Differences in tissue-specific and embryonic expression of mouse Ceacam1 and Ceacam2 genes

The intercellular adhesion molecule CEACAM1, also known as C-CAM1 (where CAM is cell-adhesion molecule), can function as a tumour suppressor in several carcinomas, including those of the prostate, breast, bladder and colon. This suggests that CEACAM1 may play an important role in the regulation of normal cell growth and differentiation. However, there is no direct evidence to support this putative function of CEACAM1. To elucidate its physiological function by targeted gene deletion, we isolated the Ceacam genes from a mouse 129 Sv/Ev library. Although there is only one Ceacam1 gene in humans and one in rats, two homologous genes (Ceacam1 and Ceacam2) have been identified in the mouse. Our sequence analysis revealed that the genes encoded nine exons and spanned approx. 16-17 kb (Ceacam1) and 25 kb (Ceacam2). The genes were highly similar (79.6%). The major differences in the protein-coding regions were located in exons 2, 5 and 6 (76.9%, 87.0% and 78.5% similarity respectively). In addition, introns 2, 5 and 7 were also significantly different, being 29.7%, 59.8% and 64.5% similar respectively. While most of these differences were due to nucleotide substitutions, two insertions of 418 and 5849 bp occurred in intron 2 of Ceacam2, and another two insertions of 1384 and 197 bp occurred in introns 5 and 7 respectively. To determine whether functional redundancy exists between Ceacam1 and Ceacam2, we examined their expression in 16 mouse tissues by using semi-quantitative reverse transcription-PCR. As in human and rat, in the mouse Ceacam1 mRNA was highly abundant in the liver, small intestine, prostate and spleen. In contrast, Ceacam2 mRNA was only detected in kidney, testis and, to a lesser extent, spleen. Reverse transcription-PCR using testis RNA indicated that Ceacam2 in the testis is an alternatively spliced form containing only exons 1, 2, 5, 6, 8 and 9. In the mouse embryo, Ceacam1 mRNA was detected at day 8.5, disappeared between days 9.5 and 12.5, and re-appeared at day 19. On the other hand, no Ceacam2 mRNA was detected throughout embryonic development. The different tissue expression patterns and regulation during embryonic development suggest that the CEACAM1 and CEACAM2 proteins, although highly similar, may have different functions both during mouse development and in adulthood.