Simon Boulton: Projects

DNA is highly susceptible to damage and must be repaired correctly to prevent genome instability. Failure to correctly repair DNA damage is the underlying cause of a number of hereditary cancer predisposition syndromes such as Fanconi Anemia and Blooms. The long-term aim of my group is to understand how DNA double-strand break (DSB) repair pathways, such as non-homologous end joining (NHEJ) and homologous recombination (HR), are regulated in mitotic cells and during meiosis. We also have an active interest in understanding how these pathways impact on human diseases such as cancer.

RIF1 is a 53BP1 co-factor that promotes NHEJ by blocking DSB resection

To counter the potential deleterious impact of DSBs, cells have evolved distinct DSB repair pathways, of which non-homologous end joining (NHEJ) and homologous recombination (HR) are the best understood in eukaryotic cells. The initial processing of the DSB ends is a key determinant of DSB repair pathway choice and is tightly regulated during the cell cycle. 53BP1 and BRCA1 directly influence the choice between NHEJ and HR by regulating 5' end resection but how this is achieved remains uncertain. In recent work, we established that Rif1-/- mice are severely compromised for 53BP1-dependent NHEJ during class switch recombination (CSR) and during the fusion of dysfunctional telomeres.

Similar to loss of 53BP1, we discovered that deletion of RIF1 also suppresses the toxic NHEJ induced by PARP inhibition in BRCA1 deficient cells. Consistent with this finding, we showed that BRCA1 antagonises the inappropriate accumulation of RIF1 at DSBs in S-phase, which is important for DNA end resection. At a mechanistic level we established that RIF1 is recruited to DSBs via the N-terminal ATM phospho-SQ/TQ domain of 53BP1 and DSBs generated by IR or during CSR are hyper-resected in the absence of RIF1. Taken together, our results revealed that RIF1 and 53BP1 cooperate to block DSB resection to promote NHEJ in G1, which is antagonised by BRCA1 in S-phase to ensure a switch of DSB repair mode to homologous recombination (Chapman et al., 2013; Mol Cell. 49(5): 858-71).

Figure 1

Figure 1: RIF1 and 53BP1 block DSB end resection to promote NHEJ in the G1 phase of the cell cycle. (Click to view larger image)

HELQ promotes RAD51 paralog-dependent repair to avert germ cell attrition and tumourigenesis

Detection and faithful repair of damaged DNA is essential for genome integrity. Repair of interstrand crosslinks (ICLs) requires the coordinate action of the intra-S phase checkpoint and the Fanconi Anemia (FA) pathway, which promote ICL incision, translesion synthesis, and homologous recombination. Previous studies have implicated the 3'-5' superfamily 2 helicase HELQ/Hel308 in ICL repair in D. melanogaster (known as Mus301 or Spn-C) and C. elegans (known as Helq-1 or Hel-308). While in vitro analysis suggests that HELQ preferentially unwinds synthetic replication fork substrates with 3' ssDNA overhangs and also disrupts protein/DNA interactions while translocating along DNA, little was known regarding its functions in mammalian organisms.

In recent work, we established that HELQ deficient mice exhibit subfertility, germ cell attrition, ICL sensitivity and tumour predisposition, with HelQ heterozygous mice exhibiting a similar, albeit less severe, phenotype than the null, indicative of haploinsufficiency. We discovered that HELQ interacts directly with the RAD51 paralog BCDX2 complex and functions in parallel to the FA pathway to promote efficient HR at damaged replication forks. These results uncovered a critical role for HELQ in replication coupled DNA repair, germ cell maintenance and tumour suppression in mammals (Adelman et al., 2013; Nature. 502(7471): 381-4). Our findings may help to explain the prevalence of non-synonymous variants in HELQ, which are significantly associated with upper aerodigestive tract cancers, particularly among smokers; and variants in HELQ associated with early menopause, which may reflect the germ cell defects and ovarian dysgenesis observed in HELQ deficient mice.

RTEL1 facilitates genome-wide and telomere replication

RTEL1 is a helicase that was originally identified by mapping of loci that control telomere length differences between M. musculus and M. spretus. RTEL1 plays a critical role in genome stability as knockout mice are embryonic lethal and cells derived from these mice exhibit telomere fragility and loss. We previously identified RTEL1 as a key regulator of HR in a genetic screen for anti-recombinases. RTEL1 mutant worms and RTEL1 depleted human cells are hyper-recombinogenic and DNA damage sensitive and biochemical studies revealed that human RTEL1 promotes the disassembly of D loop recombination intermediates in vitro.

Subsequent work from the lab established that RTEL1 also functions in meiosis to limit excessive crossing over and, disassembles T-loops and suppresses telomere fragility to maintain integrity of the chromosome end. In our most recent work, we employed a proteomic approach to gain further insight into the function and regulation of RTEL1 in cells. This strategy revealed that RTEL1 can also associate with the replisome and does so via direct binding to PCNA. To investigate the functional importance of this interaction, we established mouse cells disrupted for the RTEL1-PCNA interaction (PIP mutant). Unexpectedly, the PIP mutant cells exhibited accelerated senescence, replication fork instability, reduced replication fork extension rates, and increased origin usage, thus revealing an important role for RTEL1 during DNA replication. Although T-loop disassembly at telomeres was unaffected in the PIP mutant cells, telomere replication was compromised leading to fragile sites at telomeres. Finally, we showed that although RTEL1-PIP mutant mice are viable, loss of the RTEL1-PCNA protein interaction significantly accelerates the onset of tumourigenesis and predisposes to medulloblastomas in p53 deficient mice. Taken together, these data established that RTEL1 plays a critical role in both telomere and genome-wide replication, which is crucial for genetic stability and tumour avoidance (Vannier et al., 2013; Science. 342(6155): 239-42).

The lab currently exploits a combination of genetic approaches in C. elegans, proteomic approaches in mammalian cells, mouse genetics and biochemistry to investigate the mechanisms of DSB metabolism in mitotic and meiotic cells.

 

Simon Boulton

simon.boulton@crick.ac.uk
+44 (0)20 379 61774

  • Qualifications and history
  • 1998 PhD, University of Cambridge, UK
  • 1998 HFSP and EMBO Postdoctoral Fellow, Harvard Medical School, USA
  • 2002 Established lab at the London Research Institute, Cancer Research UK
  • 2015 Group Leader, The Francis Crick Institute, London, UK