Genetic Variation associated with Ischemic Heart Failure: A HuGE Review and Meta-Analysis
by Georgios Kitsios¹ and Elias Zintzaras^1,2

¹ Department of Biomathematics, University of Thessaly School of Medicine, Larissa, Greece
² Center for Clinical Evidence Synthesis, Institute for Clinical Research and Health Policy Studies, Department of Medicine, Tufts-New England Medical Center, Tufts University School of Medicine, Boston, MA

Correspondence to Prof. Elias Zintzaras, Center for Clinical Evidence Synthesis, Institute for Clinical Research and Health Policy Studies, Tufts-New England Medical Center, 750 Washington Street, Tufts-NEMC #63, Boston, MA 02111 (e-mail: ezintzaras@tufts-nemc.org ).

Received for publication January 24, 2007. Accepted for publication March 21, 2007.

The ischemic etiology of heart failure is an independent prognostic factor associated with worse long-term outcome. Recent evidence indicates a role for genetic susceptibility to ischemic heart failure. The authors systematically reviewed all known case-control studies that investigated the association between genetic variants and ischemic heart failure. Twenty-two articles, which examined 24 gene polymorphisms, were identified. In 22 polymorphisms, the variant form had a functional effect. Twenty-two polymorphisms were variants of genes involved in the maladaptive neurohormonal activation. Seven polymorphisms (ACE I/D, AGT M235T, ADRA2C Del322-325, ADRB2 Arg16Gly, ADRB2 Gln27Glu, EDN1 Lys198Asn, VEGF G-405C) showed a significant association in individual studies. Five polymorphisms (ACE I/D, ADRB1 Arg389Gly, ADRB2 Arg16Gly, ADRB2 Gln27Glu, TNF G308A) were examined by more than one study, and meta-analyses were performed. The meta-analyses showed no significant sign of heterogeneity. In all settings, there was no significant association, except for polymorphism ADRB2 Arg16Gly under a recessive model (fixed-effects odds ratio = 1.32, 95% confidence interval: 1.05, 1.65). Taking into account that ischemic heart failure is a complex disease with multifactorial etiology, a minor contributing pathogenetic role of the investigated gene polymorphisms cannot be totally excluded. Case-control studies that investigate gene-gene and gene-environment interactions might further elucidate the genetics of ischemic heart failure.

Indexing terms: epidemiology; heart failure, congestive; meta-analysis; myocardial ischemia; polymorphism, genetic; variation (genetics)

Abbreviations: CI, confidence interval; HWE, Hardy-Weinberg equilibrium; IHF, ischemic heart failure; SNP, single nucleotide polymorphism

Heart failure is a complex clinical syndrome that can result from any structural or functional cardiac disorder that impairs the ability of the ventricle to fill with or eject blood (1). Heart failure is a relatively common disorder, and the diagnosis is clinical (2). Patients with heart failure are classified broadly into two groups on the basis of the etiology of the left ventricular dysfunction: patients with ischemic (40–74 percent) and nonischemic (26–35 percent) heart disease (3–5). Ischemic etiology has been shown to be independently associated with worse long-term outcome in heart failure patients in a variety of studies (6, 7). Clinically, patients are classified as having heart failure of ischemic etiology on the basis of a history of myocardial infarction or on objective evidence of coronary artery disease such as angiography or functional testing, although a more standardized definition for ischemic heart failure (IHF) has been proposed for use in research (8).

Although significant progress has been made in elucidating the genetics of coronary artery disease/myocardial infarction with a large number of family-based (whole-genome scans) and association studies (9–11), the evidence for a genetic basis of IHF susceptibility is limited (12). Nevertheless, a genetic basis could be indicated by the ethnic diversity in disease prevalence (13), the interindividual variability in IHF susceptibility (14, 15), the familial clustering of heart failure (16), and the experimental data from animal models (17, 18).

The genetic association studies of IHF under the "candidate gene" approach have produced inconclusive or inconsistent results so far. To address this issue, we reviewed the literature for genetic studies investigating the association of genetic variation with the risk of developing clinically evident IHF.

