Clinical Information

Antibodies to U1 ribonucleoprotein particle (U1-RNP) are central to the diagnosis of mixed connective tissue disease and are also associated with other antinuclear antibody (ANA)-associated connective tissue diseases (CTDs) such as systemic lupus erythematosus, systemic sclerosis, undifferentiated CTD, and CTD overlap syndromes.(1-5) Immunologic characterization studies suggest that anti-U1RNP antibodies are directed toward both discontinuous and linear epitopes that are either contained in the protein sequence or are post-translationally modified. These antibodies mainly target the RNP68 or RNP70, RNPA (33 kD), and occasionally RNPC (22 kD) proteins.(4-9)

Originally described by Mattioli et al (3) by immunodiffusion assay using calf thymus extract, current solid-phase immunoassays now use diverse analytes (purified or recombinant proteins, synthetic peptides of dominant epitopes) of the 3 main proteins (RNP68 or RNP70, RNPA, and RNPC) either singly or in any of the various combinations.(4-10) Because of the use of these different antigens and combinations thereof, the nomenclature, reporting, and interpretation of anti-U1RNP antibodies remain obscure.(10) In the absence of standardized nomenclature for anti-U1-RNP antibody assays, familiarity of the analytes in specific assays and use of Hep-2 substrate by indirect immunofluorescence assay for ANA are required for appropriate interpretation. In addition, low level anti-U1-RNP antibodies in the absence of ANA have a low predictive value for ANA-CTD. The U1-RNP antibody test offered by Mayo Clinic detects antibodies to both RNP68 and RNPA proteins. Combined response is more sensitive and less specific than assays to the Sm (Smith)/RNP.(10)

For more information see Connective Tissue Disease Cascade.