chrisdellavedova.com

“All I ask for when I pray,
Steady rollin woman gonna come my way.
Need a woman gonna hold my hand
And tell me no lies, make me a happy man.”

- Led Zeppelin - “Black Dog”

I knew as soon as I saw this paper in Science this week that I wouldn’t be able to resist. The upcoming, and most likely unfortunate, Led Zeppelin reunion has been in the news in Britain and as a dog owner I’m always a sucker for canine genetics. In fact, one of the most interesting scientific talks I’ve ever heard was by Robert Wayne of UCLA at the Genetics Society in London a couple of years ago. His premise at the time was that the domestic dog evolved from gray wolves in different places and at different times but always under the same evolutionary pressure. It is easier, if you’re a dog, to live as a scavenger with humans than to do the hunting yourself. Dogs aren’t dumb.

That’s neither here nor there; the research published this week in Science by Greg Barsh’s group at Stanford University focuses on coat color in dogs. Specifically they describe the characterization and cloning of a dominant mutation in dogs that renders a black coat - creatively named dominant black. Mammalian coat color is an excellent system for the study of basic genetics because, like Mendel’s peas, phenotypes are easy to follow and they tend to involve only a handful of genes in well behaved pathways. All of the diversity that is present in coat color in mammals, including humans, stems from variation in the levels of two pigments - yellow or red pheomelanin and brown or black eumelanin. The genetic regulation of these pigments’ synthesis is fairly well characterized and in most mammals is under the control of two genes - Agouti and Melanocortin 1 receptor (Mcr1). The latter gene encodes a protein that sits in the cell membrane and acts as a receptor for the protein encoded by Agouti. Typically the ligand Agouti binds to the receptor Mcr1 resulting in the production of the yellow/red pheomelanin.

The family dog has always been an exception to this neat two gene model. Researchers have known for some time, by using classical genetic analysis, that dominant black (KB*) is not an allele of either Agouti or Mcr1. This mutation is one of three alleles of the K locus - the dominant black KB (rendering black coat color) is dominant to kbr (brindle color) which is dominant to ky (yellow). The recent sequencing of the dog genome allowed Barsh’s group to clone the K locus - and reveals a curious candidate.

To get at the gene responsible for dominant black, Barsh’s group used genetic mapping to roughly localize the mutation in the genome. They were able to exploit the fact that most dog breeds were created in the last 200 years from a very small population to localize the gene more specifically. Their assumption - that mutations within a breed are identical - allowed them to use association mapping to map dominant black to one of 16 genes, an impressive mapping experiment. This was followed by sequencing of the relevant regions to confirm that KB results from a small deletion in CBD103, one of a cluster of B-defensin genes. These genes encode small antibiotic proteins that are secreted from the skin and are important in immunity.

A mutation in a immune system gene is an odd candidate for a factor in coat pigmentation, so Barsh’s group did a number of experiments to confirm that they had the right candidate. These included looking at expression of CBD103 in different mutant backgrounds, expressing it in a mouse model system and establishing the binding affinity of the protein encoded by CBD103 to components of the Mcr1-Agouti pathway. All of these experiments provide support to the hypothesis that the dominant KB allele results from a the deletion in CBD103 and an increase in the amount of the mutated protein. The binding experiments show that the CBD103 protein can, like Agouti, bind Mcr1.

These result lead Barsh and his colleagues to propose that CBD103 can also act as a ligand for Mcr1. When the ky allele is present, not enough CBD103 is produced to affect the Agouti-Mcr1 interaction. However, in the dominant KB mutants, CBD103 is produced at a higher lever and competes with Agouti for binding to Mcr1. Under these circumstances, the darker eumelanin pigments are synthesized - and, voila, a black dog. But why would a gene involved in protection from microbes also play a role in coat color. The authors propose, rather unsatisfyingly, that beyond the role that they play in immunity, B-defensins may have been important during mammalian evolution to provide camouflage by producing novel, environment specific coat patterns and colors.

That just leaves me to include a gratuitous picture of my dog, Timmins. He is a white - not albino - Siberian husky. I was curious how, with only yellow and black pigment to work with, you can get white coats in dogs. You may have been looking at your pooch wondering the same thing - what explains the massive diversity in coat colors and patterns in domestic dogs. Unfortunately, it’s a little more complicated than the story told above. There are parallel genetic pathways that control different patterns of color and presence or absence of pigments.

Just a little editorializing, if I may. I’m not sure why this paper made it into such a high impact journal. The research is well designed and controlled, but ultimately it is just the description of the mapped based cloning of a gene responsible for a previously described mutation. Lower impact journals like Genetics are full of these sorts of papers. Yes, the link between immune response and coat color is new but it is a stretch to see this kind of paper in Science. Perhaps what attracted me to this story is the same thing that attracted the editors of Science - a soft spot for our faithful companions.

Image credits:

Yellow & black labs

Segregating litter

* I have not learned how to do superscript in HTML yet, so apologies - to those of you in the know - for the dodgy notation on these alleles. I did my best to make clear dominance and color.

 

 Led Zeppelin - "Black Dog" [4:55m]: Play Now | Play in Popup | Download