Polyoma small T antigen triggers cell death via mitotic catastrophe.

Polyoma small T antigen (PyST), an early gene product of the polyoma virus, has been shown to cause cell death in a number of mammalian cells in a protein phosphatase 2A (PP2A)-dependent manner. In the current study, using a cell line featuring regulated expression of PyST, we found that PyST arrests cells in mitosis. Live-cell and immunofluorescence studies showed that the majority of the PyST expressing cells were arrested in prometaphase with almost no cells progressing beyond metaphase. These cells exhibited defects in chromosomal congression, sister chromatid cohesion and spindle positioning, thereby resulting in the activation of the spindle assembly checkpoint. Prolonged mitotic arrest then led to cell death via mitotic catastrophe. Cell cycle inhibitors that block cells in G1/S prevented PyST-induced death. PyST-induced cell death that occurs during M is not dependent on p53 status. These data suggested, and our results confirmed, that PP2A inhibition could be used to preferentially kill cancer cells with p53 mutations that proliferate normally in the presence of cell cycle inhibitors.

Abnormalities of the inactive X chromosome are a common feature of BRCA1 mutant and sporadic basal-like breast cancer.

As a clinical entity, breast cancer appears to be a series of subforms, each with a relatively specific molecular phenotype. Among the characteristics that differentiate these subforms are sex hormone receptor expression, HER2 expression, p53 mutation, high-grade histopathology, and particular gene expression array patterns. Sporadic basal-like breast cancer is one such form. It is a relatively common, high-grade, hormone receptor and HER2-expression-negative, p53 mutation-bearing tumor and is particularly lethal. Although wild type for BRCA1, it is a sporadic phenocopy of most cases of BRCA1(/) breast cancer. Not only do the cells of the two tumors resemble one another with respect to the above-noted characteristics, they also share a defect in the maintenance of an intact, inactive X chromosome (Xi). Other high-grade and most low-grade tumors are rarely defective at Xi. This evidence suggests that an Xi defect contributes to the evolution of both sporadic and BRCA1(/) basal-like breast tumors.

Functional communication between endogenous BRCA1 and its partner, BARD1, during Xenopus laevis development.

The breast and ovarian susceptibility protein 1 (BRCA1) heterodimerizes with its structural relative, the BRCA1-associated RING domain protein (BARD1), which may have tumor suppressing function in its own right. Both proteins have evolved from a common evolutionary ancestor, and both exist in Xenopus laevis where, similar to their mammalian homologs, they form functional heterodimers. Depleting frog embryos of either BARD1 or BRCA1 led to similar and widely defective developmental phenotypes as well as depletion of the other polypeptide due to its decreased stability. Thus, each protein, in part, controls the abundance, stability, and function of the other, and these effects are heterodimerization-dependent. The interdependent nature of BRCA1 and BARD1 function supports the view that BARD1/BRCA1 heterodimers play a major role in breast and ovarian cancer suppression.

The p400 complex is an essential E1A transformation target.

Here, we report the identification of a new E1A binding protein complex that is essential for E1A-mediated transformation. Its core component is a SWI2/SNF2-related, 400 kDa protein (p400). Other components include the myc- and p/CAF-associated cofactor, TRRAP/PAF400, the DNA helicases TAP54alpha/beta, actin-like proteins, and the human homolog of the Drosophila Enhancer of Polycomb protein. An E1A mutant, defective in p400 binding, is also defective in transformation. Certain p400 fragments partially rescued this phenotype, underscoring the role of E1A-p400 complex formation in the E1A transforming process. Furthermore, E1A and c-myc each alter the subunit composition of p400 complexes, implying that physiological p400 complex formation contributes to transformation suppression.

Self-excising retroviral vectors encoding the Cre recombinase overcome Cre-mediated cellular toxicity.

The Cre-lox system is often used to manipulate sequences in mammalian genomes. We have observed that continuous expression of the Cre recombinase in cultured cells lacking exogenous lox sites caused decreased growth, cytopathic effects, and chromosomal aberrations. Cre mutants defective in DNA cleavage were not geno- or cytotoxic. A self-excising retroviral vector that incorporates a negative feedback loop to limit the duration and intensity of Cre expression avoided measurable toxicity, retained the ability to excise a target sequence flanked by lox sites, and may provide the basis of a less toxic strategy for the use of Cre or similar recombinases.

Rescue of a telomere length defect of Nijmegen breakage syndrome cells requires NBS and telomerase catalytic subunit.

Nijmegen breakage syndrome (NBS) is a rare human disease displaying chromosome instability, radiosensitivity, cancer predisposition, immunodeficiency, and other defects [1, 2]. NBS is complexed with MRE11 and RAD50 in a DNA repair complex [3-5] and is localized to telomere ends in association with TRF proteins [6, 7]. We show that blood cells from NBS patients have shortened telomere DNA ends. Likewise, cultured NBS fibroblasts that exhibit a premature growth cessation were observed with correspondingly shortened telomeres. Introduction of the catalytic subunit of telomerase, TERT, was alone sufficient to increase the proliferative capacity of NBS fibroblasts. However, NBS, but not TERT, restores the capacity of NBS cells to survive gamma irradiation damage. Strikingly, NBS promotes telomere elongation in conjunction with TERT in NBS fibroblasts. These results suggest that NBS is a required accessory protein for telomere extension. Since NBS patients have shortened telomeres, these defects may contribute to the chromosome instability and disease associated with NBS patients.

