DNA recombination and cancer

Principal investigator
Dr. Sylvie Noordermeer

Our genome is constantly threatened by many DNA damaging chemicals and processes. One of the most cytotoxic types of DNA damage are DNA double-strand breaks (DSBs). These breaks are a dangerous consequence of errors arising during DNA replication, but can also be caused by reactive oxygen species, exogenous ionizing irradiation (IR) or DNA damaging drugs. DSBs are highly cytotoxic lesions and can lead to deletions, duplications and translocations and eventually cancer, if repaired incorrectly.

To maintain genomic stability, cells are equipped with efficient ways of repairing these breaks via several mechanisms. BRCA1 is a key protein for the repair of DSBs via Homologous Recombination (HR). This faithful mechanism of repair uses the information of the sister chromatid for repair during S and G2 of the cell cycle. BRCA1 mutations occur in a wide variety of tumours, including hereditary forms of breast and ovarian cancer, but also lung and gastric cancer. Next to many truncating mutations, leading to a fully dysfunctional BRCA1, many missense mutations and also variants of unknown significance have been identified in tumours.  BRCA1-mutated tumours are characterised by gross chromosomal instability. However, how all these different mutations in BRCA1 are involved in cancer development and therapy response is often still unclear. 

One of the main complexities in studying BRCA1 function is the fact that it forms several multi-protein complexes via its different protein domains, most of them stimulating HR, but others – counterintuitively – inhibiting HR. The main question in my lab is to better understand how all these different BRCA1-complexes cooperate to maintain genomic stability and how mutations disrupting single complexes affect tumorigenesis. My lab uses a combination of systematic approaches such as genome-wide CRISPR-Cas9 screens and proteomics, with dedicated functional assays in mammalian cell systems to study the mechanisms of BRCA1 function.

Key Research Topics:

1. Identification and characterisation of BRCA1-complexes
Using proteomics, we study DSB-dependent and –independent BRCA1-interactors and characterise the architecture of the complexes with BRCA1. To better understand the individual role of and interplay between the different BRCA1-complexes, we characterise and compare the spatial and temporal dynamics of recruitment of BRCA1 and its distinct complexes to DSBs. By functional testing of complex-disrupting separation-of-function mutations, we aim to uncover the unique roles of the different complexes in DSB repair. 

2. Improving personalized treatment options for patients with specific complex-disrupting BRCA1 mutations
We correlate the functional studies on complex-disrupting BRCA1 mutations to clinical data of cancer patients with such mutations to improve risk stratification of patients with BRCA1-mutated tumours. Furthermore, we study the genetic relationship between BRCA1 and its different complex members via a collection of genome-wide CRISPR/Cas9 synthetic lethality screens. This will enhance our understanding of the unique and common functions of these complexes in cellular homeostasis. With the data of the screens, we are identifying novel druggable pathways and targets to treat cancer patients with specific complex-disrupting mutations.

3. Studying the mechanisms involved in PARP inhibitor resistance of BRCA1-mutated tumours
We performed a genome wide CRISPR/Cas9 positive selection screen to identify factors that are involved in PARP inhibitor (PARPi) resistance of BRCA1-deficient cells (Noordermeer et al., Nature, 2018). We are now characterising the role of novel hits of this screen in PARPi sensitivity and are studying the effect of individual BRCA1 missense mutations on the acquisition of PARPi resistance. Furthermore, we study the effect of BRCA1 complex-disrupting mutations on de novo or acquired PARP inhibitor resistance.

4. Mechanistic studies on DNA double-strand break repair pathway choice
The cell is equipped with multiple repair pathways to resolve breaks. These pathways vary in their fidelity for repair. When a break occurs, the cell needs to make a choice which pathway to activate. Although we already know that many cellular aspects, such as cell cycle stage, chromatin context and break aetiology, affect pathway choice, we still do not fully understand how a cell makes this decision. Our lab investigates this pathway choice by performing high resolution live cell imaging experiments to better understand and predict the outcome of repair of breaks with different origins.

Figure legend:
BRCA1 forms multiple protein-complexes each with a unique role in maintaining genomic stability