RAD51 paralogs promote genomic integrity and chemoresistance in cancer by facilitating homologous recombination

The role of RAD51 regulators and variants in primary ovarian insufficiency, endometriosis, and polycystic ovary syndrome

The study of RAD51 regulators in female reproductive diseases has novel biomarker potential and implications for therapeutic advancement. Regulators of RAD51 play important roles in maintaining genome integrity and variations in these genes have been identified in female reproductive diseases including primary ovarian insufficiency (POI), endometriosis, and polycystic ovary syndrome (PCOS). RAD51 modulators change RAD51 activity in homologous recombination, replication stress, and template switching pathways. However, molecular implications of these proteins in primary ovarian insufficiency, endometriosis, and polycystic ovary syndrome have been understudied. For each reproductive disease, we provide its definition, current diagnostic and therapeutic treatment strategies, and associated genetic variations. Variants were discovered in RAD51, and regulators including DMC1, RAD51B, SWS1, SPIDR, XRCC2 and BRCA2 linked with POI. Endometriosis is associated with variants in XRCC3, BRCA1 and CSB genes. Variants in BRCA1 were associated with PCOS. Our analysis identified novel biomarkers for POI (DMC1 and RAD51B) and PCOS (BRCA1). Further biochemical and cellular analyses of RAD51 regulator functions in reproductive disorders will advance our understanding of the pathogenesis of these diseases.

Homologous recombination pathway gene variants identified by tumor-only sequencing assays in lung carcinoma patients

Background

The homologous recombination (HR) repair pathway plays a key role in double-stranded DNA break repair, and germline HR pathway gene variants are associated with increased risk of several cancers, including breast and ovarian cancer. HR deficiency is also a therapeutically targetable phenotype.

Methods

Somatic (tumour-only) sequencing was performed on 1,109 cases of lung tumors, and the pathological data were reviewed to filter for lung primary carcinomas. Cases were filtered for variants (disease-associated or of uncertain significance) in 14 HR pathway genes, including BRCA1, BRCA2, and ATM. The clinical, pathological and molecular data were reviewed.

Results

Sixty-one HR pathway gene variants in 56 patients with primary lung cancer were identified. Further filtering by variant allele fraction (VAF) of ≥30% identified 17 HR pathway gene variants in 17 patients. ATM gene variants were most the commonly identified (9/17), including two patients with c.7271T>G (p.V2424G), a variant in the germline that is associated with increased familial cancer risk. Four (4/17) patients had a family history of lung cancer, among which three patients had ATM gene variants suspected to be germline in origin. In three other patients with BRCA1/2 or PALB2 gene variants who had undergone germline testing, the variants were confirmed to be germline; lung cancer was the sentinel cancer in two of these patients with a BRCA1 or PALB2 variant.

Conclusions

Genomic variants in the HR repair pathway identified in tumor-only sequencing and occurring at higher VAFs (i.e., ≥30%) may suggest a germline origin. Correlating with personal and family history, a subset of these variants is also suggested to be associated with familial cancer risks. Patient age, smoking history and driver mutation status are expected to be a poor screening tool in identifying these patients. Finally, the relative enrichment for ATM variants in our cohort suggests a possible association between ATM mutation and lung cancer risk.

Role of ionizing radiation activated NRF2 in lung cancer radioresistance

Radiotherapies are commonly used to target remaining tumor niches after surgery of solid tumors but are restricted due to therapeutic resistance. Several pathways of radioresistance have been reported in various cancers. This study investigates the pivotal role of Nuclear factor-erythroid 2-related factor 2 (NRF2) in the activation of DNA damage repair in lung cancer cells after x-rays exposure. To explore the NRF2 activation after ionizing irradiations, this study uses a knockdown of NRF2, which shows potential DNA damage after x-rays irradiation in lung cancers. This work further shows that NRF2 knockdown disrupts damaged DNA repair by inhibiting DNA-dependent protein kinase catalytic subunit. At the same time, NRF2 knockdown by shRNA considerably disparate homologous recombination by interfering with Rad51 expression. Further investigation of the associated pathway reveals that NRF2 activation mediates DNA damage response via the mitogen-activated protein kinase (MAPK) pathway as the knockout of NRF2 directly enhances intracellular MAPK phosphorylation. Similarly, both NAC and constitutive knockout of NRF2 disrupt DNA-dependent protein kinase catalytic subunit, while NRF2 knockout failed to upregulate Rad51 expression after irradiation in-vivo. Taken together, these findings advocate NRF2 plays a critical role in the development of radioresistance by upregulating DNA damage response via the MAPK pathway, which can be of great significance.

