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

Skin lightness affects ultraviolet A-induced oxidative stress: Evaluation using ultraweak photon emission measurement

The human skin is usually exposed to ultraviolet A (UVA) in the sunlight and experiences oxidative stress associated with skin disorders and aging. Although oxidative stress caused by UVA exposure is assumed to be dependent on skin colour, few studies have demonstrated this dependency. We investigated the effects of skin colour on UVA-induced oxidative stress using ultraweak photon emission (UPE) generated from the skin during oxidation processes. The UPE intensities of skin samples were detected using a photomultiplier tube every second without any labelling. We irradiated skin tissue of different colours with UVA and measured UPE over time. UVA-induced UPE could be detected from immediately after irradiation to 2 h after irradiation, indicating persistent oxidative stress. Skin lightness (L*) positively correlates with UPE intensity. Lighter-coloured skin exhibited more UVA-induced UPE, indicating higher oxidative stress. Additionally, oxidative stress persisted significantly more in lighter skin compared with darker skin. Skin tissues exhibited pigment darkening after UVA irradiation. Our results suggest that skin lightness affects oxidative stress induced by UV irradiation. Our study demonstrated the relationship between skin lightness and UVA-induced oxidative stress for the first time and offers new photodermatological insights into the human skin.

The evolution of human skin pigmentation involved the interactions of genetic, environmental, and cultural variables

The primary biological role of human skin pigmentation is as a mediator of penetration of ultraviolet radiation (UVR) into the deep layers of skin and the cutaneous circulation. Since the origin of Homo sapiens, dark, protective constitutive pigmentation and strong tanning abilities have been favored under conditions of high UVR and represent the baseline condition for modern humans. The evolution of partly depigmented skin and variable tanning abilities has occurred multiple times in prehistory, as populations have dispersed into environments with lower and more seasonal UVR regimes, with unique complements of genes and cultural practices. The evolution of extremes of dark pigmentation and depigmentation has been rare and occurred only under conditions of extremely high or low environmental UVR, promoted by positive selection on variant pigmentation genes followed by limited gene flow. Over time, the evolution of human skin pigmentation has been influenced by the nature and course of human dispersals and modifications of cultural practices, which have modified the nature and actions of skin pigmentation genes. Throughout most of prehistory and history, the evolution of human skin pigmentation has been a contingent and non-deterministic process.

Erythema and Acclimatization Following Repeated Sun Exposure: A Modeling Study

The development of a computational model is described that allows time-varying erythema and acclimatization to be determined following repeated exposure to solar ultraviolet (UV) radiation. Good agreement was observed between experimental data and model predictions. Results are presented that illustrate how daily, adventitious sun exposure over the course of a year can result in occasional erythema generally in the late spring/early summer, while at the same time showing how the photoadaptive response of the skin changes throughout the year at different latitudes and for different sun-reactive skin types. The model is predicated on three physiological responses; the erythemal response as a function of increasing UV dose, the time courses of erythema and adaptation in response to a single UV exposure. Mathematical models complement observational data and the outputs from the model may help in planning future experimental studies, as well as providing insights into mechanistic responses.

Skin Photosensitivity is Associated with 25-Hydroxyvitamin D and BMD but not Fractures Independent of Melanin Density in Older Caucasian Adults

