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Clinical relevance of measurement of antibodies to individual snU1-RNP proteins.

Anti-ribonucleoprotein (RNP) antibodies are found in mixed connective tissue disease (MCTD), a syndrome characterized by features of systemic lupus erythematosus (SLE), inflammatory muscle disease, and scleroderma (1). High titers of anti-RNP antibodies support the diagnosis of MCTD, and testing should be ordered when the diagnosis is suspected (2). Anti-RNP antibodies are also found in rheumatic diseases such as SLE, Sjogren syndrome, rheumatoid arthritis, polymyositis, and systemic sclerosis (2).

The antigens to which anti-RNP antibodies react reside in many proteins (70 kD, protein A, and protein C) complexed with small nuclear U1-RNA. This complex is called small nuclear U1 ribonucleoprotein (snU1-RNP). Similarly, the antigen to which anti-Sm antibodies bind is composed of a complex of proteins (B, B′, D, E, F, and G) and snRNAs (U1, U2, U4–U6, and U5). Anti-Sm antibodies are highly specific for SLE (2). The snRNPs (U1-RNP/Sm) are involved in the splicing of precursor messenger RNA.

Traditionally, anti-RNP antibodies have been detected by techniques such as passive hemagglutination, immunodiffusion, counterimmunoelectrophoresis, and ELISA using purified antigen. More recently, recombinant antigens have been used increasingly to identify anti-RNP antibodies, thus allowing identification of antibodies to the individual proteins of the snU1-RNP complex (70 kD, protein …