Deficiency of the dual ubiquitin/SUMO ligase Topors results in genetic instability and an increased rate of malignancy in mice

Mice homozygous for a mutant Topors allele exhibited a high rate of perinatal mortality and decreased lifespan. In addition, heterozygotes were found to have an increased incidence of malignancy, involving a variety of tissues. Consistent with this finding, primary embryonic fibroblasts lacking Topors exhibited an increased rate of malignant transformation, associated with aneuploidy and defective chromosomal segregation. While loss of Topors did not alter sensitivity to DNA-damaging or microtubule-targeting agents, cells lacking Topors exhibited altered pericentric heterochromatin, manifested by mislocalization of HP1α and an increase in transcription from pericentric major satellite DNA. Topors-deficient cells exhibited a transcriptional profile similar to that of cells treated with histone deacetylase inhibitors, and were resistant to the anti-proliferative effects of the histone deacetylase inhibitor trichostatin A.These results indicate a unique role for Topors in the maintenance of genomic stability and pericentric heterochromatin, as well as in cellular sensitivity to histone deacetylase inhibitors.Topors is nuclear protein that is widely expressed in human tissues [1], and is the first example of protein that is capable of functioning as both a ubiquitin and SUMO E3 ligase [2-4]. Furthermore, expression analyses and genetic studies have implicated TOPORS as a tumor suppressor in colon, lung, brain, and prostate malignancies [5,1,6].In proliferating cells, Topors localizes in nuclear foci, many of which co-localize with PML nuclear bodies [7]. While identified originally as a topoisomerase I- and p53-binding protein, human Topors and a Drosophila ortholog were shown to function as RING-dependent E3 ubiquitin ligases, with substrates including p53, the Hairy transcription factor, and the homeodomain protein NKX3.1 [2,8,9]. Additional studies indicated that Topors could act as an E3 SUMO ligase for p53 and topoisomerase I, with the RING domain dispensable for t