Regulation of DNA repair throughout the cell cycle. Nat Rev Mol Cell Biol. 2008;9:297-308. [PMID: 18285803 DOI: 10.1038/nrm2351]

Adeno-associated virus replication induces a DNA damage response coordinated by DNA-dependent protein kinase

The parvovirus adeno-associated virus (AAV) contains a small single-stranded DNA genome with inverted terminal repeats that form hairpin structures. In order to propagate, AAV relies on the cellular replication machinery together with functions supplied by coinfecting helper viruses such as adenovirus (Ad). Here, we examined the host cell response to AAV replication in the context of Ad or Ad helper proteins. We show that AAV and Ad coinfection activates a DNA damage response (DDR) that is distinct from that seen during Ad or AAV infection alone. The DDR was also triggered when AAV replicated in the presence of minimal Ad helper proteins. We detected autophosphorylation of the kinases ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and signaling to downstream targets SMC1, Chk1, Chk2, H2AX, and XRCC4 and multiple sites on RPA32. The Mre11 complex was not required for activation of the DDR to AAV infection. Additionally, we found that DNA-PKcs was the primary mediator of damage signaling in response to AAV replication. Immunofluorescence revealed that some activated damage proteins were found in a pan-nuclear pattern (phosphorylated ATM, SMC1, and H2AX), while others such as DNA-PK components (DNA-PKcs, Ku70, and Ku86) and RPA32 accumulated at AAV replication centers. Although expression of the large viral Rep proteins contributed to some damage signaling, we observed that the full response required replication of the AAV genome. Our results demonstrate that AAV replication in the presence of Ad helper functions elicits a unique damage response controlled by DNA-PK.

Differential regulation of homologous recombination at DNA breaks and replication forks by the Mrc1 branch of the S-phase checkpoint

The Rad52 pathway has a central function in the recombinational repair of chromosome breaks and in the recovery from replication stress. Tolerance to replication stress also depends on the Mec1 kinase, which activates the DNA replication checkpoint in an Mrc1-dependent manner in response to fork arrest. Although the Mec1 and Rad52 pathways are initiated by the same single-strand DNA (ssDNA) intermediate, their interplay at stalled forks remains largely unexplored. Here, we show that the replication checkpoint suppresses the formation of Rad52 foci in an Mrc1-dependent manner and prevents homologous recombination (HR) at chromosome breaks induced by the HO endonuclease. This repression operates at least in part by impeding resection of DNA ends, which is essential to generate 3' ssDNA tails, the primary substrate of HR. Interestingly, we also observed that the Mec1 pathway does not prevent recombination at stalled forks, presumably because they already contain ssDNA. Taken together, these data indicate that the DNA replication checkpoint suppresses genomic instability in S phase by blocking recombination at chromosome breaks and permitting helpful recombination at stalled forks.

Wolbachia-mediated cytoplasmic incompatibility is associated with impaired histone deposition in the male pronucleus

Wolbachia is a bacteria endosymbiont that rapidly infects insect populations through a mechanism known as cytoplasmic incompatibility (CI). In CI, crosses between Wolbachia-infected males and uninfected females produce severe cell cycle defects in the male pronucleus resulting in early embryonic lethality. In contrast, viable progeny are produced when both parents are infected (the Rescue cross). An important consequence of CI–Rescue is that infected females have a selective advantage over uninfected females facilitating the rapid spread of Wolbachia through insect populations. CI disrupts a number of prophase and metaphase events in the male pronucleus, including Cdk1 activation, chromosome condensation, and segregation. Here, we demonstrate that CI disrupts earlier interphase cell cycle events. Specifically, CI delays the H3.3 and H4 deposition that occurs immediately after protamine removal from the male pronucleus. In addition, we find prolonged retention of the replication factor PCNA in the male pronucleus into metaphase, indicating progression into mitosis with incompletely replicated DNA. We propose that these CI-induced interphase defects in de novo nucleosome assembly and replication are the cause of the observed mitotic condensation and segregation defects. In addition, these interphase chromosome defects likely activate S-phase checkpoints, accounting for the previously described delays in Cdk1 activation. These results have important implications for the mechanism of Rescue and other Wolbachia-induced phenotypes.

