Histone H2AX: a dosage-dependent suppressor of oncogenic translocations and tumors

A hypomorphic Artemis human disease allele causes aberrant chromosomal rearrangements and tumorigenesis

The Artemis gene encodes a DNA nuclease that plays important roles in non-homologous end-joining (NHEJ), a major double-strand break (DSB) repair pathway in mammalian cells. NHEJ factors repair general DSBs as well as programmed breaks generated during the lymphoid-specific DNA rearrangement, V(D)J recombination, which is required for lymphocyte development. Mutations that inactivate Artemis cause a human severe combined immunodeficiency syndrome associated with cellular radiosensitivity. In contrast, hypomorphic Artemis mutations result in combined immunodeficiency syndromes of varying severity, but, in addition, are hypothesized to predispose to lymphoid malignancy. To elucidate the distinct molecular defects caused by hypomorphic compared with inactivating Artemis mutations, we examined tumor predisposition in a mouse model harboring a targeted partial loss-of-function disease allele. We find that, in contrast to Artemis nullizygosity, the hypomorphic mutation leads to increased aberrant intra- and interchromosomal V(D)J joining events. We also observe that dysfunctional Artemis activity combined with p53 inactivation predominantly predisposes to thymic lymphomas harboring clonal translocations distinct from those observed in Artemis nullizygosity. Thus, the Artemis hypomorphic allele results in unique molecular defects, tumor spectrum and oncogenic chromosomal rearrangements. Our findings have significant implications for disease outcomes and treatment of patients with different Artemis mutations.

The role of mechanistic factors in promoting chromosomal translocations found in lymphoid and other cancers

Recurrent chromosomal abnormalities, especially chromosomal translocations, are strongly associated with certain subtypes of leukemia, lymphoma and solid tumors. The appearance of particular translocations or associated genomic alterations can be important indicators of disease prognosis, and in some cases, certain translocations may indicate appropriate therapy protocols. To date, most of our knowledge about chromosomal translocations has derived from characterization of the highly selected recurrent translocations found in certain cancers. Until recently, mechanisms that promote or suppress chromosomal translocations, in particular, those responsible for their initiation, have not been addressed. For translocations to occur, two distinct chromosomal loci must be broken, brought together (synapsed) and joined. Here, we discuss recent findings on processes and pathways that influence the initiation of chromosomal translocations, including the generation fo DNA double strand breaks (DSBs) by general factors or in the context of the Lymphocyte-specific V(D)J and IgH class-switch recombination processes. We also discuss the role of spatial proximity of DSBs in the interphase nucleus with respect to how DSBs on different chromosomes are justaposed for joining. In addition, we discuss the DNA DSB response and its role in recognizing and tethering chromosomal DSBs to prevent translocations, as well as potential roles of the classical and alternative DSB end-joining pathways in suppressing or promoting translocations. Finally, we discuss the potential roles of long range regulatory elements, such as the 3'IgH enhancer complex, in promoting the expression of certain translocations that are frequent in lymphomas and, thereby, contributing to their frequent appearance in tumors.

DNA damage recognition via activated ATM and p53 pathway in nonproliferating human prostate tissue

DNA damage response (DDR) pathways have been extensively studied in cancer cell lines and mouse models, but little is known about how DNA damage is recognized by different cell types in nonmalignant, slowly replicating human tissues. Here, we assess, using ex vivo cultures of human prostate tissue, DDR caused by cytotoxic drugs (camptothecin, doxorubicin, etoposide, and cisplatin) and ionizing radiation (IR) in the context of normal tissue architecture. Using specific markers for basal and luminal epithelial cells, we determine and quantify cell compartment-specific damage recognition. IR, doxorubicin, and etoposide induced the phosphorylation of H2A.X on Ser(139) (γH2AX) and DNA damage foci formation. Surprisingly, luminal epithelial cells lack the prominent γH2AX response after IR when compared with basal cells, although ATM phosphorylation on Ser(1981) and 53BP1 foci were clearly detectable in both cell types. The attenuated γH2AX response seems to result from low levels of total H2A.X in the luminal cells. Marked increase in p53, a downstream target of the activated ATM pathway, was detected only in response to camptothecin and doxorubicin. These findings emphasize the diversity of pathways activated by DNA damage in slowly replicating tissues and reveal an unexpected deviation in the prostate luminal compartment that may be relevant in prostate tumorigenesis. Detailed mapping of tissue and cell type differences in DDR will provide an outlook of relevant responses to therapeutic strategies.

Cellular context-dependent effects of H2ax and p53 deletion on the development of thymic lymphoma

H2AX and Artemis each cooperate with p53 to suppress lymphoma. Germline H2ax(-/-)p53(-/-) mice die of T-cell receptor-β(-) (TCR-β(-)) thymic lymphomas with translocations and other lesions characteristic of human T-cell acute lymphoblastic leukemia. Here, we demonstrate that mice with inactivation of H2ax and p53 in thymocytes die at later ages to TCR-β(-) or TCR-β(+) thymic lymphomas containing a similar pattern of translocations as H2ax(-/-)p53(-/-) tumors. Germline Artemis(-/-) p53(-/-) mice die of lymphomas with antigen receptor locus translocations, whereas Artemis(-/-)H2ax(-/-)p53(-/-) mice die at earlier ages from multiple malignancies. We show here that Artemis(-/-) mice with p53 deletion in thymocytes die of TCR-β(-) tumors containing Tcrα/δ translocations, other clonal translocations, or aneuploidy, as well as Notch1 mutations. Strikingly, Artemis(-/-) mice with H2ax and p53 deletion in thymocytes exhibited a lower rate of mortality from TCR-β(-) tumors, which harbored significantly elevated levels of genomic instability. Our data reveal that the cellular origin of H2ax and p53 loss impacts the rate of mortality from and developmental stage of thymic lymphomas, and suggest that conditional deletion of tumor suppressor genes may provide more physiologic models for human lymphoid malignancies than germline inactivation.

H2AX Phosphorylation: Its Role in DNA Damage Response and Cancer Therapy

Double-strand breaks (DSBs) are the most deleterious DNA lesions, which, if left unrepaired, may have severe consequences for cell survival, as they lead to chromosome aberrations, genomic instability, or cell death. Various physical, chemical, and biological factors are involved in DSB induction. Cells respond to DNA damage by activating the so-called DNA damage response (DDR), a complex molecular mechanism developed to detect and repair DNA damage. The formation of DSBs triggers activation of many factors, including phosphorylation of the histone variant H2AX, producing gammaH2AX. Phosphorylation of H2AX plays a key role in DDR and is required for the assembly of DNA repair proteins at the sites containing damaged chromatin as well as for activation of checkpoints proteins which arrest the cell cycle progression. In general, analysis of gammaH2AX expression can be used to detect the genotoxic effect of different toxic substances. When applied to clinical samples from cancer patients, evaluation of gammaH2AX levels may allow not only to monitor the efficiency of anticancer treatment but also to predict of tumor cell sensitivity to DNA damaging anticancer agents and toxicity of anticancer treatment toward normal cells.

