Comparison of the different mechanisms of cytotoxicity induced by checkpoint kinase I inhibitors when used as single agents or in combination with DNA damage

Morin overcomes doxorubicin resistance in human breast cancer by inducing DNA damage and modulating the LKB1/AMPK/mTORC1 signaling pathway

Breast cancer chemoresistance hampers chemotherapy efficacy; researchers investigate the pharmacological activities of natural products for potential solutions. This study aimed to determine the effect of morin, a bioflavonoid isolated from Maclura pomifera, on two Dox-resistant human breast cancer cell lines MDA-MB-231 (MDA-DR) and MCF-7 (MCF-DR). Sulforhodamine B and colony-forming assays demonstrated the cytotoxic effect of morin on both cell lines. Morin induced DNA damage and reduced the DNA repair mechanism, a feature of chemoresistance. In addition, morin reduced the protein expressions of cell cycle regulators, such as cyclin D1, CDK4, cyclin E1, cyclin B1, and p-Rb, thereby halting cell cycle progression. Moreover, morin slightly reduced PARP and Bcl-xL expressions but left LC3-II and RIPK3 expressions unchanged. Annexin-V/7-AAD analysis showed morin increased 7-AAD positive cells and annexin-V positive cells among MDA-DR and MCF-DR cells, respectively. In addition, morin increased p-AMPK and p-LKB1 levels; and, thus, inhibited phosphorylation of the mTOR pathway, but decreased t-AMPK levels by inducing lysosomal degradation, and AICAR, an AMPK activator, reduced Raptor, cyclin D1, CDK4, cyclin E1 and phosphorylated, and total mTOR levels, indicating AMPK is a key player in inducing cell death. Also, morin modulated MAPK phosphorylation and attenuated p-Akt and p-GSK3αβ levels; and thus, inhibited cell survival. In addition, morin suppressed tumor growth in our MDA-DR xenografted mouse model. These findings indicate that morin is a potential treatment for Dox-resistant breast cancer and that it does so by inducing DNA damage and modulating the LKB1/AMPK/mTORC1 pathway, along with regulating the MAPK, and Akt/GSK3αβ signaling pathways.

GRK2-mediated AKT activation controls cell cycle progression and G2 checkpoint in a p53-dependent manner

Cell cycle checkpoints, activated by stressful events, halt the cell cycle progression, and prevent the transmission of damaged DNA. These checkpoints prompt cell repair but also trigger cell death if damage persists. Decision-making between these responses is multifactorial and context-dependent, with the tumor suppressor p53 playing a central role. In many tumor cells, p53 alterations lead to G1/S checkpoint loss and the weakening of the G2 checkpoint, rendering cell viability dependent on the strength of the latter through mechanisms not fully characterized. Cells with a strong pro-survival drive can evade cell death despite substantial DNA lesions. Deciphering the integration of survival pathways with p53-dependent and -independent mechanisms governing the G2/M transition is crucial for understanding G2 arrest functionality and predicting tumor cell response to chemotherapy. The serine/threonine kinase GRK2 emerges as a signaling node in cell cycle modulation. In cycling cells, but not in G2 checkpoint-arrested cells, GRK2 protein levels decline during G2/M transition through a process triggered by CDK2-dependent phosphorylation of GRK2 at the S670 residue and Mdm2 ubiquitination. We report now that this downmodulation in G2 prevents the unscheduled activation of the PI3K/AKT pathway, allowing cells to progress into mitosis. Conversely, higher GRK2 levels lead to tyrosine phosphorylation by the kinase c-Abl, promoting the direct association of GRK2 with the p85 regulatory subunit of PI3K and AKT activation in a GRK2 catalytic-independent manner. Hyperactivation of AKT is conditioned by p53's scaffolding function, triggering FOXO3a phosphorylation, impaired Cyclin B1 accumulation, and CDK1 activation, causing a G2/M transition delay. Upon G2 checkpoint activation, GRK2 potentiates early arrest independently of p53 through AKT activation. However, its ability to overcome the G2 checkpoint in viable conditions depends on p53. Our results suggest that integrating the GRK2/PI3K/AKT axis with non-canonical functions of p53 might confer a survival advantage to tumor cells.