Role of T-bet in commitment of TH1 cells before IL-12-dependent selection.

How cytokines control differentiation of helper T (TH) cells is controversial. We show that T-bet, without apparent assistance from interleukin 12 (IL-12)/STAT4, specifies TH1 effector fate by targeting chromatin remodeling to individual interferon-gamma (IFN-gamma) alleles and by inducing IL-12 receptor beta2 expression. Subsequently, it appears that IL-12/STAT4 serves two essential functions in the development of TH1 cells: as growth signal, inducing survival and cell division; and as trans-activator, prolonging IFN-gamma synthesis through a genetic interaction with the coactivator, CREB-binding protein. These results suggest that a cytokine does not simply induce TH fate choice but instead may act as an essential secondary stimulus that mediates selective survival of a lineage.

BACH1, a novel helicase-like protein, interacts directly with BRCA1 and contributes to its DNA repair function.

BRCA1 interacts in vivo with a novel protein, BACH1, a member of the DEAH helicase family. BACH1 binds directly to the BRCT repeats of BRCA1. A BACH1 derivative, bearing a mutation in a residue that was essential for catalytic function in other helicases, interfered with normal double-strand break repair in a manner that was dependent on its BRCA1 binding function. Thus, BACH1/BRCA1 complex formation contributes to a key BRCA1 activity. In addition, germline BACH1 mutations affecting the helicase domain were detected in two early-onset breast cancer patients and not in 200 matched controls. Thus, it is conceivable that, like BRCA1, BACH1 is a target of germline cancer-inducing mutations.

p19ARF targets certain E2F species for degradation.

p19ARF suppresses the growth of cells lacking p53 through an unknown mechanism. p19ARF was found to complex with transcription factors E2F1, -2, and -3. Levels of endogenous or ectopically expressed E2F1, -2, and -3, but not E2F6, were reduced after synthesis of p19ARF, through a mechanism involving increased turnover. p19ARF-induced degradation of E2F1 depended on a functional proteasome, and E2F1 was relocalized to nucleoli when coexpressed with p19ARF. Consistent with reduced levels of E2F1 and E2F3, the proliferation of cells defective for p53 function was suppressed by p19ARF, and the effect was partially reversed by ectopic overexpression of E2F1. These results suggest a broader role for p19ARF as a tumor suppressor, in which targeting of certain E2F species may cooperate with stimulation of the p53 pathway to counteract oncogenic growth signals.

Toward mechanism-based cancer care.

In the past 25 years, research has elucidated molecular mechanisms directing key aspects of tumor cell behavior. Detailed understanding of these mechanisms has already changed methods for diagnosis, prognosis, and treatment. With continuing advances in cancer science and the emergence of new technologies for applying basic science to clinical practice, new methods based on molecular mechanisms will dominate cancer care and prevention.

E2F4 is exported from the nucleus in a CRM1-dependent manner.

E2F is a family of transcription factors required for normal cell cycle control and for cell cycle arrest in G1. E2F4 is the most abundant E2F protein in many cell types. In quiescent cells, it is localized to the nucleus, where it is bound to the retinoblastoma-related protein p130. During entry into the cell cycle, the protein disappears from the nucleus and appears in the cytoplasm. The mechanism by which this change occurs has, in the past, been unclear. We have found that E2F4 is actively exported from the nucleus and that leptomycin B, a specific inhibitor of nuclear export, inhibits this process. E2F4 export is mediated by two hydrophobic export sequences, mutations in either of which result in export failure. Individual export mutants of E2F4, but not a mutant with inactivation of both export signals, can be efficiently excluded from the nucleus by forced coexpression of the nuclear export receptor CRM1. Similarly, CRM1 overexpression can prevent cell cycle arrest induced by the cyclin kinase inhibitor p16(INK4a), an E2F4-dependent process. Taken together, these data suggest that nuclear export contributes to the regulation of E2F4 function, including its ability to regulate exit from G1 in association with a suitable pocket protein.

Meiotic alterations in CAG repeat tracts.

We have investigated meiotic changes in CAG repeat tracts embedded in a yeast chromosome. Repeat tracts undergo either conversion events between homologs or expansion and contraction events that appear to be confined to a single chromatid. We did not find evidence for conversion of tract interruptions or excess exchange of flanking markers.

In search of the tumour-suppressor functions of BRCA1 and BRCA2.