Both ERK1 and ERK2 kinases promote G2/M arrest in etoposide-treated MCF7 cells by facilitating ATM activation

The MEK-ERK pathway plays a role in DNA damage response (DDR). This has been thoroughly studied by modulating MEK activation. However, much less has been done to directly examine the contributions of ERK1 and ERK2 kinases to DDR. Etoposide induces G2/M arrest in a variety of cell lines, including MCF7 cells. DNA damage-induced G2/M arrest depends on the activation of the protein kinase ataxia-telangiectasia mutated (ATM). ATM subsequently activates CHK2 by phosphorylating CHK2 threonine 68 (T68) and CHK2 inactivates CDC25C via phosphorylation of its serine 216 (S216), resulting in G2/M arrest. To determine the contribution of ERK1 and ERK2 to etoposide-induced G2/M arrest, we individually knocked-down ERK1 and ERK2 in MCF7 cells using specific small interfering RNA (siRNA). Knockdown of either kinases significantly reduced ATM activation in response to etoposide treatment, and thereby attenuated phosphorylation of the ATM substrates, including the S139 of H2AX (gammaH2AX), p53 S15, and CHK2 T68. Consistent with these observations, knockdown of either ERK1 or ERK2 reduced etoposide-induced CDC25C S216 phosphorylation and significantly compromised etoposide-induced G2/M arrest in MCF7 cells. Taken together, we demonstrated that both ERK1 and ERK2 kinases play a role in etoposide-induced G2/M arrest by facilitating activation of the ATM pathway. These observations suggest that a cellular threshold level of ERK kinase activity is required for the proper checkpoint activation in MCF7 cells.

ATM-deficient thymic lymphoma is associated with aberrant tcrd rearrangement and gene amplification

Ataxia telangiectasia mutated (ATM) deficiency predisposes humans and mice to T lineage lymphomas with recurrent chromosome 14 translocations involving the T cell receptor alpha/delta (Tcra/d) locus. Such translocations have been thought to result from aberrant repair of DNA double-strand breaks (DSBs) during Tcra locus V(D)J recombination, and to require the Tcra enhancer (Ealpha) for Tcra rearrangement or expression of the translocated oncogene. We now show that, in addition to the known chromosome 14 translocation, ATM-deficient mouse thymic lymphomas routinely contain a centromeric fragment of chromosome 14 that spans up to the 5' boundary of the Tcra/d locus, at which position a 500-kb or larger region centromeric to Tcra/d is routinely amplified. In addition, they routinely contain a large deletion of the telomeric end of one copy of chromosome 12. In contrast to prior expectations, the recurrent translocations and amplifications involve V(D)J recombination-initiated breaks in the Tcrd locus, as opposed to the Tcra locus, and arise independently of the Ealpha. Overall, our studies reveal previously unexpected mechanisms that contribute to the oncogenic transformation of ATM-deficient T lineage cells.

Conserved gammaherpesvirus kinase and histone variant H2AX facilitate gammaherpesvirus latency in vivo

Many herpesvirus-encoded protein kinases facilitate viral lytic replication. Importantly, the role of viral kinases in herpesvirus latency is less clear. Mouse gammaherpesvirus-68 (MHV68)-encoded protein kinase orf36 facilitates lytic replication in part through activation of the host DNA damage response (DDR). Here we show that MHV68 latency was attenuated in the absence of orf36 expression. Unexpectedly, our study uncovered enzymatic activity-independent role of orf36 in the establishment of MHV68 latency following intraperitoneal route of infection. H2AX, an important DDR protein, facilitates MHV68 lytic replication and may be directly phosphorylated by orf36 during lytic infection. In this study, H2AX deficiency, whether systemic or limited to infected cells, attenuated the establishment of MHV68 latency in vivo. Thus, our work reveals viral kinase-dependent regulation of gammaherpesvirus latency and illuminates a novel link between H2AX, a component of a tumor suppressor DDR network, and in vivo latency of a cancer-associated gammaherpesvirus.

Mi-2/NuRD complex making inroads into DNA-damage response pathway

In eukaryotic cells, packaging of DNA into highly condensed chromatin presents a significant obstacle to DNA-based processes. Cells use two major strategies including histone modifications and ATP-dependent chromatin remodeling to alter chromatin structure that allows protein factors to gain access to nucleosomal DNA. Beyond their well-established role in transcription, histone modifications and several classes of ATP-dependent chromatin-remodeling complex have been functionally linked to efficient DNA repair. Mi-2/nucleosome remodeling and histone deacetylation (NuRD) complex uniquely possess both nucleosome remodeling and histone deacetylation activities, which play a vital role in regulating transcription. However, the role of the Mi-2/NuRD complex in DNA damage response remains largely unexplored until now. Recent findings reveal that metastasis-associated protein 1 (MTA1), an integral component of the Mi-2/NuRD complex, has successfully made inroads into DNA damage response pathway, and thus, links two previously unconnected Mi-2/NuRD complex and DNA damage response research areas. In this review, we will summarize recent progress concerning the functions of histone modifications and chromatin remodeling in DNA repair, and discuss new role of Mi-2/NuRD complex in DNA damage response.

The chromosomal instability pathway in colon cancer

The acquisition of genomic instability is a crucial feature in tumor development and there are at least 3 distinct pathways in colorectal cancer pathogenesis: the chromosomal instability (CIN), microsatellite instability, and CpG island methylator phenotype pathways. Most cases of colorectal cancer arise through the CIN pathway, which is characterized by widespread imbalances in chromosome number (aneuploidy) and loss of heterozygosity. It can result from defects in chromosomal segregation, telomere stability, and the DNA damage response, although the full complement of genes underlying CIN remains incompletely described. Coupled with the karyotypic abnormalities observed in CIN tumors are the accumulation of a characteristic set of mutations in specific tumor suppressor genes and oncogenes that activate pathways critical for colorectal cancer initiation and progression. Whether CIN creates the appropriate milieu for the accumulation of these mutations or vice versa remains a provocative and unanswered question. The goal of this review is to provide an updated perspective on the mechanisms that lead to CIN and the key mutations that are acquired in this pathway.

