The heterogeneus nuclear ribonucleoprotein (hnRNP) D, which is also known as AU-rich element binding factor 1 (AUF1), decreases stability of many short-lived mRNAs (including mRNAs of IL-1 and TNFa) by binding to adenosine and uridine rich sequences (ARE) contained in their 3'-untranslated regions. Previous studies had indicated this protein to be recognized by sera from patients RA, SLE and MCTD.
To investigate this autoimmune response in greater detail 356 sera from patients (pts) with various rheumatic disorders were tested by immunoblotting employing the recombinant 45 kDa variant of D/AUF1 (the largest of the 4 known D/AUF1 proteins). Autoantibodies (aab) were detected in 20% of RA pts (n = 105), 34% of SLE pts (n = 70), 17% of MCTD pts (n = 31) and in 25% of pts with primary Sjoegren's syndrome (n = 21), but in less than 5% of 129 pts with other rheumatic disorders and not at all in healthy controls. Importantly, anti-D/AUF1 aab were already present in 25% of 60 patients with early RA of less than 6 months duration, whereas only one of 40 non-RA patients with other forms of early arthritis showed this antibody. Epitope mapping studies showed the aab to be directed to conformational epitopes in the N-terminal part of hnRNP-D/AUF1 known to be indispensable for the protein's function. However, aab did not interfere with RNA binding as assessed by gel shift assays employing the ARE of the TNFa mRNA. Instead, they were able to supershift protein-RNA complexes indicating binding sites for RNA and aab to be distinct.
Thus, these findings suggest that anti-D/AUF1 aab target the mRNA decay complex which may form another large ribonucleoprotein target structure in systemic autoimmunity. We are tempted to speculate that increased formation of such complexes (e.g. due to overexpression of instable mRNAs such as those for IL-1 and TNFa as seen in RA) may lead to pathologic autoimmune reactions against D/AUF1 and other proteins of mRNA decay complexes.