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).