Genomic instability underlies various diseases including cancer. The maintenance of
genomic stability requires accurate replication of the genome, proper segregation of
duplicated DNA to progeny cells, and the capacity to respond effectively to DNA
damage. Early sections of this thesis focus on the response to DNA double-strand
breaks (DSBs) within compact regions of chromatin (heterochromatin). Here,
methodology was optimised for monitoring the repair of site-specific DSBs within
regions likely to be enriched for heterochromatin. This system was exploited to examine
the function of the Artemis endonuclease in heterochromatic DSB repair. Later sections
focus on factors involved in DNA replication and the response to replication stress.
Among the various mechanisms involved in the DNA damage response (DDR) to
replication stress, the licensing of excess origins of replication has been proposed to
safeguard against replication failure. Here, the impact of diminished origin licensing
capacity on the response to replication stress was compared in tumour and non-tumour
cell lines. I present findings demonstrating that depletion of origin licensing factors
causes hypersensitisation of tumour-derived but not non-tumour cell lines to replication
stress-inducing agents. Further, combining diminished origin licensing capacity with
depletion of the tumour suppressor, p53, or overexpression of the c-Myc oncogene
impairs viability under conditions of replication stress in non-tumour fibroblasts. These
findings suggest that tumour cells have a greater reliance on origin licensing capacity,
raising the possibility that licensing factors might represent suitable targets for drugbased
cancer therapy. Factors involved in replication origin licensing have also been
implicated in the establishment of heterochromatin. Here, I examined higher-order
chromatin structure and the ionizing radiation (IR)-induced DDR in cells from patients
harbouring mutations in origin licensing factors. Findings from these studies provide
evidence for the first time that origin licensing complex (ORC)-deficient Meier-Gorlin
Syndrome (MGS) may be classified as a disordered chromatin syndrome
