Paternal monoallelic expression of PEG3 in the human placenta

Genomic imprinting is the phenomenon whereby mono-allelic expression of cer tain genes occurs depending on their parental origin, The observation that imprinting only occurs in placental mammals has led to the suggestion that it may play a role in this form of reproduction, In the present study we ha ve investigated the pattern of expression of the human PEGS gene in the ear ly to term placenta, as well as the uterus and ovary, using RT-PCR, norther n blot and in situ hybridization, A comparison is made with the expression of Peg3 in the mouse by histochemical staining in beta geo knock out mice, We have demonstrated high levels of PEG3 in the human placenta and have loc alized the signal to the layer of villous cytotrophoblast cells, In contras t, the pattern of expression of Peg3 in the mouse placenta is less restrict ed, the message being present in all trophoblast populations. Thus, express ion of PEG3/Peg3 in the human and mouse placenta is not directly comparable . We have also detected PEGS message in the ovarian stroma, We have sequenc ed the human PEGS gene from exon 3 to exon 9, By utilizing a polymorphism d etected in exon 9, we have established that only the paternal allele is exp ressed in human placenta, Human PEGS is therefore maternally imprinted as i n mouse.