Title: Life, Death, and Tax: Role of HTLV-I Oncoprotein in Genetic Instability and Cellular Transformation
Abstract: Human T-cell leukemia virus type I (HTLV-I) 1The abbreviations used are: HTLV-I, human T-cell leukemia virus type I; ATL, adult T-cell leukemia; LTR, long terminal repeat; IL, interleukin; CDK, cyclin-dependent kinase; BER, base excision repair; NER, nucleotide excision repair; PCNA, proliferating cell nuclear antigen; MSC, mitotic spindle assembly checkpoint; AML, acute myeloid leukemia. causes adult T-cell leukemia (ATL) (1Poiesz B.J. Ruscetti F.W. Gadzar A.F. Bunn P.A. Minna J.D. Gallo R.C. Proc. Natl. Acad. Sci. U. S. A. 1980; 77: 7415-7419Google Scholar, 2Hinuma Y. Nagata K. Misoka M. Nakai T. Matsumoto T. Kiroshita K. Shirakwa S. Miyoshi I. Proc. Natl. Acad. Sci. U. S. A. 1981; 78: 6476-6480Google Scholar, 3Matsuoka M. Oncogene. 2003; 22: 5131-5140Google Scholar). The virus is also associated with a neuropathy/myelopathy termed HTLV-associated myelopathy and tropical spastic paraparesis. ATL develops in 2–5% of HTLV-I-infected individuals after a long latent period, suggesting a multistage process of immortalization and transformation of T-lymphocytes. Extant data suggest that 8 discrete events likely occur serially in vivo before an HTLV-I-infected cell becomes immortalized and transformed (4Okamoto T. Ohno Y. Tsugane S. Watanabe S. Shimoyama M. Tajima K. Miwa M. Shimotohno K. Jpn. J. Cancer Res. 1989; 80: 191-195Google Scholar). How HTLV-I infection progresses from clinical latency to T-cell malignancy is not well understood but involves the unique viral transactivator/oncoprotein, Tax (Fig. 1). Tax has been shown to be singly sufficient for immortalizing T-lymphocytes (5Grassmann R. Dengler C. Muller-Fleckenstein I. McGuire K. Dokhelar M.C. Sodroski J.G. Haseltine W.A. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 3351-3355Google Scholar, 6Ross T.M. Pettiford S.M. Green P.L. J. Virol. 1996; 70: 5194-5202Google Scholar) and transforming rat fibroblasts (7Tanaka A. Takahashi G. Yamaoka S. Nosaka T. Maki M. Hatanaka M. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 1071-1075Google Scholar). Further, transgenic mice expressing Tax (driven by the HTLV-I long terminal repeat (LTR)) develop neurofibroma, a tumor of mesenchymal tissue (8Nerenberg M. Hinrichs S.H. Reynolds R.K. Khoury G. Jay G. Science. 1987; 237: 1324-1329Google Scholar). Finally, large granular lymphocytic leukemia has been found in mice transgenic for Tax expressed from the T-cell specific, granzyme B promoter (9Grossman W.J. Kimata J.T. Wong F.H. Zutter M. Ley T.J. Ratner L. Proc. Natl. Acad. Sci. U. S. A. 1995; 14: 1057-1061Google Scholar). It is estimated that cells in the human body divide 1016 times during a lifetime. To control and prevent errors in cell divisions, mammalian cells have evolved “gatekeepers” and “caretakers” to regulate the rate of cell growth and the fidelity by which cellular genetic information is transmitted to progenies (10Kinzler K.W. Vogelstein B. Cell. 1996; 87: 159-170Google Scholar). Gatekeepers monitor the net proliferative capacity of a cell, whereas caretakers act to eliminate DNA damages. Accordingly, one perspective is that transformation occurs when both gatekeeper and caretaker functions are abrogated. Using HTLV-I as a model, we review in a non-exhaustive fashion current thoughts on how Tax perturbs normal cellular regulation and engenders cellular transformation. HTLV-I belongs to the Deltaretrovirus genera of the Orthoretrovirinae family. In vivo, the virus has a tropism for CD4+ T-cells (11Richardson J.H. Edwards A.J. Cruickshank J.K. Rudge P. Dalgleish A.G. J. Virol. 