Molecular genetics of the blood group I system and the regulation of I antigen expression during erythropoiesis and granulopoiesis

Purpose of review The molecular genetics of the blood group I system and the regulation mechanism for I antigen expression in postnatal red blood cells are intriguing. This review summarizes their elucidation and recent findings. Recent findings Accumulating data from the molecular analysis of individuals with the adult i phenotype supports the proposed molecular genetic mechanism for the partial association of the adult i phenotype with congenital cataracts. Recent investigations have shown that the regulation of I antigen formation during erythropoiesis is determined by transcription factor CCAAT/enhancer binding protein-alpha (C/EBP alpha) and the phosphorylation status of C/EBP alpha Ser-21 residue. Summary The human I locus is organized such that it has an uncommon genetic architecture and expresses three different I transcript forms. The results obtained from molecular analysis of two adult i groups, with and without congenital cataracts, demonstrate that the molecular background accounts for the partial association between these two traits and suggest that an I gene defect may lead directly to the development of congenital cataracts. Analysis of the regulation for I antigen expression shows that the regulation during erythropoiesis and granulopoiesis share a common mechanism, with dephosphorylation of the Ser-21 residue on C/EBP alpha playing the critical role.