Structure and evolution of the mouse pregnancy-specific glycoprotein (Psg) gene locus

Background: The pregnancy-specific glycoprotein (Psg) genes encode
proteins of unknown function, and are members of the
carcinoembryonic antigen (Cea) gene family, which is a member of
the immunoglobulin gene (Ig) superfamily. In rodents and primates,
but not in artiodactyls (even-toed ungulates/hoofed mammals), there
have been independent expansions of the Psg gene family, with all
members expressed exclusively in placental trophoblast cells. For
the mouse Psg genes, we sought to determine the genomic
organisation of the locus, the expression profiles of the various
family members, and the evolution of exon structure, to attempt to
reconstruct the evolutionary history of this locus, and to
determine whether expansion of the gene family has been driven by
selection for increased gene dosage, or diversification of
function. Results: We collated the mouse Psg gene sequences
currently in the public genome and expressed-sequence tag (EST)
databases and used systematic BLAST searches to generate complete
sequences for all known mouse Psg genes. We identified a novel
family member, Psg31, which is similar to Psg30 but, uniquely
amongst mouse Psg genes, has a duplicated N1 domain. We also
identified a novel splice variant of Psg16 (bCEA). We show that
Psg24 and Psg30/Psg31 have independently undergone expansion of
N-domain number. By mapping BAC, YAC and cosmid clones we described
two clusters of Psg genes, which we linked and oriented using
fluorescent in situ hybridisation ( FISH). Comparison of our Psg
locus map with the public mouse genome database indicates good
agreement in overall structure and further elucidates gene order.
Expression levels of Psg genes in placentas of different
developmental stages revealed dramatic differences in the
developmental expression profile of individual family members.
Conclusion: We have combined existing information, and provide new
information concerning the evolution of mouse Psg exon
organization, the mouse Psg genomic locus structure, and the
expression patterns of individual Psg genes. This information will
facilitate functional studies of this complex gene family.