An Informative Guide to RNP Antibody
When your immune system is functioning properly, your body produces protective proteins called antibodies. These proteins usually target antigens, or foreign substances. However, when you have an autoimmune disease, some of your antibodies mistakenly attack your own cells. We call antibodies that target an individual’s own tissue autoantibodies.
One such autoantibody is the anti-U1-ribonucleoprotein (RNP) antibody, or U1-RNP antibody. As its name suggests, the RNP antibody targets ribonucleoproteins in your body. Although these proteins sound very technical, they are simply RNA–protein complexes that help your cells regulate protein production. RNA is a molecule, similar to DNA, that helps make proteins in our bodies. (Source)
In this post, we explore the role of the RNP antibody, how the RNP antibody is linked to mixed connective tissue disease (MCTD), and what an RNP antibody test can tell you about your immune health.
How Does the RNP Antibody Affect Me?
Researchers are not yet sure how the RNP antibody disrupts protein production. However, the RNP antibody is thought to affect the nucleus of cells, where DNA is stored. The nucleus is also where ribonucleoproteins, the proteins this antibody targets, take part in protein production. Because of this role, the RNP antibody is grouped with other antinuclear antibodies. (Source)
Although there has been limited research done on the RNP antibody, we do know there are correlations between high levels of RNP antibody and certain systemic rheumatic diseases such as mixed connective tissue disease (MCTD) and systemic lupus erythematosus (SLE). RNP antibody is also correlated with Raynaud’s phenomenon, in which extremities (especially fingers and toes) have increased sensitivity to the cold, often reacting with a change in color. (Source, Source, Source)
What is an RNP Antibody Test?
It may take years to diagnose an autoimmune disease, but antibody tests can speed up this process. Testing for RNP antibody is especially helpful in diagnosing MCTD and rheumatic diseases such as:
- systemic lupus erythematosus (SLE)
- Sjögren’s syndrome
- rheumatoid arthritis
- systemic sclerosis
There are different antibody tests that can be performed to check for the presence of RNP antibody including:
- enzyme-linked immunosorbent assay (ELISA), considered the gold standard
- immunoblot (Western blot)
- counterimmunoelectrophoresis (CIE)
How to Understand Your RNP Antibody Test Results
The results of an RNP antibody test may be considered negative, borderline, or positive. As with other tests, normal ranges may vary according to the lab, but the following general guidelines apply:
- negative (less than 20 U): If you receive a negative RNP antibody test result, you do not have elevated levels of RNP antibody and do not have MCTD or an autoimmune disease associated with this antibody.
- borderline (20 U to 25 U): A borderline RNP antibody test result is inconclusive. It is likely your health care provider will take other test results into consideration and may repeat this test later.
- positive (26 U or greater): If you receive a positive RNP antibody test result, you have elevated levels of RNP antibody and may have MCTD. In addition to MCTD, a positive RNP antibody test is sometimes seen with:
- SLE (systemic lupus)
- discoid lupus erythematosus (a kind of lupus that affects the skin)
- rheumatoid arthritis
The Link Between RNP Antibody and MCTD
What is MCTD?
We talk about mixed connective tissue disease, or MCTD, but what does this term mean?
Autoimmune connective tissue diseases, such as SLE and dermatomyositis, are diseases in which the immune system targets connective tissue in the body, such as skin or bones. While these diseases have some common characteristics, they are distinct from one another.
MCTD, however, is a type of overlap syndrome because individuals with MCTD experience symptoms that are characteristic of multiple autoimmune connective tissue diseases. The symptoms overlap and make it difficult to determine which autoimmune disease an individual may have.
Is MCTD a Separate Disease?
There is a great deal of debate on whether MCTD should be considered a distinct autoimmune disease. Patients who initially appear to have MCTD may eventually be diagnosed with one specific autoimmune connective tissue disease. There is no consensus on whether MCTD should be an initial diagnosis, or whether a diagnosis should wait until the patient has symptoms of an established connective tissue disease.
Another issue is that the definition of MCTD is muddled and health care professionals are not yet sure of the prognosis for patients with MCTD. Overall, MCTD looks to be a placeholder name until connective tissue diseases are better understood. (Source, Source)
Do High Levels of RNP Antibody Mean I Have MCTD?
An estimated 95% to 100% of people diagnosed with MCTD have very high levels of RNP antibody. If your RNP antibody level is high, you may very well receive a diagnosis of MCTD, although you may later be determined to have a specific connective tissue disease. Overall, a diagnosis of MCTD is made on the basis of your symptoms as well as clinical features such as laboratory testing results. However, there is no clear diagnostic criteria. (Source, Source)
Symptoms of MCTD
MCTD is considered by some to be a distinct autoimmune disease with symptoms characteristic of the autoimmune connective tissue diseases SLE, systemic sclerosis, and polymyositis. Symptoms vary but commonly include:
- Raynaud’s syndrome (decreased blood flow to fingers, toes, nose, or ears)
- swelling of the hands
- pain in multiple joints
- muscle pain and inflammation
- weak swallow
- interstitial lung disease (lung scarring)