Wolbachia are among the most successful of all intracellular bacteria, infecting an estimated 65% of insect species. Wolbachia are also present in filarial nematodes and are the cause of African river blindness. Wolbachia's success is due in part to its ability to induce a conditional form of sterility known as cytoplasmic incompatibility (CI), endowing infected females with a tremendous selective advantage. CI results in the severe reduction in progeny from crosses between uninfected females and Wolbachia-infected males. However, Wolbachia-infected females can mate with either infected or uninfected males with no reduction in progeny. CI may drive speciation and is intensively being pursued as a means to control insect-borne human disease. In spite of its biological and medical significance, the molecular basis of CI is not understood. We take advantage of newly generated chromatin reagents to demonstrate that prior to the well-documented defects in chromosome condensation and segregation, CI produces a delay in recruiting the replication-independent histone H3.3/H4 complex to the male pronucleus. There is great interest in histone H3.3 because of its general role in transcription and in remodeling of the sperm chromatin following fertilization. In addition, these findings may provide insight into other Wolbachia–host interactions such as CI–Rescue and male-killing.

Screening a genome-wide S. pombe deletion library identifies novel genes and pathways involved in genome stability maintenance

The maintenance of genome stability is essential for an organism to avoid cell death and cancer. Based on screens for mutant sensitivity against DNA damaging agents a large number of DNA repair and DNA damage checkpoint genes have previously been identified in genetically amenable model organisms. These screens have however not been exhaustive and various genes have been, and remain to be, identified by other means. We therefore screened a genome-wide Schizosaccharomyces pombe deletion library for mutants sensitive against various DNA damaging agents. Screening the library on different concentrations of these genotoxins allowed us to assign a semi-quantitative score to each mutant expressing the degree of sensitivity. We isolated a total of 229 mutants which show sensitivity to one or more of the DNA damaging agents used. This set of mutants was significantly enriched for processes involved in DNA replication, DNA repair, DNA damage checkpoint, response to UV, mating type switching, telomere length maintenance and meiosis, and also for processes involved in the establishment and maintenance of chromatin architecture (notably members of the SAGA complex), transcription (members of the CCR4-Not complex) and microtubule related processes (members of the DASH complex). We also identified 23 sensitive mutants which had previously been classified as "sequence orphan" or as "conserved hypothetical". Among these, we identified genes showing extensive homology to CtIP, Stra13, Ybp1/Ybp2, Human Fragile X mental retardation interacting protein NUFIP1, and Aprataxin. The identification of these homologues will provide a basis for the further characterisation of the role of these conserved proteins in the genetically amenable model organism S. pombe.

Eukaryotic translesion polymerases and their roles and regulation in DNA damage tolerance

DNA repair and DNA damage tolerance machineries are crucial to overcome the vast array of DNA damage that a cell encounters during its lifetime. In this review, we summarize the current state of knowledge about the eukaryotic DNA damage tolerance pathway translesion synthesis (TLS), a process in which specialized DNA polymerases replicate across from DNA lesions. TLS aids in resistance to DNA damage, presumably by restarting stalled replication forks or filling in gaps that remain in the genome due to the presence of DNA lesions. One consequence of this process is the potential risk of introducing mutations. Given the role of these translesion polymerases in mutagenesis, we discuss the significant regulatory mechanisms that control the five known eukaryotic translesion polymerases: Rev1, Pol zeta, Pol kappa, Pol eta, and Pol iota.