Whether skin photosensitivity modulates sun exposure behaviours, consequent vitamin D status and skeletal health outcomes independently of constitutive pigmentation have not been systematically investigated. 1072 community-dwelling adults aged 50-80 years had skin photosensitivity quantified by questionnaire and melanin density by spectrophotometry. Bone mineral density (BMD), falls risk and 25-hydroxyvitamin D (25OHD) were measured using DXA, short form physiological profile assessment and radioimmunoassay, respectively. Sun exposure and symptomatic fractures were assessed by questionnaire. Participants were followed up at 2.5 (n = 879), 5 (n = 767) and 10 (n = 571) years. Higher resistance to sunburn and greater ability to tan were associated with reduced sun protection behaviours (RR 0.87, p < 0.001 & RR 0.88, p < 0.001), higher lifetime discretionary sun exposure in summer (RR 1.05, p = 0.001 & RR 1.07, p = 0.001) and winter (RR 1.07, p = 0.001 & RR 1.08, p = 0.02) and fewer lifetime sunburns (RR 0.86, p < 0.001 & RR 0.91, p = 0.001). Higher resistance to sunburn was associated with lower total body (β =  - 0.006, p = 0.047) and femoral neck (β = - 0.006, p = 0.038) BMD, but paradoxically, fewer prevalent fractures (RR 0.94, p = 0.042). Greater ability to tan was associated with higher 25OHD (β = 1.43, p = 0.04), lumbar spine (β = 0.014, p = 0.046) and total body (β = 0.013, p = 0.006) BMD, but not fracture or falls risk. These associations were independent of constitutive melanin density. Cutaneous photosensitivity was associated with sun exposure behaviours, cutaneous sequelae and, consequently, 25OHD and BMD in older Caucasian adults independent of constitutive melanin density. There was no consistent association with fracture outcomes, suggesting environmental factors are at least as important.

Photoadaptation to ultraviolet B TL01 in psoriatic patients

BACKGROUND

In the biologic era, narrow-band ultraviolet B (NB-UVB) phototherapy still remains a valuable, effective, inexpensive, safe anti-psoriatic treatment. Patients can lose response to NB-UVB over time due to photoadaptation. This phenomenon is the tendency of the skin to respond to ultraviolet (UV) exposure by undergoing changes that may result in a decreased future response to an equivalent dose of radiation, thus leading to the need for an increased exposure during phototherapy course.

AIM

To characterize and quantify the determinants of photoadaptation in NB-UVB treated psoriatic patients.

METHODS

We enrolled 57 adult patients with moderate plaque psoriasis. Patients underwent 24 sessions of NB-UVB phototherapy delivered thrice a week. Dosing was started with 70% of the minimal erythema dose (MED) with percentage-based dose increments every two treatments. MED as well as change in the erythema and melanin index (MI) were measured at baseline and at the end of phototherapy course. Moreover, an adaptation factor (AF) was calculated for each patient.

RESULTS

Adaptation factor was not influenced by both baseline MED and skin type. We found a weak correlation between higher cumulative dosages and the initial MED (Spearman's rho = 0.32, P = 0.0154) as well as with the mean initial MI (Spearman's rho = 0.25, P = 0.0624, statistically borderline). Clearance and mean number of treatments were correlated (Spearman's rho = 0.48, P < 0.001).

CONCLUSION

Photoadaptation is a physiological skin response that negatively influences NB-UVB responsiveness and is not predictable by the baseline MED and skin type. Thus, starting with more aggressive protocols and increasing rapidly dosage progression to prevent AF may increase NB-UVB response.

The necessity of antinuclear antibody investigation in pre-phototherapy vitiligo patients: A retrospective study

BACKGROUND

Narrowband UVB (NBUVB) phototherapy is the cornerstone treatment for vitiligo. Before its initiation, some experts recommend antinuclear antibody (ANA) screening out of concern for either photosensitivity to NBUVB or autoimmune disease exacerbation during treatment. As vitiligo is considered an autoimmune disorder, ANAs can be positively found in the disease without any clinical importance. The necessity for ANA investigations for pre-phototherapy vitiligo patients is therefore questioned.

METHODS

We conducted a retrospective study to investigate vitiligo patients who had been checked for ANA before commencing NBUVB phototherapy. Demographic data-including vitiligo type and age of onset-were collected. Samples of ANA, anti-thyroglobulin, and anti-thyroid peroxidase were obtained. The phototherapy treatment protocol and cutaneous reactions to the phototherapy were also recorded.

RESULTS

Among 85 Thai vitiligo patients, the ANA prevalence was 35.3%. The speckled ANA pattern was the most common, and the large majority of patients (80%) had a titer of ≤1:100. Factors associated with positive ANA were female gender and positive anti-thyroglobulin. There were no statistical differences between the phototoxic reactions or phototoxic doses of NBUVB of the ANA-positive vitiligo and ANA-negative vitiligo groups. No cases of SLE were detected in ANA-positive group.