1990; 64: 5682-5687Google Scholar) although CD8+ T-cells may also serve as a reservoir (12Nagai M. Brennan M.B. Sakai J.A. Mora C.A. Jacobson S. Blood. 2001; 98: 1858-1861Google Scholar). HTLV-I infection is primarily transmitted via cell-cell contact (13Okochi K. Sato H. Princess Takamatsu Symp. 1984; 15: 129-135Google Scholar, 14Igakura T. Stinchcombe J.C. Goon P.K. Taylor G.P. Weber J.N. Griffiths G.M. Tanaka Y. Osame M. Bangham C.R. Science. 2003; 299: 1713-1716Google Scholar). Recently, the human Glut1 glucose transporter has been identified as a receptor for infection by cell-free virus (15Manel N. Kim F.J. Kinet S. Taylor N. Sitbon M. Battini J.L. Cell. 2003; 115: 449-459Google Scholar). The proviral genome of HTLV-I is roughly 9 kbp, and like other retroviruses, contains two LTRs flanking structural genes encoding Gag, Pol, and Env (Fig. 1). An additional region located between env and the 3′-LTR, known as the pX region, encodes accessory proteins. The pX region has four partially overlapping reading frames (ORF, Fig. 1), of which ORF IV encodes Tax. Tax is predominantly a nuclear phosphoprotein (16Semmes O.J. Jeang K.T. J. Virol. 1996; 70: 6347-6357Google Scholar), which can shuttle into the cytoplasm using a nuclear export signal (17Burton M. Upadhyaya C.D. Maier B. Hope T.J. Semmes O.J. J. Virol. 2000; 74: 2351-2364Google Scholar). The mechanism of this shuttling is unclear; however, recent findings that Tax binds tristetrapolin (18Twizere J.C. Kruys V. Lefebvre L. Vanderplasschen A. Collete D. Debacq C. Lai W.S. Jauniaux J.C. Bernstein L.R. Semmes O.J. Burny A. Blackshear P.J. Kettmann R. Willems L. J. Natl. Cancer Inst. 2003; 95: 1846-1859Google Scholar) and that tristetrapolin associates with nucleoporin Nup214 (19Carman J.A. Nadler S.G. Biochem. Biophys. Res. Commun. 2004; 315: 445-449Google Scholar) raise the possibility that tristetrapolin may serve as a possible nucleocytoplasmic transporter for Tax. Nevertheless, the primary nuclear activity of Tax is to modulate transcription from the HTLV-I LTR (20Seiki M. Inoue J. Takeda T. Yoshida M. EMBO J. 1986; 5: 561-565Google Scholar, 21Brady J. Jeang K.T. Duvall J. Khoury G. J. Virol. 1987; 61: 2175-2181Google Scholar, 22Jeang K.T. Boros I. Brady J. Radonovich M. Khoury G. J. 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Princler G.L. Shimozato O. Taylor G.P. Bangham C.R. Derse D. Blood. 2003; 102: 4130-4136Google Scholar) among others. Indeed the breadth of Tax's transcriptional reprogramming of host cell genes was verified by DNA array studies which showed that of 2000 assayed genes the expression profiles of ∼300 were significantly altered (31Ng P.W. Iha H. Iwanaga Y. Bittner M. Chen Y. Jiang Y. Gooden G. Trent J.M. Meltzer P. Jeang K.T. Zeichner S.L. Oncogene. 2001; 20: 4484-4496Google Scholar). Tax influences so many promoters through its capacity to act in four discrete signaling pathways: CREB/ATF (reviewed in Ref. 32Mesnard J.M. Devaux C. Virology. 1999; 257: 277-284Google Scholar); NF-κB (reviewed in Ref. 33Sun S.C. Ballard D.W. Oncogene. 1999; 18: 6948-6958Google Scholar); AP-1 (34Jeang K.T. Chiu R. Santos E. Kim S.J. Virology. 1991; 181: 218-227Google Scholar); and SRF (35Fujii M. Tsuchiya J. Chuhjo T. Akizawa T. Seiki M. Genes Dev. 1992; 6: 2066-2076Google Scholar). These Tax signaling cascades are discussed in greater detail elsewhere (36Jeang K.T. Cytokine Growth Factor Rev. 2001; 12: 207-217Google Scholar). In the course of transforming cells, viral oncoproteins such as E1A, HPV E7, and SV40 T Ag profoundly dysregulate cell cycle controls (37Helt A.M. Galloway D.A. Carcinogenesis. 2003; 24: 159-169Google Scholar, 38Duensing S. Munger K. Crit. Rev. Eukaryotic Gene Expression. 