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Title: Time to quit? : non-genetic heterogeneity in cell fate propensity after DNA damage
Author: Campbell, Callum James
ISNI:       0000 0004 7229 6103
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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Cellular checkpoints are typically considered to both facilitate the ordered execution of the cell cycle and to act as a barrier to oncogene driven cell cycles and the transmission of unresolved genetic lesions from one phase to the next. Furthermore, these mechanisms are also believed to underpin the responses of cells, both in normal and cancerous tissues, to those therapies that either directly or indirectly generate DNA damage. In recent studies however, it has become clear these checkpoints permit the passage of significant genomic aberrations into subsequent cell cycle phases and even descendant cells, and that heterogeneous responses are apparent amongst genetically identical cells. The consequences of this checkpoint ‘negligence’ remain relatively uncharacterised despite the importance of checkpoints in current models for how genomic instability is avoided in the face of ubiquitous DNA damage. Unresolved DNA damage is presumably inherited by subsequent cell cycle phases and descendant cells yet characterisation of the consequences of this has been relatively limited to date. I therefore utilised microscopy-based lineage tracing of cells expressing genetically encoded fluorescent sensors, particularly the Fluorescent Ubiquitination-based Cell Cycle Indicator (FUCCI) probes (Sakaue-Sawano et al., 2008), with semi-automated image analysis to characterise the response of single cells and their descendants to DNA lesions across multiple cell cycle generations. This approach, complemented by generational tracing by flow cytometry, permitted me to characterise the timing of cell fate determination in treated and descendant cells, the non-genetic heterogeneity in checkpoint responses and overall lineage behaviour, correlations between cells (similarly to Sandler et al., 2015) and cell cycle timing dependencies in the response to DNA damaging agents. With these single cell analytical approaches I show that the consequences of DNA damage on descendant cell fate is dramatic, suggesting checkpoint mechanisms may have consequences and even cooperate across phases and generations. U2OS cell lineages traced for three generations following the induction of DNA damage in the form of strand breaks showed greatly induced cell death in the daughters and granddaughters of DNA damaged cells, termed delayed death. Furthermore, lineage behaviour was characterised as highly heterogeneous in when and whether cell death occurred. Complementary flow cytometric approaches validated the findings in U2OS cells and suggested HeLa cells may show similar behaviour. These findings indicate that checkpoint models need to incorporate multigenerational behaviour in order to better describe the response of cells to DNA damage. Understanding the processes governing cell fate determination in descendant cells will impact upon our understanding of the development of genomic instability during carcinogenesis and how DNA-damaging chemotherapeutics drive cells to ‘quit’ the cell cycle.
Supervisor: Venkitaraman, Ashok ; Esposito, Alessandro Sponsor: Cambridge Cancer Centre
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Cell cycle ; DNA damage ; Lineage tracing ; microscopy ; FUCCI ; checkpoint ; G1 ; G2 ; S phase ; mitosis