Thr40 and Met122 are new partial loss-of-function natural mutations of the human melanocortin 1 receptor

Melanocortin-1 receptor mutations and pigmentation: Insights from large animals

The melanocortin-1 receptor (MC1R) is a G protein-coupled receptor expressed in cutaneous and hair follicle melanocytes, and plays a central role in coat color determination in vertebrates. Numerous MC1R variants have been identified in diverse species. Some of these variants have been associated with specific hair and skin color phenotypes in humans as well as coat color in animals. Gain-of-function mutations of the MC1R gene cause dominant or partially dominant black/dark coat color, and loss-of-function mutations of the MC1R gene cause recessive or partially recessive red/yellow/pale coat color phenotypes. These have been well documented in a large number of mammals, including human, dog, cattle, horse, sheep, pig, and fox. Higher similarities between large mammals and humans makes them better models to understand pathogenesis of human diseases caused by MC1R mutations. High identities in MC1Rs and similar variants identified in both humans and large mammals also provide an opportunity for receptor structure and function study. In this review, we aim to summarize the naturally occurring mutations of MC1R in humans and large animals.

A genome-wide scan study identifies a single nucleotide substitution in MC1R gene associated with white coat colour in fallow deer (Dama dama)

Background

The coat colour of fallow deer is highly variable and even white animals can regularly be observed in game farming and in the wild. Affected animals do not show complete albinism but rather some residual pigmentation resembling a very pale beige dilution of coat colour. The eyes and claws of the animals are pigmented. To facilitate the conservation and management of such animals, it would be helpful to know the responsible gene and causative variant. We collected 102 samples from 22 white animals and from 80 animals with wildtype coat colour. The samples came from 12 different wild flocks or game conservations located in different regions of Germany, at the border to Luxembourg and in Poland. The genomes of one white hind and her brown calf were sequenced.

Results

Based on a list of colour genes of the International Federation of Pigment Cell Societies (http://www.ifpcs.org/albinism/), a variant in the MC1R gene (NM_174108.2:c.143 T > C) resulting in an amino acid exchange from leucine to proline at position 48 of the MC1R receptor protein (NP_776533.1:p.L48P) was identified as a likely cause of coat colour dilution. A gene test revealed that all animals of the white phenotype were of genotype CC whereas all pigmented animals were of genotype TT or TC. The study showed that 14% of the pigmented (brown or dark pigmented) animals carried the white allele.

Conclusions

A genome-wide scan study led to a molecular test to determine the coat colour of fallow deer. Identification of the MC1R gene provides a deeper insight into the mechanism of dilution. The gene marker is now available for the conservation of white fallow deer in wild and farmed animals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00950-3.

Prediction of the damage-associated non-synonymous single nucleotide polymorphisms in the human MC1R gene

The melanocortin 1 receptor (MC1R) is involved in the control of melanogenesis. Polymorphisms in this gene have been associated with variation in skin and hair color and with elevated risk for the development of melanoma. Here we used 11 computational tools based on different approaches to predict the damage-associated non-synonymous single nucleotide polymorphisms (nsSNPs) in the coding region of the human MC1R gene. Among the 92 nsSNPs arranged according to the predictions 62% were classified as damaging in more than five tools. The classification was significantly correlated with the scores of two consensus programs. Alleles associated with the red hair color (RHC) phenotype and with the risk of melanoma were examined. The R variants D84E, R142H, R151C, I155T, R160W and D294H were classified as damaging by the majority of the tools while the r variants V60L, V92M and R163Q have been predicted as neutral in most of the programs The combination of the prediction tools results in 14 nsSNPs indicated as the most damaging mutations in MC1R (L48P, R67W, H70Y, P72L, S83P, R151H, S172I, L206P, T242I, G255R, P256S, C273Y, C289R and R306H); C273Y showed to be highly damaging in SIFT, Polyphen-2, MutPred, PANTHER and PROVEAN scores. The computational analysis proved capable of identifying the potentially damaging nsSNPs in MC1R, which are candidates for further laboratory studies of the functional and pharmacological significance of the alterations in the receptor and the phenotypic outcomes.

A large French case-control study emphasizes the role of rare Mc1R variants in melanoma risk

BACKGROUND

The MC1R gene implicated in melanogenesis and skin pigmentation is highly polymorphic. Several alleles are associated with red hair and fair skin phenotypes and contribute to melanoma risk.

OBJECTIVE

This work aims to assess the effect of different classes of MC1R variants, notably rare variants, on melanoma risk. Methods. MC1R coding region was sequenced in 1131 melanoma patients and 869 healthy controls. MC1R variants were classified as RHC (R) and non-RHC (r). Rare variants (frequency < 1%) were subdivided into two subgroups, predicted to be damaging (D) or not (nD).

RESULTS

Both R and r alleles were associated with melanoma (OR = 2.66 [2.20-3.23] and 1.51 [1.32-1.73]) and had similar population attributable risks (15.8% and 16.6%). We also identified 69 rare variants, of which 25 were novel. D variants were strongly associated with melanoma (OR = 2.38 [1.38-4.15]) and clustered in the same MC1R domains as R alleles (intracellular 2, transmembrane 2 and 7).

CONCLUSION

This work confirms the role of R and r alleles in melanoma risk in the French population and proposes a novel class of rare D variants as important melanoma risk factors. These findings may improve the definition of high-risk subjects that could be targeted for melanoma prevention and screening.

Colorful DNA polymorphisms in humans

In this review article we summarize current knowledge on how variation on the DNA level influences human pigmentation including color variation of iris, hair, and skin. We review recent progress in the field of human pigmentation genetics by focusing on the genes and DNA polymorphisms discovered to be involved in determining human pigmentation traits, their association with diseases particularly skin cancers, and their power to predict human eye, hair, and skin colors with potential utilization in forensic investigations.

Differential and competitive regulation of human melanocortin 1 receptor signaling by β-arrestin isoforms

The melanocortin 1 receptor (MC1R) is a G protein-coupled receptor (GPCR) crucial for the regulation of melanocyte proliferation and differentiation. MC1R activation by melanocortin hormones triggers the cAMP pathway and stimulates the extracellular signal-regulated protein kinases ERK1 and ERK2 to promote synthesis of photoprotective eumelanin pigments among other effects. Signaling from most GPCRs is regulated by the β-arrestin (ARRB) family of cytosolic multifunctional adaptor proteins which mediate signal termination and endocytosis of GPCR-agonist complexes. The ubiquitously expressed non-visual β-arrestin1 (ARRB1) and β-arrestin2 (ARRB2) are highly homologous but not functionally equivalent. Their role in the regulation of MC1R is unknown. Using a combination of co-immunoprecipitation, gel filtration chromatography, confocal microscopy, siRNA-mediated knockdown and functional assays, we demonstrated agonist-independent competitive interactions of ARRB1 and ARRB2 with MC1R, which might also be independent of phosphorylation of MC1R C-terminal Ser/Thr residues. The effects of ARRBs were isoform-specific. ARRB2 inhibited MC1R agonist-dependent cAMP production but not ERK activation, stimulated internalization and showed prolonged co-localization with the receptor in endocytic vesicles. Conversely, ARRB1 had no effect on internalization or functional coupling, but competed with ARRB2 for binding MC1R, which might increase signaling by displacement of inhibitory ARRB2. These data suggest a novel mechanism of MC1R functional regulation based on the relative expression of ARRB isoforms, with possible activatory ARRB1-dependent effects arising from partial relief of inhibitory ARRB2-MC1R interactions. Thus, competitive displacement of inhibitory ARRBs by functionally neutral ARRB isoforms might exert a paradigm-shifting signal-promoting effect to fine-tune signaling downstream of certain GPCRs.

