Balancing Pathways in DNA Double Strand Break Repair

All information a cell needs to live and survive is stored in the genomic DNA. Maintenance of an intact and uncompromised genome is of vital importance for cell survival. Damaged DNA can block transcription and replication, processes essential for cell viability. Persistent DNA damage can result in cell death and the accumulation of mutations, which may lead to accelerated ageing or malignant transformation (i.e. cancer). To counteract the deleterious effects of DNA damage, cells evolved an intricate network of pathways to detect and repair DNA damage, the DNA Damage Response (DDR). Chapter 1 gives a general introduction into the topic of DNA repair and the DDR. For the repair of DNA double strand breaks, one of the most deleterious types of DNA damage, there are two pathways for repair: homologous recombination and non-homologous end joining. Assays to measure the repair via these pathways and the importance of a proper balance between the two are reviewed in chapter 2.
Mutations in proteins involved in DNA repair can predispose to specific types of cancer. For example, women with a heterozygous mutation in the homologous recombination proteins BRCA1 and BRCA2 are more prone to develop breast and ovarian cancer. These mutations cause a defect in homologous recombination and make BRCA1 or BRCA2 mutant tumours exquisitely sensitive to PARP inhibitors. The treatment with PARP inhibitors is very promising, but a fraction of the tumours eventually becomes resistant to the treatment. In chapter 3 we therefore set out to identify novel mechanism of resistance in BRCA1 deficient mouse mammary tumour cells. We identified loss of REV7 as a cause of resistance and in chapter 4 investigate the importance of the different roles of Rev7 in the restoration of homologous recombination and mediating PARP inhibitor resistance In chapter 5 we studied BRCA2 and its interactions partners. In a mass spectrometry screen, a new BRCA2 interacting protein, HSF2BP (Heat Shock Factor 2 Binding Protein), was identified. HSF2BP overexpression resulted in hypersensitivity to the interstrand-crosslinking agent mitomycin C (MMC) and caused a phenotype that was similar to Fanconi Anemia.
Chapter 6, the general discussion, examines the results and implications of this thesis for the treatment of cancer and also how new findings can be implemented in the clinic or which steps should be taken before this can be done.