Regulation of RAD51 at the Transcriptional and Functional Levels: What Prospects for Cancer Therapy?

Abstract

The RAD51 recombinase is a critical effector of Homologous Recombination (HR), which is an essential DNA repair mechanism for double-strand breaks. The RAD51 protein is recruited onto the DNA break by BRCA2 and forms homopolymeric filaments that invade the homologous chromatid and use it as a template for repair. RAD51 filaments are detectable by immunofluorescence as distinct foci in the cell nucleus, and their presence is a read out of HR proficiency. RAD51 is an essential gene, protecting cells from genetic instability. Its expression is low and tightly regulated in normal cells and, contrastingly, elevated in a large fraction of cancers, where its level of expression and activity have been linked with sensitivity to genotoxic treatment. In particular, BRCA-deficient tumors show reduced or obliterated RAD51 foci formation and increased sensitivity to platinum salt or PARP inhibitors. However, resistance to treatment sets in rapidly and is frequently based on a complete or partial restoration of RAD51 foci formation. Consequently, RAD51 could be a highly valuable therapeutic target. Here, we review the multiple levels of regulation that impact the transcription of the RAD51 gene, as well as the post-translational modifications that determine its expression level, recruitment on DNA damage sites and the efficient formation of homofilaments. Some of these regulation levels may be targeted and their impact on cancer cell survival discussed.

Keywords: DNA break; DNA repair; RAD51; cancer therapy; genomic instability; homologous recombination.

Figure 1

Representation of DNA double-strand break repair choice in the S/G2 phase between homologous recombination non-homologous end joining. The scheme details the steps leading homologous strand invasion and D loop formation. Step 1: 3′ end resection by the MRN complex, RPA loading to protect single-strand DNA (ssDNA) from degradation. Step 2: BRCA2 is recruited by BRCA1/PALB2 and loads RAD51 onto the ssDNA overhangs and RPA is dislodged. Step 3: formation of the RAD51 Homopolymeric filament which wind around the ssDNA. Step 4: invasion of the homologous chromatid and search for homology in view of recombination and repair.

Figure 2

The schematic view of the RAD51 protein structure. The Walker A and B domains of the RAD51 family are depicted as green and blue boxes. The different interaction and post-translational modification sites are represented.

Figure 3

Identified regulators of RAD51 transcription. The DNA sequences containing the RAD51 promoter and the gene are represented as a purple box and a green box, respectively.

Figure 4

Post-translational modifications that impinge on RAD51 function. (A) Modifications leading to the activation and loading of RAD51: phosphorylation cascade at Ser14 and Thr13 by PLK1 and CK2, phosphorylation at Thr307/309 by CHK1, ubiquitination at Lys56/57/63 by UCHL3. (B) Modifications leading to the inactivation of RAD51: ubiquitination at Lys58/64 by FBH1, polyubiquitination by FBK05 or RFWD3 leading to the degradation of RAD51 by the proteasome. Blue bubbles (P) represent phosphorylation, red bubbles (Ub) represent ubiquitination.

Figure 5

Intervention of RAD51 cofactors in RAD51 loading and stabilization of the filaments. The BCDX2 intervenes at early stages of RAD51 loading, while CX3 stabilizes the filaments onto the ssDNA.