Functional status and relationships of melanocortin 1 receptor signaling to the cAMP and extracellular signal-regulated protein kinases 1 and 2 pathways in human melanoma cells

Melanocortin 1 receptor (MC1R), a major determinant of skin phototype frequently mutated in melanoma, is a Gs protein-coupled receptor that regulates pigment production in melanocytes. MC1R stimulation activates cAMP synthesis and the extracellular signal-regulated (ERK) ERK1 and ERK2. In human melanocytes, ERK activation by MC1R relies on cAMP-independent transactivation of the c-KIT receptor. Thus MC1R functional coupling to the cAMP and ERK pathways may involve different structural requirements giving raise to biased effects of skin cancer-associated mutations. We evaluated the impact of MC1R mutations on ERK activation, cAMP production and agonist binding. We found that MC1R mutations impair cAMP production much more often than ERK activation, suggesting less stringent requirements for functional coupling to the ERK pathway. We examined the crosstalk of the cAMP and ERK pathways in HBL human melanoma cells (wild-type for MC1R, NRAS and BRAF). ERK activation by constitutively active upstream effectors or pharmacological inhibition had little effect on MC1R-stimulated cAMP synthesis. High cAMP levels were compatible with normal ERK activation but, surprisingly, the adenylyl cyclase activator forskolin abolished ERK activation by MC1R, most likely by a cAMP-independent mechanism. These results indicate little crosstalk of the cAMP and ERK pathways in HBL melanoma cells. Finally, we studied cAMP accumulation in a panel of 22 human melanoma cell lines stimulated with MC1R agonists or forskolin. cAMP synthesis was often inhibited, even in cells wild-type for MC1R and NRAS. Therefore, the cAMP pathway is more frequently impaired in melanoma than could be predicted by the MC1R or NRAS genotype.

Melanocortin-1 receptor, skin cancer and phenotypic characteristics (M-SKIP) project: study design and methods for pooling results of genetic epidemiological studies

Background

For complex diseases like cancer, pooled-analysis of individual data represents a powerful tool to investigate the joint contribution of genetic, phenotypic and environmental factors to the development of a disease. Pooled-analysis of epidemiological studies has many advantages over meta-analysis, and preliminary results may be obtained faster and with lower costs than with prospective consortia.

Design and methods

Based on our experience with the study design of the Melanocortin-1 receptor (MC1R) gene, SKin cancer and Phenotypic characteristics (M-SKIP) project, we describe the most important steps in planning and conducting a pooled-analysis of genetic epidemiological studies. We then present the statistical analysis plan that we are going to apply, giving particular attention to methods of analysis recently proposed to account for between-study heterogeneity and to explore the joint contribution of genetic, phenotypic and environmental factors in the development of a disease. Within the M-SKIP project, data on 10,959 skin cancer cases and 14,785 controls from 31 international investigators were checked for quality and recoded for standardization. We first proposed to fit the aggregated data with random-effects logistic regression models. However, for the M-SKIP project, a two-stage analysis will be preferred to overcome the problem regarding the availability of different study covariates. The joint contribution of MC1R variants and phenotypic characteristics to skin cancer development will be studied via logic regression modeling.

Discussion

Methodological guidelines to correctly design and conduct pooled-analyses are needed to facilitate application of such methods, thus providing a better summary of the actual findings on specific fields.

ASSESSMENT OF MC1R AND α-MSH GENE SEQUENCES IN IRANIAN VITILIGO PATIENTS

Background:

Vitiligo is an acquired pigmentary disorder of the skin that is caused by unknown factors and is characterized by white and depigmented patches that enlarge and become more numerous with time. Genetic factors, oxidative stress, autoimmunity, and neurochemical agents, such as catecholamines might also contribute to vitiligo. Cutaneous pigmentation is determined by the amounts of eumelanin and pheomelanin synthesized by the epidermal melanocytes and interference of melanocortin-1 receptor (MC1R), a G-protein coupled receptor, its normal agonist, alpha-melanocyte stimulating hormone (α-MSH), and key enzymes, such as tyrosinase, to protect against sun-induced DNA damage. The MC1R, a 7 pass trans-membrane G-protein coupled receptor, is a key control point in melanogenesis. Loss-of-function mutations at the MC1R are associated with a switch from eumelanin to pheomelanin production, resulting in a red or yellow coat color.

Aim:

In this research, we aim to examine the genetic variety of MC1R and α-MSH gene in 20 Iranian vitiligo patients and 20 healthy controls.

Materials and Methods:

Analysis of the MC1R coding gene was performed with direct sequencing.

Results:

We found the following 9 MC1R coding region variants: Arg163Gl (G488A), Arg227Leu (G680A), Val 97Phe (G289T), Asp184Asn (G550A), Arg227Lys (G680A), Arg142His (G425A), Val60Leu (G178T), Val247Met (C739A), and Val174Ile (G520A). We also found 2 frameshift changes: one of them was the Insertion of C (frameshift in Pro136, stop at Trp148) and the other, Insertion of G (frameshift in Pro256, stop at Trp 333). Of all the changes, the most common was Val60Leu at 5% in patients vs 20% in controls, Val247Met at 15% in patients vs 0% in controls and Val174Ile at 15% in controls and 0% in patients. The other variants showed a frequency <5% in both patients and controls. Also in this study, we have examined the frequency of single nucleotide polymorphisms within the α-MSH genes with direct sequencing in 20 patients and 20 healthy subjects but found no changes along this gene.

Conclusion:

We could not find any relationship between MC1R and α-MSH genes and their effect on the disease in Iranian vitiligo patients.

N-glycosylation of the human melanocortin 1 receptor: occupancy of glycosylation sequons and functional role

The melanocortin 1 receptor (MC1R), a major determinant of skin pigmentation and phototype, mediates the actions of α-melanocyte-stimulating hormone on melanocytes and is critical for melanocyte proliferation and differentiation. MC1R has two putative N-glycosylation targets, Asn15 and Asn29. It has been shown that MC1R is a glycoprotein with an unusual sensitivity to endoglycosidase H digestion. However, the occupancy and functional importance of each specific glycosylation sequon remains unknown. We demonstrate that MC1R is N-glycosylated at Asn15 and Asn29, with structurally and functionally different glycan chains. N-glycosylation is not necessary for high affinity agonist binding or functional coupling but has a strong effect on the availability of MC1R molecules on the plasma membrane, most likely by a combination of improved forward trafficking and decreased internalization. Finally, we found that MC1R variants exhibit different degrees of glycosylation which do not show a simple correlation with their functional status or intracellular trafficking.

