|Home | About | Journals | Submit | Contact Us | Français|
The year 2003 marks the 50th anniversary of the publication of “Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid” by James Watson and Francis Crick.1 In just 50 years, the human genome has been sequenced, along with those of more than 1000 viruses and 100 microbes (see the National Center of Biotechnology Information’s list of completed and ongoing projects2). The stage is set for the identification of the approximately 30 000 genes thought to be present in the human genome. There are as yet only 1300 genes listed in the databases for human diseases (see the Human Gene Mutation Database at the Institute of Medical Genetics in Cardiff3). One would expect that within the next 5 to 10 years, all genes in the human genome will be identified. Additional decades will be needed to determine their roles in human diseases and develop techniques to manipulate them to the benefit of human beings. Nonetheless, the genetic revolution is well underway, and we in the cardiovascular world must prepare for its application and consequences. The 21st century is likely to be earmarked for its achievements in genetic and restorative biology.
Revolutions in science are often nonuniform but kindled by sparks of success. The success in genetics of cardiomyopathies could be such a spark for the cardiovascular world. The first gene for hypertrophic cardiomyopathy (HCM) was identified in 1991,4 and in one decade, more than 10 genes and over 200 mutations have been identified.5,6 The article in the present issue of Circulation by Richard et al7 is a tour de force in terms of attempts to identify these mutations in preparation for clinical application. The authors are to be congratulated for analyzing the entire coding sequence of 9 major causal genes for mutations in a relatively large number of index cases. The results, while largely in accord with existing literature, have significant implications for genetic screening and risk stratifications. Unlike some genetic disorders, such as cystic fibrosis, in which the majority of the cases are caused by a single deletion mutation, no single mutation predominates in HCM, and the frequency of each causal mutation is low. This is largely due to the independent origin of the mutations, which arise de novo and do not share a common ancestor.7–10 The finding of novel mutations in approximately 60% of the cases in the present study, although not surprising, is quite remarkable. It emphasizes the need for the comprehensive screening of the complete sequence of known causal genes in each individual, rather than restricting the screening to known mutations. It is somewhat disappointing to clinicians and patients alike that despite extensive screening, the causal mutations could not be identified in 37% of the cases. As the authors acknowledge, the inability to detect mutations may reflect the imperfection of the screening technique. Richard et al7 used single-stranded conformation polymorphism, which is probably the most widely used and is relatively cheap and efficient. Unfortunately, because single-stranded conformation polymorphism has a sensitivity of approximately 60% to 85%,11 up to 20% to 30% of the causal mutations could have been missed. Thus, one may speculate that a more sensitive screening technique would have detected the causal mutations in up to 80% of the study population. Given our knowledge about the molecular genetics of HCM, it would be considered appropriate to restrict screening to coding and exon-intron boundary sequences, because no causal mutation in 5′ or 3′ regulatory sequences or in intron alone has been identified or would be expected to cause HCM. Mutations in other causal genes appear to be uncommon. One potential notable exception is the gene encoding titin, which is a very large gene and has yet to be systematically screened. Finally, a phenotype of hypertrophy in the absence of an increased external load, which defines HCM, could also occur because of mutations in AMP-activated protein kinase and mitochondrial DNA, triplet repeat syndromes, and metabolic disorders (reviewed in Marian et al12). Collectively, these factors could account for why causal mutations were identified in only 63% of the population in the present study.
The results are also remarkable for the finding of double and compound heterozygosity and homozygosity in 6 families. Although this not a novel finding,9,13,14 it further complicates the feasibility of routine genetic screening. The direct implication of this finding is that it is prudent not to stop screening once a mutation is identified, which makes the task quite tedious. Another notable finding was the observation that 7 of 40 (17%) mutations in the β-myosin heavy chain (MyHC) were in the rod domain of the protein. While this is also not new,15,16 it indicates the need for inclusion of the rod domain in genetic screening. Overall, the results indicate the complexity of genetic screening but also suggest that a tiered screening approach, which gives preference to genes with a high pretest likelihood of carrying a causal mutation, is prudent. This approach will be facilitated with accurate knowledge of the frequency of the causal genes. The true prevalence, however, is unknown and will require large-scale epidemiological studies in the general population without considerations for the presence or absence of the clinical phenotype. Because expression of cardiac hypertrophy, the clinical hallmark of HCM, is age dependent and variable among causal genes, population selection is important. Screening of index cases in whom hypertrophy was established in early life will lead to underestimation of the frequency of mutations that are associated with late onset of hypertrophy. In contrast, screening of older subjects could lead to underestimation of mutations that confer a survival disadvantage. Nonetheless, the present as well as the previous data suggest MYH7 and MYBPC3 genes, encoding the β-MyHC and myosin-binding protein C (MyBP-C), respectively, are the most common causal genes, accounting for at least half of all HCM cases. Thus, the initial approach could focus on screening of the coding regions and intron-exon boundaries of MYH7 and MBPC3 genes, followed by screening of less common genes such as TNNT2 and TNNI3. Nevertheless, the task of routine screening still remains daunting because screening of MYH7, MYBPC3, TNNT2, and TNNI3 alone would require analysis of at least 15 kb of DNA sequence in each individual, which is impractical with existing technology. While this approach would lead to identification of the causal mutations in approximately 70% to 80% of the HCM cases, it is less than the threshold considered desirable for a screening test. The current mutation screening techniques are part of the problem with routine genetic screening; although a variety of techniques are available, none is perfect. A desirable screening test—namely, one that is highly accurate, simple to use, and cost-effective—is yet to be developed for routine application of genetic screening.