Selection of studies
Literature for this review was systematically identified by searching PubMed (National Library of Medicine, Bethesda, Maryland) for all English-language articles published up to November 2006 related to IHF and genetic polymorphisms. As search criteria, we used combinations of the following terms: "ischemic heart failure," "IHF," "ischemic cardiomyopathy," "heart failure," "cardiac failure," "cardiomyopathy," "polymorphism," "gene variant," "genetic variant," "susceptibility," and "genetic association study." Bibliographies in articles provided further references.

Our review included genetic association studies fulfilling the following inclusion criteria: 1) providing cases with clinically diagnosed IHF and controls free of heart failure, 2) providing information on genotype frequency or risk estimates, 3) using validated molecular methods for genotyping, and 4) including subjects who were human. Studies investigating progression, severity, phenotype modification, response to treatment, or survival were excluded from our study. Case reports, editorials, and review articles were also excluded. Finally, family-based studies were excluded because of different design settings (19).

Data extraction
From each study, the following information was extracted: first author, journal, year of publication, ethnicity of the study population, demographics, definition of cases and controls, matching, blinded genotyping, validity of the genotyping method, and number of cases and controls for each genotype. The frequencies of the alleles and the genotypic distributions were extracted or calculated for both the cases and the controls. The two investigators independently extracted data, discussed all disagreements, and reached consensus on all items.

Data synthesis
In this review, the associations are indicated as odds ratios with the corresponding 95 percent confidence intervals. When more than one genetic association study investigated the same polymorphism, a meta-analysis of published results was carried out. The meta-analysis examined the overall association of the allele contrast and the recessive and dominant models for the allele of interest. For a polymorphism with two alleles (A and a), the allele contrast is defined as *a vs. *A, the recessive model for allele a is defined as aa vs. Aa + AA, and the dominant model is defined as aa + Aa vs. AA (20, 21). In the meta-analysis, then, pooled odds ratios were estimated based on the individual odds ratios produced by the individual studies. Heterogeneity between studies was tested by using the Q statistic, a weighted sum of squares of the deviations of individual study odds ratio estimates from the overall (pooled) estimate (22, 23). If p < 0.10, then heterogeneity was considered statistically significant. Heterogeneity was quantified with the I² metric, which is independent of the number of studies in the meta-analysis. I2 takes values between 0 percent and 100 percent, with higher values denoting a greater degree of heterogeneity (19, 24).

The pooled odds ratio was estimated by using fixed-effects (Mantel-Haenszel) and random-effects (DerSimonian and Laird) models (25). Random-effects modeling assumes a genuine diversity in the results of various studies, and it incorporates into the calculations a between-study variance. Hence, when there is heterogeneity between studies, the pooled odds ratio is preferably estimated by using the random-effects model (26). Studies with controls not in Hardy-Weinberg equilibrium (HWE; p ≥ 0.05) (27) were subjected to a sensitivity analysis (26, 28). Analyses were performed by using Meta-Analyst (Joseph Lau, Tufts-New England Medical Center, Boston, Massachusetts), StatsDirect (Microsoft Corporation, Redmond, Washington), and CVF90 with the IMSL library (26, 29, 30).

The literature review identified 693 titles in PubMed that met the search criteria. The abstracts of these articles were independently read by the two investigators to assess their appropriateness for this review. The results were compared, and disagreements were resolved by consensus. Sixty-four articles remained after abstract selection. The full articles for the remaining studies were evaluated for compliance with the inclusion criteria. Data from 22 articles describing 37 studies that investigated the association between polymorphisms and IHF met the inclusion criteria (31–52), and they were included in our review. The diagnosis criteria were similar in the reviewed studies, although not standardized ((8), table 1). Overall, 17 candidate genes and 24 polymorphisms were found to have been investigated in association with IHF (table 2).

