Alexander disease Labrador Retriever Type (AxD)
Alexander disease Labrador retriever type is a rare neurodegenerative disorder characterized by astrocyte dysfunction. The disorder is also known as Leukodystrophy in Labrador retrievers. Alexander disease is generally not common in dogs, but it has been recognized in several species. It has been diagnosed in humans, dogs, and sheep. Among dogs, AxD has been reported in Labrador retriever, Bernese mountain dogs, Scottish Terriers, miniature poodles and Chihuahua. The causative mutation has been identified in Labrador retriever breed.
Alexander disease can be divided into three types (infantile, juvenile and adult), dependent on the age of onset and the clinical signs. Another classification system can be based on clinical signs, by which we recognize type I and type II AxD. Alexander disease in Labrador retrievers can be compared with the juvenile type of the disorder in man, based on symptoms and age of their onset.
Characteristics and Symptoms
Astrocytes are a type of glial cells in the brain and spinal cord. One of the main compounds of astrocytes is the glial fibrillary acidic protein (GFAP). Astrocytic protein aggregates are a pathological feature termed as Rosenthal fibers. These fibers mainly contain GFAP and are a result of mutation within GFAP coding gene, which is specific for the Alexander disease.
Affected Labrador retrievers start to show symptoms by 4 to 6 weeks of age. Symptoms include severe tetraplegia, spastic forelimbs, and mild vestibular signs. Histopathological examination reveals numerous intra-astrocytic Rosenthal fibers in the brain and spinal cord with large GFAP accumulation. Due to the severity of the neurological symptoms, affected puppies usually need to be euthanized.
Leukodystrophy Labrador retriever type or Alexander disease is caused by a mutation in the GFAP gene. The mutation causes a G to A nucleotide substitution which results in an Arginine to Histidine amino acid substitution.
The disorder is inherited in an autosomal dominant pattern. This means that one copy of the mutated gene is enough for a dog to develop the symptoms of the disorder.
Van Poucke, M., Martlé, V., Van Brantegem, L., Ducatelle, R., Van Ham, L., Bhatti, S., & Peelman, L. J. (2016). A canine orthologue of the human GFAP c.716G>A (p.Arg239His) variant causes Alexander disease in a Labrador retriever. European Journal of Human Genetics, 24(6), 852–856. http://doi.org/10.1038/ejhg.2015.223