Molecular mechanism of protein assembly on DNA double-strand breaks in the non-homologous end-joining pathway

Non-homologous end-joining (NHEJ) is the major repair pathway for DNA double-strand breaks (DSBs) in mammalian species. Upon DSB induction, a living cell quickly activates the NHEJ pathway comprising of multiple molecular events. However, it has been difficult to analyze the initial phase of DSB responses in living cells, primarily due to technical limitations. Recent advances in real-time imaging and site-directed DSB induction using laser microbeam allow us to monitor the spatiotemporal dynamics of NHEJ factors in the immediate-early phase after DSB induction. These new approaches, together with the use of cell lines deficient in each essential NHEJ factor, provide novel mechanistic insights into DSB recognition and protein assembly on DSBs in the NHEJ pathway. In this review, we provide an overview of recent progresses in the imaging analyses of the NHEJ core factors. These studies strongly suggest that the NHEJ core factors are pre-assembled into a large complex on DSBs prior to the progression of the biochemical reactions in the NHEJ pathway. Instead of the traditional step-by-step assembly model from the static view of NHEJ, a novel model for dynamic protein assembly in the NHEJ pathway is proposed. This new model provides important mechanistic insights into the protein assembly at DSBs and the regulation of DSB repair.

Regulated degradation of FANCM in the Fanconi anemia pathway during mitosis

The 13 Fanconi anemia (FA) proteins cooperate in a common DNA repair pathway. Eight of these proteins are assembled into a multisubunit E3 ligase called the FA core complex. During S phase, the FA core complex is loaded by the FANCM protein into chromatin where it monoubiquitinates its substrates. In mitosis, the FA core complex is released from FANCM by an unknown mechanism. Here we show that FANCM is hyperphosphorylated and degraded during mitosis. beta-TRCP and Plk1 are the key regulators of FANCM degradation. Nondegradable mutant forms of FANCM retain the FA core complex in the chromatin and disrupt the FA pathway. Our data provide a novel mechanism for the cell cycle-dependent regulation of the FA pathway.

Gene expression profile of human endometrial receptivity: comparison between natural and stimulated cycles for the same patients

BACKGROUND

The adjunction of exogenous hormones for controlled ovarian stimulation (COS) may alter endometrial receptiveness. In order to identify the genes misregulated under COS, we compared the endometrium gene expression profiles, from the same patients, in a natural cycle and in a subsequent COS cycle.

METHODS

For the same normal-responder patients (n = 21), endometrial biopsies (n = 84) were collected during the pre-receptive (LH + 2) and receptive stages (LH + 7) of a natural cycle and, subsequently, on oocyte retrieval day (hCG + 2) and on transfer day (hCG + 5) of a stimulated cycle. Samples were analyzed using DNA microarrays. Gene expression profiles and biological pathways involved in endometrial receptivity were analyzed.

RESULTS

Although endometrium transition profiles from pre-receptive to receptive phases are similar between patients, COS regimens alter endometrial receptivity in comparison with natural cycle. Under COS conditions, two endometrial profiles were identified and were associated either with a moderately altered receptivity profile for the majority of the patients or a strongly altered profile for a sub-category of patients. The receptive endometrium transcription profile under COS was defective for biological functions such as TGFbeta signaling, leukocyte transendothelial migration and the cell cycle.

CONCLUSIONS

Gonadotrophin treatments in COS cycles led to disruptions of the transcriptional activation of genes involved in normal endometrial receptivity. We propose that when the receptiveness of the endometrium is seriously compromised by the COS protocol, fresh embryo replacement should be cancelled, the embryo frozen and thawed embryo replacement should be performed under natural cycles.

Repair of ionizing radiation-induced DNA double-strand breaks by non-homologous end-joining

