Pl^A2 WHITE PAPER: Pl^A2 Test: Introduction to Clinical Practice.

Pascal J. Goldschmidt, July 2000

The predisposition to developing coronary artery disease and consequent thromboembolic complications is genetically determined. It is now believed that each of us is born with a specific set of genes that define our susceptibility to develop the most prevalent condition in the western world, coronary heart disease (CHD). This is not to say that environmental factors do not have a substantial impact on the disease process, but instead, in a given set of environmental conditions, such as cigarette smoking and antecedent of casual infections like chlamydia and cytomegalovirus, an individual's susceptibility to develop CHD is defined at the genetic level. Unlike disease processes that are driven by a major gene such as Duchenne's muscular dystrophy, which develops as a consequence of defects in the dystrophin gene, complex traits like CHD are controlled by a multitude of gene variants, whose individual impact on the disease process is rather small. As a consequence, the linkage of the CHD trait to a specific locus of the genome would not be possible, except in rare situations like in familial hypercholesterolemia and in homozygous homocystinuria, where the defective gene has such a powerful impact on the disease process that it can be traced readily. However, for most patients with CHD, such a link cannot be established.

The most common variations in the human genome from individual to individual correspond to the single nucleotide polymorphism (Snps). The switch of one base pair for another is a common occurrence and its frequency has been found to be approximately one per thousand base pair. Quite often, such variants can occur within a non-coding region of the genome and outside from the regulatory domain of any gene. While the impact of such variant, if any, has been difficult to establish, it could be related to the superstructure of the genome and in particular its interaction with regulatory proteins such as histones and enzymes. Alternatively, Snp can occur within the coding region of the gene or within the regulatory (promoter) domain of the gene. On occasion, variants occurring within the coding region of the gene can induce a missense polymorphism where one aminoacid is switched for another.

The Pl^A2 variant of GPllb-IIIa is a paradigm for missence polymorphisms that can change not only the structure of the protein whose gene they affect, but also, to a certain extent, the function of the protein. Pl^A2 induces a switch from a leucine to a proline, an aminoacid 33 of the GPIIIa subunit of the fibrinogen and von Willebrand factor receptor. This switch has been associated with a gain of antigenicity, and was found to be the cause of a severe form of neonatal thrombocytopenia. We rediscovered the Pl^A2 polymorphism while studying the siblings of patients with evidence of coronary artery disease prior to age 60. We also reported on the high prevalence of Pl^A2 polymorphism in patients admitted to a coronary care unit because of complex thromboembolic complications in the context of acute coronary syndromes. Since our reports, the effect of the Pl^A2 polymorphism on the function of GPIIb-IIIa has been carefully investigated. The Pl^A2 polymorphism appears to increase platelet responsiveness, perhaps by strengthening the interaction between GPIIb-IIIa and extracellular ligands. Taken together these data suggested that the Pl^A2 polymorphism might be a gene variant that impacts the clinical manifestations of athersclerosis and in particular, coronary thromboembolic complications. Consistent with this finding was the fact that young individuals who died of sudden death due to coronary thrombosis are found to have a markedly elevated prevalence of the Pl^A2 polymorphism.

While in some conditions the Pl^A2 test might be helpful in predicting individuals at risk for premature coronary thrombosis, it seems at this point premature to recommend the test in the absence of a large prospective clinical trial to confirm the association between the Pl^A2 polymorphism and sudden death.

It does appear, however, that the Pl^A2 test could identify patients at increased risk for stent thrombosis following angioplasty with stent placement. Thus, patients who are homozygous for the Pl^A2 polymorphism, have a markedly increased risk of stent thrombosis. Furthermore, in the absence of anti-platelet therapy with at least a combination of aspirin and a thienopyridine derivative, even heterozygous Pl^A2 patients appear to be at risk for stent thrombosis. In a large trial, it was also shown that Pl^A2 positive patients are at increased risk for restenosis following angioplasty. There is currently a lack of scientific information necessary for the design of anti-thrombolic therapy following angioplasty. Quite often, patients are treated during the angioplasty with a combination of anti-platelet drugs that include aspirin, thienopyridine, and with a highly variable frequency, a parenteral GPIIb-IIIa blocker. Following the intervention, the patients are maintained on a combination of aspirin and thienopyridine for a period of four-to-six weeks, at which point, the patients are switched to aspirin as their only anti-thrombotic drug treatment. The dose of aspirin also varies substantially from patient-to-patient, anywhere between 81mg to 325mg and sometimes 650mg. It is highly likely that some patients would benefit from prolonged anti-thrombotic therapy that includes aspirin and a thienopyridine derivative (clopidogrel most likely considering the worst side effects of ticlopidine). However, there is no available mechanistic approach that would allow the clinician to select effectively the patients that should be maintained on the combination of aspirin and clopidogrel for extended periods of time.

We propose to use the Pl^A2 polymorphism as a means to determine which patients should be maintained on the aspirin/clopidogrel combination. Thus, patients with a positive Pl^A2 test (either homozygous or heterozygous patients) should be maintained on the aspirin/clopidogrel combination for at least six months. Patients who have a negative Pl^A2 test could discontinue their anti-throbotic treatment after six weeks as has been the routine. The hope with such an approach is not only to reduce acute thromboembolic complications in Pl^A2 patients, but reduce their heightened risk of restenosis. Now, it is clear that even the PLA1/A1 might benefit from prolonged combination of antiplatelet therapy. However, if the standard of care were to discontinue clopidogrel after four-to-six weeks, we would recommend using the Pl^A2 test to identify patients that are most likely to benefit from sustained combination therapy.

Finally, we would also recommend to use the Pl^A2 test to define the optimal dose of aspirin in patients. Patients with a positive Pl^A2 test appear to be more sensitive to aspirin than patients with a negative Pl^A2 test. Therefore, the Pl^A2 test can be used to choose wisely the appropriate dose of aspirin required for individual patients. In secondary prevention, patients with a positive Pl^A2 test would be recommended aspirin at a dose of 160mg qd, while patients with negative Pl^A2 test would be advised to take 325mg qd. The use of the Pl^A2 test for the dosage for aspirin as a means of primary prevention will require a large scale trial to establish its effectiveness.

In conclusion, the Pl^A2 test detects a missence polymorphism in the key receptor on platelets, GPIIb-IIIa, that controls thromboembolic complications in patients with acute coronary syndromes. The Pl^A2 polymorphism increases thrombolic diathesis within coronary vessels, but its overall power as a risk for myocardial infarction is limited by the fact that many myocardial infarctions occur in the absence of a large thrombus. In contrast, the Pl^A2 test can be helpful in the selection of anti-thrombotic regimens for patients undergoing stent angioplasty of a coronary vessel, and in secondary prevention. Thus, patients with a positive Pl^A2 test should be maintained on the combination of clopidogrel and aspirin for an extended period of time beyond the usual four-to-six weeks, and probably for a period of six months to a year. Moreover, for those laboratories that do not use routinely a GPIIb-IIIa blocker (abciximab, aggrastat or eptafibitide) while performing angioplasty with stent placement, the Pl^A2 test would definitely recognize patients at very high risk for acute stent thrombosis, i.e. the Pl^A2 homozygous patients. These patients, particularly when other risk factors for acute thromboembolic complications are present such as diabetes, female gender, or old age, have been shown to have a five-to-six fold increase risk of stent thrombosis, and therefore, should be protected with a GPIIb-IIIa blocker when undergoing angioplasty with and without stent placement. Finally, as mentioned above, the Pl^A2 test may be helpful in selecting the right dose of aspirin in secondary prevention for coronary artery disease.

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