Inhibition of DNA Topoisomerase Ι by Flavonoids and Polyacetylenes Isolated from Bidens pilosa L

Human DNA topoisomerase I (Topo I) is an essential enzyme in regulating DNA supercoiling during transcription and replication, and it is an important therapeutic target for anti-tumor agents. Bidens pilosa L. is a medicinal herb that is used as a folk medicine for cancers in China. A new flavonoid (1) and a new polyacetylene (20), along with eighteen flavonoids (2-19) and nine polyacetylenes (21-29), were isolated and identified from the methanol extract of the whole plant of B. pilosa, and some of the compounds (4, 5, 6 and 7) exhibited potent cytotoxicity against a panel of five human cancer cell lines. The DNA relaxation assay revealed that some flavonoids and polyacetylenes exerted inhibitory activities on human DNA Topo I, among them compounds 1, 2, 5, 6, 7, 8, 15, 19, 20, 22, and 24 were the most active ones, with IC50 values of 393.5, 328.98, 145.57, 239.27, 224.38, 189.84, 89.91, 47.5, 301.32, 178.03, and 218.27 μM, respectively. The structure-activity analysis of flavonoids was performed according to the results from the Topo I inhibition assay. The DNA content analysis revealed that 5, 6, and 7 potently arrested cell cycle at the G1/S and G2/M phases in human colon cancer cell DLD-1 depending on the concentration of the inhibitors. The levels of protein expression related to the G1/S and G2/M cell cycle checkpoints were in accordance with the results from the DNA content analysis. These findings suggest that flavonoids are one of the key active ingredients accounting for the anti-tumor effect of B. pilosa.

Inhibition of Chk1 stimulates cytotoxic action of platinum-based drugs and TRAIL combination in human prostate cancer cells

Checkpoint kinase 1 (Chk1) plays an important role in regulation of the cell cycle, DNA damage response and cell death, and represents an attractive target in anticancer therapy. Small-molecule inhibitors of Chk1 have been intensively investigated either as single agents or in combination with various chemotherapeutic drugs and they can enhance the chemosensitivity of numerous tumor types. Here we newly demonstrate that pharmacological inhibition of Chk1 using potent and selective inhibitor SCH900776, currently profiled in phase II clinical trials, significantly enhances cytotoxic effects of the combination of platinum-based drugs (cisplatin or LA-12) and TRAIL (tumor necrosis factor-related apoptosis inducing ligand) in human prostate cancer cells. The specific role of Chk1 in the drug combination-induced cytotoxicity was confirmed by siRNA-mediated silencing of this kinase. Using RNAi-based methods we also showed the importance of Bak-dependent mitochondrial apoptotic pathway in the combined anticancer action of SCH900776, cisplatin and TRAIL. The triple drug combination-induced cytotoxicity was partially enhanced by siRNA-mediated Mcl-1 silencing. Our findings suggest that targeting Chk1 may be used as an efficient strategy for sensitization of prostate cancer cells to killing action of platinum-based chemotherapeutic drugs and TRAIL.

Review of Personalized Medicine and Pharmacogenomics of Anti-Cancer Compounds and Natural Products

In recent years, the FDA has approved numerous anti-cancer drugs that are mutation-based for clinical use. These drugs have improved the precision of treatment and reduced adverse effects and side effects. Personalized therapy is a prominent and hot topic of current medicine and also represents the future direction of development. With the continuous advancements in gene sequencing and high-throughput screening, research and development strategies for personalized clinical drugs have developed rapidly. This review elaborates the recent personalized treatment strategies, which include artificial intelligence, multi-omics analysis, chemical proteomics, and computation-aided drug design. These technologies rely on the molecular classification of diseases, the global signaling network within organisms, and new models for all targets, which significantly support the development of personalized medicine. Meanwhile, we summarize chemical drugs, such as lorlatinib, osimertinib, and other natural products, that deliver personalized therapeutic effects based on genetic mutations. This review also highlights potential challenges in interpreting genetic mutations and combining drugs, while providing new ideas for the development of personalized medicine and pharmacogenomics in cancer study.