CONCLUSIONS

ANA positivity was not correlated with the incidence or dose of phototoxic reaction in phototherapy treated vitiligo, and it may not a predictive factor for SLE diagnosis in vitiligo. ANA might therefore not need to be routinely checked in pre-phototherapy in vitiligo, unless there are clinical suspicions of an autoimmune disease. However, ANA might be involved in part of the cutaneous photoadaptation response to phototherapy.

Clinical and Spectrophotometric Evaluation of Skin Photoadaptation in Vitiligo Patients after a Short Cycle of NB-UVB Phototherapy

BACKGROUND

The phenomenon of photoadaptation to narrow-band ultraviolet B (NB-UVB) radiation has been previously described in vitiligo and has usually been clinically measured by the assessment of the minimal erythema dose (MED) after phototesting.

OBJECTIVES

To assess the photoadaptive response in vitiligo and healthy skin after NB-UVB phototherapy not only clinically, but also by spectrophotometry.

MATERIALS AND METHODS

Fourteen patients affected by generalized vitiligo underwent NB-UVB phototherapy twice weekly for 12 weeks. Before and after phototherapy, a phototesting procedure was administered on vitiligo patches and adjacent healthy skin with a solar simulated radiation (SSR). Visual assessment of the MED took place after 24 h. A spectrophotometer was used to assess the a* value and the melanin index (MI*), as signs of skin erythema and pigmentation. The photoadaptation factor (MED-PF) and the a* photoadaptation factor (a*-PF) were calculated.

RESULTS

After NB-UVB phototherapy, both vitiligo and healthy skin showed an increase in MED and MI* values and a reduction of skin erythema compared to baseline (p < 0.05). MED-PF data showed a photoadaptation in 10 (71.4%) vitiligo lesions and in 12 (85.7%) healthy skin areas. The assessment of the a*-PF showed a negative mean percentage value in all affected and unaffected skin areas.

CONCLUSIONS

A short cycle of NB-UVB phototherapy can induce photoadaptation in vitiligo by increasing the MED and reducing skin erythema after stimulation with SSR. This is most likely due to the physical filter function induced by ultraviolet radiation.

Effect of A Sun Protection Intervention on the Immune Response to Measles Booster Vaccination in Infants in Rural South Africa

The incidence of many serious childhood infections can be reduced by vaccination. High sun exposure at the time of vaccination has been associated with a reduced antigen-specific immune response. We hypothesized that providing sun protection advice and equipment to mothers of children who were waiting to be vaccinated would result in a more robust immunization response. We conducted a pilot study in 2015/2016 (data analyzed in 2017-2018) among 98 Black African children (~18 months of age) receiving the booster measles vaccination at two clinics in South Africa. Clinics were randomized to receive (or not) sun protection advice and equipment. We recorded demographic information on children and mothers and data on the child's usual sun exposure. At approximately 4 weeks' postmeasles vaccination, we measured measles immunoglobulin G levels in children. All children with blood results (n = 87, 89%) across both groups had antibody titers higher than 200 mIU mL^-1 which was considered the protective antibody concentration. There was no statistically significant difference in titers between groups: geometric difference in mean titers 1.13 mIU mL^-1 (95% CI 0.85, 1.51; P = 0.39) and 1.38 mIU mL^-1 (95% CI 0.90, 2.11, P = 0.14) for unadjusted and adjusted analyses, respectively. This study demonstrated that a sun protection intervention study could be performed in a developing-world pediatric vaccination setting. Although the sun protection intervention around the time of vaccination was not associated with a higher antibody level, given the potential importance of such an effect, a larger study should be considered.

'My child did not like using sun protection': practices and perceptions of child sun protection among rural black African mothers

BACKGROUND

Photodamage is partially mitigated by darker skin pigmentation, but immune suppression, photoaging and cataracts occur among individuals with all skin types.

METHODS

To assess practices and acceptability to Black African mothers of sun protection equipment for their children living in a rural area, participants were recruited at the time of their child's 18-month vaccinations. Mothers completed a baseline questionnaire on usual sun behaviours and sun protection practices. They were then provided with sun protection equipment and advice. A follow-up questionnaire was administered two weeks later.

