Ro(SSA)

Autoantibodies to the Ro(SSA) antigen are one of the most frequent serological markers of autoimmunity in rheumatic diseases. They are present in the serum of 50-80% of patients with Sjögren’s syndrome, 30-40% of patients with systemic lupus erythematosus (SLE), and 3-5% of patients with rheumatoid arthritis (RA). Ro(SSA) autoantibodies have also been reported in Sjögren’s syndrome-lupus overlap disease, subacute cutaneous lupus erythematosus, lupus with complement component deficiencies, neonatal lupus syndrome, multiple myeloma, polymyositis, progressive systemic sclerosis and primary biliary cirrhosis.

The incidence of anti-Ro (SSA) antibodies in the population of normal women of reproductive age is in the order of 1%. The presence of anti-La (SSB) autoantibodies usually coincides with the presence of anti-Ro (SSA).

Ro(SSA) antibodies target protein antigens associated with small RNA molecules known as hY-RNAs. These protein-RNA complexes are referred to as Ro-ribonucleoproteins (Ro-RNPs) and their biological function has yet to be elucidated. It is generally acknowledged that a 60kDa protein (Ro60) intimately associated with Ro-RNP constitutes the major Ro(SSA) antigen. The binding of autoantibodies to Ro60 is heterogenous: some sera recognise both the native and denatured antigen while others recognise the native antigen conformation only. The association of a second putative Ro(SSA) antigen of 52kDa (Ro52) with Ro-RNP remains unclear.

Although it has been proposed that Ro52 may be indirectly associated with Ro-RNP through Ro60, it has been conversely demonstrated that Ro52 does not copurify with Ro-RNPs during chromatographic separation. Furthermore true monospecific anti-Ro52 sera are known to be rare, they do not display a typical Ro(SSA) immunofluorescence pattern and are negative in immunodiffusion.

 

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