Jo-1

Jo-1 (histidyl tRNA synthetase) is a member of the amino acyl-tRNA synthetase family of enzymes found in all nucleated cells^[1]. The amino acid sequence of human Jo-1 antigen has been determined by cloning and sequencing its cDNA10. It contains three structural motifs typical of class IIa aminoacyl transferases and two signature regions common to histidyltRNA synthetases. The human enzyme amino acid sequence shows significant homology to those of the enzymes from yeast (47.5%) and E. coli.Although the primary structure of bovine HRS has not yet been reported, the use of bovine antigen for the detection of anti-Jo-1 autoantibodies has been described elsewhere.

Anti-Jo-I antibody, an autoantibody directed at the cellular enzyme histidyl-tRNA synthetase and inhibit its catalytic activity in vitro, is found in 25% of patients with myositis but not in other myopathies and only rarely in other connective tissue diseases^[2]. It has long been recognised that a subset of patients with myositis have other features such as serositis and Raynaud's phenomenon, but, until a recent report associating anti-Jo-i antibody with interstitial pulmonary fibrosis, lung disease was thought to be uncommon.In these patients the detection of anti-Jo-1 antibody is particularly useful for the diagnosis of idiopathic inflammatory myopathy and for disease monitoring since the serum levels of anti-Jo-1 antibodies strongly correlate with myositis disease activity, relating also to the therapy.

Reference Values

• <1.0 U (negative)
• > or =1.0 U (positive)

Positive results for jo-1 antibodies are consistent with the diagnosis of polymyositis, suggesting an increased risk of pulmonary fibrosis in these patients. A negative test for Jo-1 does not rule out a diagnosis of polymyositis or dermatomyositis.

References

1. Targoff I: Autoantibodies in polymyositis. Rheum Dis Clin North Am 1992;18:455
2. Leff R, Sherman J, Plotz P: Chapter 65: Inflammatory muscle diseases. In Clinical Immunology Principles and Practice, Second edition. Edited by R Rich, T Fleisher, W Shearer, B Kotzin, et al. St. Louis, Mosby-Year Book, 2001, pp 65.1-65.