What is U1-nRNP antibodies test?
U1-nRNP antibodies, also known as anti-RNP or anti-ribonucleoprotein antibodies, are a type of autoantibodies produced against ribonucleoprotein (RNP) - one of the proteins present in the nucleus of every cell of our body.
Normally, our immune system produces antibodies against invading foreign substances, such as bacteria or viruses. However, sometimes the immune system mistakes our own tissues or cells as harmful and starts producing antibodies against them. These antibodies are called autoantibodies. The presence of autoantibodies is usually associated with autoimmune diseases.
U1-nRNP antibodies are found in 95-100% of people with mixed connective tissue disease (MCTD). MCTD is a rare autoimmune condition characterised by a combination of features of the following disorders:
- Systemic lupus erythematosus (SLE): It is a long-term autoimmune condition that leads to inflammation of certain connective tissues, such as cartilage and the walls of blood vessels. U1-nRNP antibodies and anti-Sm (anti-Smith) antibodies are seen in about 20-30% of people with SLE. (Read more: Anti-Sm antibody test)
- Polymyositis: Polymyositis is an autoimmune inflammatory disease that affects the muscles and tissues associated with it, for example, the blood vessels that supply the muscles.
- Scleroderma: It is an autoimmune condition which causes thickening and tightening of the skin along with scarring of body tissues. Scleroderma may sometimes be referred to as progressive systemic sclerosis (PSS). About 20-25% of people with PSS may have U1-nRNP antibodies.
An ANA (anti-nuclear antibody) test is usually the first test performed to detect an autoimmune condition. If an individual has a positive ANA test, then an extractable nuclear antigen (ENA) panel test is performed to check for the type of autoimmune condition. The U1-nRNP antibodies test is one of the ENA panel tests that is done to confirm the presence of MCTD in suspected individuals.