The SKP2-p27 axis defines susceptibility to cell death upon CHK1 inhibition

Checkpoint kinase 1 (CHK1; encoded by CHEK1) is an essential gene that monitors DNA replication fidelity and prevents mitotic entry in the presence of under-replicated DNA or exogenous DNA damage. Cancer cells deficient in p53 tumor suppressor function reportedly develop a strong dependency on CHK1 for proper cell cycle progression and maintenance of genome integrity, sparking interest in developing kinase inhibitors. Pharmacological inhibition of CHK1 triggers B-Cell CLL/Lymphoma 2 (BCL2)-regulated cell death in malignant cells largely independently of p53, and has been suggested to kill p53-deficient cancer cells even more effectively. Next to p53 status, our knowledge about factors predicting cancer cell responsiveness to CHK1 inhibitors is limited. Here, we conducted a genome-wide CRISPR/Cas9-based loss-of-function screen to identify genes defining sensitivity to chemical CHK1 inhibitors. Next to the proapoptotic BCL2 family member, BCL2 Binding Component 3 (BBC3; also known as PUMA), the F-box protein S-phase Kinase-Associated Protein 2 (SKP2) was validated to tune the cellular response to CHK1 inhibition. SKP2 is best known for degradation of the Cyclin-dependent Kinase Inhibitor 1B (CDKN1B; also known as p27), thereby promoting G1-S transition and cell cycle progression in response to mitogens. Loss of SKP2 resulted in the predicted increase in p27 protein levels, coinciding with reduced DNA damage upon CHK1-inhibitor treatment and reduced cell death in S-phase. Conversely, overexpression of SKP2, which consequently results in reduced p27 protein levels, enhanced cell death susceptibility to CHK1 inhibition. We propose that assessing SKP2 and p27 expression levels in human malignancies will help to predict the responsiveness to CHK1-inhibitor treatment.

MCDB: A comprehensive curated mitotic catastrophe database for retrieval, protein sequence alignment, and target prediction

Mitotic catastrophe (MC) is a form of programmed cell death induced by mitotic process disorders, which is very important in tumor prevention, development, and drug resistance. Because rapidly increased data for MC is vigorously promoting the tumor-related biomedical and clinical study, it is urgent for us to develop a professional and comprehensive database to curate MC-related data. Mitotic Catastrophe Database (MCDB) consists of 1214 genes/proteins and 5014 compounds collected and organized from more than 8000 research articles. Also, MCDB defines the confidence level, classification criteria, and uniform naming rules for MC-related data, which greatly improves data reliability and retrieval convenience. Moreover, MCDB develops protein sequence alignment and target prediction functions. The former can be used to predict new potential MC-related genes and proteins, and the latter can facilitate the identification of potential target proteins of unknown MC-related compounds. In short, MCDB is such a proprietary, standard, and comprehensive database for MC-relate data that will facilitate the exploration of MC from chemists to biologists in the fields of medicinal chemistry, molecular biology, bioinformatics, oncology and so on. The MCDB is distributed on http://www.combio-lezhang.online/MCDB/index_html/.

