Melanocortin 1 receptor variants in an Irish population

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.

Identification of novel functional variants of the melanocortin 1 receptor gene originated from Asians

Human melanocortin 1 receptor (MC1R) is a seven transmembrane G-coupled protein receptor that upregulates the cAMP pathway. Several functional variants of MC1R that show an impaired ability to activate the cAMP pathway are strongly associated with fair skin and red hair in Europeans and European descendants. The sequence variations of the MC1R gene were repeatedly investigated against worldwide populations; however, there was no evidence that functional variant of MC1R exists in non-European descendants. We report the presence of novel functional variants of MC1R with Asian origins. Three novel variants of MC1R, Phe147Delta, Thr157Ile, and Pro159Thr, were identified in our screening for the sequence variations of the MC1R gene against 995 individuals from 30 Asian and Oceanian populations; there was a single case for the Pro159Thr variant allele and two instances of Phe147Delta and Thr157Ile variant alleles. Our pharmacological assay revealed that Phe147Delta, Thr157Ile, and Pro159Thr variant showed similar or more dramatically impaired activities in comparison with Arg151Cys, which is a major functional variant of MC1R in Europeans. These functional variant alleles were geographically localized in relatively high latitudes, which suggest that the adaptation to ambient UV light intensity may play an important role in shaping the geographical distribution of MC1R alleles in Asia and Oceania.

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.

The patterns of natural variation in human genes

Currently, more than 10 million DNA sequence variations have been uncovered in the human genome. The most detailed variation discovery efforts have focused on candidate genes involved in cardiovascular disease or in susceptibilities associated with exposure to environmental agents. Here we provide an overview of natural genetic variation from the literature and in 510 human candidate genes resequenced for variation discovery. The average human gene contains 126 biallelic polymorphisms, 46 of which are common (> or =5% minor allele frequency) and 5 of which are found in coding regions. Using this complete picture of genetic diversity, we explore conservation, signatures of selection, and historical recombination to mine information useful for candidate gene association studies. In general, we find that the patterns of human gene variation suggest that no one approach will be appropriate for genetic association studies across all genes. Therefore, many different approaches may be required to identify the elusive genotypes associated with common human phenotypes.

Alpha-melanocyte-stimulating hormone suppresses antigen-induced lymphocyte proliferation in humans independently of melanocortin 1 receptor gene status

Studies in mice indicate that alpha-melanocyte-stimulating hormone (alphaMSH) is immunosuppressive, but it is not known whether alphaMSH suppresses human immune responses to exogenous Ags. Human PBMCs, including monocytes, express the melanocortin 1 receptor (MC1R), and it is thought that the ability of alphaMSH to alter monocyte-costimulatory molecule expression and IL-10 release is mediated by this receptor. However, the MC1R gene is polymorphic, and certain MC1R variants compromise receptor signaling via cAMP, resulting in red hair and fair skin. Here, we have investigated whether alphaMSH can suppress Ag-induced lymphocyte proliferation in humans and whether these effects are dependent on MC1R genotype. alphaMSH suppressed streptokinase-streptodornase-induced lymphocyte proliferation, with maximal inhibition at 10(-13)-10(-11) M alphaMSH. Anti-IL-10 Abs failed to prevent suppression by alphaMSH, indicating that it was not due to MC1R-mediated IL-10 release by monocytes. Despite variability in the degree of suppression between subjects, similar degrees of alphaMSH-induced immunosuppression were seen in individuals with wild-type, heterozygous variant, and homozygous/compound heterozygous variant MC1R alleles. RT-PCR of streptokinase-streptodornase-stimulated PBMCs for all five melanocortin receptors demonstrated MC1R expression by monocytes/macrophages, MC1R and MC3R expression by B lymphocytes, but no melanocortin receptor expression by T lymphocytes. In addition, alphaMSH did not significantly inhibit anti-CD3 Ab-induced lymphocyte proliferation, whereas alphaMSH and related analogs (SHU9119 and MTII) inhibited Ag-induced lymphocyte proliferation in monocyte-depleted and B lymphocyte-depleted assays. These findings demonstrate that alphaMSH, acting probably via MC1R on monocytes and B lymphocytes, and possibly also via MC3R on B lymphocytes, has immunosuppressive effects in humans but that suppression of Ag-induced lymphocyte proliferation by alphaMSH is independent of MC1R gene status.

Regulatory elements of the melanocortin 1 receptor

Among more than 120 genes that are now known to regulate mammalian pigmentation, one of the key genes is MC1R, which encodes the melanocortin 1 receptor, a seven transmembrane G protein-coupled receptor expressed on the surface of melanocytes. Since the monoexonic sequence of the gene was cloned and characterized more than a decade ago, tremendous efforts have been dedicated to the extensive genotyping of mostly red-haired populations all around the world, thus providing allelic variants that may or may not account for melanoma susceptibility in the presence or absence of ultraviolet (UV) exposure. Soluble factors, such as proopiomelanocortin (POMC) derivatives, agouti signal protein (ASP) and others, regulate MC1R expression, leading to improved photoprotection via increased eumelanin synthesis or in contrast, inducing the switch to pheomelanin. However, there is an obvious lack of knowledge regarding the numerous and complex regulatory mechanisms that govern the expression of MC1R at the intra-cellular level, from gene transcription in response to an external stimulus to the expression of the mature receptor on the melanocyte surface.

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.

