Page 29 - Copper-associated hepatitis in the Labrador retriever diagnosis, treatment and genetics

deregulations. Another important factor that can be controlled for in dog populations is dietary copper intake. The majority of privately kept dogs is fed a kibble diet, in which copper concentrations are relatively stable and copper intake can be estimated more precisely than in humans.
Some pitfalls are present when applying GWAS studies in canine copper toxicosis and they have to be taken into account in study design and data analysis. As stressed before, correct phenotyping is of utmost importance in the design of a genetic study. Phenocopies can occur as a result of hepatic copper accumulation due to reduced bile flow and this has to be distinguished from primary copper accumulation resulting from a genetic defect. In primary forms of copper accumulation, copper is localized around the central veins in the liver lobule, whereas copper accumulation due to cholestasis is present in hepatocytes in the periportal areas. In advanced stages of copper toxicosis, when liver cirrhosis is present, the architecture of the liver is disturbed and localization of copper within the liver lobe becomes a challenge. Also, in advanced, untreated cases, hepatic copper concentrations may actually decrease, due to replacement of hepatocytes with fibrotic tissue and regenerative nodules that have not yet accumulated copper. In these cases, it may become difficult to distinguish between primary copper toxicosis and idiopathic cirrhosis. In conclusion, for correct phenotyping an experienced veterinary pathologist and reliable methods for quantitative copper determination are indispensable.
In the GWAS data analysis, it is important to look for population sub-structuring and cryptic relatedness in the dog sample as this can cause false association signals. The use of mixed models in the data analyses, for example implemented in the software GenABEL144, can elegantly correct for underlying population or family structure. In addition, the use of this kind of models has the advantage that traits can be analyzed quantitatively, for example hepatic copper level, and that modifying factors such as age of onset, sex, and dietary copper intake can be implemented in the model. There is a high level of conservation of copper metabolism genes over species; therefore it is likely that genetic studies into canine copper toxicosis will contribute to an increased knowledge into mammalian copper metabolism and human copper storage diseases. It is clear that upon identification of copper metabolism associated new genes in purebred dog populations the translational step to human disease phenotypes
General introduction
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Chapter 1