DNA DSBs (double-strand breaks) are considered the most cytotoxic type of DNA lesion. They can be introduced by external sources such as IR (ionizing radiation), by chemotherapeutic drugs such as topoisomerase poisons and by normal biological processes such as V(D)J recombination. If left unrepaired, DSBs can cause cell death. If misrepaired, DSBs may lead to chromosomal translocations and genomic instability. One of the major pathways for the repair of IR-induced DSBs in mammalian cells is NHEJ (non-homologous end-joining). The main proteins required for NHEJ in mammalian cells are the Ku heterodimer (Ku70/80 heterodimer), DNA-PKcs [the catalytic subunit of DNA-PK (DNA-dependent protein kinase)], Artemis, XRCC4 (X-ray-complementing Chinese hamster gene 4), DNA ligase IV and XLF (XRCC4-like factor; also called Cernunnos). Additional proteins, including DNA polymerases mu and lambda, PNK (polynucleotide kinase) and WRN (Werner's Syndrome helicase), may also play a role. In the present review, we will discuss our current understanding of the mechanism of NHEJ in mammalian cells and discuss the roles of DNA-PKcs and DNA-PK-mediated phosphorylation in NHEJ.

Normal tissue reactions to radiotherapy: towards tailoring treatment dose by genotype

A key challenge in radiotherapy is to maximize radiation doses to cancer cells while minimizing damage to surrounding healthy tissue. As severe toxicity in a minority of patients limits the doses that can be safely given to the majority, there is interest in developing a test to measure an individual's radiosensitivity before treatment. Variation in sensitivity to radiation is an inherited genetic trait and recent progress in genotyping raises the possibility of genome-wide studies to characterize genetic profiles that predict patient response to radiotherapy.

The Saccharomyces cerevisiae Esc2 and Smc5-6 proteins promote sister chromatid junction-mediated intra-S repair

Recombination is important for DNA repair, but it can also contribute to genome rearrangements. RecQ helicases, including yeast Sgs1 and human BLM, safeguard genome integrity through their functions in DNA recombination. Sgs1 prevents the accumulation of Rad51-dependent sister chromatid junctions at damaged replication forks, and its functionality seems to be regulated by Ubc9- and Mms21-dependent sumoylation. We show that mutations in Smc5-6 and Esc2 also lead to an accumulation of recombinogenic structures at damaged replication forks. Because Smc5-6 is sumoylated in an Mms21-dependent manner, this finding suggests that Smc5-6 may be a crucial target of Mms21 implicated in this process. Our data reveal that Smc5-6 and Esc2 are required to tolerate DNA damage and that their functionality is critical in genotoxic conditions in the absence of Sgs1. As reported previously for Sgs1 and Smc5-6, we find that Esc2 physically interacts with Ubc9 and SUMO. This interaction is correlated with the ability of Esc2 to promote DNA damage tolerance. Collectively, these data suggest that Esc2 and Smc5-6 act in concert with Sgs1 to prevent the accumulation of recombinogenic structures at damaged replication forks, likely by integrating sumoylation activities to regulate the repair pathways in response to damaged DNA.

Esc2 and Sgs1 act in functionally distinct branches of the homologous recombination repair pathway in Saccharomyces cerevisiae

Esc2 is a member of the RENi family of SUMO-like domain proteins and is implicated in gene silencing in Saccharomyces cerevisiae. Here, we identify a dual role for Esc2 during S-phase in mediating both intra-S-phase DNA damage checkpoint signaling and preventing the accumulation of Rad51-dependent homologous recombination repair (HRR) intermediates. These roles are qualitatively similar to those of Sgs1, the yeast ortholog of the human Bloom's syndrome protein, BLM. However, whereas mutation of either ESC2 or SGS1 leads to the accumulation of unprocessed HRR intermediates in the presence of MMS, the accumulation of these structures in esc2 (but not sgs1) mutants is entirely dependent on Mph1, a protein that shows structural similarity to the Fanconi anemia group M protein (FANCM). In the absence of both Esc2 and Sgs1, the intra-S-phase DNA damage checkpoint response is compromised after exposure to MMS, and sgs1esc2 cells attempt to undergo mitosis with unprocessed HRR intermediates. We propose a model whereby Esc2 acts in an Mph1-dependent process, separately from Sgs1, to influence the repair/tolerance of MMS-induced lesions during S-phase.

