From human genetics to radiobiology : in vitro radiosensitivity in individuals with a germline defect in DNA damage response genes

All currently known high to intermediate risk "breast cancer genes", including BRCA1 and BRCA2, are involved in the DNA damage response pathway. Heterozygous germline mutations in these genes predispose to breast and ovarian cancer. In addition, such mutations may also result in enhanced radiosensitivity mediated by chromosomal instability after exposure to ionizing radiation, leading to a higher risk to develop radiation-induced breast cancer. However, results of currently available clinical studies evaluating carcinogenesis and in vitro studies comparing chromosomal radiosensitivity in mutation carriers and non-carriers are inconclusive. Nevertheless, insights into the radiosensitive phenotype of healthy tissues of mutation carriers is of the utmost importance for the safe use of ionizing radiation for diagnostic purposes or radiotherapy treatment. In this thesis, we evaluated in vitro radiosensitivity in carriers of a mutation in DNA damage response genes by means of two different assays. The first assay, the G2 micronucleus assay, is a cytogenetic assay in which MN are analyzed in cells irradiated in the G2 phase of the cell cycle. This assay was developed to evaluate radiosensitivity in cells with a heterozygous BRCA1 or BRCA2 mutation. BRCA1 and BRCA2 have a function in homologous recombination (HR), the main DNA double strand break repair pathway activated in late S and G2 phase of the cell cycle. Furthermore, BRCA1 is also involved in the G2/M cell cycle checkpoint. The G2 micronucleus assay allows evaluation of both functions by means of two distinct endpoints: (1) the radiation-induced micronucleus yield, which reflects DNA double strand break repair capacity and (2) the G2/M checkpoint efficiency ratio, which allows evaluation of the G2 arrest capacity. Before applying the G2 micronucleus assay on BRCA mutation carriers, the assay was validated in a patient with Ataxia Telangiectasia (AT). AT patients are characterized by a manifest increased radiosensitivity. AT patients show biallelic inactivation of ATM, involved in both DNA double strand break repair by means of HR and G2/M checkpoint activation. We demonstrated a severely increased radiosensitivity with both endpoints when applying the G2 micronucleus assay in lymphocytes of this AT patient. In lymphocytes of healthy relatives with a heterozygous ATM mutation the radiosensitivity observed with this assay was intermediate between the AT patient and the control cohort. When applying the G2 micronucleus assay on lymphocytes of healthy BRCA1/2 mutation carriers, we demonstrated significantly enhanced radiation-induced MN yields in both BRCA1 and BRCA2 germline mutation carriers, pointing to an impaired DNA double strand break repair capacity in both groups. Furthermore, an impaired G2 arrest capacity was observed in BRCA1 mutation carriers. In healthy relatives who did not inherit the familial mutation, no enhanced radiosensitivity was observed. Although a significantly enhanced radiosensitivity was demonstrated for the cohort of BRCA1 and BRCA2 mutation carriers compared to the control cohort, individual radiosensitivity evaluation was less straightforward due to overlap in micronucleus yields between both cohorts. Therefore, a scoring system to evaluate individual radiosensitivity was implemented. As both BRCA1 and BRCA2 are involved in HR, we evaluated if the accumulation of RAD51, a key protein involved in this pathway, at the double strand break site can be used to assess HR functionality and radiosensitivity. To this end, a radiation-induced RAD51 foci assay was optimized in a breast epithelial cell line (MCF10A) expressing ±50% reduced BRCA1 and BRCA2 protein levels, obtained by RNA interference. RAD51 foci were analyzed in cells synchronized in S phase by aphidicolin as HR is upregulated during this phase of the cell cycle. We demonstrated significantly reduced RAD51 foci formation, and thus impaired HR capacity, in response to the induction of radiation-induced double strand breaks in the BRCA knockdown cells compared to control cells. As no overlap in RAD51 foci distribution is observed between knockdown and control cells, we think that this assay could better differentiate between normal cells and cells with a heterozygous BRCA1 or BRCA2 mutation than the G2 micronucleus assay. This will be further explored in synchronized lymphocytes of heterozygous germline mutation carriers. In addition to the detection of unequivocal deleterious mutations in BRCA1 and BRCA2, variants of unknown clinical significance (VUS) are detected during diagnostic screening. The associated breast cancer risk is unknown, which creates a challenge for genetic counselling. mRNA analysis to assess variants that might impair proper RNA splicing, a highly regulated process, are widely used. We evaluated the outcome at cDNA level of 21 putative splicing variants in BRCA1 and BRCA2 and demonstrated aberrant splicing for 12 variants, suggesting that these are likely pathogenic. Furthermore, we demonstrated that in silico prediction tools might assist in the evaluation of these putative splicing variants. However, further optimization is warranted to allow reliable application outside the highly conserved consensus splice sites. The results obtained in this thesis may indicate that care should be taken when applying ionizing radiation for diagnostic or therapeutic purposes in individuals with a germline mutation in BRCA1 or BRCA2 as they may be at higher risk of developing radiation-induced breast cancer.

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