2003; 13: 9-23Google Scholar, 39Lavia P. Mileo A.M. Giordano A. Paggi M.G. Oncogene. 2003; 22: 6508-6516Google Scholar). Transition from one phase of the cell cycle to the next is normally governed by cyclin-dependent kinases (CDKs) partnered with cyclins. These CDK-cyclin complexes are in turn modulated by phosphorylation mediated through CDK-activating kinases and phosphatases, and through physical sequestration by CDK inhibitory proteins (reviewed in Refs. 40Hunter T. Pines J. Cell. 1994; 79: 573-582Google Scholar and 41Sherr C.J. Roberts J.M. 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Fig. 1B summarizes several key cell cycle factors that have been experimentally shown to be influenced by Tax. For instance, Tax can directly bind p16INK4a, CycD2, pro-IL-16, and Cdk4 (49Suzuki T. Kitao S. Matsushime H. Yoshida M. EMBO J. 1996; 15: 1607-1614Google Scholar, 50Low K.G. Dorner L.F. Fernando D.B. Grossman J. Jeang K.T. Comb M.J. J. Virol. 1997; 71: 1956-1962Google Scholar, 51Iwanaga R. Ohtani K. Hayashi T. Nakamura M. Oncogene. 2001; 20: 2055-2067Google Scholar, 52Huang Y. Ohtani K. Iwanaga R. Matsumura Y. Nakamura M. Oncogene. 2001; 20: 1094-1102Google Scholar, 53Akagi T. Ono H. Shimotohno K. Oncogene. 1996; 12: 1645-1652Google Scholar, 54de La Fuente C. Santiago F. Chong S.Y. Deng L. Mayhood T. Fu P. Stein D. Denny T. Coffman F. Azimi N. Mahieux R. Kashanchi F. J. Virol. 2000; 74: 7270-7283Google Scholar, 55Haller K. Wu Y. Derow E. Schmitt I. Jeang K.T. Grassmann R. Mol. Cell. Biol. 2002; 22: 3327-3338Google Scholar, 56Wilson K.C. Center D.M. Cruikshank W.W. Zhang Y. 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Azimi N. Kashanchi F. J. Virol. 1999; 73: 9917-9927Google Scholar, 61Kawata S. Ariumi Y. Shimotohno K. J. Virol. 2003; 77: 7291-7299Google Scholar, 62Chowdhury I.H. Farhadi A. Wang X.F. Robb M.L. Birx D.L. Kim J.H. Int. J. Cancer. 2003; 107: 603-611Google Scholar, 63Schavinsky-Khrapunsky Y. Huleihel M. Aboud M. Torgeman A. Oncogene. 2003; 22: 5315-5324Google Scholar), and E2F (64Lemasson I. Thebault S. Sardet C. Devaux C. Mesnard J.M. J. Biol. Chem. 1998; 273: 23598-23604Google Scholar, 65Neuveut C. Low K.G. Maldarelli F. Schmitt I. Majone F. Grassmann R. Jeang K.T. Mol. Cell. Biol. 1998; 18: 3620-3632Google Scholar, 66Ohtani K. Iwanaga R. Arai M. Huang Y. Matsumura Y. Nakamura M. J. Biol. Chem. 2000; 275: 11154-11163Google Scholar) are regulated by Tax via transcriptional induction/repression (see Fig. 1B). Finally, Tax via an unknown mechanism influences the phosphorylation of CycD3 (65Neuveut C. Low K.G. Maldarelli F. Schmitt I. Majone F. Grassmann R. Jeang K.T. Mol. Cell. Biol. 1998; 18: 3620-3632Google Scholar). To properly consider this complex pattern of interactions, one should appreciate that the context of Tax's up- or down-regulation matters. An instructive example is presented by Tax-p21CIP1/WAF1 interaction. Various studies agree that p21CIP1/WAF1 levels are significantly elevated in Tax-expressing cells (53Akagi T. Ono H. Shimotohno K. Oncogene. 1996; 12: 1645-1652Google Scholar, 54de La Fuente C. Santiago F. Chong S.Y. Deng L. Mayhood T. Fu P. Stein D. Denny T. Coffman F. Azimi N. Mahieux R. Kashanchi F. J. Virol. 2000; 74: 7270-7283Google Scholar, 61Kawata S. Ariumi Y. Shimotohno K. J. Virol. 2003; 77: 7291-7299Google Scholar, 62Chowdhury I.H. Farhadi A. Wang X.F. Robb M.L. Birx D.L. Kim J.H. Int. J. Cancer. 2003; 107: 603-611Google Scholar, 63Schavinsky-Khrapunsky Y. Huleihel M. Aboud M. Torgeman A. Oncogene. 