Hereditary breast and ovarian cancer syndromes can be caused by loss-of-function germline mutations in one of two tumour-suppressor genes, BRCA1 and BRCA2 (ref. 1). Each gene product interacts with recombination/DNA repair proteins in pathways that participate in preserving intact chromosome structure. However, it is unclear to what extent such functions specifically suppress breast and ovarian cancer. Here we analyse what is known of BRCA gene function and highlight some unanswered questions in the field.

Suppression of tumor growth through disruption of hypoxia-inducible transcription.

Chronic hypoxia, a hallmark of many tumors, is associated with angiogenesis and tumor progression. Strategies to treat tumors have been developed in which tumor cells are targeted with drugs or gene-therapy vectors specifically activated under hypoxic conditions. Here we report a different approach, in which the normal transcriptional response to hypoxia is selectively disrupted. Our data indicate that specific blockade of the interaction of hypoxia-inducible factor with the CH1 domain of its p300 and CREB binding protein transcriptional coactivators leads to attenuation of hypoxia-inducible gene expression and diminution of tumor growth. Thus, disrupting the normal co-activational response to hypoxia may be a new and useful therapeutic strategy.

E2F4 and E2F5 play an essential role in pocket protein-mediated G1 control.

E2F transcription factors are major regulators of cell proliferation. The diversity of the E2F family suggests that individual members perform distinct functions in cell cycle control. E2F4 and E2F5 constitute a defined subset of the family. Until now, there has been little understanding of their individual biochemical and biological functions. Here, we report that simultaneous inactivation of E2F4 and E2F5 in mice results in neonatal lethality, suggesting that they perform overlapping functions during mouse development. Embryonic fibroblasts isolated from these mice proliferated normally and reentered from Go with normal kinetics compared to wild-type cells. However, they failed to arrest in G1 in response to p16INK4a. Thus, E2F4 and E2F5 are dispensable for cell cycle progression but necessary for pocket protein-mediated G1 arrest of cycling cells.

p300 interacts with the nuclear proto-oncoprotein SYT as part of the active control of cell adhesion.

Complexes containing p300, but not CBP, and the nuclear proto-oncoprotein SYT were detected in confluent cultures of G1-arrested cells but not in sparse cells or during S or G2. SYT sequences constitute the N-terminal segment of a fusion oncogene product, SYT-SSX, routinely detected in synovial sarcoma, an aggressive human tumor. SYT/p300 complex formation promotes cell adhesion to a fibronectin matrix, as reflected by compromise of this process in cells expressing SYT dl mutants that retain p300 binding activity and in the primary fibroblasts of p300 but not CBP heterozygous null mice. The mechanism linking the action of SYT/p300 complexes to adhesion function is, at least in part, transcription activation-independent and results in proper activation of beta1 integrin, a major adhesion receptor.

The impact of lagging strand replication mutations on the stability of CAG repeat tracts in yeast.

We have examined the stability of long tracts of CAG repeats in yeast mutants defective in enzymes suspected to be involved in lagging strand replication. Alleles of DNA ligase (cdc9-1 and cdc9-2) destabilize CAG tracts in the stable tract orientation, i.e., when CAG serves as the lagging strand template. In this orientation nearly two-thirds of the events recorded in the cdc9-1 mutant were tract expansions. While neither DNA ligase allele significantly increases the frequency of tract-length changes in the unstable orientation, the cdc9-1 mutant produced a significant number of expansions in tracts of this orientation. A mutation in primase (pri2-1) destabilizes tracts in both the stable and the unstable orientations. Mutations in a DNA helicase/deoxyribonuclease (dna2-1) or in two RNase H activities (rnh1Delta and rnh35Delta) do not have a significant effect on CAG repeat tract stability. We interpret our results in terms of the steps of replication that are likely to lead to expansion and to contraction of CAG repeat tracts.

ATM phosphorylation of Nijmegen breakage syndrome protein is required in a DNA damage response.

Nijmegen breakage syndrome (NBS) is characterized by extreme radiation sensitivity, chromosomal instability and cancer. The phenotypes are similar to those of ataxia telangiectasia mutated (ATM) disease, where there is a deficiency in a protein kinase that is activated by DNA damage, indicating that the Nbs and Atm proteins may participate in common pathways. Here we report that Nbs is specifically phosphorylated in response to gamma-radiation, ultraviolet light and exposure to hydroxyurea. Phosphorylation of Nbs mediated by gamma-radiation, but not that induced by hydroxyurea or ultraviolet light, was markedly reduced in ATM cells. In vivo, Nbs was phosphorylated on many serine residues, of which S343, S397 and S615 were phosphorylated by Atm in vitro. At least two of these sites were underphosphorylated in ATM cells. Inactivation of these serines by mutation partially abrogated Atm-dependent phosphorylation. Reconstituting NBS cells with a mutant form of Nbs that cannot be phosphorylated at selected, ATM-dependent serine residues led to a specific reduction in clonogenic survival after gamma-radiation. Thus, phosphorylation of Nbs by Atm is critical for certain responses of human cells to DNA damage.