rs1408799 — TYRP1 Intron variant

TYRP1 and the Eumelanin Route to Eye, Hair, and Skin Color

TYRP1 (tyrosinase-related protein 1) is a melanocyte-specific enzyme that sits at a critical junction in the eumelanin biosynthesis pathway. Inside melanosomes — the specialized organelles that produce and store pigment — TYRP1 catalyzes the oxidation of DHICA¹¹ DHICA
5,6-dihydroxyindole-2-carboxylic acid, a key intermediate in brown-black eumelanin production, while simultaneously stabilizing tyrosinase (the rate-limiting enzyme) and maintaining the structural integrity of the melanosome membrane itself. rs1408799 is an intronic variant in TYRP1 on chromosome 9p23 that modulates how much eumelanin — the brown-black pigment responsible for photoprotection — your melanocytes produce. The C allele is nearly fixed in Northern Europeans (frequency ~69%) but extremely rare in East Asians (~2%), reflecting its role in the adaptive depigmentation that occurred as populations moved to lower-UV environments.

The Mechanism

rs1408799 sits within an intron and does not directly alter the TYRP1 protein sequence. Its biological effect appears to be mediated through linkage disequilibrium with nearby functional variants — it is in strong LD (D'>0.7) with rs683 (a 3'UTR variant) and rs2733836, both of which are incorporated into forensic eye-color prediction models. The net effect of the C-allele haplotype is reduced eumelanin output: less brown-black pigment in irises, hair follicles, and skin. Reduced eumelanin shifts the melanocyte balance toward pheomelanin (the yellow-red pigment), lightening overall coloration and diminishing the natural photoprotective shield that dense eumelanin provides. Pheomelanin is a pro-oxidant that generates reactive oxygen species even without UV exposure²² Pheomelanin is a pro-oxidant that generates reactive oxygen species even without UV exposure
unlike eumelanin, which absorbs and dissipates UV energy harmlessly, meaning lower eumelanin does not merely reduce protection — it actively amplifies oxidative damage in skin and iris tissue.

The Evidence

The foundational evidence comes from an Icelandic genome-wide association study³³ Icelandic genome-wide association study
Sulem et al., Nature Genetics, 2007 with replication in additional Icelandic and Dutch participants. The C allele was associated with blue versus nonblue eyes (OR 1.41, p=1.5×10⁻⁹) and showed a suggestive association with blond versus brown hair. The study is also replicated in eye-color prediction research from forensic genetics, where rs1408799 is one of two TYRP1 variants included in pigmentation prediction tools alongside the major HERC2/OCA2 locus (rs12913832).

For melanoma, the same research group (Gudbjartsson et al., Nature Genetics, 2008⁴⁴ Gudbjartsson et al., Nature Genetics, 2008
2,121 melanoma cases, 40,000+ controls) found the C allele associated with cutaneous melanoma risk (OR 1.15, p=4.6×10⁻⁴), and critically, this association remained significant even after statistical adjustment for pigmentation phenotypes — suggesting that the C-allele haplotype's effect on melanoma risk is not entirely explained by lighter visible pigmentation alone. A nested case-control study in Caucasian women⁵⁵ nested case-control study in Caucasian women
Nan et al., 2009, 218 melanoma cases and 870 controls found a protective trend for the T allele (OR 0.77, 95% CI 0.60–0.98), though this did not survive Bonferroni correction. The overall body of evidence indicates a modest but real risk contribution from the C-allele haplotype, consistent with reduced eumelanin as a biological mechanism.

The population frequency pattern itself tells part of the story: the C allele rose from ~28% in African populations (which have the highest eumelanin levels) to ~69% in Europeans (who have lower photoprotection needs due to reduced UV at high latitudes). East Asian populations, who achieved light skin through different genes (SLC24A5, SLC45A2), show only ~2% C-allele frequency — an elegant example of convergent evolution where multiple genetic pathways reached similar phenotypic endpoints.

Practical Implications

This variant is an additive risk modifier: each C allele slightly reduces eumelanin production, shifts the balance toward lighter pigmentation, and modestly increases UV-induced melanoma risk. For CC homozygotes (the most common European genotype), the effect is most pronounced. For TT homozygotes, higher eumelanin provides a natural photoprotective advantage at this locus.

The practical take-away differs from "just use sunscreen" (a recommendation that applies to everyone). Carriers of one or two C alleles have a specific eumelanin deficit that increases their sensitivity to UV-induced oxidative DNA damage and their melanoma susceptibility beyond what visible skin tone alone would predict. The melanoma risk remaining after adjustment for pigmentation phenotypes means that even individuals who don't look especially light-skinned but carry CC at TYRP1 may face elevated risk. This makes genotype-informed photoprotection monitoring more valuable than relying on a clinician's visual assessment of skin type alone.

Interactions

The most significant documented interaction is between rs1408799 in TYRP1 and rs12913832 in HERC2. Pospiech et al. (2011)⁶⁶ Pospiech et al. (2011)
718 European participants, Journal of Human Genetics identified a novel synergistic (epistatic) interaction between these two loci specifically for green eye color determination. In individuals already homozygous for the blue-eye HERC2 allele (rs12913832:GG), TYRP1 rs1408799 modulates residual variation in iris color — explaining why some GG individuals have green rather than blue irises. This interaction is not captured by either variant alone and requires co-occurrence of specific alleles at both loci. The combination of HERC2 rs12913832 (the dominant blue-eye switch) and TYRP1 rs1408799 (a eumelanin volume dial) together create the conditions for green iris pigmentation.

TYRP1 also interacts with TYR (rs1042602), SLC45A2 (rs16891982), and IRF4 (rs12203592) in melanoma risk. Individuals carrying high-risk alleles at multiple pigmentation loci face compounding — not merely additive — risk increases that substantially exceed what any single variant predicts. The compound effect of low eumelanin from multiple independent genetic routes creates both phenotypic and oncological risk that warrants intensified dermatology surveillance beyond what any single test result would recommend.