Ribonucleoproteins (RNP) is composed of RNA and protein and is localised in the cell nucleus. The RNA components are described as U1 to U6 depending on their chromatographic behaviour. Apart from the individual RNA component, each U-n(nuclear)RNP particle displays six various core proteins (B, B', D, E, F, G). Antibodies against RNP in high titer and clinical symptom are use to diagnose Sharp syndrome (mixed connective tissue disease, MCTD). This syndrome is a multi-symptomatic and multiform mixed connective tissue disease combining symptoms of rheumatoid arthritis, SLE, systemic sclerosis, CREST syndrome and vasculitides.

Antibodies against RNP occur in approximately 50% of patients with lupus erythematosus (LE) and in patients with other connective tissue diseases, notably mixed connective tissue disease (MCTD). MCTD is characterized by high levels of RNP antibodies without detectable Sm or double-stranded DNA (dsDNA) antibodies. MCTD resembles LE but is not accompanied by renal involvement. High anti-nRNP/Sm titer are characteristic for Sharp syndrome, whereby the titer correlates with the disease activity. Anti-nRNP/Sm antibodies are also detected in patient with SLE, SSc and PM/DM.

Manufacturer’s reagent package insert, Anti-nRNP/SM ELISA (IgG) Test instruction, February 2014, EUROIMMUN, D-23560 Lübeck Germany.