The melanocortin-1 receptor gene polymorphism and association with human skin cancer

The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

Mahogunin ring finger-1 (MGRN1) E3 ubiquitin ligase inhibits signaling from melanocortin receptor by competition with Galphas

Mahogunin ring finger-1 (MGRN1) is a RING domain-containing ubiquitin ligase mutated in mahoganoid, a mouse mutation causing coat color darkening, congenital heart defects, high embryonic lethality, and spongiform neurodegeneration. The melanocortin hormones regulate pigmentation, cortisol production, food intake, and body weight by signaling through five G protein-coupled receptors positively coupled to the cAMP pathway (MC1R-MC5R). Genetic analysis has shown that mouse Mgrn1 is an accessory protein for melanocortin signaling that may inhibit MC1R and MC4R by unknown mechanisms. These melanocortin receptors (MCRs) regulate pigmentation and body weight, respectively. We show that human melanoma cells express 4 MGRN1 isoforms differing in the C-terminal exon 17 and in usage of exon 12. This exon contains nuclear localization signals. MGRN1 isoforms decreased MC1R and MC4R signaling to cAMP, without effect on beta(2)-adrenergic receptor. Inhibition was independent on receptor plasma membrane expression, ubiquitylation, internalization, or stability and occurred upstream of Galpha(s) binding to/activation of adenylyl cyclase. MGRN1 co-immunoprecipitated with MCRs, suggesting a physical interaction of the proteins. Significantly, overexpression of Galpha(s) abolished the inhibitory effect of MGRN1 and decreased co-immunoprecipitation with MCRs, suggesting competition between MGRN1 and Galpha(s) for binding to MCRs. Although all MGRN1s were located in the cytosol in the absence of MCRs, exon 12-containing isoforms accumulated in the nuclei upon co-expression with the receptors. Therefore, MGRN1 inhibits MCR signaling by a new mechanism involving displacement of Galpha(s), thus accounting for key features of the mahoganoid phenotype. Moreover, MGRN1 might provide a novel pathway for melanocortin signaling from the cell surface to the nucleus.

Identification and functional analysis of novel variants of the human melanocortin 1 receptor found in melanoma patients

The melanocortin 1 receptor, a Gs protein-coupled receptor expressed in epidermal melanocytes, is a major determinant of skin pigmentation and phototype and an important contributor to melanoma risk. MC1R activation stimulates synthesis of black, strongly photoprotective eumelanin pigments. Several MC1R alleles are associated with red hair, fair skin, increased sensitivity to ultraviolet radiation, and increased skin cancer risk. The MC1R gene is highly polymorphic, but only a few naturally occurring alleles have been functionally characterized, which complicates the establishment of accurate correlations between the signaling properties of mutant alleles and defined cutaneous phenotypes. We report the functional characterization of six MC1R alleles found in Spanish melanoma patients. Two variants (c.152T>C, p.Val51Ala and c.865T>C, p.Cys289Arg) have never been described, and the others (c.112G>A, p.Val38Met; c.122C>T, p.Ser41Phe; c.383T>C, p.Met128Thr; and c.842A>G, p.Asn281Ser) have not been analyzed for function. p.Asn281Ser corresponds to a functionally silent polymorphism. The other mutations are associated with varying degrees of loss of function (LOF), from moderate decreases in coupling to the cAMP pathway (p.Val38Met and p.Val51Ala) to nearly complete absence of functional coupling (p.Ser41Phe, p.Met128Thr, and p.Cys289Arg). The LOF p.Met128Thr and p.Cys289Arg mutants are trafficked to the cell surface, but are unable to bind agonists efficiently. Conversely, LOF of p.Val38Met, p.Ser41Phe, and p.Val51Ala is due to reduced cell surface expression as a consequence of retention in the endoplasmic reticulum (ER). Therefore, LOF of MC1R alleles is frequently associated with aberrant forward trafficking and accumulation within the ER or with inability to bind properly the activatory ligand.

Aberrant trafficking of human melanocortin 1 receptor variants associated with red hair and skin cancer: Steady-state retention of mutant forms in the proximal golgi

The melanocortin 1 receptor (MC1R), a Gs protein-coupled receptor (GPCR) expressed in melanocytes, is a major determinant of skin pigmentation and phototype. MC1R activation stimulates melanogenesis and increases the ratio of black, strongly photoprotective eumelanins to reddish, poorly photoprotective pheomelanins. Several MC1R alleles are associated with red hair, fair skin, increased sensitivity to ultraviolet radiation (the RHC phenotype) and increased skin cancer risk. Three highly penetrant RHC variants, R151C, R160W, and D294H are loss-of-function MC1R mutants with altered cell surface expression. In this study, we show that forward trafficking was normal for D294H. Conversely, export traffic was impaired for R151C, which accumulated in the endoplasmic reticulum (ER), and for R160W, which was enriched in the cis-Golgi. This is the first report of steady-state retention in a post-ER secretory compartment of a GPCR mutant found in the human population. Residues R151 and R160 are located in the MC1R second intracellular loop (il2). Two other mutations in il2, T157A preventing T157 phosphorylation and R162P disrupting a (160)RARR(163) motif, also caused intracellular retention. Moreover, T157 was phosphorylated in wild-type MC1R and a T157D mutation mimicking constitutive phosphorylation allowed normal traffic, and rescued the retention phenotype of R160W and R162P. Therefore, MC1R export is likely regulated by T157 phosphorylation and the (160)RARR(163) arginine-based motif functions as an ER retrieval signal. These elements are conserved in mammalian MC1Rs and in all five types of human melanocortin receptors. Thus, members of this GPCR subfamily might share common mechanisms for regulation of plasma membrane expression.

Characterisation of the melanocortin-1 receptor gene in alpaca and identification of possible markers associated with phenotypic variations in colour

The aim of this study was to determine if any correlation exists between melanocortin-1 receptor (MC1R) polymorphisms and skin and fibre colour in alpacas. Primers capable of amplifying the entire alpaca MC1R gene were designed from a comparative alignment of Bos taurus and Mus musculus MC1R gene sequences. The complete MC1R gene of 41 alpacas exhibiting a range of fibre colours, and which were sourced from farms across Australia, was sequenced from PCR products. Twenty-one single nucleotide polymorphisms were identified within MC1R. Two of these polymorphisms (A82G and C901T) have the potential to reduce eumelanin production by disrupting the activity of MC1R. No agreement was observed between fibre colour alone and MC1R genotype in the 41 animals in this study. However, when the animals were assigned to groups based on the presence or absence of eumelanin in their fibre and skin, only animals that had at least one allele with the A82/C901 combination expressed eumelanin. We propose that A82/C901 is the wild-type dominant ‘E’ MC1R allele, while alpacas with either G82/T901 or G82/Y901 are homozygous for the recessive ‘e’ MC1R allele and are therefore unable to produce eumelanin.