SUMOylation regulates Rad18-mediated template switch

Replication by template switch is thought to mediate DNA damage-bypass and fillings of gaps. Gap-filling repair requires homologous recombination as well as Rad18- and Rad5-mediated proliferating cell nuclear antigen (PCNA) polyubiquitylation. However, it is unclear whether these processes are coordinated, and the physical evidence for Rad18-Rad5-dependent template switch at replication forks is still elusive. Here we show, using genetic and physical approaches, that in budding yeast (Saccharomyces cerevisiae) Rad18 is required for the formation of X-shaped sister chromatid junctions (SCJs) at damaged replication forks through a process involving PCNA polyubiquitylation and the ubiquitin-conjugating enzymes Mms2 and Ubc13. The Rad18-Mms2-mediated damage-bypass through SCJs requires the small ubiquitin-like modifier (SUMO)-conjugating enzyme Ubc9 and SUMOylated PCNA, and is coordinated with Rad51-dependent recombination events. We propose that the Rad18-Rad5-Mms2-dependent SCJs represent template switch events. Altogether, our results unmask a role for PCNA ubiquitylation and SUMOylation pathways in promoting transient damage-induced replication-coupled recombination events involving sister chromatids at replication forks.

DNA damage alters DNA polymerase delta to a form that exhibits increased discrimination against modified template bases and mismatched primers

Human DNA polymerase δ (Pol δ4), a key enzyme in chromosomal replication, is a heterotetramer composed of the p125, p50, p68 and p12 subunits. Genotoxic agents such as UV and alkylating chemicals trigger a DNA damage response in which Pol δ4 is converted to a trimer (Pol δ3) by degradation of p12. We show that Pol δ3 has altered enzymatic properties: it is less able to perform translesion synthesis on templates containing base lesions (O⁶-MeG, 8-oxoG, an abasic site or a thymine-thymine dimer); a greater proofreading activity; an increased exonuclease/polymerase activity ratio; a decreased tendency for the insertion of wrong nucleotides, and for the extension of mismatched primers. Overall, our findings indicate that Pol δ3 exhibits an enhanced ability for the detection of errors in both primers and templates over its parent enzyme. These alterations in Pol δ3 show that p12 plays a major role in Pol δ4 catalytic functions, and provides significant insights into the rationale for the conversion of Pol δ4 to Pol δ3 in the cellular response to DNA damage.

The INO80 chromatin remodeling complex in transcription, replication and repair

The Ino80 ATPase is a member of the SNF2 family of ATPases and functions as an integral component of a multisubunit ATP-dependent chromatin remodeling complex. Although INO80 complexes from yeast and higher eukaryotes share a common core of conserved subunits, the complexes have diverged substantially during evolution and have acquired new subunits with apparently species-specific functions. Recent studies have shown that the INO80 complex contributes to a wide variety of chromatin-dependent nuclear transactions, including transcription, DNA repair and DNA replication.

Chk2-dependent phosphorylation of XRCC1 in the DNA damage response promotes base excision repair

The DNA damage response (DDR) has an essential function in maintaining genomic stability. Ataxia telangiectasia-mutated (ATM)-checkpoint kinase 2 (Chk2) and ATM- and Rad3-related (ATR)-Chk1, triggered, respectively, by DNA double-strand breaks and blocked replication forks, are two major DDRs processing structurally complicated DNA damage. In contrast, damage repaired by base excision repair (BER) is structurally simple, but whether, and how, the DDR is involved in repairing this damage is unclear. Here, we demonstrated that ATM-Chk2 was activated in the early response to oxidative and alkylation damage, known to be repaired by BER. Furthermore, Chk2 formed a complex with XRCC1, the BER scaffold protein, and phosphorylated XRCC1 in vivo and in vitro at Thr(284). A mutated XRCC1 lacking Thr(284) phosphorylation was linked to increased accumulation of unrepaired BER intermediate, reduced DNA repair capacity, and higher sensitivity to alkylation damage. In addition, a phosphorylation-mimic form of XRCC1 showed increased interaction with glycosylases, but not other BER proteins. Our results are consistent with the phosphorylation of XRCC1 by ATM-Chk2 facilitating recruitment of downstream BER proteins to the initial damage recognition/excision step to promote BER.

