Homologous recombination preferentially repairs heat-induced DNA double-strand breaks in mammalian cells

Genetic and epigenetic alterations in DNA repair genes and treatment outcome of chemoradiotherapy in cervical cancer

Cervical cancer (CaCx) is the deadliest malignancy among women which is caused by human papillomavirus (HPV) and anthro-demographical/clinicopathological factors. HPV oncoproteins E6 and E7 target p53 and RB (retinoblastoma) protein degradation, Ataxia telangiectasia mutated (ATM), ATM-RAD3-related (ATR) inactivation and subsequent impairment of non-homologous end joining (NHEJ), homologous recombination, and base excision repair pathways. There is also an accumulation of genetic and epigenetic alterations in Tumor Growth Suppressors (TGS), oncogenes, and DNA repair genes leading to increased genome instability and CaCx development. These alterations might be responsible for differential clinical response to Cisplatin-based chemoradiotherapy (CRT) in patients. This review explores HPV-mediated DNA damage as a risk factor in CaCx development, the mechanistic role of genetic and epigenetic alterations in DNA repair genes and their association with CRT and outcome, It also explores new possibilities for the development of genetic and epigenetic-based biomarkers for diagnostic, prognostic, and molecular therapeutic interventions.

Chimeric chromosome landscapes of human somatic cell cultures show dependence on stress and regulation of genomic repeats by CGGBP1

Genomes of somatic cells in culture are prone to spontaneous mutations due to errors in replication and DNA repair. Some of these errors, such as chromosomal fusions, are not rectifiable and subject to selection or elimination in growing cultures. Somatic cell cultures are thus expected to generate background levels of potentially stable chromosomal chimeras. A description of the landscape of such spontaneously generated chromosomal chimeras in cultured cells will help understand the factors affecting somatic mosaicism. Here we show that short homology-associated non-homologous chromosomal chimeras occur in normal human fibroblasts and HEK293T cells at genomic repeats. The occurrence of chromosomal chimeras is enhanced by heat stress and depletion of a repeat regulatory protein CGGBP1. We also present evidence of homologous chromosomal chimeras between allelic copies in repeat-rich DNA obtained by methylcytosine immunoprecipitation. The formation of homologous chromosomal chimeras at Alu and L1 repeats increases upon depletion of CGGBP1. Our data are derived from de novo sequencing from three different cell lines under different experimental conditions and our chromosomal chimera detection pipeline is applicable to long as well as short read sequencing platforms. These findings present significant information about the generation, sensitivity and regulation of somatic mosaicism in human cell cultures.

Immune biological rationales for the design of combined radio- and immunotherapies

Cancer immunotherapies are promising treatments for many forms of cancer. Nevertheless, the response rates to, e.g., immune checkpoint inhibitors (ICI), are still in low double-digit percentage. This calls for further therapy optimization that should take into account combination of immunotherapies with classical tumor therapies such as radiotherapy. By designing multimodal approaches, immune modulatory properties of certain radiation schemes, additional immune modulation by immunotherapy with ICI and hyperthermia, as well as patient stratification based on genetic and immune constitutions have to be considered. In this context, both the tumor and its microenvironment including cells of the innate and adaptive immune system have to be viewed in synopsis. Knowledge of immune activation and immune suppression by radiation is the basis for well-elaborated addition of certain immunotherapies. In this review, the focus is set on additional immune stimulation by hyperthermia and restoration of an immune response by ICI. The impact of radiation dose and fractionation on immune modulation in multimodal settings has to be considered, as the dynamics of the immune response and the timing between radiotherapy and immunotherapy. Another big challenge is the patient stratification that should be based on matrices of biomarkers, taking into account genetics, proteomics, radiomics, and “immunomics”. One key aim is to turn immunological “cold” tumors into “hot” tumors, and to eliminate barriers of immune-suppressed or immune-excluded tumors. Comprehensive knowledge of immune alterations induced by radiation and immunotherapy when being applied together should be utilized for patient-adapted treatment planning and testing of innovative tumor therapies within clinical trials.

Deficiency of the Fanconi anemia E2 ubiqitin conjugase UBE2T only partially abrogates Alu-mediated recombination in a new model of homology dependent recombination

The primary function of the UBE2T ubiquitin conjugase is in the monoubiquitination of the FANCI-FANCD2 heterodimer, a central step in the Fanconi anemia (FA) pathway. Genetic inactivation of UBE2T is responsible for the phenotypes of FANCT patients; however, a FANCT patient carrying a maternal duplication and a paternal deletion in the UBE2T loci displayed normal peripheral blood counts and UBE2T protein levels in B-lymphoblast cell lines. To test whether reversion by recombination between UBE2T AluYa5 elements could have occurred in the patient's hematopoietic stem cells despite the defects in homologous recombination (HR) in FA cells, we constructed HeLa cell lines containing the UBE2T AluYa5 elements and neighboring intervening sequences flanked by fluorescent reporter genes. Introduction of a DNA double strand break in the model UBE2T locus in vivo promoted single strand annealing (SSA) between proximal Alu elements and deletion of the intervening color marker gene, recapitulating the reversion of the UBE2T duplication in the FA patient. To test whether UBE2T null cells retain HR activity, the UBE2T genes were knocked out in HeLa cells and U2OS cells. CRISPR/Cas9-mediated genetic knockout of UBE2T only partially reduced HR, demonstrating that UBE2T-independent pathways can compensate for the recombination defect in UBE2T/FANCT null cells.

