Bing Xia

RAP80 Targets BRCA1 to Specific Ubiquitin Structures at DNA Damage Sites

Mutations affecting the BRCT domains of the breast cancer–associated tumor suppressor BRCA1 disrupt the recruitment of this protein to DNA double-strand breaks (DSBs). The molecular structures at DSBs recognized by BRCA1 are presently unknown. We report the interaction of the BRCA1 BRCT domain with RAP80, a ubiquitin-binding protein. RAP80 targets a complex containing the BRCA1-BARD1 (BRCA1-associated ring domain protein 1) E3 ligase and the deubiquitinating enzyme (DUB) BRCC36 to MDC1-γH2AX–dependent lysine6- and lysine63-linked ubiquitin polymers at DSBs. These events are required for cell cycle checkpoint and repair responses to ionizing radiation, implicating ubiquitin chain recognition and turnover in the BRCA1-mediated repair of DSBs.

Fanconi anemia is associated with a defect in the BRCA2 partner PALB2

The Fanconi anemia and BRCA networks are considered interconnected, as BRCA2 gene defects have been discovered in individuals with Fanconi anemia subtype D1. Here we show that a defect in the BRCA2-interacting protein PALB2 is associated with Fanconi anemia in an individual with a new subtype. PALB2-deficient cells showed hypersensitivity to cross-linking agents and lacked chromatin-bound BRCA2; these defects were corrected upon ectopic expression of PALB2 or by spontaneous reversion.

A recurrent mutation in PALB2 in Finnish cancer families

BRCA1, BRCA2 and other known susceptibility genes account for less than half of the detectable hereditary predisposition to breast cancer. Other relevant genes therefore remain to be discovered. Recently a new BRCA2-binding protein, PALB2, was identified. The BRCA2–PALB2 interaction is crucial for certain key BRCA2 DNA damage response functions as well as its tumour suppression activity. Here we show, by screening for PALB2 mutations in Finland that a frameshift mutation, c.1592delT, is present at significantly elevated frequency in familial breast cancer cases compared with ancestry-matched population controls. The truncated PALB2 protein caused by this mutation retained little BRCA2-binding capacity and was deficient in homologous recombination and crosslink repair. Further screening of c.1592delT in unselected breast cancer individuals revealed a roughly fourfold enrichment of this mutation in patients compared with controls. Most of the mutation-positive unselected cases had a familial pattern of disease development. In addition, one multigenerational prostate cancer family that segregated the c.1592delT truncation allele was observed. These results indicate that PALB2 is a breast cancer susceptibility gene that, in a suitably mutant form, may also contribute to familial prostate cancer development.

PALB2 Links BRCA1 and BRCA2 in the DNA-Damage Response

BRCA1 and BRCA2 are often mutated in familial breast and ovarian cancer. Both tumor suppressors play key roles in the DNA-damage response. However, it remains unclear whether these two tumor suppressor function together in the same DNA-damage response pathway. Here, we show that BRCA1 associates with BRCA2 through PALB2/FANCN, a major binding partner of BRCA2. The interaction between BRCA1 and BRCA2 is abrogated in PALB2-deficient Fanconi anemia cells and in the cells depleted of PALB2 by small interfering RNA. Moreover, we show that BRCA1 promotes the concentration of PALB2 and BRCA2 at DNA-damage sites and the interaction between BRCA1 and PALB2 is important for the homologous recombination repair. Taken together, our results indicate that BRCA1 is an upstream regulator of BRCA2 in the DNA-damage response, and PALB2 is the linker between BRCA1 and BRCA2.

A function for cyclin D1 in DNA repair uncovered by protein interactome analyses in human cancers

Cyclin D1 is a component of the core cell cycle machinery. Abnormally high levels of cyclin D1 are detected in many human cancer types. To elucidate the molecular functions of cyclin D1 in human cancers, we performed a proteomic screen for cyclin D1 protein partners in several types of human tumours. Analyses of cyclin D1 interactors revealed a network of DNA repair proteins, including RAD51, a recombinase that drives the homologous recombination process. We found that cyclin D1 directly binds RAD51, and that cyclin D1–RAD51 interaction is induced by radiation. Like RAD51, cyclin D1 is recruited to DNA damage sites in a BRCA2-dependent fashion. Reduction of cyclin D1 levels in human cancer cells impaired recruitment of RAD51 to damaged DNA, impeded the homologous recombination-mediated DNA repair, and increased sensitivity of cells to radiation in vitro and in vivo. This effect was seen in cancer cells lacking the retinoblastoma protein, which do not require D-cyclins for proliferation. These findings reveal an unexpected function of a core cell cycle protein in DNA repair and suggest that targeting cyclin D1 may be beneficial also in retinoblastoma-negative cancers which are currently thought to be unaffected by cyclin D1 inhibition.

