Human pigmentation genes: identification, structure and consequences of polymorphic variation

Prediction of Skin Color Using Forensic DNA Phenotyping in Asian Populations: A Focus on Thailand

Forensic DNA phenotyping (FDP) has emerged as an essential tool in criminal investigations, enabling the prediction of physical traits based on genetic information. This review explores the genetic factors influencing skin pigmentation, particularly within Asian populations, with a focus on Thailand. Key genes such as Oculocutaneous Albinism II (OCA2), Dopachrome Tautomerase (DCT), KIT Ligand (KITLG), and Solute Carrier Family 24 Member 2 (SLC24A2) are examined for their roles in melanin production and variations that lead to different skin tones. The OCA2 gene is highlighted for its role in transporting ions that help stabilize melanosomes, while specific variants in the DCT gene, including single nucleotide polymorphisms (SNPs) rs2031526 and rs3782974, are discussed for their potential effects on pigmentation in Asian groups. The KITLG gene, crucial for developing melanocytes, includes the SNP rs642742, which is linked to lighter skin in East Asians. Additionally, recent findings on the SLC24A2 gene are presented, emphasizing its connection to pigmentation through calcium regulation in melanin production. Finally, the review addresses the ethical considerations of using FDP in Thailand, where advances in genetic profiling raise concerns about privacy, consent, and discrimination. Establishing clear guidelines is vital to balancing the benefits of forensic DNA applications with the protection of individual rights.

Molecular Basis of MC1R Activation: Mutation-Induced Alterations in Structural Dynamics

The MC1R protein is a receptor found in melanocytes that plays a role in melanin synthesis. Mutations in this protein can impact hair color, skin tone, tanning ability, and increase the risk of skin cancer. The MC1R protein is activated by the alpha-melanocyte-stimulating hormone (α-MSH). Previous studies have shown that mutations affect the interaction between MC1R and α-MSH; however, the mechanism behind this process is poorly understood. Our study aims to shed light on this mechanism using molecular dynamics (MD) simulations to analyze the Asp84Glu and Asp294His variants. We simulated both the wild-type (WT) protein and the mutants with and without ligand. Our results reveal that mutations induce unique conformations during state transitions, hindering the switch between active and inactive states and decreasing cellular levels of cAMP. Interestingly, Asp294His showed increased ligand affinity but decreased protein activity, highlighting that tighter binding does not always lead to increased activation. Our study provides insights into the molecular mechanisms underlying the impact of MC1R mutations on protein activity.

A novel source of biologically active compounds - The leaves of Serbian herbaceous peonies

In order to gain further insight into how various extraction techniques (maceration, microwave-, and ultrasound-assisted extractions) affect the chemical profile and biological activities of leaf extracts from Paeonia tenuifolia L., Paeonia peregrina Mill., and Paeonia officinalis L., this research was performed. The targeted chemical characterization of the extracts was achieved using the Ultra-High-Performance-Liquid-Chromatography-Linear-Trap-Mass-Spectrometry OrbiTrap instrumental technique, while Fourier Transform Infrared Spectroscopy was conducted to investigate the structural properties of the examined leaf extracts. According to the results, the species P. officinalis, Božurna locality as the origin of the plant material, and microwave-assisted extraction produced the maximum polyphenol yield, (491.9 ± 2.7 mg gallic acid equivalent (GAE)/mL). The ethanolic extracts exhibited moderate antioxidant activity as evaluated by DPPH (2,2-diphenyl-1-picrylhydrazyl) and phosphomolybdenum tests. With MIC values of 0.125 mg/mL, the leaf extracts produced by ultrasound-assisted extraction and maceration (Deliblato sands and Bogovo gumno) had the best antibacterial activity against Pseudomonas aeruginosa and Salmonella Typhimurium. Ultrasound-assisted extraction has proven to produce the most effective antimicrobial agents. Inhibitory potential towards glucosidase, amylase, cholinesterases, and tyrosinase was evaluated in enzyme inhibition assays and molecular docking simulations. Results show that leaves of P. tenuifolia L. obtained by ultrasound-assisted extraction had the highest acetylcholinesterase and butyrylcholinesterase inhibitory activity. Namely, the complexity of the polyphenol structures, the extraction method, the used locality, and the different mechanisms of the reactions between bioactives from leaf extracts and other components (free radicals, microorganisms, and enzymes) are the main factors that influence the results of the antioxidant tests, as well as the antibacterial and enzyme-inhibitory activities of the extracts. Hydroxymethyl-phenyl pentosyl-hexoside and acetyl-hydroxyphenyl-hexoside were the first time identified in the leaf extract of the Paeonia species. Due to their proven biological activities and the confirmed existence of bioactive compounds, leaf extracts may find use in foodstuffs, functional foods, and pharmaceutical products.

Irradiation-induced hair graying in mice: an experimental model to evaluate the effectiveness of interventions targeting oxidative stress, DNA damage prevention, and cellular senescence

Hair graying, also known as canities or achromotrichia, is a natural phenomenon associated with aging and is influenced by external factors such as stress, environmental toxicants, and radiation exposure. Understanding the mechanisms underlying hair graying is an ideal approach for developing interventions to prevent or reverse age-related changes in regenerative tissues. Hair graying induced by ionizing radiation (γ-rays or X-rays) has emerged as a valuable experimental model to investigate the molecular pathways involved in this process. In this review, we examine the existing evidence on radiation-induced hair graying, with a particular focus on the potential role of radiation-induced cellular senescence. We explore the current understanding of hair graying in aging, delve into the underlying mechanisms, and highlight the unique advantages of using ionizing-irradiation-induced hair graying as a research model. By elucidating the molecular pathways involved, we aim to deepen our understanding of hair graying and potentially identify novel therapeutic targets to address this age-related phenotypic change.

PMEL p.L18del associates with beef quality of Kumamoto sub-breed of Japanese Brown cattle

Japanese Brown cattle is the second most popular breed among Wagyu breeds and raised mainly in Kumamoto and Kochi Prefectures. Typical coat color of the Kumamoto sub-breed is solid brown, but individuals with diluted coat color are sometimes born. We previously detected four SNPs in PMEL gene and identified p.L18del as the causative polymorphism of this diluted phenotype. The current study examined the association between the SNPs in PMEL gene and carcass traits of the Kumamoto sub-breed. Our association analysis revealed that p.L18del had significant effects on BMS (p = 0.0263), meat brightness (p = 0.0179), meat firmness (p = 0.0102), and meat texture (p = 0.0252) and that del allele of this SNP might be useful to improve these carcass traits.

