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Chromatin Collapse During Caspase-dependent Apoptotic Cell Death Requires DFF40/CAD-mediated 3'-OH Single-strand DNA Breaks
Published 2013 · Biology
Abstract Apoptotic nuclear morphology and oligonucleosomal double-strand DNA fragments (also known as DNA ladder) are considered the hallmarks of apoptotic cell death. From a classic point of view, these two processes occur concomitantly. Once activated, DFF40/CAD endonuclease hydrolyzes the DNA into oligonucleosomal-size pieces, facilitating the chromatin package. However, the dogma that the apoptotic nuclear morphology depends on DNA fragmentation has been questioned. Here, we use different cellular models, including MEF CAD-/- cells, to unravel the mechanism by which DFF40/CAD influences chromatin condensation and nuclear collapse during apoptosis. Upon apoptotic insult, SK-N-AS cells display caspase-dependent apoptotic nuclear alterations in the absence of internucleosomal DNA degradation. The overexpression of a wild-type form of DFF40/CAD endonuclease, but not of different catalytic-null mutants, restores the cellular ability to degrade the chromatin into oligonucleosomal-length fragments. We show that apoptotic nuclear collapse requires a 3′-OH endonucleolytic activity, even though the internucleosomal DNA degradation is impaired. Moreover, the alkaline unwinding electrophoresis and the ISEL/ISNT assays reveal that the apoptotic DNA damage observed in the DNA ladder-deficient SK-N-AS cells is characterized by the presence of single-strand nicks/breaks (SSBs). Apoptotic SSBs can be impaired by DFF40/CAD knockdown, abrogating nuclear collapse and disassembly. In conclusion, the highest order of chromatin compaction observed in the later steps of caspase-dependent apoptosis relies on DFF40/CAD-mediated DNA damage by generating 3′-OH ends in single-strand rather than double-strand DNA nicks/breaks.