Widespread Divergence of the CEACAM/PSG Genes in Vertebrates and Humans Suggests Sensitivity to Selection

Chang, Chia Lin; Semyonov, Jenia; Cheng, Po Jen; Huang, Shang Yu; Park, Jae Il; Tsai, Huai-Jen; Lin, Cheng-Yung; Grützner, Frank; Soong, Yung Kuei; Cai, James J.; Hsu, Sheau Yu Teddy

Carcinoembryonic antigen cell adhesion molecules (CEA-CAMs) and pregnancy-specific glycoproteins (PSGs; also known as Schwangerschafts Protein 1) are characterized by an N-terminal immunoglobulin variable domain-like region followed by a varied number of immunoglobulin constant domain-like structures. The prototypic member, CEA (or CEACAM5), was initially discovered as a membrane-anchored tumor antigen [1] . CEACAM subfamily proteins have since been shown to play important roles in the regulation of immune responses, angiogenesis, the differentiation of mammary glands, insulin signaling turnover, tumorigenesis, and metastasis [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] . By contrast, PSGs have been characterized as highly glycosylated major proteins from human syncytiotrophoblasts during pregnancy. High levels of human PSGs have been detected in maternal serum as early as 3 days postfertilization, coinciding with the attachment of the blastocyst to the uterine wall, and were believed to be critical for protecting the semiallotypic fetus from the maternal immune system during pregnancy in, at least, primates [13, 14, 15, 16, 17, 18, 19] .