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.