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. Author manuscript; available in PMC: 2023 Mar 16.
Published in final edited form as: Cancer Res. 2022 Jul 12:can.22.1535. doi: 10.1158/0008-5472.CAN-22-1535

BRCA1-dependent and independent recruitment of PALB2-BRCA2-RAD51 in the DNA damage response and cancer

Tzeh Keong Foo 1, Bing Xia 1,*
PMCID: PMC9481714  NIHMSID: NIHMS1824684  PMID: 35819255

Abstract

The BRCA1-PALB2-BRCA2 axis plays essential roles in the cellular response to DNA double strand breaks (DSB), maintenance of genome integrity, and suppression of cancer development. Upon DNA damage, BRCA1 is recruited to DSBs, where it facilitates end resection and recruits PALB2 and its associated BRCA2 to load the central recombination enzyme RAD51 to initiate homologous recombination (HR) repair. In recent years, several BRCA1-independent mechanisms of PALB2 recruitment have also been reported. Collectively, these available data illustrate a series of hierarchical, context-dependent, and cooperating mechanisms of PALB2 recruitment that is critical for HR and therapy response either in the presence or absence of BRCA1. Here, we review these BRCA1-dependent and independent mechanisms and their importance in DSB repair, cancer development, and therapy. As BRCA1-mutant cancer cells regain HR function, for which PALB2 is generally required, and become resistant to targeted therapies, such as PARP inhibitors, targeting BRCA1-independent mechanisms of PALB2 recruitment represents a potential new avenue to improve treatment of BRCA1-mutant tumors.

DNA double stand breaks (DSBs) are the most hazardous form of DNA damage which, if unrepaired or misrepaired, can lead to cell death or genome instability that drives tumorigenesis. The major breast cancer tumor suppressor genes BRCA1 and BRCA2 encode very large and completely distinct proteins (Figure 1) that both play essential roles in the DNA damage response (DDR), especially in the repair of DSBs by homologous recombination (HR) (1). In the HR pathway, BRCA1 functions upstream of BRCA2 and facilitates the resection of DSB ends, while BRCA2 loads the central recombination enzyme RAD51 to resected single-stranded DNA (ssDNA) to initiate strand invasion (1) (Figure 2A). The two BRCA proteins are physically and functionally linked by a third tumor suppressor protein, PALB2, which was discovered as a major BRCA2 binding partner that is required for its stability, chromatin association and HR function (2). Like BRCA1/2, inherited monoallelic mutations in PALB2 cause high risk of breast cancer and also increase the risks of ovarian, pancreatic, and certain other cancers (3,4). Moreover, biallelic mutations in each of the 3 genes cause Fanconi anemia (FA), a rare genetic syndrome characterized by developmental defects, progressive bone marrow failure, and cancer susceptibility, with BRCA1 being FANCS, BRCA2 being FANCD1, and PALB2 being FANCN (5).

Figure 1.

Figure 1.

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Domain structures of BRCA1, BARD1, PALB2 and BRCA2. Their key interacting partners involved in the DNA damage response are shown and their binding sites indicated. The triple lines between BRCA1 and BARD1 indicate a stoichiometric complex formation of the two proteins. NCP, nucleosome core particle.

Figure 2.

Figure 2.

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HR initiation upon DSB formation and mechanisms of PALB2-BRCA2-RAD51 recruitment under different conditions. (A) Cartoon showing steps of HR initiation and the involvement of the relevant proteins in this review. (B-E) Cartoons showing PALB2-BRCA2 recruitment and the extent of RAD51 loading in cells with different BRCA1 and 53BP1 status. Panel B illustrates “canonical” PALB2 recruitment in normal cells by a direct interaction with BRCA1, with RNF168 serving as a stabilizing factor aside from its function as a histone E3 ligase; panel C depicts a blockade of PALB2 recruitment by 53BP1 and associated proteins in BRCA1 null cells; panel D shows recruitment of PALB2 through its interaction with RNF168 and the nucleosome core particle (via ChAM) in BRCA1/53BP1 double null cells; and panel E shows residual PALB2 recruitment by RNF168 in cells with disrupted PALB2-BRCA1 interaction. MRG15-mediated PALB2 tethering to actively transcribed chromatin regions (marked by H3K36me3) is also shown in B. A BRCA1 CC mutation affecting BRCA1-PALB2 interaction is indicated as a red dot in E. See text for details.