Combination bromo- and extraterminal domain and poly (ADP-ribose) polymerase inhibition synergistically enhances DNA damage and inhibits neuroblastoma tumorigenesis

PURPOSE

JQ1 is a bromo- and extraterminal (BET) domain inhibitor that downregulates MYC expression and impairs the DNA damage response. Poly (ADP-ribose) polymerase (PARP) inhibitors prevent DNA damage sensing and repair. We hypothesized that JQ1 would promote a DNA repair-deficient phenotype that sensitizes neuroblastoma cells to PARP inhibition.

METHODS

Four human neuroblastoma cell lines were examined: two MYCN-amplified (BE(2)-C and IMR-32), and two non-MYCN-amplified (SK-N-SH and SH-SY5Y). Cells were treated with JQ1 (BET inhibitor), Olaparib (PARP inhibitor), or in combination to assess for therapeutic synergy of JQ1 and Olaparib. Treated cells were harvested and analyzed. Quantitative assessment of combination treatment synergy was performed using the median effect principle of Chou and Talalay.

RESULTS

Combination treatment with Olaparib decreased the IC₅₀ of JQ1 by 19.9-fold, 2.0-fold, 12.1-fold, and 2.0-fold in the BE(2)-C, IMR-32, SK-N-SH, and SH-SY5Y cell lines, respectively. In the MYCN-amplified cell lines, BE(2)-C and IMR-32, combination treatment decreased gene expression of MYCN relative to single-drug treatment alone or control. Combination treatment decreased protein expression of DNA repair proteins Ku80 and RAD51, led to accumulation of DNA damage marker phospho-histone H2A.X, and increased caspase activity. In the non-MYCN-amplified cell lines, SK-N-SH and SH-SY5Y, combination treatment induced G0/G1 cell cycle arrest.

CONCLUSIONS

Combination BET and PARP inhibition synergistically inhibited neuroblastoma tumorigenesis in vitro. In MYCN-amplified neuroblastoma cells, this effect may be induced by downregulation of MYCN transcription, defects in DNA repair, accumulation of DNA damage, and apoptosis. In non-MYCN-amplified cell lines, combination treatment induced cell cycle arrest.

Genome downsizing after polyploidy: mechanisms, rates and selection pressures

An analysis of over 10 000 plant genome sizes (GSs) indicates that most species have smaller genomes than expected given the incidence of polyploidy in their ancestries, suggesting selection for genome downsizing. However, comparing ancestral GS with the incidence of ancestral polyploidy suggests that the rate of DNA loss following polyploidy is likely to have been very low (4-70 Mb/million years, 4-482 bp/generation). This poses a problem. How might such small DNA losses be visible to selection, overcome the power of genetic drift and drive genome downsizing? Here we explore that problem, focussing on the role that double-strand break (DSB) repair pathways (non-homologous end joining and homologous recombination) may have played. We also explore two hypotheses that could explain how selection might favour genome downsizing following polyploidy: to reduce (i) nitrogen (N) and phosphate (P) costs associated with nucleic acid synthesis in the nucleus and the transcriptome and (ii) the impact of scaling effects of GS on cell size, which influences CO₂ uptake and water loss. We explore the hypothesis that losses of DNA must be fastest in early polyploid generations. Alternatively, if DNA loss is a more continuous process over evolutionary time, then we propose it is a byproduct of selection elsewhere, such as limiting the damaging activity of repetitive DNA. If so, then the impact of GS on photosynthesis, water use efficiency and/or nutrient costs at the nucleus level may be emergent properties, which have advantages, but not ones that could have been selected for over generational timescales.