The chromosomal instability pathway in colon cancer

The acquisition of genomic instability is a crucial feature in tumor development and there are at least 3 distinct pathways in colorectal cancer pathogenesis: the chromosomal instability (CIN), microsatellite instability, and CpG island methylator phenotype pathways. Most cases of colorectal cancer arise through the CIN pathway, which is characterized by widespread imbalances in chromosome number (aneuploidy) and loss of heterozygosity. It can result from defects in chromosomal segregation, telomere stability, and the DNA damage response, although the full complement of genes underlying CIN remains incompletely described. Coupled with the karyotypic abnormalities observed in CIN tumors are the accumulation of a characteristic set of mutations in specific tumor suppressor genes and oncogenes that activate pathways critical for colorectal cancer initiation and progression. Whether CIN creates the appropriate milieu for the accumulation of these mutations or vice versa remains a provocative and unanswered question. The goal of this review is to provide an updated perspective on the mechanisms that lead to CIN and the key mutations that are acquired in this pathway.

Focus on histone variant H2AX: to be or not to be

Phosphorylation of histone variant H2AX at serine 139, named gammaH2AX, has been widely used as a sensitive marker for DNA double-strand breaks (DSBs). gammaH2AX is required for the accumulation of many DNA damage response (DDR) proteins at DSBs. Thus it is believed to be the principal signaling protein involved in DDR and to play an important role in DNA repair. However, only mild defects in DNA damage signaling and DNA repair were observed in H2AX-deficient cells and animals. Such findings prompted us and others to explore H2AX-independent mechanisms in DNA damage response. Here, we will review recent advances in our understanding of H2AX-dependent and independent DNA damage signaling and repair pathways in mammalian cells.

Wip1 directly dephosphorylates gamma-H2AX and attenuates the DNA damage response

The integrity of DNA is constantly challenged throughout the life of a cell by both endogenous and exogenous stresses. A well-organized rapid damage response and proficient DNA repair, therefore, become critically important for maintaining genomic stability and cell survival. When DNA is damaged, the DNA damage response (DDR) can be initiated by alterations in chromosomal structure and histone modifications, such as the phosphorylation of the histone H2AX (the phosphorylated form is referred to as gamma-H2AX). gamma-H2AX plays a crucial role in recruiting DDR factors to damage sites for accurate DNA repair. On repair completion, gamma-H2AX must then be reverted to H2AX by dephosphorylation for attenuation of the DDR. Here, we report that the wild-type p53-induced phosphatase 1 (Wip1) phosphatase, which is often overexpressed in a variety of tumors, effectively dephosphorylates gamma-H2AX in vitro and in vivo. Ectopic expression of Wip1 significantly reduces the level of gamma-H2AX after ionizing as well as UV radiation. Forced premature dephosphorylation of gamma-H2AX by Wip1 disrupts recruitment of important DNA repair factors to damaged sites and delays DNA damage repair. Additionally, deletion of Wip1 enhances gamma-H2AX levels in cells undergoing constitutive oncogenic stress. Taken together, our studies show that Wip1 is an important mammalian phosphatase for gamma-H2AX and shows an additional mechanism for Wip1 in the tumor surveillance network.

Origin of chromosomal translocations in lymphoid cancer

Aberrant fusions between heterologous chromosomes are among the most prevalent cytogenetic abnormalities found in cancer cells. Oncogenic chromosomal translocations provide cells with a proliferative or survival advantage. They may either initiate transformation or be acquired secondarily as a result of genomic instability. Here, we highlight recent advances toward understanding the origin of chromosomal translocations in incipient lymphoid cancers and how tumor-suppressive pathways normally limit the frequency of these aberrant recombination events. Deciphering the mechanisms that mediate chromosomal fusions will open new avenues for developing therapeutic strategies aimed at eliminating lesions that lead to the initiation, maintenance, and progression of cancer.

Differential requirement for H2AX and 53BP1 in organismal development and genome maintenance in the absence of poly(ADP)ribosyl polymerase 1

Combined deficiencies of poly(ADP)ribosyl polymerase 1 (PARP1) and ataxia telangiectasia mutated (ATM) result in synthetic lethality and, in the mouse, early embryonic death. Here, we investigated the genetic requirements for this lethality via analysis of mice deficient for PARP1 and either of two ATM-regulated DNA damage response (DDR) factors: histone H2AX and 53BP1. We found that, like ATM, H2AX is essential for viability in a PARP1-deficient background. In contrast, deficiency for 53BP1 modestly exacerbates phenotypes of growth retardation, genomic instability, and organismal radiosensitivity observed in PARP1-deficient mice. To gain mechanistic insights into these different phenotypes, we examined roles for 53BP1 in the repair of replication-associated double-strand breaks (DSBs) in several cellular contexts. We show that 53BP1 is required for DNA-PKcs-dependent repair of hydroxyurea (HU)-induced DSBs but dispensable for RPA/RAD51-dependent DSB repair in the same setting. Moreover, repair of mitomycin C (MMC)-induced DSBs and sister chromatid exchanges (SCEs), two RAD51-dependent processes, are 53BP1 independent. Overall, our findings define 53BP1 as a main facilitator of nonhomologous end joining (NHEJ) during the S phase of the cell cycle, beyond highly specialized lymphocyte rearrangements. These findings have important implications for our understanding of the mechanisms whereby ATM-regulated DDR prevents human aging and cancer.

Prediction of breast cancer sensitivity to neoadjuvant chemotherapy based on status of DNA damage repair proteins

Introduction

Various agents used in breast cancer chemotherapy provoke DNA double-strand breaks (DSBs). DSB repair competence determines the sensitivity of cells to these agents whereby aberrations in the repair machinery leads to apoptosis. Proteins required for this pathway can be detected as nuclear foci at sites of DNA damage when the pathway is intact. Here we investigate whether focus formation of repair proteins can predict chemosensitivity of breast cancer.

Methods

Core needle biopsy specimens were obtained from sixty cases of primary breast cancer before and 18-24 hours after the first cycle of neoadjuvant epirubicin plus cyclophosphamide (EC) treatment. Nuclear focus formation of DNA damage repair proteins was immunohistochemically analyzed and compared with tumor response to chemotherapy.

Results

EC treatment induced nuclear foci of γH2AX, conjugated ubiquitin, and Rad51 in a substantial amount of cases. In contrast, BRCA1 foci were observed before treatment in the majority of the cases and only decreased after EC in thirteen cases. The presence of BRCA1-, γH2AX-, or Rad51-foci before treatment or the presence of Rad51-foci after treatment was inversely correlated with tumor response to chemotherapy. DNA damage response (DDR) competence was further evaluated by considering all four repair indicators together. A high DDR score significantly correlated with low tumor response to EC and EC + docetaxel whereas other clinicopathological factors analyzed did not.

