A novel role of DNA fragmentation factor as a tumor suppressor through maintaining genomic stability.

10023 Background: DNA fragmentation is a hallmark of apoptosis. However, the biological function of apoptotic DNA fragmentation remains unclear. Genes encoding the nuclease responsible for DNA fragmentation, DFF40/CAD and DFF45/ICAD, are deleted, mutated, or aberrantly expressed in many human tumor types. Abnormalities in this gene are associated with aggressive tumors and poor prognosis in cancer patients. Tumor-specific DFF45 gene mutations were identified in human tumors, indicating the involvement of this gene in tumor development. METHODS: We studied genetic instability, cellular transformation and tumorigenesis in cells and mice deficient in DNA fragmentation. We applied two methods to inhibit DNA fragmentation, overexpressing a modified ICAD which inhibits DNA fragmentation during apoptosis in stable cell lines and disrupting cad gene in transgenic mice. Genetic instability was studied by gene amplification assay, mutation assay and cytogenetic analysis including chromosomal aberrations and translocations. Cellular transformation was studied by soft agar assay using CAD-/- MEF cells. Tumorigenesis in DFF/CAD-knockout mice was studied by radiation carcinogenesis and two-stage skin chemical carcinogenesis. RESULTS: (1) Human and mouse cancer cell lines with their CAD activity inhibited exhibited significantly more genetic instability under spontaneous or mutagen (ionizing radiation) induced conditions. These are reflected as elevated frequencies of chromosomal aberrations, gene amplifications, and gene mutations; (2) Mouse cells with targeted disruption of the cad gene exhibit a similar, increased level of genetic instability when the host animal is irradiated; (3) Mechanistically, CAD maintains genetic instability through the removal of cells with DNA damage, a role that is similar to genomic "gatekeepers" like p53; (4) CAD-null mouse embryo fibroblasts exhibited enhanced cellular transformation; (5) Significantly enhanced susceptibility to chemical and radiation-induced tumor development was observed in mice with targeted disruption of the CAD gene, indicating that it plays a role in suppressing tumor development. CONCLUSIONS: CAD plays an important role in maitaining genetic stability and suppressing tumor development. No significant financial relationships to disclose.

Duke Scholars

Author Chuan-Yuan Li Dermatology