Mitochondrial dysfunction can cause cells to senesce even without nuclear DNA damage or free radical accumulation.

“We recently showed, for example, that having bad mitochondria in the absence of DNA damage...so this is just mitochondrial dysfunction if you will. And we could show that you could cause mitochondrial dysfunction by any number of means. Five or six different ways of causing the mitochondria to fail to produce the energy that they need and to produce more free radicals, even in the absence of those free radicals getting to the nucleus cause the cells to senesce, so they will senesce in response to bad mitochondria.”