By Gesa Junge, PhD
Our cells are subject to damage every minute of every day, be it from endogenous factors such as reactive oxygen species generated as part of normal cell respiration, or exogenous factors such as UV radiation from the sun. Together, these factors can lead to as many as 60 000 damaged DNA bases per cell per day. Most of these are changes to the DNA bases or single strand breaks (SSBs), which only affect one strand of the double helix, and can usually be repaired before the DNA is replicated and the cell divides. However, about 1% of SSBs escape and become double stand breaks (DSBs) upon DNA replication. DSBs are highly toxic, and a single DSB can be lethal to a cell if not repaired.
Usually, cells are well-equipped to deal with DNA damage and have several pathways that can remove damaged DNA bases and restore the DNA sequence. Nucleotide excision repair (NER, e.g. for UV damage) and base excision repair (BER, for oxidative damage) are the main SSB repair pathways, and homologous recombination (HR) and non-homologous enjoining (NHEJ) repair most DSBs. HR is the more accurate pathways for DSB repair, as it relies on a homologous DNA sequence on the sister chromosome to restore the damaged bases, whereas NHEJ simply relegates the ends of the break, potentially losing genetic information. However, NHEJ can function at any time in the cell cycle whereas HR requires a template and is only active once the DNA is replicated (i.e. in G2 and S-phase).
Depending on the severity of the damage, cells can either stop the cell cycle to allow for repair to take place or, if the damage is too severe, undergo apoptosis and die, which in a multicellular organism is generally favourable to surviving with damaged DNA. If cells are allowed to replicate with unrepaired DNA damage, they pass this damage on to their daughter cells in mitosis, and mutations in the DNA accumulate. While mutations are essential to evolution, they can also be problematic. Genomic instability, and mutations in genes such as those that control the cell cycle and the DNA damage response can increase the risk of developing cancer. For example, germline mutations in ATM, a key protein in HR pathway of DSB repair, leads to Ataxia Telangiectasia (AT), a neurodegenerative disorder. AT sufferers are hypersensitive to DSB-inducing agents such as x-rays, and have a high risk of developing cancer. Deficiencies in NER proteins lead to conditions such as Xeroderma Pigmentosa or Cockayne Syndrome which are characterised by hypersensitivity to UV radiation and an increased risk of skin cancer, and mutations BRCA2, another key HR protein, increase a woman’s risk of developing breast cancer to 60-80% (compared to 13% on average).
Even though deficiencies in DNA repair can predispose to cancer, DNA repair is also emerging as a viable target for cancer therapy. For example, DNA repair inhibitors can be used to sensitise cancer cells to chemotherapy- or radiation-induced damage, making it possible to achieve more tumour cell kill with the same dose of radiation or chemotherapy. However, this approach is not yet used clinically and a major complication is that it often increases both the efficacy as well as the toxicity of treatment.
Another approach is the idea of “synthetic lethality”, which relies on a cancer cell being dependent on a specific DNA repair pathway because it is defective in another, such that deficiency of either one of two pathways is sustainable, but loss of both leads to cell death. This concept was first described by Calvin Bridges in 1922 in a study of fruit flies and is now used in the treatment of breast cancer in the form of an inhibitor of Poly-ADP ribose polymerase (PARP), a key enzyme in the repair of SSBs. Loss of PARP function leads to increased DSBs after cell division due to unrepaired SSBs, which in normal tissue are removed by the DSB repair system. However, BRCA2-deficient tumours are defective in HR and cannot repair the very toxic DSBs, leading to cell death. Therefore, BRCA2-deficient tumours are hypersensitive to PARP inhibitors, which are now an approved therapy for advanced BRCA2-deficient breast and ovarian cancer.
PARP inhibitors are a good example of a so-called “target therapy” for cancer, which is the concept of targeting the molecular characteristics that distinguish the tumour cell from healthy cells (in this case, BRCA2 deficiency), as opposed to most older, cytotoxic chemotherapies, which generally target rapidly dividing cells by inducing DNA damage, and can actually lead to second cancers. With an improved understanding of the molecular differences between normal and tumour cells, cancer therapy is slowly moving away from non-specific cytotoxic drugs towards more tolerable and effective treatments.