2003; 22: 5315-5324Google Scholar). However, depending on whether p21CIP1/WAF1 complexes with CycD/Cdk2 or CycA/Cdk2, it has been noted that the resulting ternary complex either promotes or inhibits G1/S progression (67LaBaer J. Garrett M.D. Stevenson L.F. Slingerland J.M. Sandhu C. Chou H.S. Fattaey A. Harlow E. Genes Dev. 1997; 11: 847-862Google Scholar, 68Zhang H. Hannon G.J. Beach D. Genes Dev. 1994; 8: 1750-1758Google Scholar, 69Kehn K. Deng L. de la Fuente C. Strouss K. Wu K. Maddukkuri A. Baylor S. Rufner R. Pumfery A. Bottazzi M.E. Kashanchi F. Retrovirology. 2004; 1: 6Google Scholar). These observations, if correct, help to explain seemingly opposing effects of Tax on CycD (up-regulated (31Ng P.W. Iha H. Iwanaga Y. Bittner M. Chen Y. Jiang Y. Gooden G. Trent J.M. Meltzer P. Jeang K.T. Zeichner S.L. Oncogene. 2001; 20: 4484-4496Google Scholar, 51Iwanaga R. Ohtani K. Hayashi T. Nakamura M. Oncogene. 2001; 20: 2055-2067Google Scholar, 52Huang Y. Ohtani K. Iwanaga R. Matsumura Y. Nakamura M. Oncogene. 2001; 20: 1094-1102Google Scholar, 53Akagi T. Ono H. Shimotohno K. Oncogene. 1996; 12: 1645-1652Google Scholar, 54de La Fuente C. Santiago F. Chong S.Y. Deng L. Mayhood T. Fu P. Stein D. Denny T. Coffman F. Azimi N. Mahieux R. Kashanchi F. J. Virol. 2000; 74: 7270-7283Google Scholar, 55Haller K. Wu Y. Derow E. Schmitt I. Jeang K.T. Grassmann R. Mol. Cell. Biol. 2002; 22: 3327-3338Google Scholar)) and CycA (down-regulated (58Kibler K.V. Jeang K.T. J. Virol. 2001; 75: 2161-2173Google Scholar)) transcription. Indeed, enhanced transcription of CycD in the face of repressed transcription of CycA would tip the balance toward more G1/S transition-promoting p21CIP1/WAF1/CycD/Cdk2 at the expense of G1/S transition-inhibiting p21CIP1/WAF1/CycA/Cdk2 moieties. To date, collective evidence do support that Tax has evolved diverse means to defuse various cellular brakes that guard against accelerated G1/S progression. The ability of Tax to shorten the length of G1 and to accelerate cells into S (70Lemoine F.J. Marriott S.J. J. Biol. Chem. 2001; 276: 31851-31857Google Scholar) embodies a constitutive (i.e. DNA damage-independent) and a DNA damage-induced component. Thus, direct Tax binding of Cdk4 and its enhancement of CycD-Cdk4 activity (55Haller K. Wu Y. Derow E. Schmitt I. Jeang K.T. Grassmann R. Mol. Cell. Biol. 2002; 22: 3327-3338Google Scholar) occur constitutively and are independent of any DNA damage-triggered events. At the same time, Tax can also subvert DNA damage-induced G1 arrest enforced through p53 (71Pise-Masison C.A. Choi K.S. Radonovich M. Dittmer J. Kim S.J. Brady J.N. J. Virol. 1998; 72: 1165-1170Google Scholar, 72Akagi T. Ono H. Tsuchida N. Shimotohno K. FEBS Lett. 1997; 406: 263-266Google Scholar, 73Takemoto S. Trovato R. Cereseto A. Nicot C. Kislyakova T. Casareto L. Waldmann T. Torelli G. Franchini G. Blood. 2000; 95: 3939-3944Google Scholar, 74Van P.L. Yim K.W. Jin D.Y. Dapolito G. Kurimasa A. Jeang K.T. J. Virol. 2001; 75: 396-407Google Scholar) (see more below). Currently, how Tax affects other phases of the cell cycle is less clear. Emerging findings suggest that this viral oncoprotein can also impair the DNA damage-induced checkpoint in G2/M transition (75Haoudi A. Daniels R.C. Wong E. Kupfer G. Semmes O.J. J. Biol. Chem. 2003; 278: 37736-37744Google Scholar, 76Liang M.H. Geisbert T. Yao Y. Hinrichs S.H. Giam C.Z. J. Virol. 2002; 76: 4022-4033Google Scholar). Cancer is a genetic disease. It is estimated that cancer cells can contain more than 100,000 discrete mutations (77Perucho M. J. Biol. Chem. 1996; 377: 677-684Google Scholar). All cancers can be broadly divided into two groups (reviewed in Ref. 78Loeb K.R. Loeb L.A. Carcinogenesis. 2000; 21: 379-385Google Scholar): those arising from loss of DNA repair function (and therefore have structurally damaged chromosomes) and those with chromosomal instability (and therefore have polypoidy and/or aneuploidy). Clastogenic DNA damage is frequently found in HTLV-I-transformed cells (79Marriott S.J. Lemoine F.J. Jeang K.T. J. Biomed. Sci. 2002; 9: 292-298Google Scholar) and cells transfected to express Tax (80Majone F. Semmes O.J. Jeang K.T. Virology. 