Variants of the MATP/SLC45A2 gene are protective for melanoma in the French population

In this study, we investigated whether variants in three key pigmentation genes-MC1R, MATP/SLC45A2, and OCA2--were involved in melanoma predisposition. A cohort comprising 1,019 melanoma patients (MelanCohort) and 1,466 Caucasian controls without skin cancers were studied. A total of 10 polymorphisms, including five functional MC1R alleles (p.Asp84Glu, p.Arg142His, p.Arg151Cys, p.Arg160Trp, and p.Asp294His), two nonsynonymous SLC45A2 variants (p.Phe374Leu and p.Glu272Lys), and three intronic OCA2 variants previously shown to be strongly associated with eye color (rs7495174 T>C, rs4778241 G>T, and rs4778138 T>C) were genotyped. As expected, MC1R variants were closely associated with melanoma risk (P value <2.20.10(-16); odds ratio [OR]=2.29 [95% confidence interval, CI=1.85-2.82 and OR=3.3 [95% CI=2.00-5.45], for the presence of one or two variants, respectively). Interestingly, the SLC45A2 variant p.Phe374Leu was significantly and strongly protective for melanoma (P-value=2.12.10(-15); OR=0.35 [95% CI=0.26-0.46] and OR=0.32 [95% CI=0.24-0.43], considering the genotypes Phe/Leu and Leu/Leu, respectively). MC1R and SLC45A2 variants had additive effects on melanoma risk, and after adjusting for pigmentation characteristics, the risk was persistent, even though both genes had a strong impact on pigmentation. Future studies may show whether genetic information could provide a useful complement to physical examination in predicting melanoma risk.

Comprehensive evaluation of allele frequency differences of MC1R variants across populations

The melanocortin 1 receptor (MC1R), a member of the G protein-coupled receptors superfamily, mediates the response to melanocortins and is currently the best-described contributor to normal pigment variation in humans. A remarkably large number of natural polymorphisms, or variants, of the MC1R gene have been identified in different populations. Some of these variants have been associated with specific hair and skin color phenotypes, the presence of freckling, and melanoma and nonmelanoma skin cancer risk. Interestingly, some MC1R variants have been associated with skin cancer beyond their effects on pigmentation. Although the red hair color variants (RHC variants) have been associated with skin cancer risk in the Celtic population, studies in darkly-pigmented Caucasian populations have demonstrated the importance of non-RHC MC1R variants on skin cancer risk as well. We have reviewed and compared allele frequency differences of MC1R variants across geographic regions. We observed large differences in the distribution of variants across populations, with a prominent difference between lightly and darkly-pigmented individuals. Moreover, among Caucasian groups, there were seven variants (p.V60L, p.V92M, p.D84E, p.R151C, p.R160W, p.R163Q, and p.D294H) with significantly different allele frequencies. Exploring differences in allele frequencies of MC1R variants across populations with varying pigmentation and differing skin cancer risk may improve our understanding of the complex relationship between MC1R, pigmentation, and carcinogenesis.

MC1R: three novel variants identified in a malignant melanoma association study in the Spanish population

The human melanocortin-1 receptor (MC1R) gene, which plays a crucial role in pigmentation, also appears to be important in malignant melanoma (MM). This case-control study in the Spanish population included 116 consecutive MM patients and 188 controls frequency matched for sex and age. Sequence analysis of the entire coding region of MC1R was performed, identifying 21 variants, all of them previously reported except for three novel non-synonymous changes: Ser41Phe, Met128Thr and Asn281Ser. Simulated structural analyses suggested disruption of the local structure around Phenylalanine 41, possible destabilization of the hydrophobic interior of the molecule in Threonine 128 and that Asparagine 281 could be in a region of functional importance. The fact that these three novel variants were not present in 1,000 healthy individuals tested adds further weight to them having putative adverse effects on the functional protein. Six variants, all non-synonymous changes, were individually associated with MM risk (Arg160Trp, Asp294His, Val60Leu, Val92Met, Ile155Thr and Arg163Gln). Carrying two non-synonymous variants was associated with much higher risk of MM (odds ratio: 10.44, 95% confidence interval = 4.48-24.33, P = 5 x 10(-8)) and haplotype analysis, verified by cloning, confirmed that this is predominantly due to carrying each on a different chromosome. Our results suggest that both red hair colour (RHC) and non-red hair colour variants, and possibly other rare non-synonymous variants, in MC1R are implicated in the development of MM. In addition to carrying MC1R variant alleles, having blond/red hair and childhood sunburns were independent risk factors for MM.

Regulation of human melanocortin 1 receptor signaling and trafficking by Thr-308 and Ser-316 and its alteration in variant alleles associated with red hair and skin cancer

The melanocortin 1 receptor (MC1R), a G protein-coupled receptor (GPCR) positively coupled to adenylyl cyclase, is a key regulator of melanocyte proliferation and differentiation and a determinant of pigmentation, skin phototype, and skin cancer risk. MC1R activation stimulates melanogenesis and increases the ratio of black, strongly photoprotective eumelanins to yellowish and poorly photoprotective pheomelanin pigments. Desensitization and internalization are key regulatory mechanisms of GPCR signaling. Agonist-induced desensitization usually depends on phosphorylation by a GPCR kinase (GRK) followed by receptor internalization in endocytic vesicles. We have shown that MC1R desensitization is mediated by two GRKs expressed in melanocytes and melanoma cells, GRK2 and GRK6. Here we show that in contrast with this dual specificity for desensitization, GRK6 but not GRK2 mediated MC1R internalization. Mutagenesis studies suggested that the targets of GRK6 are two residues located in the MC1R cytosolic C terminus, Thr-308 and Ser-316. A T308D/S316D mutant mimicking their phosphorylated state was constitutively desensitized and associated with endosomes, whereas a T308A/S316A mutant was resistant to desensitization and internalization. We studied the desensitization and internalization of three variant MC1R forms associated with red hair and increased skin cancer risk: R151C, R160W, and D294H. These variants showed a less efficient desensitization. Moreover, D294H was resistant to internalization, thus accounting for its abnormally high surface expression. Co-expression of variant and wild type MC1R modified its desensitization and internalization behavior. These data suggest that MC1R might be regulated by novel mechanisms including differential effects of GRKs and altered desensitization rates of certain allelic combinations.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Regulation of constitutive and UVR‐induced skin pigmentation by melanocortin 1 receptor isoforms

Melanin synthesized by epidermal melanocytes protects the skin against UVR-induced DNA damage and skin cancer. Exposure to UVR increases the synthesis of the photoprotective eumelanin on activation of MC1R, a melanoma susceptibility gene. We studied the expression of MC1R under UVR and alpha-MSH stimulation in skin of different ethnic origins and in melanocytes of various pigmentary levels. This study identifies and characterizes a novel MC1R isoform (MC1R350) generated by alternative splicing of the classically known MC1R (MC1R317). We demonstrate that the melanin content of melanocytes shows a significant positive correlation with MC1R317 levels but correlates inversely with the amount of MC1R350, suggesting that this latter isoform could act as a negative regulator of melanin synthesis. We confirmed that hypothesis by showing that while MC1R317 signaling significantly increases the expression of MITF and tyrosinase, two key factors in the melanin synthesis pathway, MC1R350 dramatically hampers their expression. In the skin, we show that UVR does not increase MC1R350 expression but does significantly increase MC1R317. Taken together, our results strongly suggest that MC1R350 acts as a negative regulator of skin pigmentation and demonstrate for the first time that MC1R isoform-specific expression is closely related to skin pigmentation and photoprotection.

