DNA, Diseases and Diagnostic Tests in the Staffordshire Bull Terrier
Introduction
The Animal Health Trust is a charity that has been helping dogs, cats and horses for more than half a century. We provide specialist veterinary clinical, diagnostic and surgical services and our successes in research have ranged from major breakthroughs in anaesthesia and surgical techniques to the development of vaccines against diseases such as canine distemper and equine influenza. Our scientists and veterinarians, many of whom are world leaders in their field, work alongside bringing together a wide range of expertise for a co-ordinated attack on animal diseases and injuries. By publishing scientific papers, speaking at conferences and talking to other veterinary surgeons about the cases dealt with, this knowledge is passed on to benefit the maximum number of animals.
The canine genetics group is a small team of skilled personnel that liases with clinicians, other scientists within and outside the Trust and dog owners and breeders alike. This unique partnership is what makes the AHT’s research and DNA Diagnostic Service so successful. The genetics group strives to understand the genetic basis of inherited canine disease. The long-term goal of the work we undertake is to identify the genetic mutations that cause inherited disease and develop DNA tests that identify those dogs which carry these causal mutations. Breeders can use the information gleaned from these tests to instigate appropriate breeding programmes that avoid the production of affected dogs and aide in the eventual elimination of inherited disease from breeds at risk.
Basic Genetics
Selective breeding for desirable phenotypic and behavioural traits has produced more than 300 distinct breeds of domestic dog worldwide with each breed being defined by a narrow set of specific physical and behavioural characteristics. Not surprisingly, because many of these key features are maintained by crossing closely related individuals, inherited diseases are very common among purebred dogs. Inherited diseases are caused by mistakes (mutations) within the DNA of genes.
Chromosomes
Each dog has 38 pairs of ‘autosomes’ and a single pair of ‘sex chromosomes’ in nearly every cell within its body. The autosomes are the same in males and females whereas females have 2 ‘X’ sex chromosomes and males have an ‘X’ and a ‘Y’ chromosome. One of each pair of chromosomes is inherited from the dog’s mother and the other from its father.
DNA & Genes
The chromosomes are made up of deoxyribonucleic acid or DNA. DNA is a long, double stranded, very complex molecule composed of 4 different building blocks or nucleotides (abbreviated to C, G, T and A). It is divided into ‘genes’; every gene is literally a written instruction that codes for the production of a single protein. There are probably about 30 - 40,000 different genes within the DNA of every dog. The dog has 2 copies (or alleles) of each gene, one on the chromosome it inherited from its mother and one on its paternal chromosome. The proteins that are coded for by these genes interact with one another in very specific ways within the body to perform all the functions necessary for life. The order of As, Gs, Cs and Ts within a gene needs to be exactly right for the cells to make the correct proteins; ‘misprints’ or mutations within genes can lead to ‘faulty’ or non-functional proteins and these, in turn, can give rise to disease.
Inherited Disease in the Staffordshire Bull Terrier
As with other breeds of dog the Staffordshire bull terrier suffers from its fair share of inherited disease. The AHT has been researching the genetic basis of two such diseases over the last few years, Hereditary Cataract (HC) and L-2-Hydroxyglutaric Aciduria (L-2-HGA) and is pleased to announce the development of DNA Diagnostic Tests for both these conditions are now available at the AHT.
L-2-Hydroxyglutaric aciduria
L-2-HGA (L-2-hydroxyglutaric aciduria) in Staffordshire Bull Terriers is a neurometabolic disorder characterised by elevated levels of L-2-hydroxyglutaric acid in urine, plasma and cerebrospinal fluid. L-2-hydroxyglutarate is normally metabolised to α-ketoglutarate but in affected dogs it is not, and builds up in the body with devastating results. L-2-HGA affects the central nervous system, with clinical signs usually apparent between 6 months and one year (although they can appear later). Symptoms include epileptic seizures, "wobbly" gait, tremors, muscle stiffness as a result of exercise or excitement and altered behavior.
Hereditary Cataract
Hereditary cataract in the Staffordshire bull terrier was first reported in the United Kingdom in 1976. The condition is not congenital, so puppies are born with normal eyes, but cataracts begin to appear at a few weeks to months in age, progressing to total cataracts by 2 to 3 years of age. This cataract is always bilateral, symmetrical in the two eyes, and progressive until total with resultant blindness.
Autosomal Recessive Disease
Both HC and L-2-HGA are autosomal recessive conditions. This type of disease is caused by a mutation within a single gene located on one of the 38 pairs of autosomes. Mutations causing recessive diseases can be small (for example a single incorrect nucleotide, or the insertion or deletion of a small number of nucleotides) or large (such as the deletion of a large number of nucleotides). Because the mutations are within genes located on the autosomes both males and females suffer from the disease with equal frequency.
A dog has to have a mutation in each copy of the gene (i.e. the copy on each of its chromosomes) before it will actually develop symptoms of the disease. This is known as being homozygous for the disease allele. If it has one mutated copy of the gene and one normal copy it will be a carrier of the disease but will never actually develop symptoms. It can, however, pass the mutation onto future generations. For a dog to be affected with an autosomal recessive disease, both its parents have to be either carriers or affected. If two carriers are mated together on average one in four of their offspring will be affected, one in four will be genetically clear and half will be carriers.
DNA Diagnostic Tests
After several years of research into the genetic basis of these two inherited conditions the AHT is pleased to announce the identification of the mutations that cause both diseases. The research was made possible by funding from the Kennel Club Health Foundation Fund, the American Kennel Club Canine Health Foundation, the Staffordshire Bull Terrier Breed Council and PetSavers. DNA samples from affected dogs and their close relatives were donated by Staffordshire Bull terrier owners and breeders and the AHT is sincerely grateful to all those who donated samples from their dogs.