1993; 193: 456-459Google Scholar) (Fig. 2A). Clastogenic changes (point mutations, deletions, substitutions, translocations) arise and persist when defects in DNA repair mechanisms co-exist in a cell with a loss in checkpoint functions that would normally eliminate damaged DNA. All cells acquire DNA damage at a low frequency as they transit the cell cycle. Several mechanisms, including base excision repair (BER), nucleotide excision repair (NER), recombination, and direct repair of nicks by DNA ligation act to correct genetic mistakes. In 1990, the first clue that HTLV-I subverts cellular DNA repair came from the finding that Tax repressed the expression of DNA polymerase β, an enzyme involved in BER (81Jeang K.T. Widen S.G. Semmes O.J. Wilson S.H. Science. 1990; 247: 1082-1084Google Scholar). Subsequently, reduced BER activity was confirmed in HTLV-I, HTLV-II, and bovine leukemia virus-transformed cells (82Philpott S.M. Buehring G.C. J. Natl. Cancer Inst. 1999; 91: 933-942Google Scholar). Next, Tax was found to suppress the NER normally observed following UV irradiation of cells (83Kao S.Y. Marriott S.J. J. Virol. 1999; 73: 4299-4304Google Scholar). NER requires DNA polymerases δ and ϵ and uses proliferating cell nuclear antigen (PCNA) as a cofactor. Excessive PCNA can prompt DNA polymerase δ to synthesize inappropriately new DNA past template lesions, resulting in nucleotide misincorporation (84Mozzherin D.J. Shibutani S. Tan C.K. Downey K.M. Fisher P.A. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 6126-6131Google Scholar). Tax is believed to inhibit NER through its transcriptional up-regulation of PCNA (85Lemoine F.J. Kao S-Y. Marriott S.J. AIDS Res. Hum. Retroviruses. 2000; 16: 1623-1627Google Scholar); this inhibition of NER also depends, in part, on Tax's inactivation of p53 function (71Pise-Masison C.A. Choi K.S. Radonovich M. Dittmer J. Kim S.J. Brady J.N. J. Virol. 1998; 72: 1165-1170Google Scholar, 72Akagi T. Ono H. Tsuchida N. Shimotohno K. FEBS Lett. 1997; 406: 263-266Google Scholar, 73Takemoto S. Trovato R. Cereseto A. Nicot C. Kislyakova T. Casareto L. Waldmann T. Torelli G. Franchini G. Blood. 2000; 95: 3939-3944Google Scholar, 74Van P.L. Yim K.W. Jin D.Y. Dapolito G. Kurimasa A. Jeang K.T. J. Virol. 2001; 75: 396-407Google Scholar). There is no evidence that Tax interferes with DNA ligation (86Kao S-Y. Marriott S.J. J. Biol. Chem. 2000; 275: 35926-35931Google Scholar) or DNA recombination. However, recent data suggest that Tax represses the expression of human telomerase (hTert) (87Gabet A.S. Mortreux F. Charneau P. Riou P. Duc-Dodon M. Wu Y. Jeang K.T. Wattel E. Oncogene. 2003; 22: 3734-3741Google Scholar). Repression of telomerase is significant because the telomeric repeats of chromosomes normally prevent aberrant end-to-end fusions (Fig. 2B) and protect the ends from degradation by exonucleases. Furthermore, de novo double-stranded breaks in chromosomes can also be stabilized by the transient addition of telomeric repeats (88Morin G.B. Nature. 1991; 353: 454-456Google Scholar, 89Wilkie A.O. Lamb J. Harris P.C. Finney R.D. Higgs D.R. Nature. 1990; 346: 868-871Google Scholar, 90Flint J. Craddock C.F. Villegas A. Bentley D.P. Williams H.J. Galanello R. Cao A. Wood W.G. Ayyub H. Higgs D.R. Am. J. Hum. Genet. 