The Impact of the Kozak Sequence Polymorphism of the Glycoprotein Ibα Gene on the Risk and Extent of Coronary Heart Disease

Glycoprotein Ib-alpha Kozak polymorphism in ischemic stroke

BACKGROUND

Recently, a T/C polymorphism in the Kozak sequence of glycoprotein Ib-alpha (GPIb-alpha) gene at position -5 from the initiator ATG codons, has been identified. The presence of -5C allele increases the surface expression of GPIb-IX-V complex in a gene dosage-dependent manner. It has been suggested that higher receptor levels might increase the adhesiveness of the platelets and confer risk for thrombosis. In this study, we aimed to investigate the association between GPIb-alpha Kozak polymorphism and ischemic stroke.

METHODS

We prospectively and consecutively recruited 231 patients (118 women and 113 men; mean age: 65 ± 14.2 years) with first ever ischemic stroke admitted to Istanbul Faculty of Medicine Edip Aktin Stroke Unit between April 2007 and June 2009. Demographic features, risk factors, clinical, and etiological subtypes were analyzed. As the control group, 220 unrelated healthy subjects were included.

RESULTS

We found that 156 patients had TT, 70 patients had TC, and 5 patients had CC genotype. At least one copy of C allele carriers were overrepresented in the ischemic stroke group (32.5%) compared with controls (23%) [odds ratio (OR): 0.61; 95% confidence interval (CI): 0.40-0.93; P = 0.03]. Among etiologic subtypes, the distribution of C allele carriers was the highest in patients with undetermined etiology (45%) and it was significantly higher than controls (OR: 0.36; 95% CI: 0.20-0.65; P = 0.0008). In other subtypes, there was no association with Kozak -5C allele.

CONCLUSION

In conclusion, these encouraging preliminary results show that GPIb-alpha T/C polymorphism might increase the risk of ischemic stroke, especially in those with undetermined etiology.

Platelet functional implications of glycoprotein Ibalpha polymorphisms in African Americans

The platelet glycoprotein Ibalpha is crucial in the binding of platelets to Von Willebrand Factor within areas of high stress. A single nucleotide polymorphism of GP Ibalpha gives rise to the Ko(a) (HPA-2b) and the -5C Kozak polymorphism. The presence of these polymorphisms has been associated with an increased risk for atherothrombotic disease. The Ko(a) polymorphism has been shown to have a higher prevalence in African Americans compared to American Caucasians. However, very little is known regarding any functional consequences of these platelet polymorphisms in African Americans. We assessed the prevalence of the Ko and -5C Kozak polymorphisms in a population of both African American and American Caucasian patients with and without CAD and determined whether there were platelet functional consequences in both groups. We studied 99 patients of which 22 were African American and 77 were American Caucasian. Aggregations were performed and shear induced platelet plug formation was tested using a platelet function analyzer. The HPA-2b allele was significantly higher in African Americans when compared to Caucasians (P = 0.001). Genotype frequencies of the -5C Kozak polymorphism were not significantly different between the two groups. We found no differences in platelet aggregation in African Americans who were either heterozygous or homozygous for the HPA-2b allele or the -5C Kozak allele when compared to American Caucasians of the same category. We found no significant differences in PFA-100 testing. We conclude from our study that these polymorphisms do not lead to altered platelet function in African Americans.

Genetics of platelet glycoprotein receptors: risk of thrombotic events and pharmacogenetic implications

Platelet aggregation and coronary thrombosis have a central role in the development of acute coronary syndromes and myocardial infarction (MI). Therapies aimed at inhibiting platelet aggregation have shown great benefit in individuals with coronary disease or with multiple risk factors for coronary disease. Genetic variation in platelet surface receptors mediating thrombus formation has been suggested to be associated with platelet hyperreactivity, with increased risk of MI and possibly with the benefit received from various antithrombotic drug treatments. This review focuses on discrepancies and their likely explanations in studies on platelet glycoprotein genetics. Current knowledge on important issues concerning coronary event phenotypes and pharmacogenetics is analyzed. Possible future applicability of these data to patient treatment is also discussed.

