Ali J. Marian

IDENTIFICATION OF A GENETIC LOCUS FOR FAMILIAL ATRIAL FIBRILLATION

BACKGROUND Atrial fibrillation, the most common sustained cardiac-rhythm disturbance, affects over 2 million Americans and accounts for one third of all strokes in patients over 65 years of age. The molecular basis for atrial fibrillation is unknown, and palliative therapy is used to control the ventricular rate and prevent systemic emboli. We identified a family of 26 members of whom 10 had atrial fibrillation which segregated as an autosomal dominant disease. We subsequently identified two additional families in which the disease was linked to the same locus. METHODS We screened the human genome with 300 polymorphic dinucleotide-repeat markers using an unconventional strategy of pooling the DNA samples into two groups (affected and unaffected), which reduced the sample size by approximately 90 percent, before performing linkage analysis to map the locus. This made it possible to identify potential loci within a few weeks. RESULTS The lod scores for markers D10S569 and D10S607, located at 10q22-q24, were 3.60 in Family 1. The disease locus in Families 2 and 3 was also linked to the same markers, with lod scores of 6.02 and 5.35 for markers D10S569 and D10S607, respectively, when data on all three families were combined. Haplotype analysis of the three families showed that the locus was between D10S1694 and D10S1786, an interval of 11.3 centimorgans. CONCLUSIONS Identification of the gene for familial atrial fibrillation will help to elucidate the molecular basis of the disease and provide insights into acquired forms. The strategy of pooling DNA samples for analysis is more time and cost effective than conventional screening and should accelerate the process of gene mapping in the future.

Tissue Doppler Imaging Consistently Detects Myocardial Abnormalities in Patients With Hypertrophic Cardiomyopathy and Provides a Novel Means for an Early Diagnosis Before and Independently of Hypertrophy

Background—Left ventricular hypertrophy (LVH), the clinical hallmark of familial hypertrophic cardiomyopathy (FHCM), is absent in a significant number of subjects with causal mutations. In transgenic rabbits that fully recapitulate the FHCM phenotype, reduced myocardial tissue Doppler (TD) velocities accurately identified the mutant rabbits, even in the absence of LVH. We tested whether humans with FHCM also consistently showed reduced myocardial TD velocities, irrespective of LVH. Methods and Results—We performed 2D and Doppler echocardiography and TD imaging in 30 subjects with FHCM, 13 subjects who were positive for various mutations but did not have LVH, and 30 age- and sex-matched controls (all adults; 77% women). LV wall thickness and mass were significantly greater in FHCM subjects (P <0.01 versus those without LVH and controls). There were no significant differences in 2D echocardiographic, mitral, and pulmonary venous flow indices between mutation-positives without LVH and controls. In contrast, systolic and early diastolic TD velocities were significantly lower in both mutation-positives without LVH and in FHCM patients than in controls (P <0.001). Reduced TD velocities had a sensitivity of 100% and a specificity of 93% for identifying mutation-positives without LVH. Conclusions—Myocardial contraction and relaxation velocities, detected by TD imaging, are reduced in FHCM, including in those without LVH. Before and independently of LVH, TD imaging is an accurate and sensitive method for identifying subjects who are positive for FHCM mutations.

Suppression of canonical Wnt/β-catenin signaling by nuclear plakoglobin recapitulates phenotype of arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVC) is a genetic disease caused by mutations in desmosomal proteins. The phenotypic hallmark of ARVC is fibroadipocytic replacement of cardiac myocytes, which is a unique phenotype with a yet-to-be-defined molecular mechanism. We established atrial myocyte cell lines expressing siRNA against desmoplakin (DP), responsible for human ARVC. We show suppression of DP expression leads to nuclear localization of the desmosomal protein plakoglobin and a 2-fold reduction in canonical Wnt/beta-catenin signaling through Tcf/Lef1 transcription factors. The ensuing phenotype is increased expression of adipogenic and fibrogenic genes and accumulation of fat droplets. We further show that cardiac-restricted deletion of Dsp, encoding DP, impairs cardiac morphogenesis and leads to high embryonic lethality in the homozygous state. Heterozygous DP-deficient mice exhibited excess adipocytes and fibrosis in the myocardium, increased myocyte apoptosis, cardiac dysfunction, and ventricular arrhythmias, thus recapitulating the phenotype of human ARVC. We believe our results provide for a novel molecular mechanism for the pathogenesis of ARVC and establish cardiac-restricted DP-deficient mice as a model for human ARVC. These findings could provide for the opportunity to identify new diagnostic markers and therapeutic targets in patients with ARVC.

