Inactivation of 53BP1 in BRCA1 ES cells led to an increase in the two nucleolytic DNA end processing and RPA phosphoryl ation. ATM inhibition in BRCA1 53BP1 cells decreased RPA phosphorylation and Rad51 foci formation, indicating that ATM dependent resection makes it possible for partial restoration of HR. Figure 4 presents a plausible model to accommodate these findings. One particular ended DSBs that are formed during replication require BRCA1 to stimulate resection. In BRCA1 deficient cells, 53BP1 prevents resection of DNA ends, leading to aberrant diversion of breaks to NHEJ. This generates dead finish solutions or inappropriate joining to distant sequences leading to chromosomal translocations. Two ended DSBs, then again, call for 53BP1 to limit resec tion and allow efficient NHEJ.
Overactive resection in 53BP1 cells may lead to aberrant HR reactions or choice finish joining pathways, producing microhomology mediated transloca tions and/or junctions with excessive deletions. Further insight in to the purpose of BRCA1 and 53BP1 in repair pathway selection was lately obtained applying super resolution microscopy of IR induced foci. The core from the target contained additional reading mostly 53BP1 mole cules within the G1 phase in the cell cycle, possibly repre senting repair through NHEJ. In S phase, however, the core on the IRIF was full of BRCA1 and 53BP1 formed a ring close to this core, suggesting that BRCA1 phys ically excludes 53BP1 through the break to allow repair by way of HR. BRCA1 deficient cells are exquisitely sensitive to PARP inhibitors, which inhibit single strand break restore.
The rationale for this observation is that rep lication of DNA with single strand breaks leads to formation kinase inhibitor AZD4547 of single DNA ends, which demand HR for their fix. As described above, deletion of 53BP1 in BRCA1 deficient cells rescues embryonic lethality. Nevertheless, loss of 53BP1 also leads to resist ance to PARP inhibition. While in the BRCA1 deficient cells that have also lost 53BP1, the amount of chromosome and chromatid breaks is decreased and checkpoint activation is diminished in contrast to cells which are only BRCA1 deficient, suggesting that the regained HR capability in these cells is largely suffi cient to restore genomic stability. A subset BRCA1 and BRCA2 mutant tumors demonstrates reduction of 53BP1, indicating that therapy resistance via loss of 53BP1 might be clinic ally related. Ubiquitylation and sumoylation Ubiquitin as well as small ubiquitin like modifier are little polypeptides that could be attached to proteins as a posttranslational modification. Right after activation of ubiquitin or SUMO by an E1 enzyme, they’re trans ferred to an E2 conjugating enzyme. With the help of an ubiquitin ligase the modification is connected to the substrate.