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Abstract Human T-cell Leukemia Virus type 1 (HTLV-1) and 2 (HTLV-2) are two closely related human retroviruses. HTLV-1 is associated with an aggressive Adult T-cell Leukemia (ATL) while there is no evidence for an association of HTLV-2 with any human malignancies. The two viruses encode transactivator proteins, Tax-1 and Tax-2 respectively. In ATL, Tax-1 is thought to play a central role in the transformation of a normal T-cell into a leukemic cell; however, it has not been entirely clear how post-translational modifications of Tax-1 influence its transforming activity. Here, we discuss three recent papers that report on the ubiquitination and sumoylation of Tax-1 and Tax-2. We comment on their divergent findings implicating the importance (or lack of importance) of these modifications and other events on Tax activation of NF-κB as related to cellular transformation.

Abstract Background Human T-cell Leukemia Virus type 1 (HTLV-1) infects 20 million individuals world-wide and causes Adult T-cell Leukemia/Lymphoma (ATLL), a highly aggressive T-cell cancer. ATLL is refractory to treatment with conventional chemotherapy and fewer than 10% of afflicted individuals survive more than 5 years after diagnosis. HTLV-1 encodes a viral oncoprotein, Tax, that functions in transforming virus-infected T-cells into leukemic cells. All ATLL cases are believed to have reduced p53 activity although only a minority of ATLLs have genetic mutations in their p53 gene. It has been suggested that p53 function is inactivated by the Tax protein. Results Using genetically altered mice, we report here that Tax expression does not achieve a functional equivalence of p53 inactivation as that seen with genetic mutation of p53 (i.e. a p53−/− genotype). Thus, we find statistically significant differences in tumorigenesis between Tax+p53+/+versus Tax+p53−/− mice. We also find a role contributed by the cellular Wip1 phosphatase protein in tumor formation in Tax transgenic mice. Notably, Tax+Wip1−/− mice show statistically significant reduced prevalence of tumorigenesis compared to Tax+Wip1+/+ counterparts. Conclusions Our findings provide new insights into contributions by p53 and Wip1 in the in vivo oncogenesis of Tax-induced tumors in mice.

Abstract Background Adult T cell leukemia results from the malignant transformation of a CD4+ lymphoid clone carrying an integrated HTLV-1 provirus that has undergone several oncogenic events over a 30-60 year period of persistent clonal expansion. Both CD4+ and CD8+ lymphocytes are infected in vivo; their expansion relies on CD4+ cell cycling and on the prevention of CD8+ cell death. Cloned infected CD4+ but not CD8+ T cells from patients without malignancy also add up nuclear and mitotic defects typical of genetic instability related to theexpression of the virus-encoded oncogene tax. HTLV-1 expression is cancer-prone in vitro, but in vivo numerous selection forces act to maintain T cell homeostasis and are possibly involved in clonal selection. Results Here we demonstrate that the HTLV-1 associated CD4+ preleukemic phenotype and the specific patterns of CD4+ and CD8+ clonal expansion are in vivo selected processes. By comparing the effects of recent (1 month) experimental infections performed in vitro and those observed in cloned T cells from patients infected for >6-26 years, we found that in chronically HTLV-1 infected individuals, HTLV-1 positive clones are selected for tax expression. In vivo, infected CD4+ cells are positively selected for cell cycling whereas infected CD8+ cells and uninfected CD4+ cells are negatively selected for the same processes. In contrast, the known HTLV-1-dependent prevention of CD8+ T cell death pertains to both in vivo and in vitro infected cells. Conclusions Therefore, virus-cell interactions alone are not sufficient to initiate early leukemogenesis in vivo.