Human skin color is a highly genetically complex trait and one of the most important phenotypic characteristics of modern humans. It plays a critical role in protection against the harmful effects of ultraviolet radiation (UVR) and other environmental stressors (Jablonski and Chaplin, 2000; Basang et al., 2021; Chen et al., 2022; Yang et al., 2022). The color depth of the skin is determined by the type and amount of melanin produced in melanosomes and subcellular lysosome-like organelles of melanocytes located in the basal layer of the epidermis (Marks and Seabra, 2001; Lin and Fisher, 2007; Yardman-Frank and Fisher, 2021). Melanin synthesis in melanocytes is determined genetically and is influenced by a variety of extrinsic and intrinsic factors, such as hormonal changes, inflammation, and exposure to UVR (Slominski et al., 2004; Jablonski and Chaplin, 2010).

In general, human skin color includes constitutive skin color (baseline pigmentation, skin areas not exposed to sunlight, such as the buttocks and underarms) and facultative skin color (pigmentation of exposed skin areas, such as the hands) (Quevedo et al., 1975; Jimbow et al., 1976; Yang et al., 2022). Several genetic studies on constitutive skin color have suggested that pigmentation genes are important factors in human skin pigmentation (Lamason et al., 2005; Izagirre et al., 2006; Yuasa et al., 2006; Lao et al., 2007; Norton et al., 2007; Soejima and Koda, 2007; Stokowski et al., 2007; Han et al., 2008; Edwards et al., 2010; Beleza et al., 2013; Yang et al., 2016; Crawford et al., 2017; Martin et al., 2017; Yang et al., 2018; Adhikari et al., 2019; Fan et al., 2023). UVR (especially UVB) exposure induces facultative tanning, an acquired trait that contributes to skin color (Schallreuter et al., 1999; Jablonski and Chaplin, 2010). People living in areas with strong UVR have dark facultative skin color (e.g. Africans and Tibetans), which can protect against UVR damage (Crawford et al., 2017; Martin et al., 2017; Yang et al., 2022). In areas at high latitudes, tanning behaviors in European populations are relatively common, and tanning reaction responses vary for different populations (including different genders) (Robinson et al., 1997; Tadokoro et al., 2003; Tadokoro et al., 2005; Warthan et al., 2005; Damian et al., 2008; Harrington et al., 2011; Mays et al., 2017; Iliescu et al., 2018). Previous GWAS studies on light-skinned Europeans reveals there is a genetic contribution of UVR exposure to pigmentation through the tanning response. Several genes (SLC45A2, IRF4, TYR, OCA2/HERC2, MC1R, RALY/ASIP, PDE4B, RIPK5, PA2G4P4, PPARGC1B, AHR/AGR3, TRPS1, TYRP1, BNC2, EMX2, TPCN2, DCT, ATP11A, SLC24A4, KIAA0930, GRM6, ATF1, WNT1, and SILV/Pmel17) associated with tanning ability in European populations suggest that Europeans have a unique tanning mechanism (Sulem et al., 2007; Sulem et al., 2008; Visconti et al., 2018). However, most people in East Asia prefer sun protection to tanning to avoid more UVR, and previous studies have indicated that different pigmentation genes play an important role in convergent evolution of skin lightening between East Asians and Europeans (Kung and Lee, 2006; Norton et al., 2007; Edwards et al., 2010; Jang et al., 2013; Yang et al., 2016). Collectively, the tanning mechanism in East Asians may different from that in Europeans. Further studies on tanning-related genes in East Asians are necessary to better understand skin color changes in East Asians.

A genome-wide study found PAH and OCA2 with higher genetic divergence between light-skinned Han Chinese and dark-skinned Africans and Cambodians (Yang et al., 2016). However, unlike OCA2 which plays a critical role in constitutive skin color, the functional effect of PAH in East Asian populations remains unclear. PAH is one of the enzymes involved in the production of melanin substrate tyrosine under UVB treatment (Schallreuter and Wood, 1999). Therefore, we hypothesized that PAH may affect the facultative skin color under the influence of external UVR, and its mechanism for evolutionary skin lightening in the East Asian populations is different from the previous pigmentation genes.

In the current study, we observed a strong signal of Darwinian positive selection for the pigmentation gene PAH in the Han Chinese (a major East Asian population). Skin color association analysis showed that intronic SNP rs10778203 in PAH was significantly associated with facultative skin color of the back of the hand in Han Chinese males (P <0.05, additive model). Furthermore, a dual-luciferase reporter gene assay and transcription factor binding assays showed that the ancestral allele of rs10778203 could bind to SMAD2 and had significant enhancer activity for PAH in human melanoma A375 cell lines. The regulatory activity of the ancestral C allele was enhanced by UVB stimulation, but that of the derived T allele was not. Compared to the ancestral-type C allele, the derived T allele of rs10778203 (a major allele in East Asians) decreased the UVB response and resulted in lighter skin pigmentation in transgenic zebrafish embryos under UVB treatment. These results suggest that PAH is a key pigmentation gene for skin lightening and alters tanning ability in East Asians, suggesting a unique genetic mechanism for evolutionary skin lightening in East Asians during recent human evolutionary history.