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

Hierarchical modeling of melanocortin 1 receptor variants with skin cancer risk

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

Functional melanocortin 1 receptor Mc1r is not necessary for an inflammatory response to UV radiation in adult mouse skin

The G-protein-coupled receptor, Mc1r, plays a major role in pigment production and has been reported to be important in the inflammatory response. We have investigated the effect of deficiency in Mc1r on UV-induced inflammation. Mice on the same genetic background were used - C57BL/6-c (albino), C57BL/6 (black), C57BL/6-Mc1r(e/e) deficient (yellow). FACS analysis of disaggregated skin showed a similar dose-dependent increase in Ly6G(+) and CD11b(+) cells in response to UV radiation in all groups. No differences in UV-induced edema or in DNA damage were detected between groups. The contact hypersensitivity response, neonatal immune tolerance and UV immunosuppression were all similar in C57BL/6 and C57BL/6-Mc1r(e/e) mice. We conclude that the absence of Mc1r does not impair the inflammatory response to UV radiation or the generation of immunosuppression.

PGC-1 coactivators regulate MITF and the tanning response

The production of pigment by melanocytes tans the skin and protects against skin cancers. UV-exposed keratinocytes secrete α-MSH, which then activates melanin formation in melanocytes by inducing the microphthalmia-associated transcription factor (MITF). We show that PPAR-γ coactivator (PGC)-1α and PGC-1β are critical components of this melanogenic system in melanocytes. α-MSH signaling strongly induces PGC-1α expression and stabilizes both PGC-1α and PGC-1β proteins. The PGC-1s in turn activate the MITF promoter, and their expression correlates strongly with that of MITF in human melanoma cell lines and biopsy specimens. Inhibition of PGC-1α and PGC-1β blocks the α-MSH-mediated induction of MITF and melanogenic genes. Conversely, overexpression of PGC-1α induces pigment formation in cell culture and transgenic animals. Finally, polymorphism studies reveal expression quantitative trait loci in the PGC-1β gene that correlate with tanning ability and protection from melanoma in humans. These data identify PGC-1 coactivators as regulators of human tanning.

The physiological and phenotypic determinants of human tanning measured as change in skin colour following a single dose of ultraviolet B radiation

Experimental study of the in vivo kinetics of tanning in human skin has been limited by the difficulties in measuring changes in melanin pigmentation independent of the ultravioletinduced changes in erythema. The present study attempted to experimentally circumvent this issue. We have studied erythemal and tanning responses following a single exposure to a range of doses of ultraviolet B irradiation on the buttock and the lower back in 98 subjects. Erythema was assessed using reflectance techniques at 24 h and tanning measured as the L* spectrophotometric score at 7 days following noradrenaline iontophoresis. We show that dose (P < 0.0001), body site (P < 0.0001), skin colour (P < 0.0001), ancestry (P = 0.0074), phototype (P = 0.0019) and sex (P = 0.04) are all independent predictors of erythema. Quantitative estimates of the effects of these variables are reported, but the effects of ancestry and phototype do not appear solely explainable in terms of L* score. Dose (P < 0.0001), body site (P < 0.0001) and skin colour (P = 0.0365) or, as an alternative to skin colour, skin type (P = 0.0193) predict tanning, with those with lighter skin tanning slightly more to a defined UVB dose. If erythema is factored into the regression, then only dose and body site remain significant predictors of tanning: therefore neither phototype nor pigmentary factors, such as baseline skin colour, or eye or hair colour, predict change in skin colour to a unit erythemal response.

Quantitative analysis of MC1R gene expression in human skin cell cultures

To address the issue of melanocortin-1 receptor (MC1R) expression in non-melanocytic cells, we have quantitatively evaluated the relative expression levels of both MC1R mRNA and protein in a subset of different cell types. Using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) at high cycle numbers, we detected MC1R mRNA in all cell types examined, including human embryonic kidney-293 (HEK 293) cells, a cell type widely used as a negative control in melanocortin expression studies. Quantitative real-time PCR revealed the highest levels of MC1R transcripts were in melanocytic cells, whereas the keratinocyte and fibroblast cell cultures examined had only a low level of expression, similar to that of HEK 293 cells. Antibody mediated detection of MC1R protein in membrane extracts demonstrated exogenous receptor in MC1R transfected cell lines, as well as endogenous MC1R in melanoma cells. However, radioligand binding procedures were required to detect MC1R protein of normal human melanocytes and no surface expression of MC1R was detected in any of the non-melanocytic cells examined. This was consistent with their low level of mRNA, and suggests that, if present, the levels of surface receptor are significantly lower than that in melanocytes. The capacity of such limited levels of MC1R protein to influence non-melanocytic skin cell biology would likely be severely compromised. Indeed, the MC1R agonist [NIe(4), D-Phe(7)] alpha-melanocyte stimulating hormone (NDP-MSH) was unable to elevate intracellular cyclic adenosine monophosphate (cAMP) levels in the keratinocyte and fibroblast cells examined, whereas a robust increase was elicited in melanocytes. Although there are a variety of cell types with detectable MC1R mRNA, the expression of physiologically significant levels of the receptor may be more restricted than the current literature indicates, and within epidermal tissue may be limited to the melanocyte.