Inhibition of Protein Synthesis Induced by CHK1 Inhibitors Discriminates Sensitive from Resistant Cancer Cells

The DNA-damage-activated checkpoint protein CHK1 is required to prevent replication or mitosis in the presence of unrepaired DNA damage. Inhibitors of CHK1 (CHK1i) circumvent this checkpoint and enhance cell killing by DNA-damaging drugs. CHK1i also elicit single-agent cytotoxicity in a small subset of cell lines. Resolving the mechanisms underlying the single-agent activity may permit patient stratification and targeted therapy against sensitive tumors. Our recent comparison of three CHK1i demonstrated that they all inhibited protein synthesis only in sensitive cells. LY2606368, the most selective of these CHK1i, was used in the current study. Comparison across a panel of cell lines demonstrated that sensitive cells died upon incubation with LY2606368, whereas resistant cells underwent growth inhibition and/or cytostasis but failed to die. Sensitive cells exhibited inhibition of protein synthesis, elevated DNA damage, impaired DNA repair, and subsequently death. The consequence of CHK1 inhibition involved activation of cyclin A/CDK2 and MUS81, resulting in DNA damage. This damage led to activation of AMPK, dephosphorylation of 4E-BP1, and inhibition of protein synthesis. Inhibition of MUS81 prevented activation of AMPK, while inhibition of AMPK enhanced DNA repair and cell survival. The activation of AMPK may involve a combination of LKB1 and CaMKKβ. This study raises questions concerning the potential importance of the inhibition of protein synthesis in response to other drugs, alone or in combination with CHK1i. It also highlights the importance of clearly discriminating among growth inhibition, cytostasis, and cell death, as only the latter is likely to result in tumor regression.

Genetic vulnerabilities upon inhibition of DNA damage response

Because of essential roles of DNA damage response (DDR) in the maintenance of genomic integrity, cellular homeostasis, and tumor suppression, targeting DDR has become a promising therapeutic strategy for cancer treatment. However, the benefits of cancer therapy targeting DDR are limited mainly due to the lack of predictive biomarkers. To address this challenge, we performed CRISPR screens to search for genetic vulnerabilities that affect cells’ response to DDR inhibition. By undertaking CRISPR screens with inhibitors targeting key DDR mediators, i.e. ATR, ATM, DNAPK and CHK1, we obtained a global and unbiased view of genetic interactions with DDR inhibition. Specifically, we identified YWHAE loss as a key determinant of sensitivity to CHK1 inhibition. We showed that KLHL15 loss protects cells from DNA damage induced by ATM inhibition. Moreover, we validated that APEX1 loss sensitizes cells to DNAPK inhibition. Additionally, we compared the synergistic effects of combining different DDR inhibitors and found that an ATM inhibitor plus a PARP inhibitor induced dramatic levels of cell death, probably through promoting apoptosis. Our results enhance the understanding of DDR pathways and will facilitate the use of DDR-targeting agents in cancer therapy.

Chk1 and the Host Cell DNA Damage Response as a Potential Antiviral Target in BK Polyomavirus Infection

The human BK polyomavirus (BKPyV) is latent in the kidneys of most adults, but can be reactivated in immunosuppressed states, such as following renal transplantation. If left unchecked, BK polyomavirus nephropathy (PyVAN) and possible graft loss may result from viral destruction of tubular epithelial cells and interstitial fibrosis. When coupled with regular post-transplant screening, immunosuppression reduction has been effective in limiting BKPyV viremia and the development of PyVAN. Antiviral drugs that are safe and effective in combating BKPyV have not been identified but would be a benefit in complementing or replacing immunosuppression reduction. The present study explores inhibition of the host DNA damage response (DDR) as an antiviral strategy. Immunohistochemical and immunofluorescent analyses of PyVAN biopsies provide evidence for stimulation of a DDR in vivo. DDR pathways were also stimulated in vitro following BKPyV infection of low-passage human renal proximal tubule epithelial cells. The role of Chk1, a protein kinase known to be involved in the replication stress-induced DDR, was examined by inhibition with the small molecule LY2603618 and by siRNA-mediated knockdown. Inhibition of Chk1 resulted in decreased replication of BKPyV DNA and viral spread. Activation of mitotic pathways was associated with the reduction in BKPyV replication. Chk1 inhibitors that are found to be safe and effective in clinical trials for cancer should also be evaluated for antiviral activity against BKPyV.

