These mutations could help physicians predict which cancer patients will respond to the therapy and who will likely become resistant. With this insight, doctors can decide the most appropriate treatment approach.
The study, “Genome-wide and high-density CRISPR-Cas9 screens identify point mutations in PARP1 causing PARP inhibitor resistance,” was published in the journal Nature Communications.
Lynparza, an oral medicine developed by AstraZeneca, is approved in the U.S for the treatment of HER2-negative, metastatic breast cancer patients who have mutations in the BRCA genes and have received prior chemotherapy treatment. The medicine is also approved as a maintenance treatment for advanced ovarian cancer.
The compound is an inhibitor of PARP enzymes – particularly PARP1 – which are key factors for repairing DNA damage.
Because it targets DNA-repairing proteins, the treatment is most effective in tumors that lack other DNA-repairing proteins, such as those with mutations in the BRCA genes. It is believed that suppressing PARP activity leads to the accumulation of DNA damage and ultimately to the death of these cancer cells.
However, if cancer cells alter their PARP1 activity or structure, they will most likely become resistant to PARP inhibitors, as these treatments will no longer be able to eliminate the tumor.
In a study funded by Cancer Research UK and Breast Cancer Now, researchers investigated the effects of PARP1 mutations and examined how these alterations could stop PARP inhibitors from being effective.
“PARP inhibitors are hugely exciting new drugs which are especially effective in women with BRCA mutations—but unfortunately as with many other treatments it is common for cancer cells to eventually develop resistance,” Stephen Pettitt, PhD, staff scientist at The Institute of Cancer Research in London, and first author of the study, said in a news release.
“Studies like this can tell us how and why drug resistance occurs, and give us new ways of predicting the likely response to new-style targeted drugs,” added Chris Lord, a professor at The Institute of Cancer Research and a study leader.
The team used a modern technology called CRISPR-Cas9 gene editing – a defense mechanism found in bacteria that allows researchers to edit specific parts of the DNA – to generate several small mutations across the PARP1 gene. The mutant proteins were also tagged with a fluorescent protein to enable its tracking inside the cells.
Using this approach, researchers were able to examine how specific mutations on PARP1 influenced the sensitivity of cancer cells to PARP inhibitors, including olaparib and talazoparib.
Talazoparib is currently being evaluated in a Phase 3 clinical trial (NCT01945775) for the treatment of advanced or metastatic breast cancer in patients with BRCA mutations. Results so far have shown that talazoparib significantly extends the time until disease progression or death.
Researchers identified several PARP1 mutations that made cells resistant to talazoparib. The data indicated that mutations that prevented PARP1 from being trapped in the DNA were likely to lead to resistance to PARP inhibitors. In fact, a mutant cell line with this type of mutation was also resistant to Lynparza.
To confirm that PARP1 mutations could also confer resistance in breast tumors, the team tested the response to talazoparib in a mouse model of breast cancer with PARP1 and BRCA mutations.
Tumors mutated for PARP1 were significantly resistant to talazoparib, growing at the same pace with or without treatment.
In parallel, researchers identified a PARP1 mutation in an ovarian cancer patient who developed resistance to Lynparza. In agreement with their findings, the mutation impaired PARP1 DNA-trapping.
Researchers also found that, contrary to what was thought, tumors with certain BRCA1 mutations still retain residual levels of BRCA1 protein, which can also enable them to resist PARP inhibitors.
The team stressed that additional research is needed to identify more PARP1 mutations in patients that develop resistance to Lynparza. Nonetheless, this approach could also be applied to understanding the mechanisms that drive resistance to other cancer therapies.
“This important finding could in the future allow clinicians to determine who would benefit most from these drugs, or to track when they are becoming less effective and when a change of treatment might be appropriate,” said Baroness Delyth Morgan, chief executive at Breast Cancer Now.
“We hope our research will help doctors use the best drug right from the outset, respond quickly to early signs of resistance, and work out the best ways to combine treatments to overcome drug resistance,” Lord said.