Genetic Association and Cellular Function of MC1R Variant Alleles in Human Pigmentation

Abstract

We have examined MC1R variant allele frequencies in the general population of South East Queensland and in a collection of adolescent dizygotic and monozygotic twins and family members to define statistical associations with hair and skin color, freckling, and mole count. Results of these studies are consistent with a linear recessive allelic model with multiplicative penetrance in the inheritance of red hair. Four alleles, D84E, R151C, R160W, and D294H, are strongly associated with red hair and fair skin with multinomial regression analysis showing odds ratios of 63, 118, 50, and 94, respectively. An additional three low-penetrance alleles V60L, V92M, and R163Q have odds ratios 6, 5, and 2 relative to the wild-type allele. To address the cellular effects of MC1R variant alleles in signal transduction, we expressed these receptors in permanently transfected HEK293 cells. Measurement of receptor activity via induction of a cAMP-responsive luciferase reporter gene found that the R151C and R160W receptors were active in the presence of NDP-MSH ligand, but at much reduced levels compared with that seen with the wild-type receptor. The ability to stimulate phosphorylation of the cAMP response element binding protein (CREB) transcription factor was also apparent in all stimulated MC1R variant allele-expressing HEK293 cell extracts as assessed by immunoblotting. In contrast, human melanoma cell lines showed wide variation in the their ability to undergo cAMP-mediated CREB phosphorylation. Culture of human melanocytes of known MC1R genotype may provide the best experimental approach to examine the functional consequences for each MC1R variant allele. With this objective, we have established more than 300 melanocyte cell strains of defined MC1R genotype.

Novel MC1R variants in Ligurian melanoma patients and controls

“…Variant p.R160Q is also at a previously described site , and we found it in association with fair skin and fair hair. We suggest that it will similarly affect MC1R function by analogy to reports on variant p.R160W, which is known to be impaired in its ability to stimulate cAMP production (Schioth et al, 1999) and consistent with more recent genetic associations with red hair (Sturm et al, 2003a).…”

“…Mutations noted in the text with amino acid terminology are described also at the DNA level in Table 1 below. (Sturm et al, 2003a)MC1R cDNA sequence from GenBank (Accession number AF 326275) was used as areference sequence, where the A of the ATG translation initiation start site represents nucleotide +1.…”

“…High-penetrance alleles were defined as "R" variants while low-penetrance alleles were defined as "r" variants (R= R142H, R151C, R160W, D294H; r=V60L, V92M, R163Q) (Sturm et al, 2003a). Mutations noted in the text with amino acid terminology are described also at the DNA level in Table 1. MC1R cDNA sequence from GenBank (Accession number AF 326275) was used as a reference sequence, where the A of the ATG translation initiation start site represents nucleotide +1.…”

“…Recently several variant forms of MC1R (MIM# 155555) have been associated with the recessive inheritance of this trait including the common p.R151C, p.R160W, p.D294H, and the less frequent p.D84E, p.R142H, p.I155T, c.86_87insA, and c.537_538insC alleles . These variants have been defined as high-penetrance red hair alleles compared to other common variants that are of low-penetrance, which include p.V60L, p.V92M and p.R163Q (Sturm et al, 2003a), whose association with phenotype is as yet unclear. The impact of MC1R variants on risk of melanoma has been found to be mediated largely through the action of high-penetrance alleles (Palmer et al, 2000;Kennedy et al, 2001) also independently of their effect on skin type.…”

Anesthetic Requirement Is Increased in Redheads

“…The functional significance of mutation D84E, carried by one of the remaining two red haired subjects, remains unknown but this mutation is also strongly associated with red hair, having an odds ratio of 63 for red hair relative to the consensus MC1R allele. 22 In the dark haired group there was no clear evidence for the effect of heterozygosity on anesthetic requirement.…”