Mismatch repair (MMR) is a DNA repair system that corrects base-base misincorporations generated by the replicative DNA polymerases. Previous publications have shown the involvement of the MMR... Show moreMismatch repair (MMR) is a DNA repair system that corrects base-base misincorporations generated by the replicative DNA polymerases. Previous publications have shown the involvement of the MMR pathway in control of DNA damage responses. The work presented in this thesis suggests that the regulation of DNA damage induced mutagenesis and signaling may lie in the complex interplay between translesion synthesis (TLS), a subset of error-prone polymerases capable of replicating damaged DNA, and MMR. This thesis suggests that MMR may reduce DNA damage induced mutagenesis via two ways: (i) recruit the somewhat less error-prone TLS polymerases to the DNA lesion and (ii) post-replicative removal of mismatched nucleotides. Of note, inheriting a heterogenous defect in an MMR gene results in increased risk of developing cancer, in particular colorectal cancer. The reason for this specific cancer tropism is, as of yet, unclear. The colorectal tract is continuously exposed to DNA damaging agents and indeed data in this thesis shows that MMR is important in protecting against diet-derived DNA damaging agents by controlling DNA damage induced mutagenesis and signaling. As such the colorectal cancer tropism of MMR of LS may partly be explained by the role of MMR in controlling the DNA damage response. Show less
Pathogenic variants in PALB2 and CHEK2 are associated with an increased risk of breast cancer. By contrast, for missense variants of uncertain significance (VUS) in these genes, the associated... Show morePathogenic variants in PALB2 and CHEK2 are associated with an increased risk of breast cancer. By contrast, for missense variants of uncertain significance (VUS) in these genes, the associated breast cancer risk is often unclear. To aid in their clinical classification, functional assays that determine the impact of missense VUS on PALB2 and CHK2 protein function have been performed in this thesis. By means of these functional analyses, numerous missense VUS (in both PALB2 and CHEK2) have been identified that are, from a functional viewpoint, just as damaging as known pathogenic (i.e., truncating) variants. In agreement, we observe that the level of impaired protein function correlates with the degree of increased breast cancer risk. Overall, these findings suggest that damaging PALB2 and CHEK2 missense VUS are associated with a risk of breast cancer similar to that of protein-truncating variants in these genes. This indicates the urgency of expanding the functional characterization of missense VUS in both PALB2 and CHEK2 to further understand the associated cancer risk. Show less
Evidence is accumulating that infliction of DNA damage can lead to spatial repositioning of chromatin. Mitomycin C (MMC)-induced interchanges between homologous chromosomes have been taken as... Show moreEvidence is accumulating that infliction of DNA damage can lead to spatial repositioning of chromatin. Mitomycin C (MMC)-induced interchanges between homologous chromosomes have been taken as cytological evidence for somatic pairing of human chromosomes. Pairing of specific chromosomal regions might differ dependent on biological states or activities, i.e. cell type, cell cycle phase, differentiation stage and diseases. However, the biological relevance of pairing is unclear. The aim of this thesis was to dissect the mechanisms that underlie the interaction between homologous chromosomes observed in metaphase and the interphase of the cell cycle. To reach this goal, we assessed the effect of genotoxic agents such as X-rays, UV radiation and MMC on the positioning of homologous chromosomal regions within the nucleus of human cells. Moreover, we examined the relationship between interphase pairing and processing of DNA damage that leads to exchange formation. We found that: (i) pairing of heterochromatic regions is a general response to genotoxic stress, (ii) pairing is correlated with exchanges between homologous chromosomes after exposure to the anticancer drug mitomycin C (MMC), (iii) pairing is genetically controlled by genes involved in the DNA damage response and (iv) processing of MMC-induced DNA damage is initiated in non-dividing cells. Show less
The initial damage recognizing complex in bacterial nucleotide excision repair consists of two UvrA and two UvrB molecules. Of these proteins UvrB is the main damage recognition protein and is... Show moreThe initial damage recognizing complex in bacterial nucleotide excision repair consists of two UvrA and two UvrB molecules. Of these proteins UvrB is the main damage recognition protein and is capable of recognizing various structurally unrelated types of damage. To do so, the protein must recognize a common alteration of the DNA structure induced by these different damages. For UvrB sterical hindrance of the present DNA modification prevents it from passing behind a _-hairpin structure present in the protein, thereby conferring damage recognition. Upon stalling of the protein at the site of damage in this way we show that two nucleotides become extrahelical: the nucleotide directly 3__ to the lesion and its base-pairing partner in the non-damaged strand. In contrast to other repair enzymes however the damaged nucleotide itself does not become extrahelical. Flipping in one of the two DNA strands has been shown to be important in preventing stable binding of the protein to undamaged DNA. Flipping in the other DNA strand however is required for efficient 3__ incision by UvrC, the nuclease of the system. A second incision at the 5__ side of the damage by the same protein facilitates removal of the damage-containing DNA oligonucleotide. Show less