There is also significant enthusiasm for the use of genetic screening for risk stratification. Richard et al7 also have attempted to characterize the prognostic significance of the causal mutations and report that one third of β-MyHC mutations were associated with a malignant prognosis, while 90% of the MyBP-C mutations were associated with a benign or intermediary prognosis. The results are in agreement with published data17–21 and suffer from similar limitations as the previous studies—namely, the small size of the families, low frequency of each causal mutation, intraindividual variability of the phenotypic expression within the same family or among families with identical mutations, and the confounding effects of modifier genes and environmental factors. All in all, the results of genotype-phenotype correlation studies suggest mutations in the β-MyHC are expressed at a younger age and are associated with more extensive hypertrophy and a higher incidence of sudden cardiac death (SCD), as compared with those in the MyBP-C, which are associated with late onset and usually a mild phenotype.18,21–23 Mutations in TNNT2 generally exhibit less severe hypertrophy but a relatively high incidence of SCD.24 However, there is a significant degree of variability and no particular clinical phenotype is mutation specific.25 As such, a high incidence of SCD in families with mutations in MyBP-C also has been reported.26 Data in the present study, albeit in a small number of cases, suggest the presence of an additional mutation could affect the severity of the phenotype, as subjects with compound, double, or homozygous mutations appeared to have a more severe phenotype.13,27 Thus, in view of the presence of a significant degree of phenotypic and genotypic variability, a comprehensive approach for risk stratification would include not only the causal genes and mutations but also the modifier genes, as well as clinical and environmental factors.
The spectrum of mutations differs for the two most common genes for HCM, namely MYH7 and MYBPC3. Although the vast majority of mutations in MYH7 are missense, those in MYBPC3 comprise not only missense but also deletion/insertion and splice junction mutations.21,23,28 The reasons for the relative preponderance of the frame-shift and truncation mutations in the MyBP-C are unknown but could reflect differences in the complexity of the structures of these two genes. Alternatively, it may reflect the differences in the functional significance of the encoded proteins, which could lead to ascertainment bias if one includes only those who already have developed hypertrophy. Frame-shift or deletion mutations in the β-MyHC or cTnT could induce an early embryonic death and thus lower frequency in adult population. In contrast, similar mutations in MyBP-C are compatible with survival and are found in adults with HCM.
In considering the genetic screening, one must ask what is to be gained. The most beckoning reason at present is to provide genetic counseling and answer the need to know. Fear often arises not from knowledge but from lack of it. Genetic screening could potentially have significant impact on early identification, risk stratification, and implementation of measures to prevent or attenuate evolving phenotype. Perhaps the impact of genetic (and biochemical) screening in conjunction with counseling is best exemplified in the case of Tay-Sachs disease, an autosomal recessive storage disorder in Ashkenazi Jews, the incidence of which has diminished by 90% to 95% over a period of 20 years.29,30 Genetic screening has not been applied for an early diagnosis of HCM but is expected to confer substantial benefits not only for an early diagnosis but also for prevention of SCD in the young. HCM is the most common cause of SCD in young competitive athletes, and SCD is often the first manifestation.31 In Italy, routine electrocardiographic and echocardiographic screening of young athletes has been implicated in reducing the mortality from HCM during the past 20 years.32 Given the imperfect sensitivity of ECG and echocardiography in early diagnosis of HCM and their inaccuracy in differentiating HCM from the athletes’ hearts, it is likely that genetic screening could prove to have a greater impact. The issue of genetic testing also pertains to affected parents wanting to know who among their children have inherited the mutation and are at risk. Because HCM is an autosomal dominant disorder, only 50% of the offspring will inherit the mutant gene and be at risk for developing the disease. The pressing demand is to identify those who have inherited the causal mutations and should avoid participating in competitive sports. Physicians face the daunting task of providing accurate counseling and are often the subject of a flurry of inquiries about genetic screening whenever a tragic death occurs in a young individual participating in high school, college, or professional sports.
