Novel cartilage-specific splice variants of fibronectin.

OBJECTIVE

To determine the nature of alternatively spliced isoforms of fibronectin expressed in healthy bovine articular cartilage and cartilage derived from human osteoporotic and osteoarthritic joints.

DESIGN

Our study focused on a single alternatively spliced region of the fibronectin gene, the variable region. Bovine cartilage samples were obtained within 12h of slaughter and human cartilage samples were obtained within 8h of the time of joint replacement surgery. RNA was extracted and alternatively spliced isoforms of fibronectin were amplified using RT-PCR.

RESULTS

Two novel alternatively spliced forms of fibronectin designated (V+I-10)(-) and (V+III-15)(-) were identified in bovine articular cartilage. Fibronectin is composed of multiple repeats of three types of homologous units and these two novel isoforms specifically splice out single individual repeating units. Expression of all these isoforms was dependent upon the presence of an extracellular matrix. In the human samples the expression profiles obtained with osteoporotic hip and osteoarthritic knee cartilage was not uniform. The (V+C)(-) isoform was present in all samples and the (V+I-10)(-) isoform was distributed between both osteoporotic and osteoarthritic cartilage. However, the (V+III-15)(-) isoform was shown to be associated with osteoarthritic cartilage with statistical significance (P< 0.015 ). In addition a third novel splice variant was identified and designated as III-15X. Translation of the III-15X isoform results in a truncated form of fibronectin lacking a significant portion of the C-terminus. Further RT-PCR analysis of several other tissue types suggests that these variants are cartilage specific.

CONCLUSION

Our results demonstrate the existence of three new cartilage specific isoforms of fibronectin and indicate that expression of one or more may be associated with osteoarthritis. Published by Elsevier Science Ltd. All rights reserved.