Renin-angiotensin-aldosterone system polymorphisms: a role or a hole in occurrence and long-term prognosis of acute myocardial infarction at young age

Five to ten percent of new acute myocardial infarction (AMI) occur in individuals younger than 45 years [1]. An Italian population registry (GISSI study) [2] reported a similar, even if slightly lower, incidence (about 2%) in our country at the end of the XXI century. At this age, AMI is characterized by low mortality rates, less extensive coronary artery disease (CAD), better residual left ventricular function and a favourable prognosis in short and medium follow-up [3].

As known, CAD is a multifactorial disease influenced by environmental and genetic factors, in fact, in a younger population smoking, dyslipidemia and familiarity are more frequent than in older patients [2–4].

Although the role of these environmental factors in the development of AMI has been clearly established, the role of non-conventional risk factors remains undefined. In the last few years a great interest has been focused on genetic factors with the intent to find common markers that could identify a subgroup of patients at higher risk of death or with a worse prognosis in which new therapeutic timings and interventions could be tested.

Particular interest has been focused on the renin-angiotensin-aldosterone system (RAAS) [5, 6] in view of its physiological role and of the established benefits of ACE inhibitors therapy.

In the enzymatic cascade, angiotensinogen (AGT) is cleaved by renin to produce angiotensin I, which is further converted in the bioactive octapeptide angiotensin II (ATII) through the action of angiotensin I converting enzyme (ACE), a membrane-bound, zinc metallo-endopeptidase involved in the metabolism of many small peptides. The cellular effects of ATII in human adults are mainly mediated by the angiotensin II type 1 receptor (AGTR1). Aldosterone synthase (CYP11B2), sensible to the effects of ATII, catalyses the final step of aldosterone biosyntesis in adrenal glomerulosa. Because of the various physiologic effects of ATII, including vasoconstriction, promotion of vascular smooth muscle cells growth and increase of extracellular collagen matrix synthesis, genetic variation of the level of each RAAS component could probably affect a wide variety of clinical phenotypes.

AGTR1 is known to play a pivotal role in the physiopatology of several cardiovascular abnormalities [7–10], and ACE and AGT are important in blood pressure homeostasis [11, 12], so it is not surprising that the genes coding for members of the RAAS and its products are being investigated in relation with AMI.

Aim of this study is to investigate the effects of ACE insertion/deletion (ACE I/D), AGT1R A1166C and CYP11B2 C-344T polymorphisms on the occurrence and the long-term prognosis of AMI at young age in an Italian population.

The study population consisted in 201 patients and 201 controls (mean age 40 ± 4 years, range 25–45 years).

Genotype frequencies in the overall sample fit the Hardy-Weiberg equilibrium and were similar to those found in other studies on Caucasian population.

We found a statistically significant association between AMI and ACE polymorphism. The DD genotype alone, reaching borderline significance in the univariate analysis (42% in cases vs 31% in controls; p = 0.056) remained significant also after adjustment for conventional risk factors (p = 0.005) with a independent role on predicting events (OR 2.39 CI 1.25–4.58, p = 0.008); whereas the CYP11B2 polymorphism (CC genotype, 27% vs 17%, p = 0.054) reached borderline significance only in the crude analysis, but lost an independent role after adjustment for confounders.

The worst prognosis of patients with the AC genotype is successively confirmed by the Kaplan-Meier event free curves (Figure 1) considering the time course from the first AMI event to the different end-points (cardiac death, myocardial infarction, revascularization procedures).

Furthermore in the multiple logistic regression analyses the heterozygous AC genotype resulted as a protective polymorphisms on the onset of events at follow-up also when correcting for conventional risk factors (OR 0.48 CI 0.24–0.96, p = 0.038). Instead prevalence of DD homozygous genotype of ACE and CC genotype of CYP11B2 did not diverge enough in the two groups to reach statistical significance.

Epidemiological and clinical data from studies performed on young adults with premature AMI have shown that the occurrence of AMI in young Italian people is uncommon (2% in the GISSI study) [1, 2] and that the prognosis in short-term outcome is good (mortality rate lower than 2%). For the risk factors profile, young patients have significantly higher rates of smoking, dyslipidemia and family history for CAD, compared with older patients [2]. A promising but inherently difficult area of study is the identification of genes that predispose to CAD.