Distinct cAMP Signaling Microdomains Differentially Regulate Melanosomal pH and Pigmentation

cAMP signaling is a well-established regulator of melanin synthesis. Two distinct cAMP signaling pathways-the transmembrane adenylyl cyclase pathway, activated primarily by the MC1R, and the soluble adenylyl cyclase (sAC) pathway-affect melanin synthesis. The sAC pathway affects melanin synthesis by regulating melanosomal pH, and the MC1R pathway affects melanin synthesis by regulating gene expression and post-translational modifications. However, whether MC1R genotype affects melanosomal pH is poorly understood. We now report that loss of function MC1R does not affect melanosomal pH. Thus, sAC signaling appears to be the only cAMP signaling pathway that regulates melanosomal pH. We also addressed whether MC1R genotype affects sAC-dependent regulation of melanin synthesis. Although sAC loss of function in wild-type human melanocytes stimulates melanin synthesis, sAC loss of function has no effect on melanin synthesis in MC1R nonfunctional human and mouse melanocytes or skin and hair melanin in e/e mice. Interestingly, activation of transmembrane adenylyl cyclases, which increases epidermal eumelanin synthesis in e/e mice, leads to enhanced production of eumelanin in sAC-knockout mice relative to that in sAC wild-type mice. Thus, MC1R- and sAC-dependent cAMP signaling pathways define distinct mechanisms that regulate melanosomal pH and pigmentation.

Ssc-mir-221-3p regulates melanin production in Xiang pigs melanocytes by targeting the TYRP1 gene

BACKGROUND

MicroRNAs (miRNAs) are small endogenous non-coding RNAs that regulate gene expression by down-regulating it. Several studies have suggested that miRNAs plays a crucial role in mammalian skin color production. The TYRP1 gene, a member of the tyrosine family, is an important candidate gene that affects melanogenesis. This study aimed to identify genes and miRNAs that affect melanin production in Xiang pigs by transcriptome sequencing, and to validate their targeted regulatory relationships.

RESULTS

17 miRNAs and 1,230 genes were significantly differentially expressed (P < 0.05) in the black and white skin tissues of Jianbai Xiang pigs. miRNA-221-3p was identified as a candidate miRNA for melanin formation and its target gene, TYRP1, was selected. The TYRP1 gene is a member of the TYR gene family, which evolved from the TYR gene through chromosome segmental duplication. The function of the gene was highly conserved throughout the evolutionary process. overexpression of TYRP1 gene significantly increased the expression of TYR, TYRP1, and DCT genes P < 0.01, which led to an increase in the relative content of melanin. Silencing of TYRP1 through the use of TYRP1-siRNA significantly reduced the expression of TYR, TYRP1, and DCT genes in Jianbai Xiang pig melanocytes P < 0.01, which in turn decreased the relative melanin content. The targeted binding relationship between ssc-miR-221-3p and TYRP1 gene was validated. After transfection of porcine melanocytes with ssc-miR-221-3p mimic, the expression of ssc-miR-221-3p was significantly up-regulated (P < 0.01). Furthermore, the mRNA and protein levels of TYR, TYRP1, and DCT genes were significantly down-regulated (P < 0.01), and melanin content in cells was significantly reduced (P < 0.01).

CONCLUSION

The TYRP1 gene affects melanogenesis in melanocytes of Jianbai Xiang pigs, and ssc-miR-221-3p targets the TYRP1 gene to regulate melanogenesis in melanocytes of Jianbai Xiang pigs.

Recent Advances and Progress on Melanin: From Source to Application

Melanin is a biological pigment formed by indoles and phenolic compounds. It is widely found in living organisms and has a variety of unique properties. Due to its diverse characteristics and good biocompatibility, melanin has become the focus in the fields of biomedicine, agriculture, the food industry, etc. However, due to the wide range of melanin sources, complex polymerization properties, and low solubility of specific solvents, the specific macromolecular structure and polymerization mechanism of melanin remain unclear, which significantly limits the further study and application of melanin. Its synthesis and degradation pathways are also controversial. In addition, new properties and applications of melanin are constantly being discovered. In this review, we focus on the recent advances in the research of melanin in all aspects. Firstly, the classification, source, and degradation of melanin are summarized. Secondly, a detailed description of the structure, characterization, and properties of melanin is followed. The novel biological activity of melanin and its application is described at the end.

The evolution of human skin pigmentation: A changing medley of vitamins, genetic variability, and UV radiation during human expansion

This review examines putative, yet likely critical evolutionary pressures contributing to human skin pigmentation and subsequently, depigmentation phenotypes. To achieve this, it provides a synthesis of ideas that frame contemporary thinking, without limiting the narrative to pigmentation genes alone. It examines how geography and hence the quality and quantity of UV exposure, pigmentation genes, diet-related genes, vitamins, anti-oxidant nutrients, and cultural practices intersect and interact to facilitate the evolution of human skin color. The article has a strong focus on the vitamin D-folate evolutionary model, with updates on the latest biophysical research findings to support this paradigm. This model is examined within a broad canvas that takes human expansion out of Africa and genetic architecture into account. A thorough discourse on the biology of melanization is provided (includes relationship to BH4 and DNA damage repair), with the relevance of this to the UV sensitivity of folate and UV photosynthesis of vitamin D explained in detail, including the relevance of these vitamins to reproductive success. It explores whether we might be able to predict vitamin-related gene polymorphisms that pivot metabolism to the prevailing UVR exposome within the vitamin D-folate evolutionary hypothesis context. This is discussed in terms of a primary adaptive phenotype (pigmentation/depigmentation), a secondary adaptive phenotype (flexible metabolic phenotype based on vitamin-related gene polymorphism profile), and a tertiary adaptive strategy (dietary anti-oxidants to support the secondary adaptive phenotype). Finally, alternative evolutionary models for pigmentation are discussed, as are challenges to future research in this area.

Investigating the morphology and genetics of scalp and facial hair characteristics for phenotype prediction

Microscopic traits and ultrastructure of hair such as cross-sectional shape, pigmentation, curvature, and internal structure help determine the level of variations between and across human populations. Apart from cosmetics and anthropological applications, such as determining species, somatic origin (body area), and biogeographic ancestry, the evidential value of hair has increased with rapid progression in the area of forensic DNA phenotyping (FDP). Individuals differ in the features of their scalp hair (greying, shape, colour, balding, thickness, and density) and facial hair (eyebrow thickness, monobrow, and beard thickness) features. Scalp and facial hair characteristics are genetically controlled and lead to visible inter-individual variations within and among populations of various ethnic origins. Hence, these characteristics can be exploited and made more inclusive in FDP, thereby leading to more comprehensive, accurate, and robust prediction models for forensic purposes. The present article focuses on understanding the genetics of scalp and facial hair characteristics with the goal to develop a more inclusive approach to better understand hair biology by integrating hair microscopy with genetics for genotype-phenotype correlation research.

