Molecular studies of trophoblast HLA-G: polymorphism, isoforms, imprinting and expression in preimplantation embryo

There is considerable interest in human HLA-G arising from the observation that it is expressed selectively on the surface of extravillous trophoblast, the fetal cell population directly in contact with the mother. We investigated several aspects of the molecular biology of this unusual molecule. Limited polymorphism at the nucleotide level, and even more restricted variation at the amino acid level, was found in our Caucasian population. A further unusual aspect of HLA-G is the occurrence of alternatively spliced mRNAs. Spliced messages that could give rise to either membrane-bound or soluble proteins have been reported and six of these alternative forms were detected in all first trimester and term placentae, highly purified villous and extravillous trophoblast and the cell lines, JEG-3 and 221-G. An additional novel splice variant involving loss of part of the 3'-untranslated region was observed with two alleles. Using a sensitive RNase protection assay higher levels of the membrane-bound RNAs as compared to the soluble forms were detected in first trimester and term placentae as well as in JEG-3. Contrary to previous findings our term samples taken from the maternal aspect showed higher levels of both mRNA species when compared to first trimester placenta. The question of imprinting was addressed through the detection of heterozygotes both in placental tissue and, more tellingly, in the purified trophoblast cells. There was no evidence of imprinting. In addition we did not find mRNA for HLA-G in human two to eight-cell embryos or in blastocyst or in sperm samples.