The Precision Oncology Program for Cancer of the Prostate (POPCaP) Network: A Veterans Affairs/Prostate Cancer Foundation Collaboration

Precision Oncology for Prostate Cancer

A series of studies interrogating the genomics of metastatic prostate cancer have been critical to defining the relevance of precision oncology for prostate cancer. Most of what is known about the genomics of prostate cancer has been derived from analysis of samples from the prostate itself. These samples may not reflect the biology of metastasis and genetic evolution in response to treatment pressure, so the genomic alterations in metastatic disease remained incompletely characterized. Two large research teams supported by grants from the American Association for Cancer Research, Stand Up 2 Cancer, and Prostate Cancer Foundation (PCF) focused their efforts on sampling and analyzing metastatic tissue to define the most relevant genomic alterations in advanced prostate cancer.

These efforts defined a broad range of relatively common alterations in the androgen receptor, as well as the tumor suppressors TP53 and PTEN.^2,3 Important subsets of less common alterations in pathways that were potentially targetable were also found, including new alterations in PIK3CA/B, BRAF/RAF1, and β-catenin. Most surprisingly, alterations of DNA repair pathways, including mismatch repair and homologous recombination were found in 20% of tumors, and half of these tumors contained germline alterations. The same groups performed a follow up analysis of germline DNA from men with metastatic prostate cancer, which confirmed that 12% of these patients carry a pathogenic germline alteration in a DNA repair pathway gene.⁴ These efforts immediately invigorated precision oncology clinical trials for prostate cancer and spurred an effort to find the molecular alterations that could be leveraged to improve care for men with advanced prostate cancer.

Targetable Alterations

Currently a number of genomic alterations are immediately actionable. There are several agents approved by the US Food and Drug Administration (FDA) that exploit these Achilles heels of prostate cancer. Mismatch repair deficiency occurs when any of a group of genes responsible for proofreading the fidelity of DNA replication is compromised by mutation or deletion. Imperfect reading and correction subsequently lead to many DNA mutations in a tissue (hypermutation), which then increases the risk of developing malignancy. If a defective gene in the mismatch repair pathway is inherited, a patient has a genetic predisposition to specific malignancies that are part of the Lynch syndrome.⁵ Prostate cancer is a relatively rare manifestation of Lynch syndrome, although it is considered one of the malignancies in the Lynch syndrome spectrum.⁶

Alteration of one of the mismatch repair genes also can occur spontaneously in a tumor, resulting in the same high frequency of spontaneous DNA mutations. Overall, between 3% and 5% of metastatic prostate cancers contain mismatch repair deficiency. The majority of these cases are a result of spontaneous loss or mutation of the relevant gene, but 1 in 5 of these tumors occurs as a component of Lynch syndrome.⁷ Identification of mismatch repair deficiency is critical because the resulting hypermutation makes these tumors particularly susceptible to intervention with immunotherapy. Up to half of patients with metastatic prostate cancer can have durable responses. This finding is consistent with the experience treating other malignancies with mismatch repair deficiency.⁸ Although screening for mismatch repair deficiency is standard of care for patients with malignancies such as colorectal cancer, few patients with prostate cancer may receive the mismatch repair deficiency screening (based on unpublished data). In contrast, screening is routine for patients with adenocarcinoma of the lung because their proportion of ROS1 and ALK alterations is similar to the frequency of mismatch repair deficiency when compared with patients with prostate cancer.⁹

Homologous recombination is another mechanism by which cells repair DNA damage and is responsible for repairing double strand breaks, the type of DNA damage most likely to lead to carcinogenesis. In advanced prostate cancer, BRCA2, ATM, BRCA1 and other members of the Fanconi Anemia/BRCA gene family are altered 20% of the time. These genes also are the most common germline alterations implicated in the development of prostate cancer.^2,10 Prostate cancer is considered a BRCA-related cancer much like breast, ovarian, and pancreatic cancers. Defects in homologous recombination repair make BRCA-altered prostate cancers susceptible to DNA damaging chemotherapy, such as platinum and to the use of poly–(adenosine diphosphate–ribose) polymerase (PARP) inhibitors because cancer cells then accumulate cytotoxic and apoptotic levels of DNA.¹¹

In May 2020, the FDA approved the use of PARP inhibitors for the treatment of prostate cancers that contain BRCA and other DNA repair alterations. Rucaparib received accelerated approval for the treatment of prostate cancers containing BRCA alterations and olaparib received full approval for treatment of prostate cancers containing an array of alterations in DNA repair genes.^12,13 Both approvals were the direct result of the cited landmark studies that demonstrated the frequency of these alterations in advanced prostate cancer.^2,3

Beyond mismatch and homologous recombination repair, there are a large number of potentially targetable alterations found in advanced prostate cancer. It is thus critical that we put systems into place both to find germline and somatic alterations that will inform a veteran’s clinical care and to provide veterans access to precision oncology clinical trials.