Identification of Candidate Genes for Pigmentation in Camels Using Genotyping-by-Sequencing

The coat color of dromedary is usually uniform and varies from black to white, although dark- to light-brown colors are the most common phenotypes. This project was designed to gain knowledge on novel color-related variants using genotyping-by-sequencing (GBS). The association between the SNPs and coat color was tested using MLM (mixed linear models) with kinship matrix. Three GWAS models including white color vs. non-white color, black vs. non-black color, and light-brown vs. dark-brown color were performed. There were no distinct genetic clusters detected based on the color phenotypes. However, admixture occurred among all individuals of the four different coat color groups. We identified nine significant SNPs associated with white color after Bonferroni correction, located close to ANKRD26, GNB1, TSPYL4, TEKT5, DEXI, CIITA, TVP23B, CLEC16A, TMPRSS13, FXYD6, MPZL3, ANKRD26, HFM1, CDC7, TGFBR3, and HACE1 genes in neighboring flanking regions. The 13 significant SNPs associated with black color and the candidate genes were: CAPN7, CHRM4, CIITA, CLEC16A, COL4A4, COL6A6, CREB3L1, DEXI, DGKZ, DGKZ, EAF1, HDLBP, INPP5F, MCMBP, MDK, SEC23IP, SNAI1, TBX15, TEKT5, TMEM189, trpS, TSPYL4, TVP23B, and UBE2V1. The SNAI1 gene interacted with MCIR, ASIP and KIT genes. These genes play a key role in the melanin biosynthetic and pigmentation biological process and melanogenesis biological pathway. Further research using a larger sample size and pedigree data will allow confirmation of associated SNPs and the identified candidate genes.

Genetic Connections and Convergent Evolution of Tropical Indigenous Peoples in Asia

Tropical indigenous peoples in Asia (TIA) attract much attention for their unique appearance, whereas their genetic history and adaptive evolution remain mysteries. We conducted a comprehensive study to characterize the genetic distinction and connection of broad geographical TIAs. Despite the diverse genetic makeup and large interarea genetic differentiation between the TIA groups, we identified a basal Asian ancestry (bASN) specifically shared by these populations. The bASN ancestry was relatively enriched in ancient Asian human genomes dated as early as ∼50,000 years before the present and diminished in more recent history. Notably, the bASN ancestry is unlikely to be derived from archaic hominins. Instead, we suggest it may be better modeled as a survived lineage of the initial peopling of Asia. Shared adaptations inherited from the ancient Asian ancestry were detected among the TIA groups (e.g., LIMS1 for hair morphology, and COL24A1 for bone formation), and they are enriched in neurological functions either at an identical locus (e.g., NKAIN3), or different loci in an identical gene (e.g., TENM4). The bASN ancestry could also have formed the substrate of the genetic architecture of the dark pigmentation observed in the TIA peoples. We hypothesize that phenotypic convergence of the dark pigmentation in TIAs could have resulted from parallel (e.g., DDB1/DAK) or genetic convergence driven by admixture (e.g., MTHFD1 and RAD18), new mutations (e.g., STK11), or notably purifying selection (e.g., MC1R). Our results provide new insights into the initial peopling of Asia and an advanced understanding of the phenotypic convergence of the TIA peoples.

GWAS Identifies Multiple Genetic Loci for Skin Color in Korean Women

Human skin color is largely determined by genetic factors. Recent GWASs have reported several genetic variants associated with skin color, mostly in European and African populations. In this study, we performed GWAS in 17,019 Korean women to identify genetic variants associated with facial skin color, quantitatively measured as CIELAB color index. We identified variants in three, one, and six genomic loci associated with facial skin color index L∗, a∗, and b∗ values, respectively, and replicated the associations (combined analysis P-value < 5.0 × 10^-8). The significant loci included variants in known genes (OCA2 rs74653330, BNC2 rs16935073, rs72620727 near KITLG, and SLC6A17 rs6689641) and to our knowledge previously unreported genes (SCARB1 rs10846744, SYN2 rs12629034, and LINC00486 rs6543678). This is GWAS to elucidate genetic variants of facial skin color in a Korean female population. Further functional characterizations of the investigated genes are warranted to elucidate their contribution to skin pigmentation-related traits.

Measurement of Melanin Metabolism in Live Cells by [U-13C]-L-Tyrosine Fate Tracing Using Liquid Chromatography-Mass Spectrometry

Melanin synthesis occurs within a specialized organelle called the melanosome. Traditional methods for measuring melanin levels rely on the detection of chemical degradation products of melanin by high-performance liquid chromatography. Although these methods are robust, they are unable to distinguish between melanin synthesis and degradation and are best suited to measure melanin changes over long periods of time. We developed a method that actively measures both eumelanin and pheomelanin synthesis by fate tracing [U-13C] L-tyrosine using liquid chromatography-mass spectrometry. Using this method, we confirmed the previous reports of the differences in melanin synthesis between melanocytes derived from individuals with different skin colors and MC1R genotype and uncovered new information regarding the differential de novo synthesis of eumelanin and pheomelanin, also called mixed melanogenesis. We also revealed that distinct mechanisms that alter melanosomal pH differentially induce new eumelanin and pheomelanin synthesis. Finally, we revealed that the synthesis of L-3,4-dihydroxyphenylalanine, an important metabolite of L-tyrosine, is differentially controlled by multiple factors. Because L-tyrosine fate tracing is compatible with untargeted liquid chromatography-mass spectrometry‒based metabolomics, this approach enables the broad measurement of cellular metabolism in combination with melanin metabolism, and we anticipate that this approach will shed new light on multiple mechanisms of melanogenesis.

Single organelle measurements of melanosome pH using the novel ratiometric indicator RpHiMEL

Melanocytes are specialized cells that produce melanin pigments responsible for skin, hair, and eye pigmentation. The synthesis and storage of melanin occurs in unique lysosome-related organelles called melanosomes, which regulate melanin production via complex regulatory mechanisms. Maintenance of the melanosome luminal ionic environment and pH is crucial for proper function of the main melanogenic enzymes. Defects in genes encoding pH-regulating melanosomal proteins result in oculocutaneous albinism, which is characterized by hypopigmentation, impaired vision, and increased susceptibility to skin and eye cancers. We recently uncovered several ion channels and transporters that modulate melanin synthesis by acidifying or neutralizing the luminal pH of melanosomes. However, our understanding of how melanosomes and other related organelles maintain their luminal pH is far from complete. The study of melanosome pH regulation requires robust imaging and quantification tools. Despite recent advances in the development of such methods, many limitations remain, particularly for quantitative analysis of individual organelle pH. In this chapter, we will provide an overview of the available methods used for melanosome pH determination, including their advantages, limitations, and challenges. To address the critical, unmet need for reliable melanosome pH quantification tools, we engineered a novel genetically encoded, ratiometric pH sensor for melanosomes that we named RpHiMEL. Here, we describe the design and optimization of RpHiMEL, and provide a pH quantification method for individual melanosomes in live cells. We demonstrate that RpHiMEL is a highly versatile tool with the potential to advance our understanding of pH regulation in melanosomes and related organelles.

