Homology of lubricin and superficial zone protein (SZP): products of megakaryocyte stimulating factor (MSF) gene expression by human synovial fibroblasts and articular chondrocytes localized to chromosome 1q25

We have previously identified megakaryocyte stimulating factor (MSF) gene e xpression by synovial fibroblasts as the origin of lubricin in the synovial cavity. Lubricin is a mucinous glycoprotein responsible for the boundary l ubrication of articular cartilage. MSF has a significant homology to vitron ectin and is composed of 12 exons. RNA was purified from human synovial fib roblasts and articular chondrocytes grown in vitro from tissue explants obt ained from subjects without degenerative joint disease. RT-PCR was used wit h multiple complimentary primer pairs spanning the central mucin expressing exon 6 of the MSF gene and individual exons on both the N- and C-terminal sides of exon 6. Exons 2, 4 and 5 appear to be variably expressed by synovi al fibroblasts and articular chondrocytes. Lubricating mucin, in the form o f MSF, is expressed by both chondrocytes and synovial fibroblasts in vitro. Both lubricin and superficial zone protein (SZP), a related proteoglycan, share a similar primary structure but could differ in posttranslational mod ifications with O-linked oligosaccharides which are predominant in lubricin and with limited amounts chondroitin and keratan sulfate found in SZP. Sin ce most of the MSF exons are involved in the expression of lubricating muci n, a strong homology to vitronectin persists. It is therefore appropriate t o consider that both SZP and lubricin occupy a new class of biomolecules te rmed tribonectins. Screening of a human genome bacterial artificial chromso me (BAC) library with a cDNA primer pair complimentary for exon 6 identifie d two clones. Both clones were complimentary for chromosome 1q25 by in situ hybridization. This same locus was previously implicated in camptodactyl-a rthropatliy-pericarditis syndrome (CAP) by genetic mapping. It is hypothesi zed that CAP, a large joint arthropathy, may be associated with ineffective boundary lubrication provided by synovial fluid. (C) 2001 Orthopaedic Rese arch Society. Published by Elsevier Science Ltd. All rights reserved.