Worldwide polymorphism at the MC1R locus and normal pigmentation variation in humans

While there have been many advances in our understanding of the genetics of pathological skin pigmentation in humans, our knowledge about what determines variation in normal skin color is still incomplete. Variation in one gene, melanocortin 1 receptor (MC1R), has been associated with red hair and fair skin in Europeans. However, this gene might also play an important role in shaping pigmentation of other human populations, where it experiences different selective pressures. Below we review what is currently known about polymorphism and selection at the MC1R coding and promoter regions in human populations, the pattern of MC1R evolution in nonhuman primates, and the interaction of MC1R with other genes.

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.

Role of G protein-coupled receptor kinases in the homologous desensitization of the human and mouse melanocortin 1 receptors

The melanocortin 1 receptor, a G protein-coupled receptor positively coupled to adenylyl cyclase, is a key regulator of epidermal melanocyte proliferation and differentiation and a determinant of human skin phototype and skin cancer risk. Despite its potential importance for regulation of pigmentation, no information is available on homologous desensitization of this receptor. We found that the human melanocortin 1 receptor (MC1R) and its mouse ortholog (Mc1r) undergo homologous desensitization in melanoma cells. Desensitization is not dependent on protein kinase A, protein kinase C, calcium mobilization, or MAPKs, but is agonist dose-dependent. Both melanoma cells and normal melanocytes express two members of the G protein-coupled receptor kinase (GRK) family, GRK2 and GRK6. Cotransfection of the receptor and GRK2 or GRK6 genes in heterologous cells demonstrated that GRK2 and GRK6 impair agonist-dependent signaling by MC1R or Mc1r. However, GRK6, but not GRK2, was able to inhibit MC1R agonist-independent constitutive signaling. Expression of a dominant negative GRK2 mutant in melanoma cells increased their cAMP response to agonists. Agonist-stimulated cAMP production decreased in melanoma cells enriched with GRK6 after stable transfection. Therefore, GRK2 and GRK6 seem to be key regulators of melanocortin 1 receptor signaling and may be important determinants of skin pigmentation.

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.

The genetics of sun sensitivity in humans

Humans vary >100-fold in their sensitivity to the harmful effects of ultraviolet radiation. The main determinants of sensitivity are melanin pigmentation and less-well-characterized differences in skin inflammation and repair processes. Pigmentation has a high heritability, but susceptibility to cancers of the skin, a key marker of sun sensitivity, is less heritable. Despite a large number of murine coat-color mutations, only one gene in humans, the melanocortin 1 receptor (MC1R), is known to account for substantial variation in skin and hair color and in skin cancer incidence. MC1R encodes a 317-amino acid G-coupled receptor that controls the relative amounts of the two major melanin classes, eumelanin and pheomelanin. Most persons with red hair are homozygous for alleles of the MC1R gene that show varying degrees of diminished function. More than 65 human MC1R alleles with nonsynonymous changes have been identified, and current evidence suggests that many of them vary in their physiological activity, such that a graded series of responses can be achieved on the basis of (i) dosage effects (of one or two alleles) and (ii) individual differences in the pharmacological profile in response to ligand. Thus, a single locus, identified within a Mendelian framework, can contribute significantly to human pigmentary variation.

Assessment of Polymorphic Variants in the Melanocortin-1 Receptor Gene with Cutaneous Pigmentation Using an Evolutionary Approach

The melanocortin-1 receptor gene (MC1R) encodes a membrane-bound receptor protein that is central to melanin synthesis. The coding region of MC1R is highly polymorphic and associations of variants with pigmentation phenotypes and risk for cutaneous neoplasms have been reported. We sought to determine the distribution and frequency of MC1R variants and their relationship to pigmentation characteristics in 179 Caucasian controls from the United States. One hundred thirty-five (75.4%) subjects carried one or more variants, and we determined that carriage of the previously designated “red hair color” (RHC) alleles, R151C, R160W, and D294H was strongly associated with fair pigmentation phenotypes including light hair and eye color, tendency to burn, decreased tendency to tan, and freckling. We used SIFT software to define MC1R protein positions that were predicted intolerant to amino acid substitutions; detected variants that corresponded to intolerant substitutions were D84E, R142H, R151C, I155T, R160W, and D294H. Carriage of one or more of these putative functionally important variants or the frameshift variant ins86A was significantly associated with fair pigmentation phenotypes. Analyses limited to carriage of ins86A and the three non-RHC alleles identified by SIFT were attenuated and no longer reached statistical significance. This is the first study to describe MC1R variants among control subjects from the U.S. Our results indicate that the frequency of variants is similar to that previously observed among non-U.S. Caucasians. Risk variants defined by either the published literature or by evolutionary criteria are strongly and significantly associated with all fair pigmentation phenotypes that were measured.

Quantitative measures of the effect of the melanocortin 1 receptor on human pigmentary status

Variation in human hair and skin color is the most striking visible aspect of human genetic variation. The only gene known to exert an effect on pigmentary within the normal population is the melanocortin-1 receptor (MC1R). Previous studies have used a Mendelian framework to relate MC1R genotype to phenotype, by measuring pigmentary status using categorical scales. Such approaches are inadequate. We report results using direct measures of hair color using objective colorimetric dimensions and HPLC determined hair melanins. We have linked MC1R genotype with chemical measures of melanin quantity and type and objective phenotype measures of color. MC1R genotype was predictive of hair melanin expressed as the ratio of the loge of eumelanin to pheomelanin ratio, with a dosage effect evident: MC1R homozygote mean, 1.46; heterozygote, 4.44; and wild type, 5.81 (p<0.001). Approximately 67% of the variance in this model could be accounted for in terms of MC1R genotype. There was also a relation between MC1R status and hair color, most prominently for the b* axis (p<0.001), but also for the a* and L* scales (L*a*b*, CIE). We show for one of the most polymorphic human traits that it is possible to demonstrate meaningful relations between various physical characteristics: DNA sequence diversity, hair-wavelength-specific reflectance patterns, and chemical melanin assays.

