Our cells constantly face threats to their DNA, the genetic material guiding all cellular functions. Damage can arise from metabolic processes, environmental toxins, or radiation, disrupting cell operations. Accurate repair processes are essential.
Homologous recombination repair (HRR) is a highly precise repair mechanism. This pathway fixes severe DNA damage by using an undamaged copy of genetic information as a template. It faithfully restores the original sequence, safeguarding the cell’s blueprint.
The Mechanism of Homologous Recombination
One of the most disruptive forms of DNA damage is a double-strand break (DSB), where both strands of the DNA helix are completely severed. These breaks can arise from errors during DNA replication or exposure to harmful agents like ionizing radiation. If left unrepaired, DSBs can lead to large-scale chromosomal rearrangements, threatening cellular stability.
Homologous recombination repair (HRR) is the primary pathway for accurately fixing these double-strand breaks, particularly during the S and G2 phases of the cell cycle when a sister chromatid, an identical copy of the damaged DNA, is available. The repair process begins with the cell recognizing the break and initiating end resection, where enzymes trim back one strand from each broken end, creating single-stranded DNA overhangs.
Following this, the cell undertakes a “homology search,” seeking an identical, undamaged DNA sequence, typically on the sister chromatid, to serve as a template. One of the single-stranded overhangs from the broken DNA then invades this homologous template, forming a structure known as a D-loop. New DNA is then synthesized using the undamaged template to accurately fill in the missing genetic information at the break site.
Finally, the newly synthesized DNA strands are annealed back to the original broken ends, and any remaining gaps are sealed, restoring the DNA molecule to its original, undamaged state. This reliance on an undamaged template ensures HRR is a highly accurate repair mechanism, preventing the loss or alteration of genetic information.
Critical Proteins in the Repair Pathway
Homologous recombination repair is orchestrated by a team of specialized proteins, each performing a distinct role. These proteins identify DNA damage, process broken ends, and facilitate the accurate reconstruction of the DNA sequence.
BRCA1 and BRCA2 are key proteins in HRR. BRCA1 acts as an early responder to DNA double-strand breaks, signaling the presence of damage and coordinating the recruitment of other repair proteins to the site.
BRCA2 functions as a chaperone for the RAD51 protein, guiding and loading it onto the single-stranded DNA overhangs at the break site. RAD51 is a central player, forming a filament on the DNA that is responsible for the crucial “homology search” and strand invasion steps. It facilitates the pairing of the damaged DNA with the undamaged homologous template, allowing for accurate DNA synthesis.
Connection to Cancer and Genetic Disease
While homologous recombination repair is effective, its function depends on the proper functioning of the proteins involved. When genes coding for these repair proteins, such as BRCA1 and BRCA2, acquire mutations, the HRR pathway can become impaired or non-functional. These mutations can be inherited.
If homologous recombination repair cannot correctly fix double-strand breaks, other, less accurate repair pathways may be utilized, or the damage may accumulate. This accumulation of unrepaired or incorrectly repaired DNA damage leads to genomic instability, increasing the rate of mutations and chromosomal rearrangements within cells.
Over time, this unchecked accumulation of genetic alterations can drive cells towards uncontrolled growth and division, leading to cancer. Individuals inheriting faulty BRCA1 or BRCA2 genes face a significantly elevated lifetime risk of developing certain cancers, including breast, ovarian, prostate, and pancreatic cancers.
Exploiting Repair Pathways for Therapy
Understanding the vulnerabilities created by a faulty homologous recombination repair system in cancer cells has opened new avenues for targeted therapies. This approach leverages the specific defects present in cancer cells, aiming to destroy them while sparing healthy cells.
A prime example of this therapeutic strategy involves the use of PARP inhibitors. PARP (Poly ADP-ribose polymerase) is another enzyme involved in DNA repair, primarily fixing single-strand breaks. In healthy cells, if PARP is inhibited, single-strand breaks can escalate into double-strand breaks, but the cell can then rely on its intact homologous recombination repair pathway to fix them.
However, in cancer cells with a BRCA mutation, the homologous recombination repair pathway is already deficient. When a PARP inhibitor blocks the backup single-strand break repair pathway, these cancer cells lose their remaining ability to fix DNA damage effectively. This leads to an accumulation of DNA damage, causing the cancer cells to undergo programmed cell death. This concept is known as “synthetic lethality,” where two non-lethal defects (the BRCA mutation and PARP inhibition) become lethal when combined in the same cell.