Assessment and relevance of the putative DNA/RNA helicase Schlafen-11 in ovarian and breast cancer

Abstract in English Schlafen 11 (SLFN11) is a putative DNA/RNA helicase, first described for its role in thymocyte development and differentiation in mouse models [1]. SLFN11 is part of a family of proteins with various degree of homology across species, but intriguingly being consistently present only in vertebrates and especially in mammals. Recently the role of this putative DNA/RNA helicase, SLFN11, was causal association with sensitivity to DNA damaging agents, such as platinum salts, topoisomerase I and II inhibitors, and other alkylators in the NCI-60 panel of cancer cell lines.13 In the first study, we validate an anti-SLFN11 antibody in formalin-fixed paraffin-embedded (FFPE) high-grade serous ovarian carcinoma (HGSOC) samples, developing a immunohistochemistry (IHC) protocol in order to determinate the expression of SLFN11 in our series of HGSOC. Indeed, we tested and validated a reliable SLFN 11 antibody (Ab) in IHC choosing between two anti-SLFN11 Ab used normally for Western Blot (WB) in culture cell block (CCB) of ovarian carcinoma and in an independent series of HGSOCs tissue micro-array (TMA). For each case, we evaluated both the Intensity Score (IS) and the Distribution Score (DS) evaluating at least 300 cells. A Histological Score (HS) was obtained as follow: HS=IS x DS. Successively, we applied our protocol to a large case series of HGSOC samples to confirm our preliminary results. We found one antibody to be reliable in CCB and TMA series allowing to determinate clearly IHC expression of SLFN11. These results were confirmed in our large case series of FFPE HGSOC samples. Briefly, as for TMA independent series, we found that the HS for SLFN11 expression presents a normal distribution with a prevalent (≈ 60%) intermediate expression. Parallel SLFN11 was not expressed in practically 40% of cases that clinically corresponded to the platinum resistant patients in about 60% of cases (16/27). So, we believe that low IHC expression of SLFN 11 should be correlated to response to the platinum based chemotherapy. In the second study, we investigate the transcriptional landscape of SLFN11 in breast cancer performing a gene expression microarray meta-analysis of more than 7000 cases from 35 publicly available data sets. By correlation analysis, we identified 537 transcripts in the top 95th percentile of Pearson’s coef- ficients with SLFN11 identifying “immune response”, “lymphocyte activation”and “T cell activation” as top Gene Ontology enriched processes. Furthermore, we reported very strong association of SLFN11 with immune signatures in breast cancer through penalized maximum like-lihood lasso regression Finally, through multiple corresponde analysis we discovered a subgroup of patients, defined “SLF11-hot cluster”, characterized by high SLFN11 levels, estrogen receptor negativity, basal-like phenotype, elevated CD3D, STAT1 signature, and young age and using Cox proportional hazard regression, we characterized SLFN11 high levels, high proliferation index, and ER negativity as independent parameters for longer disease-free interval in patients undergoing chemotherapy. We believe that our work supports proof of concept that: i) A clear and specific role for SLFN11 in breast cancer, in likely connection with the immune system modulation in such disease entity, ii) a strong correlation between high SFLN 11 and specific molecular subtype of breast cancer (estrogen receptor negativity, basal-like phenotype).