Heterogeneity Exhibited in Cells that Escape from Drug-Induced Senescence

Maintaining genomic integrity is key in the prevention of cancer. To limit the accumulation of mutations over time and stop damaged cells from continuing to proliferate and further gain subsequent mutations, intrinsic terminal cellular programs are activated. Senescence, an evolutionarily conserved stress state that traditionally has been viewed as an irreversible state of cell cycle arrest, acts as a tumour suppressive barrier. However, senescence acts as a double-edged sword, as senescent cells accumulate and contribute to age-associated diseases by altering tissue microenvironments and disrupting tissue homeostasis. Atrophied telomeres, mitotic errors, diminished DNA damage response and loss of nuclear envelope integrity all contribute to the increased rate of the formation of senescent cells during aging. In this study, I observed an age-associated accumulation of cytoplasmic DNA, in the form of micronuclei and chromatin bridges. I extended these finding to rare segmental accelerated aging diseases such as classical and non-classical Hutchinson-Gilford progeria syndrome, and Dyskeratosis Congentia. I further reveal that age-associated micronuclei and chromatin bridges undergo DNA damage and are recognized by cGAS that likely initiates an innate immune inflammatory response. My findings, along with recent reports showing chromosomal instability in senescent cells, led me to hypothesize that senescence may be reversible. To address this question, I utilized a drug-induced senescence model in BJ fibroblast and leukemia cells. Performing cloning experiments and monitoring single cells, I show that escape from a drug-induced senescent-like state occurs in a rare sub-population of cells for both transformed and normal cells. The escaped population displayed phenotypic heterogeneity, with respect to drug-response, gene expression, rate of proliferation and duration of arrest. RNA-sequencing highlighted the extensive transcriptional and metabolic reprogramming senescent cells undergo. Inhibition of lysosome function with FDA approved drug, chloroquine, specifically kills some senescent leukemia cells. By stratifying acute myeloid leukemia patients based on expression of senescent-associated gene signatures, I show that senescent gene signatures are associated with poorer overall survival. My findings provide evidence challenging the previous notion that senescence is a permanent state of cell cycle arrest and may be a mechanism which cancer cells exploit to survive therapy.