2008阿司匹林抵抗基因

Aspirin acts by irreversibly inhibiting the cyclooxygenase-1(COX-1)enzyme through acetylating the serine residue at position 529.COX-1catalyses the con-version of arachidonic acid to prostaglandins G 2and H 2, which are subsequently converted by thromboxane syn-thase to thromboxane A2(TXA2),a potent vasoconstrictor and activator of platelet aggregation. However, the anti-platelet effects of aspirin may not be equal in all indivi-duals. A proportion of patients prescribed aspirin suffer recurrent thromboembolic vascular events, giving rise to the term ‘aspirinresistance’. This term, however, is probably misleading, because there may be a number of reasons why patients do not respond to aspirin, such as poor adherence to therapy, and it may be more appropriate for this situation to be definedas ‘clinicaltreatment failure’rather than resistance [5].

Aspirin resistance is probably better definedbiochemi-cally and/orfunctionally using measures such as:(i)light transmission aggregometry [6],(ii)bleeding time [7],(iii)platelet function analyzer-100(PFA-100)[8],(iv)VerifyNow Aspirinsystem [9],and (v)levels of serum TXB2or urinary 11-dehydroTXB2(spontaneousdegradation products of TXA2) [10].Each method has its own advantages and dis-advantages [11,12].The problems with determining drug adherence [13]and the different methodologies available for determining the biochemical and functional effects of aspirin have compounded the problems of attempting to defineaspirin resistance, with some investigators suggest-ing that the term should be used only when production of TXA2(orits breakdown products) is blocked regardless of platelet function [10].Since there is no single definitionof, or validated method to identify patients with, aspirin resis-tance, its reported prevalence varies greatly in different studies,from 5to 45%[14],andin one study as high as 60%[15].

Aspirin resistance is likely to be multifactorial in origin. Reduced absorption and/orincreased metabolism of aspirin may contribute, as may biosynthesis of TXA2from pathways not inhibited by aspirin as well as alternative pathways involved in platelet activation not blocked by aspirin [e.g.those involving adenosine diphosphate (ADP),collagen, epinephrine and thrombin].Moreover, some of the literature suggests that a majority of aspirin resistance reported may be the result of poor adherence [16–18].A genetic aetiology to aspirin resistance has also been proposed.A number of studies have examined the associa-tion of aspirin resistance with single nucleotide polymor-phisms (SNPs)in the genes for COX-1and for several receptors on the surface of platelets [19].These individual studies have been too small to allow reliable conclusions to be drawn, and have rarely taken into account the differ-ent available biochemical and functional methodologies, thus giving rise to conflictingresults.

In order to clarify what, if any, genetic basis exists for aspirin resistance, we have undertaken a comprehensive systematic review of all genetic studies on aspirin resis-

Genetics of aspirin resistance

tance, encompassing both patients with cardiovascular disease and healthy subjects.

Methods

Data sources

Electronic databases (MEDLINE,EMBASE and Google Scholar) were searched up to 1December 2007for all case–controlstudies evaluating any candidate gene and aspirin resistance in humans. Letters and abstracts were included in the systematic review. The Medical Subject Headings terms and text words used for the search were ‘aspirin’, ‘acetylsalicylicacid’, ‘aspirinresistance’and ‘aspirinnon-responder’in combination with ‘genetic’, ‘polymor-phism’, ‘mutation’, ‘genotype’, or ‘gene’. The search results were limited to human. All languages were searched and included. The references of all computer-identifiedpubli-cations were hand-searched for any additional studies,and the MEDLINE option related articles was used to identify other relevant articles. Studies were required to have measured aspirin resistance using validated laboratory methods described previously.Studies that definedaspirin resistance from a clinical perspective but did not confirmthis using laboratory methods were excluded from the analysis.

Data extraction

The primary search generated 35potentially relevant articles, of which 31met the inclusion criteria. Data for analysis were extracted independently and entered into separate databases.

Statistical analysis

Data were analysed using software for preparing and maintaining Cochrane reviews (ReviewManager, version 4.1; Cochrane Collaboration, Syracuse, NY, USA) and meta-analysis software (ComprehensiveMeta-analysis,version 2; Biostat, Englewood, NJ, USA). For each genetic marker (polymorphism)for which data were available for at least two studies, a meta-analysis was carried out. For each gene variant, a pooled odds ratio (OR)was calculated using fixed-and random-effects models, along with the 95%confidenceinterval (CI)to measure the strength of the genetic association. Fixed-effects summary ORs were cal-culated using the Mantel–Haenszelmethod [20,21]and the DerSimonian and Laird method was used to calculate random-effects summary ORs [22].

Tests for heterogeneity were performed for each meta-analysis (withsignificanceset at P

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subjects as aspirin resistant, diluting the apparent effect of the PlA1/A2polymorphism on aspirin resistance. Con-versely, light transmission aggregometry may be too insensitive in detecting aspirin resistance.Arachidonic acid aggregation directly measures the degree of inhibition of COX-1, whereas PFA-100measures the activation of platelets through ADP , collagen and epinephrine, path-ways that are not specificallyinhibited by aspirin. It could therefore be argued that PFA-100is a measurement of platelet activity rather than of aspirin resistance.

