DNA Damage Tolerance

DNA Damage Tolerance (DDT) is the collection of mechanisms that allow DNA replication to proceed past unrepaired lesions in the template strand without first removing the damage, deferring actual repair of the lesion to a later point while prioritizing completion of genome duplication — a fundamentally different strategy from the excision-and-resynthesis repair pathways discussed elsewhere in this topic area, which remove damage before replication encounters it rather than allowing replication to continue in its presence.

Why Tolerance Exists as a Distinct Strategy

Repair Cannot Always Keep Pace With Replication

Despite the combined activity of base excision repair, nucleotide excision repair, and other damage-removal pathways, some fraction of DNA lesions inevitably persists unrepaired by the time a replication fork encounters them — DNA damage tolerance exists specifically to prevent this residual, unrepaired damage from causing catastrophic replication fork stalling and collapse, providing a way to complete genome duplication even when repair has not kept pace with damage accumulation.

Tolerance Is Not Repair

A critical distinguishing feature of DDT is that it does not remove or correct the underlying lesion — the damaged base or structure remains in the DNA even after tolerance mechanisms have allowed replication to proceed past it, meaning the lesion must still eventually be addressed by an actual repair pathway at some later point, or else it will simply persist as a permanent feature of that DNA strand going forward.

Translesion Synthesis

Specialized Polymerases for Damaged Templates

Translesion synthesis (TLS) uses a specialized category of DNA polymerases — including Pol η, Pol ι, Pol κ, and REV1, distinct from the high-fidelity replicative polymerases used for normal genome duplication — capable of inserting a nucleotide opposite a damaged or distorted template base that would otherwise stall a replicative polymerase entirely.

The Fidelity Cost of Translesion Synthesis

TLS polymerases generally have substantially lower intrinsic fidelity than replicative polymerases, both because their active sites are structurally adapted to accommodate distorted, damage-containing template bases (a structural accommodation that also reduces their ability to select the correct nucleotide) and because some, like Pol η, lack proofreading exonuclease activity — this fidelity tradeoff is a deliberate feature of the pathway's design, prioritizing completed replication over perfect accuracy at the specific lesion site.

Lesion-Specific Polymerase Specialization

Different TLS polymerases show differing efficiency and accuracy when replicating past different lesion types — Pol η, for instance, is specifically adept at accurately bypassing UV-induced cyclobutane pyrimidine dimers, meaning the choice of which TLS polymerase engages a given lesion has direct consequences for whether that specific bypass event introduces a mutation or, in some cases, is accomplished with reasonable accuracy despite the lesion's presence.

PCNA Ubiquitination as the Regulatory Switch

Monoubiquitination Triggers the Polymerase Switch

Stalling of the replicative polymerase at a lesion triggers monoubiquitination of PCNA, the replication processivity clamp, which serves as the molecular signal recruiting TLS polymerases to replace the stalled replicative polymerase at the lesion site — this ubiquitination-based switch is the central regulatory mechanism determining when and where translesion synthesis is engaged, rather than TLS polymerases acting constitutively throughout the genome.

Polyubiquitination and the Template-Switching Alternative

Further polyubiquitination of PCNA, rather than simple monoubiquitination, instead promotes a distinct DDT sub-pathway called template switching, in which the nascent strand temporarily uses the newly synthesized sister strand as an alternative, undamaged template to bypass the lesion, rather than directly synthesizing past the damaged template base itself.

Template Switching as an Error-Free Alternative

Avoiding Direct Synthesis Across the Lesion

Template switching circumvents the need to synthesize DNA directly opposite a damaged base at all, instead temporarily borrowing sequence information from the newly replicated sister strand — because the sister strand is, at that point, undamaged and accurately synthesized, this route is generally considered comparatively error-free relative to translesion synthesis, at the cost of requiring the more complex strand-switching mechanics involved.

Mechanistic Relationship to Recombination-Based Processes

Template switching shares conceptual similarity with the strand-invasion and templated-synthesis logic underlying homologous recombination repair and recombination-based telomere maintenance, again illustrating a recurring theme across genome maintenance biology in which borrowing sequence information from an intact, homologous DNA source provides a more accurate alternative wherever the local template itself cannot be trusted.

Consequences for Mutation and Genome Instability

A Direct Source of Point Mutations

Because translesion synthesis frequently inserts an incorrect nucleotide opposite a damaged template base, DDT — despite successfully preventing fork collapse — represents a direct, mechanistically distinct source of point mutations in the genome, contributing its own characteristic mutational signatures depending on which TLS polymerase and which lesion type were involved in a given bypass event.

Balancing Fork Progression Against Mutation Risk

DDT exemplifies a broader theme recurring across DNA damage response biology — a tradeoff between completing an essential process (replication) promptly and accurately preserving sequence fidelity, with the specific balance struck by DDT deliberately favoring completion, on the premise that a mutated but intact genome is generally a better outcome for cell survival than a fork collapse leading to a double-strand break and its more severe genome instability consequences.

Clinical and Research Relevance

TLS Polymerase Dysregulation in Cancer

Altered expression or activity of specific TLS polymerases has been observed across cancer contexts, with implications both for a tumor's baseline mutational signature and for its response to DNA-damaging chemotherapy, since TLS activity directly influences how effectively a cancer cell can replicate past — and thereby survive — the DNA damage such treatments induce.

Therapeutic Targeting of Translesion Synthesis

Because TLS activity can allow cancer cells to tolerate and survive otherwise lethal levels of DNA-damaging chemotherapy by replicating past the induced damage rather than succumbing to fork collapse, inhibiting specific TLS components has been explored as a strategy to sensitize tumors to DNA-damaging treatment, aiming to remove this tolerance escape route and force damaged replication forks toward collapse and cell death instead.

Practical Significance

DNA Damage Tolerance provides a distinct, damage-deferring strategy for completing DNA replication in the presence of unrepaired lesions, through translesion synthesis using specialized low-fidelity polymerases or through comparatively error-free template switching, regulated by the PCNA ubiquitination status at the stalled fork. Its deliberate tradeoff of sequence fidelity for replication completion makes it a direct contributor to mutagenesis and a mechanistically distinct axis of genome alteration from the excision-based repair pathways discussed elsewhere, while its role in allowing cancer cells to survive DNA-damaging chemotherapy makes it an active area of therapeutic interest.