Conclusions

High performing DDR focus formation resulted in tumor resistance to DNA damage-inducing chemotherapy. Our results suggested an importance of evaluation of DDR competence to predict breast cancer chemosensitivity, and merits further studying into its usefulness in exclusion of non-responder patients.

Bisbenzamidine derivative, pentamidine represses DNA damage response through inhibition of histone H2A acetylation

Background

MRE11 is an important nuclease which functions in the end-resection step of homologous recombination (HR) repair of DNA double-strand breaks (DSBs). As MRE11-deficient ATLD cells exhibit hyper radio-sensitivity and impaired DSB repair, MRE11 inhibitors could possibly function as potent radio-sensitizers. Therefore, we investigated whether a bisbenzamidine derivative, pentamidine, which can inhibit endoexonuclease activity, might influence DSB-induced damage responses via inhibition of MRE11.

Results

We first clarified that pentamidine inhibited MRE11 nuclease activity and also reduced ATM kinase activity in vitro. Pentamidine increased the radio-sensitivity of HeLa cells, suggesting that this compound could possibly influence DNA damage response factors in vivo. Indeed, we found that pentamidine reduced the accumulation of γ-H2AX, NBS1 and phospho-ATM at the sites of DSBs. Furthermore, pentamidine decreased HR activity in vivo. Pentamidine was found to inhibit the acetylation of histone H2A which could contribute both to inhibition of IR-induced focus formation and HR repair. These results suggest that pentamidine might exert its effects by inhibiting histone acetyltransferases. We found that pentamidine repressed the activity of Tip60 acetyltransferase which is known to acetylate histone H2A and that knockdown of Tip60 by siRNA reduced HR activity.

Conclusion

These results indicate that inhibition of Tip60 as well as hMRE11 nuclease by pentamidine underlies the radiosensitizing effects of this compound making it an excellent sensitizer for radiotherapy or chemotherapy.

High-risk neuroblastoma tumors with 11q-deletion display a poor prognostic, chromosome instability phenotype with later onset

Analysis of chromosomal aberrations is used to determine the prognosis of neuroblastomas (NBs) and to aid treatment decisions. MYCN amplification (MNA) alone is an incomplete poor prognostic factor, and chromosome 11q status has recently been included in risk classification. We analyzed 165 NB tumors using high-density SNP microarrays and specifically compared the high-risk groups defined by MNA (n = 37) and 11q-deletion (n = 21). Median patient age at diagnosis was 21 months for MNA tumors and 42 months for 11q-deletion tumors, and median survival time after diagnosis was 16 months for MNA and 40 months for 11q deletion. Overall survival (at 8 years) was approximately 35% in both groups. MNA and 11q deletion were almost mutually exclusive; only one case harbored both aberrations. The numbers of segmental aberrations differed significantly; the MNA group had a median of four aberrations, whereas the 11q-deletion group had 12. The high frequency of chromosomal breaks in the 11q-deletion group is suggestive of a chromosomal instability phenotype gene located in 11q; one such gene, H2AFX, is located in 11q23.3 (within the 11q-deletion region). Furthermore, in the groups with segmental aberrations without MNA or 11q deletion, the tumors with 17q gain have worse prognosis than those with segmental aberrations without 17q gain, which have a favorable outcome. This study has implications for therapy in different risk groups and stresses that genome-wide microarray analyses should be included in clinical management to fully evaluate risk, aid diagnosis, and guide treatment.

gammaH2AX marks the main events of the spermatogenic process

Recent findings reveal that chromatin reorganization requires formation and repair of DNA double strand breaks (DSBs). Therefore, dynamic changes in chromatin structure taking place along the mitotic and the meiotic processes should be accompanied by histone H2AX phosphorylation and formation of gamma H2AX, a prominent event occurring at DSB sites. Accordingly, gamma H2AX has been detected at multiple developmental steps in adult germ cells. Nevertheless, no comprehensive study focusing on the main biological events occurring in the seminiferous epithelium has been carried out in mammals. Here I analyze H2AX phosphorylation in immunostained testis sections comparing PAS/cresyl violet counterstained, noncounterstained, and immuno-fluorescence preparations and show several waves of H2AX phosphorylation/dephosphorylation coupled to various developmental phases of spermatogonia and spermatocytes as well as to spermatid differentiation. The combined detection of DNA replication or free 3'-OH DNA ends highlight the possible significance of gamma H2AX at these locations.

Aggressive chronic lymphocytic leukemia with elevated genomic complexity is associated with multiple gene defects in the response to DNA double-strand breaks

PURPOSE

Genomic complexity is present in approximately 15% to 30% of all chronic lymphocytic leukemia (CLL) and has emerged as a strong independent predictor of rapid disease progression and short remission duration in CLL. We conducted this study to advance our understanding of the causes of genomic complexity in CLL.

EXPERIMENTAL DESIGN

We have obtained quantitative measurements of radiation-induced apoptosis and radiation-induced ATM autophosphorylation in purified CLL cells from 158 and 140 patients, respectively, and have used multivariate analysis to identify independent contributions of various biological variables on genomic complexity in CLL.

RESULTS

Here, we identify a strong independent effect of radiation resistance on elevated genomic complexity in CLL and describe radiation resistance as a predictor for shortened CLL survival. Furthermore, using multivariate analysis, we identify del17p/p53 aberrations, del11q, del13q14 type II (invariably resulting in Rb loss), and CD38 expression as independent predictors of genomic complexity in CLL, with aberrant p53 as a predictor of approximately 50% of genomic complexity in CLL. Focusing on del11q, we determined that normalized ATM activity was a modest predictor of genomic complexity but was not independent of del11q. Through single nucleotide polymorphism array-based fine mapping of del11q, we identified frequent monoallelic loss of Mre11 and H2AFX in addition to ATM, indicative of compound del11q-resident gene defects in the DNA double-strand break response.