Cohesion by topology: sister chromatids interlocked by DNA

Sister chromatid cohesion is coupled with chromosome replication and influences chromosome segregation and intra-S repair. Specialized proteins, the cohesins, together with other pathways contribute to tether sister chromatids. In this issue of Genes & Development, Wang and colleagues (pp. 2426-2433 demonstrate that TopoIV, a type II DNA topoisomerase, modulates cohesion in Escherichia coli, by removing interlocked DNA junctions between sister chromatids. They propose that DNA precatenanes, arising during replication fork progression, hold sister chromatids together.

Rad51 suppresses gross chromosomal rearrangement at centromere in Schizosaccharomyces pombe

Centromere that plays a pivotal role in chromosome segregation is composed of repetitive elements in many eukaryotes. Although chromosomal regions containing repeats are the hotspots of rearrangements, little is known about the stability of centromere repeats. Here, by using a minichromosome that has a complete set of centromere sequences, we have developed a fission yeast system to detect gross chromosomal rearrangements (GCRs) that occur spontaneously. Southern and comprehensive genome hybridization analyses of rearranged chromosomes show two types of GCRs: translocation between homologous chromosomes and formation of isochromosomes in which a chromosome arm is replaced by a copy of the other. Remarkably, all the examined isochromosomes contain the breakpoint in centromere repeats, showing that isochromosomes are produced by centromere rearrangement. Mutations in the Rad3 checkpoint kinase increase both types of GCRs. In contrast, the deletion of Rad51 recombinase preferentially elevates isochromosome formation. Chromatin immunoprecipitation analysis shows that Rad51 localizes at centromere around S phase. These data suggest that Rad51 suppresses rearrangements of centromere repeats that result in isochromosome formation.

Human Wrnip1 is localized in replication factories in a ubiquitin-binding zinc finger-dependent manner

Wrnip1 (Werner helicase-interacting protein 1) has been implicated in the bypass of stalled replication forks in bakers' yeast. However, the function(s) of human Wrnip1 has remained elusive so far. Here we report that Wrnip1 is distributed inside heterogeneous structures detectable in nondamaged cells throughout the cell cycle. In an attempt to characterize these structures, we found that Wrnip1 resides in DNA replication factories. Upon treatments that stall replication forks, such as UVC light, the amount of chromatin-bound Wrnip1 and the number of foci significantly increase, further implicating Wrnip1 in DNA replication. Interestingly, the nuclear pattern of Wrnip1 appears to extend to a broader landscape, as it can be detected in promyelocytic leukemia nuclear bodies. The presence of Wrnip1 into these heterogeneous subnuclear structures requires its ubiquitin-binding zinc finger (UBZ) domain, which is able to interact with different ubiquitin (Ub) signals, including mono-Ub and chains linked via lysine 48 and 63. Moreover, the oligomerization of Wrnip1 mediated by its C terminus is also important for proper subnuclear localization. Our study is the first to reveal the composite and regulated topography of Wrnip1 in the human nucleus, highlighting its potential role in replication and other nuclear transactions.

Human glyceraldehyde-3-phosphate dehydrogenase plays a direct role in reactivating oxidized forms of the DNA repair enzyme APE1

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has diverse biological functions including its nuclear translocation in response to oxidative stress. We show that GAPDH physically associates with APE1, an essential enzyme involved in the repair of abasic sites in damaged DNA, as well as in the redox regulation of several transcription factors. This interaction allows GAPDH to convert the oxidized species of APE1 to the reduced form, thereby reactivating its endonuclease activity to cleave abasic sites. The GAPDH variants C152G and C156G retain the ability to interact with but are unable to reactivate APE1, implicating these cysteines in catalyzing the reduction of APE1. Interestingly, GAPDH-small interfering RNA knockdown sensitized the cells to methyl methane sulfonate and bleomycin, which generate lesions that are repaired by APE1, but showed normal sensitivity to 254-nm UV. Moreover, the GAPDH knockdown cells exhibited an increased level of spontaneous abasic sites in the genomic DNA as a result of diminished APE1 endonuclease activity. Thus, the nuclear translocation of GAPDH during oxidative stress constitutes a protective mechanism to safeguard the genome by preventing structural inactivation of APE1.

