC3 encoding Arg502Trp has been repeatedly recognized,1,8,10–12 but its pathogenicity in HCM remains untested. Here, we demonstrate that MYBPC3 Arg502Trp is a pathogenic missense mutation and the most common cause of HCM in a diverse primarily European descent population. We report the clinical consequences of this mutation alone and in combination with other HCM mutations.
Standard methods for DNA isolation and analyses were used and are described in the expanded Methods section, available in the Online Data Supplement at http://circres.ahajournals.org.
Thirty-four of 1414 (2.4%) consecutive unrelated HCM probands who underwent genetic testing (Supplement Methods), harbored the MYBPC3 Arg502Trp variant. MYBPC3 residue 502 has been highly conserved during evolution (Online Figure I). Insertion of tryptophan at position 502 substitutes a nonpolar hydrophobic residue for the polar uncharged residue arginine.
MYBPC3 Arg502Trp cosegregated with HCM in families harboring only this sarcomere protein gene variant (Online Figure II). MYBPC3 Arg502Trp was absent in 545 healthy control subjects (n=95, this study; n=1501; n=100 white Americans and n=100 African Americans8; n=10011). The combined calculated LOD score for HCM in MYBPC3 Arg502Trp families=4.04 (θ=0, assuming penetrance=85% in subjects ≥20 years of age, and allele frequency=0.001); the likelihood that MYBPC3 Arg502Trp and HCM cosegregated by random chance was less than 1/11 000. We conclude that MYBPC3 Arg502Trp is a pathogenic HCM mutation.
Haplotype analyses were performed in 17 of the 34 MYBPC3 Arg502Trp families (Online Methods and Online Figure II) to determine whether a founding mutation in a common ancestor accounted for its high prevalence, or if MYBPC3 Arg502Trp arose repeatedly in unrelated HCM families. Haplotypes were assessed using short tandem repeat sequences that flank MYBPC3, 102 kb downstream and 188 kb upstream. Four unique haplotypes were identified (Table) with the most frequent haplotype, designated 4,5, seen in 13 families.
Forty-three of 56 at-risk relatives in 34 families carried the MYBPC3 Arg502Trp mutation. Clinical information was available for 58 of 77 subjects (probands and family members) with the MYBPC3 Arg502Trp mutation (Online Table I). Forty-five of 58 mutation carriers (78%) had clinical evidence of HCM, including hypertrophy, sudden cardiac death, or implanted cardioverter defibrillators (ICDs).
The age of HCM diagnosis for Arg502Trp carriers relative to MYBPC3 truncation mutations and to MYH7 Arg719Trp, a mutation that causes severe HCM, carriers was compared (Figure, A). There was no significant difference in ages at diagnosis for 48 subjects who carried only the MYBPC3 Arg502Trp mutation versus carriers of MYBPC3 truncation mutations (P=0.5). However, the age at diagnosis of HCM in carriers of MYH7 Arg719Trp was significantly earlier than for carriers of MYBPC3 Arg502Trp (P<0.0001). Whereas overt disease was recognized in nearly all carriers of MYH7 Arg719Trp by age 30, ≈50% of MYBPC3 Arg502Trp carriers lacked clinical evidence of HCM until after age 45 (Figure, A).
Eight HCM subjects harbored both MYBPC3 Arg502Trp and another sarcomere protein gene mutation (Online Table III; Online Figure II, B). Based on patient age at the time of a significant adverse event (eg, age at sudden cardiac death, ICD implantation, myectomy, or cardiac transplantation), the clinical course of subjects with compound MYBPC3 Arg502Trp mutations was similar to subjects with the severe MYH7 Arg719Trp mutation and was significantly worse (P≤0.0001) than the clinical course of subjects with an isolated MYBPC3 Arg502Trp mutation (Figure, B). Approximately 30% of HCM patients with an isolated MYBPC3 Arg502Trp mutation had a significant cardiac event by age 50, with 10% of these occurring before age 20. In comparison, 75% of carriers of MYBPC3 Arg502Trp and another sarcomere protein gene mutation experienced an adverse cardiac event by age 20.
We demonstrate that MYBPC3 Arg502Trp is a prevalent pathogenic mutation occurring in 2.4% of 1414 unrelated primarily European descent patients. Haplotype data indicate that this incidence is accounted for by recurrent mutation of this residue rather than a founding MYBPC3 Arg502Trp mutation. Clinical manifestations in mutation carriers indicate that in isolation, MYBPC3 Arg502Trp causes relatively mild HCM, with expression that can be delayed until middle age. However, when MYBPC3 Arg502Trp is combined with another sarcomere protein gene mutation, HCM is severe with adverse clinical outcomes occurring at young ages. Recognition of this prognostic information is particularly important given the prevalence of MYBPC3 Arg502Trp.
