Molecular studies of trophoblast HLA-G: polymorphism, isoforms, imprintingand 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 investi gated several aspects oi the molecular biology of this unusual molecule. Li mited polymorphism at the nucleotide level, and even more restricted variat ion at the amino acid level, was found in our Caucasian population A furthe r 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 detecte d in all first trimester and term placentae, highly purified villous and ex travillous trophoblast and the cell lines, JEG-3 and 221-G. An additional n ovel splice variant involving loss oi part of the 3'-untranslated region wa s observed with two alleles, Using a sensitive RNase protection assay highe r levels of the membrane-bound RNAs as compared to the soluble forms were d etected in first trimester and term placentas as well as in JEG-3. Contrary to previous findings our term samples taken from the maternal aspect showe d higher levels of both mRNA species when compared to first trimester place nta, The question of imprinting was addressed through the detection of hete rozygotes both in placental tissue and, more tellingly, in the purified tro phoblast 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.