Inherited structural and quantitative polymorphisms of C3b receptor (CR1) in normals and patients with glomerular diseases.

The erythrocyte C3b receptor (CR1) has been studied for its structural and quantitative polymorphisms in normal Indian individuals and in patients with glomerular diseases. In the normal Indian population, purification of CR1 by immunoprecipitation or C3b-Sepharose affinity column and subjecting it to electrophoresis showed the existence of two types of structural polymorphic patterns with M(r) of 190 kDa and 220 kDa, and with gene frequencies of 0.975 and 0.025, respectively. The gene frequencies of these alleles remain unaltered in the patient population. Evaluation of CR1 levels in the normal Indian population revealed a trimodal distribution of CR1 number suggesting a co-dominant allelic pattern (L and H alleles) for the quantitative expression of CR1 with gene frequencies of 0.523 and 0.477, respectively. In our earlier study we have shown that there is a decreased expression of CR1 on the erythrocytes of patients with acute glomerulonephritis. Since this decrease in the CR1 level in patients is an acquired characteristic, it may not be the level controlled by the LL homozygous alleles. The discrepancy in the gene frequencies of the structural and quantitative polymorphic alleles in normal individuals show that they are not linked to each other. In our earlier study, we showed that the affinity constant of C3b-CR1 binding in different individuals remains the same irrespective of the number of CR1 on the erythrocyte surface. Comparison of this result with the present investigation shows that there is no functional difference among various structural polymorphic forms of CR1 and the susceptibility to glomerular diseases is not associated with any of the CR1 polymorphic patterns.