How Breeders Can Benefit From the DNA Diagnostic Tests
Owners can now submit a DNA sample from any dog for testing and find out whether the dog is clear, a carrier or affected with either, or both, HC and L-2-HGA. The owner will receive a certificate that states whether the dog is Clear, Carrier or Affected, with the following explanations:
CLEAR: the dog has 2 copies of the normal gene and will neither develop HC / L-2-HGA, nor pass a copy of the HC / L-2-HGA mutation to any of its offspring.
CARRIER: the dog has one copy of the normal gene and one copy of the mutant gene that causes HC / L-2-HGA. It will not develop HC / L-2-HGA but will pass on the HC / L-2-HGA mutation to 50% (on average) of its offspring.
AFFECTED: the dog has two copies of the HC / L-2-HGA mutation and is affected with HC / L-2-HGAand will develop HC / L-2-HGA during its lifetime.
Samples should be sent with a completed DNA Testing Form and a cheque for £60-00 (inc VAT) per sample for Hereditary Cataract or L-2-HGA testing alone, or £100-00 (inc VAT) per sample for Hereditary Cataract and L-2-HGA DNA testing together, to Genetic Services, Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk CB8 7UU.
DNA testing forms can be downloaded from our web site (http://www.aht.org.uk).
DNA testing forms can also be obtained by contacting Vikki Lett 08700 50 91 44 ext 1223 or via e-mail to vikki.lett@aht.org.uk.
Blood samples which have already been tested for L-2-HGA can also be tested for Hereditary Cataracts - please provide a completed DNA Testing Form quoting the Sample Number from the L-2-HGA certificate, with a cheque for £60-00 inc VAT. DNA testing forms can also be obtained by contacting Vikki Lett 01638 750659 ext 1223 or via e-mail to vikki.lett@aht.org.uk.
The Way Forward
Now that genetic tests are available for both HC and L-2-HGA it is possible to avoid breeding any more dogs that are affected with either of these conditions. Initial L-2-HGA test results indicate that approximately 15% of dogs tested are carriers of the L-2-HGA mutation and it is likely that a similar proportion of dogs will also carry the HC mutation. Some dogs will carry both mutations but as many as 20-25% of Staffordshire bull terriers could carry at least one mutation. Therefore, geneticists at the AHT advise against eliminating carriers from the breeding population because doing so will result in an unacceptable reduction in the gene pool that can never be recovered.
Instead, our advice would be to test all dogs that might be used for breeding, and if a dog is identified as being a carrier of either HC or L-2-HGA they should only be mated to dogs that have tested clear and any resulting puppies that may be used for breeding should also be tested. This practice will allow gradual elimination of the mutations from the breeding population, while also allowing those desirable characteristics exhibited by the carriers to be passed to future generations. As long as carriers are always bred to clear dogs no affected puppies will be produced and although some carriers will be bred they will be perfectly healthy with respect to both these diseases. Below is a list of various combinations of dogs, their expected outcomes, and whether they are recommended:
Combination of Dogs Outcome Acceptable Breeding Practice
Clear X Clear All puppies will be clear No need to test puppies Yes
Clear X Carrier 50% of puppies will be clear 50% of puppies will be carriers Test all puppies to be used for breeding Yes
Clear x Affected All puppies will be carriers No need to test puppies Yes
Carrier x Carrier 25% of puppies will be clear 25% of puppies will be affected 50% of puppies will be carriers No
Carrier x Affected 50% of puppies will be affected 50% of puppies will be carriers No
Affected x Affected All puppies will be affected No
The ANIMAL HEALTH TRUST would like to thank the following organisations for contributing funds towards Hereditary Cataract and L-2-HGA research:
The Kennel Club Health Foundation Fund
The American Kennel Club Canine Health Foundation
The Staffordshire Bull Terrier Breed Council
PetSavers
The ANIMAL HEALTH TRUST would also like to express its sincere thanks to all the Staffordshire bull terrier owners who contributed DNA samples from their dogs and who readily shared information about their dogs with us. It would not have been possible to develop either of these DNA tests without their help.
Our new Health Officer is myself Mrs Debbie Taylor.
A new health issue has recently arose and is being looked into as I type, it is called
Syringomyelia, it is a condition generally found in Cavaliers but has surfaced in the odd Stafford. It is a condition whereby fluid filled cavities develop within the spinal cord. Some refer to it as “neck scratcher’s disease” because scratching in the air near the neck is a common sign.
The most important sign of syringomyelia is pain. This is most commonly found in the neck region but may be difficult to define or may be intermittent. Owners often report their dog is worse at night, when first getting up, during hot or cold temperature extremes, when excited, or related to posture eg. Preferring to sleep with their heads elevated. They may seem sensitive to touch on one side of the neck/ear/shoulder/sternum. In addition some affected dogs scratch at one area of the shoulder, ear, neck or sternum. This is typically one side only, while the dog is moving and sometimes without making skin contact. Some dogs, more commonly younger patients, develop a twisted spine. Some severe cases may have other neurological deficits such as fore and hind limb weakness and wobbliness. Facial nerve paralysis, deafness and seizures have also been associated with the condition but a link has yet to be proven. Clinical signs of syringomyelia to iccopital hypoplasia are usually recognized between 6 months to 3 years of age, however dogs of any age may be presented and dogs with more severe signs tend to be presented before 2 years of age.
Unfortunately the joys of owning a Stafford, please be vigilant to the above.
