Nucleotide Excision Repair

Nucleotide excision repair is a "cut-and-patch" DNA repair mechanism built to handle helix-distorting lesions that are too structurally diverse for a lock-and-key recognition system. Where base excision repair uses a specific glycosylase enzyme shaped to fit one damaged base (like 8-oxoguanine or a deaminated cytosine), NER instead has recognition factors that sense an abnormal shape or flexibility in the double helix — a bend, an unwound region, a bulky adduct sticking out of the minor groove. This shape-based strategy means the same excision machinery can remove UV-induced cyclobutane pyrimidine dimers (CPDs), 6-4 photoproducts, bulky chemical adducts from carcinogens like benzo[a]pyrene (found in tobacco smoke), and platinum-DNA crosslinks formed by the chemotherapy drug cisplatin. The pathway operates through two distinguishable but mechanistically convergent branches. Global genome NER (GG-NER) continuously surveys the entire genome, including transcriptionally silent regions, searching for helix distortions independent of transcriptional activity. Transcription-coupled NER (TC-NER) is triggered specifically when RNA polymerase II, moving along the template strand during transcription, physically stalls at a lesion; this branch repairs the transcribed strand of active genes preferentially and faster, because a stalled polymerase itself serves as the damage signal. Both branches differ only in how damage is initially detected — once the lesion is verified, a shared core machinery performs unwinding, dual incision, excision, and repair synthesis. The functional unit of NER damage — historically defined in bacteria and conserved conceptually in humans — is not the excision of a single base but of an oligonucleotide fragment spanning roughly a dozen nucleotides in bacteria and roughly 24–32 nucleotides in humans, with the lesion positioned asymmetrically within it. This "cut twice, remove the strip, resynthesize" logic, rather than a single-nucleotide swap, is the mechanistic signature that separates NER from base excision repair and mismatch repair.

Step 1 — Damage recognition (two entry points)

In E. coli</strong>, the UvrA₂B₂ complex (two UvrA subunits, two UvrB subunits) diffuses along DNA searching for helix distortion. UvrA has ATPase activity and acts as the initial sensor; once it finds a distortion, it recruits and hands off to UvrB, which uses its own ATP-dependent conformational change to insert a β-hairpin into the DNA, flipping out and verifying the damaged strand while UvrA departs. This produces a stable UvrB-DNA "preincision complex."

Two entry points, one convergence point

The elegance of NER is that GG-NER and TC-NER differ only in the sensor module — XPC/UV-DDB versus stalled Pol II/CSB/CSA — while sharing the entire downstream machinery: TFIIH recruitment, XPA/RPA verification, XPF-ERCC1 and XPG dual incision, and Pol δ/ε-mediated gap filling. This modularity means a mutation in a shared "core" gene (like XPB, XPD, XPF, XPG) disables both branches, while a mutation in a branch-specific sensor (XPC or CSB) disables only that branch — a distinction with direct clinical consequences seen in different NER-deficiency syndromes.

Discovering excision repair: Setlow and Carrier, and the "cut-patch" model (1964)

Richard Setlow and Robert Carrier, and independently William Rupp and colleagues, working with UV-irradiated E. coli, made a foundational observation: after UV exposure, a small fraction of the DNA's thymine dimers disappeared from high-molecular-weight DNA and reappeared in short, acid-soluble oligonucleotide fragments in the surrounding medium — but only in wild-type bacteria, not in UV-sensitive mutants. This directly showed that repair proceeds by physically excising a small DNA fragment containing the lesion, rather than by reversing the chemical lesion in place (as photolyase does). This was the first direct biochemical evidence for a "cut-and-patch," or excision-repair, mechanism distinct from direct reversal.

