BRCA1/FANCD2/BRG1-Driven DNA Repair Stabilizes the Differentiation State of Human Mammary Epithelial Cells

NF-κB at the Crossroads of Normal Mammary Gland Biology and the Pathogenesis and Prevention of BRCA1-Mutated Breast Cancer

Recent studies have shown that progesterone receptor (PR)-expressing cells respond to progesterone in part through the induction of the receptor activator of NF-κB ligand (RANKL), which acts in a paracrine manner to induce expansion of a RANK-expressing luminal progenitor cell population. The RANK⁺ population in human breast tissue from carriers of BRCA1 mutations (BRCA1^mut/+) as well as the luminal progenitor population in Brca1-deficient mouse mammary glands is abnormally amplified. Remarkably, mouse Brca1^+/- and human BRCA1^mut/+ progenitor cells are able to form colonies in vitro in the absence of progesterone, demonstrating a hormone-independent proliferative capacity. Our research has demonstrated that proliferation in BRCA1-deficient cells results in a DNA damage response (DDR) that activates a persistent NF-κB signal, which supplants progesterone/RANKL signaling for an extended time period. Thus, the transcriptional targets normally activated by RANKL that promote a proliferative response in luminal progenitors can contribute to the susceptibility of mammary epithelial cells to BRCA1-mutated breast cancers as a consequence of DDR-induced NF-κB. Together, these latest findings mark substantial progress in uncovering the mechanisms driving high rates of breast tumorigenesis in BRCA1 mutation carriers and ultimately reveal possibilities for nonsurgical prevention strategies. Cancer Prev Res; 11(2); 69-80. ©2017 AACR.

SWI/SNF aberrations sensitize pancreatic cancer cells to DNA crosslinking agents

While gemcitabine has been the mainstay therapy for advanced pancreatic cancer, newer combination regimens (e.g. FOLFIRINOX) have extended patient survival, though carry greater toxicity. Biomarkers are needed to better stratify patients for appropriate therapy. Previously, we reported that one-third of pancreatic cancers harbor deletions or deleterious mutations in key subunits of the SWItch/Sucrose NonFermentable (SWI/SNF) chromatin remodeling complex. The SWI/SNF complex mobilizes nucleosomes on DNA, and plays a key role in modulating DNA transcription and repair. Thus, we hypothesized that pancreatic cancers with SWI/SNF aberrations might exhibit compromised DNA repair, and show increased sensitivity to DNA damaging agents. Here, we studied human pancreatic cancer cell lines with deficient (or else exogenously reconstituted) SWI/SNF subunits, as well as normal pancreatic epithelial cells following SWI/SNF subunit knockdown. Cells were challenged with DNA damaging agents, including those used in current combination regimens, and then cell viability assayed. We found that pancreatic cells with SWI/SNF dysfunction showed markedly increased sensitivity to DNA damaging agents, and in particular DNA crosslinking agents (cisplatin and oxaliplatin). Assaying clearance of γH2AX confirmed that SWI/SNF dysfunction impaired DNA damage response/repair. Finally, by analyzing pancreatic cancer patient data from The Cancer Genome Atlas, we found that pancreatic cancers with SWI/SNF deficiency (subunit mutation and/or decreased expression) were associated with extended patient survival specifically when treated with platinum containing regimens. Thus, SWI/SNF dysfunction sensitizes pancreatic cancer cells to DNA crosslinking agents, and SWI/SNF mutation status may provide a useful biomarker to predict which patients are likely to benefit from platinum-containing chemotherapy regimens.

DNA2-An Important Player in DNA Damage Response or Just Another DNA Maintenance Protein?

