Advances in the understanding of the Fanconi anemia tumor suppressor pathway

Research progress on the fanconi anemia signaling pathway in non-obstructive azoospermia

Non-obstructive azoospermia (NOA) is a disease characterized by spermatogenesis failure and comprises phenotypes such as hypospermatogenesis, mature arrest, and Sertoli cell-only syndrome. Studies have shown that FA cross-linked anemia (FA) pathway is closely related to the occurrence of NOA. There are FA gene mutations in male NOA patients, which cause significant damage to male germ cells. The FA pathway is activated in the presence of DNA interstrand cross-links; the key step in activating this pathway is the mono-ubiquitination of the FANCD2-FANCI complex, and the activation of the FA pathway can repair DNA damage such as DNA double-strand breaks. Therefore, we believe that the FA pathway affects germ cells during DNA damage repair, resulting in minimal or even disappearance of mature sperm in males. This review summarizes the regulatory mechanisms of FA-related genes in male azoospermia, with the aim of providing a theoretical reference for clinical research and exploration of related genes.

Focal Point of Fanconi Anemia Signaling

Among human genetic diseases, Fanconi Anemia (FA) tops all with its largest number of health complications in nearly all human organ systems, suggesting the significant roles played by FA genes in the maintenance of human health. With the accumulated research on FA, the encoded protein products by FA genes have been building up to the biggest cell defense signaling network, composed of not only 22+ FA proteins but also ATM, ATR, and many other non-FA proteins. The FA D2 group protein (FANCD2) and its paralog form the focal point of FA signaling to converge the effects of its upstream players in response to a variety of cellular insults and simultaneously with downstream players to protect humans from contracting diseases, including aging and cancer. In this review, we update and discuss how the FA signaling crucially eases cellular stresses through understanding its focal point.

A new look at molecular biology of breast cancer

In the past 25 years, incidence rates of breast cancer have risen about 30% in westernized countries. Mutations in BRCA1 and BRCA2 are the most prominent cause of breast cancer. However, these cancer susceptibility genes (BRCAs) only account for a few percent of women suffering breast tumor. With our understanding that BRCAs are Fanconi Anemia (FA) genes, investigations into the FA signaling network should provide a previously unrecognized key to unlock in-depth insights into both etiology and treatment of breast cancer. Here, we discuss utilization of the FA signaling as a unique genetic model system to expand our knowledge about the molecular biology of breast cancer and potential applications of the gained knowledge to enable preventive and therapeutic approaches for breast cancer patient care.

BLM promotes the activation of Fanconi Anemia signaling pathway

Mutations in the human RecQ helicase, BLM, causes Bloom Syndrome, which is a rare autosomal recessive disorder and characterized by genomic instability and an increased risk of cancer. Fanconi Anemia (FA), resulting from mutations in any of the 19 known FA genes and those yet to be known, is also characterized by chromosomal instability and a high incidence of cancer. BLM helicase and FA proteins, therefore, may work in a common tumor-suppressor signaling pathway. To date, it remains largely unclear as to how BLM and FA proteins work concurrently in the maintenance of genome stability. Here we report that BLM is involved in the early activation of FA group D2 protein (FANCD2). We found that FANCD2 activation is substantially delayed and attenuated in crosslinking agent-treated cells harboring deficient Blm compared to similarly treated control cells with sufficient BLM. We also identified that the domain VI of BLM plays an essential role in promoting FANCD2 activation in cells treated with DNA crosslinking agents, especially ultraviolet B. The similar biological effects performed by ΔVI-BLM and inactivated FANCD2 further confirm the relationship between BLM and FANCD2. Mutations within the domain VI of BLM detected in human cancer samples demonstrate the functional importance of this domain, suggesting human tumorigenicity resulting from mtBLM may be at least partly attributed to mitigated FANCD2 activation. Collectively, our data show a previously unknown regulatory liaison in advancing our understanding of how the cancer susceptibility gene products act in concert to maintain genome stability.