Genetic variability of platelet glycoprotein Ibalpha gene

Platelet membrane glycoprotein (GP) Ibalpha is a critical component of platelet adhesion complex to subendothelium structures following tissue injury or pathological surfaces, such as atherosclerotic plaques. Polymorphisms of the GPIbalpha gene have been associated with a high risk for occlusive vascular disease, and its distribution varies considerably among distinct populations. These polymorphisms comprise the human platelet antigen (HPA)-2 system, the -5C/T dimorphism of the Kozak sequence, and the variable number of tandem 39-bp repeats (VNTR). Here we report the prevalence of the GPIbalpha gene polymorphisms among Brazilians, a highly ethnically diverse population. We analyzed 492 subjects of European, African, or Indigenous origin. It was possible to determine ten distinct haplotypes. The most common ( reverse similar 40%) haplotype was the Kozak-TT/HPA-2aa/VNTR-CC for both Caucasian and African descent. However, among Indigenous, Kozak-TT/HPA-2aa/VNTR-CC and Kozak-TC/HPA-2aa/VNTR-CC were equally present. Although a strong linkage disequilibrium between VNTR and HPA-2 polymorphism had also been observed, here we determined incomplete linkage disequilibrium in 10% of subjects from all ethnic groups. VNTR-E, a rare variant lacking the 39-bp repeat, was identified in two unrelated subjects, and functional platelet studies revealed no abnormalities. The VNTR-A allele, the largest variant containing four copies of the repeats, was not identified in this population. However, homozygosity for the VNTR-A allele (Kozak-TT/HPA-2aa/VNTR-AA) was determined in two distinct species of nonhuman primates. These results suggest a greater complex evolutionary mechanism in the macroglycoprotein region of the GPIbalpha gene and may be useful in the design of gene-disease association studies for vascular disease.

Vienna Stroke Registry. Impact of the platelet glycoprotein Ib alpha Kozak polymorphism on the risk of ischemic cerebrovascular events

We performed a multicenter case-control study to evaluate the impact of the glycoprotein 1b alpha (-5)T/C Kozak polymorphism on the risk of ischemic cerebrovascular events. The genetic analysis in 1399 patients (745 men; median age, 70 years; interquartile ratio, 58-78) and 1066 control subjects (549 men; median age, 47 years; interquartile ratio, 39-59) was carried out with mutagenically separated polymerase chain reaction. Homozygous C/C genotype carriers had a 3.5-fold increased risk for ischemic cerebrovascular events (95% confidence interval, 1.5-7.9, P = 0.003) over T/T or T/C genotype carriers together. The effect was independent of well-established atherosclerotic risk factors. Our findings could be explained by the reported gene dose effect of the Kozak polymorphism on the density of the glycoprotein 1b alpha/IX/V receptors on platelets. According to our data, the (-5)C Kozak allele may represent a candidate genetic susceptibility factor for ischemic cerebrovascular events.

Platelet receptor and clotting factor polymorphisms as genetic risk factors for thromboembolic complications in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune mediated adverse reaction to heparin treatment often associated with limb- and/or life-threatening thromboembolic complications (TECs). Presently, no prognostic marker has been identified that allows differentiation between mildly (isolated thrombocytopenia) and severely (TECs) affected patients. This study assesses the impact of platelet glycoprotein- and clotting factor polymorphisms in HIT-patients with isolated thrombocytopenia compared to HIT-patients with TECs. Sixty-three HIT-patients with isolated thrombocytopenia and 79 HIT-patients with HIT-related TECs were genotyped for GPIIb-IIIa polymorphisms (HPA-1, HPA-3), GPIa-IIa polymorphisms (HPA-5, GPIaC807T), GPIb-IX-V polymorphisms (HPA-2, Kozak-5, VNTR), and clotting factor polymorphisms (FV-Leiden R506Q, prothrombin PT-G20210A and MTHFR C677T). Women more often presented with TECs than men (P = 0.04). No differences in genotype frequencies could be seen on comparing HIT-patients with and without TECs. Analysing men and women separately, the C allele of the Kozak polymorphism was overrepresented in men who developed TECs (P = 0.034). The enhanced risk of women to develop HIT-associated TECs remains unexplained but it is potentially important in view of recent data on sex-hormone related changes of haemostasis. There was no correlation between platelet glycoprotein- and clotting factor polymorphisms and the risk to develop HIT-associated TECs. An association between the development of TECs and the Kozak-5C allele could be seen among male patients. However, this would need to be assessed in further larger studies. Most likely, the high levels of thrombin generation during acute HIT are so procoagulant that less pronounced risk factors such as polymorphisms are overshadowed.

