HTLV-I leukemic cells often present numerous genomic alterations, but the genesis and contribution of these chromosomal defects is unclear. The viral oncoprotein Tax has an important role in the initial stages of cellular transformation, inactivates tumor suppressors and stimulates cellular proliferation by inactivating several cell cycle checkpoints. In addition, several studies have shown that Tax inhibits multiple DNA repair pathways. We recently demonstrated that HTLV-I p30 disrupts the formation of the MRN complex onto DDSB foci induced during DNA replication in S phase. However, unlike Tax, our studies showed that p30 itself cannot induce DNA breaks directly, suggesting that it may cooperate with another viral gene to stimulate genetic instability. The data reported here indicate that Tax expression is sufficient to promote accumulation of DDSB in cells passing through the S phase. In this regard, it is important to note that Tax has been reported to inhibit PCNA, DNA Topoisomerase and DNA β-polymerase 
, all known to be involved in DNA replication and to sequester DNA-PK, BRCA1, and MDC1 
. Previous studies have reported that Tax can inhibit p-ATM 
, although similar levels of p-ATM were observed 30 minutes after irradiation of control or HTLV-I cells. It is possible that 30 minutes is sufficient to allow activation of downstream targets and initiate DNA repair. Additional evidence of this study was derived from rat fibroblasts expressing Tax or high dose irradiated cells. Other studies found significant levels of p-ATM in HTLV-I-transformed MT-2 cells in the absence of irradiation or other treatment 
, which is consistent with results presented here and constitutive activation of the DNA repair response in Tax-expressing cells. Discrepancy between these studies may be related to the use of high dose irradiation or different lines being tested (MT-4 and Hut102 versus MT-2 and C10MJ). Our studies suggest that in HTLV-I infected cells Tax impairs HR and favors NHEJ DNA repair. An important component of the NHEJ pathway is Ku80. A previous study reported that Tax is associated with quantitative reduction of Ku80 expression but not depletion 
and this explains why we were able to detect expression by immunofluorescence. Our data reveal a significant qualitative difference in Ku80 distribution within JPX9 cells following induction of Tax expression. In the absence of Tax, Ku80 appear diffuse, while in the presence of Tax and formation of DNA breaks, Ku80 accumulated at DNA break foci as previously reported 
Although our data suggest that Single strand annealing (SSA) or microhomology-mediated end joining (MMEJ) are not frequently used for the repair of DDSB in Tax-expressing cells, we cannot formally exclude the possibility that alt-NHEJ is being used at a lower frequency or specific breaks due to local chromatin structure, and additional studies will be needed to answer these questions. In contrast to HR and NHEJ, RAD52 and ERCC1 are important to promote SSA. Whether Tax affects RAD52 and/or ERCC1 remain to be demonstrated. How Tax-mediated NF-kB activation interferes with HR DNA repair is currently under investigation. An interesting possibility is the effect of Tax on p53. Independent studies showed that p53 can down-regulate HR activity and that this effect of p53 is independent from p53 transcription 
but related to relative levels of expression, because mutations in the transcriptional domain of p53 do not affect inhibition of HR 
. Interestingly, both Tax and Tax mutant M47 that inhibit HR activate NF-kB but also stabilize p53 protein levels. Whether Tax-mediated p53 protein stabilization plays a role in HR inhibition mediated by Tax warrants additional studies. Alternatively, it is possible that Tax expression prevents the recruitment of Rad51 to DDSB.
Our study describes the finding of a novel mechanism for retrovirus-associated genome instability. On one hand, HTLV-I Tax promotes DDSB during DNA replication and simultaneously prevents the repair machinery from using the conservative HR repair, thereby stimulating accumulation of genetic mutations and deletions. Surprisingly, HTLV-I also encodes another viral protein that inhibits HR repair and favors NHEJ. Abuse of unfaithful DNA repair through NHEJ in Tax-expressing cells is likely to have oncogenic consequences. Exactly how HR is inhibited and whether NHEJ is used as a default repair mechanism or is actively stimulated by Tax remain to be investigated. Our findings reveal an intriguing mechanism that could be involved in retrovirus-mediated cellular transformation and could explain the low incidence of HTLV-I-associated diseases, usually detected in 1 to 2% of infected individuals, and the very long latency, decades, before onset of the disease.