Is there more than one road to melanoma?

CONTEXT

Sunlight is the main environmental cause of most cutaneous melanomas. Exposure to intense bursts of ultraviolet radiation, especially in childhood, starts the transformation of benign melanocytes into a malignant phenotype. Paradoxically, outdoor workers have a decreased risk of melanoma compared with indoor workers, suggesting that chronic sunlight exposure can have a protective effect. Further, some melanomas form on sun-exposed regions; others do not. Although some melanomas arise from pre-existing melanocytic naevi (moles), many arise de novo. These observations suggest that melanoma arises from multiple pathways, with initiating and promoting factors differing for each.

STARTING POINT

Janet Maldonado and colleagues recently studied the distribution of BRAF gene mutations in 115 patients with invasive primary melanomas (J Natl Cancer Inst 2003; 95: 1878-80). These researchers found that BRAF mutations were statistically significantly more common in melanomas occurring on intermittently sun-exposed skin than elsewhere. By contrast, BRAF mutations in melanomas on chronically sun-damaged skin were rare. These findings strongly suggest that distinct genetic pathways lead to melanoma. WHERE NEXT? The study of gene-environment interactions is clearly the next arena for epidemiological research into melanoma. The recent identification of polymorphisms in the melanocortin-1 receptor could open up an avenue of investigation into a molecular distinction between those individuals whose melanomas arise on chronic sun-exposed skin from those in whom tumours will develop on sun-protected skin or from melanocytic naevi. If a dual pathway for melanoma is supported by other investigations, public-health messages can be tailored to the population at risk.

Genetics of hair and skin color

Differences in skin and hair color are principally genetically determined and are due to variation in the amount, type, and packaging of melanin polymers produced by melanocytes secreted into keratinocytes. Pigmentary phenotype is genetically complex and at a physiological level complicated. Genes determining a number of rare Mendelian disorders of pigmentation such as albinism have been identified, but only one gene, the melanocortin 1 receptor (MCR1), has so far been identified to explain variation in the normal population such as that leading to red hair, freckling, and sun-sensitivity. Genotype-phenotype relations of the MC1R are reviewed, as well as methods to improve the phenotypic assessment of human pigmentary status. It is argued that given advances in model systems, increases in technical facility, and the lower cost of genotype assessment, the lack of standardized phenotype assessment is now a major limit on advance.

Sequences Associated With Human Iris Pigmentation

To determine whether and how common polymorphisms are associated with natural distributions of iris colors, we surveyed 851 individuals of mainly European descent at 335 SNP loci in 13 pigmentation genes and 419 other SNPs distributed throughout the genome and known or thought to be informative for certain elements of population structure. We identified numerous SNPs, haplotypes, and diplotypes (diploid pairs of haplotypes) within the OCA2, MYO5A, TYRP1, AIM, DCT, and TYR genes and the CYP1A2-15q22-ter, CYP1B1-2p21, CYP2C8-10q23, CYP2C9-10q24, and MAOA-Xp11.4 regions as significantly associated with iris colors. Half of the associated SNPs were located on chromosome 15, which corresponds with results that others have previously obtained from linkage analysis. We identified 5 additional genes (ASIP, MC1R, POMC, and SILV) and one additional region (GSTT2-22q11.23) with haplotype and/or diplotypes, but not individual SNP alleles associated with iris colors. For most of the genes, multilocus gene-wise genotype sequences were more strongly associated with iris colors than were haplotypes or SNP alleles. Diplotypes for these genes explain 15% of iris color variation. Apart from representing the first comprehensive candidate gene study for variable iris pigmentation and constituting a first step toward developing a classification model for the inference of iris color from DNA, our results suggest that cryptic population structure might serve as a leverage tool for complex trait gene mapping if genomes are screened with the appropriate ancestry informative markers.

Pharmacogenetic candidate genes for melanoma

The incidence of melanoma is rising at an alarming rate and has become an important public health concern. If detected early, melanoma carries an excellent prognosis after appropriate surgical resection. Unfortunately, advanced melanoma has a poor prognosis and is notoriously resistant to radiation and chemotherapy. The relative resistance of melanoma to a wide-range of chemotherapeutic agents and high toxicity of current therapies has prompted a search for effective alternative treatments that would improve prognosis and limit side effects. Advances in molecular genetics are revealing in increasing detail the mechanisms responsible for the development of melanoma. Hopefully, elucidation of these pathways will provide a means of screening high-risk individuals and allow new drug development for prevention and treatment by identification of specific pharmacological targets. This review will summarize the genetics of melanoma with the goal of providing insights into potential pharmacogenetic candidate genes.

Cutaneous photobiology. The melanocyte vs. the sun: who will win the final round?

