Up to a fifth of women with breast cancer may benefit from drugs currently reserved for less common cases of inherited disease caused by faulty genes, say scientists.
A new study suggests that thousands of "common" breast cancers are biochemically similar to rarer cases involving BRCA1 and BRCA2 gene mutations.
Only 1% to 5% of the 55,000 women diagnosed with breast cancer in the UK each year are thought to have BRCA-driven disease.
They qualify for a class of drugs called PARP inhibitors that are specifically designed to target tumours with the defective genes. But according to the new findings, some 8,000 more patients may respond to the drugs than are receiving them at present.
Lead scientist Dr Serena Nik-Zainal, from the Wellcome Trust Sanger Institute in Hinxton, Cambridgeshire, said: "In the past, clinical trials for PARP inhibitors have focused mainly on the 1-5% of women with breast cancer.
"However, our study shows that there are many more people who have cancers that look like they have the same signatures and same weakness as patients with faulty BRCA1 and BRCA2 genes. We should explore if they could also benefit from PARP inhibitors.
"The results suggest that clinical trials now need to look at cancer patients who share the same genetic signature in their cancer. This could change how clinical trials are designed in the future."
The team, whose results are published in the journal Nature Medicine, analysed DNA from breast cancer tissue samples taken from 560 patients.
A piece of computer software called HRDetect was used to identify genetic code "fingerprints" revealing biochemical pathways like those associated with mutant BRCA1 and 2 genes.
Of the total group of patients, 22 had previously diagnosed BRCA 1 and 2 mutations. Another 55 had unexpected BRCA mutations, including some very unusual ones that were not inherited.
A further 47 had BRCA genes with no recognised mutations, yet in these patients the repair mechanisms controlled by the genes were still faulty.
"It's possible there are other ways of turning off BRCA 1 and 2 that we don't understand, perhaps involving other genes," said Dr Nik-Zainal.
She added: "PARP inhibitors are important for quality of life because they specifically target cancer cells and so are well tolerated.
"I feel so strongly about the fact that one in five women might benefit from these drugs. A lot of people who could be getting these treatments are not being offered them."
But she pointed out that a lot more evidence had to be collected before guidelines on the use of PARP inhibitors could be changed.
The drugs block the action of an enzyme cancer cells with faulty BRCA genes use to ensure their survival, without affecting healthy cells.
Professor Sir Mike Stratton, director of the Sanger Institute, said: "This work uses mutational signatures to identify the complete set of cancers that will respond to certain drugs that are already known to be effective in a subset.
"To translate these results into treatments, further sequencing of cancer genomes and more clinical trials are urgently needed, but this is a most promising start."
Baroness Delyth Morgan, chief executive of the charity Breast Cancer Now, said: "PARP inhibitors are a very promising treatment on the horizon and the suggestion that more patients may be able to benefit from them is greatly exciting.
"Crucially, this study is an early but encouraging step towards being able to offer women treatments targeted to the genetic make-up of their breast cancer.
"The discovery that many women may have tumours genetically similar to patients with faulty BRCA genes, without them actually having a BRCA mutation, is somewhat of a revelation.
"We hope it could now lead to a watershed moment for the use of mutational signatures in treating the disease."
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