Grant No. 903-A:  Candidate Gene Investigation of Cairn Terrier Ocular Melanosis

Submitted by Linda Heiner

PI: Simon Petersen-Jones DVetMed PhD DVOphthal DipECVO MRCVS
Start Date: April 1st 2007.
Report Due: September 30th 2008. (Includes no cost extension)
Grant duration: one year
Report: Final report

Stated Specific Objectives

Specific Objective 1.  Identification of SNPs polymorphic in the Cairn Terrier breed within candidate genes for ocular melanosis.

Specific Objective 2. Genotyping of ocular melanosis-affected and -unaffected Cairn Terriers for the candidate gene SNPs and examination of results for association of one allele with the disease.

Our strategy has been to identify markers that map within or close to each candidate gene and are polymorphic within the Cairn Terrier breed.  We have identified these in a number of different ways, namely resequencing – which consists of sequencing intronic regions of candidate genes from several Cairn Terriers to identify polymorphisms which are then typed in the affected dogs; identifying published microsatellites close to the candidate genes; identifying published SNPs close or within the candidate genes.  We make the assumption that there is a single founder animal for Cairn Terrier Ocular Melanosis and that all affected dogs have inherited a chromosomal region that is identical by descent.  We also assume that there is one necessary locus for Ocular Melanosis in the breed.  Our analysis of pedigrees in which Ocular Melanosis is segregating suggest that the condition is inherited in an autosomal recessive fashion.  Given those assumptions we expect each affected dog to share at least one version of every DNA polymorphism that predates the Ocular Melanosis mutation for the chromosomal region surrounding the mutant gene that is identical by descent to the founder animal.  If we identify affected dogs that are homozygous for both versions of a bi-allelic marker such as a SNP that suggests there is no association between that locus and the Ocular Melanosis locus.  To make a likely exclusion of a locus we wish to have two markers that exclude that locus and we want to see the exclusion based on more than one affected dog.


Our results have not shown any association between the candidate genes loci and the Ocular Melanosis locus.


We have not published the results of the candidate gene screening yet.


We identified 11 potential candidate genes for Ocular Melanosis based on their known function.  To investigate whether the genes were linked to the Ocular Melanosis locus we identified genetic variations close to the candidate genes.  These consisted of single-nucleotide polymorphisms, insertion/deletions and microsatellites.  Analysis of these DNA markers suggested that the candidate genes were not linked to the Ocular Melanosis locus.

Breed Club Report

As part of our ongoing studies of Ocular Melanosis in Cairn Terriers we identified 11 genes that were potential candidate genes for the condition.  A candidate gene for a particular disease is a gene that if it was not functioning normally the disease under investigation would be likely to develop.  Based on our previous work to characterize Ocular Melanosis we have a good idea of the likely role of the gene that is defective in the condition.  Genes that are known to be involved in the function of pigment cells (melanocytes) within the eye are potential candidate genes.  Looking at similar conditions in experimental mice and by reviewing the information about pigment cell function we identified 11 genes we considered as good candidates for the condition.  The current study was to investigate whether any one of these genes was likely to cause Ocular Melanosis in Cairn Terriers.

We also have good evidence that the condition is dominantly inherited meaning that to develop the condition a dog would only need to inherit one defective copy of the gene (which it would receive from one parent) the other copy of the gene (received from the other parent) could be normal and yet the dog would still get Ocular Melanosis.  We also make the assumption that Ocular Melanosis in Cairn Terriers resulted from the development of a single mutation in the causal gene at some stage in the past.  All the current affected dogs would have inherited that defective gene from the original founder animal.  The DNA coding for the defective gene and the DNA flanking the gene will be identical between all affected dogs (for the one defective gene copy).  The approach that we took was to identify DNA markers that were within the candidate gene, or very close to it (and therefore likely to be inherited along with the defective gene) and see what version affected dogs and older unaffected dogs had for the DNA marker.  The DNA markers that we most commonly used had two versions (that we will call A and B – a dog could therefore be AA, AB or BB for that marker).  We would expect all Ocular Melanosis affected dogs to share one version of any DNA marker within or close to the gene causing the condition.  When we find a marker where some affected dogs are AA and some are BB then that suggests that the particular location around the marker is not involved in the disease condition.  In other words, that candidate gene is unlikely to be the Ocular Melanosis gene that we are looking for.  We look for at least two different DNA markers that exclude each candidate gene and more than one dog that excludes the locus before we conclude that it is unlikely that the particular candidate gene is the Ocular Melanosis gene.

We have worked through the 11 candidate genes that we have identified and unfortunately we have not shown any association between the markers for the genes and Ocular Melanosis.  Our next approach is to use a mapping approach to show which of the 37 pairs of dog non sex chromosomes the Ocular Melanosis gene is on and then after further narrowing down the location to identify likely genes that are positioned in that region.