Anti–U1 snRNP Antibodies

SARDs are a heterogeneous group of disorders characterised by dysregulation of the immune system, which begins to attack 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 rashes. Diagnosis relies on the integration of clinical assessment and specific autoantibody testing. Early diagnosis is crucial to initiate appropriate therapy and slow disease progression.

Sm and RNP antigens are part of a nuclear macromolecular complex composed of RNA and proteins located in the spliceosome, an essential cellular structure for pre-mRNA processing. Spliceosome-associated proteins can be divided into two groups: the common Sm complex 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. Their presence is considered a strong indicator of mixed connective tissue disease (MCTD), a systemic connective tissue disease that combines clinical features of systemic lupus erythematosus (SLE), systemic sclerosis and polymyositis. Anti-U1RNP antibodies are present in nearly 100% of MCTD patients and are a key diagnostic marker.

Although anti-U1RNP can also occur in other connective tissue diseases (such as SLE), it should be noted that isolated high-titer anti-U1RNP in a compatible clinical context is highly specific for the diagnosis of MCTD.