Non-Invasive Radiofrequency Field Treatment of 4T1 Breast Tumors Induces T-cell Dependent Inflammatory Response

Previous work using non-invasive radiofrequency field treatment (RFT) in cancer has demonstrated its therapeutic potential as it can increase intratumoral blood perfusion, localization of intravenously delivered drugs, and promote a hyperthermic intratumoral state. Despite the well-known immunologic benefits that febrile hyperthermia can induce, an investigation of how RFT could modulate the intra-tumoral immune microenvironment had not been studied. Thus, using an established 4T1 breast cancer model in immune competent mice, we demonstrate that RFT induces a transient, localized, and T-cell dependent intratumoral inflammatory response. More specifically we show that multi- and singlet-dose RFT promote an increase in tumor volume in immune competent Balb/c mice, which does not occur in athymic nude models. Further leukocyte subset analysis at 24, 48, and 120 hours after a single RFT show a rapid increase in tumoral trafficking of CD4+ and CD8+ T-cells 24 hours post-treatment. Additional serum cytokine analysis reveals an increase in numerous pro-inflammatory cytokines and chemokines associated with enhanced T-cell trafficking. Overall, these data demonstrate that non-invasive RFT could be an effective immunomodulatory strategy in solid tumors, especially for enhancing the tumoral trafficking of lymphocytes, which is currently a major hindrance of numerous cancer immunotherapeutic strategies.

BRCA2 protects mammalian cells from heat shock

PURPOSE

Heat shock induces DNA double-strand breaks (DSBs) in mammalian cells. Mammalian cells are capable of repairing DSBs by utilising the homologous recombination (HR) pathway. Breast cancer susceptibility gene 2 (BRCA2) is known to regulate the HR pathway. Here, we investigate the role of BRCA2 in repairing DNA damage induced by heat shock.

MATERIALS AND METHODS

Chinese hamster lung fibroblast cell lines and human tongue squamous cell carcinoma SAS cells were used. RAD51 foci formation assay was used as an HR indicator. Heat sensitivity was analysed with colony forming assays. Phosphorylated histone H2AX (γH2AX) intensity, which correlates with the number of DSBs, was analysed with flow cytometry.

RESULTS

RAD51 foci appeared with heat shock, and the number of cells with RAD51 foci was maximal at about 4 h after heat shock. Heat-induced RAD51 foci co-localised with γH2AX foci. BRCA2-deficient cells were sensitive to heat when compared to their parental wild-type cells. Heat-induced γH2AX was higher in BRCA2-deficient cells compared to parental cells. In SAS cells, cells transfected with BRCA2-siRNA were more sensitive to heat than cells transfected with negative control siRNA. Apoptotic bodies increased in number more rapidly in BRCA2-siRNA transfected cells than in cells transfected with negative control siRNA when cells were observed at 48 h after a heat treatment. In addition, cells deficient in BRCA2 were incapable of activating heat-induced G₂/M arrest.

CONCLUSION

BRCA2 has a protecting role against heat-induced cell death. BRCA2 might be a potential molecular target for hyperthermic cancer therapy.

Enhancing radiosensitisation of BRCA2-proficient and BRCA2-deficient cell lines with hyperthermia and PARP1-i

Poly(ADP-ribose)polymerase1 (PARP1) is an important enzyme in regulating DNA replication. Inhibition of PARP1 can lead to collapsed DNA forks which subsequently causes genomic instability, making DNA more susceptible in developing fatal DNA double strand breaks. PARP1-induced DNA damage is generally repaired by homologous recombination (HR), in which BRCA2 proteins are essential. Therefore, BRCA2-deficient tumour cells are susceptible to treatment with PARP1-inhibitors (PARP1-i). Recently, BRCA2 was shown to be down-regulated by hyperthermia (HT) temporarily, and this consequently inactivated HR for several hours. In this study, we investigated whether HT exclusively interferes with HR by analysing thermal radiosensitisation of BRCA2-proficient and deficient cells. After elucidating the equitoxicity of PARP1-i on BRCA2-proficient and deficient cells, we studied the cell survival, apoptosis, DNA damage (γ-H2AX foci and comet assay) and cell cycle distribution after different treatments. PARP1-i sensitivity strongly depends on the BRCA2 status. BRCA2-proficient and deficient cells are radiosensitised by HT, indicating that HT does not exclusively act by inhibition of HR. In all cell lines, the addition of HT to radiotherapy and PARP1-i resulted in the lowest cell survival, the highest levels of DNA damage and apoptotic levels compared to duo-modality treatments. Concluding, HT not only inhibits HR, but also has the capability of radiosensitising BRCA2-deficient cells. Thus, in case of BRCA2-mutation carriers, combining HT with PARP1-i may boost the treatment efficacy. This combination therapy would be effective for all patients with PARP1-i regardless of their BRCA status.