Since its discovery in 2006, the critical role of PALB2 as an enabler of BRCA2 function and an intermediary between BRCA1 and BRCA2 in HR and cell cycle checkpoint response has been well established. In this review, we focus on the role of the BRCA1-PALB2 interaction in the DDR and cancer, recent findings of multiple BRCA1-independent “backup” mechanisms of PALB2 recruitment to DSBs, and potential strategies to target PALB2 recruitment to combat drug resistance of BRCA1 mutant cancers or induce “BRCAness” in HR proficient cancers.

The “canonical” BRCA1-PALB2-BRCA2-RAD51 axis in the DNA damage response

BRCA1 has a RING domain at its N terminus, a tandem BRCT domain at the C terminus, and a coiled-coil (CC) motif in the middle (Figure 1). The RING domain forms a complex with its stoichiometric partner BARD1, the CC motif mediates its interaction with PALB2, and the BRCT domain binds several other DDR proteins, including ABRAXAS, BRIP1 and CtIP (6) (Figure 1). BRCA1 has long been known to be recruited through its BRCT domain and binding to ABRAXAS, which is recruited via RAP80 to K63-linked ubiquitin chains generated around DSBs by RNF8 and RNF168 (7,8). Interestingly, upon RAP80 depletion, cells lose large BRCA1 foci but retain small ones, and HR activity is increased (9,10). To date, available data support a role for RAP80-ABRAXAS as a suppressor of resection (10,11); alternatively, it has also been suggested that RAP80-ABRAXAS may prevent otherwise exaggerated BRCA1-driven recombination (9). Very recently, it has been shown that BARD1 can recognize K13/K15 monoubiquitinated H2A at DSBs, thereby recruiting BRCA1 independent of its BRCT domain (12). Loss of RAP80 further reduces BRCA1 focus formation in cells selectively expressing BARD1 mutants that are unable to bind K13/K15-monoubiquitinated H2A (12); however, how this impacts HR is unclear. As such, we speculate that most BRCA1 molecules associated with ABRAXAS-RAP80 may be sequestered in an inactive state, whereas those molecules recruited by BARD1 may play a more direct role in HR, but much still remains to be learned. Upon recruitment to DSBs, BRCA1 performs two major functions: first, it facilitates end resection by excluding 53BP1-RIF1-Shiedin complex and modulates CtIP activity; second, it recruits PALB2, which brings in BRCA2 and its associated RAD51 to initiate HR (1) (Figure 2A). Moreover, BRCA1-BARD1 can promote strand exchange and D-loop formation downstream of RAD51 loading (13).

The BRCA1-PALB2 interaction is a hydrophobic interaction between the N-terminal coiled-coil motif (aa 9–42) of PALB2 and a CC motif of BRCA1 (aa 1397–1424) (14,15) (Figure 1). Under normal conditions, the BRCA1-PALB2 interaction is crucial for HR, as mutations that disrupt this hydrophobic interface greatly impairs PALB2-BRCA2-RAD51 recruitment, severely diminishes HR activity, and results in sensitivity to therapeutic agents that target HR deficiency (HRD), such as PARP inhibitors (PARPi) and platinum salts (14,16,17). Notably, this interaction suppresses single-strand annealing (SSA), a deletion-causing repair mechanism that also utilizes extensively resected DNA ends, thereby dictating the repair pathway choice downstream of BRCA1 and resection (17). Moreover, the BRCA1-PALB2 interaction promotes the G2/M checkpoint control after DNA damage (18,19).

