Fanconi anemia (FA) and crosslinker sensitivity: Re-appraising the origins of FA definition

Moonlighting functions of the ubiquitin-like protein, Hub1/UBL-5

The faithful splicing of pre-mRNA is critical for accurate gene expression. Dysregulation of pre-mRNA splicing has been associated with several human diseases including cancer. The ubiquitin-like protein Hub1/UBL5 binds to the substrates non-covalently and promotes pre-mRNA splicing. Additionally, UBL5 promotes the common fragile sites stability and the Fanconi anemia pathway of DNA damage repair. These functions strongly suggests that UBL5 could potentially be implicated in cancer. Therefore, we analyzed the UBL5 expression in TCGA tumor sample datasets and observed the differences between tumor and normal tissues among different tumor subtypes. We have noticed the alteration frequency of UBL5 in TCGA tumor samples. Altogether, this review summarizes the UBL5 functions and discusses its putative role in tumorigenesis.

Fanconi anemia: current insights regarding epidemiology, cancer, and DNA repair

Fanconi anemia is a genetic disorder that is characterized by bone marrow failure, as well as a predisposition to malignancies including leukemia and squamous cell carcinoma (SCC). At least 22 genes are associated with Fanconi anemia, constituting the Fanconi anemia DNA repair pathway. This pathway coordinates multiple processes and proteins to facilitate the repair of DNA adducts including interstrand crosslinks (ICLs) that are generated by environmental carcinogens, chemotherapeutic crosslinkers, and metabolic products of alcohol. ICLs can interfere with DNA transactions, including replication and transcription. If not properly removed and repaired, ICLs cause DNA breaks and lead to genomic instability, a hallmark of cancer. In this review, we will discuss the genetic and phenotypic characteristics of Fanconi anemia, the epidemiology of the disease, and associated cancer risk. The sources of ICLs and the role of ICL-inducing chemotherapeutic agents will also be discussed. Finally, we will review the detailed mechanisms of ICL repair via the Fanconi anemia DNA repair pathway, highlighting critical regulatory processes. Together, the information in this review will underscore important contributions to Fanconi anemia research in the past two decades.

FANCD2 modulates the mitochondrial stress response to prevent common fragile site instability

Common fragile sites (CFSs) are genomic regions frequently involved in cancer-associated rearrangements. Most CFSs lie within large genes, and their instability involves transcription- and replication-dependent mechanisms. Here, we uncover a role for the mitochondrial stress response pathway in the regulation of CFS stability in human cells. We show that FANCD2, a master regulator of CFS stability, dampens the activation of the mitochondrial stress response and prevents mitochondrial dysfunction. Genetic or pharmacological activation of mitochondrial stress signaling induces CFS gene expression and concomitant relocalization to CFSs of FANCD2. FANCD2 attenuates CFS gene transcription and promotes CFS gene stability. Mechanistically, we demonstrate that the mitochondrial stress-dependent induction of CFS genes is mediated by ubiquitin-like protein 5 (UBL5), and that a UBL5-FANCD2 dependent axis regulates the mitochondrial UPR in human cells. We propose that FANCD2 coordinates nuclear and mitochondrial activities to prevent genome instability.

Re-definition and supporting evidence toward Fanconi Anemia as a mitochondrial disease: Prospects for new design in clinical management

Fanconi anemia (FA) has been investigated since early studies based on two definitions, namely defective DNA repair and proinflammatory condition. The former definition has built up the grounds for FA diagnosis as excess sensitivity of patients’ cells to xenobiotics as diepoxybutane and mitomycin C, resulting in typical chromosomal abnormalities. Another line of studies has related FA phenotype to a prooxidant state, as detected by both in vitro and ex vivo studies. The discovery that the FA group G (FANCG) protein is found in mitochondria (Mukhopadhyay et al., 2006) has been followed by an extensive line of studies providing evidence for multiple links between other FA gene products and mitochondrial dysfunction. The fact that FA proteins are encoded by nuclear, not mitochondrial DNA does not prevent these proteins to hamper mitochondrial function, as it is recognized that most mitochondrial proteins are of nuclear origin. This body of evidence supporting a central role of mitochondrial dysfunction, along with redox imbalance in FA, should lead to the re-definition of FA as a mitochondrial disease. A body of literature has demonstrated the beneficial effects of mitochondrial cofactors, such as α-lipoic acid, coenzyme Q10, and carnitine on patients affected by mitochondrial diseases. Altogether, this re-definition of FA as a mitochondrial disease and the prospect use of mitochondrial nutrients may open new gateways toward mitoprotective strategies for FA patients. These strategies are expected to mitigate the mitochondrial dysfunction and prooxidant state in FA patients, and potentially protect transplanted FA patients from post-transplantation malignancies.

Celastrol-induced degradation of FANCD2 sensitizes pediatric high-grade gliomas to the DNA-crosslinking agent carboplatin

Background

Pediatric high-grade gliomas (pHGG) are the leading cause of cancer-related death during childhood. Due to their diffuse growth characteristics, chemoresistance and location behind the blood-brain barrier (BBB), the prognosis of pHGG has barely improved in the past decades. As such, there is a dire need for new therapies that circumvent those difficulties. Since aberrant expression of DNA damage-response associated Fanconi anemia proteins play a central role in the onset and therapy resistance of many cancers, we here investigated if FANCD2 depletion could sensitize pHGG to additional DNA damage.

