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Retention of the Matricellular Protein SPARC in the Endoplasmic Reticulum of Chondrocytes from Patients with Pseudoachondroplasia

Jacqueline T. Hecht and E. Helene Sage

Department of Pediatrics, University of Texas Medical School at Houston, Houston, Texas (JTH), and Hope Heart Program, Benaroya Research Institute at Virginia Mason, Seattle, Washington (EHS)

Correspondence to: Dr. E. Helene Sage, Benaroya Research Institute at Virginia Mason, 1201 Ninth Avenue, Seattle, WA 98101. E-mail: hsage{at}benaroyaresearch.org

Pseudoachondroplasia (PSACH) is an autosomal dominant disease characterized by dwarfism, morphological irregularities of long bones and hips, and early-onset osteoarthritis. This disease has been attributed to mutations in a structural protein of the cartilage extracellular matrix (ECM), cartilage oligomeric matrix protein (COMP), which result in its selective retention in the chondrocyte rough endoplasmic reticulum (ER). Accumulation of excessive amounts of mutated COMP might reflect a defect in protein trafficking by PSACH chondrocytes. Here we identify the matricellular protein SPARC as a component of this trafficking deficit. SPARC was localized to the hypertrophic chondrocytes in the normal human tibial growth plate and in cultured control cartilage nodules. In contrast, concentrated intracellular depots of SPARC were identified in nodules cultured from three PSACH patients with mutations in COMP. The accumulated SPARC was coincident with COMP and with protein disulfide isomerase, a resident chaperone of the rough ER, whereas SPARC and COMP were not coincident in the ECM of control or PSACH nodules. SPARC-null mice develop severe osteopenia and degenerative intervertebral disc disease, and exhibit attenuation of collagenous ECM. The retention of SPARC in the ER of chondrocytes producing mutant COMP indicates a new intracellular function for SPARC in the trafficking/secretion of cartilage ECM. (J Histochem Cytochem 54:269–274, 2006)

Key Words: SPARC • pseudoachondroplasia • matricellular • cartilage oligomeric matrix protein • cartilage • endoplasmic reticulum • thrombospondin 5

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