The ECTIM study [11] first demonstrated that the homozygote form of a deletion polymorphism of ACE could be an independent risk factor for AMI.

In the current study, we compared the distributions of conventional risk factors and RAAS polymorphisms (ACE, AGTR1, CYP11B2) in a population of young patients with AMI during the hospital recovery for acute event and after a long-term follow-up.

ACE is expressed on the surface of the cell in many tissues (particularly in lungs), but also circulates in a soluble form obtained by the cleavage of the extracellular portion from the endothelial cells [22]. The analyzed polymorphism is a 287 base pair Alu insertion/deletion (I/D) in intron 16 of ACE gene in chromosome 17; DD genotype is associated with increased serum and cellular concentrations of ACE, that results in enhanced convertion of angiotensin I in angiotensin II [19].

Cambien [5] first demonstrated a possible role of DD genotype as cardiovascular risk factor; most of the subsequent studies have shown no significant effects of this polymorphism on the extent of CAD but mainly on the onset of acute coronary syndrome, data suggesting a possible role in the mechanism involving plaque instability, ulceration and thrombosis [22].

In a recent study Palmer et al. [23] conclude that ACE I/D genotype may provide additional prognosis information in patients with AMI, suggesting an association with DD genotype and mortality after cardiac events that was excluded in previous studies [24, 25].

In our work we support the role of DD as a relevant risk factor in the acute phase, with an independent role also when adjusting for conventional risk factors. This result is of high interest if thought as a new identifiable and modifiable (ACE inhibitors, since the DD genotype enhances cellular concentration of ACE) risk factor in specific subgroups (family history for CAD) of patients. Conversely, accordingly to previous literature data [24, 25], we did not find any association between DD and mortality or other events (new AMI, revascularization procedures, angioplasty or coronary bypass graft surgery) after AMI.

AGTR1 is expressed mainly in vascular smooth muscle cells and in the myocardium. A polymorphism in the 3' untranslated region of AGTR1 gene was identified [11], corresponding to an A→C transversion at nucleotide position 1166 of the mRNA sequence.

An association between AGTR1-C allele and a modified response of the receptor to angiotensin II [5] has been suggested, thus justifying the reported statistically significant correlation of AGT1R-CC with increased cardiovascular risk, together with ACE DD polymorphism [5, 6], in a synergistic effect.

However, Andrikopoulos et al. [10] found that there was no significant association among patients with the A1166C polymorphism in the AGTR1 gene and increased mortality for AMI.

Our study, instead, after a long-term follow-up shows an unfavourable role of AC genotype after the acute coronary event; however, this result needs to be confirmed in larger studies.

The human aldosterone synthase (CYP11B2) gene is located on chromosome 8 and the polymorphism was studied in the promoter region of the gene at nucleotide 344 from the translation start site, where the residue could be cytosine or thymidine.

The activity of the CYP11B2 gene is primarily regulated by the renin-angiotensin system through the actions of angiotensin II.

This polymorphism has been correlated to increased circulating levels of aldosterone, thus influencing arterial hypertension, cardiac fibrosis and, consequently, both diastolic dysfunction and ventricular remodelling evolution after AMI [26].

Kupari et al observed, in young Finnish subjects, that the CC genotype of CYP11B2 polymorphism is a strong predictor of left ventricular diameters, mass and ventricular filling fraction.

It has been showed that smoking and dislypidemia are more potent risk factors for nonfatal MI in males who have the C allele of CYP11B2 [27].

In the present study, we found a statistically significant association between CYP11B2 polymorphisms and AMI, suggesting a possible role as cardiac risk factor, but not independently from the conventional ones.

In conclusion our work, although if based on a limited sample size, has the potential of a long follow-up period after the first cardiac event. Our results show that polymorphisms in RAAS genes could be important in the onset of a first AMI event in young patients (ACE, CYP11B2 polymorphisms) as in the disease progression.

Many contradictory results have been published on RAAS polymorphisms in the attempt to find out new associations with clinical-therapeutic features or biochemical determinations.

In a multifactorial disease like CAD, in which the weight of conventional risk factors is so high especially in young people, and in which genetic profile is so heterogeneous due to many low penetrant candidate genes possibly involved, the clinical usefulness of genetic findings needs to be carefully discussed.

The role of different polymorphisms in individual risk assessment for primary and secondary prevention programs is strictly linked to the possible interaction with many known conventional risk factors.