An innate immune response to DNA damage in prostate cancer

Abstract

Each year in the UK 47,300 men are diagnosed with prostate cancer. Recent research has shown that up to 25% of men with mCRPC harbour mutations in DNA Repair genes BRCA1/ BRCA2 and ATM. This group has previously identified a molecular subtype in solid tumors characterised by STING mediated immune signalling due to abnormal DNA as a result of loss of DNA repair mechanisms. This subtype is characterised by activation of cytokines, such as CXCL10, which attract lymphocytic infiltration and up regulate immune checkpointing genes like PD-L1. Identified by a 44-gene expression signature, the DNA damage response deficient (DDRD) assay has been shown to predict response to anthracyclines and platinum agents in breast, oesophageal and ovarian cancer. We hypothesize that there is a DDRD positive subgroup in prostate cancer, which may fail to benefit from taxane agents, which are the current standard of care.

Methods.
siRNA-mediated knockdown was used to identify if knockdown of common DNA repair genes in metastatic prostate cell lines led to up regulation of key genes associated with the DDRD signature by qPCR and also to assess response platinum, PARP inhibitors and taxanes. Subsequently a siRNA mediated screen was completed to try and identify novel gene mutations, which may contribute to BRCAness phenotype. Formalin-fixed, paraffin- embedded (FFPE) diagnostic core biopsies were obtained from 52 men with Castrate Resistant Prostate Cancer (CRPC) treated with docetaxel in the NI Cancer Centre. Samples were microarray profiled, signature scored and defined as DDRD positive or negative.

Results
siRNA mediated knockdown of BRCA1, BRCA2 and ATM resulted in up regulations of DDRD associated chemokines and a lack of response to docetaxel and increased sensitivity to cisplatin and olaparib. Ten patients (19.23%) were DDRD positive and 42 (80.76%) were DDRD negative; 80% of DDRD positive and 47% of DDRD negative patients failed to benefit from docetaxel. DDRD positive tumour samples demonstrated an association with poorer overall survival post-docetaxel (HR 3.72; 95% CI 1.40 to 9.89 ; p=0.009; Median survival DDRD positive 12.43months vs. DDRD negative 21.83 months).

Conclusions.
The DDRD positive molecular subtype of prostate cancer, characterised by an immune response to DNA damage, has a reduced benefit from docetaxel. We intend to validate this observation in a prospective clincal trial, investigating advanced prostate cancer patients who received docetaxel as primary therapy. These studies may lead to clinical trials where DDRD positive patients receive specific DNA damaging agents like carboplatin, olaparib.

Date of Award	Dec 2020
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Richard Kennedy (Supervisor), Suneil Jain (Supervisor) & Nuala McCabe (Supervisor)