Overlooked FANCD2 variant encodes a promising, portent tumor suppressor, and alternative polyadenylation contributes to its expression

Fanconi Anemia (FA) complementation group D2 protein (FANCD2) is the center of the FA tumor suppressor pathway, which has become an important field of investigation in human aging and cancer. Here we report an overlooked central player in the FA pathway, FANCD2 variant 2 (FANCD2-V2), which appears to perform more potent tumor suppressor-function compared to the known variant of FANCD2, namely, FANCD2-V1. Detailed analysis of the FANCD2 gene structure indicated a proximal and distal polyadenylation site (PAS), associated with V2 and V1 transcripts accordingly. RNA polymerase II Chromatin immunoprecipitation (ChIP) targeting the two PAS-regions determined lesser binding of RNA pol II to DNA fragments in the distal PAS region in non-malignant cells compared to malignant cells. Conversely, the opposite occurred in the proximal PAS region. Moreover, RNA immunoprecipitation (RIP) identified that U2 snRNP, a major component of RNA splicing complex that interacts with the 3′end of an intron, showed greater binding to the last intron of the FANCD2-V1 transcript in malignant cells compared to the non-malignant cells. Importantly, our data showed that in human tissue samples, the ratio of V2 /V1 expression in lung, bladder, or ovarian cancer correlates inversely with the tumor stages/grades. Therefore, these findings provide a previously unrecognized central player FANCD2-V2 and thus novel insights into human tumorigenesis, and indicate that V2/V1 can act as an effective biomarker in assisting the recognition of tumor malignance.

FANCD2 and DNA Damage

Investigators have dedicated considerable effort to understanding the molecular basis underlying Fanconi Anemia (FA), a rare human genetic disease featuring an extremely high incidence of cancer and many congenital defects. Among those studies, FA group D2 protein (FANCD2) has emerged as the focal point of FA signaling and plays crucial roles in multiple aspects of cellular life, especially in the cellular responses to DNA damage. Here, we discuss the recent and relevant studies to provide an updated review on the roles of FANCD2 in the DNA damage response.

Involvement of FANCD2 in Energy Metabolism via ATP5α

Growing evidence supports a general hypothesis that aging and cancer are diseases related to energy metabolism. However, the involvement of Fanconi Anemia (FA) signaling, a unique genetic model system for studying human aging or cancer, in energy metabolism remains elusive. Here, we report that FA complementation group D2 protein (FANCD2) functionally impacts mitochondrial ATP production through its interaction with ATP5α, whereas this relationship was not observed in the mutant FANCD2 (K561R)-carrying cells. Moreover, while ATP5α is present within the mitochondria in wild-type cells, it is instead located mostly outside in cells that carry the non-monoubiquitinated FANCD2. In addition, mitochondrial ATP production is significantly reduced in these cells, compared to those cells carrying wtFANCD2. We identified one region (AA42-72) of ATP5α, contributing to the interaction between ATP5α and FANCD2, which was confirmed by protein docking analysis. Further, we demonstrated that mtATP5α (∆AA42-72) showed an aberrant localization, and resulted in a decreased ATP production, similar to what was observed in non-monoubiquitinated FANCD2-carrying cells. Collectively, our study demonstrates a novel role of FANCD2 in governing cellular ATP production, and advances our understanding of how defective FA signaling contributes to aging and cancer at the energy metabolism level.

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.

Advances in the understanding of Fanconi Anemia Complementation Group D2 Protein (FANCD2) in human cancer

Fanconi anemia (FA) is a rare human genetic disease, resulting from dysfunction in any of 17 known complementation proteins: FANC-A, B, C, D1, D2, E, F, G, I, J, L, M, N, O, P, Q & S, and other unknowns. Besides the severe bone marrow failure, an extremely high incidence of cancer as well as many other clinic symptoms associated with FA patients, FA cells are known of insufficiency in homologous recombination, DNA mismatch repair, nucleotide excision repair, translesion DNA synthesis, and other molecular defects, leading to genome instability. Those similar molecular and cellular/tissue features show that all FA proteins function in one common signaling pathway, namely, the FA pathway. The monoubiquitination of FANCD2 is the central step of the FA pathway activation upon DNA damage or during DNA replication. The molecular functions of FANCD2 emerge as a very attractive filed of investigation in cancer research. Herein, we review the recent progresses in FANCD2 functions at these rapidly progressed aspects.

