Human epithelial tumor antigen cDNA sequences. Differential splicing may generate multiple protein forms.

The isolation and characterization of complementary DNAs (cDNAs) which code for an epithelial antigen aberrantly expressed in human breast tumor tissue are described here. The only information regarding the primary structure of this potentially important antigen has been a 20-amino-acid repeat motif. We now report the complete amino acid sequences of different forms of the human epithelial tumor antigen as deduced from the nucleotide sequence of isolated non-repeat cDNAs. The diversity of protein forms is generated by a series of alternative splicing events that occur in the regions located upstream and downstream to a central tandem repeat array. Isolated cDNAs coding for the upstream region show that differential usage of alternative splice acceptor sites may generate two protein forms containing putative signal peptides of varying hydrophobicities. The complexity of possible antigen forms is further compounded by alternative splicing events occurring in the region 3' to the repeat array. The isolated cDNAs 3' to the tandem repeats indicate that whereas one mRNA transcript is colinear with the gene, and defines an open reading frame (ORF) containing 160 amino acids downstream to the repeat array, a second cDNA correlates with a mRNA that is generated by a series of splicing events. The deduced amino acid sequence of the spliced cDNA contains an ORF that is identical for 149 amino acids downstream to the repeat array with the amino acid sequence of the unspliced cDNA. At this point it diverges and continues for an additional 179 amino acids. The sequence contains a highly hydrophobic 28-amino-acid peptide, located towards the carboxyl terminus, that may correspond to a transmembrane region. The cDNAs and deduced amino acid sequences, presented here, define the complete amino acid sequences of the epithelial tumor antigen and demonstrate the existence of multiple protein forms that probably localize to different cellular and extracellular compartments.