CONCLUSIONS

Our quantitative analysis links multiple molecular defects, including for the first time del11q and large 13q14 deletions (type II), to elevated genomic complexity in CLL, thereby suggesting mechanisms for the observed clinical aggressiveness of CLL in patients with unstable genomes.

p53 suppresses structural chromosome instability after mitotic arrest in human cells

The p53 tumor suppressor inhibits the proliferation of cells which undergo prolonged activation of the mitotic checkpoint. However, the function of this antiproliferative response is not well defined. Here we report that p53 suppresses structural chromosome instability following mitotic arrest in human cells. In both HCT116 colon cancer cells and normal human fibroblasts, DNA breaks occurred during mitotic arrest in a p53-independent manner, but p53 was required to suppress the proliferation and structural chromosome instability of the resulting polyploid cells. In contrast, cells made polyploid without mitotic arrest exhibited neither significant structural chromosome instability nor p53-dependent cell cycle arrest. We also observed that p53 suppressed both the frequency and structural chromosome instability of spontaneous polyploids in HCT116 cells. Furthermore, time-lapse videomicroscopy revealed that polyploidization of p53^−/− HCT116 cells is frequently accompanied by mitotic arrest. These data suggest that a function of the p53-dependent postmitotic response is the prevention of structural chromosome instability following prolonged activation of the mitotic checkpoint. Accordingly, our study suggests a novel mechanism of tumor suppression for p53, as well as a potential role for p53 in the outcome of antimitotic chemotherapy.

MRE11-RAD50-NBS1 complex dictates DNA repair independent of H2AX

DNA double-strand breaks (DSBs) represent one of the most serious forms of DNA damage that can occur in the genome. Here, we show that the DSB-induced signaling cascade and homologous recombination (HR)-mediated DSB repair pathway can be genetically separated. We demonstrate that the MRE11-RAD50-NBS1 (MRN) complex acts to promote DNA end resection and the generation of single-stranded DNA, which is critically important for HR repair. These functions of the MRN complex can occur independently of the H2AX-mediated DNA damage signaling cascade, which promotes stable accumulation of other signaling and repair proteins such as 53BP1 and BRCA1 to sites of DNA damage. Nevertheless, mild defects in HR repair are observed in H2AX-deficient cells, suggesting that the H2AX-dependent DNA damage-signaling cascade assists DNA repair. We propose that the MRN complex is responsible for the initial recognition of DSBs and works together with both CtIP and the H2AX-dependent DNA damage-signaling cascade to facilitate repair by HR and regulate DNA damage checkpoints.

The p53-inducible gene 3 (PIG3) contributes to early cellular response to DNA damage

The p53-inducible gene 3 (PIG3) is originally isolated as a p53 downstream target gene, but its function remains unknown. Here, we report a role of PIG3 in the activation of DNA damage checkpoints, after UV irradiation or radiomimetic drug neocarzinostatin (NCS). We show that depletion of endogenous PIG3 sensitizes cells to DNA damage agents, and impaired DNA repair. PIG3 depletion also allows for UV- and NCS-resistant DNA synthesis and permits cells to progress into mitosis, indicating that PIG3 knockdown can suppress intra-S phase and G2/M checkpoints. PIG3-depleted cells show reduced Chk1 and Chk2 phosphorylation after DNA damage, which may directly contribute to checkpoint bypass. PIG3 exhibited diffuse nuclear staining in the majority of untreated cells and forms discrete nuclear foci in response to DNA damage. PIG3 colocalizes with gamma-H2AX and 53BP1 to sites of DNA damage after DNA damage, and binds to a gamma-H2AX. Notably, PIG3 depletion decreases the efficient induction and maintenance of H2AX phosphorylation after DNA damage. Moreover, PIG3 contributes to the recruitment of 53BP1, Mre11, Rad50 and Nbs1 to the sites of DNA break lesions in response to DNA damage. Our combined results suggest that PIG3 is a critical component of the DNA damage response pathway and has a direct role in the transmission of the DNA damage signal from damaged DNA to the intra-S and G2/M checkpoint machinery in human cells.

AID produces DNA double-strand breaks in non-Ig genes and mature B cell lymphomas with reciprocal chromosome translocations

Cancer-initiating translocations such as those associated with lymphomas require the formation of paired DNA double-strand breaks (DSBs). Activation-induced cytidine deaminase (AID) produces widespread somatic mutation in mature B cells; however, the extent of "off-target" DSB formation and its role in translocation-associated malignancy is unknown. Here, we show that deregulated expression of AID causes widespread genome instability, which alone is insufficient to induce B cell lymphoma; transformation requires concomitant loss of the tumor suppressor p53. Mature B cell lymphomas arising as a result of deregulated AID expression are phenotypically diverse and harbor clonal reciprocal translocations involving a group of Immunoglobulin (Ig) and non-Ig genes that are direct targets of AID. This group includes miR-142, a previously unknown micro-RNA target that is translocated in human B cell malignancy. We conclude that AID produces DSBs throughout the genome, which can lead to lymphoma-associated chromosome translocations in mature B cells.

ATM and p53 are essential in the cell-cycle containment of DNA breaks during V(D)J recombination in vivo

V(D)J recombination is essential for the maturation of lymphocytes. Because of the involvement of cutting and joining DNA double strands, this recombination activity is strictly contained within the noncycling phases of the cell cycle. Such containment is crucial for the maintenance of genomic integrity. The ataxia telangiectasia mutated (ATM) gene is known to have a central role in sensing general DNA damage and mediating cell-cycle checkpoint. In this study, we investigated the role of ATM and its downstream targets in the cell-cycle control of V(D)J recombination in vivo. Our results revealed the persistence of double-strand breaks (DSBs) throughout the cell cycle in ATM(-/-) and p53(-/-) thymocytes, but the cell-cycle regulation of a V(D)J recombinase, Rag-2, was normal. The histone variant H2AX, which is phosphorylated during normal V(D)J recombination, was dispensable for containing DSBs. H2AX was still phosphorylated at V(D)J loci in the absence of ATM. Therefore, V(D)J recombination, a physiological DNA rearrangement process, activates the ATM/p53 pathway to contain DNA breaks within the noncycling cells and surprisingly this pathway is not important for containing Rag-2 activity. This study shows the dynamic multiple functions of ATM in maintaining genomic stability and preventing tumorigenesis in developing lymphocytes.

Colocalization of somatic and meiotic double strand breaks near the Myc oncogene on mouse chromosome 15

Both somatic and meiotic recombinations involve the repair of DNA double strand breaks (DSBs) that occur at preferred locations in the genome. Improper repair of DSBs during either mitosis or meiosis can lead to mutations, chromosomal aberration such as translocations, cancer, and/or cell death. Currently, no model exists that explains the locations of either spontaneous somatic DSBs or programmed meiotic DSBs or relates them to each other. One common class of tumorigenic translocations arising from DSBs is chromosomal rearrangements near the Myc oncogene. Myc translocations have been associated with Burkitt lymphoma in humans, plasmacytoma in mice, and immunocytoma in rats. Comparing the locations of somatic and meiotic DSBs near the mouse Myc oncogene, we demonstrated that the placement of these DSBs is not random and that both events clustered in the same short discrete region of the genome. Our work shows that both somatic and meiotic DSBs tend to occur in proximity to each other within the Myc region, suggesting that they share common originating features. It is likely that some regions of the genome are more susceptible to both somatic and meiotic DSBs, and the locations of meiotic hotspots may be an indicator of genomic regions more susceptible to DNA damage.

