GENOMIC ORGANIZATION, SPLICE VARIANTS AND EXPRESSION OF CGM1, A CD66-RELATED MEMBER OF THE CARCINOEMBRYONIC ANTIGEN GENE FAMILY

The tumor marker carcinoembryonic antigen (CEA) belongs to a family of proteins which are composed of one immunoglobulin variable domain and a varying number of immunoglobulin constant-like domains. Most of the membrane-bound members, which are anchored either by a glycosylphosphatidylinositol moiety or a transmembrane domain, have been shown to convey cell adhesion in vitro. Here we describe two splice variants of CGM1, a transmembrane member of the CEA family without immunoglobulin constant-like domains. CGM1a and CGM1c contain cytoplasmic domains of 71 and 31 amino acids, respectively. The cytoplasmic region of CGM1a is encoded by four exons (Cyt1-Cyt4). Differential splicing of the Cyt1 exon (53 bp) leads to the formation of CGM1c. The presence or absence of potential protein kinase phosphorylation sites in the cytoplasmic domains and a sequence consensus motif involved in signal transduction in multichain immune recognition receptors indicates that this splice event is of functional importance. CGM1a mRNA, the predominant CGM1 transcript, was found in the granulocytic lineage, but not in monocytes, lymphocytes nor in a number of tumors derived from all three germ layers. Weak staining using monoclonal antibodies Tu2 and 73 in fluorescence-activated cell scan analyses indicate low concentrations of CGM1 protein on the surface of granulocytes. The CGM1 protein is also recognized by CD66 antibodies. Therefore, the granulocyte-specific CD66 epitope is present on at least four CEA family members: CGM1, CEA, NCA-50/90 and NCA-160.