In separate analyses of each PSEL polymorphism, we found the 290NN homozygous genotype to be a significant predictor of CHD in whites, but not in African-Americans. No significant findings were observed for ischemic stroke in either whites or African Americans. PSEL haplotype analyses in whites resulted in eight common haplotypes that accounted for 99.8% of all haplotypes observed. The NNVP haplotype was associated with increased risk of incident CHD in whites. No significant haplotype findings were observed in African-Americans.
Tregouet and colleagues performed haplotype analyses using a maximum likelihood method in two European populations (Belfast and France) comprised of the same four PSEL polymorphisms investigated in the current study [23
]. They found the simultaneous presence of the 290N and N562 alleles to be associated with a significantly increased risk of MI in both the Belfast and French populations (OR=2.84 and 2.09, respectively). However, they found this to be significant only on the NNVT haplotype. In the current study, we found the NNVT haplotype to be associated, non-significantly, with an increased risk of CHD (HRR=1.15). Furthermore, in Tregouet's study, the NNVP haplotype was only observed in the French population and was associated, non-significantly, with an increased risk of MI (OR=1.59). It is hypothesized that the S290N and N562D polymorphisms, both located within the consensus repeat domain of the PSEL protein, may affect the binding of PSEL to PSGL-1 on leukocytes, resulting in a protein that is more efficient at recruiting leukocytes to the endothelium [23
]. We further hypothesize that this effect on PSEL/PSGL-1 binding is additionally influenced by the presence of the T715P polymorphism, also located in the consensus repeat domain of PSEL. Functional studies investigating the implications of these polymorphisms on PSEL/PSGL-1 binding have not been performed.
For independent analyses of each PSGL-1 polymorphism, we found the 62II homozygous genotype to be a significant predictor of incident CHD, while the 62MI heterozygous genotype was a significant predictor of ischemic stroke in African Americans, but not in whites. Borderline significance was observed for both PSGL-1 haplotypes carrying the 62I allele (GI and AI haplotypes) for ischemic stroke in African Americans. These results suggest the M62I polymorphism to independently contribute to disease risk regardless of the haplotype on which it is carried.
The most commonly studied genetic variation in the PSGL-1 gene is a variable number of tandem repeats (VNTR) shown to affect the binding efficacy of PSEL to PSGL-1 and leukocyte-platelet interaction [25
]. Lozano and colleagues demonstrated that the shorter VNTR alleles resulted in less efficient binding and were associated with a two-fold lower risk for cerebrovascular disease, but not CHD [25
]. The PSGL-1 M62I polymorphism is positioned upstream of this VNTR, just bordering the PSEL binding domain. Although the functional consequence of the M62I polymorphism is unknown, the proximity of this variant to the binding domain suggests that it too may have an effect on PSEL binding [14
]. To our knowledge, the PSGL-1 M62I polymorphism has not been studied with regards to stroke, and only two case-control studies have investigated the PSGL-1 M62I polymorphism with regards to CHD: one study showed no association [14
] while the other study showed the 62I allele to be protective against CHD [24
]. Our finding of an inverse relationship between the number of 62I alleles and risk of incident CHD and ischemic stroke is consistent with the suggestion that this polymorphism results in lower PSEL/PSGL-1 binding capacity, thus resulting in fewer leukocyte-endothelium and leukocyte-platelet complexes. Although we did not find a significant association of the 62II homozygous genotype in whites, the HRRs for both incident CHD and ischemic stroke showed a similar trend in the inverse direction (HRR 0.72 and 0.64, respectively). The lack of a significant association in whites may be due to the small number of persons with the 62II homozygous genotype in this racial group (N=43).
As is commonly observed in genetic association studies investigating different racial/ethnic groups, we detected associations with disease status in one racial group and not another. In the current study, the PSEL S290N polymorphism was associated with incident CHD in whites but not African Americans, while the PSGL-1 M62I polymorphism was associated with both incident CHD and stroke in African Americans but not whites. Possible explanations for these race-specific findings include the marked variant allele frequency difference between whites and African Americans for PSEL S290N (0.33 and 0.18, respectively) and PSGL-1 M62I (0.06 and 0.27, respectively) may relate to discrepancies in underlying genetic susceptibility to disease with regards to cell adhesion molecules. Additionally, cell adhesion molecules may be more important in the development of disease in one racial group, with other factors acting through different mechanisms being more important in the other group [37
]. Moreover, we cannot determine by our study if the polymorphisms studied have a direct causative effect; they may be in linkage disequilibrium with the true functional variant(s). Owing to the fact that linkage disequilibrium patterns are different between whites and African Americans, if we are not investigating the true functional variant, we may detect associations with ‘marker’ polymorphisms in one race and not another. We must also acknowledge that our results may be due to chance findings when one considers multiple testing and that significant findings were not consistent between racial groups. Therefore, results should be taken in context and note that all findings have been stated to be suggestive.
Results from the current study suggest that the PSEL S290N polymorphism, most notably on a haplotypic background with the N562, V599 and 715P alleles, to be a predictor of incident CHD in whites, while the PSGL-1 M62I polymorphism was determined to be a predictor of both incident CHD and ischemic stroke in African Americans. Possible mechanism by which these polymorphisms may affect disease risk is by altering the binding affinity of PSEL to PSGL-1, thus attenuating/increasing the recruitment and adhesion of leukocytes and platelets to the endothelium wall and/or reducing/enhancing leukocyte-platelet aggregates. Further studies are warranted to replicate the current findings in other large US populations of whites and African Americans and to determine the functional consequence of the PSGL-1 M62I polymorphism, the PSEL S290N polymorphism and the PSEL NNVP haplotype.