Pigmentation, melanocortins and red hair

Influence of Ethnicities and Skin Color Variations in Different Populations: A Review

BACKGROUND

In the world scientific tradition, skin color is the primary physical characteristic used to divide humans into groups. Human skin has a wide range of tones and colors, which can be seen in a wide range of demographic populations. Many factors influence the color of people's skin, but the pigment melanin is by far the most important. Melanin is produced by cells called melanocytes in the skin and is the primary determinant of skin color in people with darker skin. Indeed, >150 genes have now been identified as having a direct or indirect effect on skin color. Vitamin D has recently been discovered to regulate cellular proliferation and differentiation in a variety of tissues, including the skin. The mechanisms through which the active vitamin D metabolite 1,25 dihydroxyvitamin D3 (or calcitriol) affects keratinocyte development are numerous and overlap with the mechanisms by which calcium influences keratinocyte differentiation. Ultraviolet (UV) is the most major modifiable risk factor for skin cancer and many other environmental-influenced skin disorders when it is abundant in the environment. Although the UV component of sunlight is known to cause skin damage, few researches have looked at the impact of non-UV solar radiation on skin physiology in terms of inflammation, and there is less information on the role of visible light in pigmentation.

SUMMARY

The quantity and quality of melanin are regulating by the expression of genes. The enzyme tyrosinase is primarily responsible for the genetic mechanism that controls human skin color. Genetics determines constitutive skin color, which is reinforced by facultative melanogenesis and tanning reactions. High quantities of melanin and melanogenic substances are typically accepted in darker skin to protect against UV radiation-induced molecular damage. Previous research has proposed that skin color variation is caused by a dynamic genetic mechanism, contributing to our understanding of how population demographic history and natural selection shape human genetic and phenotypic diversity. However, the most significant ethnic skin color difference is determined by melanin content. This current review aimed to assess the influence of skin color variations in skin structure and functions as well as difference in dermatological disease patterns. Also, this article reviewed several cases of skin color adaptation in different populations. Key Messages: Skin color impacts the composition and activity. Therefore, the contrast of dermatological ailments between distinct race-related categories is remarkable. Skin color adaptation is a challenging procedure. Refinement of skin color is an age-old craving of humans with ever-evolving drifts.

Evolutionary genetics of skin pigmentation in African populations

Skin color is a highly heritable human trait, and global variation in skin pigmentation has been shaped by natural selection, migration and admixture. Ethnically diverse African populations harbor extremely high levels of genetic and phenotypic diversity, and skin pigmentation varies widely across Africa. Recent genome-wide genetic studies of skin pigmentation in African populations have advanced our understanding of pigmentation biology and human evolutionary history. For example, novel roles in skin pigmentation for loci near MFSD12 and DDB1 have recently been identified in African populations. However, due to an underrepresentation of Africans in human genetic studies, there is still much to learn about the evolutionary genetics of skin pigmentation. Here, we summarize recent progress in skin pigmentation genetics in Africans and discuss the importance of including more ethnically diverse African populations in future genetic studies. In addition, we discuss methods for functional validation of adaptive variants related to skin pigmentation.

The Evolutionary History of Human Skin Pigmentation

Skin pigmentation is a complex, conspicuous, highly variable human trait that exhibits a remarkable correlation with latitude. The evolutionary history and genetic basis of skin color variation has been the subject of intense research in the last years. This article reviews the major hypotheses explaining skin color diversity and explores the implications of recent findings about the genes associated with skin pigmentation for understanding the evolutionary forces that have shaped the current patterns of skin color variation. A major aspect of these findings is that the genetic basis of skin color is less simple than previously thought and that geographic variation in skin pigmentation was influenced by the concerted action of different types of natural selection, rather than just by selective sweeps in a few key genes.

Adaptation of human skin color in various populations

BACKGROUND

Skin color is a well-recognized adaptive trait and has been studied extensively in humans. Understanding the genetic basis of adaptation of skin color in various populations has many implications in human evolution and medicine.

DISCUSSION

Impressive progress has been made recently to identify genes associated with skin color variation in a wide range of geographical and temporal populations. In this review, we discuss what is currently known about the genetics of skin color variation. We enumerated several cases of skin color adaptation in global modern humans and archaic hominins, and illustrated why, when, and how skin color adaptation occurred in different populations. Finally, we provided a summary of the candidate loci associated with pigmentation, which could be a valuable reference for further evolutionary and medical studies.

