Molecular and genetic characterization of sex-linked orange coat color in the domestic cat
The Sex-linked orange mutation in domestic cats causes variegated patches of reddish/yellow hair and is a defining signature of random X-inactivation in female tortoiseshell and calico cats. Unlike the situation for most coat color genes, there is no apparent homolog for Sex-linked orange in other mammals. We show that the Sex-linked orange is caused by a 5 kb deletion that leads to ectopic and melanocyte-specific expression of the Rho GTPase Activating Protein 36 (Arhgap36) gene. Single cell RNA-seq studies from fetal cat skin reveal that red/yellow hair color is caused by reduced expression of melanogenic genes that are normally activated by the Melanocortin 1 receptor (Mc1r)--cyclic adenosine monophosphate (cAMP)--protein kinase A (PKA) pathway, but the Mc1r gene and its ability to stimulate cAMP accumulation is intact. Instead, we show that increased expression of Arhgap36 in melanocytes leads to reduced levels of the PKA catalytic subunit (PKAC); thus, Sex-linked orange is genetically and biochemically downstream of Mc1r. Our findings solve a comparative genomic conundrum, provide in vivo evidence for the ability of Arhgap36 to inhibit PKA, and reveal a molecular explanation for a charismatic color pattern with a rich genetic history.