Study of the Retrotransposon-Derived Human PEG10 Protease ^†

Paternally expressed gene 10 (PEG10) is a human retrotransposon-derived imprinted gene. Previous works have demonstrated that a mutation in the coding sequence of this gene is lethal with regard to embryological age due to defects of placental development. In addition, PEG10 is implicated in several malignancies, such as pancreatic cancer and hepatocellular carcinoma. The PEG10 gene encodes two protein isoforms, which are translated by a typical retroviral frameshift mechanism. The Gag-like protein (RF1_PEG10) is encoded by reading frame 1, whilst reading frames 1 and 2 accounts for the Gag-Pol-like polyprotein (RF1/RF2_PEG10). The protease (PR) domain of RF2_PEG10 contains an -Asp-Ser-Gly- sequence, which refers to the conservative -Asp-Ser/Thr-Gly- active-site motif of retroviral aspartic proteases. The function of the aspartic protease domain of RF2_PEG10 remains unclear. In order to further investigate the function of the PEG10 protease (PR_PEG10), a frameshift mutant was generated (_fsRF1/RF2_PEG10) for comparison with the RF1/RF2_PEG10 form. To study the effects of PR_PEG10 on cellular proliferation and viability, mammalian HEK293T and HaCaT cells were transfected with plasmids encoding for either the frameshift mutant (_fsRF1/RF2_PEG10) or a PR active-site (D370A) mutant _fsRF1/RF2_PEG10. Based on our findings, an _fsRF1/RF2_PEG10 overexpression resulted in an increased cellular proliferation, compared to the mutant form. Interestingly, transfection with _fsRF1/RF2_PEG10 had a detrimental effect on cell viability. We hypothesize that PR_PEG10 may play a cardinal role in the function of this retroviral remnant, possibly implicated in cellular proliferation and the inhibition of apoptosis.