Histone H2AX stabilizes broken DNA strands to suppress chromosome breaks and translocations during V(D)J recombination

The H2AX core histone variant is phosphorylated in chromatin around DNA double strand breaks (DSBs) and functions through unknown mechanisms to suppress antigen receptor locus translocations during V(D)J recombination. Formation of chromosomal coding joins and suppression of translocations involves the ataxia telangiectasia mutated and DNA-dependent protein kinase catalytic subunit serine/threonine kinases, each of which phosphorylates H2AX along cleaved antigen receptor loci. Using Abelson transformed pre–B cell lines, we find that H2AX is not required for coding join formation within chromosomal V(D)J recombination substrates. Yet we show that H2AX is phosphorylated along cleaved Igκ DNA strands and prevents their separation in G1 phase cells and their progression into chromosome breaks and translocations after cellular proliferation. We also show that H2AX prevents chromosome breaks emanating from unrepaired RAG endonuclease-generated TCR-α/δ locus coding ends in primary thymocytes. Our data indicate that histone H2AX suppresses translocations during V(D)J recombination by creating chromatin modifications that stabilize disrupted antigen receptor locus DNA strands to prevent their irreversible dissociation. We propose that such H2AX-dependent mechanisms could function at additional chromosomal locations to facilitate the joining of DNA ends generated by other types of DSBs.

A common gain of function of p53 cancer mutants in inducing genetic instability

The critical tumor suppressor p53 is mutated in over half of all human cancers. The majority of p53 cancer mutations are missense mutations, which can be classified into contact mutations that directly disrupt the DNA-binding of p53 but have modest impact on p53 conformation and structural mutations that greatly disrupt p53 conformation. Many p53 cancer mutants, including the hot spot mutations (R175H, R248W and R273H), not only lose p53-dependent tumor-suppressor activities, but also acquire new oncogenic activities to promote cancer. Therefore, it is critical to elucidate the gain of oncogenic function of p53 cancer mutants. Using humanized p53-mutant knock-in mouse models, we have identified a gain of oncogenic function shared by the most common p53 contact mutants (R273H and R248W) and structural mutant (R175H). This gain of function inactivates Mre11/ATM-dependent DNA damage responses, leading to chromosomal translocation and defective G(2)/M checkpoint. Considering the critical roles of ATM in maintaining genetic stability and therapeutic responses to many cancer treatments, the identification of this common gain of function of p53 cancer mutants will have important implication on the drug resistance of a significant portion of human cancers that express either the contact or structural p53 cancer mutants.

Assembly of checkpoint and repair machineries at DNA damage sites

The remarkably coordinated nature of the DNA damage response pathway relies on numerous mechanisms that facilitate the assembly of checkpoint and repair factors at DNA breaks. Post-translational modifications on and around chromatin have critical roles in allowing the timely and sequential assembly of DNA damage responsive elements at the vicinity of DNA breaks. Notably, recent advances in forward genetics and proteomics-based approaches have enabled the identification of novel components within the DNA damage response pathway, providing a more comprehensive picture of the molecular network that assists in the detection and propagation of DNA damage signals.

Chipping away at gamma-H2AX foci

The mammalian histone H2AX protein functions as a dosage-dependent genomic caretaker and tumor suppressor. Phosphorylation of H2AX to form gamma-H2AX in chromatin around DNA double strand breaks (DSBs) is an early event following induction of these hazardous lesions. For a decade, mechanisms that regulate H2AX phosphorylation have been investigated mainly through two-dimensional immunofluorescence (IF). We recently used chromatin immunoprecipitation (ChIP) to measure gamma-H2AX densities along chromosomal DNA strands broken in G(1) phase mouse lymphocytes. Our experiments revealed that (1) gamma-H2AX densities in nucleosomes form at high levels near DSBs and at diminishing levels farther and farther away from DNA ends, and (2) ATM regulates H2AX phosphorylation through both MDC1-dependent and MDC1-independent means. Neither of these mechanisms were discovered by previous if studies due to the inherent limitations of light microscopy. Here, we compare data obtained from parallel gamma-H2AX ChIP and three-dimensional IF analyses and discuss the impact of our findings upon molecular mechanisms that regulate H2AX phosphorylation in chromatin around DNA breakage sites.

Efficacy of citicoline as an acute stroke treatment

Citicoline (cytidine-5'-diphosphocholine or CDP-choline) is a precursor essential for the synthesis of phosphatidylcholine, one of the cell membrane components that is degraded during cerebral ischemia to free fatty acids and free radicals. Animal studies suggest that citicoline may protect cell membranes by accelerating resynthesis of phospholipids and suppressing the release of free fatty acids, stabilizing cell membranes, and reducing free radical generation. Numerous experimental stroke studies with citicoline have shown improved outcome and reduced infarct size in both ischemic and hemorrhagic stroke models. Citicoline has been studied worldwide in both ischemic and hemorrhagic clinical stroke with excellent safety and possibly efficacy found in several trials. A meta-analysis of four randomized US clinical citicoline trials concluded that treatment with oral citicoline within the first 24 h after a moderate to severe stroke is safe and increases the probability of complete recovery at 3 months. Citicoline clinical efficacy trials are now continuing outside of the US in both ischemic and hemorrhagic stroke. A citicoline supplement is now available from several sources on the internet.

Mechanisms of double-strand break repair in somatic mammalian cells

DNA chromosomal DSBs (double-strand breaks) are potentially hazardous DNA lesions, and their accurate repair is essential for the successful maintenance and propagation of genetic information. Two major pathways have evolved to repair DSBs: HR (homologous recombination) and NHEJ (non-homologous end-joining). Depending on the context in which the break is encountered, HR and NHEJ may either compete or co-operate to fix DSBs in eukaryotic cells. Defects in either pathway are strongly associated with human disease, including immunodeficiency and cancer predisposition. Here we review the current knowledge of how NHEJ and HR are controlled in somatic mammalian cells, and discuss the role of the chromatin context in regulating each pathway. We also review evidence for both co-operation and competition between the two pathways.

