European Heart Journal Advance Access originally published online on August 3, 2006
European Heart Journal 2006 27(17):2031-2033; doi:10.1093/eurheartj/ehl157
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Deoxyribonuclease I: exploring the hidden side of myocardial infarction
Centro Regionale per la Prevenzione dell'Aterosclerosi, Ce.S.I., University of Chieti ‘G.d'Annunzio’, ‘G.d'Annunzio’ University Foundation, via Colle dell'Ara, 66013 Chieti, Italy
Corresponding author. Tel: +39 0871 541360; fax: +39 0871 541491. E-mail address: fcipollone{at}unich.it
This editorial refers to ‘Association of Gln222Arg polymorphism in the deoxyribonuclease I (DNase I) gene with myocardial infarction in Japanese patients’
by T. Kumamoto et al., on page 2081
In the last years, the role of individual genetic background is contemplated in many aspects of medicine, such as receptiveness to diseases, disease evolution, and responses to drug treatment (pharmacogenomic).1
Genomes from different individuals are 99.9% identical, with only 0.1% of the genome showing mutations.2 Nevertheless, the rapid improvement in genome analysis has stimulated a colossal search for clinically relevant genomic polymorphisms, particularly single-nucleotide polymorphisms (SNPs), which consist of substitutions of one nucleotide for another in a DNA sequence.1 The possibility that SNPs may be useful in identifying candidate disease genes and individuals at risk of disease had led to extensive projects aimed at discovery and organization of SNPs into databases. At this time, about 2–3 million SNPs have been found in the human genome and are available in the public dbSNP databases,3 suggesting that if the promise of polymorphism analysis is realized, we are entering into an ‘era of personalized medicine’.4 Almost all genes contain SNPs, but only a limited number of SNPs really produce amino acid variation in proteins (functional SNPs). An additional level of complexity is at the protein level, as one gene may produce up to five different proteins as a result of alternative splicing.1 Furthermore, post-translational changes, such as specific assembly or glycosylation, may additionally increase the diversity of proteins. Thus, as a consequence of these complexities, it is difficult, if not impossible, to predict the real biological and clinical effect of a (functional) SNP in a given gene.1
Myocardial infarction (MI) is caused by the rupture of vulnerable atherosclerotic plaques.5 It is a complex disorder that is the result of a multifaceted interaction between a person's genetic makeup and environmental and behavioural factors.6 In fact, it is now clear that conventional risk factors for MI, such as diabetes, obesity, dyslipidaemia, and hypertension, all result from the interaction between multiple susceptibility loci with behaviour and the environment.1 However, the investigation about genes contributing to the conventional risk factors has been extensively conducted in the last 10 years, whereas relatively little is still known about the role of genes regulating the essential cellular functions in the pathophysiology of MI.
Deoxyribonuclease I (DNase I) is an enzyme that preferentially attacks double-stranded DNA by Ca2+- and Mg2+/Mn2+-dependent endonucleolytic cleavage to produce oligonucleotides with 5'-phosphoryl and 3'-hydroxy termini.7 DNase I is considered to play a major role in the digestion of dietary DNA, because in vertebrates it is secreted by exocrine/endocrine glands such as the pancreas and parotid gland into the alimentary tract.7 However, the presence of the enzyme in mammalian tissues other than the digestive organs suggests that it might have other functions in vivo; endogenous DNase I has been regarded as a candidate endonuclease responsible for internucleosomal DNA degradation during apoptosis.7 Furthermore, it is now known that extracellular (serum) DNase I participates in the chromatin breakdown of necrotic cells, achieved by its diffusion from the extracellular fluid into the cytoplasm and nucleus of such cells.7 Recently, Kawai et al.8 demonstrated that an abrupt elevation of serum DNase I activity occurs within 3 h of the onset of symptoms in patients with MI and that DNase I activity in serum then exhibits a marked time-dependent decline within 12 h, returning to basal levels within 24 h. The increase in serum DNase I activity is not related with changes in the other conventional cardiac markers such as creatine kinase isoenzyme MB and cardiac troponin T, thus suggesting that myocardial ischaemia rather than myocardial injury induces such elevation in serum DNase I activity probably by influencing the transcriptional regulation of human DNase I gene and that therefore DNase I could be potentially involved in the pathophysiology of MI. In this light, the recent demonstration that human DNase I is genetically polymorphic and is controlled by six codominant alleles at chromosome 16p13.3 raises the interesting possibility that functional SNPs in this gene could affect susceptibility to MI.
