Abstract 21P
Background
PARP inhibitors olaparib, niraparib, rucaparib are approved for treatment of mCRPC harboring Homologous Recombination Deficiency (HRD). However, not all patients who receive these therapies respond and many ultimately develop resistance. This study aims to identify new PARP alternative targetable pathways to improve outcomes of HRD Prostate Cancer (PCa) patients.
Methods
PCa samples from TCGA were divided in 2 groups on the basis of HR genomic status (HRD and non-HRD) and further divided in POLQ high and POLQ low according to POLQ median gene expression. Human PCa cell lines (LNCaP, C42) were transfected with si-scr or si-POLQ. Cell viability was evaluated by colony formation assay. POLQ expression was evaluated by RT-PCR. Immune signatures were used to assess immune activation. Kaplan-Meier and logrank test were used to analyze Progression Free Survival (PFS). ANOVA and t-test were used for groups comparisons.
Results
GSEA of the TCGA dataset showed high activation of of translesion (TLS) DNA pathways in HRD PCa (p<0.05; FDR <0.25). Polymerase theta (Polθ encoded by POLQ gene) was the only TLS polymerase to have higher expression in HRD vs HRR samples (p=2.508e-4). POLQ expression was associated with higher TMB especially in the HRD subgroup (p<0.0001). PFS of was significantly shorter in the POLQ high patients compared to POLQ low patients, regardless the HRD status (p=1.037e-5). Si-POLQ significantly decreased LNCaP cells spheroid growth (p<0.0001). POLQ silencing also leaded to higher sensitivity to olaparib in both LNCaP and C42 cells (IC50: 6.38 vs 3.4 uM for LNCaP and 13.5 vs 7.7 uM in C42; p<0.05). Moreover, POLQ mRNA levels significantly increased in the olaparib resistant LNCaP and C42 cells (p<0.0001). POLQ-silencing was able to overcome olaparib resistance in LNCaP (IC50: 47.3 uM vs 26.46 uM; p<0.05). Immune activation signature was not significantly different in the HRD vs non-HRD PCa samples. However, POLQ was highly expressed in the HRD and immune hot PCa samples (p=4.637e-4). The HRD PCa samples with high POLQ expression demonstrated the highest immune scores (p<0.0001).
Conclusions
Polθ is over expressed in HRD PCa and is associated with olaparib resistance and immune activation representing a potential therapeutic target for HRD PCa patients.
Clinical trial identification
Editorial acknowledgement
Legal entity responsible for the study
The authors.
Funding
Prostate Cancer Foundation Young Investigator Award.
Disclosure
L. Nappi: Non-Financial Interests, Personal, Invited Speaker: Pfizer, Ipsen, Bayer, Merck, AstraZeneca, Thersera, EMD Serono, Janssen. All other authors have declared no conflicts of interest.