A unique gene encodes spliceoforms of the B-cell adhesion molecule cell surface glycoprotein of epithelial cancer and of the Lutheran blood group glycoprotein

Abstract

Two new members of the Ig superfamily, the Lutheran (Lu) blood group glycoprotein and the B-cell adhesion molecule (B-CAM) epithelial cancer antigen, have been recently cloned from human placenta and colon cancer HT29 cell line, respectively. Although amino acid sequences deduced from cDNA analysis suggested that B-CAM should represent an abridged form of the Lu glycoprotein lacking the last 40 amino acids of the putative cytoplasmic tail, the relationship between the genes encoding these polypeptides has not been determined. In the present report, we showed by Southern blot analysis that the Lu and B-CAM cDNAs derived from a unique LU gene which exhibited an HindIII RFLP associated with the Lua/Lub blood group polymorphism. Accordingly, in situ hybridization of the Lu cDNA probe confirmed the localization of the Lutheran blood group locus to chromosome 19 q13.2-13.3, as previously shown for a B-CAM DNA probe. Sequence comparison between cDNA and genomic PCR fragments indicated that the Lu and B-CAM transcripts previously isolated are generated through the alternative use of internal splice donor and acceptor sites within an exon located at the 3' end of the LU gene. These spliceoforms corresponded to 2.5 kb and 4.0 kb mRNA species detectable by Northern blot in all tissues and cell lines in which the LU gene is expressed; their primary structures are consistent with the presence of both the Lu and B-CAM antigens on two glycoprotein isoforms. However, the 4.0 kb transcript was very poorly expressed as compared to the 2.5 kb species except in the colon carcinoma HT29 cell line, suggesting a differential regulation of the Lu/B-CAM messenger RNA in some tumor tissues.