For decades, radiotherapy and chemotherapy were the two only approaches exploiting DNA repair processes to fight against cancer. Nowadays, cancer therapeutics can be a major challenge when it comes to seeking personalized targeted medicine that is both effective and selective to the malignancy. Over the last decade, the discovery of new targeted therapies against DNA damage signalling and repair has offered the possibility of therapeutic improvements in oncology. In this review, we summarize the current knowledge of DNA damage signalling and repair inhibitors, their molecular and cellular effects, and future therapeutic use.

The partner and localiser of BRCA2 (PALB2) plays important roles in the maintenance of genome integrity and protection against cancer. Although PALB2 is commonly described as a repair factor recruited to sites of DNA breaks, recent studies provide evidence that PALB2 also associates with unperturbed chromatin. Here, we investigated the previously poorly described role of chromatin-associated PALB2 in undamaged cells. We found that PALB2 associates with active genes through its major binding partner, MRG15, which recognizes histone H3 trimethylated at lysine 36 (H3K36me3) by the SETD2 methyltransferase. Missense mutations that ablate PALB2 binding to MRG15 confer elevated sensitivity to the topoisomerase inhibitor camptothecin (CPT) and increased levels of aberrant metaphase chromosomes and DNA stress in gene bodies, which were suppressed by preventing DNA replication. Remarkably, the level of PALB2 at genic regions was frequently decreased, rather than increased, upon CPT treatment. We propose that the steady-state presence of PALB2 at active genes, mediated through the SETD2/H3K36me3/MRG15 axis, ensures an immediate response to DNA stress and therefore effective protection of these regions during DNA replication. This study provides a conceptual advance in demonstrating that the constitutive chromatin association of repair factors plays a key role in the maintenance of genome stability and furthers our understanding of why PALB2 defects lead to human genome instability syndromes.nherited mutations in the partner and localiser of BRCA2 (PALB2) gene predispose to breast and pancreatic cancer (1-3) and also congenital malformations, growth retardation, and early childhood cancer in a rare subgroup of Fanconi anemia (FA-N) patients (4, 5). The best characterized function of PALB2 is to physically link the two main breast cancer susceptibility gene products, BRCA1 and BRCA2, at sites of DNA damage (6-8), thus playing a pivotal role in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR). BRCA1 facilitates DNA-end resection to produce single-stranded (ss) DNA (9) and concomitantly attracts PALB2, together with BRCA2 and RAD51, to DSB sites (6-8). In turn, BRCA2 promotes the loading of the RAD51 recombinase onto ssDNA (10-12), which is critical for the strand invasion and exchange phase of HR (13). This mechanism of repair factor recruitment is important for the timely activation of HR at sites of DNA damage, and its perturbation in individuals with impaired PALB2 function is presumed to cause human pathologies. Although PALB2 is commonly described as a repair factor recruited to sites of DNA breaks, we and others have provided evidence that PALB2 also associates with chromatin in the absence of DNA damage (14-17). A recent genome-wide analysis of PALB2 chromatin occupancy revealed enrichment at highly active genes, with PALB2 occupying the entire body of these genes (18). PALB2 was shown to support the transcription of a subset of NF-κB-and retinoic acid-responsive genes. However, PALB2 does...

DNA double-strand breaks (DSB) elicit a ubiquitylation cascade that controls DNA repair pathway choice. This cascade involves the ubiquitylation of histone H2A by the RNF168 ligase and the subsequent recruitment of RIF1, which suppresses homologous recombination (HR) in G1 cells. The RIF1-dependent suppression is relieved in S/G2 cells, allowing PALB2-driven HR to occur. With the inhibitory impact of RIF1 relieved, it remains unclear how RNF168-induced ubiquitylation influences HR. Here, we uncover that RNF168 links the HR machinery to H2A ubiquitylation in S/G2 cells. We show that PALB2 indirectly recognizes histone ubiquitylation by physically associating with ubiquitin-bound RNF168. This direct interaction is mediated by the newly identified PALB2-interacting domain (PID) in RNF168 and the WD40 domain in PALB2, and drives DNA repair by facilitating the assembly of PALB2-containing HR complexes at DSBs. Our findings demonstrate that RNF168 couples PALB2-dependent HR to H2A ubiquitylation to promote DNA repair and preserve genome integrity.DOI: http://dx.doi.org/10.7554/eLife.20922.001

APRIN (PDS5 cohesin associated factor B) interacts with both the cohesin complex and the BRCA2 tumor suppressor. How APRIN influences cohesion and DNA repair processes is not well understood. Here, we show that APRIN is recruited to DNA damage sites. We find that APRIN interacts directly with RAD51, PALB2 and BRCA2. APRIN stimulates RAD51-mediated DNA strand invasion. APRIN also binds DNA with an affinity for D-loop structures and single-strand (ss) DNA. APRIN is a new homologous recombination (HR) mediator as it counteracts the RPA inhibitory effect on RAD51 loading to ssDNA. We show that APRIN strongly improves the annealing of complementary-strand DNA and that it can stimulate this process in synergy with BRCA2. Unlike cohesin constituents, its depletion has no impact on class switch recombination, supporting a specific role for this protein in HR. Furthermore, we show that low APRIN expression levels correlate with a better survival in ovarian cancer patients and that APRIN depletion sensitizes cells to the PARP inhibitor Olaparib in xenografted zebrafish. Our findings establish APRIN as an important and specific actor of HR, with cohesin-independent functions.

One typical mechanism to promote genomic instability, a hallmark of cancer, is to inactivate tumor suppressors, such as PALB2. It has recently been reported that mutations in PALB2 increase the risk of breast cancer by 8–9-fold by age 40 and the life time risk is ∼3–4-fold. To date, predicting the functional consequences of PALB2 mutations has been challenging as they lead to different cancer risks. Here, we performed a structure–function analysis of PALB2, using PALB2 truncated mutants (R170fs, L531fs, Q775X and W1038X), and uncovered a new mechanism by which cancer cells could drive genomic instability. Remarkably, the PALB2 W1038X mutant, harboring a mutation in its C-terminal domain, is still proficient in stimulating RAD51-mediated recombination in vitro, although it is unusually localized to the cytoplasm. After further investigation, we identified a hidden NES within the WD40 domain of PALB2 and found that the W1038X truncation leads to the exposure of this NES to CRM1, an export protein. This concept was also confirmed with another WD40-containing protein, RBBP4. Consequently, our studies reveal an unreported mechanism linking the nucleocytoplasmic translocation of PALB2 mutants to cancer formation.