I醣抗原轉化基因的突變與白內障的生成（3/3）

Abstract

The human i and I antigens are characterized as linear and branched repeats of N-acetyllactosamine, respectively. Conversion of the i to the I structure requires I-branching β-1,6-N-acetylglucosaminyltransferase activity. It has been noted that the null phenotype of I, the adult i phenotype, is associated with congenital cataracts in Asians. Previously, the identification of molecular changes in the IGnT gene, associated with the adult i phenotype, has been reported. In the present study, it has been demonstrated that the human I locus express three IGnT forms, designated IGnTA, IGnTB, and IGnTC, which have different exon 1, but identical exons 2 and 3, coding regions. The molecular genetics proposed for the I locus offers a new perspective of the formation and expression of the I antigen in different cells, and provides an insight into the questions derived from investigation of the adult i phenotype. Molecular genetic analyses of the I loci of the two adult i groups, with and without congenital cataracts, were performed, and enzyme function assays and expression patterns for the three IGnT transcripts in reticulocytes and lens-epithelium cells were analyzed. The results suggest a molecular genetic mechanism which may explain the partial association of the adult i phenotype with congenital cataracts, and indicating that a defect in the I locus may lead directly to the development of congenital cataracts. The results also suggest that the human blood group I gene should be re-assigned to the IGnTC form, not the IGnTB, as has previously been described.