Enhancing radiotherapy through a greater understanding of homologous recombination

Abstract

Radiotherapy for the treatment of cancer can cause a wide range of cellular effects, the most biologically potent of which is the double-strand break in DNA. The process of repairing DNA double-strand breaks involves 1 of 2 major mechanisms: nonhomologous end joining or homologous recombination. In this review, we review the molecular mechanisms of homologous recombination, in particular as it relates to the repair of DNA damage from ionizing radiation. We also present specific situations in which homologous recombination may be dysfunctional in human cancers and how this functional abnormality can be recognized. We also discuss the therapeutic opportunities that can be exploited based on deficiencies in homologous recombination at various steps in the DNA repair pathway. Side-by-side with these potential therapeutic opportunities, we review the contemporary clinical trials in which strategies to exploit these defects in homologous recombination can be enhanced by the use of radiotherapy in conjunction with biologically targeted agents. We conclude that the field of radiation oncology has only scratched the surface of a potentially highly efficacious therapeutic strategy.

Figure 1

The basic mechanism of homologous recombination (HR) involves processing a double-strand break to reveal a 3’-single strand tail. In the left panel, RPA binds to the single strand tail. By mechanisms that are still not fully understood, recombinase mediators such as BRCA2 and Rad52 help convert the RPA-coated ssDNA into Rad51 filaments. Rad51 filaments are capable of homology search and single strand invasion, which constitutes the basic mechanism of HR. In the right panel, the number of two duplex structures that can be encountered in the cell are shown. In A, the double-strand break is shown; in I, the collapsed replication fork is shown: both are substrates for HR. B-D represents synthesis-dependent strand annealing, in which no 4-strand structure is formed. B to E represents the formation of a Holliday junction, and E to F represents second-end capture and the formation of a double Holliday junction. I-L represents the restart of a collapsed replication fork, referred to as break-induced replication, in which strand invasion by HR recreates the replication fork structure. Holliday junction resolution is required for F-H and K-M.

Figure 2

The key proteins controlling homologous recombination. The main engine of HR is the Rad51 recombinase, which can often be observed by the formation of Rad51 foci in cells. Immediately upstream of Rad51 are BRCA2 and Rad52, which mediates the conversion of RPA-coated ssDNA into Rad51 filaments. The Rad51 paralogues (Rad51B/C/D, XRCC2/3) assist in this process in ways that still need to be defined. Between BRCA1 and BRCA2, there are a number of mediator proteins that support Rad51 function, such as BACH1, PALB2 and CtIP. The Fanconi Anemia proteins are playing a role in HR, particularly for collapsed replication forks in ways that still need to be determined. Upstream damage sensors send important signals to mediators of HR, such as Chk2-mediated phosphorylation of BRCA1. Any of the points in this pathway of HR are potentially targetable sites for enhancing the therapeutic effect of ionizing radiation.