H2AX: functional roles and potential applications

Upon DNA double-strand break (DSB) induction in mammals, the histone H2A variant, H2AX, becomes rapidly phosphorylated at serine 139. This modified form, termed gamma-H2AX, is easily identified with antibodies and serves as a sensitive indicator of DNA DSB formation. This review focuses on the potential clinical applications of gamma-H2AX detection in cancer and in response to other cellular stresses. In addition, the role of H2AX in homeostasis and disease will be discussed. Recent work indicates that gamma-H2AX detection may become a powerful tool for monitoring genotoxic events associated with cancer development and tumor progression.

Haploinsufficiency and acquired loss of Bcl11b and H2AX induces blast crisis of chronic myelogenous leukemia in a transgenic mouse model

Chronic myelogenous leukemia (CML) is a hematological malignancy that begins as indolent chronic phase (CP) but inevitably progresses to fatal blast crisis (BC). p210BCR/ABL, a chimeric protein with enhanced kinase activity, initiates CML CP, and additional genetic alterations account for progression to BC, but the precise mechanisms underlying disease evolution are not fully understood. In the present study, we investigated the possible contribution of dysfunction of Bcl11b, a zinc-finger protein required for thymocyte differentiation, and of H2AX, a histone protein involved in DNA repair, to the transition from CML CP to BC. For this purpose, we crossed CML CP-exhibiting p210BCR/ABL transgenic (BA(tg/-)) mice with Bcl11b heterozygous (Bcl11b(+/-)) mice and H2AX heterozygous (H2AX(+/-)) mice. Interestingly, p210BCR/ABL transgenic, Bcl11b heterozygous (BA(tg/-)Bcl11b(+/-)) mice and p210BCR/ABL transgenic, H2AX heterozygous (BA(tg/-)H2AX(+/-)) mice frequently developed CML BC with T-cell phenotype and died in a short period. In addition, whereas p210BCR/ABL was expressed in all of the leukemic tissues, the expression of Bcl11b and H2AX was undetectable in several tumors, which was attributed to the loss of the residual normal allele or the lack of mRNA expression. These results indicate that Bcl11b and H2AX function as tumor suppressor and that haploinsufficiency and acquired loss of these gene products cooperate with p210BCR/ABL to develop CML BC.

Faithful after break-up: suppression of chromosomal translocations

Chromosome integrity in response to chemically or radiation-induced chromosome breaks and the perturbation of ongoing replication forks relies on multiple DNA repair mechanisms. However, repair of these lesions may lead to unwanted chromosome rearrangement if not properly executed or regulated. As these types of chromosomal alterations threaten the cell's and the organism's very own survival, multiple systems are developed to avoid or at least limit break-induced chromosomal rearrangements. In this review, we highlight cellular strategies for repressing DNA break-induced chromosomal translocations in multiple model systems including yeast, mouse, and human. These pathways select proper homologous templates or broken DNA ends for the faithful repair of DNA breaks to avoid undesirable chromosomal translocations.

Formation of dynamic gamma-H2AX domains along broken DNA strands is distinctly regulated by ATM and MDC1 and dependent upon H2AX densities in chromatin

A hallmark of the cellular response to DNA double-strand breaks (DSBs) is histone H2AX phosphorylation in chromatin to generate gamma-H2AX. Here, we demonstrate that gamma-H2AX densities increase transiently along DNA strands as they are broken and repaired in G1 phase cells. The region across which gamma-H2AX forms does not spread as DSBs persist; rather, gamma-H2AX densities equilibrate at distinct levels within a fixed distance from DNA ends. Although both ATM and DNA-PKcs generate gamma-H2AX, only ATM promotes gamma-H2AX formation to maximal distance and maintains gamma-H2AX densities. MDC1 is essential for gamma-H2AX formation at high densities near DSBs, but not for generation of gamma-H2AX over distal sequences. Reduced H2AX levels in chromatin impair the density, but not the distance, of gamma-H2AX formed. Our data suggest that H2AX fuels a gamma-H2AX self-reinforcing mechanism that retains MDC1 and activated ATM in chromatin near DSBs and promotes continued local phosphorylation of H2AX.

Connection between histone H2A variants and chromatin remodeling complexes

The organization of the eukaryotic genome into chromatin makes it inaccessible to the factors required for gene transcription and DNA replication, recombination, and repair. In addition to histone-modifying enzymes and ATP-dependent chromatin remodeling complexes, which play key roles in regulating many nuclear processes by altering the chromatin structure, cells have developed a mechanism of modulating chromatin structure by incorporating histone variants. These variants are incorporated into specific regions of the genome throughout the cell cycle. H2A.Z, which is an evolutionarily conserved H2A variant, performs several seemingly unrelated and even contrary functions. Another H2A variant, H2A.X, plays a very important role in the cellular response to DNA damage. This review summarizes the recent developments in our understanding of the role of H2A.Z and H2A.X in the regulation of chromatin structure and function, focusing on their functional links with chromatin modifying and remodeling complexes.

Bringing H2AX into the angiogenesis family

The cell's ability to sense and respond to DNA damage is critical to maintain homeostasis and prevent the development of cancer. Paradoxically, Economopoulou et al. recently reported that a DNA damage response protein, H2AX, promotes tumor growth and angiogenesis.

N-acetyl cysteine protects against ionizing radiation-induced DNA damage but not against cell killing in yeast and mammals

Ionizing radiation (IR) induces DNA strand breaks leading to cell death or deleterious genome rearrangements. In the present study, we examined the role of N-acetyl-L-cysteine (NAC), a clinically proven safe agent, for it's ability to protect against gamma-ray-induced DNA strand breaks and/or DNA deletions in yeast and mammals. In the yeast Saccharomyces cerevisiae, DNA deletions were scored by reversion to histidine prototrophy. Human lymphoblastoid cells were examined for the frequency of gamma-H2AX foci formation, indicative of DNA double strand break formation. DNA strand breaks were also measured in mouse peripheral blood by the alkaline comet assay. In yeast, NAC reduced the frequency of IR-induced DNA deletions. However, NAC did not protect against cell death. NAC also reduced gamma-H2AX foci formation in human lymphoblastoid cells but had no protective effect in the colony survival assay. NAC administration via drinking water fully protected against DNA strand breaks in mice whole-body irradiated with 1Gy but not with 4Gy. NAC treatment in the absence of irradiation was not genotoxic. These data suggest that, given the safety and efficacy of NAC in humans, NAC may be useful in radiation therapy to prevent radiation-mediated genotoxicity, but does not interfere with efficient cancer cell killing.