1994; 55: 505-512Google Scholar). Indeed, we have documented that Tax prevents such addition of telomeric repeats to new double-stranded breaks (91Majone F. Jeang K.T. J. Biol. Chem. 2000; 275: 32906-32910Google Scholar) and in this way potentially interferes with a protective mechanism used to prevent inappropriate breakages-fusions (Fig. 2B). The combined effects of Tax on BER, NER, DNA end stability, telomerase, and cell cycle progression create a setting in which repair of mistakes is compromised. These combined dysregulations might explain the observed 2.8-fold increase in genomic mutation frequency (92Miyake H. Suzuki T. Hirai H. Yoshida M. Virology. 1999; 253: 155-161Google Scholar) in HTLV-I-infected cells. The majority of cancers are aneuploid (93Cahill D.P. Lengauer C. Yu J. Riggins G.J. Willson J.K. Markowitz S.D. Kinzler K.W. Vogelstein B. Nature. 1998; 392: 300-303Google Scholar). In transformed cells, numerical chromosomal changes that include losses or gains of entire chromosomes (aneuploidy) generally co-exist with structural chromosomal damage. 2F. Mitelman, B. Johansson, and F. Mertens, personal communication. Although controversial, increasingly aneuploidy is thought to be a cause, rather than a consequence, of transformation (95Rasnick D. Cancer Genet. Cytogenet. 2002; 136: 66-72Google Scholar). During normal mitosis, human diploid cells maintain euploidy by precisely partitioning 23 pairs of chromosomes from a mother cell to two daughter cells. ATL cells, by contrast, are famously aneuploid (reviewed in Ref. 79Marriott S.J. Lemoine F.J. Jeang K.T. J. Biomed. Sci. 2002; 9: 292-298Google Scholar). Their nuclei are highly lobulated or convoluted, earning them the name of “flower” cells. This suggests that a cellular mechanism that guards against chromosomal missegregation in mitosis is also subverted by HTLV-I. The mitotic spindle assembly checkpoint (MSC) (96Musacchio A. Hardwick K.G. Nat. Rev. Mol. Cell. Biol. 2002; 3: 731-741Google Scholar) is a key guardian of euploidy. Interestingly, when several ATL cell lines were tested ex vivo, all were found to be deficient in MSC function (97Kasai T. Iwanaga Y. Iha H. Jeang K.T. J. Biol. Chem. 2002; 277: 5187-5193Google Scholar). A potential explanation for this loss arises from two findings: (a) Tax binds human Mad1 (98Jin D.Y. Spencer F. Jeang K.T. Cell. 1998; 93: 81-91Google Scholar, 99Iwanaga Y. Kasai T. Kibler K. Jeang K.T. J. Biol. Chem. 2002; 277: 31005-31013Google Scholar) and (b) Mad1 is an integral constituent of the MSC (96Musacchio A. Hardwick K.G. Nat. Rev. Mol. Cell. Biol. 2002; 3: 731-741Google Scholar). That impairment of Mad1 function by Tax may contribute to ATL pathogenesis finds intriguing support in the clinical courses of non-HTLV-I acute myeloid leukemia (AML). In two large AML series (1213 and 1612 patients, respectively), loss of a single chromosome 7 (note that the gene for human Mad1 maps to chromosome 7 (100Jin D.Y. Kozak C.A. Pangilinan F. Spencer F. Green E.D. Jeang K.T. Genomics. 1999; 55: 363-364Google Scholar)) prognosticated an extremely poor outcome (101Byrd J.C. Mrozek K. Dodge R.K. Carroll A.J. Edwards C.G. Pettenati M.J. Patil S.R. Rao K.W. Watson M.S. Koduru P.R. Moore J.O. Stone R.M. Mayer R.J. Feldman E.J. Davey F.R. Schiffer C.A. Larson R.A. Bloomfield C.D. Cancer and Leukemia Group B (CALGB 8461) Blood. 2002; 100: 4325-4336Google Scholar, 102Grimwade D. Walker H. Oliver F. Wheatley K. Harrison C. Harrison G. Rees J. Hann I. Stevens R. Burnett A. Goldstone A. Blood. 1998; 92: 2322-2333Google Scholar). In these two studies, whereas all AML patients had 5-year overall survival rates of 24–44%, counterpart AML patients with monosomy 7 had survival rates of 0–10%, respectively (101Byrd J.C. Mrozek K. Dodge R.K. Carroll A.J. Edwards C.G. Pettenati M.J. Patil S.R. Rao K.W. Watson M.S. Koduru P.R. Moore J.O. Stone R.M. Mayer R.J. Feldman E.J. Davey F.R. Schiffer C.A. Larson R.A. Bloomfield C.D. Cancer and Leukemia Group B (CALGB 8461) Blood. 