Genetic and structural polymorphism of complement receptor 1 in normal Indian subjects

The human complement receptor 1 (C3b/C4b receptor, CD35, CR1), a polymorphic membrane bound glycoprotein, is differentially expressed on erythrocytes, eosinophils, monocytes, B and T-lymphocytes, dendritic cells and kidney podocytes. It also occurs in the plasma as soluble CR1 (sCR1) and in urine as urinary CR1 (uCR1). Different population studies have suggested the functional and physiological significance of the structural (CR1-A/190, CR1-B/220, CR1-C/160 and CR1-D/250 kDa) and genomic (HH, high erythrocyte CR1 expression, HL, intermediate and LL, low expression) polymorphisms in health and disease. However, simultaneous study on the structural and genomic polymorphism in the same group of study subjects is lacking. This is the first study on both quantitative and structural polymorphism in 101 healthy volunteers from different parts of India by random sampling. In our study, AA phenotype was found to be expressed in 84.2% of individuals and 14.8% carried AB phenotype. One individual (0.9%) was found to possess BB phenotype. Homozygous BB pattern was identified for the first time in Indian subjects. The relative gene frequencies for A and B allele were found to be 0.916 and 0.084, respectively. Pertaining to quantitative polymorphism, percentage distribution for HH, HL and LL phenotypes was found to be 23.7, 54.45 and 21.79%, respectively and the gene frequencies were 0.51 and 0.49 for H and L allele, respectively. The observations for quantitative as well as structural polymorphism showed a good probability of fitness with Hardy-Weinberg equilibrium, thus proving that both the types of CR1 polymorphic forms are encoded by autosomal co-dominant alleles. We found a higher frequency of HL and AA phenotypes in the study subjects. Our findings are unique as we found that gene frequencies for structural and quantitative polymorphism in our study subjects were a combination of those found in Caucasian and Oriental populations.