MGMT activated by Wnt pathway promotes cisplatin tolerance through inducing slow-cycling cells and nonhomologous end joining in colorectal cancer

Chemotherapy resistance plays a pivotal role in the prognosis and therapeutic failure of patients with colorectal cancer(CRC).Cisplatin(DDP)-resistant cells exhibit an inherent ability to evade the toxic chemotherapeutic drug effects which are characterized by the activation of slow-cycle programs and DNA repair.Among the elements that lead to DDP resistance,O^(6)-methylguanine(O^(6)-MG)-DNA-methyltransferase(MGMT),a DNA-repair enzyme,performs a quintessential role.In this study,we clarify the significant involvement of MGMT in conferring DDP resistance in CRC,elucidating the underlying mechanism of the regulatory actions of MGMT.A notable upregulation of MGMT in DDP-resistant cancer cells was found in our study,and MGMT repression amplifies the sensitivity of these cells to DDP treatment in vitro and in vivo.Conversely,in cancer cells,MGMT overexpression abolishes their sensitivity to DDP treatment.Mechanistically,the interaction between MGMT and cyclin dependent kinase 1(CDK1)inducing slow-cycling cells is attainted via the promotion of ubiquitination degradation of CDK1.Meanwhile,to achieve nonhomologous end joining,MGMT interacts with XRCC6 to resist chemotherapy drugs.Our transcriptome data from samples of 88 patients with CRC suggest that MGMT expression is co-related with the Wnt signaling pathway activation,and several Wnt inhibitors can repress drug-resistant cells.In summary,our results point out that MGMT is a potential therapeutic target and predictive marker of chemoresistance in CRC.

Enzalutamide and olaparib synergistically suppress castration-resistant prostate cancer progression by promoting apoptosis through inhibiting nonhomologous end joining pathway

Recent studies revealed the relationship among homologous recombination repair(HRR),androgen receptor(AR),and poly(adenosine diphosphate-ribose)polymerase(PARP);however,the synergy between anti-androgen enzalutamide(ENZ)and PARP inhibitor olaparib(OLA)remains unclear.Here,we showed that the synergistic effect of ENZ and OLA significantly reduced proliferation and induced apoptosis in AR-positive prostate cancer cell lines.Next-generation sequencing followed by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses revealed the significant effects of ENZ plus OLA on nonhomologous end joining(NHEJ)and apoptosis pathways.ENZ combined with OLA synergistically inhibited the NHEJ pathway by repressing DNA-dependent protein kinase catalytic subunit(DNA-PKcs)and X-ray repair cross complementing 4(XRCC4).Moreover,our data showed that ENZ could enhance the response of prostate cancer cells to the combination therapy by reversing the anti-apoptotic effect of OLA through the downregulation of anti-apoptotic gene insulin-like growth factor 1 receptor(IGF1R)and the upregulation of pro-apoptotic gene death-associated protein kinase 1(DAPK1).Collectively,our results suggested that ENZ combined with OLA can promote prostate cancer cell apoptosis by multiple pathways other than inducing HRR defects,providing evidence for the combined use of ENZ and OLA in prostate cancer regardless of HRR gene mutation status.

Flexibility in the order of action and in the enzymology of the nuclease, polymerases, and ligase of vertebrate non-homologous DNA end joining： relevance to cancer, aging, and the immune system

Nonhomologous DNA end joining （NHEJ） is the primary pathway for repair of double-strand DNA breaks in human cells and in multicellular eukaryotes. The causes of double-strand breaks often fragment the DNA at the site of damage, resulting in the loss of information there. NHEJ does not restore the lost information and may resect additional nucleotides during the repair process. The ability to repair a wide range of overhang and damage configurations reflects the flexibility of the nuclease, polymerases, and ligase of NHEJ. The flexibility of the individual components also explains the large number of ways in which NHEJ can repair any given pair of DNA ends. The loss of information locally at sites of NHEJ repair may contribute to cancer and aging, but the action by NHEJ ensures that entire segments of chromosomes are not lost.

The biological principles and advanced applications of DSB repair in CRISPR-mediated yeast genome editing

To improve the performance of yeast cell factories for industrial production,extensive CRISPR-mediated genome editing systems have been applied by artificially creating double-strand breaks(DSBs)to introduce mutations with the assistance of intracellular DSB repair.Diverse strategies of DSB repair are required to meet various demands,including precise editing or random editing with customized gRNAs or a gRNA library.Although most yeasts remodeling techniques have shown rewarding performance in laboratory verification,industrial yeast strain manipulation relies only on very limited strategies.Here,we comprehensively reviewed the molecular mechanisms underlying recent industrial applications to provide new insights into DSB cleavage and repair pathways in both Saccharomyces cerevisiae and other unconventional yeast species.The discussion of DSB repair covers the most frequently used homologous recombination(HR)and nonhomologous end joining(NHEJ)strategies to the less well-studied illegitimate recombination(IR)pathways,such as single-strand annealing(SSA)and microhomology-mediated end joining(MMEJ).Various CRISPR-based genome editing tools and corresponding gene editing efficiencies are described.Finally,we summarize recently developed CRISPR-based strategies that use optimized DSB repair for genome-scale editing,providing a direction for further development of yeast genome editing.

Drosophila RecQ5 is required for efficient SSA repair and suppression of LOH in vivo

RecQ5 in mammalian cells has been suggested to suppress inappropriate homologous recombination.However,the specific pathway(s)in which it is involved and the underlining mechanism(s)remain poorly understood.We took advantage of genetic tools in Drosophila to investigate how Drosophila RecQ5(dRecQ5)functions in vivo in homologous recombination-mediated double strand break(DSB)repair.We generated null alleles of dRecQ5 using the targeted recombination technique.The mutant animals are homozygous viable,but with growth retardation during development.The mutants are sensitive to both exogenous DSB-inducing treatment,such as gamma-irradiation,and endogenously induced double strand breaks(DSBs)by I-Sce I endonuclease.In the absence of dRecQ5,single strand annealing(SSA)-mediated DSB repair is compromised with compensatory increases in either inter-homologous gene conversion,or non-homologous end joining(NHEJ)when inter-chromosomal homologous sequence is unavailable.Loss of function of dRecQ5 also leads to genome instability in loss of heterozygosity(LOH)assays.Together,our data demonstrate that dRecQ5 functions in SSA-mediated DSB repair to achieve its full efficiency and in suppression of LOH in Drosophila.