2002; 100: 4325-4336Google Scholar, 102Grimwade D. Walker H. Oliver F. Wheatley K. Harrison C. Harrison G. Rees J. Hann I. Stevens R. Burnett A. Goldstone A. Blood. 1998; 92: 2322-2333Google Scholar). Other explanations not excluded, a tantalizing parallel between the two leukemias is that one (ATL) impairs Mad1 function through viral oncoprotein subversion whereas the other (AML) does so through physical loss of chromosome 7 (i.e. monosomy 7). Is loss of MSC the sole reason for aneuploidy in ATL cells? The answer appears to be “no.” Conceptually, one recognizes that loss of checkpoint can explain the tolerance of mistakes by cells, but checkpoint loss cannot create de novo mistakes. Recent studies suggest that Tax might directly trigger chromosomal separation errors in two ways. First, Tax can promote the unscheduled degradation of securin and cyclin B1 most likely through the premature activation of the CDC20-associated anaphase promoting complex (103Liu B. Liang M.H. Kuo Y.L. Liao W. Boro I. Kleinberger T. Blancato J. Giam C.Z. Mol. Cell. Biol. 2003; 23: 5269-5281Google Scholar), thereby leading to faulty mitosis. Second, like the human papilloma virus E7 oncoprotein (38Duensing S. Munger K. Crit. Rev. Eukaryotic Gene Expression. 2003; 13: 9-23Google Scholar), Tax can also induce aberrant centrosomal multiplication in G1. 3K. Haller and K. T. Jeang, unpublished data. Generating supernumerary centrosomes results in multipolar mitosis, which is another mechanism for creating aneuploidy (104Storchova Z. Pellman D. Nat. Rev. Mol. Cell. Biol. 2004; 5: 45-54Google Scholar). Finally, there is a school of thought that suggests polyploidy as the precursor of aneuploidy (104Storchova Z. Pellman D. Nat. Rev. Mol. Cell. Biol. 2004; 5: 45-54Google Scholar). Relevant to this notion, we note that Tax expression does facilely create multinucleated (i.e. polyploid) cells (76Liang M.H. Geisbert T. Yao Y. Hinrichs S.H. Giam C.Z. J. Virol. 2002; 76: 4022-4033Google Scholar, 98Jin D.Y. Spencer F. Jeang K.T. Cell. 1998; 93: 81-91Google Scholar). Add to this the fact that Tax can inactivate p53 and Rb (65Neuveut C. Low K.G. Maldarelli F. Schmitt I. Majone F. Grassmann R. Jeang K.T. Mol. Cell. Biol. 1998; 18: 3620-3632Google Scholar, 71Pise-Masison C.A. Choi K.S. Radonovich M. Dittmer J. Kim S.J. Brady J.N. J. Virol. 1998; 72: 1165-1170Google Scholar, 72Akagi T. Ono H. Tsuchida N. Shimotohno K. FEBS Lett. 1997; 406: 263-266Google Scholar, 73Takemoto S. Trovato R. Cereseto A. Nicot C. Kislyakova T. Casareto L. Waldmann T. Torelli G. Franchini G. Blood. 2000; 95: 3939-3944Google Scholar, 74Van P.L. Yim K.W. Jin D.Y. Dapolito G. Kurimasa A. Jeang K.T. J. Virol. 2001; 75: 396-407Google Scholar), two factors essential to a G1 tetraploid/polypoid checkpoint (105Margolis R.L. Lohez O.D. Andreassen P.R. J. Cell. Biochem. 2003; 88: 673-683Google Scholar), and one then can further envision how this might be yet another route traveled by HTLV-I/Tax/ATL cells toward aneuploidy (Fig. 3). A long standing cancer paradox is that overexpression of oncogenes does not simply provide proliferative advantages to cells but frequently also triggers cells to undergo apoptosis. Findings from oncogenic transcription factors such as Myc, E1A, and E2F-1 show this duality to be the rule rather than the exception (reviewed in Ref. 106Nilsson J.A. Cleveland J.L. Oncogene. 