Risk of Neoplasia in Pediatric Patients Receiving Growth Hormone Therapy--A Report From the Pediatric Endocrine Society Drug and Therapeutics Committee

CONTEXT

GH and IGF-1 have been shown to affect tumor growth in vitro and in some animal models. This report summarizes the available evidence on whether GH therapy in childhood is associated with an increased risk of neoplasia during treatment or after treatment is completed.

EVIDENCE ACQUISITION

A PubMed search conducted through February 2014 retrieved original articles written in English addressing GH therapy and neoplasia risk. Subsequent searches were done to include additional relevant publications.

EVIDENCE SYNTHESIS

In children without prior cancer or known risk factors for developing cancer, the clinical evidence does not affirm an association between GH therapy during childhood and neoplasia. GH therapy has not been reported to increase the risk for neoplasia in this population, although most of these data are derived from postmarketing surveillance studies lacking rigorous controls. In patients who are at higher risk for developing cancer, current evidence is insufficient to conclude whether or not GH further increases cancer risk. GH treatment of pediatric cancer survivors does not appear to increase the risk of recurrence but may increase their risk for subsequent primary neoplasms.

CONCLUSIONS

In children without known risk factors for malignancy, GH therapy can be safely administered without concerns about an increased risk for neoplasia. GH use in children with medical diagnoses predisposing them to the development of malignancies should be critically analyzed on an individual basis, and if chosen, appropriate surveillance for malignancies should be undertaken. GH can be used to treat GH-deficient childhood cancer survivors who are in remission with the understanding that GH therapy may increase their risk for second neoplasms.

A novel role for non-ubiquitinated FANCD2 in response to hydroxyurea-induced DNA damage

Fanconi anemia (FA) is a genetic disease of bone marrow failure, cancer susceptibility, and sensitivity to DNA crosslinking agents. FANCD2, the central protein of the FA pathway, is monoubiquitinated upon DNA damage, such as crosslinkers and replication blockers such as hydroxyurea (HU). Even though FA cells demonstrate unequivocal sensitivity to crosslinkers, such as mitomycin C (MMC), we find that they are largely resistant to HU, except for cells absent for expression of FANCD2. FANCD2, RAD51 and RAD18 form a complex, which is enhanced upon HU exposure. Surprisingly, although FANCD2 is required for this enhanced interaction, its monoubiquitination is not. Similarly, non-ubiquitinated FANCD2 can still support proliferation cell nuclear antigen (PCNA) monoubiquitination. RAD51, but not BRCA2, is also required for PCNA monoubiquitination in response to HU, suggesting that this function is independent of homologous recombination (HR). We further show that translesion (TLS) polymerase PolH chromatin localization is decreased in FANCD2 deficient cells, FANCD2 siRNA knockdown cells and RAD51 siRNA knockdown cells, and PolH knockdown results in HU sensitivity only. Our data suggest that FANCD2 and RAD51 have an important role in PCNA monoubiquitination and TLS in a FANCD2 monoubiquitination and HR-independent manner in response to HU. This effect is not observed with MMC treatment, suggesting a non-canonical function for the FA pathway in response to different types of DNA damage.

Association of human papillomavirus with Fanconi anemia promotes carcinogenesis in Fanconi anemia patients

Fanconi anemia (FA) is a rare recessive disorder associated with chromosomal fragility. FA patients are at very high risk of cancers, especially head and neck squamous cell carcinomas and squamous cell carcinomas caused by infection of human papillomaviruses (HPVs). By integrating into the host genome, HPV oncogenes E6 and E7 drive the genomic instability to promote DNA damage and gene mutations necessary for carcinogenesis in FA patients. Furthermore, E6 and E7 oncoproteins not only inhibit p53 and retinoblastoma but also impair the FANC/BRCA signaling pathway to prevent DNA damage repair and alter multiple signals including cell-cycle checkpoints, telomere function, cell proliferation, and interference of the host immune system leading to cancer development in FA patients. In this review, we summarize recent advances in unraveling the molecular mechanisms of FA susceptibility to HPV-induced cancers, which facilitate rational preventive and therapeutic strategies.