Novel immunological and genetic factors associated with vitiligo: A review

Vitiligo is a skin disorder characterized by depigmentation of the skin due to a lack of melanin. This condition affects men and woman of all ages and its incidence is not restricted by ethnicity or region. Vitiligo is a multifactorial disease, in which melanocytes, which serve important functions in skin pigmentation and immune processes, are impaired. There is sufficient evidence that immunological and genetic factors are primarily responsible for the destruction and dysfunction of melanocytes. Therefore, genetic DNA sequence variants that participate in skin homeostasis, pigmentation and immune response regulation, as well as altered expression patterns, may contribute to the risk of developing vitiligo. The current review presented an overview of the mechanism of pigmentation and of currently known factors involved in depigmentation, as well as the classification, epidemiology, associated comorbidities, risk factors, immunopathogenesis and several genetic and molecular changes associated with vitiligo.

Stimulation of melanin synthesis by UVB is mediated by NO/cGMP/PKG cascade targeting PAK4 in vitro

The coat color of mammals is primarily determined by the type, quantity, and distribution of melanin in the skin and hair. As an endogenous gas molecule, nitric oxide (NO) regulates tyrosinase production by modulating the cGMP-dependent protein kinase (PKG) pathway, which enhances melanin synthesis. However, some interrelationships have not been fully elucidated. In the present study, mouse melanocytes co-cultured with mouse keratinocytes in vitro, or as monocultures, were used as research models. The results indicated that ultraviolet B irradiation increased nitric oxide synthase (NOS) activity and NO production, and increased PKG, p21-activated kinase 4 (PAK4), and microphthalmia-associated transcription factor (MITF) levels, as well as tyrosinase (TYR), tyrosinase-related protein 1 and 2 expression, and melanin synthesis. During PKG inhibition, the expression of NO-regulated PAK4 and MITF was decreased. Pigment production was also affected, but remained higher than that in the control and NO inhibitor groups. These findings suggest that ultraviolet light regulates melanin production by activating the NO/cGMP/PKG pathway, which mediates the expression of PAK4, affecting melanin synthesis. On this basis, further elucidation of this regulatory network may improve our understanding of patterns of animal hair color formation.

Genome-wide association study in almost 195,000 individuals identifies 50 previously unidentified genetic loci for eye color

Human eye color is highly heritable, but its genetic architecture is not yet fully understood. We report the results of the largest genome-wide association study for eye color to date, involving up to 192,986 European participants from 10 populations. We identify 124 independent associations arising from 61 discrete genomic regions, including 50 previously unidentified. We find evidence for genes involved in melanin pigmentation, but we also find associations with genes involved in iris morphology and structure. Further analyses in 1636 Asian participants from two populations suggest that iris pigmentation variation in Asians is genetically similar to Europeans, albeit with smaller effect sizes. Our findings collectively explain 53.2% (95% confidence interval, 45.4 to 61.0%) of eye color variation using common single-nucleotide polymorphisms. Overall, our study outcomes demonstrate that the genetic complexity of human eye color considerably exceeds previous knowledge and expectations, highlighting eye color as a genetically highly complex human trait.

Evaluation of the effect of narrow band-ultraviolet B on the expression of tyrosinase, TYRP-1, and TYRP-2 mRNA in vitiligo skin and their correlations with clinical improvement: A retrospective study

Narrowband-ultraviolet B (NB-UVB) is considered one of the main therapeutic tools in vitiligo, which is able to induce repigmentation and halt depigmentation. However, little remains known about the effect of NB-UVB on TYR gene family, the main pigmentary genes, in vitiligo patients. To assess the effect of NB-UVB on expression of some genes related to the pigmentary problem of vitiligo; tyrosinase (TYR), tyrosinase related protein 1 (TYRP1) and tyrosinase related protein 2 (TYRP2), mRNA levels of those genes were quantitatively evaluated by Real-Time quantitative Polymerase Chain Reaction (RT-qPCR) in skin biopsies obtained from 30 patients with nonsegmental vitiligo and five healthy controls. Vitiligo patients were classified into two groups; group 1, involving 12 untreated vitiligo patients and group 2, including 18 vitiligo patients treated by NB-UVB. The levels of TYR, TYRP-1, and TYRP-2 mRNAs in untreated group were significantly lower than in control subjects (P < .001). In NB-UVB treated group, the three genes were significantly higher than in group 1 (P < .001), however, they were still significantly lower than in the control subjects (P < .001). A significant positive correlation was detected between TYR and TYRP-2 genes in group 2 (P = .03). This study demonstrated that mRNA level of TYR, TYRP-1, and TYRP-2, which decreased in vitiligo, was significantly increased upon treatment with NB-UVB. Accordingly, the mechanism of depigmentation in vitiligo disease and repigmentation by NB-UVB treatment may be related to the changes in the expression of these genes.

Genetics and Evolution of Mammalian Coat Pigmentation

The diversity of mammalian coat colors, and their potential adaptive significance, have long fascinated scientists as well as the general public. The recent decades have seen substantial improvement in our understanding of their genetic bases and evolutionary relevance, revealing novel insights into the complex interplay of forces that influence these phenotypes. At the same time, many aspects remain poorly known, hampering a comprehensive understanding of these phenomena. Here we review the current state of this field and indicate topics that should be the focus of additional research. We devote particular attention to two aspects of mammalian pigmentation, melanism and pattern formation, highlighting recent advances and outstanding challenges, and proposing novel syntheses of available information. For both specific areas, and for pigmentation in general, we attempt to lay out recommendations for establishing novel model systems and integrated research programs that target the genetics and evolution of these phenotypes throughout the Mammalia.

Iris Colour and the Risk of Developing Uveal Melanoma

Uveal melanoma (UM) is a global disease which especially occurs in elderly people. Its incidence varies widely between populations, with the highest incidence among Caucasians, and a South-to-North increase in Europe. As northern Europeans often have blond hair and light eyes, we wondered whether iris colour may be a predisposing factor for UM and if so, why. We compared the distribution of iris colour between Dutch UM patients and healthy Dutch controls, using data from the Rotterdam Study (RS), and reviewed the literature regarding iris colour. We describe molecular mechanisms that might explain the observed associations. When comparing a group of Dutch UM patients with controls, we observed that individuals from Caucasian ancestry with a green/hazel iris colour (Odds Ratio (OR) = 3.64, 95% Confidence Interval (CI) 2.57-5.14) and individuals with a blue/grey iris colour (OR = 1.38, 95% CI 1.04-1.82) had a significantly higher crude risk of UM than those with brown eyes. According to the literature, this may be due to a difference in the function of pheomelanin (associated with a light iris colour) and eumelanin (associated with a brown iris colour). The combination of light-induced stress and aging may affect pheomelanin-carrying melanocytes in a different way than eumelanin-carrying melanocytes, increasing the risk of developing a malignancy.

