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Research Xeroderma Pigmentosum, DDB and XP-E RevisitedAssistant Professor Office: 1169 Medical Laboratories Xeroderma Pigmentosum (XP) is a rare syndrome characterized by a high incidence of skin cancer. Originally it was defined as a hypersensitivity to UV light, but this definition has changed as forms have been discovered that do not demonstrate this hypersensitivity. About 1 in 100,000 people are affected by XP. Preliminary diagnosis is based on the fact that XP cells have lowered levels of Unscheduled DNA Synthesis (UDS). This test, however, is used for its convenience and simplicity, not its accuracy. Final confirmation must come from determination of actual gene mutation.  XP is actually broken down into eight different genetic diseases: XP-A through XP-G, with a variant group XP-V. XP-A through XP-G, excluding XP-E, are all deficient in different essential factors (labeled XPA through XPG) for nucleotide excision repair (NER), making them all hypersensitive to UV light. XP-V has a deficient DNA polymerase η, slowing and inhibiting translesion synthesis and replicative DNA synthesis (RDS) after UV irradiation. XP-E is a rare and relatively mild form of XP caused by mutations in the DDB2 gene. XP-E cells, unlike the other XP forms A through G, are deficient in apoptosis.DDB Damage-specific DNA binding (DDB) protein is a heterodimer composed of a larger subunit (DDB1) and a smaller subunit (DDB2). DDB1 is found in nearly all eukaryotes, and is usually found in the cytoplasm until recruited by DDB2 and brought to the nucleus. DDB2 exists almost exclusively in vertebrates, and is generally located in the nucleus. It was thought that DDB was directly involved in DNA repair, but newer data suggest that DDB plays other critical roles. Although the exact mechanism is still unclear, it is apparent that one role of DDB2 is in the regulation of the tumor suppressor p53, which controls various cellular functions, including apoptosis following UV irradiation. XP-E revisited  As stated above, XP-E is a condition caused by mutations in the DDB2 gene. Different mutations have given rise to different cell lines that demonstrate varying degrees of the XP-E phenotype. Three notable lines are 82TO, Ops1, and 3RO. 82TO cells have a missense mutation of the DDB2 gene. The mutant protein formed still retains some of its function, causing both clinical and cellular phenotypes to be relatively mild. Ops1 cells have a nonsense mutation in the DDB2 gene, producing a truncated protein. 3RO cells have a missense mutation in the DDB2 gene, producing a mutant protein that does not function. The graph to the left demonstrates the differences between these three cell lines with respect to their expression of DDB2 mRNA and DDB2 protein following UV irradiation. The diagram to the right highlights the difference between XP-E cells and normal cells in their reaction to UV light. As can be seen, in normal cells the level of RNA coding for DDB2 increases, as does the level of the protein itself. In 3RO, a line of XP-E cells, the RNA message increases 30-50 fold, but the protein is still undetectable in the cell. Because DDB2 regulates the levels of p53 in a cell, cells without functional DDB2 have greatly reduced levels of p53. When exposed to UV light, these cells are unable to demonstrate the usual apoptotic response, leading to neoplastic cells and cancer. Current Research Currently, Dr. Itoh’s research lab is focused on XP-E and understanding the relationships between DDB2, p53, and carcinogenesis. RESEARCH STAFF sachiyo-iwashita@uiowa.edu Research Assistant II 1160 ML 335-7544 Francisco Lovan francisco-lovan@uiowa.edu Student Lab Assistant 1160 ML 335-7544 Mitchell Stephenson mitchell-stephenson@uiowa.edu Student Lab Assistant 1160 ML 353-4235 David Payne Undergraduate Research Assistant 335-7544 |