DNA polymerase 3′–5′ exonuclease (3′–5′ Exo)

DNA polymerase 3′–5′ exonuclease activity, commonly known as the proofreading function, is a critical enzymatic process primarily associated with high-fidelity DNA polymerases such as Polymerase Epsilon (POLE) and Polymerase Delta (POLD1) (UniProt P27677, P28907). This activity ensures genomic integrity by identifying and removing mispaired nucleotides at the 3' end of a growing DNA strand before further elongation proceeds, thereby reducing the error rate of DNA replication by several orders of magnitude. In a clinical context, germline or somatic mutations within the exonuclease domain of these polymerases lead to a loss of proofreading capability, resulting in an 'ultramutated' genomic profile and a strong predisposition to various cancers, most notably colorectal and endometrial malignancies (Heitzer & Tomlinson, 2014; PubMed 24870022). While not a traditional drug target in the sense of a receptor, this activity is a major determinant of sensitivity to nucleoside analog chemotherapies, as the exonuclease can excise these drugs from the DNA, leading to resistance. Furthermore, the loss of this activity serves as a potent biomarker for the efficacy of immune checkpoint inhibitors like pembrolizumab, as the resulting high mutation burden generates numerous neoantigens that trigger a robust anti-tumor immune response (Rayner et al., 2016; PubMed 26823170).

Other names

Proofreading domainDNA polymerase proofreading activity3'-5' exonuclease domainDNA-directed DNA polymerase exonucleasePOLE exonuclease domainPOLD1 exonuclease domain

The 3′–5′ exonuclease activity provides a proofreading mechanism that removes misincorporated or damaged nucleotides from the 3' terminus of the nascent DNA strand during replication. Therapeutic strategies involve either exploiting the loss of this activity (using immune checkpoint inhibitors for ultramutated tumors) or potentially inhibiting the activity to prevent the excision of therapeutic nucleoside analogs, thereby enhancing their cytotoxic effect by ensuring they remain incorporated in the DNA.