Mucopolysaccharidosis type VI (Maroteaux-Lamy disease) is an inherited lysosomal disease caused by arylsulfatase B deficiency, which prevents the breakdown of glycosaminoglycans (mucopolysaccharides). The accumulation affects bones, the cornea, heart valves and internal organs: affected cats grow poorly, with coarse facial features, corneal opacity and skeletal deformities. The disease is progressive and has no curative treatment in clinical practice; control is based on genetic testing of breeding animals.

Classically described in Siamese and related breeds (Balinese, Javanese, Oriental Shorthair, Peterbald, Seychellois, Thai, Tonkinese); there are also cases in European Shorthair and in non-Siamese cats. The frequency of the D520N allele in the Siamese population was estimated at around 11.4% (Crawley et al. 2003); that of the L476P allele is low. Current frequencies by breed: limited data.

- Dwarfism with a short trunk and deformed limbs
- Coarse facial features: broad head, depressed nasal bridge
- Progressive bilateral corneal opacity
- Thick toes and joint stiffness
- Thoracic deformities (pectus) and hernias in some cases
- Heart murmur due to valvular thickening in advanced cases

Feline MPS VI was characterised from a colony of Siamese cats in which two mutations of the ARSB gene with distinct phenotypes were described: Yogalingam et al. (1996) identified the severe variant c.1427T>C (p.L476P) and Yogalingam et al. (1998), the mild variant c.1558G>A (p.D520N). Crawley et al. (1998) showed that combinations of both alleles produce three different clinical phenotypes, and Crawley et al. (2003) estimated the prevalence of the mutations in the Siamese population. The cat became a reference model for the human disease and for testing enzyme and gene replacement therapies. Lyons et al. (2016) clarified aspects of genetic testing in the species.

- Test breeding animals of the included breeds: it distinguishes clear, carrier and affected animals.
- Never mate two carriers; a carrier may be mated to a clear animal without risk of affected offspring.
- Test the offspring that will continue in breeding and record the result in the pedigree.
- Affected animals require palliative management (ocular, cardiac and orthopaedic monitoring) and should not be bred.

Differential diagnosis with other mucopolysaccharidoses (MPS VII, MPS I), skeletal dwarfisms of other causes and congenital corneal opacities. Measurement of glycosaminoglycans in urine and arylsulfatase B activity in leukocytes provide guidance; molecular testing confirms and identifies the variant. In the case of an affected animal, study both parents and assess the whole litter, including apparently healthy animals. Severity should not be inferred from the gene alone, but from the complete genotype.

1. Yogalingam G et al. 1996. Feline mucopolysaccharidosis type VI. Characterization of recombinant N-acetylgalactosamine 4-sulfatase and identification of a mutation causing the disease. J Biol Chem. PMID: 8910299
2. Crawley AC et al. 1998. Two mutations within a feline mucopolysaccharidosis type VI colony cause three different clinical phenotypes. J Clin Invest. PMID: 9421472
3. Yogalingam G et al. 1998. Mild feline mucopolysaccharidosis type VI. Identification of an N-acetylgalactosamine-4-sulfatase mutation causing instability and increased specific activity. J Biol Chem. PMID: 9593674
4. Crawley AC et al. 2003. Prevalence of mucopolysaccharidosis type VI mutations in Siamese cats. J Vet Intern Med. PMID: 12892300
5. Lyons LA et al. 2016. Mucopolysaccharidosis VI in cats - clarification regarding genetic testing. BMC Vet Res. PMID: 27370326
6. Lyons LA. 2015. DNA mutations of the cat: the good, the bad and the ugly. J Feline Med Surg. PMID: 25701860
7. OMIA:000666-9685. Mucopolysaccharidosis VI in Felis catus. Online Mendelian Inheritance in Animals.