Upregulation of Poly (ADP-Ribose) Polymerase-1 (PARP1) in Triple-Negative Breast Cancer and Other Primary Human Tumor Types

Abstract

Poly (ADP-ribose) polymerase-1 (PARP1) is a key facilitator of DNA repair and is implicated in pathways of tumorigenesis. PARP inhibitors have gained recent attention as rationally designed therapeutics for the treatment of several malignancies, particularly those associated with dysfunctional DNA repair pathways, including triple-negative breast cancer (TNBC). We investigated the PARP1 gene expression profile in surgical samples from more than 8,000 primary malignant and normal human tissues. PARP1 expression was found to be significantly increased in several malignant tissues, including those isolated from patients with breast, uterine, lung, ovarian, and skin cancers, and non-Hodgkin's lymphoma. Within breast infiltrating ductal carcinoma (IDC) samples tested, mean PARP1 expression was significantly higher relative to normal breast tissue, with over 30% of IDC samples demonstrating upregulation of PARP1, compared with 2.9% of normal tissues. Because of known DNA repair defects, including BRCA1 dysfunction, associated with TNBC, exploration of PARP1 expression in breast cancers related to expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) led to the observation that negative expression of any of the 3 receptors was associated with upregulation of PARP1 expression, compared with receptor-positive tissues. To validate these observations, an independent set of breast adenocarcinomas was evaluated and demonstrated >2-fold upregulation of PARP1 in approximately 70% of primary breast adenocarcinomas, including TNBC, compared with syngeneic nonmalignant breast tissues. Immunohistochemistry (IHC) showed that upregulation of the PARP1 gene was consistent with increased protein expression in TNBC. These analyses suggest a potential biological role for PARP1 in several distinct malignancies, including TNBC. Further investigation of PARP1 as a biomarker for the therapeutic activity of PARP inhibitor-based therapy is warranted.

Keywords: BRCA; PARP; cancer; targeted therapies; triple-negative breast cancer.

Figure 1.

PARP1 mRNA expression is elevated in human primary cancers of the breast, endometrium, lung, ovary, and skin. Microarray analysis demonstrating (A) relative PARP1 and (B) PARP2 expression in primary cancers compared with normal tissues. Levels of mRNA from a tissue database, consisting of representative samples of primary normal tissues and metastatic and benign tumors, were quantified using Affymetrix U133A and B GeneChips. Solid lines represent median mRNA levels, bars indicate range, and boxes indicate interquartile range.

Figure 2.

Statistical representation of PARP1 upregulation in ovarian, uterine, lung, and prostate cancers. PARP1 upregulation was defined based on levels of PARP1 mRNA, quantified by microarray analysis, which exceeded the 90%, 95%, 99%, and 99.9% UCLs of normal tissue distribution. Tumor types were grouped according to pathological subtype and subcategorized according to clinical characteristics. Data are represented by the percentage of samples that reach these criteria. UCL = upper confidence limit.

Figure 3.

PARP1 gene expression is elevated in syngeneic normal and IDC breast samples. Quantitative real-time multiplex RT-PCR was used to quantify (A) mean PARP1 mRNA expression in normal and malignant tissues from individual patients with breast IDC. Number represents patient code. Results are mean ± standard deviation of pooled data from 3 independent experiments (see Materials and Methods). (B) Distribution of PARP1, PARP2, Ki-67, and TOP2A mRNA expression in syngeneic sets of malignant and normal tissue samples. RNA was isolated from FFPE sections or frozen tissues, and gene expression levels were quantified and normalized to the expression of GUSB (β-glucuronidase gene). Data were reported as the mean and standard deviation of 3 independent assessments for each sample. Solid lines represent median mRNA levels, bars indicate range, and boxes indicate interquartile range. N = normal tissue; T = tumor.

Figure 4.

Increased levels of PARP1 mRNA and protein in triple-negative breast adenocarcinoma. (A) PARP1, HER2, ER, PR, Ki-67, and TOP2A gene expression was determined by multiplex RT-PCR (see Materials and Methods). RNA was isolated from FFPE sections or frozen tissues, and gene expression levels were quantified and normalized to the expression of GUSB (β-glucuronidase gene). Data were reported as the mean and standard deviation of 3 independent assessments for each sample. (B) Immunohistochemistry of syngeneic normal and triple-negative breast carcinoma tissues. FFPE sections were stained for PARP1, ER, PR, and HER2 protein (see Materials and Methods). Images were taken at 40x magnification; scale bar represents 100 µm.