The BRCA1-PALB2 interaction is regulated by multiple post-translational modifications with consequences on DSB repair and checkpoint response. The PALB2 N terminus is subject to monoubiquitination on multiple sites within the CC motif that collectively blocks BRCA1 binding and inhibits HR, especially in the G1 phase of the cell cycle (20), whereas ATR phosphorylation of S59 near the CC motif promotes BRCA1 binding and HR in combination with a dephosphorylated S64, a CDK site (21). Moreover, we recently showed that ATR/ATM phosphorylation of BRCA1 T1394 at the N-terminal end of BRCA1 CC motif promotes HR, suppresses SSA, and promotes G2/M checkpoint control (22). Interestingly, mutation of BRCA1 T1394 produces an outcome that is consistent with partial loss of BRCA1-PALB2 interaction but does not appear to affect the stable association of BRCA1 with PALB2 as assayed by co-immunoprecipitation (IP), raising the question whether there is an unstable or transient interaction between the two proteins that also contributes to the DDR.

Potentially complicating the interpretation of experimental data on the BRCA1-PALB2 interaction is the fact that the PALB2 CC-motif can self-associate to form homodimers (23–25). Biochemically, PALB2 can bind DNA via its N-terminus and physically interact with RAD51 to stimulate RAD51-mediated D-loop formation (26–28). Note that the significance of PALB2’s own biochemical activity to stimulate RAD51 nucleoprotein filament and D-loop formations is not yet clear, given the strong HR defect in BRCA2 deficient cells with wt PALB2. Deletion of the N-terminal 40 aa of PALB2, which contains the CC motif, results in a monomeric PALB2 protein with increased ssDNA binding affinity and increased ability to promote RAD51 nucleoprotein filament formation in vitro (25). Interestingly, the BRCA1 CC motif appears to have a higher binding affinity to the PALB2 CC motif than does the latter itself; overexpression of wild-type (wt) PALB2 CC-motif alone is sufficient to sequester endogenous BRCA1 and suppress RAD51 foci formation in cells treated with IR or hydroxyurea (HU) (23,25). These findings suggest that PALB2 may be held at a dimeric, less active state before DNA damage and that after damage, increased BRCA1-PALB2 association not only enhances PALB2 recruitment but also its activity to promote HR by disrupting its self-association. Given the above results, if a PALB2 mutation disrupts BRCA1 association and reduces HR activity, it should be safe to conclude that the reduced HR is due to disrupted PALB2-BRCA1 interaction whether or not PALB2 self-association is affected.

Role of the BRCA1-PALB2 interaction in development and cancer

Patient-derived variants that affect the BRCA1-PALB2 interaction have been identified in both BRCA1 and PALB2 in breast cancer patients. These include M1400V, L1407P and M1411T in BRCA1 (14), and L35P and Y28C in PALB2 (16). The proline residues in L1407P and L35P are “helix breakers” that severely or completely disrupt the conformation of the CC motifs and therefore the BRCA1-PALB2 interaction, leading to strongly diminished PALB2 recruitment and HR activity. BRCA1 M1411T is almost as disruptive as BRCA1 L1407P, whereas BRCA1 M1400V and Y28C are hypomorphic. Detailed clinical information is only available for PALB2 L35P, in which case the (germline) variant segregates with disease in a family with strong history of breast cancer, and the resulting tumor harbors a second, somatic nonsense mutation in PALB2 that presumably inactivates the wt allele (16), conforming to the classic “two-hit” model and indicative of the pathogenic nature of the variant.

Several Palb2 or Brca1 mutant mouse strains have been generated to study the importance of the BRCA1-PALB2 interaction in vivo. At first, we generated a Palb2CC6 allele with a mutation in the CC domain (24LKK26 to 24AAA26) that disrupts its BRCA1 binding (29). The mutant mice have approximately 25% reduction in body weight but otherwise largely normal development, except that mutant males show moderate fertility defect in a C57BL/6 × 129sv mixed background (29). Further studies revealed that these mice are more susceptible to both spontaneous and ionizing radiation-induced tumorigenesis (30,31). Interestingly, Palb2CC6/CC6 mutant males are more susceptible to hepatocellular carcinoma (HCC), and HCC tumors from the mutant mice show cGAS-STING pathway activation, which results in T cell infiltration and also PD-L1 mediated immunosuppression; these features of the HCC tumors and tumor microenvironment set up for a good response to anti-PD-1 treatment (32).