MC1R and PTCH Gene Polymorphism in French Patients with Basal Cell Carcinomas

In this study, we assessed the role of melanocortin 1 receptor (MC1R) variants and of two patched (PTCH) polymorphisms (c.3944C>T (P1315L), insertion 18 bp IVS1-83) as risk factors for basal cell carcinoma (BCC) in the French population. The population investigated comprised 126 BCC patients who were enrolled on the basis of specific criteria (multiple and/or familial BCC and/or onset before the age of 40 years and/or association with another tumor)--and 151 controls matched for ethnicity, age, and sex. MC1R variants appeared as a moderate risk factor for BCC (odds ratio (OR) for one and two variants, 2.17 [1.28-3.68] and 7.72 [3.42-17.38], respectively), independently of pigmentation characteristics (OR = 2.53 [1.34-4.8]). Interestingly, in addition to the predictable red hair color (RHC) alleles, two non-RHC alleles (V60L and V92M) were also closely associated with BCC risk (OR 3.21 [1.91-5.38] and 2.87 [1.5-5.48], respectively), which differs from the situation in the Celtic population. In addition, the PTCH c.3944C/C genotype was also associated with BCC risk (OR 1.94 [1.2-3.1]), especially in the subgroup of patients with multiple tumors (OR 2.16 [1.3-3.6]). Thus, our data show that MC1R and PTCH variants are associated with BCC risk in the French population. We further suggest that assessing MC1R and PTCH status could be useful, combined with the assessment of clinical risk factors, in identifying high-risk patients to be targeted for prevention or more rigorous surveillance.

A polymorphism in the agouti signalling protein (ASIP) is associated with decreased levels of mRNA

To date, a role for agouti signalling protein (ASIP) in human pigmentation has not been well characterized. It is known that agouti plays a pivotal role in the pigment switch from the dark eumelanin to the light pheomelanin in the mouse. However, because humans do not have an agouti banded hair pattern, its role in human pigmentation has been questioned. We previously identified a single polymorphism in the 3'-untranslated region (UTR) of ASIP that was found at a higher frequency in African-Americans compared with other population groups. To compare allele frequencies between European-Australians and indigenous Australians, the g.8818A --> G polymorphism was genotyped. Significant differences were seen in allele frequencies between these groups (P < 0.0001) with carriage of the G allele highest in Australian Aborigines. In the Caucasian sample set a strong association was observed between the G allele and dark hair colour (P = 0.004) (odds ratio 4.6; 95% CI 1.4-15.27). The functional consequences of this polymorphism are not known but it was postulated that it might result in message instability and premature degradation of the transcript. To test this hypothesis, ASIP mRNA levels were quantified in melanocytes carrying the variant and non-variant alleles. Using quantitative real-time polymerase chain reaction the mean ASIP mRNA ratio of the AA genotype to the AG genotype was 12 (P < 0.05). This study suggests that the 3'-UTR polymorphism results in decreased levels of ASIP and therefore less pheomelanin production.

Predisposición genética en el melanoma cutáneo

The incidence of cutaneous melanoma has increased worldwide in the last 20 years. Research on potential risk factors, both environmental and genetic, has led us to some new and interesting conclusions. Ultraviolet radiation is clearly the main environmental risk factor for melanoma, but its relationship is complex and controversial. With regard to genetic factors, the discovery of two types of genes was a great advance in further understanding the biology of the melanocyte. CDKN2A (p16) is the prototype of the high-penetrance, low-prevalence gene related to melanoma. This gene has been studied in some families in which several members have been diagnosed with melanoma. In the general population with non-familial melanoma, low-penetrance, high-prevalence genes such as MC1R seem to be more interesting. Studies on the MC1R gene have not only shown its importance in skin and hair pigmentation, but also in the development of melanoma. Functional studies on CDKN2A and MC1R have led us to new and important conclusions. The analysis of data from studies on families, twins and control cases, with the collaboration of several countries, will lead us to new discoveries. For the primary and secondary prevention of this tumor, we must promote public health campaigns on the dangers of sun exposure and the identification of individuals at high risk.

Contribution of melanocortin-1 receptor gene variants to sporadic cutaneous melanoma risk in a population in central Italy: a case–control study

The melanocortin-1 receptor (MC1R) gene is a key determinant of the physiological variation in human skin pigmentation. It is highly polymorphic, and specific MC1R allelic variants have been shown to be low-penetrance melanoma susceptibility alleles. We investigated the contribution of the MC1R genotype to the risk of sporadic cutaneous melanoma in a population in central Italy. One hundred patients with sporadic cutaneous melanoma of any stage and 100 unrelated control individuals were consecutively recruited between 1 September 2000 and 31 December 2001. Information on ethnic background and residential history, phenotypic risk factors for melanoma and ultraviolet exposure habits was collected through a standardized questionnaire and total skin examination. Sequence analysis of the entire coding region of the MC1R gene was performed. A total of 26 MC1R variants, including a novel 123_124insT allele, was identified in our population, with the most frequent allele being V60L. Carriers of high-penetrance 'R' MC1R alleles, that define MC1R variants strongly associated with the red hair colour phenotype, showed a statistically significant increase in melanoma risk [odds ratio (OR), 2.55; 95% confidence interval (CI), 1.19-5.55]. No significant association with melanoma risk was observed for carriers of 'r' variants (OR, 0.90; 95% CI, 0.45-1.82). Amongst individual MC1R variants, the R151C allele was significantly associated with melanoma, with an OR of 2.94 (95% CI, 1.04-8.31). After stratification for clinical and ultraviolet exposure risk factors, the melanoma risk associated with high-penetrance 'R' variants appeared to increase significantly, mainly in the presence of clinically atypical naevi, more than 50 melanocytic naevi, high recreational sun exposure and occupational sun exposure. These results support the contribution of high-penetrance MC1R variant alleles to genetic predisposition to sporadic cutaneous melanoma in a population in central Italy.

Effect of MELANOTAN, [Nle(4), D-Phe(7)]-alpha-MSH, on melanin synthesis in humans with MC1R variant alleles

MELANOTAN (NDP-MSH) binds the MC1 receptor to significantly increase the eumelanin content of human skin cells. In this study of 77 Caucasian individuals, we investigated the effects of MELANOTAN in individuals with variant MC1R genotypes, as it has been suggested through in vitro studies that variant alleles decrease MELANOTAN binding efficacy, which would subsequently affect the synthesis of melanin. Administration of MELANOTAN produced a significant (p<0.001) increase in melanin density in treated, compared to placebo, individuals. Importantly, MELANOTAN increased the melanin density to a greater extent in individuals carrying the variant alleles Val60Leu, Asp84Glu, Val92Met, Arg142His, Arg151Cys, and Arg160Trp than in individuals with no variant alleles. This study demonstrates that MELANOTAN effectively increases the melanin content of skin in those individuals with MC1R variant alleles and therefore, those most in need of photoprotection.