Simvastatin Induces Regression of Cardiac Hypertrophy and Fibrosis and Improves Cardiac Function in a Transgenic Rabbit Model of Human Hypertrophic Cardiomyopathy

Background—Hypertrophic cardiomyopathy is a genetic disease characterized by cardiac hypertrophy, myocyte disarray, interstitial fibrosis, and left ventricular (LV) dysfunction. We have proposed that hypertrophy and fibrosis, the major determinants of mortality and morbidity, are potentially reversible. We tested this hypothesis in &bgr;-myosin heavy chain–Q403 transgenic rabbits. Methods and Results—We randomized 24 &bgr;-myosin heavy chain–Q403 rabbits to treatment with either a placebo or simvastatin (5 mg · kg−1 · d−1) for 12 weeks and included 12 nontransgenic controls. We performed 2D and Doppler echocardiography and tissue Doppler imaging before and after treatment. Demographic data were similar among the groups. Baseline mean LV mass and interventricular septal thickness in nontransgenic, placebo, and simvastatin groups were 3.9±0.7, 6.2±2.0, and 7.5±2.1 g (P <0.001) and 2.2±0.2, 3.1±0.5, and 3.3±0.5 mm (P =0.002), respectively. Simvastatin reduced LV mass by 37%, interventricular septal thickness by 21%, and posterior wall thickness by 13%. Doppler indices of LV filling pressure were improved. Collagen volume fraction was reduced by 44% (P <0.001). Disarray was unchanged. Levels of activated extracellular signal-regulated kinase (ERK) 1/2 were increased in the placebo group and were less than normal in the simvastatin group. Levels of activated and total p38, Jun N-terminal kinase, p70S6 kinase, Ras, Rac, and RhoA and the membrane association of Ras, RhoA, and Rac1 were unchanged. Conclusions—Simvastatin induced the regression of hypertrophy and fibrosis, improved cardiac function, and reduced ERK1/2 activity in the &bgr;-myosin heavy chain–Q403 rabbits. These findings highlight the need for clinical trials to determine the effects of simvastatin on cardiac hypertrophy, fibrosis, and dysfunction in humans with hypertrophic cardiomyopathy and heart failure.

A variant of human paraoxonase/arylesterase (HUMPONA) gene is a risk factor for coronary artery disease

Coronary artery disease (CAD) is a complex trait caused by a number of genetic and environmental factors. Recently, paraoxonase/arylesterase (PONA) enzyme has been implicated in the pathogenesis of atherosclerosis. There is a 10-40-fold variability in the activity of this enzyme among individuals. This variability is due to the presence of an A/G polymorphism in the coding region of the gene (HUMPONA). The A and G alleles code for glutamine (A genotype) and arginine (B genotype), respectively. Individuals with A genotype have a lower enzymatic activity than those with B genotype. We determined the HUMPONA genotypes and alleles in 223 patients with angiographically documented CAD and in 247 individuals in the general population. The distribution of genotypes were in Hardy-Weinberg equilibrium in patients and in controls. Genotypes A and B were present in 120 (49%) and 28 (11%) individuals in controls and in 68 (30%) and 40 (18%) patients with CAD, respectively (chi squared= 16.5, P= 0.0003). The frequency of the A allele was 0.69 in controls and 0.56 in patients (OR= 1.7, P= 0.0001). There were no differences in the distribution of HUMPONA genotypes in the subgroups of patients with restenosis, myocardial infarction, or any of the conventional risk factors for CAD as compared with corresponding subgroups. In summary, variants of the HUMPONA gene are involved in predisposition to coronary atherosclerosis.

