Seemingly normal breast tissue from women with a genetic mutation associated with increased cancer risk can contain cells with unusual gene and chromosome patterns, which may represent a very early indicator of breast cancer, according to a study published in the January 31 issue of Science Advances.
Tracking these potential precancerous red flags, which were identified here by observing the nuances of genetic information within individual cells, may enable doctors to better predict and prevent breast cancer in women with the BRCA2 mutation. These women often face the difficult choice of whether to undergo a double mastectomy long before cancer emerges.
Mutations of the BRCA2 gene, whose encoded protein suppresses DNA damage when functioning properly, occur in one out of every 400 individuals in the United States, although it is more common among some ethnic groups than others. About one in 40 Ashkenazi Jewish men and women carry a mutation on either the BRCA1 or BRCA2 gene. Those with the mutations are at dramatically increased risk for breast cancer — some 50% to 80% of women who carry an altered BRCA2 gene are destined to become breast cancer patients.
Despite the need for more effective and less invasive approaches to prevention, this breast cancer's earliest stages of inception are poorly understood, and markers of risk are not well established.
"This is true in part because BRCA2 mutation carriers are a relatively rare population, but also because the multi-step development of breast cancer in the setting of a BRCA2 mutation may be somewhat different than what occurs for non-hereditary breast cancers, " said Leif Ellisen, co-leader of the Dana-Farber/Harvard Cancer Center Breast Cancer Program and an author of the study. "Our ongoing studies seek, ultimately, to find more direct and personalized markers of risk in this context by looking at the breast tissue itself."
Ellisen and colleagues analyzed noncancerous glands of breast tissue — which experienced physicians considered anatomically normal — from women harboring the mutation who had opted to undergo preventative mastectomies. They compared it with healthy tissue from women who had elected for cosmetic breast surgery. To look for changes in tissue before any of the cells began to take on an abnormal appearance, the researchers isolated mature luminal, luminal progenitor and basal epithelial cells — common cells from the body's surface — and performed whole-genome sequencing to look for genetic variations between cells.
Critically, the researchers performed a single-cell analysis, which provides an incredibly detailed look at the DNA and chromosomes of individual cells and only recently became possible.
"At the earliest stages of pre-cancer, cells begin to accumulate mutations in DNA, but early on each cell might have a different mutation," said Ellisen. "A traditional 'bulk' analysis of thousands or millions of cells would miss these individual mutations because they are all averaged out in the bulk specimen. The technology we employed allowed us to look at individual cell genomes in enough detail that we could see the mutations — in this case, small pieces of chromosomes gained or lost."
In the tissue from women with a BRCA2 mutation, single-cell genetic differences appeared evident — 27% of luminal progenitor cells and 13% of basal epithelial cells contained this telltale DNA damage, while the variations were only observed in one out of 90 cells of either type in tissue from the cosmetic surgery patients.
The researchers do not yet know the cause behind the chromosome and DNA damage in BRCA2 mutation carriers, but unraveling this mystery is the aim of their ongoing research with investigators from around the world.
Women with BRCA2 mutations inherit one working copy and one defective copy of the gene, losing the working copy as cancer develops and leaving only the defective gene to perform repairs. Ellisen noted that it makes sense for cells with a BRCA2 mutation to incur some DNA damage over time, since BRCA2 is tasked with repairing DNA and may struggle to perform its job well when the gene itself is damaged. However, the new study suggests that DNA-damaged cells may begin to appear at the earliest precancerous stages, when the working BRCA2 copy is still present and the tissues still appear completely normal under the microscope.
"Our study suggests that the early changes we have found may occur even before the working copy of BRCA2 is lost," said Ellisen. "We believe our findings do reflect some type of repair defect at this early stage, and understanding its precise nature will inform the development of effective cancer prevention approaches."
Certain types of hormonal interventions are already known to decrease long-term breast cancer risk, even in women who carry BRCA2 mutations. Ellisen hopes that by better understanding cancer development at its earliest stages, researchers can develop more effective methods to prevent breast cancer in this population, as well as other cancers they are at risk for developing, including ovarian, pancreatic and prostate cancer. Their findings may also benefit cancers beyond those that occur in people with the BRCA2 mutation.
"We know that a subset of cancers is 'BRCA-like,' meaning they have a DNA repair defect similar to that caused by BRCA2 mutations even in the absence of these mutations," said Ellisen. "Such cancers may be susceptible to the same future prevention strategies as we envision for BRCA-associated cancers."
[Credit for associated illustration: Chris Bickel for Science/AAAS]