Transcriptome analysis of feather follicles reveals candidate genes and pathways associated with pheomelanin pigmentation in chickens

Yellow plumage is common in chickens, especially in breeds such as the Huiyang Bearded chicken, which is indigenous to China. We evaluated plumage colour distribution in F1, F2, and F3 populations of an Huiyang Bearded chicken × White Leghorn chicken cross, the heredity of the yellow plumage trait was distinguished from that of the gold plumage and other known plumage colours. Microscopic analysis of the feather follicles indicated that pheomelanin particles were formed in yellow but not in white feathers. To screen genes related to formation of the pheomelanin particles, we generated transcriptome data from yellow and white feather follicles from 7- and 11-week-old F3 chickens using RNA-seq. We identified 27 differentially expressed genes (DEGs) when comparing the yellow and white feather follicles. These DEGs were enriched in the Gene Ontology classes ‘melanosome’ and ‘melanosome organization’ related to the pigmentation process. Down-regulation of TYRP1, DCT, PMEL, MLANA, and HPGDS, verified using quantitative reverse transcription PCR, may lead to reduced eumelanin and increased pheomelanin synthesis in yellow plumage. Owing to the presence of the Dominant white locus, both white and yellow plumage lack eumelanin, and white feathers showed no pigments. Our results provide an understanding of yellow plumage formation in chickens.

KIT is involved in melanocyte proliferation, apoptosis and melanogenesis in the Rex Rabbit

BACKGROUND

Melanocytes play an extremely important role in the process of skin and coat colors in mammals which is regulated by melanin-related genes. Previous studies have demonstrated that KIT is implicated in the process of determining the color of the coat in Rex rabbits. However, the effect of KIT on the proliferation and apoptosis of melanocytes and melanogenesis has not been clarified.

METHODS

The mRNA and protein expression levels of KIT were quantified in different coat colored rabbits by qRT-PCR and a Wes assay. To identify whether KIT functions by regulating of melanogenesis, KIT overexpression and knockdown was conducted in melanocytes, and KIT mRNA expression and melanin-related genes TYR, MITF, PMEL and DCT were quantified by qRT-PCR. To further confirm whether KIT influences melanogenesis in melanocytes, melanin content was quantified using NaOH lysis after overexpression and knockdown of KIT. Melanocyte proliferation was estimated using a CCK-8 assay at 0, 24, 48 and 72 h after transfection, and the rate of apoptosis of melanocytes was measured by fluorescence-activated cell sorting.

RESULTS

KITmRNA and protein expression levels were significantly different in the skin of Rex rabbits with different color coats (P < 0.05), the greatest levels observed in those with black skin. The mRNA expression levels of KIT significantly affected the mRNA expression of the pigmentation-related genes TYR, MITF, PMEL and DCT (P < 0.01). Melanin content was evidently regulated by the change in expression patterns of KIT (P < 0.01). In addition, KIT clearly promoted melanocyte proliferation, but inhibited apoptosis.

CONCLUSIONS

Our results reveal that KIT is a critical gene in the regulation of melanogenesis, controlling proliferation and apoptosis in melanocytes, providing additional evidence for the mechanism of pigmentation of animal fur.

The Antimelanogenic Effect of Inularin Isolated from Flowers of Inula britannica on B16F10 Melanoma Cells and Zebrafish Embryos

In the search for novel, natural melanogenesis inhibitors, a new sesquiterpene, inularin, was isolated from the flowers of Inula britannica, and the structure was determined using spectroscopic and chemical methods. The antimelanogenic effects of inularin on B16F10 melanoma cells and zebrafish embryos were evaluated. Inularin dose-dependently reduced melanocyte-stimulating hormone-induced melanin production and L-DOPA oxidation in B16F10 cells. Zebrafish embryos were used to confirm the antimelanogenic activity. Inularin significantly decreased the pigmentation of embryos compared with untreated controls.

The expression of miR-129-5p and its target genes in the skin of goats

Coat color is one of the major quality traits of animals, and miR-129-5p acts as an important regulator for melanin biosynthesis in mammals. In this study, real-time PCR and western blotting were used to examine the expression of miR-129-5p and its targets genes in the skin of different coat color goats. The results showed that the expression of miR-129-5p in the skin samples of Inner Mongolia cashmere goats (IMCG) was higher than that of Dazu black goat (DBG). Also, the target genes (tyrosinase (TYR), frizzled 6 (FZD6) and glycogen synthase kinase 3β (GSK3β)) of miR-129-5p was highly expressed in the skin samples of DBG. The expression of miR-129-5p firstly increased and then decreased with age in F1 hybrid generation of DBG and IMCG. In addition, the expression of TYR decreased with age, while the expression of MITF increased with age but then decreased. The expression of FZD6 and GSK3β in the skin samples of F1 of different ages were irregular. Our results indicated that miR-129-5p mainly affects the formation of coat color of goats by decreasing the expression of TYR. This study suggests that miR-129-5p can act as a suppressor in the formation of coat color to lay the foundation for studying the effect of miR-129-5p on melanin synthesis.

Cutaneous pharmacologic cAMP induction induces melanization of the skin and improves recovery from ultraviolet injury in melanocortin 1 receptor-intact or heterozygous skin

Homozygous loss of function of the melanocortin 1 receptor (MC1R) is associated with a pheomelanotic pigment phenotype and increased melanoma risk. MC1R heterozygosity is less well studied, although individuals inheriting one loss-of-function MC1R allele are also melanoma-prone. Using the K14-Scf C57BL/6J animal model whose skin is characterized by lifelong retention of interfollicular epidermal melanocytes like that of the human, we studied pigmentary, UV responses, and DNA repair capacity in the skin of variant Mc1r background. Topical application of forskolin, a skin-permeable pharmacologic activator of cAMP induction to mimic native Mc1r signaling, increased epidermal eumelanin levels, increased the capacity of Mc1r-heterozygous skin to resist UV-mediated inflammation, and enhanced the skin's ability to clear UV photolesions from DNA. Interestingly, topical cAMP induction also promoted melanin accumulation, UV resistance, and accelerated clearance in Mc1r fully intact skin. Together, our findings suggest that heterozygous Mc1r loss is associated with an intermediately melanized and DNA repair-proficient epidermal phenotype and that topical cAMP induction enhances UV resistance in Mc1r-heterozygous or Mc1r-wild-type individuals by increasing eumelanin deposition and by improving nucleotide excision repair.

