PML and PML-like exonucleases restrict retrotransposons in jawed vertebrates

We have uncovered a novel role for the promyelocytic leukemia (PML) gene and novel PML-like DEDDh exonucleases in the maintenance of genome stability through the restriction of LINE-1 (L1) retrotransposition in jawed vertebrates. Although the PML tumour suppressor protein in mammals is SUMOylated and forms nuclear bodies, we found that the spotted gar PML ortholog and related proteins in fish are not SUMOylated and function as cytoplasmic DEDDh exonucleases. In contrast, more closely related avian and turtle PML proteins are predicted to be SUMOylated and localized both to the cytoplasm and to nuclear bodies. We also identified PML-like exon 9 (Plex9) genes in teleost fishes that encode exonucleases sharing homology to gar PML. In an example of convergent evolution and akin to TREX1, gar PML and zebrafish Plex9 proteins suppressed L1 retrotransposition and could complement TREX1 knockout in mammalian cells. We also characterized the first non-mammalian TREX1 homologs in axolotl. Following export to the cytoplasm, the human PML-I isoform also restricted L1 through its conserved C-terminus and suppressed CGAS activation. Thus, PML first emerged as a cytoplasmic suppressor of retroelements, and this function is retained in amniotes despite its role in the assembly of nuclear bodies and the acquisition of SUMO-modification.