rs28777 — SLC45A2

The Pigmentation Tag SNP That Traces Human Skin Color Evolution

SLC45A2 encodes a melanosomal membrane-associated transporter protein¹¹ melanosomal membrane-associated transporter protein
MATP, which regulates melanosomal pH critical for tyrosinase activity that controls melanin synthesis. The rs28777 variant is an intronic SNP in SLC45A2 that emerged from a genome-wide association study of over 10,000 Europeans²² genome-wide association study of over 10,000 Europeans
Han et al. 2008, PLoS Genetics as one of the most statistically significant markers for human pigmentation variation. While the nearby missense variant rs16891982 (L374F) is the likely causal variant in this gene, rs28777 independently tags SLC45A2 haplotypes that track skin, hair, and eye pigmentation across global populations.

The Mechanism

As an intronic variant, rs28777 does not directly alter the SLC45A2 protein sequence. Instead, it sits within a haplotype block that encompasses regulatory elements influencing SLC45A2 expression levels. The derived A allele occurs on haplotypes associated with reduced SLC45A2 transporter activity, leading to altered melanosomal pH³³ altered melanosomal pH
more acidic melanosomal environment impairs tyrosinase copper binding and reduces eumelanin synthesis. This results in lower eumelanin (brown-black pigment) production, lighter constitutive pigmentation, and reduced natural photoprotection against ultraviolet radiation. The ancestral C allele maintains haplotypes supporting efficient melanin synthesis and darker pigmentation.

The A allele shows one of the most extreme population frequency differentials in the human genome: approximately 95% in Europeans but only 12% in East Asians and 19% in Africans. This pattern reflects strong positive selection for depigmentation⁴⁴ strong positive selection for depigmentation
selective sweep over the past 5,000-20,000 years favoring vitamin D synthesis at high latitudes as human populations migrated to northern latitudes with lower UV radiation.

The Evidence

The initial GWAS by Han et al.⁵⁵ Han et al.
multi-stage study of natural hair color in European ancestry identified rs28777 with genome-wide significant associations for hair color (pooled P = 8.9 x 10^-14), tanning ability (pooled P = 2.2 x 10^-10), and skin color (pooled P = 9.5 x 10^-4). The minor C allele in Europeans was associated with darker hair, darker skin, and stronger tanning response.

In a large Australian population-based case-control study⁶⁶ large Australian population-based case-control study
1,716 melanoma cases and 4,111 controls, rs28777 exhibited one of the strongest crude associations with cutaneous melanoma risk among all pigmentation variants tested (allelic OR 3.75, P < 1.0 x 10^-4). After adjustment for Northern European ancestry, the effect attenuated to OR 2.37 (P = 0.0012), and after further adjustment for pigmentary phenotype, OR 1.68 (P = 0.045). Multivariable analysis adjusting for rs16891982 rendered rs28777 non-significant, confirming strong linkage disequilibrium between the two SLC45A2 variants and suggesting rs16891982 is the primary functional driver.

A review and meta-analysis of GWAS for pigmentation and skin cancer⁷⁷ review and meta-analysis of GWAS for pigmentation and skin cancer
Sturm 2009 reported rs28777 associated with skin color at P = 1.2 x 10^-17 in a South Asian population study, placing it among the most significant pigmentation loci genome-wide. The combined evidence across multiple studies of SLC45A2 and melanoma⁸⁸ multiple studies of SLC45A2 and melanoma
meta-analyses in Southern European populations establishes this gene region as a bona fide melanoma susceptibility locus with genome-wide epidemiological credibility.

Collectively, pigmentation gene polymorphisms including SLC45A2 variants account for approximately 12% of familial melanoma risk⁹⁹ pigmentation gene polymorphisms including SLC45A2 variants account for approximately 12% of familial melanoma risk
in high-UV European-descent populations, underscoring the direct pathway from pigmentation genetics to cancer susceptibility.

Practical Implications

Your rs28777 genotype reflects your SLC45A2 haplotype background and correlates with your constitutive pigmentation level, tanning capacity, and UV vulnerability. Individuals homozygous for the A allele have the lightest baseline pigmentation, the weakest tanning response, and the highest susceptibility to UV-induced DNA damage. Those carrying one or two C alleles have progressively more melanin production, better tanning ability, and greater intrinsic photoprotection.

For AA carriers, rigorous photoprotection is medically indicated. Use broad-spectrum sunscreen SPF 30+¹⁰¹⁰ broad-spectrum sunscreen SPF 30+
blocks 97% of UVB radiation on all exposed skin daily, seek shade during peak UV hours (10 AM - 4 PM), and wear protective clothing including wide-brimmed hats and UV-blocking sunglasses. Annual dermatological screening enables early detection of suspicious lesions when treatment is most effective.

Interactions

rs28777 is in strong linkage disequilibrium with rs16891982 (SLC45A2 L374F), the missense variant that directly alters melanosomal transporter function. The two variants track overlapping but not identical haplotypes, and rs28777 provides additional tagging of SLC45A2 regulatory variation beyond what rs16891982 captures alone.

SLC45A2 variants interact epistatically with MC1R red hair color variants (rs1805007, rs1805008) to modulate melanoma risk. Individuals carrying MC1R risk alleles have decreased melanoma risk if they concurrently carry SLC45A2 dark-pigmentation alleles, as higher melanin synthesis partially offsets the impaired tanning response caused by MC1R variants.

Additional epistatic interactions have been documented with OCA2 (rs12913832), TYR (rs1042602), SLC24A5 (rs1426654), and ASIP (rs6058017). The combined effect of multiple light-pigmentation alleles across these loci compounds melanoma susceptibility beyond simple additive models, making multi-locus pigmentation profiling more informative than any single variant alone.