Extractable nuclear antigen

Extractable nuclear antigens (ENAs) are over 100 different soluble cytoplasmic and nuclear antigens. They are known as "extractable" because they can be removed from cell nuclei using saline and represent six main proteins: Ro, La, Sm, RNP, Scl 70, Jo1. Most ENAs are part of spliceosomes or nucleosomes complexes and are a type of small nuclear ribonucleoprotein (snRNPS). The location in the nucleus and association with spliceosomes or nucleosomes results in these ENAs being associated with additional RNA and proteins such as polymerases. This quality of ENAs often makes it difficult to purify and quantify their presence for clinical use.

An extractable nuclear antigen panel, or an ENA panel, tests for presence of autoantibodies in the blood that react with proteins in the cell nucleus. It is usually done as a follow up to a positive antinuclear antibody (ANA) test and when one is showing symptoms of an autoimmune disorder. The ANA tests for the presence or absence of autoantibodies, while the ENA panel evaluates which proteins in the cell nucleus the autoantibodies recognize. The ENA panel helps diagnosis, distinguish between, and monitor the progression of autoimmune diseases and is performed with a simple blood draw. While the levels of autoantibodies may fluctuate through one's life, once one develops autoantibodies, one will always have them. Autoantibodies to these antigens are associated with particular connective tissue disorders. Indeed, in 84.3% of positive anti ENA samples, ANA reagents were also found.

Since the discovery of ENAs, they have been used as a diagnostic tool in connective tissue disease. Two widely used gel based techniques were used to identify anti ENAs and their associations to disease in early work, double immunodiffusion (DID) and counterimmunoelectrophoresis (CIEP). Both of these techniques require the precipitation of antigens for valid results. Depending on the anti ENA being investigated, one technique may be used over the other. For example, Scl 70 antigen is less negatively charged, which can result in the antigen traveling in the same direction as the antibody. This would result in the antibody antigen complex not precipitating; leading to invalid results. In addition, some anti SS B antibodies commonly identified in Sjögren syndrome may not be detected with this method. However, this method is economically feasible and specific to confirm a diagnosis. There are two sensitivities to note when viewing data from these gel based techniques, assay sensitivity and disease sensitivity. Assay sensitivity is the ability to recognize when an antibody is present, while disease sensitivity is the ability to recognize the frequency in which the antibody occurs in a disease. Due to limitations of gel based techniques in disease sensitivity, other techniques have been explored in order to increase assay sensitivity without decreasing disease sensitivity. For example, in patients with Systemic lupus erythematous (SLE), only 8–40% have detectable anti SM when using gel based assays. CIEP has been shown to be more sensitive than DID.

Three additional techniques, passive hemagglutination, enzyme linked immunosorbent assay (ELISA), and western blotting (WB), can be used in order to identify ENAs and link them to specific diseases. Passive hemagglutination was popular in the late 1970s, but very few studies have been done using them and was restricted to anti Sm and anti ribonuclear protein (RNP) antibodies. The six main antigens used in immunological laboratories for detection are Ro, La, Sm, RNP, Scl 70 and Jo1, which are screened for by Ouchterlony double immuno diffusion techniques and confirmed by immunoblotting. On anti nuclear antibody tests, these antigens have a speckled pattern.