Human pigmentation genes under environmental selection

Abstract

Genome-wide association studies and comparative genomics have established major loci and specific polymorphisms affecting human skin, hair and eye color. Environmental changes have had an impact on selected pigmentation genes as populations have expanded into different regions of the globe.

Figure 1

Melanin formation in the melanosome. The conversion of phenylalanine to tyrosine by phenylalanine dehydroxylase (PAH) takes place in the cytoplasm of melanocytes and is necessary to maintain the supply of this substrate for melanogenesis to occur continuously, with its activity positively correlated with skin-type [140]. Active uptake of tyrosine by the melanosome is required, and is initiated by the process of oxidation by tyrosinase (TYR) and involves other enzymes such as DHI oxidase (TYRP1) and dopachrome tautomerase (DCT). Ion transport is critical to melanosome function, with TYR activity being pH-dependent and its absolute activity being critical for the rate of melanin production. The coupling of H⁺, Na⁺, Ca²⁺ and K⁺ transport by the V-ATP complex, with the involvement of SLC45A2, SLC24A5 and TPCN2 in the regulation of this process, is shown. Cystine as a negative regulator of melanogenesis is pumped out by CTNS. The SILV protein forms the matrix backbone through specific proteolysis of a precursor protein, upon which eumelanin is deposited. The melanosomes are then transported along thin cellular projections known as dendrites and deposited within keratinoctyes, which are the cells that take up the pigment and give the visible color.

Figure 2

Protection pathways in the skin against ultraviolet radiation (UVR). UVR induces DNA damage, which leads to activation of p53 and the formation of POMC and MC1R activation factors. MC1R action can be blocked by ASIP. Upon receptor activation of cAMP, dopachrome tautomerase (DCT) activity is upregulated and this leads to the generation of 5,6-dihydroxyindole-2-carboxylic acid (DHICA). MC1R also activates melanosome maturation and transfer to the keratinocyte. DHICA removes reactive oxygen species (ROS), and activates catalase (CAT) and peroxisome proliferator activated receptor (PPAR) in keratinocytes. Finally, DCT provides antagonistic feedback to p53 in melanocytes [112].