Fanconi anemia (FA) is a recessive genetic disorder caused by biallelic mutations in at least one of 22 FA genes. Beyond its pathological presentation of bone marrow failure and congenital abnormalities, FA is associated with chromosomal abnormality and genomic instability, and thus represents a genetic vulnerability for cancer predisposition. The cancer relevance of the FA pathway is further established with the pervasive occurrence of FA gene alterations in somatic cancers and observations of FA pathway activation-associated chemotherapy resistance. In this article we describe the role of the FA pathway in canonical interstrand crosslink (ICL) repair and possible contributions of FA gene alterations to cancer development. We also discuss the perspectives and potential of targeting the FA pathway for cancer intervention.

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Fanconi anemia pathway as a prospective target for cancer intervention

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Homology-directed Fanconi anemia pathway cross-link repair is dependent on DNA replication

Dysregulation of the homeostatic balance of histone H3 di- and tri-methyl lysine 27 (H3K27me2/3) levels caused by the mis-sense mutation of histone H3 (H3K27M) is reported to be associated with various types of cancers. In this study, we found that reduction in H3K27me2/3 caused by H3.1K27M, a mutation of H3 variants found in patients with diffuse intrinsic pontine glioma (DIPG), dramatically attenuated the presence of 53BP1 (also known as TP53BP1) foci and the capability of non-homologous end joining (NHEJ) in human dermal fibroblasts. H3.1K27M mutant cells showed increased rates of genomic insertions/deletions and copy number variations, as well as an increase in p53-dependent apoptosis. We further showed that both hypo-H3K27me2/3 and H3.1K27M interacted with FANCD2, a central player in the choice of DNA repair pathway. H3.1K27M triggered the accumulation of FANCD2 on chromatin, suggesting an interaction between H3.1K27M and FANCD2. Interestingly, knockdown of FANCD2 in H3.1K27M cells recovered the number of 53BP1-positive foci, NHEJ efficiency and apoptosis rate. Although these findings in HDF cells may differ from the endogenous regulation of the H3.1K27M mutant in the specific tumor context of DIPG, our results suggest a new model by which H3K27me2/3 facilitates NHEJ and the maintenance of genome stability.This article has an associated First Person interview with the first author of the paper.

Histone H3K27 methylation modulates the dynamics of FANCD2 on chromatin to facilitate NHEJ and genome stability

DNA inter-strand crosslinks (ICLs) threaten genomic stability by creating a physical barrier to DNA replication and transcription. ICLs can be caused by endogenous reactive metabolites or from chemotherapeutics. ICL repair in humans depends heavily on the Fanconi Anaemia (FA) pathway. A key signalling step of the FA pathway is the mono-ubiquitination of Fanconi Anaemia Complementation Group D2 (FANCD2), which is achieved by the multi-subunit E3 ligase complex. FANCD2 mono-ubiquitination leads to the recruitment of DNA repair proteins to the site of the ICL. The loss of FANCD2 mono-ubiquitination is a common clinical feature of FA patient cells. Therefore, molecules that restore FANCD2 mono-ubiquitination could lead to a potential drug for the management of FA. On the other hand, in some cancers, FANCD2 mono-ubiquitination has been shown to be essential for cell survival. Therefore, inhibition of FANCD2 mono-ubiquitination represents a possible therapeutic strategy for cancer specific killing. We transferred an 11-protein FANCD2 mono-ubiquitination assay to a high-throughput format. We screened 9,067 compounds for both activation and inhibition of the E3 ligase complex. The use of orthogonal assays revealed that candidate compounds acted via non-specific mechanisms. However, our high-throughput biochemical assays demonstrate the feasibility of using sophisticated and robust biochemistry to screen for small molecules that modulate a key step in the FA pathway. The future identification of FA pathway modulators is anticipated to guide future medicinal chemistry projects with drug leads for human disease.

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Methodology for the identification of small molecule inhibitors of the Fanconi Anaemia ubiquitin E3 ligase complex.

Fanconi anemia (FA) is an inherited disease characterized by bone marrow failure and increased cancer risk. FA is caused by mutation of any 1 of 22 genes, and the FA proteins function cooperatively to repair DNA interstrand cross-links (ICLs). A central step in the activation of the FA pathway is the monoubiquitination of the FANCD2 and FANCI proteins, which occurs within chromatin. How FANCD2 and FANCI are anchored to chromatin remains unknown. In this study, we identify and characterize a FANCD2 histone-binding domain (HBD) and embedded methyl-lysine-binding domain (MBD) and demonstrate binding specificity for H4K20me2. Disruption of the HBD/MBD compromises FANCD2 chromatin binding and nuclear focus formation and its ability to promote error-free DNA interstrand cross-link repair, leading to increased error-prone repair and genome instability. Our study functionally describes the first FA protein chromatin reader domain and establishes an important link between this human genetic disease and chromatin plasticity.

