Mucopolysaccharidosis VI (MPS VI) is a type of storage disease and has been noted to affect several breeds of cat, most commonly the Siamese. It is one of the most common storage diseases in the cat. The disease is caused by the lack of an enzyme (arylsulfatase B), leading to the incomplete break down of a glycosaminoglycan, which then accumulates within certain cells.
Two types of disease from MPS VI have been documented in cats, a mild form, and a severe form. The more severe mutation is uncommon but present in the Siamese population, however the milder mutation has been documented to be present at a prevalence of around 11% in Siamese cats from all over the world. When a cat has two copies of the severe mutation, the severe form of the disease occurs. When a cat has two copies of the mild mutation, very few changes are noticeable. However when a cat has one copy of each mutation (one mild and one severe) then the mild form of the disease is seen, where the cat generally develops severe arthritis, or degenerative joint disease in the shoulders. Less often arthritis is also seen in the knees (stifle). This arthritis tends to develop early in life (8 - 12 months of age) and can be very debilitating, however otherwise cats with the mild form of MPS VI appear normal.
The severe form of MPS VI causes a number of problems in affected cats. It affects the development of the skeleton, with facial deformities seen as well as a short body due to abnormal spinal bones, abnormal ribs and bones of the legs, often affecting the flexibility of the limb joints. The hind legs may be longer than the front legs, and some joints, such as the hip or carpus (wrist) may be dislocated. Cartilage is also affected, and can cause a progressive lameness which can eventually result in an inability to walk, due to degeneration of the joint cartilage or sometimes due to spinal collapse and paralysis.
Vision is also affected, due to accumulation of abnormal substances in the retinal cells within the eye, as well as the outer cornea of the eye becoming cloudy in appearance. Internal organs can be affected, and thickening of the valves within the heart may result in heart failure and breathing problems. Occasionally sudden death can occur.
This disease is inherited as an autosomal recessive condition, and signs may be seen from a few weeks of age, or not for a number of months, depending on the severity of the condition. The only treatment currently is expensive, involving either enzyme replacement therapy, or a bone marrow transplant. Most affected cats are euthanised. Recently, gene transfer therapy has been performed successfully in cats without the need for immunosuppressant treatment, and when used in young kittens has resulted in almost normal growth and development. This is a very promising future line of treatment.