Three separate studies have investigated the impact of BRCA1 CC motif mutations that disrupt PALB2 binding using mouse models. One generated a Brca1CC allele causing an in-frame deletion of amino acids 1361IKL1363 (equivalent to human BRCA1 1405IKL1407) (33), and the other two studies created the same L1363P mutation that is equivalent to the aforementioned L1407P “helix breaker” (34,35). Homozygous Brca1CC/CC mice are born at sub-Mendelian frequencies and show severe growth retardation, greatly reduced lifespan and FA-like phenotypes including bone marrow failure (33). The phenotypes of the Brca1L1363P mutation depend on genetic background - Brca1 L1363P/L1363P mice with C57BL/6 × 129/Sv background are viable but exhibit phenotypes similar to Brca1CC/CC mice (35); whereas in the FVB background this mutation causes embryonic lethality (34). The Brca1L1363P mutation in combination with conditional Trp53 ablation leads to mammary tumor formation to a similar extent to combined ablation of Brca1 and Trp53, and the tumors are HR defective; however, the Brca1L1363P/Δ;Trp53Δ/Δ tumors show distinct histopathologic-genomic features, notably a more stable copy number profile, and intermediate response to cisplatin and PARP inhibition compared with Brca1Δ/Δ;Trp53Δ/Δ tumors (34). Together, the above findings underscore the critical role of BRCA1-PALB2 interaction, or BRCA1-dependent PALB2 recruitment, in both normal development and the suppression of cancer and FA. They also indicate that PALB2-independent functions of BRCA1 also play important roles in genome stability and therapeutic response.

BRCA1-indepenent mechanisms of PALB2 recruitment

In BRCA1 wild-type (wt) cells, acute loss of the protein leads to attenuated resection and loss of PALB2, BRCA2 and RAD51 focus formation, rendering cells HR defective and hypersensitive to platinum salts and PARP inhibitors. At the same time, BRCA1 deficiency or mutations affecting the BRCA1-PALB2 interface may engender a selective pressure for cells to adopt alternative modes of PALB2 recruitment. Indeed, BRCA1 mutant cells often find ways to restore HR and acquire drug resistance (36,37). In addition to genetic reversions or intragenic deletions that either correct or skip the pathogenic mutations in the BRCA1 gene, a significant cause of drug resistance in BRCA1 deficient cells is the loss of components of the 53BP1-RIF1-Shieldin complex, which partially restores HR in the cells (36). Notably, this partial restoration of HR upon loss of the above 53BP1 complex is specific to BRCA1 deficient and not PALB2/BRCA2 deficient cells (38). Importantly, restoration of HR in BRCA1 deficient cells by 53BP1 loss is accompanied by reappearance of PALB2 foci and requires PALB2 (39), underscoring the importance of BRCA1-independent PALB2 recruitment for HR in this setting. In the section below, we summarize possible mechanisms of PALB2 recruitment in the absence of BRCA1 and in the setting of disrupted PALB2-BRCA1 interaction.

i. Direct binding to DNA and RPA

As noted before, PALB2 can bind DNA via its N-terminus (26–28). Mutation of the “main DNA-binding cluster” (146RRKK149) to AAAA strongly reduces its ssDNA binding and leads to reduced RAD51 focus formation and HR activity without affecting RAD51 binding (28); focus formation of PALB2 itself also appears to be reduced by the mutation, but this is not noted in the article and therefore is unclear. Moreover, it has been reported that PALB2 can also be recruited to ssDNA via an interaction with phosphorylated RPA following replication stress to promote stalled fork recovery (40); however, whether this contributes to HR and DSB repair is unclear.