Comparative genomics provides new insights into the remarkable adaptations of the African wild dog (Lycaon pictus)

Within the Canidae, the African wild dog (Lycaon pictus) is the most specialized with regards to cursorial adaptations (specialized for running), having only four digits on their forefeet. In addition, this species is one of the few canids considered to be an obligate meat-eater, possessing a robust dentition for taking down large prey, and displays one of the most variable coat colorations amongst mammals. Here, we used comparative genomic analysis to investigate the evolutionary history and genetic basis for adaptations associated with cursoriality, hypercanivory, and coat color variation in African wild dogs. Genome-wide scans revealed unique amino acid deletions that suggest a mode of evolutionary digit loss through expanded apoptosis in the developing first digit. African wild dog-specific signals of positive selection also uncovered a putative mechanism of molar cusp modification through changes in genes associated with the sonic hedgehog (SHH) signaling pathway, required for spatial patterning of teeth, and three genes associated with pigmentation. Divergence time analyses suggest the suite of genomic changes we identified evolved ~1.7 Mya, coinciding with the diversification of large-bodied ungulates. Our results show that comparative genomics is a powerful tool for identifying the genetic basis of evolutionary changes in Canidae.

Pigment genes not skin pigmentation affect UVB-induced vitamin D

The long-term UVB induced serum 25(OH)D increase was influenced by pigment gene polymorphisms rather than measured facultative skin pigmentation.

Genetic variants associated with skin photosensitivity in a southern European population from Spain

BACKGROUND/PURPOSE

Recent GWAS studies, mostly performed in populations of North European origin, have identified the genetic loci associated with pigmentation, sun sensitivity, freckling and skin cancer susceptibility. Here, we aimed at addressing the genetic determinants of sunlight sensitivity in Spain, a southern European population.

METHODS

Nine SNPs located in 8 pigmentation-related genes (IRF4, TYR, ASP, HERC2, OCA2, BNC2, SLC24A4 and SLC45A2) were genotyped in 456 Spaniards. Additionally, the complete sequence of the MC1R gene was obtained, testing each nonsynonymous mutation supported by the classification as R or r alleles. A standardised questionnaire was used to collect demographic characteristics, pigmentation and sun sensitivity traits, as well as sun exposure habits.

RESULTS

MC1R R alleles and IRF4 rs12203592 were significantly associated with sunlight sensitivity at the Bonferroni-corrected level (P-value < 4.54 × 10^-3 ). Genetic variants in SLC45A2 (rs16891982) and HERC2 (rs12913832) were also found to be significantly associated with skin photosensitivity in our Spanish sample. Interaction analysis using the MDR method revealed epistatic effects when these four variants were considered together.

CONCLUSION

MC1R, IRF4, HERC2 and SLC45A2 play a significant role in skin sensitivity to sunlight in the Spanish population. Moreover, interaction among these four loci seems to modulate the ability of the skin to respond to UV radiation.

Hierarchical modeling of melanocortin 1 receptor variants with skin cancer risk

The human MC1R gene is highly polymorphic among lightly pigmented populations, and several variants in the MC1R gene have been associated with increased risk of both melanoma and nonmelanoma skin cancers. The functional consequences of MC1R gene variants have been studied in vitro and in vivo in postulated causal pathways, such as G-protein-coupled signaling transduction, pigmentation, immune response, inflammatory response, cell proliferation, and extracellular matrix adhesion. In a case-control study nested within the Nurses' Health Study, we utilized hierarchical modeling approaches, incorporating quantitative information from these functional studies, to examine the association between particular MC1R alleles and the risk of skin cancers. Different prior matrices were constructed according to the phenotypic associations in controls, cell surface expression, and enzymatic kinetics. Our results showed the parameter variance estimates of each single nucleotide polymorphism (SNP) were smaller when using a hierarchical modeling approach compared to standard multivariable regression. Estimates of second-level parameters gave information about the relative importance of MC1R effects on different pathways, and odds ratio estimates changed depending on prior models (e.g., the change ranged from -21% to 7% for melanoma risk assessment). In addition, the estimates of prior model hyperparameters in the hierarchical modeling approach allow us to determine the relevance of individual pathways on the risk of each of the skin cancer types. In conclusion, hierarchical modeling provides a useful analytic approach in addition to the widely used conventional models in genetic association studies that can incorporate measures of allelic function.

An In/Del mutation in upstream regulatory region of MC1R gene associated with grey lethal disease in grey Shiraz sheep (Persian lamb)

In the present study, MC1R, which play an important role in normal pigmentation in Skeen and wool, was candidate to assess the lethal grey disease in Persian lamb. Blood samples (50) were collected randomly from grey Shiraz sheep, and DNA was extracted by salting out method. One of these samples showing the disease was assigned as control case. Two pairs of specific primers of P1MC1R and P2MC1R were designed to amplify two fragments from upstream regulatory region (URR) and coding sequence (CDS) of MC1R gene. After genotyping by SSCPtechnique, samples from each banding patterns were sequenced and analyzed using BioEdit and DNASIS MAX softwares. Comparing sequences from control (sick lamb) with healthy ones showed different haplotype in productsof each specific primer pairs. The multiple alignments revealed a 26 bp In/Del occurring at PCR product of P1MC1R gene in control case which was not observed in other studied lambs, and also 7 and 11 different positions were seen between sequence amplified by P1MC1R and P2MC1R primers, respectively. Further bioinformatics analysis showed that 26 bp insertion/deletion (In/Del) occurred in control case P1MC1R sequence caused deletion of gamma_IRE_CS and LBP_1_RS motifs from URR of control case. It seems this 26 bp In/Del mutation might have changed its expression and can be potential cause of grey lethal disease in studied sheep breed.

Genome-wide association meta-analysis of individuals of European ancestry identifies new loci explaining a substantial fraction of hair color variation and heritability

Hair color is one of the most recognizable visual traits in European populations and is under strong genetic control. Here we report the results of a genome-wide association study meta-analysis of almost 300,000 participants of European descent. We identified 123 autosomal and one X-chromosome loci significantly associated with hair color; all but 13 are novel. Collectively, single-nucleotide polymorphisms associated with hair color within these loci explain 34.6% of red hair, 24.8% of blond hair, and 26.1% of black hair heritability in the study populations. These results confirm the polygenic nature of complex phenotypes and improve our understanding of melanin pigment metabolism in humans.

