SA1, encoded by the STAG1 gene, is a subunit of the higher-order chromatin remodeler cohesin. SA1 deficiency has been implicated as a driver of aneuploidy and tumorigenesis (Remeseiro et al, EMBO J 2012). Furthermore, our lab has recently shown that SA1 functions as a tumor suppressor protein in early colon cancer (Wali et al, Cancer Prev Res 2016). The aim of this project was to investigate the role of SA1 in NSCLC. Combining data from The Cancer Genome Atlas (TCGA), human lung tissue, and in vitro gene knockdown, we have surprisingly identified SA1 as a potential pro-neoplastic factor in NSCLC.
Data was extracted from TCGA and plotted to characterize STAG1 gene alteration in NSCLC. Immunohistochemistry (IHC) was performed on human lung tissue microarray assay (TMA) to assess SA1 protein expression. Transient knockdowns were performed on A549 human lung adenocarcinoma cells using STAG1 siRNA (Dharmacon) with an incubation time of 48 hours. Following this, quantitative real-time polymerase chain reaction (qPCR, Life Technologies) and WST-1 cell proliferation assays (Promega) were performed as per protocol.
STAG1 alterations occur in about 3% of lung adenocarcinoma and 17% of lung squamous cell carcinoma. Of these alterations, the majority were amplifications. Furthermore, although not reaching statistical significance, there was a trend towards decreased survival with STAG1 alteration in adenocarcinoma patients (11.6 months vs. 46 months). IHC demonstrated a 1.7-fold increase in SA1 protein expression in NSCLC when compared with non-malignant lung tissue (p<0.001). Transfected A549 cells showed a 78% decrease in SA1 RNA expression (qPCR, p=0.0002) and a 50% decrease in cell proliferation (WST-1, p<0.0001). Proliferating cell nuclear antigen (PCNA), a marker of cell proliferation, was also decreased by 37% in transfected cells (p<0.001).
Using TCGA data, we found that STAG1 alteration occurs with some frequency in NSCLC and that many of these alterations are amplifications of the gene. Furthermore, there is a trend towards decreased survival with a STAG1 gene alteration. Our IHC data shows robust over-expression of SA1 in NSCLC when compared with non-malignant lung tissue. Our qPCR data showed a profound decrease in SA1 mRNA expression following transfection; accordingly, our WST-1 proliferation data suggests that knockdown of the STAG1 gene significantly decreases proliferation of A549 lung adenocarcinoma cells. Decreased PCNA confirms a true anti-proliferative effect, rather than a pro-apoptotic effect. This novel data suggests a link between SA1 and NSCLC, of which there are no prior published accounts. Even more compelling is the implication that in NSCLC, SA1 may be pro-neoplastic rather than anti-neoplastic, which introduces a new potential target for future gene therapy.
Citation Format: Michelle Zhang, Mart Dela Cruz, Navneet Momi, Sanjib Chowdhury, Hemant Roy, Adam Lerner. Stromal antigen 1 (SA1) as a potential pro-neoplastic factor in non-small-cell lung cancer (NSCLC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2562. doi:10.1158/1538-7445.AM2017-2562