Solar ultraviolet radiation (UV) is a major environmental factor that dramatically alters the homeostasis of the skin as an organ by affecting the survival, proliferation and differentiation of various cutaneous cell types. The effects of UV on the skin include direct damage to DNA, apoptosis, growth arrest, and stimulation of melanogenesis. Long-term effects of UV include photoaging and photocarcinogenesis. Epidermal melanocytes synthesize two main types of melanin: eumelanin and pheomelanin. Melanin, particularly eumelanin, represents the major photoprotective mechanism in the skin. Melanin limits the extent of UV penetration through the epidermal layers, and scavenges reactive oxygen radicals that may lead to oxidative DNA damage. The extent of UV-induced DNA damage and the incidence of skin cancer are inversely correlated with total melanin content of the skin. Given the importance of the melanocyte in guarding against the adverse effects of UV and the fact that the melanocyte has a low self-renewal capacity, it is critical to maintain its survival and genomic integrity in order to prevent malignant transformation to melanoma, the most fatal form of skin cancer. Melanocyte transformation to melanoma involves the activation of certain oncogenes and the inactivation of specific tumor suppressor genes. This review summarizes the current state of knowledge about the role of melanin and the melanocyte in photoprotection, the responses of melanocytes to UV, the signaling pathways that mediate the biological effects of UV on melanocytes, and the most common genetic alterations that lead to melanoma.

Time course of ultraviolet B-induced erythema in people with red hair harbouring homozygous melanocortin 1 receptor mutations

It has previously been reported that the time course of erythema may be delayed in those with sun-sensitive skin types and those with skin cancer. One molecular explanation for this putative phenotype would be that it is caused by mutations of the melanocortin 1 receptor (MC1R). In the present study of 20 persons, 10 of whom were MC1R homozygous, we measured erythema over a 21-day period in response to a range of ultraviolet B doses using methods that improved on previous studies. We could detect no consistent differences in ultraviolet radiation-induced erythema between the groups studied. The pharmacological mechanisms underpinning such prolonged inflammatory responses merit further investigation.

The genetics of malignant melanoma: lessons from mouse and man

Therapeutic resistance and proclivity for metastasis are hallmarks of malignant melanoma. Genetic, epidemiological and genomic investigations are uncovering the spectrum of stereotypical mutations that are associated with melanoma and how these mutations relate to risk factors such as ultraviolet exposure. The ability to validate the pathogenetic relevance of these mutations in the mouse, coupled with advances in rational drug design, has generated optimism for the development of effective prevention programmes, diagnostic measures and targeted therapeutics in the near future.

DNA polymorphism and selection at the melanocortin-1 receptor gene in normally pigmented southern African individuals

Skin pigmentation is a polygenic multifactorial trait determined by the cumulative effects of multiple genetic variants and environmental factors. Melanocortin-1 receptor (MC1R) is one of the genes involved in pigmentation, and has been implicated in the red hair and pale skin phenotype in human Caucasoid individuals. The present study was undertaken to identify variation at the MC1R locus in normally pigmented individuals in two African populations, sub-Saharan Negroids (22 unrelated individuals) and the San (17 unrelated individuals). The study showed considerable MC1R gene sequence variation with the detection of eight synonymous and three nonsynonymous mutations. This is the first report of nonsynonymous mutations in African individuals in the MC1R gene: L99I was found in a single San individual, S47I was detected in a single Negroid individual, and F196L was detected in five Negroid individuals (5/44; 0.11). The functional significance of these mutations is not known. Three of the eight synonymous mutations found, L106L (CTG --> CTA), F300F (TTC --> TTT), and T314T (ACA --> ACG) (also known as A942G), have been reported previously. T314T was the only variant that showed a significant difference between the Negroid and San populations (0.477 and 0.059, respectively; P = 1.6 x 10(-5)). Its low frequency in the San may be the result of random genetic drift in a population of small size, or selection. Several tests of neutrality of the MC1R coding region in these and other African populations were significant, suggesting that purifying selection (functional constraint) had occurred at this gene locus in Africans. This demonstrates that although some nonsynonymous MC1R mutations are tolerated in individuals with dark skin, this gene has likely played a significant role in the maintenance of dark pigmentation in Africans and normal pigment variation in non-African populations.

P-locus is a target for the melanogenic effects of MC-1R signaling: a possible control point for facultative pigmentation

Melanocortin receptor type 1 (MC-1R) is an important control point for ultraviolet ray (UVR)-induced tanning response in the skin. In this study, we show that p-locus is a downstream target for MC-1R signaling. The expression of p-locus was up-regulated by alpha-MSH as well as db-cAMP, a synthetic analogue of cAMP that mimics activation of MC-1R. Furthermore, p-locus transcript abundance was significantly increased in epidermal melanocytes of white skin with facultative (UVR-induced) pigmentation. Because p-locus product is essential for pigmentation and also has been shown to be highly polymorphic in human population, we propose that the pigmentary response to the melanocortin peptides/UVR would be affected not only by MC-1R mutations but also by the functionality of p-locus product. These factors together could account for the many different levels of tanning ability seen in the white population.

Significance of the melanocortin 1 receptor in regulating human melanocyte pigmentation, proliferation, and survival

The characterization of the melanocortin 1 receptor (MC1R) expressed on human melanocytes and the findings that certain mutations in the POMC gene or the MC1R gene result in red hair phenotype underscore the significance of melanocortins and MC1R in regulating human pigmentation. We demonstrated that human melanocytes respond to alpha-melanocortin (alpha-MSH) or ACTH with increased proliferation and melanogenesis, and to agouti signaling protein by abrogation of these effects. alpha-MSH and ACTH were equipotent and more potent than beta-MSH, and gamma-MSH was the least potent in activating the MC1R and stimulating melanogenesis and proliferation of human melanocytes. We characterized the MC1R genotype in a panel of human melanocyte cultures and identified three cultures that were homozygous for Arg160Trp, heterozygous for Arg151Cys and Asp294His, and heterozygous for Arg160Trp and Asp294His substitutions, respectively. Those cultures failed to respond to alpha-MSH with increase in cAMP levels, tyrosinase activity, or proliferation and had an exaggerated response to the cytotoxic effect of ultraviolet (UV) radiation. These loss-of-function mutations have been associated with red hair phenotype and increased risk for skin cancer. Melanocytes homozygous for Val29Met substitution in MC1R responded normally to alpha-MSH and UVB, suggesting that this variant is a polymorphism. We observed that alpha-MSH promotes human melanocyte survival by inhibiting the UV-induced apoptosis independently of melanin synthesis. This effect was absent in human melanocytes with loss of function MC1R mutations. We predict that the survival effect of alpha-MSH is caused by reduction of UV-induced DNA damage and contributes to the prevention of melanoma.

