In the realm of genetics, one of the most fascinating phenomena is DNA repair. Base excision repair (BER) is a vital mechanism that repairs single-base lesions in DNA, which can arise from exposure to various external agents like ionizing radiation, reactive oxygen species (ROS), and alkylating agents. The process involves several enzymes that correct these errors by removing and replacing the faulty bases with new ones.
The BER pathway begins when a lesion occurs due to a damaged base or sugar molecule. A specialized enzyme called glycosylase recognizes and cleaves the abnormal base from its sugar backbone, creating an apurinic/apyrimidinic (AP) site. Next comes the AP endonuclease 1 (APE1), which incises the phosphodiester bond at the 5′ side of the AP site, generating a free 3′-OH group.
Afterward, downstream proteins such as polynucleotide kinase-phosphatase (PNKP) remove any blocking groups to allow ligation via DNA ligases I or III. Polymerases β or λ then fill in any gaps with appropriate nucleotides before another round of ligation creates a continuous strand.
Several other proteins also play important roles in BER pathways: OGG1 removes oxidative damage-induced 8-oxoguanine adducts; MUTYH removes adenine opposite oxidized guanine; NEIL1 cleaves hydantoin lesions caused by oxidation products of pyrimidine bases; SMUG1 excises uracil misincorporated into DNA during replication or deamination events.
One interesting aspect of BER is that it’s not limited to nuclear DNA but also operates on mitochondrial DNA (mtDNA). In fact, mtDNA might be more vulnerable than nuclear DNA since it lacks protective histones and has higher ROS exposure due to its proximity to mitochondria’s electron transport chain activity.
For example, deficiencies in the BER pathway can lead to several mitochondrial disorders such as progressive external ophthalmoplegia (PEO) and mtDNA depletion syndrome. PEO is a rare disorder that affects the muscles controlling eye movements, leading to difficulties in coordination and double vision.
MtDNA depletion syndrome, on the other hand, results from mutations in genes involved in mtDNA replication or maintenance. It causes severe muscle weakness, liver failure, neurological symptoms like seizures and developmental delays.
Interestingly, some drugs used for treating cancer exploit BER’s mechanism by inhibiting specific enzymes involved in this process. For example, methoxyamine targets APE1 to enhance the efficacy of alkylating agents like temozolomide against glioblastoma multiforme- a type of brain cancer.
Another drug called olaparib inhibits poly ADP ribose polymerase (PARP), which plays a crucial role in repairing single-strand breaks caused by oxidative stress or genotoxic damage. By blocking PARP activity, olaparib selectively kills tumor cells with defects in homologous recombination repair pathways while sparing normal cells.
In conclusion, base excision repair is one of the vital mechanisms that protect our DNA from various insults that can cause genetic abnormalities and diseases if not corrected timely. The BER pathway involves several enzymes working together to identify lesions and replace them with new nucleotides accurately.
However, deficiencies or mutations in these enzymes can lead to severe mitochondrial disorders like PEO or mtDNA depletion syndrome. On the flip side, some drugs target specific components of BER pathways for treating cancer patients more effectively than traditional chemotherapy alone. As we unravel more about how DNA repairs itself when damaged through various research studies conducted globally every day , they will help us understand genetic disorders better and develop newer treatments targeted towards such diseases .