RESULTS

Mothers reported that during the week prior to the baseline questionnaire, children spent on average less than 1 hour of time outdoors (most often spent in the shade). Most mothers (97%) liked the sun protection equipment. However, many (78 of 86) reported that their child did not like any of the sun protection equipment and two-thirds stated that the sun protection equipment was not easy to use.

CONCLUSIONS

Among Black Africans in rural northern South Africa, we found a mismatch between parental preferences and child acceptance for using sun protection when outdoors. A better understanding of the health risks of incidental excess sun exposure and potential benefits of sun protection is required among Black Africans.

Sun protection to improve vaccine effectiveness in children in a high ambient ultraviolet radiation and rural environment: an intervention study

BACKGROUND

Vaccination is a mainstay of preventive healthcare, reducing the incidence of serious childhood infections. Ecological studies have demonstrated an inverse association between markers of high ambient ultraviolet (UV) radiation exposure (e.g., sunny season, low latitude of residence) and reduction in the vaccination-associated immune response. Higher sun exposure on the day prior to and spanning the day of vaccination has been associated with a reduced antigen-specific immune response independent of skin pigmentation. The South African Department of Health's Expanded Programme on Immunisation provides free vaccinations in government primary health care clinics. In some areas, these clinics may have only a small waiting room and patients wait outside in full sun conditions. In rural areas, patients may walk several kilometres to and from the clinic. We hypothesised that providing sun protection advice and equipment to mothers of children (from 18 months) who were waiting to be vaccinated would result in a more robust immune response for those vaccinated.

METHODS

We conducted an intervention study among 100 children receiving the booster measles vaccination. We randomised clinics to receive (or not) sun protection advice and equipment. At each clinic we recorded basic demographic data on the child and mother/carer participants, their sun exposure patterns, and the acceptability and uptake of the provided sun protection. At 3-4 weeks post-vaccination, we measured measles IgG levels in all children.

DISCUSSION

This is the first intervention study to assess the effect of sun protection measures on vaccine effectiveness in a rural, real-world setting. The novel design and rural setting of the study can contribute much needed evidence to better understand sun exposure and protection, as well as factors determining vaccine effectiveness in rural Africa, and inform the design of immunisation programmes. (TRN PACTCR201611001881114, 24 November 2016, retrospective registration).

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.

UV-based therapy and vitamin D

The ultraviolet (UV) light spectrum has long been known to induce biologic effect on the skin. For a large number of cutaneous disorders, phototherapy and photochemotherapy are effective therapeutic options with excellent safety profiles and well-documented side effects. Despite their ease of administration and benefits, phototherapeutic treatment modalities require appropriate space for the equipment, trained staff, and patient education prior to initiating treatment. However, when the initial barriers to treatment can be overcome, UV therapy can offer patients significant relief from their cutaneous disease. Furthermore, UVB-based phototherapy can produce significant alteration to vitamin D levels. With the recent research implicating association of low vitamin D levels with a variety of health conditions, whether patients receiving phototherapy or, more specifically, those getting vitamin D supplement may be protected from these diseases remains to be established.

Melanin mediated apoptosis of epidermal cells damaged by ultraviolet radiation: factors influencing the incidence of skin cancer

Ultraviolet (UV)-induced skin cancers, including melanomas and basal/squamous cell carcinomas, occur more frequently in individuals with fair skin than in those with dark skin. Melanin plays an important role in protecting the skin against UV radiation and levels of melanin correlate inversely with amounts of DNA damage induced by UV in human skin of different racial/ethnic groups. The objectives of this study are to review recent progress in our understanding of mechanisms underlying differences in cancer incidence in skins of different colors, particularly between Black and White skin. More specifically, we review DNA damage and apoptosis in various types of skin before and after exposure to UV in our human study protocols using a single UV dose, either one minimal erythema dose (MED) or a similar low dose of 180-200 J/m2. Our data and other published reports indicate that several major mechanisms underlie the increased rates of photocarcinogenesis in fair/light skin. First, UV-induced DNA damage in the lower epidermis (including keratinocyte stem cells and melanocytes) is more effectively prevented in darker skin. Second, rates of repair of DNA damage can differ significantly in individuals. Third, UV-induced apoptosis to remove potentially precancerous cells is significantly greater in darker skin. These results suggest that pigmented epidermis is an efficient UV filter and that UV damaged cells are removed more efficiently in darker skin. The combination of decreased DNA damage and more efficient removal of UV-damaged cells may play a critical role in the decreased photocarcinogenesis seen in individuals with darker skin.