Defining the quantitative contribution of the melanocortin 1 receptor (MC1R) to variation in pigmentary phenotype

The melanocortin 1 receptor (MC1R) is a key determinant of pigmentary phenotype. Several sequence variants of the MC1R have been described, many of which are associated with red hair and cutaneous sensitivity to ultraviolet radiation even in the absence of red hair. Red hair approximates to an autosomal recessive trait, and most people with red hair are compound heterozygote or homozygous for limited numbers of mutations that show impaired function in in vitro assays. There is a clear heterozygote effect on sun sensitivity (even in those without red hair) and with susceptibility to the most common forms of skin cancer.

Screening of human primary melanocytes of defined melanocortin-1 receptor genotype: pigmentation marker, ultrastructural and UV-survival studies

Recent population studies have demonstrated an association with the red-hair and fair-skin phenotype with variant alleles of the melanocortin-1 receptor (MC1R) which result in amino acid substitutions within the coding region leading to an altered receptor activity. In particular, Arg151Cys, Arg160Trp and Asp294His were the most commonly associated variants seen in the south-east Queensland population with at least one of these alleles found in 93% of those with red hair. In order to study the individual effects of these variants on melanocyte biology and melanocytic pigmentation, we established a series of human melanocyte strains genotyped for the MC1R receptor which included wild-type consensus, variant heterozygotes, compound heterozygotes and homozygotes for Arg151Cys, Arg160Trp, Val60Leu and Val92Met alleles. These strains ranged from darkly pigmented to amelanotic, with all strains of consensus sequence having dark pigmentation. UV sensitivity was found not to be associated with either MC1R genotype or the level of pigmentation with a range of sensitivities seen across all genotypes. Ultrastructural analysis demonstrated that while consensus strains contained stage IV melanosomes in their terminal dendrites, Arg151Cys and Arg160Trp homozygote strains contained only stage II melanosomes. This was despite being able to show expression of tyrosinase and tyrosinase-related protein-1 markers, although at reduced levels and an ability to convert exogenous 3,4-dihydroxyphenyl-alanine (DOPA) to melanin in these strains.

The role of melanocortin-1 receptor polymorphism in skin cancer risk phenotypes

We have examined melanocortin-1 receptor (MC1R) variant allele frequencies in the general population and in a collection of adolescent dizygotic and monozygotic twins to determine statistical associations of pigmentation phenotypes with increased skin cancer risk. This included hair and skin color, freckling, mole count and sun exposed skin reflectance. Nine variants were studied and designated as either strong R (OR = 63; 95% CI 32-140) or weak r (OR = 5; 95% CI 3-11) red hair alleles. Penetrance of each MC1R variant allele was consistent with an allelic model where effects were multiplicative for red hair but additive for skin reflectance. To assess the interaction of the brown eye color gene BEY2/OCA2 on the phenotypic effects of variant MC1R alleles we imputed OCA2 genotype in the twin collection. A modifying effect of OCA2 on MC1R variant alleles was seen on constitutive skin color, freckling and mole count. In order to study the individual effects of these variants on pigmentation phenotype we have established a series of human primary melanocyte strains genotyped for the MC1R receptor. These include strains which are MC1R wild-type consensus, variant heterozygotes, and homozygotes for strong R alleles Arg151Cys and Arg160Trp. Ultrastructural analysis demonstrated that only consensus strains contained stage III and IV melanosomes in their terminal dendrites whereas Arg151Cys and Arg160Trp homozygous strains contained only immature stage I and II melanosomes. Such genetic association studies combined with the functional analysis of MC1R variant alleles in melanocytic cells should provide a link in understanding the association between pigmentary phototypes and skin cancer risk.

Ultraviolet radiation and cutaneous malignant melanoma

Recent years have seen a steady rise in the incidence of cutaneous malignant melanoma worldwide. Although it is now appreciated that the key to understanding the process by which melanocytes are transformed into malignant melanoma lies in the interplay between genetic factors and the ultraviolet (UV) spectrum of sunlight, the nature of this relation has remained obscure. Recently, prospects for elucidating the molecular mechanisms underlying such gene-environment interactions have brightened considerably through the development of UV-responsive experimental animal models of melanoma. Genetically engineered mice and human skin xenografts constitute novel platforms upon which to build studies designed to elucidate the pathogenesis of UV-induced melanomagenesis. The future refinement of these in vivo models should provide a wealth of information on the cellular and genetic targets of UV, the pathways responsible for the repair of UV-induced DNA damage, and the molecular interactions between melanocytes and other skin cells in response to UV. It is anticipated that exploitation of these model systems will contribute significantly toward the development of effective approaches to the prevention and treatment of melanoma.