ii. Association with methylated histones through MRG15

Two earlier studies found that PALB2 associates with chromodomain containing proteins MRG15 and its paralog MRGX, which bind histone H3 trimethylated at lysine 36 (H3K36me3) and play a role in chromatin remodeling, but they provided conflicting interpretations about the role of this interaction for HR (41,42). Depletion of MRG15 leads to reduced PALB2 focus formation and cellular HR activity (41), although it is unclear whether this is due to loss of their interaction or an indirect effect such as a general change in chromatin structure. More recent studies have shown that PALB2 interacts with MRG15 and MRGX via an MRG-binding “FxLP” motif and that PALB2-MRG15 protects actively transcribed genes from DNA damage (43). To date, whether the PALB2-MRG15 complex is directly involved in HR remains to be seen.

iii. Association with RNF168 and ubiquitinated chromatin

Upon DSB formation, ubiquitin E3 ligase RNF8 modifies histone H2A and H2AX on residues K13/K15 in the nearby chromatin, and RNF168 is then engaged to amplify K63-linked ubiquitin chains to thresholds required for 53BP1 and BRCA1 retention (7,44). Although cells from Rnf168−/− mice show impaired BRCA1 and RAD51 focus formation, the mice are viable, suggestive of significant residual HR activity. Interestingly, RNF168, via a C-terminal PALB2-interacting domain (PID), can physically interact with the WD40 domain of PALB2 to help recruit PALB2 to histone-ubiquitinated chromatin, even in the absence of the RAP80-ABRAXAS-BRCA1 complex (45). Although the PID domain is dispensable for PALB2 recruitment when BRCA1 is present, Palb2CC6/CC6;Rnf168−/− and Brca1CC/CC;Rnf168−/− mice are both embryonic lethal (46,47). By comparison, loss of RNF168 in Brca1Δ2/Δ2 mice, which express a “RING-less” BRCA1 that can still interact with PALB2, are viable (47). These findings suggest that RNF168 is required for PALB2 recruitment when the BRCA1-PALB2 interaction is disrupted.

iv. Chromatin association through its ChAM motif

Separate from its DNA binding domain, PALB2 contains a conserved chromatin-association motif (ChAM) (48). Deletion of ChAM greatly reduces PALB2 chromatin association, moderately impairs its foci formation, and leads to moderately increased sensitivity to olaparib, a PARP inhibitor, but not mitomycin C in HT1080 cells (48). Interestingly, a recent study shows that ChAM can directly associate with the acidic patch region of the nucleosome core particle (NCP), independent of histone modifications, via an arginine/lysine (R/K) rich motif (39). Notably, this study finds little to no impact of ChAM deletion on either PALB2 recruitment or olaparib sensitivity in BRCA1-wt RPE-1 cells, possibly due to a compensatory role of MRG15-mediated recruitment. Importantly, this study also finds that loss of 53BP1 restores PALB2 focus formation in BRCA1 deficient cells, that PALB2 is required for the (partial) restoration of HR activity in BRCA1−/−;53BP1−/− cells, and that the ChAM motif of PALB2 is required in this setting. Thus, ChAM appears to have a limited contribution to PALB2 function in HR in the presence of intact BRCA1 and 53BP1 but plays a key role to mediate PALB2 recruitment when they are both absent.

