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Inherited diseases: genetic tests

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Introduction

  • Following completion of the domestic cat (Felis catus) genome, progress in characterization of hereditary diseases and traits in cats has advanced rapidly and there are now over 100 genetic mutations defined in the cat.
  • A mutation is a permanent alteration to the nucleotide sequence of an organism's DNA. Mutations can be small (involving single nucleotides of DNA) or large (involving many thousands of nucleotides of DNA, and sometimes affecting several adjacent genes). Many mutations have little or no effect on the health of the individual that carries them, whereas some mutations may even be beneficial - conferring a selective advantage in evolutionary terms or under artificial selection in domestic species. But some mutations are deleterious and can cause reduced fitness or disease. If the mutation occurs in the germline (egg or sperm) of an individual, the mutation may be passed on to offspring of that individual who then may also develop the associated disease.
  • The term "mutation" is the act of change in the DNA sequence, once such a change is established in a population it is called a variant or polymorphism. An individual may be said to have a "mutant allele" whereas the normal (unmutated) allele is often termed wildtype. 
  • Often, changes with no effect on fitness are referred to as polymorphisms whilst disease causing changes retain the name mutation. 
  • Recessive mutations are those which do not produce a phenotypic effect unless the individual has two copies of the mutation, termed homozygous for the mutation. If an individual carries a single copy of the mutation (termed heterozygous), then the mutation is present on only one chromosome inherited from one parent and the other is normal (from the other parent). If an individual is heterozygous for a recessive mutation, they will be a carrier but will be healthy, not presenting with any effects of the mutation. Only when an individual inherits a copy of the recessive mutation from both its parents will the individual suffer the deleterious effects of the mutation. 
  • Dominant mutations only need to be present as a single copy within the genome, on one chromosome of an individual, for that individual to suffer the deleterious effects of the mutation. 
  • Genetic tests analyze an individual's DNA for the presence or absence of a mutation that is known to cause an inherited disease/condition or be associated with other types of inherited trait, such as coat color, coat length or the presence/absence of a tail. 
  • Mutation-based tests examine an individual's DNA for the presence or absence of the precise mutation that is known to cause a particular disease. Within the limits of human error or known biological complications of testing, a well-designed mutation-based test is nearly 100% accurate in detecting the presence of the mutation. Most modern genetic tests are mutation-based. 
  • Linkage-based tests, in contrast, do not detect the disease-causing mutation, but instead analyse polymorphic DNA markers that are known to be located very close to the mutation. Because genetic recombination events can occur between the mutation and the markers that are analyzed, linkage-based tests are not usually 100% accurate. Rather they vary in accuracy depending on just how close the DNA marker is to the mutation. Commercial linkage-based tests will typically give an accurate result in at least 95% of animals tested, whilst some are considerably more accurate. 
  • The results of a genetic test will inform the veterinarian or owner whether the animal being tested carries zero, one or two copies of the mutation being tested. Depending on the mode of inheritance of the mutation, this information will predict whether or not the animal will, or is likely to, become affected by the disease itself and whether it could the mutation onto its offspring. 
  • It is important to understand that detecting the presence of a mutation is not always the same as certain prediction of the onset of a condition. There may be influences from the environment or other parts of the genome.
  • Some mutations show only partial or incomplete penetrance: that is their effects reach clinical significance in only a proportion of individuals that have the mutation - they carry a risk of disease rather than a certainty of having disease.
  • Some mutations show a dosage effect, with mild disease (or minor risk of disease) in heterozygous animals, but markedly more severe disease in homozygotes. These types of mutations are often termed co-dominant or semi-dominant.
  • The link between the test result and disease is considered in more detail below.
  • To date more than 100 mutations have been defined in the cat. Many of these involve diseases, some of which are clinically relevant, but also structural abnormalities and other phenotypic features such as coat color and texture. This number will continue to increase as more of the feline genome is annotated. 
Print off the owner factsheet on Genetic testing for cats to give to your client.

How genetic tests are undertaken

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How to use the information gained from a genetic test

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Limitations of DNA tests

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What information should be available for each genetic test

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What genetic tests are currently available?

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Further Reading

Publications

Refereed papers

Other sources of information

  • http://www.vetmed.ucdavis.edu/Catgenetics/
  • http://www.catgentics.co.uk 
  • Lyons L (2015) DNA Mutations of the Cat: The good, the bad and the ugly. J Fel Med Surg 17, 203-219 PubMed.
  • Mellersh C & Sargan D (2011) DNA testing in companion animals - what is it and why do it? In Practice 33, 442-453.
  • Lyons L (2010) Feline Genetics: Clinical Applications and Genetic Testing. Topics in Companion Animal Medicine 25 (4), 203-212 PubMed.

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