Long-lasting pigmentation more than its intensity is a reliable indicator of skin sun resistance

BACKGROUND

The currently adopted method for predicting sun sensitivity is Fitzpatrick's classification which however is based on self-reported burning tendency and tanning ability.

OBJECTIVE

Determination of the individual UV susceptibility based on non-subjective parameters.

METHOD

Minimal erythema dose (MED), intensity and duration of pigmentation on days 5, 9 and 16 following 1 MED and the levels of the melanin marker pyrrole-2,3,5-tricarboxylic acid (PTCA) were analysed in non-red-haired subjects (50, aged 20-46 years).

RESULTS

Phenotype groups or phototypes showed a good correlation with PTCA yields and the persistence of pigmentation on day 16, but not with MED values. MED values did not show a significant correlation with PTCA yields. On the other hand, high values on day 16 were exhibited only by subjects having PTCA values higher than 200 ng/mg.

CONCLUSIONS

Measurement persistence of pigmentation on day 16 represents a non-invasive and easy-to-perform method to evaluate photoprotection in those individuals escaping straightforward classification based on phenotype or anamnesis.

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.

A comparison of sunlight exposure in men with prostate cancer and basal cell carcinoma

Ultraviolet radiation exposure increases basal cell carcinoma (BCC) risk, but may be protective against prostate cancer. We attempted to identify exposure patterns that confer reduced prostate cancer risk without increasing that of BCC. We used a questionnaire to assess exposure in 528 prostate cancer patients and 442 men with basal cell carcinoma, using 365 benign prostatic hypertrophy patients as controls. Skin type 1 (odds ratio (OR)=0.47, 95% CI=0.26–0.86), childhood sunburning (OR=0.38, 95% CI=0.26–0.57), occasional/frequent sunbathing (OR=0.21, 95% CI=0.14–0.31), lifetime weekday (OR=0.85, 95% CI=0.80–0.91) and weekend exposure (OR=0.79, 95% CI=0.73–0.86) were associated with reduced prostate cancer risk. Skin type 1 (OR=4.00, 95% CI=2.16–7.41), childhood sunburning (OR=1.91, 95% CI=1.36–2.68), regular foreign holidays (OR=6.91, 95% CI=5.00-9.55) and weekend (OR=1.17, 95% CI=1.08–1.27) but not weekday exposure were linked with increased BCC risk. Combinations of one or two parameters were associated with a progressive decrease in the ORs for prostate cancer risk (OR=0.54–0.25) with correspondingly increased BCC risk (OR=1.60–2.54). Our data do not define exposure patterns that reduce prostate cancer risk without increasing BCC risk.

Photoadaptation: A Path Toward Rational Phototherapy Protocols

Photoadaptation is defined as the diminished future response to equivalent doses of irradiation. It is most often estimated in vivo by looking at changes in the minimal erythema dose with subsequent doses of UV radiation. Although photoadaptation's mechanism of action is poorly understood, Palmer et al. help clarify the clinical significance of photoadaptation for the dosimetry of UV-based phototherapy.

Prostate cancer susceptibility is mediated by interactions between exposure to ultraviolet radiation and polymorphisms in the 5' haplotype block of the vitamin D receptor gene

Vitamin D receptor (VDR) polymorphisms are prostate cancer risk candidates. We determined if SNPs in haplotype block sub-regions C2 (SNPs C2-1, G/C(3436), C2-2, A/G(3944)) or C1 (C1-1, C/T(20965), C1-2, C/T(30056)) are associated with risk in an ultraviolet radiation (UVR)-dependent manner. In men with very low exposure, SNPs in both sub-regions were associated with risk. Various haplotypes in haplotype block C including G(3436)-A(3944)-C(20965)-C(30056), (G or C)-A-C-C and G-A-(C or T)-C were significantly associated with increased risk (odds ratios between 1.95 and 2.37). These findings suggest various block C SNPs are associated with prostate cancer risk via a mechanism involving exposure to sunlight.