Hierarchical and context-dependent mechanisms of PALB2 recruitment to DSBs

It has become clear that recruitment of PALB2 to DSB sites is critical for HR both under normal conditions and in settings where BRCA1 is lost or mutated. Collective evidence indicates that in the presence of intact BRCA1, its direct interaction with PALB2 plays a dominant role for the targeting of PALB2-BRCA2-RAD51 to DSB-proximal chromatin presumably recognized by BARD1, with the interaction between RNF168 and PALB2 helping to retain the recruited PALB2 (Figure 2B). DNA binding by PALB2 and BRCA2, meanwhile, may position the complex at the precise location and orientation to load RAD51 onto resected ssDNA. The BRCA1-ABRAXAS-RAP80 complex, on the other hand, may help to enrich PALB2 in the surrounding chromatin for possible mobilization. In the absence of BRCA1, PALB2 in theory may still be recruited via its interaction with either RNF168 through its WD domain or the NCP through its ChAM domain, or by binding to resected DNA or RPA (note that resection still occurs in BRCA1 depleted cells although it may be delayed and less extensive). However, the absence of distinct PALB2 and RAD51 foci in cells acutely depleted of BRCA1 illustrate the inability of these mechanisms, even in combination, to recruit and retain PALB2 to damage sites without BRCA1 (when 53BP1-RIF1-Shieldin is present) (Figure 2C). When both BRCA1 and 53BP1 are lost, some PALB2 foci form in manner that is dependent on its ChAM domain; given the requirement of RNF168 for efficient PALB2 foci formation even in BRCA1 proficient cells, it is reasonable to speculate that the ChAM and WD domains of PALB2 function in synergy to recruit and retain PALB2 at DSB sites through their interactions with the NCP and RNF168, respectively (Figure 2D). The significant rescue of PALB2 focus formation in BRCA1/53BP1 doubly depleted or double null cells further indicates that in the absence of BRCA1, 53BP1-RIF1-Shieldin not only inhibits resection but also, perhaps even more importantly, precludes PALB2 recruitment, likely by occluding PALB2 access to the acidic patch of NCPs and resected DNA (Figure 2D). This “post-resection” role of the 53BP1 complex has similarly been suggested by another recent study (49). Finally, in cells expressing BRCA1 CC mutants that fail to bind PALB2 or a PALB2 CC mutants unable to bind BRCA1, the interaction between PALB2 and RNF168, although likely unstable, appears to be able to orchestrate a low level of PALB2 recruitment that may support mouse viability and some level of cellular drug resistance (Figure 2E). Thus, recruitment of PALB2 by BRCA1-independent backup mechanisms requires either loss of 53BP1 or the presence of a BRCA1 molecule that is able to exclude 53BP1 from the break site even if it cannot bind and recruit PALB2 directly.

Targeting PALB2 recruitment as a potential avenue to combat drug resistance

The advent of PARP inhibitors represents a major milestone for rational treatment of BRCA mutant cancers with a targeted therapy based on knowledge gained from basic research. Although BRCA1 mutant cancers tend to respond well to PARPi therapy initially, drug resistance frequently occurs (50). Over the past 15 years or so, several resistance mechanisms have been elucidated, which include restoration of BRCA1 function itself or restoration of HR without restoring BRCA1 (36,37,51,52). Importantly, HR rescue in BRCA1 deficient cells in most cases likely requires PALB2 recruitment via one of the backup mechanisms analyzed above, which mainly depend on protein-protein interactions between PALB2 and other proteins at DSB sites. Therefore, targeting these protein-protein interactions, especially the PALB2-NCP and PALB2-RNF168 interactions, may hold significant potential to reduce or eliminate PARPi resistance of a significant portion of BRCA1 mutant cancers. Given the overlapping mechanisms, this approach may also be effective for the platinum resistance of BRCA1 mutant tumors. Consistent with these hypotheses, Rnf168 deficiency suppresses Brca1-associated mammary tumorigenesis in a mouse model, and low RNF168 mRNA in HR defective human breast cancer correlate with improved patient survival (53). Additionally, certain patient-associated hypomorphic variants in the CC motif, such as L24S, Y28C and R37H of PALB2 and M1400V of BRCA1, partially affect BRCA1 binding, HR activity, and/or drug sensitivity (16,19,54,55); targeting the backup mechanisms of PALB2 recruitment may further sensitize tumors in patients carrying these and similar variants to PARPi or platinum salts.