Melanocortin 1 receptor variants and skin cancer risk

Melanocortin 1 receptor (MC1R) gene variants are associated with red hair and fair skin color. We assessed the associations of common MC1R genotypes with the risks of 3 types of skin cancer simultaneously in a nested case-control study within the Nurses' Health Study (219 melanoma, 286 squamous cell carcinoma (SCC), and 300 basal cell carcinoma (BCC) cases, and 873 controls). We found that the 151Cys, 160Trp and 294His variants were significantly associated with red hair, fair skin color and childhood tanning tendency. The MC1R variants, especially the 151Cys variant, were associated with increased risks of the 3 types of skin cancer, after controlling for hair color, skin color and other skin cancer risk factors. Carriers of the 151Cys variant had an OR of 1.65 (95% CI, 1.04-2.59) for melanoma, 1.67 (1.12-2.49) for SCC and 1.56 (1.03-2.34) for BCC. Women with medium or olive skin color carrying 1 nonred hair color allele and 1 red hair color allele had the highest risk of melanoma. A similar interaction pattern was observed for red hair and carrying at least 1 red hair color allele on melanoma risk. We also observed that the 151Cys variant contributed additional melanoma risk among red-haired women. The information on MC1R status modestly improved the risk prediction; the increase was significant for melanoma and BCC (p, 0.004 and 0.05, respectively). These findings indicated that the effects of the MC1R variants on skin cancer risk were independent from self-reported phenotypic pigmentation.

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.

Physiological variation in the erythemal response to ultraviolet radiation and photoadaptation

We have studied the cutaneous response to ultraviolet radiation, measured objectively as erythema in a sample of 12 body sites on 15 Northern European subjects with multiple doses of ultraviolet B (UVB). Skin pigmentation and the development of photoadaptation in response to five repeated doses of irradiation at three body sites was also measured. We report striking differences of up to 5-fold at different body sites to the same challenge dose (p < 0.001) and demonstrate that for this population, site variation is just as important as between-person variation. Skin color at each body site is a strong predictor of response (p < 0.001) and that this cannot be attributed to vascular differences, but instead we believe it reflects site-specific variations in melanin pigmentation. We also observed similar but smaller within-person effects for responses to another inflammatory agent, dithranol (p < 0.01). Despite this, we did not find evidence for differences in the development of photoadaptation by body site. These results have clear clinical implications for the practice of phototesting prior to commencing phototherapy, for therapeutic failure in sites such as the legs in patients with psoriasis, and perhaps for melanoma body-site distribution.

The Time Course of Photoadaptation and Pigmentation Studied Using a Novel Method to Distinguish Pigmentation from Erythema

The dynamics of human pigmentation in response to ultraviolet radiation (UVR) remain poorly characterized. In part, this is attributable to methodological issues relating to the overlap in spectra of hemoglobin and melanin. We describe a new method, based on the recording of reflectance properties following iontophoresis of a potent vasoconstrictor, noradrenaline. This removes the influence of blood, allowing measurement of pigmentation, represented as L* on the L*a*b* scale. Blood flow was separately assessed using laser Doppler flowmetry. We show that there is a clear dose response with the dose of UVR administered, that pigmentation peaks at 1 wk and declines over the following 10 wk, but does not return to baseline within this period. We show clear differences in the degree, but not the temporal pattern of pigmentation between different pigmentary groups. We also report that the relation between facultative pigment and constitutive pigment is incomplete, with a wide scatter of responses for the development of pigmentation irrespective of constitutive levels. For comparison we also document overall photoadaptation and relate changes in pigmentation to the overall changes in photoadaptation.

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.

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.

The relationship between constitutive pigmentation and sensitivity to ultraviolet radiation induced erythema is dose-dependent

The relationship between skin colour and experimental exposure to ultraviolet radiation (UVR) B, with response measured as erythema was studied. Two reflectance methods were used to measure skin colour--tristimulus colorimetry using a Minolta instrument (summarized as the alpha characteristic angle) and the melanin index based on the Diastron reflectance instrument. As expected both measures are highly correlated (0.91). A dose-dependent relationship between skin colour measured as the alpha characteristic angle and UVR was established, with the gradient increasing from 0.99 at 119 mJ to 2.7 at 300 mJ, with the relevant standard errors being 0.39 and 0.47, respectively. Similarly, for the melanin index (where the scale goes in the opposite direction) the gradient differs between -0.49 for 119 mJ and -0.91 for 300 mJ, with the standard errors being 0.14 and 0.17 respectively. The proportion of variation explained is also greater at higher UVR challenge doses. Studies relating UVR sensitivity and pigmentation need to take account of the dose of UVR administered.

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.

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.

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.