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Table 1: Studies of genetic polymorphisms and ischemic heart failure

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Table 2 : Genetic polymorphisms investigated in relation to ischemic heart failure risk

Table 1 shows the study characteristics and the results of association between the different polymorphisms and the risk of IHF for each individual study. Table 2 shows gene polymorphism characteristics. Table 3 shows the meta-analysis results. In summary, seven genetic polymorphisms (angiotensin-converting enzyme insertion/deletion (ACE I/D), angiotensinogen (AGT) M235T, α2C subtype-adrenergic receptor (ADRA2C) Del322-325, ß2-adrenergic receptor (ADRB2) Arg16Gly, ADRB2 Gln27Glu, endothelin-1 (EDN1) Lys198Asn, and vascular endothelial growth factor (VEGF) G-405C) showed significant association (31, 36, 37, 40, 41, 46). The genotype distribution in controls was not in HWE in three studies (37, 39, 40), whereas, in 10 studies (31, 38, 43, 48–50), information was not provided. The genotyping personnel were reported to be blinded to phenotype in four studies (33, 36, 52), and the reliability of the genotyping procedure was controlled in nine studies (33, 35, 36, 41, 45, 47).

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Table 3 : Odds ratios with the corresponding 95% confidence intervals and heterogeneity tests results (I2, Q test) for genetic contrasts of ACE I/D, ADRB1 Arg389Gly, ADRB2 Arg16Gly, ADRB2 Gln27Glu and TNF G308A polymorphisms* for ischemic heart failure

A meta-analysis was performed for polymorphisms ACE I/D (31–35), ß1-adrenergic receptor (ADRB1) Arg389Gly (37, 39), ADRB2 Arg16Gly (39, 40), ADRB2 Gln27Glu (39, 40), and tumor necrosis factor-α (TNF) G-308A (42, 44). Overall, only one polymorphism, ADRB2 Arg16Gly, was found to have a significant association with IHF in the meta-analysis. The results from the remaining studies were very consistent, with only the ACE I/D polymorphism, when examined under the recessive model, showing significant heterogeneity. The results from each individual meta-analysis are described below. We now analyze and further discuss the findings for each gene polymorphism in turn.

Candidate genes and biologic mechanisms
The high allele frequency of the studied genes (table 2) suggests low genotype relative risk. Such genes may contribute to the development of heart failure only in conjunction with exogenous and endogenous exposures (53). Susceptibly genes can be identified by studying the biochemical or physiological pathways postulated to be involved in heart failure pathophysiology.

IHF begins with an initial myocardial insult, for example, myocardial infarction, which sets into motion a destructive cycle in which the remaining normal myocardium undergoes changes in cell metabolism and morphology, leading to hypertrophy and fibrosis (54). Alternatively, chronic, low-grade myocardial ischemia may also result in such changes (55). These cellular changes gradually alter the ultrastructural properties of the ventricle through a process called remodeling. Although remodeling initially occurs as an adaptive response to improve cardiac performance, over time, the response becomes counterproductive and maladaptive (54). A key mediator of this process is the neurohormonal activation, including regulators such as the renin-angiotensin-aldosterone system, the sympathetic nervous system, growth factors, and inflammatory molecules. Considering the fundamental role of neurohormonal factors in the pathophysiology and progression of cardiac dysfunction and remodeling, variants of neurohormonal genes are logical candidate genes in heart failure (12).

The genes identified by the literature search can be classified in five main categories: renin-angiotensin-aldosterone system, sympathetic nervous system, genes encoding growth factors or endothelial proteins, inflammatory genes, and miscellaneous genes.

For 22 of the studied polymorphisms, functional implications are reported in the literature (table 2). In two cases, the polymorphisms were not functional (endothelin A receptor (EDNRA) C69T, VEGF C-590T), but even nonfunctional polymorphisms are likely to be in linkage disequilibrium with causative alleles (56). The reference single nucleotide polymorphism (SNP) identification numbers (rs numbers) from the Database of Single Nucleotide Polymorphisms (57), the chromosomal gene position, the nucleotide base change, the average heterozygosity, and the amino acid substitution for each polymorphism are shown on table 2.

Renin-angiotensin-aldosterone system
The role of the renin-angiotensin-aldosterone system in heart failure is well known (58). Angiotensin-converting enzyme catalyzes the production of angiotensin II and the degradation of bradykinin. A functional intronic I/D polymorphism of the ACE gene has been studied for several cardiovascular-renal outcomes (59–61). Five case-control studies to date have addressed whether the variant form of the ACE gene alters the risk of IHF. Raynolds et al. (31) reported the only, to date, positive association. Two small-scale studies were conducted among Chinese subjects, but no increased risk of IHF under any model was found (32, 35). However, in Chinese, the frequency of the DD ACE genotype is lower than in other populations; thus, any negative conclusion could be due to low statistical power (62). One study in Turks (33) and one in Whites (34) also failed to show a significant genetic effect.