Melanocortin-1 receptor structure and functional regulation

The melanogenic actions of the melanocortins are mediated by the melanocortin-1 receptor (MC1R). MC1R is a member of the G-protein-coupled receptors (GPCR) superfamily expressed in cutaneous and hair follicle melanocytes. Activation of MC1R by adrenocorticotrophin or alpha-melanocyte stimulating hormone is positively coupled to the cAMP signaling pathway and leads to a stimulation of melanogenesis and a switch from the synthesis of pheomelanins to the production of eumelanic pigments. The functional behavior of the MC1R agrees with emerging concepts in GPCR signaling including dimerization, coupling to more than one signaling pathway and a high agonist-independent constitutive activity accounting for inverse agonism phenomena. In addition, MC1R displays unique properties such as an unusually high number of natural variants often associated with clearly visible phenotypes and the occurrence of endogenous peptide antagonists. Therefore MC1R is an ideal model to study GPCR function. Here we review our current knowledge of MC1R structure and function, with emphasis on information gathered from the analysis of natural variants. We also discuss recent data on the regulation of MC1R function by paracrine and endocrine factors and by external stimuli such as ultraviolet light.

Dimerization of the Human Melanocortin 1 Receptor: Functional Consequences and Dominant-Negative Effects

The melanocortin 1 receptor (MC1R), a G(S)-protein-coupled receptor (GPCR), is a key regulator of proliferation and differentiation of epidermal melanocytes, and a determinant of human skin phototype and cancer risk. Homodimerization has been demonstrated for several GPCRs, but little information is available for MC1R. SDS-PAGE analysis of melanoma cells and heterologous cells expressing epitope-tagged MC1R revealed dimeric and oligomeric species in detergent-solubilized extracts, confirmed by co-immunoprecipitation of differentially tagged MC1R forms. Dimerization occurs early during MC1R biosynthesis, and is seen for mutants displaying intracellular retention. These mutants exerted dominant-negative effects on wild-type (WT) MC1R. Conversely, partial functional trans-complementation of selected loss-of-function mutants was observed. WT-MC1R lacks cooperativity in agonist binding, yet coexpression of WT and a C-terminal deletion mutant yielded a form of different pharmacological properties. The natural diminished function alleles R151C, R160W, and D294H, associated with red hair, displayed dimerization and heterodimerization with WT. Coexpression of WT and R151C or R160W reduced the density of binding sites on the plasma membrane of transfected cells, whereas D294H mediated a dominant-negative effect on functional coupling to adenylyl cyclase. Therefore, subtle changes of functional properties may be associated with different MC1R haplotypes, contributing to the complexity of skin phenotype.

Allele variations in the OCA2 gene (pink-eyed-dilution locus) are associated with genetic susceptibility to melanoma

The occuloalbinism 2 (OCA2) gene, localized at 15q11, encodes a melanosomal transmembrane protein that is involved in the most common form of human occulo-cutaneous albinism, a human genetic disorder characterized by fair pigmentation and susceptibility to skin cancer. We wondered whether allele variations at this locus could influence susceptibility to malignant melanoma (MM). In all, 10 intragenic single-nucleotide polymorphisms (SNPs) were genotyped in 113 patients with melanomas and in 105 Caucasian control subjects with no personal or family history of skin cancer. By comparing allelic distribution between cases and controls, we show that MM and OCA2 are associated (p value=0.030 after correction for multiple testing). Then, a recently developed strategy, the 'combination test' enabled us to show that a combination formed by two SNPs was most strongly associated to MM, suggesting a possible interaction between intragenic SNPs. In addition, the role of OCA2 on MM risk was also detected using a logistic model taking into account the presence of variants of the melanocortin 1 receptor gene (MC1R, a key pigmentation gene) and all pigmentation characteristics as melanoma risk factors. Our data demonstrate that a second pigmentation gene, in addition to MC1R, is involved in genetic susceptibility to melanoma.

Gene polymorphisms and their effects in the melanocortin system

In addition to its role in human pigmentation, components of the melanocortin system regulate appetite, energy homeostasis and hormone production. Recent studies have suggested possible roles of this system in immunity, transmission of pain signals, and reproductive potential. A number of polymorphisms have been identified in genes of the melanocortin system and are associated with pigmentation in humans, as well as being causative of disorders of adrenal hormone production and obesity. This review gives an outline of these polymorphisms, their functional significance and possible application to or impact on diagnosis and pharmacotherapy based on melanocortin pathways.

The relation between melanocortin 1 receptor (MC1R) variation and the generation of phenotypic diversity in the cutaneous response to ultraviolet radiation

The melanocortin 1 receptor (MC1R) is known to play an important role in determining physiological variation in human pigmentation, and consequently human susceptibility to ultraviolet radiation. A reason for wider interest is that the considerable phenotypic diversity has been in part generated by the effects of gene dosage, and the presence of a large number of mutations at this G-protein coupled receptor that are not functionally equivalent. Thus, a range of mutations at a single receptor locus can lead to a complex range of graded phenotypes.

Activation of the cAMP pathway by variant human MC1R alleles expressed in HEK and in melanoma cells

Alpha-melanocyte-stimulating hormone (alpha-MSH) activates the melanocortin-1 receptor (MC1R) on melanocytes to promote a switch from red/yellow pheomelanin synthesis to darker eumelanins via positive coupling to adenylate cyclase. The human MC1R locus is highly polymorphic with the specific variants associated with red hair and fair skin (RHC phenotype) postulated to be loss-of-function receptors. We have examined the ability of MC1R variants to activate the cAMP pathway in stably transfected HEK293 cells. The RHC associated variants, Arg151Cys, Arg160Trp and Asp294His, demonstrated agonist-mediated increases in cAMP and phosphorylation of cAMP-responsive element-binding protein (CREB). Whereas the Asp294His variant showed severely impaired functional responses, the Arg151Cys and Arg160Trp variants retained considerable signaling capacity. Melanoma cells homozygous for either the Arg151Cys variant or consensus sequence both elicited CREB phosphorylation in response to alpha-MSH in the presence of IBMX. The common RHC alleles, Arg151Cys, Arg160Trp and Asp294His, are neither complete loss-of-function receptors nor are they functionally equivalent.