De novo deoxyribonucleotide biosynthesis regulates cell growth and tumor progression in small-cell lung carcinoma

Deoxyribonucleotide biosynthesis from ribonucleotides supports the growth of active cancer cells by producing building blocks for DNA. Although ribonucleotide reductase (RNR) is known to catalyze the rate-limiting step of de novo deoxyribonucleotide triphosphate (dNTP) synthesis, the biological function of the RNR large subunit (RRM1) in small-cell lung carcinoma (SCLC) remains unclear. In this study, we established siRNA-transfected SCLC cell lines to investigate the anticancer effect of silencing RRM1 gene expression. We found that RRM1 is required for the full growth of SCLC cells both in vitro and in vivo. In particular, the deletion of RRM1 induced a DNA damage response in SCLC cells and decreased the number of cells with S phase cell cycle arrest. We also elucidated the overall changes in the metabolic profile of SCLC cells caused by RRM1 deletion. Together, our findings reveal a relationship between the deoxyribonucleotide biosynthesis axis and key metabolic changes in SCLC, which may indicate a possible link between tumor growth and the regulation of deoxyribonucleotide metabolism in SCLC.

The Influence of Cell Cycle Regulation on Chemotherapy

Cell cycle regulation is orchestrated by a complex network of interactions between proteins, enzymes, cytokines, and cell cycle signaling pathways, and is vital for cell proliferation, growth, and repair. The occurrence, development, and metastasis of tumors are closely related to the cell cycle. Cell cycle regulation can be synergistic with chemotherapy in two aspects: inhibition or promotion. The sensitivity of tumor cells to chemotherapeutic drugs can be improved with the cooperation of cell cycle regulation strategies. This review presented the mechanism of the commonly used chemotherapeutic drugs and the effect of the cell cycle on tumorigenesis and development, and the interaction between chemotherapy and cell cycle regulation in cancer treatment was briefly introduced. The current collaborative strategies of chemotherapy and cell cycle regulation are discussed in detail. Finally, we outline the challenges and perspectives about the improvement of combination strategies for cancer therapy.

Activation of CDC25A phosphatase is limited by CDK2/cyclin A-mediated feedback inhibition

Cyclin-dependent kinase (CDK) 1 complexed with cyclin B is a driver of mitosis, while CDK2 drives S phase entry and replicon initiation. CDK2 activity increases as cells progress through S phase, and its cyclin partner switches from cyclin E to cyclin A. Activation of CDK2 requires dephosphorylation of tyrosine-15 by CDC25A. DNA damage activates the checkpoint protein CHK1, which phosphorylates and degrades CDC25A to prevent activation of CDK2 and protect from cell cycle progression before damage is repaired. CHK1 inhibitors were developed to circumvent this arrest and enhance the efficacy of many cancer chemotherapeutic agents. CHK1 inhibition results in the accumulation of CDC25A and activation of CDK2. We demonstrate that inhibition of CDK2 or suppression of cyclin A also results in accumulation of CDC25A suggesting a feedback loop that prevents over activation of this pathway. The feedback inhibition of CDC25A targets phosphorylation of S88-CDC25A, which resides within a CDK consensus sequence. In contrast, it appears that CDK complexes with cyclin B (and possibly cyclin E) stabilize CDC25A in a feed-forward activation loop. While CDK2/cyclin A would normally be active at late S/G2, we propose that this feedback inhibitory loop prevents over activation of CDK2 in early S phase, while still leaving CDK2/cyclin E to catalyze replicon initiation. One importance of this observation is that a subset of cancer cell lines are very sensitive to CHK1 inhibition, which is mediated by CDK2/cyclin A activity in S phase cells. Hence, dysregulation of this feedback loop might facilitate sensitivity of the cells.

