A collaborative fine-mapping study from researchers across more than 450 departments worldwide has found five times more genetic variants to be associated with breast cancer than previously thought. Published in Nature Genetics, the most comprehensive map of breast cancer risk variants to date has identified 352 DNA variants that are associated with breast cancer with “reasonable confidence” that target 191 genes, five times more than were previously recognised.

Breast cancer is one of the most common types of cancer in the UK, with more than 20,000 cases per year. It’s not caused by a single gene, but rather a combination of genes that when combined, can raise the risk of developing the disease.

Previous genome-wide association studies (GWAS) have identified genetic variants that are associated with breast cancer in more than 150 genomic regions. However, only 20 genomic regions have been analysed in detail in efforts to pinpoint the specific variants and genes that drive this association.

The new fine-mapping study analysed DNA from 109,000 breast cancer patients and compared it to 88,937 healthy controls, all of European ancestry. In the 352 variants they discovered, a third were found to increase the likelihood of women developing hormone-responsive breast cancer. This is the most common type of breast cancer, with about 80% of patients developing this type that can be treated with hormonal treatments such as tamoxifen.

15% of the 352 variants were found to predispose women to rarer oestrogen-receptor-negative breast cancer. The remaining variants were found to be linked to both types of breast cancer.

The variants identified in the study each increase the risk of developing breast cancer by a small amount. When many of these variants are present in the genome, the variants together can increase the risk to levels that may be significant for breast cancer development.

These newly identified variants provide researchers with new genetic areas to study and will allow them to potentially understand their involvement in cancer risk. This would be a huge benefit for doctors and clinicians, who will then be able to provide the best advice for patients on reducing the risk of developing the disease.