Fanconi anemia gene editing by the CRISPR/Cas9 system

Genome engineering with designer nucleases is a rapidly progressing field, and the ability to correct human gene mutations in situ is highly desirable. We employed fibroblasts derived from a patient with Fanconi anemia as a model to test the ability of the clustered regularly interspaced short palindromic repeats/Cas9 nuclease system to mediate gene correction. We show that the Cas9 nuclease and nickase each resulted in gene correction, but the nickase, because of its ability to preferentially mediate homology-directed repair, resulted in a higher frequency of corrected clonal isolates. To assess the off-target effects, we used both a predictive software platform to identify intragenic sequences of homology as well as a genome-wide screen utilizing linear amplification-mediated PCR. We observed no off-target activity and show RNA-guided endonuclease candidate sites that do not possess low sequence complexity function in a highly specific manner. Collectively, we provide proof of principle for precision genome editing in Fanconi anemia, a DNA repair-deficient human disorder.

Basal level of FANCD2 monoubiquitination is required for the maintenance of a sufficient number of licensed-replication origins to fire at a normal rate

Normal DNA replication starts following the stepwise recruitment of replication initiators to assemble Mini-chromosome Maintenance (MCM) 2-7 protein complexes at an adequate amount of DNA replication origins. Under normal conditions, the monoubiquitination of Fanconi Anemia (FA) group D2 protein (FANCD2) occurs in each S-phase of cell cycle, which is the basal level of FANCD2 monoubiquitination. However, little is known regarding the roles of this basal level of monoubiquitinated FANCD2. Here we show that monoubiquitinated FANCD2 in each S-phase of normal cell cycle is essential for replication origins to fire at a normal rate. We found that the basal level of the monoubiquitinated FANCD2 can interact with replication origins as well as mini-chromosome maintenance protein 3 (MCM3) in an S-phase specific manner to secure an enough number of the licensed-origins to fire. Non-monoubiquitinated FANCD2 or mutant MCM3 lacking AA 477-480 responsible for interacting with FANCD2 can lead to an insufficient amount of licensed origins to fire and, thereby, enlarged intervals between the fired origins. Our results demonstrate that the monoubiquitinated FANCD2 in each S-phase of normal cell cycle is required to maintain an enough number of licensed origins to initiate the normal DNA replication. This finding is the first to provide insights into how FANCD2 functions under normal condition of cell cycle to maintain genome stability, as well as resulting implications in the strategic improvement for the fight against human cancer.

Molecular and cellular functions of the FANCJ DNA helicase defective in cancer and in Fanconi anemia

The FANCJ DNA helicase is mutated in hereditary breast and ovarian cancer as well as the progressive bone marrow failure disorder Fanconi anemia (FA). FANCJ is linked to cancer suppression and DNA double strand break repair through its direct interaction with the hereditary breast cancer associated gene product, BRCA1. FANCJ also operates in the FA pathway of interstrand cross-link repair and contributes to homologous recombination. FANCJ collaborates with a number of DNA metabolizing proteins implicated in DNA damage detection and repair, and plays an important role in cell cycle checkpoint control. In addition to its role in the classical FA pathway, FANCJ is believed to have other functions that are centered on alleviating replication stress. FANCJ resolves G-quadruplex (G4) DNA structures that are known to affect cellular replication and transcription, and potentially play a role in the preservation and functionality of chromosomal structures such as telomeres. Recent studies suggest that FANCJ helps to maintain chromatin structure and preserve epigenetic stability by facilitating smooth progression of the replication fork when it encounters DNA damage or an alternate DNA structure such as a G4. Ongoing studies suggest a prominent but still not well-understood role of FANCJ in transcriptional regulation, chromosomal structure and function, and DNA damage repair to maintain genomic stability. This review will synthesize our current understanding of the molecular and cellular functions of FANCJ that are critical for chromosomal integrity.