Analysis of PALB2/FANCN-associated breast cancer families

No more than ≈30% of hereditary breast cancer has been accounted for by mutations in known genes. Most of these genes, such as BRCA1, BRCA2, TP53, CHEK2, ATM, and FANCJ/BRIP1, function in DNA repair, raising the possibility that germ line mutations in other genes that contribute to this process also predispose to breast cancer. Given its close relationship with BRCA2, PALB2 was sequenced in affected probands from 68 BRCA1/BRCA2-negative breast cancer families of Ashkenazi Jewish, French Canadian, or mixed ethnic descent. The average BRCAPRO score was 0.58. A truncating mutation (229delT) was identified in one family with a strong history of breast cancer (seven breast cancers in three female mutation carriers). This mutation and its associated breast cancers were characterized with another recently reported but unstudied mutation (2521delA) that is also associated with a strong family history of breast cancer. There was no loss of heterozygosity in tumors with either mutation. Moreover, comparative genomic hybridization analysis showed major similarities to that of BRCA2 tumors but with some notable differences, especially loss of 18q, a change that was previously unknown in BRCA2 tumors and less common in sporadic breast cancer. This study supports recent observations that PALB2 mutations are present, albeit not frequently, in breast cancer families. The apparently high penetrance noted in this study suggests that at least some PALB2 mutations are associated with a substantially increased risk for the disease.

PALB2/FANCN: recombining cancer and Fanconi anemia.

Partner and localizer of BRCA2 (PALB2) was originally identified as a BRCA2-interacting protein that is crucial for key BRCA2 genome caretaker functions. It subsequently became clear that PALB2 was another Fanconi anemia (FA) gene (FANCN), and that monoallelic PALB2 mutations are associated with increased risk of breast and pancreatic cancer. Mutations in PALB2 have been identified in breast cancer families worldwide, and recent studies have shown that PALB2 also interacts with BRCA1. Here, we summarize the molecular functions and clinical phenotypes of this key DNA repair pathway component and discuss how its discovery has advanced our knowledge of both FA and adult cancer predisposition.

Penetrance Analysis of the PALB2 c.1592delT Founder Mutation

Purpose:PALB2 is a recently identified breast cancer susceptibility gene. We have previously identified in the Finnish population a PALB2 c.1592delT founder truncation mutation that is associated with an increased risk of breast cancer. In the present study, we wanted to assess in more detail the increased risk (hazard ratio, HR) and the age-specific cumulative risk (penetrance) of c.1592delT with regard to susceptibility to breast and other forms of cancer. Experimental Design: Modified segregation analyses fitted under maximum likelihood theory were used to estimate age-specific cumulative risks and HRs using the families of mutation carriers identified from a consecutive series of breast cancer cases unselected for age at onset or family history. Results: We found a substantially increased risk of breast cancer [HR, 6.1; 95% confidence interval (95% CI), 2.2-17.2; P = 0.01] equivalent to a 40% (95% CI, 17-77) breast cancer risk by age 70 years, comparable to that for carriers of mutations in BRCA2. We found marginal evidence (P = 0.06) that the HR for breast cancer decreased with age by 4.2% per year (95% CI, 0.2-8.1), from 7.5-fold at age 30 years to 2.0-fold at age 60 years. Conclusions: Our results suggest that it may be appropriate to offer PALB2 c.1592delT mutation testing to Finnish women with breast cancer, especially those with an early age at onset or a family history of breast or related cancers, and to offer carriers the option of participation in extended disease surveillance programs.

Autophagy Opposes p53-Mediated Tumor Barrier to Facilitate Tumorigenesis in a Model of PALB2-Associated Hereditary Breast Cancer

Hereditary breast cancers stem from germline mutations in susceptibility genes such as BRCA1, BRCA2, and PALB2, whose products function in the DNA damage response and redox regulation. Autophagy is an intracellular waste disposal and stress mitigation mechanism important for alleviating oxidative stress and DNA damage response activation; it can either suppress or promote cancer, but its role in breast cancer is unknown. Here, we show that similar to Brca1 and Brca2, ablation of Palb2 in the mouse mammary gland resulted in tumor development with long latency, and the tumors harbored mutations in Trp53. Interestingly, impaired autophagy, due to monoallelic loss of the essential autophagy gene Becn1, reduced Palb2-associated mammary tumorigenesis in a Trp53-wild-type but not conditionally null background. These results indicate that, in the face of DNA damage and oxidative stress elicited by PALB2 loss, p53 is a barrier to cancer development, whereas autophagy facilitates cell survival and tumorigenesis.

Heterozygous mutations in PALB2 cause DNA replication and damage response defects

Besides mutations in BRCA1/BRCA2, heterozygous defects in PALB2 are important in breast cancer predisposition. PALB2 heterozygosity increases the risk of malignancy about sixfold. PALB2 interacts with BRCA1 and BRCA2 to regulate homologous recombination and mediate DNA damage response. Here we show, by analysing lymphoblastoid cell lines from heterozygous female PALB2 mutation carriers, that PALB2 haploinsufficiency causes aberrant DNA replication/damage response. Mutation carrier cells show increased origin firing and shorter distance between consecutive replication forks. Carrier cell lines also show elevated ATR protein, but not phosphorylation levels, and a majority of them display aberrant Chk1-/Chk2-mediated DNA damage response. Elevated chromosome instability is observed in primary blood lymphocytes of PALB2 mutation carriers, indicating that the described mechanisms of genome destabilization operate also at the organism level. These findings provide a new mechanism for early stages of breast cancer development that may also apply to other heterozygous homologous recombination signalling pathway gene mutations in hereditary cancer predisposition.