Liquid biopsy results that identify DNA damage repair genes in prostate cancer may be a useful tool in clinical-decision making, especially because these genes can elucidate therapeutic vulnerabilities that could be exploited by current treatments.
Alexander Wyatt, PhD
Liquid biopsy results that identify DNA damage repair (DDR) genes in prostate cancer may be a useful tool in clinical-deci­sion making, espe­cially because these genes can elucidate therapeutic vulnera­bilities that could be exploited by current treatments.
The recognition of the role of alterations in the DDR pathwaynotably theBRCA1/2,ATM, andCDK12genesmarks a major milestone in recent prostate cancer research.BRCA2mutations, in par­ticular, have been associated with more aggressive disease, poor prog­nosis, and poor clinical responses to systemic therapy for patients with castration-resistant prostate cancer.1,2
An estimated 10% of primary tumors and 25% of meta­static prostate cancer harbor DDR defects.3,4
Alterations inBRCA2are most relevant in a cancer cell when both alleles are rendered incapable of pro­ducing functional protein.1 Evidence suggests that the majority of patients with aBRCA2germline alteration will have a deletion of the second allele, often called loss of heterozygosity (LOH) in the tumor. The use of next-generation sequencing studies is making pos­sible the characterization of rarer mutations in other members of the homologous recombination repair (HRR) pathway.
Alexander Wyatt, PhD, and colleagues at the Vancouver Prostate Centre, University of British Columbia, used liquid biopsy testing to distinguish between monoallelic and biallelic DDR defects in a sample of 882 men with metastatic prostate cancer.5The investigators focused on theBRCA2,ATM, andCDK12genes because they are most frequently altered in prostate cancer. Results were presented in a poster at the Advances in Liquid Biopsies meeting hosted by the American Association for Cancer Research, held January 13 to 16, 2020, in Miami, Florida.
“Over the last few years, it has become apparent that DNA repair defects are widespread in metastatic pros­tate cancer. This is characteristic of metastatic prostate cancer and not localized, curable disease,” said Wyatt, an assistant professor in the Department of Urologic Sciences, during an interview withTargeted Therapies in Oncology.
Disruptions inATMandCDK12can lead to a genomic landscape that is distinct fromBRCA2-deficient dis­ease, according to the investigators. “Three or 4 years ago, it was thought that all DNA repair genes were sim­ilar to each other and had the same biology and same effect on therapeutic response. But that’s not the case,” Wyatt said.
This revelation is especially important, noted Wyatt. “Treatment approaches such as PARP inhibitors are most effective in the presence of HRR defects. If your DNA repair gene is not actually in the pathway, then the therapeutic window for PARP inhibition may not be present,” he said.
For patients with >1 deleterious mutation inBRCA2,ATM, andCDK12, the second allele was disrupted in 97%, 85%, and 91% of cases, respectively. This disrup­tion could be attributed to mutation, deletion, or copy-neutral LOH.
ForBRCA2, biallelic loss occurred through a deletion of the second allele. The investigators reported 8 cases of somatic biallelic deletion ofBRCA2. Biallelic loss inATMoccurred through either mutation or deletion of the second allele. Disruption ofCDK12occurred almost exclusively through secondary mutations, though the investigators noted several cases of copy-neutral LOH.
A total of 277 somatic mutations were detected in patients with loss ofBRCA2,ATM, orCDK12. Patients withBRCA-mutant tumors displayed a higher median number of mutations than a control cohort of DDR-intact patients (3.6 vs 2.2;P<.01).
DNA repair defects are most relevant when both cop­ies of the gene are lost, called biallelic loss. “Many DNA repair genes are haplosufficient, so having 1 working copy [of the allele] results in the cell functioning as if it had 2 copies. Our work suggests that the majority of DNA repair defects are biallelic, which is probably good news from a targeted therapies standpoint,” Wyatt said.
Wyatt said an opportunity exists to encourage com­munity oncologists to take advantage of liquid biopsy assays as more tests become available. These tests will be able to detect different types of defects, which have different therapeutic vulnerabilities.
Germline defects are another factor for community oncologists to consider. Wyatt noted that National Comprehensive Cancer Network guidelines on prostate cancer recommend screening of patients for germline alterations in DNA repair genes.6The guidelines recom­mend genetic testing for the germline variantsMLH1,MSH2,MSH6, andPMS2(for Lynch syndrome) and homologous recombination genesBRCA1,BRCA2,ATM,PALB2, andCHEK2. “It’s not enough to just consider the tumor when testing; you’ve got to understand the familial implications that go beyond prostate cancer,” Wyatt said.