Variance between different measurements of aspirin resistance has been further confirmedby Lordkipanidze et al . [57].These authors investigated the prevalence of aspirin resistance in 201subjects with stable coronary artery disease. Prevalence of aspirin resistance varied greatly depending on the technique used, being relatively low with arachidonic acid-induced aggregation (4%)and VerifyNow Aspirin(6.7%),whereas it was very high with ADP-induced aggregation (51.7%)and PFA-100(59.5%).These findingsmake it especially difficultto ascertain whether the PlA1/A2polymorphism, and indeed other molecular variants, is associated with platelet activation or whether they are truly genetic determinants of the inhibi-tory effect of aspirin on platelets.

Analysis of four other polymorphisms, namely GPla (C807T),COX-1(A842G/C50T),P2Y12(H1/H2)and P2Y1(A1622G),revealed no apparent association with aspirin resistance. However, the number of studies and of subjects used was small, making it difficultto exclude definitivelyany contribution of these polymorphisms to aspirin resis-tance. Due to the inhibitory action of aspirin on COX-1, this would be the most obvious gene to study with regard to aspirin resistance. However, our analysis has provided little evidence for such an association. Recent studies by Frelinger et al . [18]and Meen et al . [58]have shown that aspirin resistance in a number of subjects may be indepen-dent of both COX-1and COX-2, although the precise mechanism is still unknown.

A recent systematic review by Krasopoulos et al . has concluded that aspirin-resistant patients are at greater risk of having further cardiovascular events than patients who respond to aspirin [59].Studies such as ours are needed to understand the mechanisms behind aspirin resistance, since such an understanding will help to address how excess cardiovascular risk can be reduced in aspirin-resistant patients.

Strengths and limitations of the study

Pooling all published data has maximized the statistical power of our study to detect the genetic association of aspirin resistance. Despite this, the number of subjects included within this systematic review is limited. This is because most studies of this type are small in size. The number of published papers also remains relatively low, as this is still a comparatively new research area. In addition, many studies could not be included within this statistical

Genetics of aspirin resistance

analysis,because insufficientdetailed information on geno-type frequency was given. These included nine studies of the PlA1/A2polymorphism. It also meant that a number of other polymorphisms could not be included within this analysis.This included the C893T SNP in the P2Y1receptor, documented by Li et al . [50]amongst others.

Furthermore, the optimal laboratory method to defineaspirin resistance has yet to be standardized. Until such time as a definitivetest can be established, it is difficultto determine with certainty the role of genes in aspirin resis-tance. The lack of standardization between the laboratory methods increases the heterogeneity and reduces the chances of findinga genetic association.When this is taken into account, the number of subjects in each analysis becomes smaller, also reducing the likelihood of determin-ing a genetic association with aspirin resistance. All results shown were analysed using the fixedeffects model. However, when the random effects model was applied, significancewas lost. This may be explained by the signifi-cant heterogeneity within the studies. Thus, larger and more robust studies are needed to truly understand whether the PlA2allele is a risk factor for aspirin resistance in healthy subjects.

At present,the number of studies published is too small to match according to gender, ethnicity, age and method-ology used to defineaspirin resistance [60].These factors will need to be taken into account in future studies in order truly to understand whether a genetic aetiology can partly explain the phenomenon of aspirin resistance.

The interpretation of any systematic review must be made within the context of its limitations, including study selection, publication bias, and variability in the method-ological quality of the included studies. Many of the indi-vidual studies included in our systematic review showed no statistical significanceand were interpreted by their authors as negative studies. In addition, a funnel plot on all included studies showed no substantial evidence of pub-lication bias in the fivepolymorphisms analysed, but clearly such bias cannot be completely excluded. There was no language restriction and meeting abstracts were included if found through the search strategy. Moreover, rigorous selection criteria (definitionof aspirin resistance and population included) enriched the meta-analyses for studies with comparable selection of participants. Thus, lack of specificityby the inclusion of studies with no clear definitionof aspirin resistance or the inclusion of studies in noncardiovascular diseases was avoided.

Conclusions

Our data support a possible genetic basis for the associa-tion between the PlA1/A2polymorphism and aspirin resis-tance in healthy subjects,with the effect diminishing in the presence of cardiovascular disease. However, further larger studies are needed to confirmour findings.These data strongly reinforce the argument that, in order truly to understand the genetic contribution to aspirin resistance,

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investigators need to agree on a standard technique to measure and defineaspirin resistance.

T.G. is funded by a studentship from the Biotechnology and Biosciences Research Council. A.F. receives funding from the British Heart Foundation. P.S. holds a UK Department of Health Senior Fellowship.

Competing interests:None declared.

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