Genome-Wide Association Studies Identify Candidate Genes for Coat Color and Mohair Traits in the Iranian Markhoz Goat

The Markhoz goat provides an opportunity to study the genetics underlying coat color and mohair traits of an Angora type goat using genome-wide association studies (GWAS). This indigenous Iranian breed is valued for its quality mohair used in ceremonial garments and has the distinction of exhibiting an array of coat colors including black, brown, and white. Here, we performed 16 GWAS for different fleece (mohair) traits and coat color in 228 Markhoz goats sampled from the Markhoz Goat Research Station in Sanandaj, Kurdistan province, located in western Iran using the Illumina Caprine 50K beadchip. The Efficient Mixed Model Linear analysis was used to identify genomic regions with potential candidate genes contributing to coat color and mohair characteristics while correcting for population structure. Significant associations to coat color were found within or near the ASIP, ITCH, AHCY, and RALY genes on chromosome 13 for black and brown coat color and the KIT and PDGFRA genes on chromosome 6 for white coat color. Individual mohair traits were analyzed for genetic association along with principal components that allowed for a broader perspective of combined traits reflecting overall mohair quality and volume. A multitude of markers demonstrated significant association to mohair traits highlighting potential candidate genes of POU1F1 on chromosome 1 for mohair quality, MREG on chromosome 2 for mohair volume, DUOX1 on chromosome 10 for yearling fleece weight, and ADGRV1 on chromosome 7 for grease percentage. Variation in allele frequencies and haplotypes were identified for coat color and differentiated common markers associated with both brown and black coat color. This demonstrates the potential for genetic markers to be used in future breeding programs to improve selection for coat color and mohair traits. Putative candidate genes, both novel and previously identified in other species or breeds, require further investigation to confirm phenotypic causality and potential epistatic relationships.

Capsaicin reverses the inhibitory effect of licochalcone A/β-Arbutin on tyrosinase expression in b16 mouse melanoma cells

Introduction:

Melanin is synthesized by melanocytes, which are located in the basal layer of the skin. After synthesis, melanin is further deposited on the surface of the skin to form black spots or chloasma. Tyrosinase is a rate-limiting enzyme that plays an important role in melanogenesis. Currently, there are many drugs that inhibit tyrosinase expression to further reduce melanogenesis. Nevertheless, some of these could reverse the pharmacological effect of other drugs, when used simultaneously.

Materials and Methods:

B16 mouse melanoma cells were treated with the tyrosinase inhibitors licochalcone A and β-arbutin, alone or in combination with capsaicin, an alkaloid found in peppers. Cytotoxicity, melanin content, and tyrosinase activity and expression were determined.

Results:

Licochalcone A/β-arbutin inhibited tyrosinase expression and further hindered melanin synthesis when applied individually to B16 mouse melanoma cells. However, licochalcone A/β-arbutin combined with 50 μmol/L capsaicin enhanced the expression of tyrosinase in these cells and further increased melanin content.

Conclusion:

Our data implied that capsaicin could reverse the inhibitory effect of licochalcone A/β-arbutin on tyrosinase expression in B16 mouse melanoma cells.

SUMMARY

B16 mouse melanoma cells were treated with the tyrosinase inhibitors licochalcone A and β-arbutin, alone or in combination with capsaicin, an alkaloid found in peppers. Cytotoxicity, melanin content, and tyrosinase activity and expression were determined. Licochalcone A/β-arbutin inhibited tyrosinase expression and further hindered melanin synthesis when applied individually to B16 mouse melanoma cells. However, licochalcone A/β-arbutin combined with 50 μmol/L capsaicin enhanced the expression of tyrosinase in these cells and further increased melanin content. Our research implied that capsaicin could reverse the inhibitory effect of licochalcone A/β-arbutin on tyrosinase expression in B16 mouse melanoma cells.

Abbreviations used: B16: B16 mouse melanoma cells; L-DOPA: 3, 4-L-dihydroxyphenylalanine; TYR: Tyrosinase; USP: United States Pharmacopeia; FBS: Fetal bovine serum; EDTA: Ethylenediaminetetraacetic acid; DMSO: Dimethyl sulfoxide; RPMI: Roswell Park Memorial Institute; MTT3: 4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide, NaOH: Sodium hydroxide; PBS: Phosphate-buffered saline; RIPA: Radio-immunoprecipitation assay; PMSF: Phenylmethanesulfonyl fluoride or phenylmethylsulfonyl fluoride; SDS: Sodium dodecyl sulfate, sodium salt; PVDF: Polyvinylidene fluoride; ECL: Enhanced chemiluminescence.

The Contribution of Neanderthals to Phenotypic Variation in Modern Humans

Assessing the genetic contribution of Neanderthals to non-disease phenotypes in modern humans has been difficult because of the absence of large cohorts for which common phenotype information is available. Using baseline phenotypes collected for 112,000 individuals by the UK Biobank, we can now elaborate on previous findings that identified associations between signatures of positive selection on Neanderthal DNA and various modern human traits but not any specific phenotypic consequences. Here, we show that Neanderthal DNA affects skin tone and hair color, height, sleeping patterns, mood, and smoking status in present-day Europeans. Interestingly, multiple Neanderthal alleles at different loci contribute to skin and hair color in present-day Europeans, and these Neanderthal alleles contribute to both lighter and darker skin tones and hair color, suggesting that Neanderthals themselves were most likely variable in these traits.

Regulatory pathway analysis of coat color genes in Mongolian horses

BACKGROUND

Studies on the molecular genetics of horse skin pigmentation have typically focused on very few genes and proteins. In this study, we used Illumina sequencing to determine the global gene expression profiles in horses with white-colored coats and those with black-colored coats, with the goal of identifying novel genes that could regulate horse coat color.

RESULTS

Genes encoding ribosomal-associated proteins were highly expressed in horse skin. We found a total of 231 unigenes that were differentially expressed between horses with white coats and horses with black coats; 119 were down-regulated, and 112 were up-regulated. Many of the up-regulated genes in black horses, such as genes related to tyrosine metabolism, may directly regulate dark coat color. Keratin genes, MIA family genes, fatty acid-related genes, and melanoma-associated genes were also differentially regulated, which suggests that they may play important roles in coat color formation.

CONCLUSIONS

These findings show that the transcription profiles from white and black horse skin provide useful information to understand the genetics underlying the control of skin melanin synthesis in horses, which may enhance our knowledge of human skin diseases, such as melanoma and albinism.

Identifying signatures of positive selection in pigmentation genes in two South Asian populations

OBJECTIVES

Skin pigmentation is a polygenic trait showing wide phenotypic variations among global populations. While numerous pigmentation genes have been identified to be under positive selection among European and East populations, genes contributing to phenotypic variation in skin pigmentation within and among South Asian populations are still poorly understood. The present study uses data from the Phase 3 of the 1000 genomes project focusing on two South Asian populations-GIH (Gujarati Indian from Houston, Texas) and ITU (Indian Telugu from UK), so as to decode the genetic architecture involved in adaptation to ultraviolet radiation in South Asian populations.

METHODS

Statistical tests included were (1) tests to identify deviations of the Site Frequency Spectrum (SFS) from neutral expectations (Tajima's D, Fay and Wu's H and Fu and Li's D* and F*), (2) tests focused on the identification of high-frequency haplotypes with extended linkage disequilibrium (iHS and Rsb), and (3) tests based on genetic differentiation between populations (LSBL).

