Patrick J. Venta
Michigan State University
CMO (craniomandibular osteopathy) is an inherited non-cancerous overgrowth of bone primarily occurring in the jaw region of affected dogs, most of which belong to particular terrier breeds. The disease generally disappears about a year after the initial onset at several months of age. It is rarely fatal, but can cause pain to the puppy and a good deal of distress to the owner. We are continuing our efforts to identify this gene, so that breeders can identify carriers by a diagnostic test and then make informed breeding decisions to avoid producing affected offspring.
There are two ways that simply inherited disease genes have been identified The first of these is the candidate gene approach. This approach has resulted in probably half of all currently identified canine mutations. For example, we recently used this approach to identify a gene that has a major influence on the length of hair among several dog breeds (see Housley et al., Animal Genetics, volume 37, pp. 309-315 ). In this approach, a particular gene is hypothesized to underlie a particular disease phenotype, most commonly based upon information that the homologous gene causes a similar phenotype in another species, or information about a biochemical pathway that might be affected by loss of the protein encoded by the gene. Paget’s disease of bone has a similar phenotype in humans, and was hypothesized by Dr. George Padgett (unrelated to the Dr. Paget for whom the disease was named) to be the cause of CMO. A gene causing this disease in some human families was mapped to a particular human chromosome (human 18) and so we tested the hypothesis that this gene causes the disease. The canine gene, located on canine chromosome 1 (CFA1), was excluded as underlying CMO (referred to in a report we made in the journal Genomics, volume 84, pp. 248-264 in 2004). As it turns out, three different genes can underlie Paget’s disease in different families; PDB1, PDB2, and PDB3). PDB1 and PDB2 are both closely linked together on CFA1 and so both are excluded as causing CMO.
Although Paget’s disease was the first gene to be hypothesized to underlie CMO because it causes a similar bone phenotype, it is somewhat different in that it persists into adulthood. A better candidate might be the gene(s) that underlie a human condition called Caffey’s disease (or, more formally, infantile cortical hyperostosis, or ICH). This human disease affects infants but then spontaneously disappears after about a year, just like CMO. We were very excited when it was reported that a gene encoding one form of collagen (COL1A1) had been identified as being mutant in a family with ICH-affected infants. We tested this gene in dogs but, unfortunately, we excluded it as being causative. We also tested a related collagen gene (COL1A2), but it was also excluded. It is unknown at this time if more than one gene can cause ICH, although we are continuing to monitor the human literature for new reports. We have also tested several other candidate genes selected because of their function in bone, but each of these has also been excluded. Because this “shortcut method” has not been successful for CMO, we plan to almost exclusively use the second approach, as described below.
The second approach to finding a disease gene is linkage analysis using many genetic markers that cover all chromosomes. This whole genome approach is very labor and time intensive, but we have been using it in parallel with the candidate gene approach. We previously reported that, using SNP markers primarily developed in our lab, we have excluded an estimated 30% of the canine genome. These markers are randomly distributed throughout the canine genome, and so there are gaps in coverage on each chromosome that need to be filled. With the continuation of funding starting in April of this year, we have begun to use another marker developed by Dr. Ewen Kirkness called polymorphic SINE insertions that cover all regions of the genome at reasonably high density. These markers are being tested on a complete chromosome-by-chromosome basis. Canine chromosomes 1 and 2 (the two largest chromosomes and accounting for 9% of the dog genome) have been excluded as containing the CMO gene, and we are now testing markers on the next largest chromosomes (3, 4, and 5). Although we currently plan to complete the work on these chromosomes, very recent developments in canine genetics may allow us to switch to a system that will cover the whole genome in single experiments per dog using a new automated system.
At the Dog and Cat Genome meeting held in Davis, California this summer, an automated system ( a “SNP array” ) was described by Dr. Kirsten Lindbald-Toh from the Broad Institute, which is associated with M.I.T. and Harvard. Dr. Lindblad-Toh directed the effort to sequence the boxer genome and, comparing this to the poodle genome sequence that was determined by Dr. Kirkness, a huge number of potential SNPs were identified About 20,000 of these SNPs were placed in an automated SNP-genotyping system developed by the Affymetrix Corporation. We have been in contact with Affymetrix and they have indicated that an improved version ( “version 2” ) of the canine SNP array should be commercially available near the beginning of November. We have requested a quote from the company for the cost of the array. From a conversation with a representative of Affymetrix on approximate costs, it appears that it will be somewhat more than available from the CHF grant. However, supplementary funding has been kindly provided by the MSU College of Veterinary Medicine that should make up the difference. Assuming that the array becomes available in the time frame indicated to us by Affymetrix, we plan to request approval from the CHF for a change in our study design to take advantage of this new tool. There would be no additional cost to the CHF or the participating clubs. There are some experimental details still to be worked out, but we are optimistic that this system would help us to find a linked marker to the CMO disease gene more quickly. In the meantime, we will continue to use our current system to search for the gene.