ATM and Alzheimer disease: Thus, a second, unrelated measure leads to the same conclusion: in vulnerable neuronal populations of the AD brain, ATM level is reduced such that the normal epigenetic landscape is changed; HDAC4 is ectopically located in the nucleus (where it deacetylates histone H3 and H4; Li et al., 2012) and H3K27me3 levels are abnormally high.