Researchers at the Johns Hopkins Kimmel Cancer Center have demonstrated during their work with breast cells that breast cells become vulnerable to cancer if a single copy of the breast cancer gene BRCA1 is inactivated. It causes genetic instability in the cells through reducing their ability to repair DNA damage.
The leading risk factor for hereditary breast cancer is an inherited mutation in the BRCA1 gene which requires close monitoring or prompt preventive mastectomy.
The breakthroughs might help researchers develop a drug that prevents hereditary breast cancer, as well as tools to identify those who benefit most from prophylactic treatments. The study is published in the Proceedings of the National Academy of Sciences Oct. 25.
Exactly how BRCA1 inactivation increases the risk of cancer has remained a mystery. BRCA1 is believed to be a “tumor suppressor” gene. Usually, cancer is not caused by the loss of one copy of such genes, as each individual is born with two copies of each gene (one from each parent), and the second copy is sufficient in keeping cells healthy in a similar way that a car can stop safely after losing control of the front brakes as the rear brakes are still working.
According to the researchers, cancer seems to develop in such cases only after the second copy of the gene is damaged, i.e. random mutation during cell division, resulting in uncontrolled cell growth.
Mouse models of BRCA-related cancers have demonstrated that damage to genes, such as TP53, occurred prior to damage to the second copy of BRCA.
Ben Ho Park, M.D., Ph.D., associate professor of oncology at the Johns Hopkins Kimmel Cancer Center, explained:
“In theory, this process would take a long time and BRCA-related breast cancers
occur at an early age.”
For the investigation, the team used novel technology in order to insert a single copy of a typical BRCA1 mutation into normal breast cells.
The main theory has been that the original inactivation of a single copy of BRCA1 produces additional DNA mutations to expand more rapidly than normal – a condition called “genomic instability.”
Park explains:
“The protein coded by BRCA1 is involved in repairing major DNA breaks, so it would make sense that its inactivation could weaken a cell’s resistance to DNA mutations.”
However, Park adds that the consequence of losing a single copy of BRCA1 was hard to model and difficult to investigate. Results from prior attempts to produce mice with single-copy BRCA1 mutations were uncertain as the mice were unable to demonstrate the pattern of human cancers. Furthermore, it has been hard for investigators to create human cell lines in which the only flaw is a single mutated copy of BRCA1.
In order to test their theory, the team first selected cell lines obtained from non-cancerous human breast epithelial cells – where BRCA1 breast cancers originate. An advanced gene-targeting method was then used to generate novel cell lines that have a typical cancer-associated BRCA1 mutation in just one copy of the gene.
Following this, tests were conducted on both cell types – cells with the BRCA1 mutation, and the original cells with two healthy copies of BRCA1 – to compare their DNA repair activity. The team demonstrated that cells with BRCA1 mutations were not as effective at carrying out the type of DNA repair known to involve the BRCA1 protein.
They found that when exposed to a DNA-damaging chemotherapy medication or radiation the BRCA1-mutated cells were more likely to die. In addition, BRCA1-mutated cells that were allowed to divide for many weeks were more likely to lose additional genes, includes those frequently mutated in breast tumors. Similar genetic losses were observed on non-cancerous breast cells taken from women with BRCA1 mutations.
Park said:
“What this shows is that having only a single working copy of BRCA1 really does bring about changes in a cell that would be expected to give rise to cancer.
We hope to use this new system to introduce other known BRCA1 mutations, to get a better idea of the relative cancer risk each individual mutation represents, because right now there are few good ways to do that. In the future, we hope to further define risk so that family members with one type of BRCA1 mutation may be advised to get preventative treatment or surgery, and those with other BRCA1 mutations could rely on careful screening.”
In addition, the cell models might be helpful in determining the susceptibility of various BRCA1 mutations to drugs, Park adds. At present, anti-cancer medications known as PARP inhibitors are in clinical trials against tumors with BRCA1-mutations.
The lifetime risks of developing breast cancer has been shown among women born with a mutated copy of BRCA1 to range between 50% to 90%. In addition, they have high, but variable risks of ovarian and other cancers.
Written by Grace Rattue