Receptor Function, Dominant Negative Activity and Phenotype Correlations for MC1R Variant Alleles

The human melanocortin-1 receptor (MC1R) is a G-protein coupled receptor involved in the regulation of pigmentation. Several MC1R variant alleles are associated with red hair, fair skin and increased skin cancer risk. We have performed a systematic functional analysis of nine common MC1R variants and correlated these results with the strength of the genetic association of each variant allele with pigmentation phenotypes. In vitro expression studies revealed that variant receptors with reduced cell surface expression, including V60L, D84E, R151C, I155T, R160W and R163Q, showed a corresponding impairment in cAMP coupling. The R142H and D294H variants demonstrated normal cell surface expression, but had reduced functional responses, indicating that altered G-protein coupling may be responsible for this loss of function. The V92M variant cAMP activation was equal to or higher than that for wild-type MC1R. In co-expression studies, the D84E, R151C, I155T and R160W variants showed a dominant negative effect on wild-type receptor cell surface expression, which was reflected in a decreased ability to elevate intracellular cAMP levels. The D294H variant also demonstrated a dominant negative effect on wild-type MC1R cAMP signalling, but had no effect on wild-type surface expression. Importantly, comparison of the in vitro receptor characteristics with skin and hair colour data of individuals both homozygous and heterozygous for MC1R variant alleles revealed parallels between variant MC1R cell surface expression, functional ability, dominant negative activity and their effects on human pigmentation. These findings show the first direct correlations between variant MC1R biochemical properties and pigmentation phenotype.