Anti-RNP/Sm antibodies

SARDs are a heterogeneous group of disorders characterized by dysregulation of the immune system, that starts to turn against the body’s own tissues. Major SARDs include systemic lupus erythematosus, rheumatoid arthritis, systemic sclerosis and Sjögren’s syndrome. These diseases can affect multiple organs and share common symptoms such as fatigue, fever, joint pain and skin rash. Diagnosis relies on a combination of clinical assessment and specific autoantibody tests. Early diagnosis is crucial to initiate appropriate therapy and slow down disease progression.

Sm and RNP antigens are part of a nuclear macromolecular complex composed of RNA and proteins located in the spliceosome, a cellular structure essential for processing pre-messenger RNA. Proteins associated with the spliceosome can be divided into two groups: the common Sm proteins, shared by all small nuclear ribonucleoproteins (snRNPs), and the specific proteins that characterise each snRNP, namely U1, U2, U4/U6, and U5.

Anti-U1RNP antibodies are directed against several protein components of the U1 snRNP complex, particularly RNP A, RNP C, and RNP 68 kDa. The presence of these autoantibodies is considered a strong indicator of mixed connective tissue disease (MCTD), a systemic connective tissue disorder combining clinical features of systemic lupus erythematosus (SLE), systemic sclerosis, and polymyositis.

Anti-Sm antibodies are autoantibodies directed against this component of the complex and represent a highly specific marker for systemic lupus erythematosus (SLE).

Anti-U1RNP antibodies are found in nearly 100% of patients with MCTD and are a key diagnostic marker. It should be noted that although they can also occur in other connective tissue diseases (such as SLE), the isolated presence of high-titer anti-U1RNP antibodies in a compatible clinical context is highly specific for the diagnosis of MCTD.