Skin pigmentation, biogeographical ancestry and admixture mapping

Ancestry informative markers (AIMs) are genetic loci showing alleles with large frequency differences between populations. AIMs can be used to estimate biogeographical ancestry at the level of the population, subgroup (e.g. cases and controls) and individual. Ancestry estimates at both the subgroup and individual level can be directly instructive regarding the genetics of the phenotypes that differ qualitatively or in frequency between populations. These estimates can provide a compelling foundation for the use of admixture mapping (AM) methods to identify the genes underlying these traits. We present details of a panel of 34 AIMs and demonstrate how such studies can proceed, by using skin pigmentation as a model phenotype. We have genotyped these markers in two population samples with primarily African ancestry, viz. African Americans from Washington D.C. and an African Caribbean sample from Britain, and in a sample of European Americans from Pennsylvania. In the two African population samples, we observed significant correlations between estimates of individual ancestry and skin pigmentation as measured by reflectometry (R(2)=0.21, P<0.0001 for the African-American sample and R(2)=0.16, P<0.0001 for the British African-Caribbean sample). These correlations confirm the validity of the ancestry estimates and also indicate the high level of population structure related to admixture, a level that characterizes these populations and that is detectable by using other tests to identify genetic structure. We have also applied two methods of admixture mapping to test for the effects of three candidate genes (TYR, OCA2, MC1R) on pigmentation. We show that TYR and OCA2 have measurable effects on skin pigmentation differences between the west African and west European parental populations. This work indicates that it is possible to estimate the individual ancestry of a person based on DNA analysis with a reasonable number of well-defined genetic markers. The implications and applications of ancestry estimates in biomedical research are discussed.

The biology of melanocytes

In veterinary medicine, our understanding of the biology and regulation of melanocytic function is mostly based on information realized from human and murine studies. Improved understanding of the biology of melanocytes is needed to develop more effective treatment regimens for malignant melanoma and other melanocytic disorders. In vertebrates, melanocytes are well known for their role in skin pigmentation, hair and feather coloration, and for their ability to produce and distribute melanin to surrounding keratinocytes. Enzymes involved in melanin synthesis are present exclusively in melanosomes. The type of melanin synthesized by melanocytes in mammals is regulated at a genetic, biochemical and environmental level. These regulatory factors affect not only the phenotypic appearance, but also the photoprotective properties of melanin. This review addresses the biology of melanocytes, melanin synthesis and the photoprotective properties of melanin.

Redundancy of a functional melanocortin 1 receptor in the anti-inflammatory actions of melanocortin peptides: studies in the recessive yellow (e/e) mouse suggest an important role for melanocortin 3 receptor

The issue of which melanocortin receptor (MC-R) is responsible for the anti-inflammatory effects of melanocortin peptides is still a matter of debate. Here we have addressed this aspect using a dual pharmacological and genetic approach, taking advantage of the recent characterization of more selective agonists/antagonists at MC1 and MC3-R as well as of the existence of a naturally defective MC1-R mouse strain, the recessive yellow (e/e) mouse. RT-PCR and ultrastructural analyses showed the presence of MC3-R mRNA and protein in peritoneal macrophages (M phi) collected from recessive yellow (e/e) mice and wild-type mice. This receptor was functional as Mphi incubation (30 min) with melanocortin peptides led to accumulation of cAMP, an effect abrogated by the MC3/4-R antagonist SHU9119, but not by the selective MC4-R antagonist HS024. In vitro M phi activation, determined as release of the CXC chemokine KC and IL-1 beta, was inhibited by the more selective MC3-R agonist gamma(2)-melanocyte stimulating hormone but not by the selective MC1-R agonist MS05. Systemic treatment of mice with a panel of melanocortin peptides inhibited IL-1 beta release and PMN accumulation elicited by urate crystals in the murine peritoneal cavity. MS05 failed to inhibit any of the inflammatory parameters either in wild-type or recessive yellow (e/e) mice. SHU9119 prevented the inhibitory actions of gamma(2)-melanocyte stimulating hormone both in vitro and in vivo while HS024 was inactive in vivo. In conclusion, agonism at MC3-R expressed on peritoneal M phi leads to inhibition of experimental nonimmune peritonitis in both wild-type and recessive yellow (e/e) mice.

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.

Regulation of the human melanocortin 1 receptor expression in epidermal melanocytes by paracrine and endocrine factors and by ultraviolet radiation

The aim of this study is to investigate the regulation of the human melanocortin 1 receptor (MC1R) expression in cultured normal human melanocytes (NHM) by specific paracrine and endocrine factors, and by ultraviolet radiation (UVR). Treatment of NHM with alpha-melanotropin [alpha-melanocyte stimulating hormone (alpha-MSH)] increased MC1R mRNA level; the response was often more pronounced in NHM with a low (NHM-c) than in NHM with a high melanin content (NHM-b). Endothelin-1 increased MC1R mRNA level in NHM regardless of their melanin content. Basic fibroblast growth factor consistently up regulated MC1R mRNA level in NHM-b but not in NHM-c. Activation of protein kinase C by 12-0-tetradecanoylphorbol-13-acetate slightly increased, while stimulation of adenylate cyclase by forskolin markedly up-regulated the MC1R mRNA level. beta-Estradiol increased, and combined treatment with beta-estradiol and alpha-MSH further elevated, MC1R mRNA level in NHM-c and NHM-b. Testosterone reduced, while progesterone had no effect on, MC1R mRNA level. Agouti signaling protein reduced, and UVR down regulated dose-dependently MC1R mRNA level in NHM-b and NHM-c. This effect was reversed 24 h after irradiation with the lower doses of 7 or 14 mJ/cm2, but not after exposure to a higher, more cytotoxic dose of UVR. We conclude that the MC1R is regulated by paracrine factors, including its own ligands, by specific endocrine sex hormones, and by UVR. Differences in the responses of NHM to some of these factors suggest differential regulation of MC1R gene expression, which may contribute to the variation in constitutive and UV-induced cutaneous pigmentation in humans.