Overall, the meta-analysis of the five published studies (31–35) for the recessive model showed high heterogeneity (p = 0.09; I² = 0.51), which is attributable mainly to Whites (p = 0.01; I² = 0.85) since there is no significant sign of heterogeneity (p ≥ 0.10; I² = 0) for East Asians. Then, overall and for Whites, the random-effects odds ratios were 0.95 (95 percent confidence interval (CI): 0.60, 1.52) and 1.16 (95 percent CI: 0.40, 3.37), respectively; for East Asians, the fixed-effects odds ratio was 0.67 (95 percent CI: 0.30, 1.50). The allele contrast and the dominant model consisted of four studies (32–35) because one study of Whites (31) did not provide enough data. For these models, the analysis showed no significant sign of heterogeneity overall, and the associations were not significant. In a sensitivity analysis (exclusion of the study with no information on HWE) (31), the pattern of results was not altered (table 3).

Angiotensinogen is the precursor of the hormone angiotensin II. Two functional SNPs of the AGT gene, a nonsynonymous SNP designated as M235T (63) and a promoter SNP symbolized as G-6A (64, 65), have been investigated in a heart failure cohort of 158 White cases (60 percent ischemic) and 200 controls (36). The results were significant for only M235T because the estimated odds ratio under the allele contrast model in the entire heart failure group was 1.35 (95 percent CI: 1.1, 1.6).

Sympathetic nervous system
The pathophysiological relevance of α- and ß-adrenergic receptors (α-AR and ß-AR, respectively) and the benefit of antiadrenergic strategies in heart failure have been thoroughly studied (66, 67). Two case-control studies (37, 38) have investigated an in-frame deletion (symbolized Del322-325) (68) in the gene coding ADRA2C for susceptibility to IHF. Under the recessive model, a positive association was found by Small et al. (37) for Black subjects only, because the respective odds ratio was 5.65 (95 percent CI: 2.67, 11.95).

A nonsynonymous functional SNP of the ß1-AR gene (ADRB1), symbolized Arg389Gly (69), has been genotyped by three case-control studies (37–39). All of them report lack of association of the Arg389Gly polymorphism with IHF, although two (38, 39) possibly used a completely overlapping set of White cases. The meta-analysis of two studies (37, 39) showed no significant sign of heterogeneity (p ≥ 0.10), and the allele contrast, the recessive model, and the dominant model produced no significant results (table 3).

Three nonsynonymous functional SNPs of the ß2-AR gene (ADRB2), designated Arg16Gly, Gln27Glu, and Thr134Ile (66, 70–75), have been investigated for a potential role in IHF risk (39, 40). Covolo et al. (39) studied the possible association of the Gly16Arg and Gln27Glu polymorphisms with IHF, but no significant effect was observed. Leineweber et al. (40) genotyped the three aforementioned SNPs in a heart failure cohort consisting of 80 percent IHF cases. A positive association for *Gly under the allele contrast and a marginal association of Glu homozygotes versus heterozygotes were found. The meta-analysis for the Arg16Gly polymorphism showed no significant sign of heterogeneity (p ≥ 0.10; I² ≤ 12) and that the recessive model reaches marginal significance with a fixed-effects odds ratio equal to 1.32 (95 percent CI: 1.05, 1.65) (table 3). The meta-analysis for the Gln27Glu polymorphism showed no significant sign of heterogeneity (p ≥ 0.10; I² ≤ 7), and the allele contrast, the recessive model, and the dominant model produced no significant results (table 3).

Growth factors and endothelial proteins
A multitude of data suggests that the endothelin system is intricately involved in the pathophysiology of heart failure (76). A functional nonsynonymous SNP (Lys198Asn) of the EDN1 gene (77, 78), and a nonfunctional synonymous SNP (C69T) (79) of the endothelin A receptor gene (EDNRA), have been genotyped by Colombo et al. (41) in 122 White heart failure (79 percent ischemic) cases and 216 controls. In comparison with that for Lys homozygotes, the odds ratio for heart failure associated with the AsnAsn genotype was 4.34 (95 percent CI: 1.7, 11.1).

