rs2228000 — XPC Ala499Val

Missense variant in the DNA damage recognition gene XPC that moderately reduces nucleotide excision repair capacity, with elevated risk for bladder and breast cancer, particularly in homozygous carriers and those with carcinogen exposure

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Every day your cells cope with thousands of DNA lesions — from ultraviolet light, environmental carcinogens, and byproducts of normal cellular metabolism. The XPC protein is the first responder in the global-genome nucleotide excision repair pathway11 global-genome nucleotide excision repair pathway
GG-NER scans the entire genome for bulky DNA lesions, including UV-induced cyclobutane pyrimidine dimers and carcinogen-DNA adducts. XPC recognizes structural distortions in DNA caused by such lesions and recruits downstream repair factors. Without efficient XPC function, damaged nucleotides persist longer before repair initiates, increasing the chance that replication across the lesion introduces a permanent mutation.

The rs2228000 variant (Ala499Val, described as C>T in older literature using coding-strand notation) lies at codon 499 of the XPC open reading frame. The ancestral Ala499 allele (G on the plus strand, ~75% globally) encodes the efficient repair form. The derived Val499 allele (A on the plus strand, ~25% globally) substitutes a nonpolar valine for alanine at a position within XPC's central domain, subtly altering local protein structure and reducing damage recognition efficiency. This SNP complements the well-studied rs2228001 (Lys939Gln) in the same gene, and the two are in partial linkage disequilibrium — together they provide a more complete picture of XPC function than either variant alone.

XPC's central domain mediates binding to the distorted minor groove of damaged DNA and facilitates loading of the downstream TFIIH helicase complex. Codon 499 sits within a region implicated in DNA substrate binding22 Codon 499 sits within a region implicated in DNA substrate binding
Structural analyses of the XPC-RAD23B complex show that the central domain contacts the undamaged strand opposite the lesion; substitutions in this region reduce the affinity and kinetics of damage recognition without abolishing it entirely. The Ala-to-Val substitution introduces a slightly larger side chain, which may modestly alter the binding geometry. Ex vivo host-cell reactivation assays show that Val499 carriers have measurably reduced DNA repair capacity compared to Ala499 homozygotes, and the effect is dose-dependent: heterozygotes have intermediate repair activity, while Val/Val homozygotes show the most pronounced deficit.

Unlike the Lys939Gln variant (rs2228001), which maps to the C-terminal TFIIH-interaction domain, Ala499Val operates at the DNA substrate binding step. These two functional domains are distinct, so compound carriers of risk alleles at both positions could face compounded impairment at sequential steps in the NER initiation sequence.

The largest analysis of rs2228000 to date is a comprehensive meta-analysis of 71 studies33 comprehensive meta-analysis of 71 studies
Liu et al. Biosci Rep 2019 — 26,835 cancer cases and 37,069 controls across multiple cancer types. For bladder cancer, homozygous Val/Val carriers (plus-strand AA) showed 68% higher risk compared to Ala/Ala carriers (TT vs CC: OR=1.68, 95% CI 1.25–2.26, p=0.001), and even the T allele in aggregate conferred measurable risk (T vs C: OR=1.25, CI 1.07–1.45). For breast cancer, Val/Val homozygotes had 33% higher risk (TT vs CC: OR=1.33, CI 1.10–1.60, p=0.003). Across all 71 studies combined, the homozygous recessive comparison reached significance (TT vs CC+CT: OR=1.11, CI 1.01–1.22).

An earlier meta-analysis of 34 studies44 meta-analysis of 34 studies
He et al. Int J Cancer 2013 — 14,877 cases and 17,888 controls found 21% higher overall cancer risk for Val/Val vs Ala/Ala (OR=1.21, 95% CI 1.07–1.36, p=0.003) with the strongest effects in breast and bladder cancer and in Asian study populations. The recessive model OR was essentially identical (OR=1.20, CI 1.08–1.34, p=0.001), indicating the risk is concentrated in homozygous carriers rather than being evenly distributed across heterozygotes.

Bladder cancer has emerged as the most robustly associated malignancy. A dedicated bladder cancer study and meta-analysis55 dedicated bladder cancer study and meta-analysis
Rashed et al. Mutat Res 2016 — 234 cases and 258 controls, plus pooled meta-analysis of 7 studies (2,893 cases / 3,056 controls) confirmed that the Val allele is an independent bladder cancer risk factor (OR=1.54, CI 1.21–1.97, p=0.001), with the effect amplified by tobacco exposure (OR=2.23 with smoking, OR=2.40 with tobacco chewing). This is mechanistically expected: the urothelium continuously excretes carcinogens from tobacco and other environmental sources, making efficient NER especially critical for this tissue.

In contrast, [lung cancer meta-analysis | PLOS One 2014 — 2,605 patients, 5 studies] found no significant association (AlaVal/ValVal OR=1.054, CI 0.95–1.17), suggesting that rs2228000 risk is tissue-specific rather than uniform across all carcinogen-exposed organs. Digestive system cancers show a more complex picture: pooled analysis of gastric and esophageal cancer studies in mostly Chinese populations found a modest protective effect of the T allele in the dominant model (OR=0.84, CI 0.76–0.94), possibly reflecting population-specific effects or confounding.