Glycoprotein Ib polymorphisms influence platelet plug formation under high shear rates

Platelet polymorphisms (Kozak, VNTR and HPA-2) within glycoprotein (GP)Ib alpha may be associated with an increased risk of arterial thrombosis. However, the functional role of these polymorphisms has not been clarified. Their influence on platelet plug formation under high shear rates was, therefore, examined in 233 healthy individuals. Collagen-adrenaline-induced closure time was shorter in carriers of the C/D versus C/C VNTR allele and in homozygotes with the (-5)T/T versus (-5)C/T Kozak genotype as determined by novel polymerase chain reaction methods. The HPA-2 genotype had no effects, and the density of GPIb alpha molecules was not influenced by GPIb alpha genotypes.

Association of C807T, PlA, and −5 C/T Kozak genotypes with density of glycoprotein receptors on platelet surface

This study investigates whether three platelet glycoprotein (GP) polymorphisms, C807T in GP Ia, Pl(A1/A2) in GP IIIa, and -5 T/C Kozak in GP Ibalpha gene, influence the density of the three important adhesion and activation receptors on the platelet surface. Fifty-four healthy donors were genotyped according to the three polymorphisms, and densities of the corresponding GPs were measured by flow cytometry. Our study confirmed the association between C807T polymorphism and platelet surface expression of GP Ia-IIa and GP Ia and demonstrated that the density of GP Ibalpha or GP IX is not associated with the Kozak polymorphism. Although the Pl(A1/A2) polymorphism did not affect the expression of GP IIb-IIIa and GP IIIa on the platelet surface, flow-cytometric analysis employing murine monoclonal antibody SZ21 against GP IIIa can be applied to distinguish Pl(A1/A1) and Pl(A1/A2) polymorphism.

The influence of platelet glycoprotein polymorphisms on receptor function and risk for thrombosis

With regard to hemostasis and thrombosis, collagens are among the most important physiologic components of the extracellular matrix in their role as platelet activators. Differences in the rate of platelet activation markedly influence normal hemostasis and the pathological outcome of thrombosis. Thus, collagen receptors, such as the integrin alpha2beta1 and indirectly, GPIb alpha, represent a relatively unexploited target of pharmacological control and are only recently becoming appreciated as potential factors in the generic risk for thrombosis. In general, the importance platelet glycoprotein polymorphisms as genetic risk factors for arterial thrombosis is a new area of human genomics that needs to be carefully addressed. Discrepancies in the degree to which they are reported to contribute to risk for clinical thrombosis will only be resolved once there is a universal standard for clinical study design. Most of the clinical studies differ by patient population size, ethnicity, bias in the selection of patients and controls, plurality in clinical endpoints and variation of environmental factors. Despite these differences, there is substantial evidence that the integrin beta3 PlA2 haplotype, the GPIb alpha Met145 haplotype, the GPIb alpha -5C haplotype and the integrin alpha2 haplotype 1 (807T) each contribute to the risk for and morbidity of thrombotic disease. There may remain dispute as to the extent of their contribution. However, well-designed, large, prospective, genetic and epidemiologic studies are needed to clarify the role of these and other platelet receptor polymorphisms. Most importantly, the cumulative effects of multiple platelet and plasma glycoproteins SNPs to thrombotic risk must be evaluated concurrently. Additional in vitro studies of the functional relevance underlying these polymorphisms are needed to provide a sound biological explanation for the results of clinical correlations. The opportunity now exists to make significant inroads into the development of strategies for the prevention of thrombotic disease.