Targeting HR proficient tumors, including relapsed BRCA mutant tumors, using PARP inhibitors in combination with agents that inhibit HR is gaining increasing attention. In this case, it is important to remember the basis of selectivity of PARP inhibitors. BRCA deficient tumors mostly arise from heterozygous mutation carriers and generally only after the wt allele is lost in the tumor initiating cells (56). Therefore, tumor cells are mostly BRCA−/− and HR defective, whereas other cells in the body are BRCA+/− and largely HR proficient, and this difference underpins the therapeutic window. As such, targeting HR proficient tumors by combining PARP inhibitors with agents that indiscriminately inhibit HR in all cells will inevitably lead to reduced selectivity and higher toxicity. Still, given that relapsed BRCA mutant cancer cells often only partially regain HR competency, there may still be a meaningful therapeutic window for such approaches. In this regard, targeting PALB2 recruitment in these tumors by means other than the backup mechanisms discussed above may also represent a viable avenue that merits investigation. For example, it has been shown that ATR inhibition overcomes PARPi and platinum resistance of BRCA1 mutant cancers in both cell and animal models (57,58). Given that ATM/ATR-mediated phosphorylation of PALB2 supports RAD51 focus formation after DNA damage (21,59) and that ATR inhibition impairs PALB2 focus formation without significantly affecting that of BRCA1 (21), inhibition of PALB2 recruitment may be a key part of the underlying mechanism for this resensitization. Further studies are warranted to determine the importance of PALB2 recruitment in these settings, as well as the toxicity and the clinical utility of this type of combination approaches.

Beside HR defect, BRCA1, BRCA2 and PALB2 deficient cells also display excessive degradation of stalled DNA replication forks after replication stress or fork blockade (60,61), and restoration of stalled fork protection can also drive PARPi resistance in BRCA deficient cells (57,62). Perplexingly, although both BRCA1 and PALB2 are important for fork protection, their interaction appears to be not required (33,34,61). Also, very recent studies suggest that sensitivity of BRCA deficient cells to platinum salts and PARPi stems from ssDNA replication gaps induced by the drugs and that suppression of gap formation may lead to therapy resistance (63,64). On a separate note, BRCA mutant and other HR deficient cells heavily rely on DNA polymerase θ (POLθ)-dependent microhomology-mediated end joining (MMEJ) for DSB repair and survival, and POLθ inhibition can selectively kill not only naïve BRCA mutant cells but also PARPi resistant BRCA mutant cells such as the BRCA1/53BP1 double null cells (65–67). Whether and how PALB2 and the PALB2-BRCA1 interaction are involved in these settings and resistance mechanisms await further investigation.

Concluding remarks

In summary, the direct interaction between BRCA1 and PALB2 is the primary mechanism for PALB2, BRCA2 and RAD51 recruitment to DSBs in normal cells, and this interaction is critical for HR, cell cycle checkpoint control, genome stability and tumor suppression. In the absence of BRCA1 or in cells with mutations in either BRCA1 or PALB2 that disrupt their interaction, PALB2 can possibly be recruited by several backup mechanisms to sustain residual HR. These secondary mechanisms depend on its interaction with RNF168 and the acidic patch of the NCP, and possibly also its MRG15 and DNA binding, and generally requires the absence of 53BP1 at break sites. As these mechanisms can (partially) restore HR and drive therapy resistance of BRCA1 mutant cancer cells, they represent logical drug targets that may be exploited to resensitize relapsed BRCA1 mutant cancers or further sensitize cancers with the afore mentioned hypomorphic BRCA1 or PALB2 mutations to PARPi, platinum salts, and similar DNA-damaging therapies. Moreover, measured inhibition of PALB2 recruitment may also sensitize other HR-proficient or semi-proficient tumors to the above therapies without excessive toxicity.

Acknowledgements

This work was supported by the National Cancer Institute (R01CA138804, R01CA262227, and P01CA250957–9485 to BX). TKF was a recipient of a postdoctoral fellowship from the New Jersey Commission of Cancer Research (NJCCR).

Footnotes

The authors declare no potential conflicts of interest.

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