CHEK1 and circCHEK1_246aa evoke chromosomal instability and induce bone lesion formation in multiple myeloma

BACKGROUND

Multiple myeloma (MM) is still incurable and characterized by clonal expansion of plasma cells in the bone marrow (BM). Therefore, effective therapeutic interventions must target both myeloma cells and the BM niche.

METHODS

Cell proliferation, drug resistance, and chromosomal instability (CIN) induced by CHEK1 were confirmed by Giemsa staining, exon sequencing, immunofluorescence and xenograft model in vivo. Bone lesion was evaluated by Tartrate-resistant acid phosphatase (TRAP) staining. The existence of circCHEK1_246aa was evaluated by qPCR, Sanger sequencing and Mass Spectrometer.

RESULTS

We demonstrated that CHEK1 expression was significantly increased in human MM samples relative to normal plasma cells, and that in MM patients, high CHEK1 expression was associated with poor outcomes. Increased CHEK1 expression induced MM cellular proliferation and evoked drug-resistance in vitro and in vivo. CHEK1-mediated increases in cell proliferation and drug resistance were due in part to CHEK1-induced CIN. CHEK1 activated CIN, partly by phosphorylating CEP170. Interestingly, CHEK1 promoted osteoclast differentiation by upregulating NFATc1 expression. Intriguingly, we discovered that MM cells expressed circCHEK1_246aa, a circular CHEK1 RNA, which encoded and was translated to the CHEK1 kinase catalytic center. Transfection of circCHEK1_246aa increased MM CIN and osteoclast differentiation similarly to CHEK1 overexpression, suggesting that MM cells could secrete circCHEK1_246aa in the BM niche to increase the invasive potential of MM cells and promote osteoclast differentiation.

CONCLUSIONS

Our findings suggest that targeting the enzymatic catalytic center encoded by CHEK1 mRNA and circCHEK1_246aa is a promising therapeutic modality to target both MM cells and BM niche.

Sensitivity of cells to ATR and CHK1 inhibitors requires hyperactivation of CDK2 rather than endogenous replication stress or ATM dysfunction

DNA damage activates cell cycle checkpoint proteins ATR and CHK1 to arrest cell cycle progression, providing time for repair and recovery. Consequently, inhibitors of ATR (ATRi) and CHK1 (CHK1i) enhance damage-induced cell death. Intriguingly, both CHK1i and ATRi alone elicit cytotoxicity in some cell lines. Sensitivity has been attributed to endogenous replications stress, but many more cell lines are sensitive to ATRi than CHK1i. Endogenous activation of the DNA damage response also did not correlate with drug sensitivity. Sensitivity correlated with the appearance of γH2AX, a marker of DNA damage, but without phosphorylation of mitotic markers, contradicting suggestions that the damage is due to premature mitosis. Sensitivity to ATRi has been associated with ATM mutations, but dysfunction in ATM signaling did not correlate with sensitivity. CHK1i and ATRi circumvent replication stress by reactivating stalled replicons, a process requiring a low threshold activity of CDK2. In contrast, γH2AX induced by single agent ATRi and CHK1i requires a high threshold activity CDK2. Hence, phosphorylation of different CDK2 substrates is required for cytotoxicity induced by replication stress plus ATRi/CHK1i as compared to their single agent activity. In summary, sensitivity to ATRi and CHK1i as single agents is elicited by premature hyper-activation of CDK2.

