Resistance to irradiation of prostate cancer

Identification of genes involved in resistance of prostate cancer cells to irradiation

J. Hodzic1, B. van Triest2, A. Geldof3, R.X. de Menezes4, W.R. Gerritsen1, M. Verheij2, V.W. van Beusechem1,5

Departments of 1Medical Oncology, 3Urology, 4Epidemiology and Biostatistics  and 5RNA Interference Functional Oncogenomics Laboratory, VU University Medical Center, Amsterdam; 2Department of Radiotherapy, The Netherlands Cancer Institute, Amsterdam (Stichting CCA)

Prostate cancer (PC) is the most commonly diagnosed cancer and the second leading cause of cancer death in Western world males. PSA testing allows early detection of localized disease. Treatment options for patients with local disease include radiotherapy and surgery. However, local tumor control rates after radiotherapy are highly variable; and ~40% of treated patients will experience disease recurrence and progression. The aim of our study is to identify molecular targets of radiation susceptibility in PCa cells to improve early local therapeutic intervention. This is done by high-throughput screening using genome-wide siRNA and miRNA libraries on the radioresistant PC-3 cell line subjected to low-dose irradiation. We conducted two genome-wide siRNA and two miRNA screens with and without irradiation in PC-3 cells. From the siRNA screen 45 candidate targets were selected for further research. In a conformation screen, multiple independent siRNAs silencing 17 genes induced ?2-fold more cell death upon 4Gy irradiation compared to a negative control siRNA. Confirmed hits were validated independently using the colony formation assay (CFA). The dose modifying factor at 80% clonogenic cell survival (DMF0.8) was calculated for each gene-specific siRNA compared to control siRNA from linear-quadratic survival curves. Sixteen candidate genes were validated with a DMF0.8 ? 1.3. Silencing of 9 genes sensitized PC-3 cells to irradiation more effectively than did silencing of the known radiation susceptibility gene PRKDC. The miRNA screen identified 2 known radiosensitizing miRNAs and 10 other miRNAs not previously associated with radio susceptibility. Of these, 8 were validated independently with miRNA mimics and the CFA. Hence, we identified novel genes and miRNAs that that could be used to potently enhance the efficacy of radiotherapy in prostate cancer. This could contribute significantly to improving overall success of prostate cancer intervention.