Agonist-independent, high constitutive activity of the human melanocortin 1 receptor

The melanocortins (alpha-melanocyte-stimulating hormone and adrenocorticotropin) act on epidermal melanocytes to increase melanogenesis, the eumelanin/pheomelanin ratio and dendricity. These actions are mediated by the heptahelical melanocortin 1 receptor (MC1R), positively coupled to adenylyl cyclase. Gain-of-function mouse Mc1r alleles are associated with a dark, eumelanic coat. Conversely, loss-of-function variants, or overexpression of agouti, a natural melanocortin antagonist, yield yellow, pheomelanic furs. In humans, loss-of-function MC1R variants are associated with fair skin, poor tanning, propensity to freckle and increased skin cancer risk. Therefore, MC1R is a key regulator of mammalian pigmentation. Several observations such as induction of constitutive pigmentation in amelanotic mouse melanoma cells following expression of MC1R indicate that the receptor might display agonist-independent activity. We report a systematic and comparative study of MC1R and Mc1r constitutive activity. We show that expression of MC1R in heterologous systems leads to an agonist-independent increase in cyclic adenosine monophophate (cAMP). Basal signalling is a function of receptor expression and is two to fourfold higher for MC1R than for Mc1r. Moreover, it is observed in human melanoma cells over-expressing the MC1R. Constitutive signalling is abolished or reduced by point mutations of MC1R impairing the response to agonists, and is only doubled by the Lys94Glu mutation, mimicking the constitutively active mouse E(so-3J) allele. Stable or transient expression of wild-type MC1R, but not of loss-of-function mutants, potently stimulates forskolin activation of adenylyl cyclase, a common feature of constitutively active Gs-coupled receptors. Therefore, human MC1R displays a strong agonist-independent constitutive activity.

Pharmacological characterization of loss of function mutations of the human melanocortin 1 receptor that are associated with red hair

Variation in skin color is the major host risk factor for melanoma and other forms of skin cancer. Individuals with red hair show an increased ratio of phaeomelanin to eumelanin in both hair and skin. This ratio is regulated by the melanocortin (MC) 1 receptor. There are several common point mutations in the human MC1 receptor that are overrepresented in North European red-heads, and in individuals with pale skin. In order to determine the functional significance of these mutations, we expressed the Asp84Glu, Val92Met, Arg163Gln, and Asp294His variants of the human MC1 receptors in eukaryotic cells and determined their ability to bind alpha-melanocyte stimulating hormone (MSH) peptides and increase intracellular cAMP. The mutants Asp84Glu and Asp294His showed a much lower response to alpha-MSH in cAMP and a slightly impaired ability to bind alpha-MSH, and the Val92Met mutant bound alpha-MSH with 100-fold lower affinity as compared with the wild-type. The Arg163Gln variant, widely found in some Asian populations, reached normal level of cAMP response but had just slightly lower potency for alpha-MSH in binding and second messenger studies. The results provide important pharmacological characterization of common MC1 receptor variants in various world populations.

Melanocortin 1 receptor variants, pigmentation, and skin cancer susceptibility

The melanocortin 1 receptor is a key regulator of variation in normal human pigmentation. Genetic variants of this receptor cause red hair and fair skin, and several case-control studies have demonstrated that these genetic variants increase the risk of skin cancer development in humans. The mechanism whereby the risks of skin cancer are increased is not entirely clear, and may be because of a combination of effects on pigmentation and non-pigmentary pathways.

Anti-inflammatory and anti-invasive effects of alpha-melanocyte-stimulating hormone in human melanoma cells

α-Melanocyte stimulating hormone (α-MSH) is known to have pleiotrophic functions including pigmentary, anti-inflammatory, antipyretic and immunoregulatory roles in the mammalian body. It is also reported to influence melanoma invasion with levels of α-, β- and γ-MSH correlated clinically with malignant melanoma development, but other studies suggest α-MSH acts to retard invasion. In the present study, we investigated the action of α-MSH on three human melanoma cell lines (HBL, A375-SM and C8161) differing in metastatic potential. α-melanocyte-simulating hormone reduced invasion through fibronectin and also through a human reconstructed skin composite model for the HBL line, and inhibited proinflammatory cytokine-stimulated activation of the NF-κB transcription factor. However, A375-SM and C8161 cells did not respond to α-MSH. Immunofluorescent microscopy and Western blotting identified melanocortin-1 receptor (MC-1R) expression for all three lines and MC-2R on HBL and A375-SM lines. Receptor binding identified a similar affinity for α-MSH for all three lines with the highest number of binding sites on HBL cells. Only the HBL melanoma line demonstrated a detectable cyclic adenosine monophosphate (cAMP) response to α-MSH, although all three lines responded to acute α-MSH addition (+(−)-N⁶-(2-phenylisopropyl)-adenosine (PIA)) with an elevation in intracellular calcium. The nonresponsive lines displayed MC-1R polymorphisms (C8161, Arg (wt) 151/Cys 151; A375-SM, homozygous Cys 151), whereas the HBL line was wild type. Stable transfection of the C8161 line with wild-type MC-1R produced cells whose invasion was significantly inhibited by α-MSH. From this data, we conclude that α-MSH can reduce melanoma cell invasion and protect cells against proinflammatory cytokine attack in cells with the wild-type receptor (HBL).

Rate limiting factors in melanocortin 1 receptor signalling through the cAMP pathway

The melanotropic actions of alpha-melanocyte-stimulating hormone (alpha-MSH) and other melanocortins are mediated by activation of the melanocortin 1 receptor (MC1R). This G protein-coupled receptor is positively coupled to Gs and triggers the cyclic adenosine mono-phosphate (cAMP) pathway. Mutations of the MC1R gene are associated with skin type and pigmentation phenotypes, and with increased risk of skin cancers. Genetic studies have demonstrated an heterozygote carrier effect for these associations, suggesting the importance of variant allele dosage. This could be accounted for, at least partially, if the number of MC1R molecules, rather than the Gs protein or the effector enzyme, adenylyl cyclase, is limiting for the activation of the signalling pathway. However, the nature of the limiting factor(s) in MC1R signalling has not been investigated. We addressed this question by comparing the cAMP output of clones of human melanoma cell lines enriched in MC1R by stable transfection. We also analysed heterologous cell systems widely used for functional studies of MC1R. We show that cAMP production in clones of Chinese hamster ovary cells stably expressing the MC1R is a linear function of receptor number up to high, supraphysiological levels of approximately 50,000 alpha-MSH binding sites per cell. Enrichment of human melanoma cell lines with MC1R also results in increased cAMP levels, with a small leftward shift of the agonist dose-response curves. Therefore, at physiological expression levels second-messenger generation is dependent on receptor density. Within melanoma cells and also likely in normal melanocytes, MC1R appears the limiting factor controlling the output of the cAMP signalling pathway.

Polymorphisms in the melanocortin-1 receptor (MC1R) and agouti signaling protein (ASIP) genes in Korean vitiligo patients

We have examined the frequency of SNP polymorphisms within the melanocortin-1 receptor (MC1R) and agouti signaling protein (ASIP) genes in 114 Korean vitiligo patients and 111 normal controls to assess the association of these loci with vitiligo risk. Using direct sequencing techniques, we found the following five MC1R coding region SNPs: Arg67Gln (G200A), Val92Met (G274A), Ile120Thr (T359C), Arg160Arg (C478A), and Gln163Arg (A488G). Of these, the most common were Val92Met at 14% in patients vs. 9% in controls (P = 0.17) and Gln163Arg at 17% in patients vs. 17% in controls (P = 0.84). Presence of the A allele of Val92Met (G274A) was higher in vitiligo patients [P = 0.12, odds ratio (OR) [95% confidence interval (CI)] = 1.68 (0.86-3.25)]. The other three variants showed a frequency <5% of both patients and controls. The ASIP 3'UTR genotype (g.8818A-G) was also assessed in the same subjects. The frequency of the G allele of 3'UTR in ASIP was 17% in vitiligo and 12% in controls [P = 0.14, OR (95% CI) = 1.49 (0.87-2.54)]. Carriage of the G allele was higher in vitiligo patients [P = 0.17, OR (95% CI) = 1.50 (0.83-2.72)], and those who also carried MC1R Val92Met were more prone to vitiligo [eight of 111 patients vs. four of 111 in controls, P = 0.14, OR (95% CI) = 2.75 (0.71-8.69)]. None of these associations, however, reached statistical significance.