Jatrophane Diterpenoids from Euphorbia peplus as Multidrug Resistance Modulators with Inhibitory Effects on the ATR-Chk-1 Pathway

Twelve undescribed jatrophane diterpenoids, euphpepluones A-L (1-12), together with seven known analogues (13-19), were isolated from the whole plant of Euphorbia peplus, and their structures were elucidated by spectroscopic studies. The absolute configurations of 1 and 4 were assigned by X-ray crystallographic analysis. All isolates were investigated for their inhibitory effects against the ATR-Chk1 pathway using a Western blotting assay. As a result, 1, 2, 5, 8, 10, and 16 were found to suppress the camptothecin (CPT)-induced phosphorylation of Chk1, indicating that these compounds inhibit the activation of the ATR-Chk1 pathway. A preliminary structure-activity relationship (SAR) study of the isolates was conducted. When compound 10 and CPT were combined, apoptosis was induced in A549 cells with PARP cleavage, while there was no apoptotic effect by treatment with CPT or 10 alone. The data obtained indicate that 10 potentiates the chemotherapeutic sensitivity of A549 cells to CPT.

Comparative Activity and Off-Target Effects in Cells of the CHK1 Inhibitors MK-8776, SRA737, and LY2606368

DNA damage activates the checkpoint protein CHK1 to arrest cell cycle progression, providing time for repair and recovery. Consequently, inhibitors of CHK1 (CHK1i) enhance damage-induced cell death. Additionally, CHK1i elicits single agent cytotoxicity in some cell lines. We compared three CHK1i that have undergone clinical trials and exhibited different toxicities. Each CHK1i inhibits other targets at higher concentrations, and whether these contribute to the toxicity is unknown. We compared their sensitivity in a panel of cell lines, their efficacy at inhibiting CHK1 and CHK2, and their ability to induce DNA damage and abrogate damage-induced S phase arrest. Published in vitro kinase analyses were a poor predictor of selectivity and potency in cells. LY2606368 was far more potent at inhibiting CHK1 and inducing growth arrest, while all three CHK1i inhibited CHK2 at concentrations 10- (MK-8776 and SRA737) to 100- (LY2606368) fold higher. MK-8776 and SRA737 exhibited similar off-target effects: higher concentrations demonstrated transient protection from growth inhibition, circumvented DNA damage, and prevented checkpoint abrogation, possibly due to inhibition of CDK2. Acquired resistance to LY2606368 resulted in limited cross-resistance to other CHK1i. LY2606368-resistant cells still abrogated DNA damage-induced S phase arrest, which requires low CDK2 activity, whereas inappropriately high CDK2 activity is responsible for sensitivity to CHK1i alone. All three CHK1i inhibited protein synthesis in a sensitive cell line correlating with cell death, whereas resistant cells failed to inhibit protein synthesis and underwent transient cytostasis. LY2606368 appears to be the most selective CHK1i, suggesting that further clinical development of this drug is warranted.

MCPH1 Lack of Function Enhances Mitotic Cell Sensitivity Caused by Catalytic Inhibitors of Topo II

The capacity of Topoisomerase II (Topo II) to remove DNA catenations that arise after replication is essential to ensure faithful chromosome segregation. Topo II activity is monitored during G2 by a specific checkpoint pathway that delays entry into mitosis until the chromosomes are properly decatenated. Recently, we demonstrated that the mitotic defects that are characteristic of cells depleted of MCPH1 function, a protein mutated in primary microcephaly, are not a consequence of a weakened G2 decatenation checkpoint response. However, the mitotic defects could be accounted for by a minor defect in the activity of Topo II during G2/M. To test this hypothesis, we have tracked at live single cell resolution the dynamics of mitosis in MCPH1 depleted HeLa cells upon catalytic inhibition of Topo II. Our analyses demonstrate that neither chromosome alignment nor segregation are more susceptible to minor perturbation in decatenation in MCPH1 deficient cells, as compared with control cells. Interestingly, MCPH1 depleted cells were more prone to mitotic cell death when decatenation was perturbed. Furthermore, when the G2 arrest that was induced by catalytic inhibition of Topo II was abrogated by Chk1 inhibition, the incidence of mitotic cell death was also increased. Taken together, our data suggest that the MCPH1 lack of function increases mitotic cell hypersensitivity to the catalytic inhibition of Topo II.