Edward E. (Ned) Patterson,     St. Paul, Minnesota

Scientific Basis of the Tests

The basis, methods, techniques, and quality control of inherited disease testing are not standardized for small animals. In addition, animal genetic testing laboratories and companies have no oversight or accreditation. Regulations and guidelines for small animal genetic testing are needed. Until these are enacted, veterinarians submitting samples for canine genetic disease testing must evaluate each test individually to determine its accuracy and reliability. A basic knowledge of the methods of the test and an evidence-based approach to its evaluation are important to guide decisions about when to use a specific test. Criteria for molecular genetic testing should be similar to those for any diagnostic medical test. Within a reasonable amount of time after development, the data and results of the test should be published in a peer-reviewed scientific journal. Ideally the test should be verified independently by an outside group. Any test, evidence, and data without the scrutiny of a peer review should be used cautiously or evaluated critically. For some of the currently available tests, only patent information is available; this information can be reviewed through the U.S. patent website (www.uspto.gov/main/patents.htm). In some cases companies offering tests are awaiting resolution of intellectual property issues.

Biochemical Tests

Biochemical tests for inherited disorders in dogs and cats have been available for many years. They continue to play an important role in diagnosis of inherited disorders in which the chromosomal location or gene for the defect has not been identified. Most biochemical tests require only a simple blood or urine sample. Biochemical tests are also necessary to help evaluate newly developed molecular genetic tests, especially those lacking documentation or presenting controversy. Examples of some of the currently available biochemical tests include those for mucopolysaccharidosis, Fanconi syndrome, erythrocyte osmotic fragility, methylmalonic aciduria, cystinuria, urinary acids, urinary amino acids, urinary carbohydrates, urinary glycosaminoglycans, urinary oligosaccharides, cobalamin malabsorption, hypersarcosinemia, and other inborn errors of metabolism performed at the University of Pennsylvania School of Veterinary Medicine (PennGen) (Table 221-1). Factor assay tests for von Willebrand’s disease and other inherited coagulopathies are performed at a number of veterinary diagnostic laboratories. In many cases biochemical tests are the best estimate of the genetic status of an individual (i.e., affected, carrier, or clear). However, test results can fall into overlapping categories, causing potential problems with classification and definition.

Deoxyribonucleic Acid–Based Tests

Many genetic markers do not code for messenger ribonucleic acid or proteins (noncoding markers). These markers are interspersed throughout all chromosomes, and many noncoding markers are near every gene. The markers tend to be variable among individuals because changes in the nucleotides do not have any known functional effect. Many of the markers used in past canine genetic studies and marker tests are repeats of nucleotides (e.g., CA repeated 10 to 30 times). Restriction enzymes cut DNA at specific, short sequences. The resulting different lengths of DNA can be detected on a gel because the fragments migrate in an electrical field in inverse proportion to their size. The varying lengths of the marker are called different alleles, just as various blood types represent different alleles of a coding gene. Single nucleotide polymorphisms (SNPs), such as a C or T at the exact same chromosomal location (locus), are another type of genetic marker. Canine SNP arrays that can genotype thousands of SNP markers for one individual have been available for a number of years. Currently, most canine genetic disease research looks for association of SNP markers with the disease of interest for particular breed or a group of related breeds. A SNP array has just been developed and now is available for feline research.

A genetic marker can be associated strongly or linked with a disease gene if it is close to a gene and the marker has more than one allele. The farther a marker is from a gene on the same chromosome, the more likely recombination is to have occurred during meiosis. The percentage of time a marker and gene have recombination between them is termed the recombination fraction. For a marker to be potentially useful as a screening genetic test, a recombination fraction of 5% or less typically is required. Figure 221-1 illustrates marker and gene linkage and recombination. A marker allele is linked to a disease; this is probably the most difficult concept pertaining to an understanding of published chromosomal locations of causative genes and molecular genetic disease testing.