Falcarindiol Enhances Cisplatin Chemosensitivity of Hepatocellular Carcinoma via Down-Regulating the STAT3-Modulated PTTG1 Pathway

Hepatocellular carcinoma (HCC) is the most frequent primary liver malignancy globally and the third leading cause of cancer-related death. Chemotherapy is one of the main methods in treating HCC, while recent studies have found that the resistance of HCC to chemotherapeutic drugs reduces the efficacy of the chemotherapy. Falcarindiol (FAD) is a cytotoxic and anti-inflammatory polyacetylenic oxylipin found in food plants of the carrot family (Apiaceae), while its role in HCC remains to be explored. Here, HCC cells (Huh7 and LM3) were treated with FAD at different doses. Cell proliferation was tested by the cell counting kit-8 (CCK-8) method and colony formation assay, while the apoptosis was monitored by flow cytometry. The profiles of apoptosis-related proteins (Bax, bcl2, and Caspase-3), DNA repair proteins (Rad51, BRCA1, and MDC1), and the signal transducer and activator of transcription 3 (STAT3)/Pituitary Tumor Transforming Gene 1 (PTTG1) were verified by western blot (WB) or quantitative reverse transcription-polymerase chain reaction (qRT-PCR). The interaction between STAT3 and PTTG1 was verified by immunoprecipitation (IP). In addition, a xenograft tumor model was constructed in mice to explore the anti-tumor effects of FAD in vivo, and immunohistochemistry (IHC) was performed to count the number of Ki67-stained cells. As a result, FAD inhibited HCC cell proliferation and DNA repair, facilitated their apoptosis, and also enhanced cisplatin (DDP) chemosensitivity. The Combination Index (CI) evaluation showed that FAD and DDP had synergistic effects in repressing HCC cell proliferation. Besides, FAD dampened the STAT3/PTTG1 pathway expression. Further studies revealed that inhibiting STAT3 enhanced the inhibitive effect of FAD on HCC cells, whereas overexpressing PTTG1 attenuated the anti-tumor effect of FAD. Overall, our study illustrated that FAD is a potential anticancer drug and strengthens the chemosensitivity of HCC cells to DDP by inhibiting the STAT3/PTTG1 pathway.

Regulation and pharmacological targeting of RAD51 in cancer

Regulation of homologous recombination (HR) is central for cancer prevention. However, too little HR can increase cancer incidence, whereas too much HR can drive cancer resistance to therapy. Importantly, therapeutics targeting HR deficiency have demonstrated a profound efficacy in the clinic improving patient outcomes, particularly for breast and ovarian cancer. RAD51 is central to DNA damage repair in the HR pathway. As such, understanding the function and regulation of RAD51 is essential for cancer biology. This review will focus on the role of RAD51 in cancer and beyond and how modulation of its function can be exploited as a cancer therapeutic.

The implication of the SUMOylation pathway in breast cancer pathogenesis and treatment

Breast cancer is the most commonly diagnosed malignancy in woman worldwide, and is the second most common cause of death in developed countries. The transformation of a normal cell into a malignant derivate requires the acquisition of diverse genomic and proteomic changes, including enzymatic post-translational modifications (PTMs) on key proteins encompassing critical cell signaling events. PTMs occur on proteins after translation, and regulate several aspects of proteins activity, including their localization, activation and turnover. Deregulation of PTMs can potentially lead to tumorigenesis, and several de-regulated PTM pathways contribute to abnormal cell proliferation during breast tumorigenesis. SUMOylation is a PTM that plays a pivotal role in numerous aspects of cell physiology, including cell cycle regulation, protein trafficking and turnover, and DNA damage repair. Consistently with this, the deregulation of the SUMO pathway is observed in different human pathologies, including breast cancer. In this review we will describe the role of SUMOylation in breast tumorigenesis and its implication for breast cancer therapy.