The article by Kumamoto et al.9 should be viewed on this background. The findings are interesting and potentially important, but the study also demonstrates the classical pitfalls of research on SNPs as markers of disease. The authors studied 311 patients with MI and 300 patients with stable angina (SA) admitted at three hospitals in Japan. SA patients were persons who were visiting the clinic for chest pain as a consequence of a significant stenosis (>50%) in at least one major epicardial coronary artery, but had no history for MI or unstable angina, clinical events related to atherosclerotic plaque rupture. The investigators compared the patients with MI with the SA patients in terms of the prevalence of polymorphisms in the DNase I gene. Thus, they found three common phenotypes determined by two co-dominant alleles, DNASE1*1 and *2. These phenotypes translated in corresponding products exhibit different properties. A multiple logistic regression analysis was performed and showed that the odds ratio (OR) of DNASE1*2 was 1.51 (95% CI 1.04–2.18). The result remained significant (P<0.003) also when the analysis was stratified according to the conventional risk factors for MI. The authors concluded that this SNP is associated with a higher incidence of MI.
The study by Kumamoto et al. is remarkable because of the accurate analysis described above. Most genetic-association studies ignore the presence of conventional risk factors, usually because of a small sample. In the present study, an attempt was made to account for this factor. In addition, many studies do not provide evidence that the studied SNP has functional consequences, and in the absence of such data, the relevance of the associations remains uncertain; in contrast, Kumamoto et al. demonstrate that this SNP is really associated with significant changes in the protein function. This is particularly relevant, as only few genetic studies integrated the association data with experimental evidences aimed to demonstrate the way by which the SNP may influence the risk of MI.10 In this study, the authors provide a clear evidence that this SNP is associated with changes in protein activity in vitro. However, additional experiments focused to confirm the same data also in vivo could further strengthen the manuscript.
However, there are also important limitations. First, the selection of the accurate control groups is critical in genetic studies. Controls should represent the cohort from which the patients were derived, for example, the general population or a birth cohort.1 If this restriction is adhered to, the difference between the groups will be the presence or absence of disease, and genetic differences may then be related to that disease. If the control group is recruited from the general population, then the SNP found to have an association with disease might be used as a marker of risk in that population. In the present study, the controls were patients with SA. However, a really healthy control group is absent, and therefore it is not possible to compare the prevalence of this SNP in patients with MI directly with the healthy population. Authors try to reduce this bias by performing an indirect comparison with data from previously studied healthy Japanese subjects and reporting that no significant difference in the frequency of the DNASE1*2 allele was observed between the SA group and the healthy group. However, this indirect comparison has (of course) many caveats and cannot truly resolve the issue. Secondly, the prevalence of the classic risk factors for MI was almost identical among MI and SA patients. This similarity would permit the identification if this SNP could cause disease under certain conditions, for example, in diabetics only. However, such identification requires separate analyses for each risk factor, but such analyses could not be reliably performed in this study for the small sample. Furthermore, SNPs that pre-dispose persons to classic risk factors, such as hypertension, hypercholesterolaemia, obesity, and diabetes, were probably eliminated from consideration in spite of being risk factors for MI. In addition, MI is a heterogeneous diagnosis, and the small number of patients enrolled may indicate the presence of selection bias, with unknown effects on the results, as well as cannot exclude the likelihood that the findings result from the play of chance. Additionally, the presence of two different recruitment phases in the MI group also introduces the specific bias regarding the recruitment of non-consecutive patients. Finally, the OR is relatively small and one cannot totally rule out the possibility of unintentional bias in the study. Another study limitation is that the authors studied a particular ethnic group, i.e. Japanese subjects all coming from the same region, and therefore the results cannot be automatically translated to the general population. In fact, we know well that many SNPs that have previously been shown to be associated with coronary disease in some ethnic groups were not found to have a significant association with disease in other populations.1 In addition, also assuming that this SNP may have the same impact on the risk of MI in all the populations, however, its different frequency in the diverse populations could deeply modify its specific weight as risk factor for MI. For example, as the authors correctly reported, the frequency of this SNP was significantly higher in the German population (as reported in a previous population-genetic survey) than in the Japanese population. Finally, despite the author's effort, the classical limit of most genetic studies, i.e. the possibility that the association of an SNP with a disease may result from its linkage with a nearby susceptibility locus, is present in this study also. This non-random association between alleles is called linkage disequilibrium. Thus, an SNP may be a marker of susceptibility to a disease without having a causal role.
In conclusion, despite for this polymorphism as well as for the majority of all discovered polymorphisms, therapeutic consequences are a long way off; however, the authors are to be commended for providing the first report demonstrating the association between polymorphism in a gene related to apoptosis and susceptibility to MI, thus contributing to reduce the hidden side of the pathophysiology of plaque rupture. The findings of the study should be used to initiate further research aimed at replication and at elucidation of the underlying mechanisms of disease.
Conflict of interest: none declared.
Footnotes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
doi:10.1093/eurheartj/ehl177 
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