In a South Indian case-control study of chronic myeloid leukemia66 South Indian case-control study of chronic myeloid leukemia
Dixit et al. Gene 2021 — 212 CML cases and 212 controls, the Val allele showed markedly elevated CML susceptibility (CT: OR=1.92, CI 1.21–3.06; TT: OR=2.84, CI 1.22–6.71) and correlated with disease progression and splenomegaly. This is a small, single-population study and should be interpreted cautiously, but it extends the signal for Ala499Val beyond solid tumors.

The actionable landscape for rs2228000 parallels the companion variant rs2228001 but with evidence concentrated specifically in bladder and breast cancer rather than the broader multi-cancer associations of Lys939Gln. For Val/Val homozygotes — roughly 6% of the global population, with higher prevalence in East Asians (~12%) — the bladder cancer risk elevation is clinically meaningful (OR=1.68), particularly when tobacco exposure is present.

The most direct intervention is reducing carcinogen input: tobacco abstinence removes the major modifiable source of urothelial carcinogen load, and the gene-environment interaction documented in the bladder cancer data (OR up to 2.40 with tobacco chewing) makes this more impactful for Val carriers than for the general population.

For breast cancer risk, the Val/Val association (OR=1.33) is modest in absolute terms and does not approach the penetrance of BRCA1/2 variants, but it is consistent across studies and can be incorporated into personalized screening timing discussions alongside other risk factors.

XPC operates in sequence with XPA (rs1800975) and ERCC2/XPD (rs13181): XPC identifies DNA damage, XPA verifies the lesion, and XPD unwinds the DNA duplex to allow excision. Carriers of risk alleles at rs2228000 (damage recognition) and rs1800975 (damage verification) may have compounded NER deficiency, with each step of the cascade functioning below optimal efficiency simultaneously.

The companion variant rs2228001 (Lys939Gln) maps to XPC's TFIIH-interaction domain, a functionally distinct region from the codon-499 DNA-binding region. Individuals carrying risk alleles at both positions may have the most compromised XPC function, as both the initial damage binding step and the subsequent TFIIH recruitment step are impaired. Haplotype analyses including both rs2228000 and rs2228001 show stronger cancer risk prediction than either variant alone in several Asian population studies.

Genotype Interpretations

What each possible genotype means for this variant.

GG Efficient Repair Normal

Ala499 homozygous — full XPC damage binding efficiency, normal NER capacity

You carry two copies of the ancestral Ala499 allele (G on the plus strand), encoding XPC protein with optimal DNA substrate binding geometry. Your global-genome NER pathway initiates repair of UV-induced lesions and environmental carcinogen adducts at full efficiency. This is the most common genotype globally, shared by approximately 57% of people. Studies consistently show Ala499 homozygotes have the highest XPC repair capacity and the baseline cancer risk for this variant.

AG Intermediate Repair Intermediate

One Val499 allele — moderately reduced XPC damage recognition with slightly elevated bladder and breast cancer susceptibility

With one Ala499 and one Val499 allele, your NER pathway recognizes DNA lesions with slightly reduced efficiency compared to Ala499 homozygotes. The Val499 protein's altered central-domain geometry modestly reduces the speed and affinity of initial damage recognition — but the functional Ala499 molecules partially compensate under typical daily exposure conditions.

The bladder cancer meta-analysis found that the T allele in aggregate (including both heterozygotes and Val/Val homozygotes) is associated with elevated risk (T vs C OR=1.25, CI 1.07–1.45), while the strongest effects are concentrated in Val/Val homozygotes. Tobacco carcinogen exposure amplifies the risk: the gene-environment interaction documented in bladder cancer studies makes tobacco exposure more consequential for carriers of even one Val allele than for Ala/Ala individuals.

AA Reduced Repair Reduced

Val499 homozygous — most pronounced XPC recognition impairment, substantially elevated bladder and breast cancer risk, especially with carcinogen exposure

With both XPC alleles encoding the Val499 form, your DNA damage recognition step in global-genome NER operates at the lowest efficiency for this position. The Val499 protein's altered central-domain binding geometry reduces initial lesion detection speed, meaning DNA adducts — particularly bulky carcinogen adducts in the urothelium and oxidative lesions in breast tissue — persist longer before XPC loads downstream repair factors. Over decades of cumulative exposure, this kinetic delay increases the probability that a lesion will be replicated before repair, potentially generating a mutation rather than a corrected sequence.

The evidence for bladder cancer risk is the most robust for this genotype: the homozygous Val/Val comparison (TT vs CC OR=1.68) is consistent across multiple meta-analyses and methodologically independent study populations. The tobacco interaction is particularly important for bladder cancer — Val499 carriers who smoke or use tobacco products accumulate urothelial DNA adducts that their impaired XPC cannot process as efficiently as Ala/Ala individuals, compounding both the damage input and the repair deficit simultaneously.

The breast cancer association (OR=1.33) is consistent and replicated but modest in absolute terms. This SNP alone does not approach the penetrance of BRCA1/2 variants, but it contributes to polygenic breast cancer risk alongside other variants. It is most relevant when integrated with other risk factors (family history, dense breast tissue, hormone exposure) rather than treated as a standalone predictor.