RNP-Sm

Autoantibodies directed against the snRNP (small nuclear ribonucleoprotein) autoantigens referred to as RNP and Sm were originally detected in the sera of systemic lupus erythematosus (SLE) patients. Anti-RNP antibodies were subsequently found in the sera of mixed connective tissue disease (MCTD) patients and it is now known that when these antibodies occur at high titre and in the absence of Sm, they are a good marker for MCTD. Such autoantibodies are also known to occur in the sera of patients with a range of other rheumatic diseases including progressive systemic sclerosis, rheumatoid arthritis, discoid lupus erythematosus, Sjögren's syndrome and various overlapping conditions.
 
The snRNPs are a group of nuclear particles comprised of several polypeptides associated with a small nuclear RNA molecule.  The most abundant snRNPs are involved in pre-mRNA-splicing. At least 13 different snRNAs have been identified in mammalian cells. Whereas autoantibodies directed against Sm are able to precipitate a wide range of snRNAs, RNP autoantibodies are only able to precipitate one particular type, referred to as U1snRNA.

Anti-RNP antibodies react with the U1 snRNP-specific polypeptides (68K, A and C antigens) whereas anti-Sm antibodies react with polypeptides associated with U1, U2, U5 and U4/U6 snRNAs (B,B', D, E, F and G antigens). While the 68K polypeptide constitutes the major MCTD RNP autoantigen, the major Sm autoantigen is represented by the D polypeptide.
 
The RNP 68K (present as a 33/35K doublet), RNP A and Sm D polypeptides represent the most abundant components of AROTEC's RNP/Sm antigen. Other snRNP subunits (RNP-C) are also detectable. Human Sm D1, D2 and D3 are identical to the bovine sequences indicating that the antigens are highly conserved across mammalian species.

 

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