Photoadaptation during Narrowband Ultraviolet-B Therapy Is Independent of Skin Type: A Study of 352 Patients

Understanding how photoadaptation differs between individuals is important when considering susceptibility to the beneficial and harmful effects of sunlight exposure and when determining optimal phototherapy regimens. Most narrowband UVB (NB-UVB) regimens start with 70% of the minimal erythema dose (MED) with 20% increments at each treatment thereafter. We retrospectively studied 352 skin types I-IV psoriatic patients having twice weekly treatment with this regimen. Patients with high skin types tended to have high MEDs (P<0.001). By session 20 the proportion of patients who had developed erythema was approximately 60% regardless of MED. Among patients who developed erythema, the number of treatments before erythema occurred did not differ between skin types (P=0.33). We conclude that patients with high skin types photoadapt approximately equally per physical unit of UVR in comparison to those with low skin types, but they have greater photoadaptation in absolute terms because they are able to tolerate a higher initial dose of radiation. Differences in skin type or MED are not associated with clinically important differences in tendency to erythema during a standard 70/20% NB-UVB twice-weekly regimen. This regimen is suitable for all skin types I-IV patients regardless of skin type or MED.

The photoadaptive response to ultraviolet exposure in human skin using ultraviolet spectrophotometry

BACKGROUND

Both pigmentation and non-pigmentary processes contribute to the development of photoadaptation yet the exact contribution of either in the resting state and in response to ultraviolet (UV) radiation is unclear. The purpose of this study was to estimate independently these changes occurring in the epidermis following repeated exposure to UV in two groups with differing degrees of constitutive pigmentation.

METHODS

We describe a mathematical model for explaining the spectral absorbance of excised human epidermis based on the absorbance of constituent chromophores. The model was applied to spectral absorbance data measured on samples of epidermis excised from pre-irradiated skin and from skin obtained following UV irradiation on 3 successive days.

RESULTS

We found that in Asian skin there was only a mild photoadaptive response, principally by a small increase in pigmentation. On the other hand, the significant adaptive response in Caucasian skin was through hyperplasia of the epidermis, with tanning contributing only to a much smaller degree.

CONCLUSION

This study has enabled us to study independently the pigmentary and non-pigmentary pathways and has shown that in those people with a lower degree of constitutive pigment, the primary mechanism of photoadaptation is via the non-pigmentary route.

Using light in dermatology: an update on lasers, ultraviolet phototherapy, and photodynamic therapy

Indications for light-based treatments, such as lasers, UV phototherapy, and photodynamic therapy, are rapidly increasing within the arena of skin disorders. Physicians can remain current in their understanding of these modalities if they understand a few basic principles outlined in this article. Once these concepts are understood, all the rapid advances can be kept in perspective and physicians can apply the most appropriate technology to the care of their patients while informing them about the limitations of overmarketed but poorly proved strategies.

Eumelanin and pheomelanin concentrations in human epidermis before and after UVB irradiation

Pheomelanin is widely thought to be causally related to susceptibility to the harmful effects of ultraviolet radiation: epidemiological studies show that those with a higher ratio of pheomelanin to eumelanin in hair have higher rates of melanoma, and work in mouse and cell culture shows that pheomelanin generates excess free radicals after UVR exposure. By contrast, based on measurements of eumelanin and pheomelanin in human skin, before and following irradiation, we now report that both pheomelanin and eumelanin are positively related to skin colour, and by inference, inversely with cancer susceptibility. The ratio of melanin classes is similar in people with widely different cancer rates and UVR sensitivity. Although our numbers are small, our results extend previous work in man, and lead us to speculate that factors other than the amount of pheomelanin may be important in determining UVR susceptibility in persons with red hair.

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.