New Understanding of Fanconi Anemia Signaling Network upon Studying FANCD2

The Fanconi Anemia (FA) pathway is activated upon replication stress and DNA damage. With the accumulated studies, this pathway has emerged as a fundamental signaling network to defend genome stability. The Fanconi Anemia complementation group D2 protein (FANCD2) sits at the center of the pathway, orchestrating many players to prevent our genome from going awry, leading to diseases including cancer. Here, we highlight recent advances in our understanding of FA signaling, emphasizing on a rarely known form of FANCD2, FANCD2-V2.

Given accumulated studies showing much crosstalk between FA and other important DNA damage repair proteins, FA signaling pathway emerges as a significant signaling network, responding to a variety of genotoxic stresses. This network is activated upon replication stress or DNA damage. FANCM is a DNA translocase and forms a protein complex with FAAP24 (FA-associated protein 24) and others. FANCM-FAAP24 recognizes stalled replication forks due to the ICL [1,2]. Histone-fold containing kinetochore protein MHF1-MHF2 allows for the stable association of FANCM to chromatin [5]. ATR checkpoint kinase phosphorylates FANCM proceeding its recruitment to the site of damage [3]. Eight FA proteins assemble and form the FA core complex (FANCA, B, C, E, F, G, L, M) in addition to FAAP100, FAAP20, and FAAP24 to act as E3 ubiquitin ligase.
Similarly, to FANCM, FANCD2 and FANCI are phosphorylated in an ATR-dependent manner. FANCT acts as an E2 together with the FA complex E3 responsible for monoubiquitinating FANCD2 and FANCI at Lys 561 and Lys 523, respectively [15]. The monoubiquitination of FANCD2-FANCDI (ID2) complex is a key step, representing FA signaling network activation. Therefore, FANCD2 must be tightly regulated to maintain DNA repair. The ID2 complex moves to the a double strand break (DSB) and translesion synthesis (TLS) allows for continued replication of the leading strand, while the lagging strand is mediated by homologous recombination (HR) [3,5,14,[17][18][19][20]. However, it remains largely unclear as to how each individual DNA damage repair mechanism works in concert to fully repair the damage within the expanded FA signaling network, including many FA and non-FA proteins that are yet to be identified.

FA Signaling and Cancer
Therefore, if the FA pathway is impaired due to mutations in involved genes, DNA remains damaged, promoting genotoxic stress, genomic instability, and tumorigenesis [3,21,22]. For instance, unresolved ICLs cause DNA breakage and chromosomal rearrangement, leading to cancer development [4]. Mutations Recently, researchers performed whole genome sequencing on three patients presented with three up to five primary cancers.
Interestingly, the varied genes in each patient are part of the FA pathway [26]. FA gene defects are found in a variety of human cancers. Of the FA genes, FANCA has the highest mutation rate and is associated with AML, pancreatic, cervical, oral and prostate cancers [1,27]. An elevated FANCA expression determines a worse outcome for patients with chronic lymphocytic leukemia (CLL) and is due to a reduction of p53 genes, p21 and ΔNp73 [28]. Another group found FANCA amplification correlates with reduced progression-free survival in head and neck squamous cell carcinoma after radiotherapy [27]. FANCC has the next highest mutation rate and is similarly associated with pancreatic, cervical and oral cancers in addition to breast cancer [1,6,29]. Like any other FA gene, mutations in FANCD2 gene are involved in a variety of malignancies, including testicular and esophageal squamous cell carcinoma [1]. However, its importance is far beyond cancer implications, as FANCD2 sits at the center of the FA signaling network, and its activation/monoubiquitination represents the activation of FA signaling. With our recent studies on FANCD2, the previous unrecognized variant of FANCD2, FANCD2-V2 appears to be a more potent tumor suppressor than the commonly known form of FANCD2, FANCD2-V1. This review will focus on our current knowledge of FANCD2-V2. The related studies will aid in further understanding the roles that FANCD2 and/or FA signaling play in the development of human cancer and other diseases.

Overlooked FANCD2 Variant
Previously, our lab reported the first study on an alternate variant of FANCD2, named FANCD2-V2 (NCBI RefSeq accession#NM_033084.4) [30]. We refer to the long-known form as

Conclusion and Prospective
In this short review, we aimed to update our understanding of FA signaling network with a focus on the center player, FANCD2. We