Mutations in Smooth Muscle Alpha-Actin (ACTA2) Cause Coronary Artery Disease, Stroke, and Moyamoya Disease, Along with Thoracic Aortic Disease

The vascular smooth muscle cell (SMC)-specific isoform of alpha-actin (ACTA2) is a major component of the contractile apparatus in SMCs located throughout the arterial system. Heterozygous ACTA2 mutations cause familial thoracic aortic aneurysms and dissections (TAAD), but only half of mutation carriers have aortic disease. Linkage analysis and association studies of individuals in 20 families with ACTA2 mutations indicate that mutation carriers can have a diversity of vascular diseases, including premature onset of coronary artery disease (CAD) and premature ischemic strokes (including Moyamoya disease [MMD]), as well as previously defined TAAD. Sequencing of DNA from patients with nonfamilial TAAD and from premature-onset CAD patients independently identified ACTA2 mutations in these patients and premature onset strokes in family members with ACTA2 mutations. Vascular pathology and analysis of explanted SMCs and myofibroblasts from patients harboring ACTA2 suggested that increased proliferation of SMCs contributed to occlusive diseases. These results indicate that heterozygous ACTA2 mutations predispose patients to a variety of diffuse and diverse vascular diseases, including TAAD, premature CAD, ischemic strokes, and MMD. These data demonstrate that diffuse vascular diseases resulting from either occluded or enlarged arteries can be caused by mutations in a single gene and have direct implications for clinical management and research on familial vascular diseases.

Angiotensin II Blockade Reverses Myocardial Fibrosis in a Transgenic Mouse Model of Human Hypertrophic Cardiomyopathy

Background —Hypertrophic cardiomyopathy (HCM), the most common cause of sudden cardiac death in the young, is characterized by cardiac hypertrophy, myocyte disarray, and interstitial fibrosis. We propose that hypertrophy and fibrosis are secondary to the activation of trophic and mitotic factors and, thus, potentially reversible. We determined whether the blockade of angiotensin II, a known cardiotrophic factor, could reverse or attenuate interstitial fibrosis in a transgenic mouse model of human HCM. Methods and Results —We randomized 24 adult cardiac troponin T (cTnT-Q92) mice, which exhibit myocyte disarray and interstitial fibrosis, to treatment with losartan or placebo and included 12 nontransgenic mice as controls. The mean dose of losartan and the mean duration of therapy were 14.2±5.3 mg · kg–1 · d–1 and 42±9.6 days, respectively. Mean age, number of males and females, and heart/body weight ratio were similar in the groups. Collagen volume fraction and extent of myocyte disarray were increased in the cTnT-Q92 mice (placebo group) compared with nontransgenic mice (9.9±6.8% versus 4.5±2.2%, P =0.01, and 27.6±10.6% versus 3.9±2.3%, P <0.001, respectively). Treatment with losartan reduced collagen volume fraction by 49% to 4.9±2.9%. The expression of collagen 1&agr; (I) and transforming growth factor-&bgr;1, a mediator of angiotensin II profibrotic effect, were also reduced by 50%. Losartan had no effect on myocyte disarray. Conclusions —Treatment with losartan reversed interstitial fibrosis and the expression of collagen 1&agr; (I) and transforming growth factor-&bgr;1 in the hearts of cTnT-Q92 mice. These findings suggest that losartan has the potential to reverse or attenuate interstitial fibrosis, a major predictor of sudden cardiac death, in human patients with HCM.