Heme oxygenase-1 is a rate-limiting enzyme in heme degradation, leading to the generation of by-products that exert potent antiproliferative and antiinflammatory effects (80, 81). A functional promoter dinucleotide repeat polymorphism [(GT)n] (82) of the heme oxygenase-1 gene (HMOX1) was investigated in relation to IHF (47). No association was found for the long (>27 repeats) or the short (≤27 repeats) version of this polymorphism.

Vascular endothelial growth factor plays a key role in angiogenesis and endothelial integrity and seems to be involved in the microvasculature abnormalities in heart failure (83, 84). Two functional promoter SNPs (G-405C and C-460T) (85, 86) of the VEGF gene have been examined by van der Meer et al. (46) in 417 IHF patients enrolled in the Metoprolol CR/XL Randomized Intervention Trial in Heart Failure study (5) and in 187 healthy controls. Only a marginal association for the –405C allele was obvious under the allele contrast.

Inflammatory genes
Evidence is accumulating that inflammation plays an important role in the development of left ventricular remodeling (87, 88).

The gene for proinflammatory cytokine tumor necrosis factor-α (TNF) is arranged in tandem with the tumor necrosis factor-ß or lymphotoxin alpha (LTA) gene. Three genetic association studies of Whites (42–44) have shown lack of association of IHF with two functional promoter SNPs (G-238A and G-308A) (89, 90) of the TNF gene and a functional intronic SNP (G252A) (91) of the LTA gene. Accordingly, the meta-analysis of two published studies of the TNF G308A polymorphism (42, 44) showed no significant sign of heterogeneity (p ≥ 0.10; I² = 0), and the allele contrast, the recessive, and the dominant model produced no significant results (table 3).

Transforming growth factor-beta (TGFß) is a multifunctional cytokine involved in the production and degradation of the extracellular matrix, important during the healing process after myocardial infarction and the transition from stable hypertrophy to heart failure (92, 93). Investigation of two functional nonsynonymous SNPs (Leu10Pro and Arg25Pro) (94, 95) of the TGFB1 gene in 144 heart transplant recipients with IHF and 94 healthy controls has shown no significant results (45).

Interleukin 4 (IL-4) and interleukin 10 (IL-10) are antiinflammatory cytokines that inhibit the synthesis of proinflammatory cytokines (96). Three published studies by the same group of investigators in the Netherlands examined whether genetic variability in the IL-4 and IL-10 genes affects individual susceptibility to IHF (48, 49). One promoter SNP (C-590T) for IL-4 (97) and two promoter SNPs (G-1082A and C-592A) for IL-10 (98, 99) were examined, but no positive association was reported. Cluster of differentiation (CD) surface molecules mediate cell activation and signaling (100). The functional promoter SNP (C-260T) of the CD14 gene (101) was genotyped by Kruger et al. (51) in 51 IHF cases and 100 healthy controls, but no increased risk of IHF was found.

Miscellaneous genes
Kolek et al. (50) hypothesized that carriers of the C34T nonsense mutation of the adenosine monophosphate deaminase gene (AMPD1) might have a relative advantage, since this mutation results in a beneficial increase in the cardioprotective molecule adenosine (102–104). They genotyped 605 IHF patients in the Beta-Blocker Evaluation of Survival Trial compared with two control groups from the Intermountain Heart Collaborative Study Registry. No protective effect of the C34T mutation was detected for carriers when compared with both the first and the second control groups.

The serotonin transporter is considered one of the determinants of depressive symptoms, which is an independent predictor of increased morbidity and mortality in patients with acute myocardial infarction (105). Nakatani et al. (52) published a cohort study from the Osaka Acute Coronary Insufficiency Study group, in which they investigated the influence of a functional I/D polymorphism of the serotonin transporter gene (SLC6A4) (106). The D allele did not confer an increased risk of developing new-onset heart failure in myocardial infarction survivors within 1 year of follow-up.

