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Glutathione S‐transferase gene polymorphism and cardiac iron overload in thalassaemia major

2008; Wiley; Volume: 142; Issue: 1 Linguagem: Inglês

10.1111/j.1365-2141.2008.07175.x

1365-2141

Raffaella Origa, S. Satta, Gildo Matta, R. Galanello,

Trace Elements in Health

The heart is the target lethal organ for iron accumulation in thalassaemia major (TM) and heart failure secondary to iron overload is still the main cause of death in transfusion-dependent patients. Currently, magnetic resonance imaging (MRI) is the only non-invasive method with the potential to assess myocardial iron. The MRI T2* parameter has proven to be a fast, simple, robust parameter and, although it has not been calibrated with chemical measurements of endomyocardial iron in humans, growing indirect evidence suggests that it reflects cardiac iron. In chelated patients, myocardial iron is usually inversely related to the compliance with chelation, whereas no significant correlation with liver iron and serum ferritin concentration measured at the time of T2* assessment was found (Tanner et al, 2006). However, in a subset of patients, low T2* values occur despite a history of good compliance with chelation therapy, suggesting the possible role of genetic factors in cardiac iron deposition. Several gene polymorphisms including apolipoprotein ε and human leucocyte antigen haplotypes have been described as protective or predisposing factors for cardiac dysfunction in patients with TM. Wu et al (2006) analysed polymorphisms of two endogenous antioxidant enzymes, glutathione S-transferase M1 (GSTM1) and glutathione S-transferase T1 (GSTT1). They found that the GSTM1 null (deleted) genotype was associated with a decreased signal intensity ratio on MRI, an index of increased myocardial iron, suggesting that genetic variations of the GSTM1 enzyme are associated with cardiac iron deposition. The current study aimed to evaluate if the GSTM1 null genotype is a predisposing factor for myocardial iron overload in TM patients with low body iron, as assessed by lifelong serum ferritin levels. Fifty TM patients who had been regularly transfused and chelated with desferrioxamine since the first or second year of life and who had heart T2* lower than 12 ms were selected based on body iron load as estimated by serum ferritin levels. Twenty-four of them had unexpectedly severe cardiac iron overload based on estimated low body iron load (mean of lifelong serum ferritin determinations 1360 ± 268 μg/l), and 26 had expected severe cardiac iron overload based on estimated high body iron load (mean of lifelong serum ferritin determinations 4724 ± 1530 μg/l). At least 4 ferritin values per year in the 18 years before the MRI assessment were considered for each patient. Age and haematological data of the two groups are reported in Table I. Patients were similar in both groups except for age and serum ferritin levels and most of them had a left ventricular ejection fraction <50% or a history of heart failure. Myocardial iron was measured with the T2* parameter, using a commercially available 1·5 T Magnet (GE, Milwaukee, WI, USA) with a torso PA coil using a multiecho breath hold sequence according to Westwood et al (2003). The blood samples were assigned a random code before genotyping for GSTM1 by the laboratory. A group of 26 healthy Sardinian patients was genotyped as control group. GSTM1 analysis was conducted using a multiplex polymerase chain reaction, according to Arand et al (1996), with albumin gene as an internal control. Statistical analysis was carried out using the Pearson's chi-squared test. P-values <0·05 were considered significant. The study was approved by the Hospital Ethics Committee. GSTM1 null genotype was found in 34·6% (9/26) of patients with TM and expected heart low T2*, in 66·7% (16/24) of the patients with unexpected heart low T2* and in 38·4% (10/26) of the healthy controls. GSTM1 is a member of the glutathione S-transferase family, which is thought to play a role in the detoxification of metabolites of many xenobiotics involved in the aetiology of cancer. Homozygous deletion of GSTM1 (null genotype) results in a lack of GSTM1 enzyme activity and has been associated with lung, bladder, prostate and other tumours. Variations in GSTM1 null allele frequency have been observed in different ethnic groups. Garte et al (2001) reported the homozygous GSTM1 null allele in 53% of 10 541 Caucasian subjects. Our Sardinian healthy controls and TM patients with expected cardiac iron overload did not show a statistically different GSTM1 null frequency from that reported by Garte et al (2001). In contrast, the GSTM1 null allele was significantly more common in TM patients with cardiac iron load and low body iron as assessed by lifelong serum ferritin levels, when compared with both patients with expected cardiac iron overload and with Sardinian healthy controls (P = 0·02 and P = 0·04, respectively), and with the normal Caucasian population (P < 0·001). It should be pointed out that the patients with unexpected cardiac iron overload were younger than those with expected cardiac iron overload. In agreement with Wu et al (2006), therefore, the GSTM1 null genotype seems to be a possible predisposing factor for myocardial iron overload and could explain why cardiac iron overload can also be detected in TM patients that have been adequately chelated since the first years of life. However, no explanation regarding the mechanisms that underlie this association has hitherto been proposed. As the members of the glutathione transferase structural family have been demonstrated to be novel inhibitors of cardiac ryanodine receptor calcium channels, we hypothesized that, when GSTM1 is deleted in presence of iron accumulation, the entry of iron into the heart is significantly enhanced. This could be due to the difference in permeability among the divalent cations of these channels, as already demonstrated for Ba2+, Sr2+ and Mg2+ (Tinker et al, 1992). This hypothesis is also supported by the finding that L-type Ca2+channels are high-capacity pathways of ferrous iron uptake into cardiomyocytes under iron overload conditions (Oudit et al, 2006). Interestingly, in the heart and smooth muscle, under physiological conditions, calcium influx through L-type Ca2+ channels influences calcium release from ryanodine sensitive internal stores, which subsequently triggers muscle contraction (Cheng et al, 1993). Although the presence of very low concentrations of Fe2+ and Fe3+ in solution makes electrophysiological measurements very difficult, investigations at the myocardial cellular level, gene knockout experiments and larger clinic studies are needed to support this model. This study was supported by Grants from L.R.11 1990 Regione Autonoma Sardegna, Ithanet, PRIN 2006.