IDDM is an autoimmune disease characterized by the gradual destruction of pancreatic beta cells. Although many factors are thought to be involved in the development of IDDM, no conclusive evidence exists as to which target molecule or gene actually causes the disease. Autoantibodies to pancreatic islet cells (ICA), are detected in patients' sera years before the onset of clinical disease. ICA consists of several different autoantibodies against different autoantigens in the islets. Determination of the identity of these autoantigens is important if we are to understand the mechanism by which these autoantibodies contribute to the pathogenesis of IDDM. In this project, we used differential subtraction techniques and the polymerase chain reaction to isolate novel genes from pancreatic beta cells. We then expressed these genes and studied their biological properties including their reactivity with autoantibodies from patients with IDDM. A human insulinoma cDNA library (ISL-153) was constructed by subtracting glucagonoma phagmid cDNA from insulinoma phagmid cDNA. A total of 153 clones was screened with mRNA probes from insulinoma, glucagonoma and HeLa cells. Clones that hybridized preferentially with end- labeled insulinoma mRNA were further screened by Northern analysis with a panel of tumor cell lines and normal tissues. cDNA sequences were matched to the GenBank DNA database. A total of five novel cDNAs, designated IA-1, IA-2, IA-2beta, PDIp, and IA-4, were isolated. IA-1 is expressed only in tumors of neuroendocrine origin, fetal brain and fetal pancreas. It possesses five zinc-finger DNA- binding motifs and acts as a transcriptional suppressor. IA-2 and IA-2beta are transmembrane protein tyrosine phosphatases and major autoantigens in IDDM. Autoantibodies to IA-2/IA-2beta autoantigens were found in approximately 75% of IDDM patients. Moreover, the presence of autoantibodies to IA-2/IA-2beta in otherwise normal individuals has proven to be a good predictive marker for identifying those individuals at high risk of ultimately coming down with clinical diabetes. PDIp is a pancreas-specific protein disulfide isomerase which functions as a catalyst for disulfide bond formation. IA-4 is a newly isolated arginine-rich basic protein from pancreatic islets and brain. Functional characterization and clinical study of these novel cDNAs are underway to elucidate their roles in the normal physiology and/or pathophysiology of pancreatic islets.
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