Identifying the bacterial genes: UV-sensitive mutant screens

Geneticists isolated E. coli mutants hypersensitive to UV light and mapped them to genes named uvrA, uvrB, and uvrC. Cells lacking any one of these genes failed to excise thymine dimers (they showed no acid-soluble oligonucleotide release after UV), directly linking these three genes to the excision step. This genetic dissection, combined with subsequent purification of the UvrA, UvrB, and UvrC proteins, allowed the full biochemical reconstitution of the "UvrABC excinuclease" system in vitro by Aziz Sancar's laboratory in the early 1980s — purified UvrA, UvrB, and UvrC proteins, mixed with UV-damaged plasmid DNA and ATP, produced the characteristic dual incision releasing a 12–13 nucleotide fragment, definitively proving that these three proteins alone (plus DNA polymerase I and ligase for the patch) are sufficient for bacterial NER. Sancar coined the term "excinuclease" specifically to distinguish this dual-incision, fragment-releasing activity from ordinary endonucleases or exonucleases.

Human NER: cell fusion genetics and xeroderma pigmentosum complementation groups

The existence of human NER, and the identity of its genes, emerged largely from a devastating natural experiment: patients with xeroderma pigmentosum (XP), who are extraordinarily UV-sensitive and develop skin cancers at a rate estimated 1,000- to 2,000-fold higher than the general population, often before age 10. Cell fusion experiments (fusing skin fibroblasts from different XP patients and testing whether UV sensitivity was rescued) defined seven complementation groups, XP-A through XP-G, each corresponding to a distinct gene later shown to encode a specific NER factor (XPA, XPB, XPC, XPD, XPE/DDB2, XPF, XPG). An eighth clinical category, "XP variant," was later shown not to be an NER defect at all but a deficiency in the translesion polymerase Pol η, which normally allows replication to bypass UV lesions — foreshadowing how translesion synthesis backs up NER when replication forks arrive at unrepaired damage before excision is complete.

Reconstituting human NER in vitro (Aziz Sancar, Richard Wood, and colleagues, late 1980s–1990s)

Building on the bacterial reconstitution, laboratories led by Aziz Sancar and Richard Wood used cell-free extracts from human cells, combined with purified recombinant factors, to biochemically reconstitute the entire human NER reaction on damaged plasmid DNA — first with crude extracts complemented by purified XPA, RPA, and TFIIH, and eventually with a fully purified, minimal set of six factors (XPC-RAD23B, TFIIH, XPA, RPA, XPF-ERCC1, XPG) sufficient to perform dual incision. This confirmed, factor by factor, that the model built from genetics (XP complementation groups) matched a biochemically sufficient, reconstitutable mechanism — the gold standard of mechanistic proof in molecular biology, because it shows the identified components are not merely necessary (genetics) but also sufficient (biochemistry) for the reaction.

Mapping the excision product size

Wood and colleagues used DNA fragments end-labeled and containing a single defined lesion (e.g., a cisplatin adduct or an acetylaminofluorene adduct at a known position) incubated with human cell extracts, then resolved the released oligonucleotide fragments on high-resolution denaturing gels. This precisely mapped human excision fragments to a range of 24–32 nucleotides, with incision occurring a defined distance 5′ and 3′ of the lesion — establishing the now-canonical asymmetric dual-incision geometry of human NER and distinguishing it quantitatively from the shorter bacterial 12–13 nt fragment.

Example 1 — Interpreting an XP complementation analysis. A clinician has skin fibroblast lines from four unrelated children with early, severe photosensitivity and skin cancers, suspected to be xeroderma pigmentosum. To determine how many distinct genes are affected, cells from each pair of patients are fused (creating heterokaryons) and tested for restored UV resistance (measured as unscheduled DNA synthesis after UV exposure).

Reasoning: Complementation (restored resistance) means the two patients carry mutations in different genes, since each fused cell supplies a working copy of what the other lacks. Failure to complement means they share a defect in the same gene. Patient 1 and Patient 2 fail to complement each other → same complementation group (call it Group A). Patient 3 and Patient 4 fail to complement → same complementation group (Group B). All cross-group fusions (1×3, 1×4, 2×3, 2×4) complement, consistent with two distinct genes. This is exactly the logic Cleaver, Kraemer, and colleagues used historically to define the seven (later eight, including the variant) XP complementation groups — each group later mapped to a specific NER gene (e.g., Group A → XPA, Group B → XPB, etc.).