CONCLUSION

Previous studies generally indicated a complex genetic mechanism underlying the skin color variation, expanding our understanding of the role of population demographic history and natural selection in shaping genetic and phenotypic diversity in humans. Future work is needed to dissect the genetic architecture of skin color adaptation in numerous ethnic minority groups around the world, which remains relatively obscure compared with that of major continental groups, and to unravel the exact genetic basis of skin color adaptation.

Intraoperative awareness risk, anesthetic sensitivity, and anesthetic management for patients with natural red hair: a matched cohort study

PURPOSE

The red-hair phenotype, which is often produced by mutations in the melanocortin-1 receptor gene, has been associated with an increase in sedative, anesthetic, and analgesic requirements in both animal and human studies. Nevertheless, the clinical implications of this phenomenon in red-haired patients undergoing surgery are currently unknown.

METHODS

In a secondary analysis of a prospective trial of intraoperative awareness, red-haired patients were identified and matched with five control patients, and the relative risk for intraoperative awareness was determined. Overall anesthetic management between groups was compared using Hotelling's T(2) statistic. Inhaled anesthetic requirements were compared between cohorts by evaluating the relationship between end-tidal anesthetic concentration and the bispectral index with a linear mixed-effects model. Time to recovery was compared using Kaplan-Meier analysis, and differences in postoperative pain and nausea/vomiting were evaluated with Chi square tests.

RESULTS

A cohort of 319 red-haired patients was matched with 1,595 control patients for a sample size of 1,914. There were no significant differences in the relative risk of intraoperative awareness (relative risk = 1.67; 95% confidence interval 0.34 to 8.22), anesthetic management, recovery times, or postoperative pain between red-haired patients and control patients. The relationship between pharmacokinetically stable volatile anesthetic concentrations and bispectral index values differed significantly between red-haired patients and controls (P < 0.001), but without clinical implications.

CONCLUSION

There were no demonstrable differences between red-haired patients and controls in response to anesthetic and analgesic agents or in recovery parameters. These findings suggest that perioperative anesthetic and analgesic management should not be altered based on self-reported red-hair phenotype.

Was skin cancer a selective force for black pigmentation in early hominin evolution?

Melanin provides a crucial filter for solar UV radiation and its genetically determined variation influences both skin pigmentation and risk of cancer. Genetic evidence suggests that the acquisition of a highly stable melanocortin 1 receptor allele promoting black pigmentation arose around the time of savannah colonization by hominins at some 1-2 Ma. The adaptive significance of dark skin is generally believed to be protection from UV damage but the pathologies that might have had a deleterious impact on survival and/or reproductive fitness, though much debated, are uncertain. Here, I suggest that data on age-associated cancer incidence and lethality in albinos living at low latitudes in both Africa and Central America support the contention that skin cancer could have provided a potent selective force for the emergence of black skin in early hominins.

Bispectral index dynamics during propofol hypnosis is similar in red-haired and dark-haired subjects

BACKGROUND

We have previously shown that red hair is associated with increased desflurane requirement for immobility, compared with dark hair. The effect of red hair on IV anesthetic requirement remains unknown. We tested the hypothesis that the propofol concentration in the effect site associated with half maximal electroencephalogram response, Ce50, is at least 50% higher in subjects with red hair.

METHODS

We modeled the propofol concentration versus electroencephalogram response relationship using a 2-step approach in 29 healthy dark- and red-haired volunteers receiving a propofol infusion to produce loss of consciousness. Bispectral Index (BIS) was the measure of drug effect. The parameters of a 3-compartment pharmacokinetic model were fit to measured arterial propofol concentrations. The relationship between effect-site propofol concentration (Ce) and BIS was characterized using a sigmoid Emax model. Model performance and accuracy of the estimated parameters were evaluated using accepted metrics and bootstrap resampling. The effect of hair color on the Ce50 for BIS response in the final model was assessed using a threshold of 6.63 (P<0.01) in reduction of -2 log likelihood. The influence of body weight on the model was also assessed.

RESULTS

The inclusion of hair color as a model covariate did not improve either the pharmacokinetic or the pharmacodynamic model. A separate analysis for the dark- and red-haired subjects estimated a median (95% confidence interval) Ce50 BIS of 2.71 μg/mL (2.28-3.36 μg/mL) and 2.57 μg/mL (1.68-3.60 μg/mL), respectively. Body weight was a significant covariate for the CL1 and V1.

CONCLUSIONS

Red hair phenotype does not affect the pharmacokinetics or pharmacodynamics of propofol.

