Happy DNA Day! Celebrated on April 25th, DNA Day commemorates both the discovery of the structure of DNA in 1953 and the completion of the Human Genome Project in 2003. Since its designation in 2003, DNA Day has been used to increase public understanding of DNA and the scientific advances that have been made as a result of these two events. I am excited to use DNA Day to explore with my readers the importance of maintaining the structure of our cells’ instruction book (the order of the words and chapters), and the roles the BRCA1 and BRCA2 genes play in this critical function. As a budding scientist in college, my interest in biological research was sparked when I learned that protecting the integrity of the genome (the genetic material present in every cell) was the most fundamental level of regulation in the cell required to ensure proper cell functioning. The realization that failure to protect DNA is the underlying cause of many diseases, including cancer, was the impetus for me to research DNA repair and chromosome-structuring mechanisms in graduate school.
In my previous article, The Double-Edged Sword of Mutation, I discussed how spontaneous mutations in our DNA drive diversity and evolution. These changes of single letters throughout our genome can change the reading of a gene (or a word in our instruction book) and the resulting protein. Changes, however, can also occur on a larger scale, which result in deleting or duplicating words and moving words around. While a single typo doesn’t affect our ability to read a book too much (although, it’s certainly annoying), rearranging the whole book does. Rearranging the genome within our cells, similarly, can lead to very serious consequences.
Cells, for example, experience DNA damage all the time. Damage can occur as a result of natural cellular processes or from sources outside of the cell, like UV rays from the sun and carcinogens from chemicals. Repairing DNA damage is of the utmost importance to the cell because failure to do so will result in the death of the cell. Within the cell there are different mechanisms for repairing damage that are specific for a particular type of damage under a particular circumstance. There are even backup mechanisms in case one pathway fails to do the job.
Regardless of the type of DNA damage or repair mechanism, DNA damage responses are similar to the local emergency responses we see in our towns and cities. When damage is sensed, an alert is sent out throughout the cell. A stop sign appears and the cell stops its progression toward division. Simultaneously, responders come to the scene of the damage. The responders have the jobs of assessing the type of damage and then fixing it. Similarly, when an emergency occurs in our communities, first responders — firefighters, police officers and paramedics — respond to a 911 call and race to the site of the incident. The first responders assess the situation, relay the message about the emergency to back-up responders and warn people in the community. They also provide immediate assistance to victims and stabilize the situation. Once the emergency has been assessed, the appropriate people can provide a second level of response that can include containment of the threat, search and rescue, continued assistance or aid, and restoration of the infrastructure.
In the event of DNA damage, the cell must assess both the type of damage and the part of the cell cycle in which the response is taking place, in order to pick the best mode of action for repair. There is a hierarchy to DNA repair mechanisms. The best-case scenario is the repair mechanism that leaves no sign of damage at the end, called homologous recombination (HR for short). This perfect repair is able to occur by using the intact copy of DNA that was formed earlier in the cell cycle during DNA replication as a template to rebuild the damaged sequence. The worst-case scenario, a mechanism called non-homologous end joining, (NHEJ for short) is akin to crisis mode and salvages the DNA to prevent loss of information by gluing broken ends of DNA back together. In the latter example, the DNA might not be reconstructed in the order in which it originally existed. The cell also has a “self destruct” mode called apoptosis that is invoked if there is too much damage to be salvaged.
In order to choose the best way to fix the DNA damage, the type of damage must first be assessed. This means many of the same “first responders” arrive at the site of damage, independent of what type of damage has occurred. Additionally, many of the same repair proteins can function in different capacities for various types of damage. The double-duty repair proteins can perform is similar to how police officers, paramedics, or firefighters are trained to respond to many different situations. Of the myriad of proteins at a DNA damage site, many of us are familiar with two of them: BRCA1 and BRCA2.