Genetic association and cellular function of MC1R variant alleles in human pigmentation

We have examined MC1R variant allele frequencies in the general population of South East Queensland and in a collection of adolescent dizygotic and monozygotic twins and family members to define statistical associations with hair and skin color, freckling, and mole count. Results of these studies are consistent with a linear recessive allelic model with multiplicative penetrance in the inheritance of red hair. Four alleles, D84E, R151C, R160W, and D294H, are strongly associated with red hair and fair skin with multinomial regression analysis showing odds ratios of 63, 118, 50, and 94, respectively. An additional three low-penetrance alleles V60L, V92M, and R163Q have odds ratios 6, 5, and 2 relative to the wild-type allele. To address the cellular effects of MC1R variant alleles in signal transduction, we expressed these receptors in permanently transfected HEK293 cells. Measurement of receptor activity via induction of a cAMP-responsive luciferase reporter gene found that the R151C and R160W receptors were active in the presence of NDP-MSH ligand, but at much reduced levels compared with that seen with the wild-type receptor. The ability to stimulate phosphorylation of the cAMP response element binding protein (CREB) transcription factor was also apparent in all stimulated MC1R variant allele-expressing HEK293 cell extracts as assessed by immunoblotting. In contrast, human melanoma cell lines showed wide variation in the their ability to undergo cAMP-mediated CREB phosphorylation. Culture of human melanocytes of known MC1R genotype may provide the best experimental approach to examine the functional consequences for each MC1R variant allele. With this objective, we have established more than 300 melanocyte cell strains of defined MC1R genotype.

Conformation-dependent post-translational glycosylation of tyrosinase. Requirement of a specific interaction involving the CuB metal binding site

Tyrosinase, the rate-limiting enzyme in mammalian melanogenesis, is a copper-containing transmembrane glycoprotein. Tyrosinase undergoes a complex post-translational processing before reaching the melanosomal membrane. This processing involves N-glycosylation in several sites, including one located in the CuB copper binding site, movement from the endoplasmic reticulum (ER) to the Golgi, copper binding, and sorting to the melanosome. Aberrant processing is causally related to the depigmented phenotype of human melanomas. Moreover, some forms of albinism and several other pigmentary syndromes are considered ER retention diseases or trafficking defects. A critical step in tyrosinase maturation is the acquisition of an ER export-competent conformation recognized positively by the ER quality control system. However, the minimal structural requirements allowing exit from the ER to the Golgi have not yet been identified for tyrosinase or other melanosomal proteins. We addressed this question by analyzing the enzymatic activity and glycosylation pattern of mouse tyrosinase point mutants and chimeric constructs, where selected portions of tyrosinase were replaced by the homologous fragments of the highly similar tyrosinase-related protein 1. We show that a completely inactive tyrosinase point mutant lacking a critical histidine residue involved in copper binding is nevertheless able to exit from the ER and undergo further processing. Moreover, we demonstrate that tyrosinase displays at least two sites whose glycosylation is post-translational and most likely conformation-dependent and that a highly specific interaction involving the CuB site is essential not only for correct glycosylation but also for exit from the ER and enzymatic activity.

Loss-of-function variants of the human melanocortin-1 receptor gene in melanoma cells define structural determinants of receptor function

The alpha-melanocyte-stimulating hormone (alphaMSH) receptor (MC1R) is a major determinant of mammalian skin and hair pigmentation. Binding of alphaMSH to MC1R in human melanocytes stimulates cell proliferation and synthesis of photoprotective eumelanin pigments. Certain MC1R alleles have been associated with increased risk of melanoma. This can be theoretically considered on two grounds. First, gain-of-function mutations may stimulate proliferation, thus promoting dysplastic lesions. Second, and opposite, loss-of-function mutations may decrease eumelanin contents, and impair protection against the carcinogenic effects of UV light, thus predisposing to skin cancers. To test these possibilities, we sequenced the MC1R gene from seven human melanoma cell (HMC) lines and three giant congenital nevus cell (GCNC) cultures. Four HMC lines and two GCNC cultures contained MC1R allelic variants. These were the known loss-of-function Arg142His and Arg151Cys alleles and a new variant, Leu93Arg. Moreover, impaired response to a superpotent alphaMSH analog was demonstrated for the cell line carrying the Leu93Arg allele and for a HMC line homozygous for wild-type MC1R. Functional analysis in heterologous cells stably or transiently expressing this variant demonstrated that Leu93Arg is a loss-of-function mutation abolishing agonist binding. These results, together with site-directed mutagenesis of the vicinal Glu94, demonstrate that the MC1R second transmembrane fragment is critical for agonist binding and maintenance of a resting conformation, whereas the second intracellular loop is essential for coupling to the cAMP system. Therefore, loss-of-function, but not activating MC1R mutations are common in HMC. Their study provides important clues to understand MC1R structure-function relationships.

Skin colour and skin cancer - MC1R, the genetic link

Pigmentary traits such as red hair, fair skin, lack of tanning ability and propensity to freckle (the RHC phenotype) have been identified as genetic risk factors for both melanoma and non-melanocytic skin cancers when combined with the environmental risk factor of high ultraviolet light exposure. The human melanocortin-1 receptor (MC1R) is a key determinant of the pigmentation process and can account in large part for the diverse range of variation in human pigmentation phenotypes and skin phototypes. The coding sequence is highly polymorphic in human populations, with several of these variant forms of the receptor now known to be associated with the RHC phenotype. We have examined variant allele frequencies in the general population and in a collection of adolescent dizygotic and monozygotic twins with defined pigmentation characteristics. Variant allele frequencies have also been determined in several case-control studies of sporadic melanoma, basal cell carcinoma and squamous cell carcinoma, and in familial melanoma kindreds collected within Australia. These studies have shown that three RHC alleles - Arg151Cys, Arg160Trp and Asp294His - were associated with increased risk in all forms of skin cancer and with penetrance and age of onset in familial melanoma in mutation carriers. There is a significant RHC allele heterozygote carrier effect on skin phototype and skin cancer risk, which indicates that variant alleles do not behave in a strictly recessive manner. Ultimately, the genetic and chemical assessment of melanin synthesis rather than skin colour will be the best indicator for skin cancer risk, and such genetic association studies combined with functional analysis of variant alleles should provide the link to understanding skin phototypes.