Interactions
As with other complex traits, development of IHF is likely determined by several genes that act collectively, and allelic variants at different genes may have either additive or contrasting effects (epistasis) (56, 107). Additionally, there are several possible interactions between genetic polymorphisms and effect modifiers such as age, gender, treatment, hypertension, or other environmental factors (108).

Gene-gene interactions
Four studies (37–39, 41) investigated possible gene-gene interactions. Small et al. (37) examined the possible interaction of the ADRA2C, Del322-325, and ADRB1 Arg389Gly polymorphisms. In Black subjects, homozygosity for *Del322-325 and *Arg was associated with a substantially increased risk of heart failure, and the estimated odds ratio was 10.11 (95 percent CI: 2.11, 48.53). A possible biologic explanation for this synergistic effect is that the combination of receptor variance results in increased synaptic norepinephrine release and in enhanced receptor function at the cardiomyocyte (37). The lack of association of this combined genotype in Whites was reported by two studies (37, 38). Covolo et al. (39) investigated the possibility of an interaction between the ADRB1 and ADRB2 polymorphisms, that is, whether homozygosity for ADRB1*Arg combined with the ADRB2 *Gly*Gln haplotype confers an increased risk. Despite their negative findings, these should be carefully interpreted because the per-stratum sample size and the associated statistical power are reduced when the number of examined genes is increased (109).

Colombo et al. (41) investigated the potential synergistic effect between the genes of the endothelin system signal transduction pathway. A two-locus analysis indicated that homozygosity for EDN1 AsnAsn and EDNRA TT was associated with a substantially increased risk of heart failure because the odds ratio was 8.6 (95 percent CI: 1.5, 48.1).

Gene-environment interactions
The conflicting results among studies investigating genetic polymorphisms and the risk of IHF may be due to the lack of information on the possible interactions with environmental factors. Akbulut et al. (33) used coronary artery disease patients without heart failure as controls, but there was a significant difference in the positive history of myocardial infarction between the cases and the controls (p < 0.05). Because the ACE DD genotype was not associated with an increased risk of myocardial infarction (p = 0.6), the DD genotype was not overrepresented in cases because of the positive history of myocardial infarction. The presence of hypertension could promote the progression of heart failure. However, Small et al. (37) demonstrated that the distribution of the risk-associated dual genotype (*Del322-325 and *Arg) was not different between hypertensive and normotensive heart failure cases (chi-square = 0.34, p = 0.95). Goldbergova et al. (36) detected an increased risk of the G-6A polymorphism of the AGT gene, only after adjustment for sex. Furthermore, for women carrying the combined genotype GGMT for the AGT gene, the odds ratio was 15.5 (95 percent CI: 1.86, 129.42) in contrast to the nonsignificant odds ratio observed for men. Such a sex-specific influence could result from the effect of steroid hormones, which affect AGT expression in a variety of tissues (110).

Understanding the role that genes play in developing IHF is essential to creating more effective screening tests for predicting which individuals are at risk of developing the disease, to implementing appropriate early-intervention preventive and therapeutic strategies, and to developing gene therapy approaches in the future (111). So far, IHF genetic association studies have been highly inconsistent. The complex nature of heart failure implies that, for individual polymorphisms, associations are likely to be modest (12). To detect such modest genetic effects, a series of important research priorities must be implemented.

Power improvement
There is clearly a loss of statistical power when the genetic effect is reduced (107). Most of the studies we analyzed included few cases and controls and consequently did not have adequate power to detect a modest genetic effect. Apart from the need for larger sample sizes, selecting cases that are genetically loaded may also improve power. By selecting cases with very early onset disease and a strong family history, cases will be weighted toward those individuals whose disease has a strong genetic etiology (112).

Stratification
Small et al. (37) were able to detect the strongest to-date genetic association in Blacks, but not in Whites. Lack of stratification in this study could have led to blurring of the genetic effect. On the contrary, there is serious concern about the possible effects of population stratification on the results of case-control studies (113). Unequal genetic admixture in the control and patient populations can result in spurious associations. An approach proposed to minimize this potential problem is to measure and adjust for genetic markers not linked to the disease under investigation (114).

Prospective design
All the analyzed studies except for one (52) were of case-control design and were retrospective. If a genetic variant not only significantly increases the risk of IHF but also influences survival, it is possible that risk-allele carriers will have advanced heart failure and die prematurely, leading to an underrepresentation of the risk genotype at the time of enrollment (115). Consequently, prospective studies are needed.