The human DNA2 (DNA replication helicase/nuclease 2) protein is expressed in both the nucleus and mitochondria, where it displays ATPase-dependent nuclease and helicase activities. DNA2 plays an important role in the removing of long flaps in DNA replication and long-patch base excision repair (LP-BER), interacting with the replication protein A (RPA) and the flap endonuclease 1 (FEN1). DNA2 can promote the restart of arrested replication fork along with Werner syndrome ATP-dependent helicase (WRN) and Bloom syndrome protein (BLM). In mitochondria, DNA2 can facilitate primer removal during strand-displacement replication. DNA2 is involved in DNA double strand (DSB) repair, in which it is complexed with BLM, RPA and MRN for DNA strand resection required for homologous recombination repair. DNA2 can be a major protein involved in the repair of complex DNA damage containing a DSB and a 5' adduct resulting from a chemical group bound to DNA 5' ends, created by ionizing radiation and several anticancer drugs, including etoposide, mitoxantrone and some anthracyclines. The role of DNA2 in telomere end maintenance and cell cycle regulation suggests its more general role in keeping genomic stability, which is impaired in cancer. Therefore DNA2 can be an attractive target in cancer therapy. This is supported by enhanced expression of DNA2 in many cancer cell lines with oncogene activation and premalignant cells. Therefore, DNA2 can be considered as a potential marker, useful in cancer therapy. DNA2, along with PARP1 inhibition, may be considered as a potential target for inducing synthetic lethality, a concept of killing tumor cells by targeting two essential genes.

Elevated PTTG and PBF predicts poor patient outcome and modulates DNA damage response genes in thyroid cancer

The proto-oncogene PTTG and its binding partner PBF have been widely studied in multiple cancer types, particularly thyroid and colorectal, but their combined role in tumourigenesis is uncharacterised. Here, we show for the first time that together PTTG and PBF significantly modulate DNA damage response (DDR) genes, including p53 target genes, required to maintain genomic integrity in thyroid cells. Critically, DDR genes were extensively repressed in primary thyrocytes from a bitransgenic murine model (Bi-Tg) of thyroid-specific PBF and PTTG overexpression. Irradiation exposure to amplify p53 levels further induced significant repression of DDR genes in Bi-Tg thyrocytes (P=2.4 × 10^-4) compared with either PBF- (P=1.5 × 10^-3) or PTTG-expressing thyrocytes (P=NS). Consistent with this, genetic instability was greatest in Bi-Tg thyrocytes with a mean genetic instability (GI) index of 35.8±2.6%, as well as significant induction of gross chromosomal aberrations in thyroidal TPC-1 cells following overexpression of PBF and PTTG. We extended our findings to human thyroid cancer using TCGA data sets (n=322) and found striking correlations with PBF and PTTG expression in well-characterised DDR gene panel RNA-seq data. In addition, genetic associations and transient transfection identified PBF as a downstream target of the receptor tyrosine kinase-BRAF signalling pathway, emphasising a role for PBF as a novel component in a pathway well described to drive neoplastic growth. We also showed that overall survival (P=1.91 × 10^-5) and disease-free survival (P=4.9 × 10^-5) was poorer for TCGA patients with elevated tumoural PBF/PTTG expression and mutationally activated BRAF. Together our findings indicate that PBF and PTTG have a critical role in promoting thyroid cancer that is predictive of poorer patient outcome.

Transfer of malignant traits as opposed to migration of cells: A novel concept to explain metastatic disease

Metastatic disease is believed to develop following dissemination of cells to target organs. Inability of this theory to effectively explain certain phenomena such as patterns of metastatic spread, late metastasis formation, different gene patterns between primary cancer and metastasis have brought forward the need for alternative models. Recent discoveries have strengthened the validity of theories supporting a humoral transfer of malignant traits as opposed to migration of malignant cells to explain metastatic disease in cancer patients. In light of this new evidence, we would like to highlight a model that offers a new perspective to explain cancer metastasis. In the system that we theorize, genetic material released by cancer cells would travel, either free or packed in exosomes, through the blood. Target cells located in organs deriving from the same embryological layer might uptake this genetic material due to expression of specific receptors. Interplay with the immune system would determine the fate of these oncofactors and would regulate their ability to circulate in the blood, integrate in the genome and be transcribed. We also hypothesize that the expression of cell membrane receptors such as integrins, to which cancer exosomes ligate might be mediated by inherited or acquired oncosuppressor mutations.