Base Excision Repair (BER) for Oxidative Damage

Core Idea

BER removes small lesions such as oxidized bases (e.g., 8-oxoguanine) and spontaneous deamination products. DNA glycosylase recognizes and removes the damaged base, creating an apurinic/apyrimidinic (AP) site. AP endonuclease then cleaves the backbone, and the gap is filled by polymerase and sealed by ligase.

How It's Best Learned

Learn the series of enzymatic steps: glycosylase excision, AP site processing, gap fill-in, and ligation. Understand how multiple glycosylases recognize different base lesions. Consider the evolutionary advantage of removing bases vs. nucleotides (BER vs. NER).

Common Misconceptions

• Assuming BER handles only oxidative damage when it also processes spontaneous deamination and other lesions.
• Not recognizing that single-strand breaks, if abundant, can be converted to double-strand breaks during replication.
• Thinking all BER is faithful; some variants use error-prone polymerases for translesion synthesis.

Explainer

From your study of DNA repair mechanisms, you know that cells face constant DNA damage and have evolved multiple repair pathways to deal with different types of lesions. Base excision repair (BER) is the pathway specialized for small, chemically subtle lesions — damaged bases that don't dramatically distort the DNA helix but would cause mutations if left unrepaired. The most common of these are oxidative lesions like 8-oxoguanine (produced thousands of times per cell per day by reactive oxygen species) and deaminated bases like uracil (produced when cytosine spontaneously loses its amino group).

The BER pathway works like a surgical extraction in four steps. First, a DNA glycosylase recognizes and removes the damaged base by cleaving the bond between the base and the sugar, leaving the sugar-phosphate backbone intact. This creates an apurinic/apyrimidinic (AP) site — a position in the DNA that has a sugar and phosphate but no base. Think of it as pulling a rotten tooth but leaving the socket. There are at least 11 different glycosylases in human cells, each specialized for recognizing specific types of base damage — this specificity is what allows BER to handle a wide variety of small lesions. Second, AP endonuclease (APE1 in humans) cleaves the backbone at the AP site, creating a single-strand nick with a free 3'-OH end. Third, DNA polymerase β fills in the one-nucleotide gap with the correct base using the undamaged complementary strand as a template. Finally, DNA ligase III (working with its partner XRCC1) seals the remaining nick, restoring the continuous double helix.

This "short-patch" pathway replaces just a single nucleotide and handles the vast majority of BER events. However, some lesions produce modified sugar residues that polymerase β cannot process. In these cases, cells switch to long-patch BER, where a replicative polymerase (Pol δ or Pol ε) displaces a flap of 2-10 nucleotides, FEN1 endonuclease trims the flap, and DNA ligase I seals the result. Long-patch BER is more complex but handles the edge cases that short-patch cannot.

The clinical significance of BER is substantial. Because oxidative damage is relentless — a byproduct of normal aerobic metabolism — any weakness in BER leads to mutation accumulation. Variants in BER genes (particularly MUTYH, a glycosylase that removes adenine mispaired with 8-oxoguanine) are associated with colorectal cancer predisposition. Understanding BER also clarifies why it differs from nucleotide excision repair (NER): BER removes the damaged *base* first and then deals with the backbone, replacing just 1-10 nucleotides, while NER excises an entire ~25-nucleotide stretch of the strand containing the lesion. BER handles small, non-distorting damage; NER handles bulky, helix-distorting lesions. The two pathways are complementary, not redundant.