RESULTS

Twenty-two pigmentation genes fall in the top 1% for at least one statistic in the GIH population, 5 of which (LYST, OCA2, SLC24A5, SLC45A2, and TYR) have been previously associated with normal variation in skin, hair, or eye color. In comparison, 17 genes fall in the top 1% for at least one statistic in the ITU population. Twelve loci which are identified as outliers in the ITU scan were also identified in the GIH population.

CONCLUSIONS

These results suggest that selection may have affected these loci broadly across the region.

Identification of a novel locus associated with skin colour in African-admixed populations

Skin pigmentation is a complex trait that varies largely among populations. Most genome-wide association studies of this trait have been performed in Europeans and Asians. We aimed to uncover genes influencing skin colour in African-admixed individuals. We performed a genome-wide association study of melanin levels in 285 Hispanic/Latino individuals from Puerto Rico, analyzing 14 million genetic variants. A total of 82 variants with p-value ≤1 × 10⁻⁵ were followed up in 373 African Americans. Fourteen single nucleotide polymorphisms were replicated, of which nine were associated with skin colour at genome-wide significance in a meta-analysis across the two studies. These results validated the association of two previously known skin pigmentation genes, SLC24A5 (minimum p = 2.62 × 10⁻¹⁴, rs1426654) and SLC45A2 (minimum p = 9.71 × 10⁻¹⁰, rs16891982), and revealed the intergenic region of BEND7 and PRPF18 as a novel locus associated with this trait (minimum p = 4.58 × 10⁻⁹, rs6602666). The most significant variant within this region is common among African-descent populations but not among Europeans or Native Americans. Our findings support the advantages of analyzing African-admixed populations to discover new genes influencing skin pigmentation.

Refining the ideas of "ethnic" skin

Skin disease occur worldwide, affecting people of all nationalities and all skin types. These diseases may have a genetic component and may manifest differently in specific population groups; however, there has been little study on this aspect. If population-based differences exist, it is reasonable to assume that understanding these differences may optimize treatment. While there is a relative paucity of information about similarities and differences in skin diseases around the world, the knowledge-base is expanding. One challenge in understanding population-based variations is posed by terminology used in the literature: including ethnic skin, Hispanic skin, Asian skin, and skin of color. As will be discussed in this article, we recommend that the first three descriptors are no longer used in dermatology because they refer to nonspecific groups of people. In contrast, "skin of color" may be used - perhaps with further refinements in the future - as a term that relates to skin biology and provides relevant information to dermatologists.

Novel quantitative pigmentation phenotyping enhances genetic association, epistasis, and prediction of human eye colour

Success of genetic association and the prediction of phenotypic traits from DNA are known to depend on the accuracy of phenotype characterization, amongst other parameters. To overcome limitations in the characterization of human iris pigmentation, we introduce a fully automated approach that specifies the areal proportions proposed to represent differing pigmentation types, such as pheomelanin, eumelanin, and non-pigmented areas within the iris. We demonstrate the utility of this approach using high-resolution digital eye imagery and genotype data from 12 selected SNPs from over 3000 European samples of seven populations that are part of the EUREYE study. In comparison to previous quantification approaches, (1) we achieved an overall improvement in eye colour phenotyping, which provides a better separation of manually defined eye colour categories. (2) Single nucleotide polymorphisms (SNPs) known to be involved in human eye colour variation showed stronger associations with our approach. (3) We found new and confirmed previously noted SNP-SNP interactions. (4) We increased SNP-based prediction accuracy of quantitative eye colour. Our findings exemplify that precise quantification using the perceived biological basis of pigmentation leads to enhanced genetic association and prediction of eye colour. We expect our approach to deliver new pigmentation genes when applied to genome-wide association testing.

Comparative Transcriptome Analysis of Raccoon Dog Skin to Determine Melanin Content in Hair and Melanin Distribution in Skin

The raccoon dog (Nyctereutes procyonoides) is an important canid fur-bearing animal species worldwide. Chinese raccoon dogs that present a white mutation, especially those with a white coat. Exploring melanin biosynthesis in the hair and skin of raccoon dogs is important for understanding the survival and evolutionary mechanisms of them. In this study, we measured the content of melanin in the hair of two types of raccoon dog and generated stained slices of skin tissue. The results indicated that melanin biosynthesis occurs in the wild-type (W) and white-type (B) raccoon dog skin, although less melanin is produced in B skin. We then sequenced the skin transcriptomes of W and B, compared the similarities and differences in expressed genes. A comparison of the gene expression showed 60 up-regulated genes and 127 down-regulated genes in B skin. We analyzed the unigenes and pathways related to the melanogenesis pathway and found that TYR, TYRP1, MC1R, SLC24a5, SLC45a2 and OCA2 were significantly down-regulated in B skin and these results were verified via qRT-PCR. We surmised that the phenotypic characteristics of the white mutation might be caused by the reduced expression of these genes and this finding provides new insights for future experiments in raccoon dogs.

Anti-Melanogenic Effect of Oenothera laciniata Methanol Extract in Melan-a Cells

We evaluated the antioxidant activity and anti-melanogenic effects of Oenothera laciniata methanol extract (OLME) in vitro by using melan-a cells. The total polyphenol and flavonoid content of OLME was 66.3 and 19.0 mg/g, respectively. The electron-donating ability, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical-scavenging activity, and superoxide dismutase (SOD)-like activity of OLME (500 μg/mL) were 94.5%, 95.6%, and 63.6%, respectively. OLME and arbutin treatment at 50 μg/mL significantly decreased melanin content by 35.5% and 14.2%, respectively, compared to control (p < 0.05). OLME and arbutin treatment at 50 μg/mL significantly inhibited intra-cellular tyrosinase activity by 22.6% and 12.6%, respectively, compared to control (p < 0.05). OLME (50 μg/mL) significantly decreased tyrosinase, tyrosinase-related protein-1 (TRP-1), TRP-2, and microphthalmia-associated transcription factor-M (MITF-M) mRNA expression by 57.1%, 67.3%, 99.0%, and 77.0%, respectively, compared to control (p < 0.05). Arbutin (50 μg/mL) significantly decreased tyrosinase, TRP-1, and TRP-2 mRNA expression by 24.2%, 42.9%, and 48.5%, respectively, compared to control (p < 0.05). However, arbutin (50 μg/mL) did not affect MITF-M mRNA expression. Taken together, OLME showed a good antioxidant activity and anti-melanogenic effect in melan-a cells that was superior to that of arbutin, a well-known skin-whitening agent. The potential mechanism underlying the anti-melanogenic effect of OLME was inhibition of tyrosinase activity and down-regulation of tyrosinase, TRP-1, TRP-2, and MITF-M mRNA expression.