Chromatin maintenance and dynamics in senescence: a spotlight on SAHF formation and the epigenome of senescent cells

Senescence is a stable proliferation arrest characterized by profound changes in cellular morphology and metabolism as well as by extensive chromatin reorganization in the nucleus. One particular hallmark of chromatin changes during senescence is the formation of punctate DNA foci in DAPI-stained senescent cells that have been called senescence-associated heterochromatin foci (SAHF). While many advances have been made concerning our understanding of the effectors of senescence, how chromatin is reorganized and maintained in senescent cells has remained largely elusive. Because chromatin structure is inherently dynamic, senescent cells face the challenge of developing chromatin maintenance mechanisms in the absence of DNA replication in order to maintain the senescent phenotype. Here, we summarize and review recent findings shedding light on SAHF composition and formation via spatial repositioning of chromatin, with a specific focus on the role of lamin B1 for this process. In addition, we discuss the physiological implication of SAHF formation, the role of histone variants, and histone chaperones during senescence and also elaborate on the more general changes observed in the epigenome of the senescent cells.