Tissue Doppler Imaging Predicts the Development of Hypertrophic Cardiomyopathy in Subjects With Subclinical Disease

Background—Systolic (Sa) and diastolic (Ea) myocardial velocities measured by tissue Doppler (TD) imaging (TDI) recently were shown to be decreased in subjects who have mutations causing hypertrophic cardiomyopathy (HCM) but who do not have left ventricular (LV) hypertrophy. By studying these subjects at a later date, we sought to determine whether TDI predicts the subsequent evolution of the HCM phenotype. Methods and Results—Serial 2D and Doppler echocardiography were performed in 12 subjects (age range, 17 to 51 years) with HCM-causing mutations on 2 occasions: before development of hypertrophy and 2 years later. Twelve age- and gender-matched family members without mutations were included as control subjects. In those with mutations, mean septal thickness and LV mass were 1.07±0.14 cm and 103.0±11 g at baseline, respectively, and increased to 1.30±0.36 cm and 193.0±78 g at follow-up (P <0.01), with 6 subjects satisfying HCM diagnostic criteria. Sa and Ea velocities in those with mutations were lower compared with control subjects at baseline and follow-up (lateral Sa, 15±1.2 versus 8.2±2.1 cm/s; lateral Ea, 16.5±2.8 versus 8.1±2.3 cm/s; P <0.01). At 2 years, left atrial volume and pulmonary venous flow indices of LV filling pressures increased, whereas TD early and late diastolic velocities decreased (all P <0.05) in those with the mutations. Control subjects had no significant interval changes of the above parameters. Conclusions—Subsequent development of HCM in subjects with initially reduced TD velocities establishes TDI as a reliable method for early identification of HCM mutation carriers.

Endothelial lipase is a major genetic determinant for high-density lipoprotein concentration, structure, and metabolism

High-density lipoprotein (HDL) protects against atherosclerosis. Endothelial lipase (EL) has been postulated to be involved in lipoprotein, and possibly HDL, metabolism, yet the evidence has been scarce and conflicting. We have inactivated EL in mice by gene targeting. EL−/− mice have elevated plasma and HDL cholesterol, and increased apolipoproteins A-I and E. NMR analysis reveals an abundance of large HDL particles. There is down-regulation of the transcripts for phospholipid transfer protein, but up-regulation of those for hepatic lipase and lipoprotein lipase. Plasma lecithin:cholesterol acyltransferase is unchanged despite an increase in hepatic mRNA; lecithin:cholesterol acyltransferase activity toward endogenous EL−/− substrate is, however, reduced by 50%. HDL clearance is decreased in EL−/− mice; both the structure of HDL and the presence of EL are factors that determine the rate of clearance. To determine ELs role in humans, we find a significant association between a single-nucleotide polymorphism 584C/T in the EL (LIPG) gene and HDL cholesterol in a well characterized population of 372 individuals. We conclude that EL is a major determinant of HDL concentration, structure, and metabolism in mice, and a major determinant of HDL concentration in humans.

Recent advances in the molecular genetics of hypertrophic cardiomyopathy

The recent evolution of molecular genetic techniques and their application in deciphering the molecular genetic basis of inherited diseases have facilitated the dawning of molecular medicine. A complete genetic map of the human genome, based on easily identifiable highly polymorphic DNA markers, has been developed.1 2 3 This achievement is a milestone in that it provides the foundation for genetic linkage analysis, a technique essential to the mapping of the chromosomal locus responsible for a disease. Before the development of multiple informative markers, identification of a disease-related gene required a priori knowledge of the defective protein, which was known for only a few diseases. The previous approach of “from a defective protein to a defective gene” has been replaced with the approach of “from a defective gene to a defective protein.” The recent availability of the highly informative STRP markers compared with previous markers based on RFLP has greatly accelerated chromosomal mapping by linkage analysis.4 Furthermore, the RFLP markers detected by Southern blotting required 5 to 7 days, whereas STRP markers are detected by PCR and require as few as 1 to 2 days.4 5 The loci for more than 400 disease-related genes have been mapped, and the responsible genes have been identified for more than 40 of these diseases.1 Theoretically, it is possible to map the chromosomal locus of any disease-related gene if a family with 10 or more living affected individuals spanning two or more generations is available.4 5 Subsequent to the chromosomal mapping of the locus by genetic linkage analysis, several techniques, such as positional cloning, are used to identify the responsible gene.6 7 Positional cloning refers to cloning of a segment of DNA with only its chromosomal position in relation to a marker known. This process of identifying the genes, …