MPS I is a variant of mucopolysaccharidosis in cats and is a rare recessive disorder due to an enzyme deficiency that causes grossly abnormal neurons in the brain and spinal cord.
The facial profile of affected cats is altered: short, broad nose, depressed nasal bridge, prominent forehead, small ears and opacity of the cornea. The affected cat sits crouched with spread forelegs. The cervical vertebrae unusually wide, asymmetrical and frequently fused. The breast-bone (sternum) is unusually concave.
Cardiac abnormalities are noted as cats age, including lesions involving the mitral and aortic valves and ascending aorta. Enlarged liver and spleen may be evident. Though the gene behaves as a recessive (lethal in homozygous cats), the enzyme deficiency can also be detected in heterozygous cats, allowing carriers to be screened out of breeding programmes.
Some responses have been reported using enzyme replacement therapy. Recombinant alpha-L-iduronidase was administered intravenously at low dose (approximately 0.1 mg/kg or 25,000 units/kg) to four cats and high dose (0.5 mg/kg or 125,000 units/kg) to two cats on a weekly basis for 3- or 6-month terms.
Clinical examinations showed distinct clearing of corneal clouding in one cat although clinical effects in the others were not evident. Biochemical studies of the cats showed that the enzyme was distributed to a variety of tissues although the liver and spleen contained the highest enzyme activities.