Identification and characterization of a novel cartilage gene product CILP, which is an early indicator of osteoarthritis

Abstract: The aim of this work was to study alterations in the matrix composition of human articular cartilage in different stages of osteoarthritis and to isolate and characterize one of the proteins that is altered in the osteoarthritic process. The metabolism of articular cartilage in different stages of osteoarthritis including early asymptomatic OA was monitored in a culture system of metabolically labeled cartilage explants. The labeled cartilage was extracted and the components separated at the analytical level in a two-dimensional system of ion exchange chromatography (Mono Q) in the first dimension, followed by an electrophoretic separation (SDS-polyacrylamide gel electrophoresis) of each effluent fractions. The labeled components were visualized by fluorography and quantitated by densitometric analysis. Several matrix components showed an altered synthesis and contents. One of those components, a 92 kDa protein, was further characterized after purification from human articular cartilage. The protein is a synthetic product of the chondrocytes and is restricted to cartilaginous tissues with a specific distribution in the middle layer of the normal articular cartilage. This specific distribution gave the name to the protein, CILP, (Cartilage Intermediate Layer Protein). Its deduced primary structure is quite unique, with very limited homology to other known proteins. In addition the gene encodes for another protein homologous to a porcine nucleotide pyrophosphohydrolase. The human CILP gene spans about 15.3 kbp of genomic DNA and it is organized in 9 exons interrupted by 8 introns. CILP is coded from exon 2 to exon 9, where the latter also codes for the entire human nucleotide pyrophosphohydrolase. Also particular to the gene was that one intron (intron 8) did not follow the "gt-ag" rule, but belongs to the minor classes of pre-mRNA introns containing "at-ac" at their 5’ and 3’ respectively. The gene was mapped to the chromosome 15q22, however no musculoskeletal diseases have been linked to this region.