New Understanding of Genetic Mechanism in BRCA-related Breast Cancer

Researchers have long hypothesized that a single healthy copy of a gene could still guard against tumor development, called the two-hit hypothesis. But for women with a heterozygous BRCA1 gene, breast cancer is far more likely to develop without the traditionally explained risk brought on by a mutation in the second healthy copy of a given heterozygous gene.

Researchers at Harvard Medical School have published new findings in Nature Genetics on the malignancy risk of a single faulty BRCA1 copy that renders breast cells more vulnerable to tumorigenesis.

In general, a single functioning gene copy is enough to ensure normal function per the two-hit hypothesis. This is why, though common, DNA breaks are usually of little consequence as there are genes responsible for monitoring and repairing DNA damage. One of these DNA-repairing genes, BRCA1, makes proteins to heal damaged DNA. Even when one copy of the BRCA1 is inactive due to inherited mutation, the other copy should be sufficient to ensure normal protein-repairing function and prevent tumorigenesis in breast cells.

To test whether or how the two-hit hypothesis explains BRCA1-driven breast cancer in heterozygous genes, the researchers followed two groups of mice. The first group had one faulty and one normal copy of BRCA1; the second group had two normal copies of the gene. The normal copy of the gene in the first group and both copies in the second group were turned off, leaving the mice without any protective BRCA1 function.

The group that started with heterozygous BRCA1 developed mammary gland tumors about 20 weeks earlier than the group that started with two normally functioning BRCA1 copies, but then lost function of both. If the two-hit hypothesis did explain cell proliferation related to cell heterozygosity, then tumors would have developed at comparable rates and around the same time in both groups.

“This indicates that the two-hit hypothesis alone does not explain the earlier incidence of breast cancer in animals with a single defective copy,” said senior study author Joan Brugge, Louise Foote Pfeiffer Professor of Cell Biology at Harvard Medical School. “If it were, cancer would have developed at the same time in both groups of mice.”

The researchers then compared mammary gland cells from both groups of mice with single-cell RNA sequencing. The heterozygous cells showed expected organization and packaging of DNA rendering certain cancer-promoting genes ready to be activated.

The alterations were specifically found in the structure and organization of chromatin, which helps package and maintain DNA in the nucleus. One of the chromatin changes rendered the WNT10A gene, which is responsible for regulating cell division and growth and more prone to overactivity. This overactivity can lead to aberrant cell growth, thus promoting cancer. Further in vivo experiments implicated that WNT10A as well as AP-1 sites are promoters of BRCA1-related breast cancer.

“These findings reveal a previously unappreciated epigenetic effect of BRCA1 haploinsufficiency in accelerating tumorigenesis, advancing our mechanistic understanding and informing potential therapeutic strategies,” the researchers concluded.