Methods

We determined the capacity of celastrol, a BBB-penetrable compound that degrades FANCD2, to sensitize glioma cells to the archetypical DNA-crosslinking agent carboplatin in vitro in seven patient-derived pHGG models. In addition, we tested this drug combination in vivo in a patient-derived orthotopic pHGG xenograft model. Underlying mechanisms to drug response were investigated using mRNA expression profiling, western blotting, immunofluorescence, FANCD2 knockdown and DNA fiber assays.

Findings

FANCD2 is overexpressed in HGGs and depletion of FANCD2 by celastrol synergises with carboplatin to induce cytotoxicity. Combination therapy prolongs survival of pHGG-bearing mice over monotherapy and control groups in vivo (P<0.05). In addition, our results suggest that celastrol treatment stalls ongoing replication forks, causing sensitivity to DNA-crosslinking in FANCD2-dependent glioma cells.

Interpretation

Our results show that depletion of FANCD2 acts as a chemo-sensitizing strategy in pHGG. Combination therapy using celastrol and carboplatin might serve as a clinically relevant strategy for the treatment of pHGG.

Funding

This study was funded by a grant from the Children Cancer-Free Foundation (KIKA, project 210). The disclosed funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Fanconi anemia protein FANCI functions in ribosome biogenesis

Fanconi anemia (FA) is a disease of DNA repair characterized by bone marrow failure and a reduced ability to remove DNA interstrand cross-links. Here, we provide evidence that the FA protein FANCI also functions in ribosome biogenesis, the process of making ribosomes that initiates in the nucleolus. We show that FANCI localizes to the nucleolus and is functionally and physically tied to the transcription of pre-ribosomal RNA (pre-rRNA) and to large ribosomal subunit (LSU) pre-rRNA processing independent of FANCD2. While FANCI is known to be monoubiquitinated when activated for DNA repair, we find that it is predominantly in the deubiquitinated state in the nucleolus, requiring the nucleoplasmic deubiquitinase (DUB) USP1 and the nucleolar DUB USP36. Our model suggests a possible dual pathophysiology for FA that includes defects in DNA repair and in ribosome biogenesis.

Maintenance of genome stability by Fanconi anemia proteins

Persistent dysregulation of the DNA damage response and repair in cells causes genomic instability. The resulting genetic changes permit alterations in growth and proliferation observed in virtually all cancers. However, an unstable genome can serve as a double-edged sword by providing survival advantages in the ability to evade checkpoint signaling, but also creating vulnerabilities through dependency on alternative genomic maintenance factors. The Fanconi anemia pathway comprises an intricate network of DNA damage signaling and repair that are critical for protection against genomic instability. The importance of this pathway is underlined by the severity of the cancer predisposing syndrome Fanconi anemia which can be caused by biallelic mutations in any one of the 21 genes known thus far. This review delineates the roles of the Fanconi anemia pathway and the molecular actions of Fanconi anemia proteins in confronting replicative, oxidative, and mitotic stress.

Genomic amplification of Fanconi anemia complementation group A (FancA) in head and neck squamous cell carcinoma (HNSCC): Cellular mechanisms of radioresistance and clinical relevance

Radio (chemo) therapy is a crucial treatment modality for head and neck squamous cell carcinoma (HNSCC), but relapse is frequent, and the underlying mechanisms remain largely elusive. Therefore, novel biomarkers are urgently needed. Previously, we identified gains on 16q23-24 to be associated with amplification of the Fanconi anemia A (FancA) gene and to correlate with reduced progression-free survival after radiotherapy. Here, we analyzed the effects of FancA on radiation sensitivity in vitro, characterized the underlying mechanisms, and evaluated their clinical relevance. Silencing of FancA expression in HNSCC cell lines with genomic gains on 16q23-24 resulted in significantly impaired clonogenic survival upon irradiation. Conversely, overexpression of FancA in immortalized keratinocytes conferred increased survival accompanied by improved DNA repair, reduced accumulation of chromosomal translocations, but no hyperactivation of the FA/BRCA-pathway. Downregulation of interferon signaling as identified by microarray analyses, enforced irradiation-induced senescence, and elevated production of the senescence-associated secretory phenotype (SASP) appeared to be candidate mechanisms contributing to FancA-mediated radioresistance. Data of the TCGA HNSCC cohort confirmed the association of gains on 16q24.3 with FancA overexpression and impaired overall survival. Importantly, transcriptomic alterations similar to those observed upon FancA overexpression in vitro strengthened the clinical relevance. Overall, FancA amplification and overexpression appear to be crucial for radiotherapeutic failure in HNSCC.

Mutated Fanconi anemia pathway in non-Fanconi anemia cancers

An extremely high cancer incidence and the hypersensitivity to DNA crosslinking agents associated with Fanconi Anemia (FA) have marked it to be a unique genetic model system to study human cancer etiology and treatment, which has emerged an intense area of investigation in cancer research. However, there is limited information about the relationship between the mutated FA pathway and the cancer development or/and treatment in patients without FA. Here we analyzed the mutation rates of the seventeen FA genes in 68 DNA sequence datasets. We found that the FA pathway is frequently mutated across a variety of human cancers, with a rate mostly in the range of 15 to 35 % in human lung, brain, bladder, ovarian, breast cancers, or others. Furthermore, we found a statistically significant correlation (p < 0.05) between the mutated FA pathway and the development of human bladder cancer that we only further analyzed. Together, our study demonstrates a previously unknown fact that the mutated FA pathway frequently occurs during the development of non-FA human cancers, holding profound implications directly in advancing our understanding of human tumorigenesis as well as tumor sensitivity/resistance to crosslinking drug-relevant chemotherapy.