Regulation of human skin pigmentation and responses to ultraviolet radiation

Pigmentation of human skin is closely involved in protection against environmental stresses, in particular exposure to ultraviolet (UV) radiation. It is well known that darker skin is significantly more resistant to the damaging effects of UV, such as photocarcinogenesis and photoaging, than is lighter skin. Constitutive skin pigmentation depends on the amount of melanin and its distribution in that tissue. Melanin is significantly photoprotective and epidermal cells in darker skin incur less DNA damage than do those in lighter skin. This review summarizes current understanding of the regulation of constitutive human skin pigmentation and responses to UV radiation, with emphasis on physiological factors that influence those processes. Further research is needed to characterize the role of skin pigmentation to reduce photocarcinogenesis and to develop effective strategies to minimize such risks.

Natural hair color and the incidence of endometriosis

OBJECTIVE

To investigate a previously hypothesized relation between natural hair color and endometriosis.

DESIGN

Prospective cohort study.

SETTING

Nurses' Health Study II with 10 years of follow-up.

PARTICIPANT(S)

A total of 90,065 women, 25-42 years old, who had never been diagnosed with endometriosis, infertility, or cancer at baseline in 1989.

MAIN OUTCOME MEASURE(S)

Incidence of laparoscopically confirmed endometriosis according to natural hair color.

RESULT(S)

During 379,422 person-years of follow-up, 1,130 cases of laparoscopically confirmed endometriosis were reported among women with no past infertility. After adjusting for age, calendar time, parity, race, and body mass index at age 18, we observed no association overall. However, compared with women with any other hair color, we observed an increased rate of endometriosis among women with naturally red hair who had never been infertile (incidence rate = 1.3, 95% confidence interval [CI] = 1.0-1.7), but a decreased rate among women with naturally red hair among women who were infertile (incidence rate = 0.4, 95% CI = 0.2-1.2); P value, test for heterogeneity = .03.

CONCLUSION(S)

Overall, we did not observe a significant relation between red hair color and the rate of endometriosis, however this prospective cohort study suggests that the relation may differ by infertility status.

Increased sensitivity to thermal pain and reduced subcutaneous lidocaine efficacy in redheads

BACKGROUND

Anesthetic requirement in redheads is exaggerated, suggesting that redheads may be especially sensitive to pain. Therefore, the authors tested the hypotheses that women with natural red hair are more sensitive to pain and that redheads are resistant to topical and subcutaneous lidocaine.

METHODS

The authors evaluated pain sensitivity in red-haired (n = 30) or dark-haired (n = 30) women by determining the electrical current perception threshold, pain perception, and maximum pain tolerance with a Neurometer CPT/C (Neurotron, Inc., Baltimore, MD). They evaluated the analogous warm and cold temperature thresholds with the TSA-II Neurosensory Analyzer (Medoc Ltd., Minneapolis, MN). Volunteers were tested with both devices at baseline and with the Neurometer after 1-h exposure to 4% liposomal lidocaine and after subcutaneous injection of 1% lidocaine. Data are presented as medians (interquartile ranges).

RESULTS

Current perception, pain perception, and pain tolerance thresholds were similar in the red-haired and dark-haired women at 2,000, 250, and 5 Hz. In contrast, redheads were more sensitive to cold pain perception (22.6 [15.1-26.1] vs. 12.6 [0-20] degrees C; P = 0.004), cold pain tolerance (6.0 [0-9.7] vs. 0.0 [0.0-2.0] degrees C; P = 0.001), and heat pain (46.3 [45.7-47.5] vs. 47.7 [46.6-48.7] degrees C; P = 0.009). Subcutaneous lidocaine was significantly less effective in redheads (e.g., pain tolerance threshold at 2,000-Hz stimulation in redheads was 11.0 [8.5-16.5] vs. > 20.0 (14.5 to > 20) mA in others; P = 0.005).

CONCLUSION

Red hair is the phenotype for mutations of the melanocortin-1 receptor. Results indicate that redheads are more sensitive to thermal pain and are resistant to the analgesic effects of subcutaneous lidocaine. Mutations of the melanocortin-1 receptor, or a consequence thereof, thus modulate pain sensitivity.