The HTLV-1 Tax interactome

The Tax1 oncoprotein encoded by Human T-lymphotropic virus type I is a major determinant of viral persistence and pathogenesis. Tax1 affects a wide variety of cellular signalling pathways leading to transcriptional activation, proliferation and ultimately transformation. To carry out these functions, Tax1 interacts with and modulates activity of a number of cellular proteins. In this review, we summarize the present knowledge of the Tax1 interactome and propose a rationale for the broad range of cellular proteins identified so far.

Cdc14B and APC/C tackle DNA damage

Mitotic exit in budding yeast is regulated by the proteins Cdc14, APC/C(Cdh1), and Plk1. In this issue, Bassermann and colleagues (2008) show that this network of proteins has been rewired in human cells to control the cell cycle in response to DNA damage.

Multiple pathways cooperate to facilitate DNA replication fork progression through alkylated DNA

Eukaryotic genomes are especially vulnerable to DNA damage during the S phase of the cell cycle, when chromosomes must be duplicated. The stability of DNA replication forks is critical to achieve faithful chromosome replication and is severely compromised when forks encounter DNA lesions. To maintain genome integrity, replication forks need to be protected by the S-phase checkpoint and DNA insults must be repaired. Different pathways help to repair or tolerate the lesions in the DNA, but their contribution to the progression of replication forks through damaged DNA is not well known. Here we show in budding yeast that, when the DNA template is damaged with the alkylating agent methyl methanesulfonate (MMS), base excision repair, homologous recombination and DNA damage tolerance pathways, together with a functional S-phase checkpoint, are essential for the efficient progression of DNA replication forks and the maintenance of cell survival. In the absence of base excision repair, replication forks stall reversibly in cells exposed to MMS. This repair reaction is necessary to eliminate the lesions that impede fork progression and has to be coordinated with recombination and damage tolerance activities to avoid fork collapse and allow forks to resume and complete chromosome replication.

P21 and p27: roles in carcinogenesis and drug resistance

Human cancers arise from an imbalance of cell growth and cell death. Key proteins that govern this balance are those that mediate the cell cycle. Several different molecular effectors have been identified that tightly regulate specific phases of the cell cycle, including cyclins, cyclin-dependent kinases (CDKs) and CDK inhibitors. Notably, loss of expression or function of two G1-checkpoint CDK inhibitors - p21 (CDKN1A) and p27 (CDKN1B) - has been implicated in the genesis or progression of many human malignancies. Additionally, there is a growing body of evidence suggesting that functional loss of p21 or p27 can mediate a drug-resistance phenotype. However, reports in the literature have also suggested p21 and p27 can promote tumours, indicating a paradoxical effect. Here, we review historic and recent studies of these two CDK inhibitors, including their identification, function, importance to carcinogenesis and finally their roles in drug resistance.

Deregulated proteolysis by the F-box proteins SKP2 and beta-TrCP: tipping the scales of cancer

The maintenance and preservation of distinct phases during the cell cycle is a highly complex and coordinated process. It is regulated by phosphorylation--through the activity of cyclin-dependent kinases (CDKs)--and protein degradation, which occurs through ubiquitin ligases such as SCF (SKP1-CUL1-F-box protein) complexes and APC/C (anaphase-promoting complex/cyclosome). Here, we explore the functionality and biology of the F-box proteins, SKP2 (S-phase kinase-associated protein 2) and beta-TrCP (beta-transducin repeat-containing protein), which are emerging as important players in cancer biogenesis owing to the deregulated proteolysis of their substrates.