An indirect and yet promising outcome of the genetic studies of cardiomyopathies has been the use of tissue Doppler imaging (TDI) for an early identification of the mutation carriers.33–35 The concept behind the use of TDI originates from our current understanding of the pathogenesis of HCM—namely, the initial defect imparted by the mutant sarcomeric protein.36 The causal mutations exert a diverse array of functional defects that alter myocardial contraction and relaxation. The functional defect provides the impetus for the subsequent development of compensatory cardiac hypertrophy and fibrosis. Thus, TDI might be expected to detect myocardial contraction and relaxation abnormalities prior to and independent of cardiac hypertrophy, a hypothesis that was initially shown in the β-MyHC-Q403 transgenic rabbits. 34 Subsequently, the results were confirmed in humans with HCM mutations who had not yet developed cardiac hypertrophy33,35 and validated by a follow-up study showing subsequent evolution of cardiac hypertrophy in those with reduced myocardial Doppler velocity (S. Nagueh, unpublished data, 2003). Thus, TDI provides an early noninvasive tool to identify mutation carriers independent of and prior to development of cardiac hypertrophy. This technique could complement the results of genetic screening studies.
Despite the demand for an early genetic diagnosis, the skeptics have questioned the clinical utility of genetic screening in the absence of specific therapies to prevent the evolving phenotypes. The existing technology does not permit correction of the underlying genetic defect. The current use of defibrillators is primarily limited to those with an established high-risk phenotype, such as those with a history of recurrent syncope or aborted SCD or with a strong family history of SCD. The prophylactic use of current pharmacological interventions in asymptomatic individuals with HCM or in those with causal mutations is not justified because none of the existing therapies has been proven to prevent SCD or induce regression of cardiac hypertrophy, fibrosis, or disarray, which are major predictors of mortality and morbidity. 37,38 Studies in genetically induced animal models of human HCM offer promising results and, for the first time, show reversibility of the evolving phenotypes in HCM.39,40 Administration of simvastatin, a pleiotropic HMG-CoA reductase inhibitor, induced regression of cardiac hypertrophy and fibrosis and improved left ventricular filling pressure in the β-MyHC-Q403 transgenic rabbit model of human HCM.40 Blockade of angiotensin II receptor 1 reduced the interstitial fibrosis in the cardiac troponin T-Q92 transgenic mice to normal levels.39 Experiments in other animal models of hypertrophy have shown the potential utility of HMG-CoA reductase inhibitors in prevention of cardiac hypertrophy and fibrosis.41 Collectively, the results in genetic animal models raise the possibility of early pharmacological interventions in humans to prevent the evolving cardiac phenotype. Clinical studies in humans with HCM will be needed to establish the utility of pharmacological interventions to prevent, attenuate, or induce regression of the cardiac phenotype. The availability of TDI to detect early manifestations of HCM may provide a necessary tool to screen individuals with the causal mutations for an early intervention.
Richard et al7 provide the first large-scale systematic screening of 9 genes for causal mutations for HCM. The results have significant implications for implementing routine genetic screening and illustrate the complexity we face in designing screening tests, interpreting the results, providing counseling, and implementing preventive measures. The time has arrived to bring the remarkable advances in the molecular genetics of HCM, achieved during such a short period of time, to clinical fruition. The technology is already available for high-throughout genetic screening but needs to be tailored to address the specific issues related to HCM. Experimental data in genetic animal models of HCM in conjunction with application of TDI in detection of early phenotypic expression of HCM beckon the need for clinical studies in humans with the hope of preventing the evolving phenotype. Mean-while, the scientific society and the practicing physicians have an obligation to educate not only themselves but also the public on the potential benefits and pitfalls of genetic screening and the application of preventive measures based on an early genetic diagnosis.
Supported by grants from the National Heart, Lung, and Blood Institute, Specialized Centers of Research (P50-HL42267) and R01-HL68884.
Reprints: Information about reprints can be found online at http://www.lww.com/reprints
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.