RAG-1 and ATM coordinate monoallelic recombination and nuclear positioning of immunoglobulin loci

Coordinated recombination of homologous antigen receptor loci is thought to be important for allelic exclusion. Here, we show that homologous Ig alleles pair in a stage-specific manner that mirrors the recombination patterns of these loci. The frequency of homologous Ig pairing was substantially reduced in the absence of the RAG1-RAG2 recombinase and was rescued in Rag1^-/- developing B cells with a transgene expressing a RAG1 active site mutant that supports DNA binding but not cleavage. The introduction of DNA breaks on one Ig allele induced ATM-dependent repositioning of the other allele to pericentromeric heterochromatin. ATM activated by the cleaved allele acts in trans on the uncleaved allele to prevent bi-allelic recombination and chromosome breaks or translocations.

Contemplating chemosensitivity of basal-like breast cancer based on BRCA1 dysfunction

Gene-expression profiling classified breast cancer to intrinsic subtypes, including luminal A and B, HER2 positive, normal-breast-like, and basal-like tumors. Of these, basal-like tumors that express basal cytokeratins and that are negative for estrogen receptor alpha, progesterone receptor, and HER2 show the most aggressive phenotype with a poor prognosis. Analyses of clinical samples and basic research indicate that basal-like breast cancer is caused by deficiencies in the breast cancer susceptibility protein, BRCA1. Indeed, conditionally deleting BRCA1 from the mammary gland causes mice to develop basal-like cancers at high rates. One of the major functions of BRCA1 is DNA double-strand break (DSB) repair, and its failure to perform causes increased sensitivity of cells to DNA damage-inducing agents, such as PARP inhibitors, DNA cross-linkers, or topoisomerase inhibitors. Therefore, BRCA1 dysfunction could be a principal target for therapeutic application of basal-like breast cancer. Recently, significant progress has been made in understanding the BRCA1 cascade in response to DSBs, where ubiquitin polymer formation plays critical roles. Ubiquitination was indeed found to be an apparent early response of breast cancer to neoadjuvant treatment with epirubicin and cyclophosphamide. Deducing the role of BRCA1 ubiquitin E3 ligase activity in this pathway is a critical challenge to further clarify its functional mechanism. In individualized treatment of breast cancer, evaluation of the DNA repair capacity by the BRCA1 pathway may be an important issue when determining proper treatment of basal-like breast cancer.

Chromatin remodelling beyond transcription: the INO80 and SWR1 complexes

Chromatin-modifying factors have essential roles in DNA processing pathways that dictate cellular functions. The ability of chromatin modifiers, including the INO80 and SWR1 chromatin-remodelling complexes, to regulate transcriptional processes is well established. However, recent studies reveal that the INO80 and SWR1 complexes have crucial functions in many other essential processes, including DNA repair, checkpoint regulation, DNA replication, telomere maintenance and chromosome segregation. During these diverse nuclear processes, the INO80 and SWR1 complexes function cooperatively with their histone substrates, gamma-H2AX and H2AZ. This research reveals that INO80 and SWR1 ATP-dependent chromatin remodelling is an integral component of pathways that maintain genomic integrity.

PTIP regulates 53BP1 and SMC1 at the DNA damage sites

Although PTIP is implicated in the DNA damage response, through interactions with 53BP1, the function of PTIP in the DNA damage response remain elusive. Here, we show that RNF8 controls DNA damage-induced nuclear foci formation of PTIP, which in turn regulates 53BP1 localization to the DNA damage sites. In addition, SMC1, a substrate of ATM, could not be phosphorylated at the DNA damage sites in the absence of PTIP. The PTIP-dependent pathway is important for DNA double strand breaks repair and DNA damage-induced intra-S phase checkpoint activation. Taken together, these results suggest that the role of PTIP in the DNA damage response is downstream of RNF8 and upstream of 53BP1. Thus, PTIP regulates 53BP1-dependent signaling pathway following DNA damage.

RAD18 transmits DNA damage signalling to elicit homologous recombination repair

To maintain genome stability, cells respond to DNA damage by activating signalling pathways that govern cell-cycle checkpoints and initiate DNA repair. Cell-cycle checkpoint controls should connect with DNA repair processes, however, exactly how such coordination occurs in vivo is largely unknown. Here we describe a new role for the E3 ligase RAD18 as the integral component in translating the damage response signal to orchestrate homologous recombination repair (HRR). We show that RAD18 promotes homologous recombination in a manner strictly dependent on its ability to be recruited to sites of DNA breaks and that this recruitment relies on a well-defined DNA damage signalling pathway mediated by another E3 ligase, RNF8. We further demonstrate that RAD18 functions as an adaptor to facilitate homologous recombination through direct interaction with the recombinase RAD51C. Together, our data uncovers RAD18 as a key factor that orchestrates HRR through surveillance of the DNA damage signal.

Histone H2A has a novel variant in fish oocytes

Histone variants and their modification have significant roles in many cellular processes. In this study, we identified and characterized the histone H2A variant h2af1o in fish and revealed its oocyte-specific expression pattern during oogenesis and embryogenesis. Moreover, posttranslational modification of H2af1o was observed that results from phosphorylation during oocyte maturation. To understand the binding dynamics of the novel core histone variant H2af1o in nucleosomes, we cloned ubiquitous gibel carp h2afx as a conventional histone control and investigated the dynamic exchange difference in chromatin by fluorescence recovery after photobleaching. H2af1o has significantly higher mobility in nucleosomes than ubiquitous H2afx. Compared with ubiquitous H2afx, H2af1o has a tightly binding C-terminal and a weakly binding N-terminal. These data indicate that fish oocytes have a novel H2A variant that destabilizes nucleosomes by protruding its N-terminal tail and stabilizes core particles by contracting its C-terminal tail. Our findings suggest that H2af1o may have intrinsic ability to modify chromatin properties during fish oogenesis, oocyte maturation, and early cleavage.

miR-24-mediated downregulation of H2AX suppresses DNA repair in terminally differentiated blood cells

Terminally differentiated cells have a reduced capacity to repair double-stranded breaks, but the molecular mechanism behind this downregulation is unclear. Here we find that miR-24 is upregulated during postmitotic differentiation of hematopoietic cell lines and regulates the histone variant H2AX, a protein that has a key role in the double-stranded break response. We show that the H2AX 3' untranslated region contains conserved miR-24 binding sites that are indeed regulated by miR-24. During terminal differentiation, both H2AX mRNA and protein levels are substantially reduced by miR-24 upregulation in in vitro differentiated cells; similar diminished levels are found in primary human blood cells. miR-24-mediated suppression of H2AX renders cells hypersensitive to gamma-irradiation and genotoxic drugs, a phenotype that is fully rescued by overexpression of miR-24-insensitive H2AX. Therefore, miR-24 upregulation in postreplicative cells reduces H2AX and makes them vulnerable to DNA damage.