The Future of Veterinary Genetic Testing

Genetically based (ie, inherited) diseases are prevalent in dogs. Purebred dogs carry the highest number of disease-causing genetic mutations, but these mutations also occur in mixed-breeds. Breed-specific genetic diseases, which are associated with most canine breeds, result from selective breeding for specific cosmetic and behavioral traits. Inbreeding to achieve desirable traits limits a breed’s gene pool and perpetuates genetic mutations, both benign and pathologic, that affect every body system.^1,2 For example, the cardiac and cranial abnormalities seen in many Cavalier King Charles spaniels can be the unintended consequences of selective cosmetic breeding.¹

Genetic testing, which has been used to identify inherited diseases in humans since the 1950s—beginning with Down syndrome and cystic fibrosis³—is becoming an important diagnostic tool in veterinary medicine. Collaborative efforts by the One Health Initiative and the National Institutes of Health, as well as genome-wide studies such as the National Human Genome Research Institute Dog Genome Project⁴ and the Broad Institute Dog Genome Project,⁵ have greatly advanced veterinary genetics in the past several decades. Efforts to map the canine genome began in the United States in 2003; the results of 2 studies—one of a poodle and one of a purebred boxer—were published in 2004.^2,6,7 Canine genetic research has since expanded to include additional diseases and breeds.

Patterns of Inheritance & Associated Genetic Diseases^2,6,19

Genetic Mutations

Most mutations occur and recur because of parental genetic influence and environmental adaptation. The mutations influence many characteristics within an offspring’s phenotype (eg, coat length and color, behavior, eye color), but some genetic coding errors during development can cause specific disease states.

A mutation may then be transferred to offspring through breeding.¹ (See Genetics 101.) A mutation in a single gene (ie, monogenic) is characterized as autosomal recessive, autosomal dominant, or sex-linked. (See Table 1.)

• Autosomal recessive: Most genetic diseases in humans and dogs are autosomal recessive. (See Table 2.) Puppies may be carriers of the genetic defect (ie, ≈50%), be affected by the disease (ie, ≈25%), or be genetically normal (ie, ≈25%).^1,6,8

• Autosomal dominant: These mutations are less prevalent but still common. The disease state is present if this type of mutation is passed to the offspring from only one parent.

• Sex-linked: These chromosomal diseases occur because of a mutation on the sex chromosomes, leading to a sex disposition in some inherited diseases.^6,8 Their prevalence is unknown; however, the number of sex-linked testable diseases is much fewer than autosomal recessive and dominant mutations.²

Mutations that occur in multiple genes may result in disease conditions more difficult to identify. Their prevalence is unknown, although research is ongoing. Current known polygenic diseases causing mutations include patent ductus arteriosus in poodles and progressive retinal atrophy in the Welsh corgi.^6,9 These mutations are difficult to identify because they require breeding studies with a large number of participants. Determining the type of inheritance (maternal vs paternal) in polygenic diseases is not specific but can indicate the prevalence of a particular polygenic mutation in a group.¹⁰

Punnett Square: Autosomal Recessive

Abbreviations: R, dominant (normal gene); r, recessive (disease-causing mutation)

Genetic Testing

Genetic testing refers to the identification of a genetic mutation, which can identify potential diseases or the possibility of offspring carrying the same mutation and disease potential.^11-15

The ability to identify carriers and affected offspring offers potential benefits to veterinarians, clients, and breeders. For example, dogs suspected of having certain genetic diseases can be identified at a younger age by minimally invasive procedures that require only cheek swabs or blood, urine, or semen samples submitted to reliable laboratories. Preventive and/or advanced care can then be initiated at an earlier age.

Genetic testing could become part of existing wellness programs to help identify genetic factors that affect the length and quality of a dog’s life. Carriers can be removed from breeding programs or bred more selectively with partners that are genetically normal to decrease the prevalence of genetic diseases.^1,11-13,16

Genetic testing in dogs is currently limited, however, and caution should be used when recommending testing to clients and breeders. Most tests check only for single gene defects, yet some diseases (eg, canine inherited cataracts) are caused by several different genetic mutations.¹⁷ Many tests identify only the genetic markers for a disease rather than the specific genetic mutation and existence of illness. Genetic markers only identify a series of genes that indicate the possibility of mutation. Testing for genetic markers can be useful, however, as their presence can indicate disease-causing genes, and the affected animal can be monitored more closely for disease signs.^6,15 Clients and breeders must be counseled on the different tests available from veterinary professionals and other advertised sources, and clients must know how best to interpret the results of these tests. (See Table 3.)

Commonly Performed Genetic Tests^17,18

Conclusion

Canine genetic testing offers a wide range of possible benefits for veterinary medicine. Ongoing research, including clinical trials studying common diseases (eg, progressive retinal atrophy, idiopathic epilepsy, glaucoma, intervertebral disc disease) and continued mapping of the canine genome will help improve diagnostic, preventive, and treatment options for genetic conditions.¹⁸

Behavioral genetics is an emerging research field that has demonstrated the inheritability of certain behaviors such as flank sucking in Dobermans and the herding ability of some breeds (eg, border collies, German shepherd dogs, Australian cattle dogs).¹⁶ Identifying dogs affected by these mutations may enable them to receive early environmental modification to counter potential negative behaviors. Improved testing accuracy and decreased testing costs will help establish canine genetic testing as a powerful diagnostic tool.

This article originally appeared in the July 2017 issue of Veterinary Team Brief.