Autocrine catecholamine biosynthesis and the beta-adrenoceptor signal promote pigmentation in human epidermal melanocytes

Earlier it has been shown that human proliferating/undifferentiated basal keratinocytes hold the full capacity for autocrine catecholamine synthesis/degradation and express beta2-adrenoceptors (beta2-AR). In this report, we show that human melanocytes also express all of the mRNA and enzymes for autocrine synthesis of norepinephrine but fail to produce epinephrine. So far, it was established that human melanocytes express alpha1-AR which are induced by norepinephrine yielding the inosine triphosphate diacylglycerol signal. The presence of catecholamine synthesis and the beta2-AR signal escaped definition at that time. Using RT-PCR, immunofluorescence and radioligand binding with the beta2-AR antagonist (-)-[3H]CGP 12177, we show here that human melanocytes express functional beta2-AR (4230 receptors per cell) with a Bmax at 129.3 and a KD of 3.19 nM but lack beta1-AR expression. beta2-AR stimulation with epinephrine 10(-6) M and salbutamol 10(-6)-10(-5) M yielded a strong cyclic adenosine monophospate (cAMP) response in association with upregulated melanin production. Taken together these results indicate that the biosynthesis and release of epinephrine (10(-6) M) by surrounding keratinocytes can provide the cAMP response leading to melanogenesis in melanocytes via the beta2-AR signal. Moreover, the discovery of this catecholaminergic cAMP response in melanocytes adds a new source for this important second messenger in melanogenesis.

Structural colouration of mammalian skin: convergent evolution of coherently scattering dermal collagen arrays

For more than a century, the blue structural colours of mammalian skin have been hypothesized to be produced by incoherent, Rayleigh or Tyndall scattering. We investigated the colour, anatomy, nanostructure and biophysics of structurally coloured skin from two species of primates – mandrill(Mandrillus sphinx) and vervet monkey (Cercopithecus aethiops) – and two species of marsupials – mouse opossum(Marmosa mexicana) and wooly opossum (Caluromys derbianus). We used two-dimensional (2-D) Fourier analysis of transmission electron micrographs (TEMs) of the collagen arrays in the primate tissues to test whether these structural colours are produced by incoherent or coherent scattering (i.e. constructive interference). The structural colours in Mandrillus rump and facial skin and Cercopithecus scrotum are produced by coherent scattering by quasi-ordered arrays of parallel dermal collagen fibres. The 2-D Fourier power spectra of the collagen arrays from Mandrillus and Cercopithecus reveal ring-shaped distributions of Fourier power at intermediate spatial frequencies,demonstrating a substantial nanostructure of the appropriate spatial frequency to produce the observed blue hues by coherent scattering alone. The Fourier power spectra and the observed reflectance spectra falsify assumptions and predictions of the incoherent, Rayleigh scattering hypothesis. Samples of blue Marmosa and Caluromys scrotum conform generally to the anatomy seen in Mandrillus and Cercopithecus but were not sufficiently well preserved to conduct numerical analyses. Colour-producing collagen arrays in mammals have evolved multiple times independently within the two clades of mammals known to have trichromatic colour vision. Mammalian colour-producing collagen arrays are anatomically and mechanistically identical to structures that have evolved convergently in the dermis of many lineages of birds, the tapetum of some mammals and the cornea of some fishes. These collagen arrays constitute quasi-ordered 2-D photonic crystals.

Comparison of phaeomelanin and its precursor 5-S-cysteinyldopa in the serum of melanoma patients

5-S-Cysteinyldopa (5-S-CD) has been used as a biochemical marker of melanoma progression. Recently we have shown that the serum level of 5-S-CD is a sensitive and specific marker in predicting distant metastases. In melanocytes and melanoma cells, cysteinyldopa isomers are oxidized to phaeomelanin, the yellow to reddish melanin pigment. In this study we have developed a new method to measure levels of phaeomelanin in serum samples and have evaluated its clinical significance. The method is based on the production of 4-amino-3-hydroxyphenylalanine (4-AHP) and 3-amino-4-hydroxyphenylalanine (3-AHP) on reductive hydrolysis of phaeomelanin with hydriodic acid. 3-AHP is also derived from 3-nitrotyrosine-containing proteins. The isomeric 4-AHP and 3-AHP can be separated by high performance liquid chromatography. The mean +/- SD serum levels of 5-S-CD in control subjects (n = 36), in melanoma patients without recurrence (n = 92) and in melanoma patients with metastases (n = 24) were 2.7 +/- 1.2 nM (median 2.3 nM), 4.0 +/- 1.6 nM (median 3.8 nM) and 72 +/- 105 nM (median 35 nM), respectively. The serum levels of 4-AHP in these three groups were 45 +/- 21 nM (median 31 nM), 80 +/- 75 nM (median 53 nM) and 306 +/- 627 nM (median 133 nM), respectively. The serum levels of 4-AHP in patients with metastases (100 samples from 15 patients with progressive disease) correlated well (r = 0.887) with serum levels of 5-S-CD, while serum levels of 3-AHP did not (r = 0.240). The serum 5-S-CD and 4-AHP levels were serially analysed in the 15 patients with progressive disease. In two patients (13%), serum 4-AHP levels were elevated to abnormal levels before the serum 5-S-CD levels exceeded the cut-off value of 10 nM. In five patients (33%), the serum 4-AHP levels rose concurrently with the serum 5-S-CD levels. In the remaining eight patients (54%), serum 4-AHP levels were of less diagnostic value. Thus, the serum phaeomelanin level appears to be less sensitive than the serum 5-S-CD level in detecting distant metastases.

