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Hyaluronan is a high-molecular-weight glycosaminoglycan (GAG) prominent in the extracellular matrix. Emerging relatively late in evolution, it may have evolved to evade immune recognition. Chondroitin is a more ancient GAG and a possible hyaluronan precursor. Epimerization of a 4-hydroxyl in N-acetylgalactosamine in chondroitin to N-acetylglucosamine of hyaluronan is the only structural difference other than chain length between these two polymers. The axial 4-hydroxyl group extends out perpendicular from the equatorial plane of N-acetylgalactosamine in chondroitin. We suspect that this hydroxyl is a prime target for immune recognition. Conversion of a thumbs-up hydroxyl group into a thumbs-down position in the plane of the sugar endows hyaluronan with the ability to avoid immune recognition. Chitin is another potential precursor to hyaluronan. But regardless whether of chondroitin or of chitin origin, an ancient chondroitinase enzyme sequence seems to have been commandeered to catalyze the cleavage of the new hyaluronan substrate. The evolution of six hyaluronidase-like sequences in the human genome from a single chondroitinase as found in Caenorhabditis elegans can now be traced. Confirming our previous predictions, two duplication events occurred, with three hyaluronidase-like sequences occurring in the genome of Ciona intestinalis (sea squirt), the earliest known chordate. This was probably followed by en masse duplication, with six such genes present in the genome of zebra fish onwards. These events occurred, however, much earlier than predicted. It is also apparent on an evolutionary time scale that in several species, this gene family is continuing to evolve.

Hyaluronidases are endoglycosidases that hydrolyze hyaluronan (HA), an abundant component of the extracellular matrix of vertebrate connective tissues. Six human hyaluronidase-related genes have been identified to date. Mutations in one of these genes cause a deficiency of hyaluronidase 1 (HYAL1) resulting in a lysosomal storage disorder, mucopolysaccharidosis (MPS) IX. We have characterized a mouse model of MPS IX and compared its phenotype with the human disease. The targeted Hyal1 allele in this model had a neomycin resistance cassette in exon 2 that replaced 753 bp of the coding region containing the predicted enzyme active site. As a result, Hyal1(-/-) animals had no detectable wild-type Hyal1 transcript, protein or serum activity. Hyal1 null animals were viable, fertile and showed no gross abnormalities at 1 year and 8 months of age. Histological studies of the knee joint showed a loss of proteoglycans occurring as early as 3 months that progressed with age. An increased number of chondrocytes displaying intense pericellular and/or cytoplasmic HA staining were detected in the epiphyseal and articular cartilage of null mice, demonstrating an accumulation of HA. Elevations of HA were not detected in the serum or non-skeletal tissues, indicating that osteoarthritis is the key disease feature in a Hyal1 deficiency. Hyal3 expression was elevated in Hyal1 null mice, suggesting that Hyal3 may compensate in HA degradation in non-skeletal tissues. Overall, the murine MPS IX model displays the key features of the human disease.