The main function of the BRCA1 and BRCA2 proteins, when not mutated, is to help repair specific types of breaks in the DNA strands (called double-strand breaks). Both BRCA1 and BRCA2 are involved in a myriad of other cellular functions, as well, including repair of other types of DNA damage. Their functions in repairing double-strand breaks through the HR repair pathway, however, are the most important for preventing changes that could lead to cancer. Research has shown that BRCA1 is involved in activation of the stop signs that go up when damage is sensed. BRCA1 is important for making the decision to repair DNA double-strand breaks by the HR pathway, meaning that it functions early in the response to help determine the best course of action. BRCA2, in contrast to BRCA1, functions later in the repair pathway and is not involved in the decision-making process. Despite their differences, both BRCA1 and BRCA2 act similarly to the coordinators in an emergency situation: neither protein is the first to show up at a damage site, nor do they physically repair the damage. BRCA1 and BRCA2 do, however, communicate with many of the different responders, help call in additional responders, and coordinate the proteins that do the actual repair.
In The Double-Edged Sword of Mutation, I discussed that BRCA1 and BRCA2 are tumor suppressor genes. Although they protect against uncontrolled cell growth that could lead to cancer, mutations alone, in either gene do not necessarily cause cancer. When the BRCA1 or BRCA2 genes contain detrimental mutations, the resulting proteins are no longer able to perform the HR perfect-repair mechanism. The inability to perform HR causes the error-prone NHEJ pathway to taking over repair. The NHEJ pathway is akin to trying to glue back together a pair of shoelaces that has been cut: the correct pieces may be re-attached to each other, but there is also a significant chance the wrong ends will be put together. Putting DNA fragments back together incorrectly through the NHEJ process causes gene rearrangements or deletions of DNA sequences.
The rearrangements of genes that can result from improper repair of DNA damage can cause other cancer-promoting genes to be dysregulated. While the steps between dysfunction of either BRCA1 or BRCA2 and cancer formation are not well defined at this point, strong efforts are being made to better understand them. Dr. David Livingston, a Harvard investigator supported by the BRCA Foundation, discusses the current efforts of scientists to map the cancer-formation progression in BRCA1- and BRCA2-deficient cells in Episode 10 of the BRCA Foundation’s Positive Perspectives podcast. Understanding the steps that occur after DNA is incorrectly repaired, but prior to cancer formation, would allow doctors to identify potentially cancerous cells and recommend treatments before the disease forms or becomes serious.
Repairing DNA damage and maintaining the integrity of the genome, as we just discussed, is one of the most important processes in the cell. BRCA1 and BRCA2 are integral players in ensuring DNA is repaired properly. When our BRCA1 or BRCA2 genes have a mutation, protein function is compromised and they cannot properly repair damage. While a mutation in BRCA1 or BRCA2 greatly increases a person’s chance of developing cancer, these mutations alone do not cause cancer. Lack of BRCA1 or BRCA2 function due to mutation increases the chance of dysregulation of other genes, which then cause cancerous properties in cells.
Understanding the connection between mutations in the BRCA1 and BRCA2 genes and the development of cancer has great potential for prevention and treatment of BRCA-related cancers. Current treatments, such as immunotherapy, target the characteristics of cancerous cells that differ from normal cells. In Episode 12 of Positive Perspectives, Dr. Joan Brugge, also a BRCA Foundation-supported investigator at the Dana Farber Cancer Institute who researches BRCA1 in collaboration with Dr. Livingston, discusses the potential benefits of understanding the steps that lead to cancer in BRCA-mutated cells. Identifying the intermediate steps that lead to cancer would allow for the development of therapies that target cells prior to becoming cancerous. Uncovering new markers early in cancer development would also allow doctors to detect potential cancers with greater accuracy, and earlier than current tests allow.
In these past two articles we explored the genome and how cells ensure the integrity of our DNA. We discussed how errors in DNA arise and roles of the BRCA1 and BRCA2 proteins in protecting against these changes. I hope you will join me in two weeks when I delve further into how our DNA is protected by PARP proteins. I will discuss research that allowed scientists to take advantage of PARP functions to develop therapies that treat, specifically, BRCA-mutated cancers.