DNA Fragmentation in Mammalian Apoptosis and Tissue Homeostasis

Abstract

Apoptosis is a highly regulated physiological process critical in development and tissue homeostasis. Abnormal apoptosis can lead to disease conditions including neurodegeneration, autoimmunity and cancer. DNA fragmentation is an integral part of apoptosis and has long been suspected to be of critical importance in cleaning up potentially antigenic DNA and genetic materials capable of inducing neoplasmic transformation in neighboring cells. Direct evidence for this role of DNA fragmentation in apoptosis however, is still lacking. The identification of a heterodimeric DNA fragmentation factor composed of a 45 and 40 kDa subunit (termed DFF45 and DFF40, or ICAD for Inhibitor of Caspase Activated DNase and CAD for Caspase Activated DNase, respectively) as well as endonuclease G (EndoG) provides a timely opportunity for addressing the physiological significance of DNA fragmentation in apoptosis and tissue homeostasis. We previously generated a DFF45 mutant mouse in which the DFF activity is abolished. We found that DFF45-deficient thymocytes are resistant to DNA fragmentation both in vivo and in cultured primary cells exposed to various apoptotic stimuli. Interestingly, DFF45-deficient thymocytes and mouse embryonic fibroblasts (MEFs) are partially resistant to apoptosis in response to several apoptotic-inducing agents. There are more granule cells in the dentate gyrus of the hippocampal formation in DFF45 mutant mice than in normal control mice. This increased neuronal cell number correlates with enhanced spatial and non-spatial learning and memory retention in DFF45 mutant mice compared with control mice. These results suggest that DFF45 is critical for DNA fragmentation and a deficiency in DFF45 can affect timely completion of apoptosis and consequently tissue homeostasis and proper cellular function. Likely due to the unaffected EndoG activity however, residual DNA fragmentation can be found in DFF45-deficient splenocytes and MEFs. In a collaborative effort, we are generating EndoG mutant mice and mice with combined deficiencies of DFF45 and EndoG to investigate how DFF and EndoG jointly function to insure proper apoptosis and tissue homeostasis.