Anti-RNP/Sm antibodies

Anti RNP are autoantibodies to the extractable nuclear antigen which is ribonucleoprotein (RNP). Anti RNP antibodies are intended for the diagnosis of systemic lupus erythematosus (SLE) and mixed connective tissue diseases (MCTD). These antibodies belong to the antinuclear antibody (ANA) group. ANA is a specific class of autoantibodies which can bind and destroy certain structures in the nucleus of cells.

Antibodies to RNP and the RNP/Sm complex are associated with mixed connective tissue disease and SLE. Overlap syndrome is a common term created to unite different inflammatory autoimmune diseases characterized by overlapping symptoms of SLE, progressive systemic scleroderma (SSS), dermato/polymyositis (DM/PM), rheumatoid arthritis (RA) and Sjogren's syndrome. Mixed connective tissue disease (MCTD, SST, Sharp syndrome) is a well-studied cross-linked syndrome characterized by specific clinical features, the presence of antibodies of different specificity, and a characteristic HLA antigens phenotype (i.e., DR2 and DR4).  An extreme clinical manifestation of MCTD is Raynaud's phenomenon, which specific symptom is vascular swelling of the fingers and hands. In addition, patients suffer from arthritis. Patients with MCTD are often found to have symptoms of various cross-tissue connective-tissue diseases. However, these may not appear in all patients. Serologically, MCTD is characterized by the presence of autoantibodies directed against the 70 kDa protein of the U1-snRNP spliceosome complex. Up to 100% of patients with MCTD have high titers of anti-RNP-70 autoantibodies. Patients with only anti-RNP-70 antibodies among the variety of anti-nuclear antibodies have classical clinical manifestations of MCTD. In addition, typical SLE antibodies are often found in these patients (e.g. anti-dsDNA, anti-Sm, anti-histone). In these cases, classic MCTD will develop into SLE. The U1-snRNP molecule (which function is pre-mRNA splicing) consists of a uridine-rich RNA complex with classic RNP antigens, i.e. protein 70 kDa, protein, A (33 kDa) and protein C (22 kDa). Specific Sm proteins are also associated with this RNA. Anti-Sm antibodies are pathognomonic markers of SLE.