Human melanocortin 1 receptor (MC1R) gene variants alter melanoma cell growth and adhesion to extracellular matrix

Pigmentation is a significant determinant of individual susceptibility to cutaneous melanoma, with fair skinned subjects at highest risk of developing this neoplasm. Melanocortin 1 receptor (MC1R) gene variants alter pigment synthesis in vivo, and are causally associated with red hair and fair skin in humans. MC1R variants are more frequent in subjects with melanoma, and increase the risk of developing this tumour in sporadic and familial cases. MC1R variants may predispose to melanoma as a result of alterations in skin pigmentation (which affords less protection against incident ultraviolet radiation). However, melanoma cells synthesize and release alpha-melanocyte stimulating hormone (alphaMSH, the ligand for MC1R), therefore MC1R variants could alter the autocrine effects of alphaMSH on melanoma cell behaviour, thereby affecting early melanoma development and progression via non-pigmentary mechanisms. B16G4F melanoma cells, which are functionally null at Mc1r, were stably transfected with wild type and variant (Arg151Cys, Arg160Trp, and Asp294His) human MC1R. At similar MC1 receptor numbers per cell, alphaMSH increased intracellular cAMP in wild type MC1R transfected melanoma cells, but the cAMP response was compromised in the variant MC1R transfected clones. In growth inhibition experiments, alphaMSH significantly reduced growth of wild type MC1R transfected cells, but had no effect on cells transfected with variant MC1R. In addition, binding to fibronectin was significantly reduced by alphaMSH in the wild type transfectants whereas this was not observed in the variant transfected clones; binding to laminin was not affected by alphaMSH in this cell line. These results provide evidence for differences in melanoma cell behaviour secondary to MC1R variants, and suggest an alternative non-pigmentary mechanism whereby MC1R variants could modify melanoma susceptibility or progression.

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.

Red hair - a desirable mutation?

Red hair is one of the most striking variants of human hair coloration and has historically been of profound social importance. Red hair in man is due to certain loss of function mutations of one of the peptide products of the pro-opiomelanocortin (POMC) gene, the melanocortin-1 receptor (MC1R, MIM 155555). Such functional mutations enable the melanocyte to produce red-yellow pheomelanin in preference to the default, black-brown eumelanin. This paper reviews the path of discovery of the MC1R in control of animal coat colour, the subsequent role of MC1R in human physiology and possibly wider role of MC1R in human skin carcinogenesis and human development through history.

Humanmelanocortin 1 receptorvariants, receptor function and melanocyte response to UV radiation

Cutaneous pigmentation is determined by the amounts of eumelanin and pheomelanin synthesized by epidermal melanocytes and is known to protect against sun-induced DNA damage. The synthesis of eumelanin is stimulated by the binding of α-melanotropin (α-melanocyte-stimulating hormone)to the functional melanocortin 1 receptor (MC1R) expressed on melanocytes. The human MC1R gene is highly polymorphic and certain allelic variants of the gene are associated with red hair phenotype, melanoma and non-melanoma skin cancer. The importance of the MC1R gene in determining skin cancer risk led us to examine the impact of specific polymorphisms in this gene on the responses of human melanocytes to α-melanotropin and UV radiation. We compared the ability of human melanocyte cultures, each derived from a single donor, to respond to α-melanotropin with dose-dependent stimulation of cAMP formation, tyrosinase activity and proliferation. In each of those cultures the MC1R gene was sequenced, and the eumelanin and pheomelanin contents were determined. Human melanocytes homozygous for Arg160Trp, heterozygous for Arg160Trp and Asp294His, or for Arg151Cys and Asp294His substitutions, but not melanocytes homozygous for Val92Met substitution, in the MC1R demonstrated a significantly reduced response toα-melanotropin. Additionally, melanocytes with a non-functional MC1R demonstrated a pronounced increase in their sensitivity to the cytotoxic effect of UV radiation compared with melanocytes expressing functional MC1R. We conclude that loss-of-function mutations in the MC1R gene sensitize human melanocytes to the DNA damaging effects of UV radiation, which may increase skin cancer risk.

Melanocyte function and its control by melanocortin peptides

Melanocytes are cells of neural crest origin. In the human epidermis, they form a close association with keratinocytes via their dendrites. Melanocytes are well known for their role in skin pigmentation, and their ability to produce and distribute melanin has been studied extensively. One of the factors that regulates melanocytes and skin pigmentation is the locally produced melanocortin peptide alpha-MSH. The effects of alpha-MSH on melanogenesis are mediated via the MC-1R and tyrosinase, the rate-limiting enzyme in the melanogenesis pathway. Binding of alpha-MSH to its receptor increases tyrosinase activity and eumelanin production, which accounts for the skin-darkening effect of alpha-MSH. Other alpha-MSH-related melanocortin peptides, such as ACTH1-17 and desacetylated alpha-MSH, are also agonists at the MC-1R and could regulate melanocyte function. Recent evidence shows that melanocytes have other functions in the skin in addition to their ability to produce melanin. They are able to secrete a wide range of signal molecules, including cytokines, POMC peptides, catecholamines, and NO in response to UV irradiation and other stimuli. Potential targets of these secretory products are keratinocytes, lymphocytes, fibroblasts, mast cells, and endothelial cells, all of which express receptors for these signal molecules. Melanocytes may therefore act as important local regulators of a range of skin cells. It has been shown that alpha-MSH regulates NO production from melanocytes, and it is possible that the melanocortins regulate the release of other signalling molecules from melanocytes. Therefore, the melanocortin signaling system is one of the important regulators of skin homeostasis.