Case selection
The inclusion criteria for cases could also be another source of bias. Firstly, all studies except for one (40) included cases with impaired systolic function only. However, there are accumulating data indicating that as much as 50 percent of heart failure is associated with a normal left ventricular ejection fraction (116). Additionally, myocardial hibernation could introduce an issue of misclassification because revascularization could improve, and even normalize, left ventricular ejection fraction and heart failure symptoms in patients with hibernating myocardium (117).

Standardization of the definition of IHF based on angiographic criteria, as proposed by Felker et al. (8), could limit variability in defining etiologic subgroups in heart failure cohorts.

Appropriateness of controls
The majority of the control groups consisted of healthy or nonischemic and non–heart failure subjects. However, the absence of coronary artery disease in controls could lead to spurious associations. The use of two control groups, with and without coronary artery disease as in the study by Kolek et al. (50), establishes the appropriate contrast to detect a possibly true genetic effect.

HWE and genotyping
In the studies with the controls not in HWE (37, 39, 40), the lack of HWE indicates genotyping errors, population stratification, and selection bias (118). In addition, lack of HWE in a population implies continued selection, migration, mutation, and absence of random mating (119, 120). Thus, the validity of the genotyping method, and the selection of controls, are questioned (119, 121). Furthermore, the lack of reporting blindness in genotyping personnel in 33 studies (31, 32, 34, 35, 37–51) and the possible lack of a validated genotyping procedure in 28 studies (31, 32, 34, 37–40, 42–44, 46, 48–52) could be potential sources of biases.

Candidate gene selection
In addition to candidate gene approaches, genomic or proteomic expression analyses can assist in the selection of candidate variants by ranking those genes that appear to be the most active in the disease process (107, 122). Recent studies have identified gene expression profiles that could accurately distinguish ischemic and nonischemic cardiomyopathy (123). This overlapping of independent sources of information has been termed "genomic convergence" and is expected to provide new insights into the cellular mechanisms involved in cardiac dysfunction (124).

Gene-environment interactions
Many environmental factors have been associated with increased risk of IHF, such as age, obesity, hypertension, myocardial infarction, anemia, diabetes mellitus, hyperlipidemia, and thyroid disorders, while a number of pharmacological and nonpharmacological interventions have been shown to alter the natural history of the syndrome (125). Despite difficulties in study design and assessment of the exposures, such parameters should be incorporated in future studies (126).

Gene-gene interactions
The search for susceptibility loci has probably been complicated by the increased number of contributing loci and susceptibility alleles (127). Elucidating the pathogenesis of the disorder would demand investigation of association for many variants of genes that constitute distinct pathophysiological pathways (128).

Large-scale genetic association studies and meta-analyses
IHF cases are usually aged (table 1), which means that recruiting large numbers of affected sib pairs or family trios, needed for wide-genome scans and family-based association studies, might be problematic (10, 129). Consequently, elucidating the genetics of IHF largely relies upon designing and undertaking rigorous genetic association studies. Moreover, future studies should be planned with the idea of their being incorporated into other similar studies in a meta-analysis. The opportunities offered by a meta-analysis are enhancement of power; the ability to place each study in the context of all others, particularly early spurious results; and the possibility of examining why studies reach different conclusions (120).

In summary, there is no evidence of strong association between genetic variants and the risk of developing IHF in the individual studies and meta-analyses. These findings suggest that the risk of IHF is not related to genes or that research to date has been insufficient to identify such associations. However, conclusions reached in the present analysis were based on relatively small numbers of studies and participants for each gene polymorphism, and their interpretation has to be cautious. Taking into account that IHF is a complex disease with multifactorial etiology, a minor contributing pathogenetic role of the investigated gene polymorphisms in specific cases, and in cooperation with other factors, cannot be totally excluded. Therefore, the relation between genetic variation and IHF still remains an unresolved issue. The results of long-term prospective and case-control studies (118, 130), designed to investigate gene-gene and gene-environment interactions, and utilizing the vast amount of data produced by genomic studies (122) might produce more conclusive claims about the genetics of IHF.

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