2003; 22: 9007-9021Google Scholar). Indeed, it is now apparent that oncogenic insults induce countervailing responses by the cell, which are reflected in cell cycle arrest and apoptosis. We reviewed, above, how Tax defeats cellular mechanisms for braking cell cycle progression. No cell cycle and/or genetic instability manifestations of Tax can confer selective growth advantage if cells fail to tolerate such phenotypic and genotypic changes and choose instead apoptotic death. Hence, disabling the cellular apoptotic response remains a requisite for transformation. By definition, the clinical presentation of ATL implies that in a subpopulation of CD4+ T-cells, HTLV-I infection tips the balance between proliferation and apoptosis toward the former. Nevertheless, how HTLV-I Tax oncoprotein influences this choice is not fully understood. Many have examined the contribution of Tax to stress-induced apoptosis. Overall findings have been controversial and divergent. Some found that Tax protects cells from stress-induced cell cycle arrest or apoptosis (107Torgeman A. Ben-Aroya Z. Grunspan A. Zelin E. Butovsky E. Hallak M. Lochelt M. Flugel R.M. Livneh E. Wolfson M. Kedar I. Aboud M. Exp. Cell Res. 2001; 271: 169-179Google Scholar, 108Brauweiler A. Garrus J.E. Reed J.C. Nyborg J.K. Virology. 1997; 231: 135-140Google Scholar, 109Copeland K.F. Haaksma A.G. Goudsmit J. Krammer P.H. Heeney J.L. AIDS Res. Hum. Retroviruses. 1994; 10: 1259-1268Google Scholar), whereas others observed that Tax sensitizes cells to stress-induced apoptosis (110Chlichlia K. Moldenhauer G. Daniel P.T. Busslinger M. Gazzolo L. Schirmacher V. Khazaei K. Oncogene. 1995; 10: 269-277Google Scholar, 111Chlichlia K. Busslinger M. Peter M.E. Walczak H. Krammer P. Schirrmacher V. Khazaie K. Oncogene. 1997; 14: 2265-2272Google Scholar, 112Kao S.Y. Leomine F.J. Marriott S.J. Oncogene. 2000; 19: 2240-2248Google Scholar, 113Kasai T. Jeang K.T. Retrovirology. 2004; 1: 7Google Scholar). Likely, the decision between proliferation and death is influenced by the cellular environment, cell type genetic background, and multiple co-existing signaling events. Depending on context, which set of genes that Tax transcriptionally activates (31Ng P.W. Iha H. Iwanaga Y. Bittner M. Chen Y. Jiang Y. Gooden G. Trent J.M. Meltzer P. Jeang K.T. Zeichner S.L. Oncogene. 2001; 20: 4484-4496Google Scholar) and/or which cluster of gene products that Tax binds (94Wu K. Bottazzi M.E. de la Fuente C. Deng L. Gitlin S.D. Maddukuri A. Dadgar S. Li H. Vertes A. Pumfery A. Kashanchi F. J. Biol. Chem. 2004; 279: 495-508Google Scholar) will mean either the normal cellular response against oncogenic stress will either prevail (i.e. apoptosis) or be subverted (i.e. proliferation) by HTLV-I. A clear understanding of factors in addition to Tax that guide this choice for HTLV-I-infected T-cells will be a major topic for future research. Over 20 million individuals globally are infected with HTLV-I. It is estimated that 2–5% of these carriers will develop ATL over their lifetime. The identification and isolation of HTLV-I 25 years ago have spurred intensive mechanistic investigations into ATL transformation. Using Tax as a model system, we have learned that viral means for transformation parallel similar mechanistic changes seen in spontaneously occurring cancers. A simplified sequence of events appears to be genetic damage initiated by oncogenic stimuli, followed by subversion of cellular checks allowing tolerance and fixation of changes into the genome, and finally selection over time for the correct mix of gene alterations that confer selective growth advantage. Clearly the process is complex and multifaceted. Fleshing out all the biological and molecular details to accompany this simplified framework will easily keep HTLV researchers busy for another 25 years. We thank Fatah Kashanchi for critical reading of the manuscript and Lan Lin for preparation of figures and text.