FANCD2 Binding to H4K20me2 via a Methyl-Binding Domain Is Essential for Efficient DNA Cross-Link Repair.

Walking a tightrope: The complex balancing act of R-loops in genome stability.

// David A. Vierra 1 , Jada L. Garzon 1 , Meghan A. Rego 2 , Morganne M. Adroved 1 , Maurizio Mauro 3 and Niall G. Howlett 1 1 Department of Cell and Molecular Biology, University of Rhode Island, Kingston, Rhode Island, U.S.A 2 Addgene, Cambridge, Massachusetts, U.S.A 3 Department of Obstetrics & Gynecology and Women’s Health, Albert Einstein College of Medicine, New York, New York, U.S.A Correspondence to: Niall G. Howlett, email: nhowlett@uri.edu Keywords: fanconi anemia, FANCD2, FANCI, monoubiquitination, histone methylation Received: January 02, 2017     Accepted: June 10, 2017     Published: July 22, 2017 ABSTRACT Fanconi anemia (FA) is a rare disease characterized by congenital defects, bone marrow failure, and atypically early-onset cancers. The FA proteins function cooperatively to repair DNA interstrand crosslinks. A major step in the activation of the pathway is the monoubiquitination of the FANCD2 and FANCI proteins, and their recruitment to chromatin-associated nuclear foci. The regulation and function of FANCD2 and FANCI, however, is poorly understood. In addition, how chromatin state impacts pathway activation is also unknown. In this study, we have examined the influence of chromatin state on the activation of the FA pathway. We describe potent activation of FANCD2 and FANCI monoubiquitination and nuclear foci formation following treatment of cells with the histone methyltransferase inhibitor BRD4770. BRD4770-induced activation of the pathway does not occur via the direct induction of DNA damage or via the inhibition of the G9a histone methyltransferase, a mechanism previously proposed for this molecule. Instead, we show that BRD4770-inducible FANCD2 and FANCI monoubiquitination and nuclear foci formation may be a consequence of inhibition of the PRC2/EZH2 chromatin-modifying complex. In addition, we show that inhibition of the class I and II histone deacetylases leads to attenuated FANCD2 and FANCI monoubiquitination and nuclear foci formation. Our studies establish that chromatin state is a major determinant of the activation of the FA pathway and suggest an important role for the PRC2/EZH2 complex in the regulation of this critical tumor suppressor pathway.

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Modulation of the Fanconi anemia pathway via chemically induced changes in chromatin structure.

SWI/SNF has been shown to have important functions in hypoxia-mediated gene expression through roles of its catalytic and core subunits. Since SWI/SNF exists as three distinct assemblies, and usage of complex specific subunits of the complex can be expected to vary within a given cell under changing environmental conditions. It remains an open question as to the compositional makeup of SWI/SNF and the roles of individual complexes in gene expression and cell viability in a hypoxic environment. In our current study, we find that hypoxia regulates levels of unique subunits that define each complex. Protein levels of ARID2 and PBRM1, members of PBAF and BRD9, a member of ncBAF, are downregulated in hypoxic cells, while members of BAF complex are retained. Our studies further show that loss of ARID1A, ARID1B and DPF2, which are unique subunits of BAF, reduces induction of HIF target genes and ARID1A or DPF2 are important for cell survival during hypoxia. Collectively, our results provide evidence that levels of SWI/SNF forms are not static within cells, but can be dynamically altered as a response to environmental changes.

Hypoxic response is driven by the BAF form of SWI/SNF

ARID1A, a subunit of SWI/SNF, has been shown to play a major role in recruitment of the chromatin remodeler to enhancers for transcriptional regulation. Mutations inARID1A have been found in various cancers, many of which form solid tumors. Recent studies have revealed vulnerabilities in cells lacking ARID1A, specifically ARID1B, an ortholog and mutually exclusive subunit, in 2D cell culture. However, identification of vulnerabilities within SWI/SNF for loss of ARID1A in a multicellular tumor spheroid, that mimic in vivo condition within tumors, has not been explored. Here we show in the absence of ARID1A in a MTS model, ARID1B continues to be a vulnerability but we have identified PBRM1 as a new vulnerability within SWI/SNF. Levels of ARID1B and PBRM1 are elevated on loss of ARID1A. Further, reduction of ARID1B and PBRM1 protein levels, decreases cell survival and reduces induction of several hypoxia regulated genes in ARID1A deficient MTSs. Our studies have identified PBRM1 as a new vulnerability in ARID1a deficient cancers and which provides a new target for therapeutic strategies.

Jada L. Garzon

Papers

Walking a tightrope: The complex balancing act of R-loops in genome stability.

FANCD2 Binding to H4K20me2 via a Methyl-Binding Domain Is Essential for Efficient DNA Cross-Link Repair.

Modulation of the Fanconi anemia pathway via chemically induced changes in chromatin structure.

Hypoxic response is driven by the BAF form of SWI/SNF

PBRM1 is a vulnerability in ARID1A deficient multicellular tumor spheroids