Allelic diversity at the primateMhc-G locus: Exon 3 bears stop codons in allCercophitecinae sequences

Abstract

Twenty-seven major histocompatibility complex (Mhc)-G exon 2, exon 3, and exon 2 and 3 allelic sequences were obtained together with 12 different intron 2 sequences. Homo sapiens, Pan troglodytes, Pan paniscus, Gorilla gorilla, Pongo pygmaeus, Macaca fascicularis, Macaca mulatta, and Cercopithecus aethiops individuals were studied. Polymorphism does not follow the classical pattern of three hypervariable regions per domain and is found in all species studied; exon 3 (equivalent to the alpha 2 protein domain) shows stop codons in the Cercopithecinae group but not in the Pongidae and human groups. Dendrograms show that cotton top tamarin (Saguinus oedipus) Mhc-G sequences are closer to Homo sapiens and Pongidae than to Cercopithecinae, probably due to the stop codons existing at exon 3 of the latter. There is a clear trans-species evolution of allelism in Cercopithecinae and also in exon 2 of all the other apes studied, but a generation of allelism within each species may be present on exon 3 sequences. This discrepancy may be due to the preferential use of exon 2 over exon 3 at the mRNA splicing level within each species in order to obtain the appropriate functional G product. Mhc-G intron 2 shows conserved motifs in all species studied, particularly a 23 base pair deletion between positions 161 and 183 which is locus specific, and some of the invariant residues, important for peptide presentation, conserved in classical class I molecules from fish and reptiles to humans were not found in Mhc-G alleles; the intron 2 dendrogram also shows a particular pattern of allelism within each species. In summary, Mhc-G has substantial differences from other classical class I genes: polymorphism patterns, tissue distribution, gene structure, splicing variability, and probably an allelism variability within each species at exon 3. The G proteins may also be different. This indicates that the Mhc-G function may not be peptide presentation to the clonotypic T-cell receptor.

DNA Archives and Our Nearest Relative: The Trichotomy Problem Revisited

“…Compared to the classical class I loci, Mhc-G is less polymorphic and is restricted in its expression to the extravillous trophoblast (Kovats et al, 1990). Although there is some evidence for transspecies polymorphism at this locus (Castro et al, 1996), it has not been well documented that the polymorphism is maintained by balancing selection. Since there is only one informative site, there is no reason to exclude this locus from the analysis.…”

Placental expression of the nonclassical MHC class I molecule Mamu-AG at implantation in the rhesus monkey

“…There is close homology of implantation and placentation among the Old World primates (35). The rhesus monkey MHC contains orthologs of the human classical MHC class I loci Mamu-A and -B, and the nonclassical loci Mamu-E, -F, and -G (14,20,25,(36)(37)(38). The identification of a novel MHC class I locus expressed in the rhesus monkey placenta that shares unique molecular characteristics of HLA-G (14) and has a tissue distribution closely matching that seen for HLA-G (16) provides a unique opportunity for the study of nonclassical MHC class I molecules in the primate placenta.…”

Mammalian non-classical major histocompatibility complex I and its receptors: Important contexts of gene, evolution, and immunity

“…MHC-G has been described in some nonhuman primates. [2728293031] It has been mentioned that in chimpanzee ( Pan troglodytes ) MHC-Ib is known to be organized in similar way as human MHC-Ib. [32] It has also been described in case of many other nonhuman primate species.…”

“…[31] Phylogenetic study comprising exon 2, exon 3, and intron 2 sequences of MHC-G of 7 nonhuman primates along with HLA-G have shown that cotton top tamarin ( Saguinus oedipus ) MHC-G sequences are more closer to human and great apes (Pongids). [30]…”

“…Similar type of observation have been noticed in phylogenetic analyses in earlier studies of MHC-F and MHC-G of human and nonhuman primates. [3037] In addition, gene sequences analysis of nonclassical MHC-I revealed that there are around 34–90% similarity in case of MHC-G, 53–98% similarity for MHC-E, whereas among MHC-F and MHC-H or HFE have 48–87% and 54–98% similarity, respectively. Similar observation has been reported for MHC-H gene of human and nonhuman primates.…”