Ultraviolet radiation and autoimmune disease: insights from epidemiological research

This review examines the epidemiological evidence that suggests ultraviolet radiation (UVR) may play a protective role in three autoimmune diseases: multiple sclerosis, insulin-dependent diabetes mellitus and rheumatoid arthritis. To date, most of the information has accumulated from population studies that have studied the relationship between geography or climate and autoimmune disease prevalence. An interesting gradient of increasing prevalence with increasing latitude has been observed for at least two of the three diseases. This is most evident for multiple sclerosis, but a similar gradient has been shown for insulin-dependent diabetes mellitus in Europe and North America. Seasonal influences on both disease incidence and clinical course and, more recently, analytical studies at the individual level have provided further support for a possible protective role for UVR in some of these diseases but the data are not conclusive. Organ-specific autoimmune diseases involve Th1 cell-mediated immune processes. Recent work in photoimmunology has shown ultraviolet B (UVB) can specifically attenuate these processes through several mechanisms which we discuss. In particular, the possible contribution of an UVR-induced increase in serum vitamin D (1,25(OH)2D3) levels in the beneficial immunomodulation of these diseases is discussed.

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).

Melanocortin-1 receptor genotype is a risk factor for basal and squamous cell carcinoma

MC1R gene variants have previously been associated with red hair and fair skin color, moreover skin ultraviolet sensitivity and a strong association with melanoma has been demonstrated for three variant alleles that are active in influencing pigmentation: Arg151Cys, Arg160Trp, and Asp294His. This study has confirmed these pigmentary associations with MC1R genotype in a collection of 220 individuals drawn from the Nambour community in Queensland, Australia, 111 of whom were at high risk and 109 at low risk of basal cell carcinoma and squamous cell carcinoma. Comparative allele frequencies for nine MC1R variants that have been reported in the Caucasian population were determined for these two groups, and an association between prevalence of basal cell carcinoma, squamous cell carcinoma, solar keratosis and the same three active MC1R variant alleles was demonstrated [odds ratio = 3.15 95% CI (1.7, 5.82)]. Three other commonly occurring variant alleles: Val60Leu, Val92Met, and Arg163Gln were identified as having a minimal impact on pigmentation phenotype as well as basal cell carcinoma and squamous cell carcinoma risk. A significant heterozygote effect was demonstrated where individuals carrying a single MC1R variant allele were more likely to have fair and sun sensitive skin as well as carriage of a solar lesion when compared with those individuals with a consensus MC1R genotype. After adjusting for the effects of pigmentation on the association between MC1R variant alleles and basal cell carcinoma and squamous cell carcinoma risk, the association persisted, confirming that presence of at least one variant allele remains informative in terms of predicting risk for developing a solar-induced skin lesion beyond that information wained through observation of pigmentation phenotype.

Evidence for variable selective pressures at MC1R

It is widely assumed that genes that influence variation in skin and hair pigmentation are under selection. To date, the melanocortin 1 receptor (MC1R) is the only gene identified that explains substantial phenotypic variance in human pigmentation. Here we investigate MC1R polymorphism in several populations, for evidence of selection. We conclude that MC1R is under strong functional constraint in Africa, where any diversion from eumelanin production (black pigmentation) appears to be evolutionarily deleterious. Although many of the MC1R amino acid variants observed in non-African populations do affect MC1R function and contribute to high levels of MC1R diversity in Europeans, we found no evidence, in either the magnitude or the patterns of diversity, for its enhancement by selection; rather, our analyses show that levels of MC1R polymorphism simply reflect neutral expectations under relaxation of strong functional constraint outside Africa.

Melanocortin-1-receptor gene and sun sensitivity in individuals without red hair

Susceptibility to sunburn, photoageing, and skin cancer is inversely related to an individual's ability to tan after sun exposure. We examined variants in the melanocortin-1-receptor (MC1R) gene in individuals from Ireland and the UK. We found evidence of an association between the degree of tanning after repeated sun exposure, and the number of variant alleles present. Heterozygotes were intermediate between wild-type individuals and those with two variant alleles. We suggest that MC1R gene status therefore determines sun sensitivity in people without red hair.