A few founding HCM mutations have been defined, and all occur in the MYBPC3 gene. The population prevalence of founding MYBPC3 mutations that have been identified in Europe (2373_2374insG), India (25-bp deletion in intron 32), and Finland (Gln1061X)11,13,14 are attributed to neutral evolutionary selection. Although analyses of the MYBPC3 Arg502Trp mutation revealed one prevalent haplotype (4,5; Table) in 75% of mutation carriers, the 4,5 haplotype was also common throughout this study cohort (data not shown). As such, we could not determine whether 1 founding mutation accounted for disease in multiple families or whether multiple independent mutations arose on this common haplotype. Regardless, because MYBPC3 Arg502Trp was also identified on 3 other haplotypes, our data are consistent with independent mutations that occurred at least four times. Further evidence that MYBPC3 codon 502 is a mutational hotspot comes from two other HCM mutations that alter arginine502 to either glutamine (Arg502Gln)7 or to leucine (Arg502Leu) (H Rehm, unpublished data, 2008). The cause for multiple mutational events at residue 502 is likely a CG dinucleotide, which has been associated with other mutational hotspots.15
Some founding mutations provide a selective advantage, such as the hemoglobin mutation Glu6Val, which causes sickle cell disease but also conveys resistance to malarial infection,16 therein accounting for its prevalence in the general population. Unlike these mutations, MYBPC3 Arg502Trp has only been associated with adverse cardiac events (Figure, B) that can reduce life expectancy. Moreover, MYBPC3 Arg502Trp has a low prevalence in the general population (estimated at 1/20 000 individuals, based on a 2.4% frequency in this HCM cohort, and the prevalence of HCM). Taken together with haplotype data, we suggest that the deleterious consequences of MYBPC3 Arg502Trp result in negative selection, but new mutation increases its prevalence among HCM patients. A prediction of this model is that MYBPC3 Arg502Trp will also be a relatively common cause of HCM in other populations.
The clinical course of HCM observed in more than 80 patients with MYBPC3 Arg502Trp reported here and elsewhere1,8,10,11 appears comparable to disease caused by MYBPC3 truncation mutations and carries a better prognosis than that associated with several MYH7 mutations (eg, MYH7 Arg719Trp; Figure A). We note, however, that even this milder spectrum of HCM has important impact on health, as 30% of MYBPC3 Arg502Trp carriers had an adverse cardiac event by age 50 (Figure B).
The high prevalence (2.4%) of MYBPC3 Arg502Trp mutation contributed to finding compound mutations in four families. In comparison to patients with only MYBPC3 Arg502Trp, patients with a second sarcomere protein gene mutation had more severe disease, and approximately 75% had an ICD, sudden cardiac death, or a myectomy before 20 years of age. Although the clinical consequences associated with these particular sarcomere mutations (MYBPC3 Ser858Asn, MYBPC3 Glu542Gln, MYBPC3 Gly148Arg, or MYH7 Asp587Asn) are unknown, they are more likely to cause typical rather than severe HCM. Rather, we suggest that a high dosage of mutant protein, due to 2 mutations, accounts for disease severity, as has been previously observed with other compound mutations.1 Consistent with this model, we note that segregation analyses in families F001849 and HCM-345 indicated that the 2 MYBPC3 mutations occurred in trans, which predicts that no normal MYBPC3 protein would be found in the hearts of these patients.
In conclusion, MYBPC3 Arg502Trp is a common pathogenic mutation that increases the risk for HCM by 340-fold. Identification of MYBPC3 Arg502Trp as an isolated HCM mutation or in combination with another sarcomere gene mutation provides prognostic information that should guide patient management and counseling.
We thank the families and physicians for their generous contribution of clinical information.
Sources of Funding
Supported by NIH and LeDuc Foundation grants (to J.G.S. and C.E.S.); the Children’s Cardiomyopathy Foundation (to W.K.C.); and the Howard Hughes Medical Institute (to C.E.S.).
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Novelty and Significance
What Is Known?
More than 800 different sarcomere protein gene mutations, of which >200 are in the myosin-binding protein C isoform 3 (MYBPC3) gene, cause hypertrophic cardiomyopathy (HCM).
Some sarcomere protein gene mutations cause worse disease than others.
What New Information Does This Article Contribute?
The MYBPC3 Arg502Trp mutation is the most common HCM-causing mutation among individuals of European descent from several North American clinics and has a penetrance of ≈50% (increased relative risk of 340) by age 45.
The high frequency of this mutation reflects a high mutation frequency rather than a founder effect.
Mutations in genes encoding components of the heart’s contractile apparatus cause the inherited human disease HCM, most commonly recognized as the cause of 50% of deaths that occur on the athletic field. Whereas some HCM-causing mutations cause severe disease, other mutations in the same genes cause milder disease. The MYBPC3 Arg502Trp mutation, the most common HCM-causing mutation found in a North American population, causes milder disease than many other HCM mutations. However 30% of mutation carriers have a serious event by age 40. The high frequency of mutation carriers reflects new mutations rather than a founder mutation.
This manuscripts was sent to Ali J. Marian, Consulting Editor, for review by expert referees, editorial decision, and final disposition.
Original received January 7, 2010; revision received March 24, 2010; accepted March 29, 2010.