The role of melanocortins in skin homeostasis

Melanocortins are structurally related bioactive peptides which are produced by many extra-neural tissues including the skin. All of the melanocortins (alpha, beta, and gamma-melanocyte-stimulating hormone and adrenocorticotropin) have melanotropic activity but can elicit many other effects on skin cells. On the basis of in vitro and in vivo findings melanocortins have been shown to regulate immune and inflammatory responses, hair growth, exocrine gland activity and extracellular matrix composition. These effects are mediated by melanocortin receptors among which the melanocortin-1 receptor is most ubiquitously expressed by human skin cells. Simultaneous expression of melanocortins and their receptors suggest a complex autocrine and/or paracrine regulatory network whose disruption invariably affects skin homeostasis. Expression of melanocortin receptors on various skin cell types further indicates novel pharmacological targets for the treatment of skin diseases.

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

Activation by melanocortins of the melanocortin 1 receptor (MC1R), expressed in epidermal melanocytes, stimulates melanogenesis. Human MC1R gene loss-of-function mutations are associated with fair skin, poor tanning and increased skin cancer risk. We identified two natural alleles: Ile40Thr, probably associated with skin types I-II, and Val122Met. Val122Met bound [(125)I][Nle(4), D-Phe(7)]-alpha-melanocyte stimulating hormone with lower affinity than the wild-type. Dose-response curves of cAMP accumulation were right-shifted for both forms. The Val122Met form failed to achieve maximal cAMP responses comparable to the wild-type or Ile40Thr receptors. Thus, the Ile40Thr and Val122Met variants are partial loss-of-function natural mutations of MC1R.

Sequence polymorphism in the human melanocortin 1 receptor gene as an indicator of the red hair phenotype

We describe a minisequencing protocol for screening DNA samples for the presence of 12 mutations in the human melanocortin 1 receptor gene (MC1R), eight of which are associated with the red hair phenotype. A minisequencing profile which shows homozygosity for one of these mutations or the presence of two different mutations would strongly indicate that the sample donor is red haired. The absence of any red hair causing mutations would indicate that the sample donor does not have red hair. We report the frequencies of MC1R variants in the British red haired population.

The relation between melanocortin 1 receptor genotype and experimentally assessed ultraviolet radiation sensitivity

Pigmentary phenotype is a key determinant of an individual's response to ultraviolet radiation with the presence of phaeomelanin thought to be of particular importance. Reports of minimal erythema testing, however, have failed to show a consistent difference between skin type I and other skin types. The melanocortin 1 receptor is a key genetic determinant of the cutaneous response to ultraviolet radiation. In this study we investigate the relation between experimentally induced erythemal response to ultraviolet radiation and the melanocortin 1 receptor genotype. Phototesting was performed in 20 redheads and 20 nonredheaded subjects, the majority of whom were also screened for the presence of melanocortin 1 receptor variants. The majority of redheads sequenced (89%) had two melanocortin 1 receptor variants previously found to be associated with red hair compared to none of the controls. There was no significant difference between the groups in minimal erythema dose: the median minimal erythema dose in redheads was 44 mJ per cm2 (interquartile range 34-56) and in the nonredheaded group was 40 mJ per cm2 (interquartile range 40-56). Objective measurements of ultraviolet-B-induced erythema were performed using reflectance instrument measurements of erythema intensity and dose-response curves constructed for each subject. The slope of the dose-response curve in the redheaded group was statistically greater than in the nonredheaded group (median in redheads 4.08 vs 3.56 for controls, 95% confidence interval for the difference between the medians being 0.01-1.23, p = 0.043). In addition the ratio D0.05:D0.025 was significantly lower for the redheaded group (median in redheads 1.22, interquartile range 1.18-1.26; median in nonreds 1.28, interquartile range 1.23-1.32; p < 0.05). Thus, although the minimal erythema dose values were not different, subjects with red hair develop greater intensity of erythema than nonredheaded individuals when doses greater than the minimal erythema dose are given. Importantly, when analyzed by genotype alone rather than phenotype, the slope of the erythema dose-response differed between those persons who were homozygous or heterozygous mutants and wildtype/pseudo-wildtype (p = 0.026).

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

The synthesis of the visible pigment melanin by the melanocyte cell is the basis of the human pigmentary system, those genes directing the formation, transport and distribution of the specialised melanosome organelle in which melanin accumulates can legitimately be called pigmentation genes. The genes involved in this process have been identified through comparative genomic studies of mouse coat colour mutations and by the molecular characterisation of human hypopigmentary genetic diseases such as OCA1 and OCA2. The melanocyte responds to the peptide hormones alpha-MSH or ACTH through the MC1R G-protein coupled receptor to stimulate melanin production through induced maturation or switching of melanin type. The pheomelanosome, containing the key enzyme of the pathway tyrosinase, produces light red/yellowish melanin, whereas the eumelanosome produces darker melanins via induction of additional TYRP1, TYRP2, SILV enzymes, and the P-protein. Intramelanosomal pH governed by the P-protein may act as a critical determinant of tyrosinase enzyme activity to control the initial step in melanin synthesis or TYRP complex formation to facilitate melanogenesis and melanosomal maturation. The search for genetic variation in these candidate human pigmentation genes in various human populations has revealed high levels of polymorphism in the MC1R locus, with over 30 variant alleles so far identified. Functional correlation of MC1R alleles with skin and hair colour provides evidence that this receptor molecule is a principle component underlying normal human pigment variation.