Example 2 — Estimating repair capacity from an excision assay. A researcher exposes a culture of 10⁶ cultured human fibroblasts to UV-C light at a dose known to induce, on average, 5 × 10⁴ cyclobutane pyrimidine dimers (CPDs) per genome (genome size ≈ 3.2 × 10⁹ bp). Using a specific antibody against CPDs, the researcher measures that after 24 hours, 60% of the initial CPD signal remains; after 48 hours, 25% remains. Calculation: Initial lesions per cell = 5 × 10⁴.

• At 24 h: 60% remain → 0.60 × 5 × 10⁴ = 3.0 × 10⁴ lesions remaining → 2.0 × 10⁴ repaired.
• At 48 h: 25% remain → 0.25 × 5 × 10⁴ = 1.25 × 10⁴ lesions remaining → 3.75 × 10⁴ repaired cumulatively.

Interpretation: The repair rate is clearly non-linear — roughly 40% removed in the first 24 h but a further 35% (not another 40%) in the second 24 h, consistent with known biology: CPDs are repaired more slowly than 6-4 photoproducts (which UV-DDB and XPC detect far more efficiently), and GG-NER kinetics in normal human fibroblasts show this characteristic slow, saturating removal curve, quite different from the fast, nearly complete repair of 6-4 photoproducts (typically >90% removed within a few hours). If this same experiment were repeated in XP-C fibroblasts (GG-NER-deficient but TC-NER-intact), CPD removal in bulk genomic DNA would be almost undetectable, while CPDs in the transcribed strands of active genes would still be removed — a diagnostic pattern that itself was historically used to demonstrate the existence of TC-NER as a distinct sub-pathway.

Aziz Sancar's path to NER was unusually direct: as a young physician in Turkey, he became fascinated by photoreactivation (light-dependent DNA repair by photolyase), which led him to a PhD in the United States studying DNA repair biochemically. In the early 1980s, working at Yale and then the University of North Carolina, Sancar cloned the E. coli uvrA, uvrB, and uvrC genes, overexpressed and purified their protein products, and reconstituted the full excinuclease reaction in a test tube using only these three purified proteins plus damaged DNA and ATP — directly proving biochemical sufficiency where genetics had only shown necessity. He went on to help establish the human NER mechanism and TC-NER, and to characterize the circadian photolyase/cryptochrome system, work for which he shared the 2015 Nobel Prize in Chemistry with Tomas Lindahl (base excision repair) and Paul Modrich (mismatch repair) — a single Nobel Prize year that recognized the three principal DNA repair pathways as a coherent, mechanistically distinct triad.

Oncology and chemotherapy resistance. Platinum-based drugs (cisplatin, carboplatin, oxaliplatin) work by forming intrastrand DNA crosslinks that are NER substrates; tumors with high ERCC1 expression tend to be more resistant to cisplatin because they excise these adducts efficiently, making ERCC1 levels a studied (though imperfect) biomarker for predicting chemotherapy response in lung and ovarian cancers.

Genetic diagnosis and counseling. XP, Cockayne syndrome, and trichothiodystrophy — all caused by mutations in overlapping sets of NER genes — are diagnosed via genetic sequencing and cellular UV-sensitivity/complementation assays, guiding strict UV-avoidance regimens and cancer-surveillance schedules for patients.

Cancer biology and synthetic lethality. Cells or tumors with specific DNA-repair deficiencies (paralleling the PARP-inhibitor strategy in BRCA-mutant, homologous-recombination-deficient cancers) are an active area of research for exploiting NER deficiency to selectively kill tumor cells with targeted agents, sparing NER-proficient normal tissue.

Dermatology and public health. Understanding NER kinetics underlies sunscreen and photoprotection guidance, since the balance between UV dose, dimer formation rate, and cellular repair capacity determines net mutation accumulation — directly informing UV-index-based public health messaging.

Forensic and environmental toxicology. NER excision assays (measuring the rate of removal of a defined carcinogen adduct from cultured cells or biopsy tissue) are used to assess genotoxic risk of environmental chemicals and to validate new mutagens/carcinogens in regulatory toxicology testing.

Ask the tutor: How does translesion synthesis handle a lesion that NER failed to repair before the replication fork arrived?