Discovery, characterization and potential roles of a novel NF-YAx splice variant in human neuroblastoma

Expression and function of NF-Y subunits in cancer

NF-Y is a Transcription Factor (TF) targeting the CCAAT box regulatory element. It consists of the NF-YB/NF-YC heterodimer, each containing an Histone Fold Domain (HFD), and the sequence-specific subunit NF-YA. NF-YA expression is associated with cell proliferation and absent in some post-mitotic cells. The review summarizes recent findings impacting on cancer development. The logic of the NF-Y regulome points to pro-growth, oncogenic genes in the cell-cycle, metabolism and transcriptional regulation routes. NF-YA is involved in growth/differentiation decisions upon cell-cycle re-entry after mitosis and it is widely overexpressed in tumors, the HFD subunits in some tumor types or subtypes. Overexpression of NF-Y -mostly NF-YA- is oncogenic and decreases sensitivity to anti-neoplastic drugs. The specific roles of NF-YA and NF-YC isoforms generated by alternative splicing -AS- are discussed, including the prognostic value of their levels, although the specific molecular mechanisms of activity are still to be deciphered.

NF-YA isoforms with alternative splicing of exon-5 in Aves

NF-YA, the regulatory subunit of the trimeric CCAAT-binding transcription factor NF-Y, is present in vertebrates in two major alternative spliced isoforms: NF-YAl and NF-YAs, differing for the presence of exon-3. NF-YAx, a third isoform without exon-3/-5, was reported only in human neuronal cells and tumors. These events affect the Trans-Activation Domain. We provide here evidence for the expression of NF-YAx and for the existence of a new isoform, NF-YAg, skipping only exon-5. These isoforms are abundant in Aves, but not in reptiles, and are the prevalent transcripts in the initial phases of embryo development in chicken. Finally, we analyzed NF-YAg and NF-YAx amino acid sequence using AlphaFold: absence of exon-5 denotes a global reduction of β-stranded elements, while removal of the disordered exon-3 sequence has limited effects on TAD architecture. These data identify an expanded program of NF-YA isoforms within the TAD in Aves, implying a role during early development.

NF-YAl drives EMT in Claudinlow tumours

NF-Y is a trimeric transcription factor whose binding site -the CCAAT box- is enriched in cancer-promoting genes. The regulatory subunit, the sequence-specificity conferring NF-YA, comes in two major isoforms, NF-YA long (NF-YAl) and short (NF-YAs). Extensive expression analysis in epithelial cancers determined two features: widespread overexpression and changes in NF-YAl/NF-YAs ratios (NF-YAr) in tumours with EMT features. We performed wet and in silico experiments to explore the role of the isoforms in breast -BRCA- and gastric -STAD- cancers. We generated clones of two Claudin^low BRCA lines SUM159PT and BT549 ablated of exon-3, thus shifting expression from NF-YAl to NF-YAs. Edited clones show normal growth but reduced migratory capacities in vitro and ability to metastatize in vivo. Using TCGA, including upon deconvolution of scRNA-seq data, we formalize the clinical importance of high NF-YAr, associated to EMT genes and cell populations. We derive a novel, prognostic 158 genes signature common to BRCA and STAD Claudin^low tumours. Finally, we identify splicing factors associated to high NF-YAr, validating RBFOX2 as promoting expression of NF-YAl. These data bring three relevant results: (i) the definition and clinical implications of NF-YAr and the 158 genes signature in Claudin^low tumours; (ii) genetic evidence of 28 amino acids in NF-YAl with EMT-promoting capacity; (iii) the definition of selected splicing factors associated to NF-YA isoforms.

Phylogeny of NF-YA trans-activation splicing isoforms in vertebrate evolution

NF-Y is a trimeric pioneer Transcription Factor (TF) whose target sequence -the CCAAT box- is present in ~25% of mammalian promoters. We reconstruct the phylogenetic history of the regulatory NF-YA subunit in vertebrates. We find that in addition to the remarkable conservation of the subunits-interaction and DNA-binding parts, the Transcriptional Activation Domain (TAD) is also conserved (>90% identity among bony vertebrates). We infer the phylogeny of the alternatively spliced exon-3 and partial splicing events of exon-7 -7N and 7C- revealing independent clade-specific losses of these regions. These isoforms shape the TAD. Absence of exon-3 in basal deuterostomes, cartilaginous fishes and hagfish, but not in lampreys, suggests that the "short" isoform is primordial, with emergence of exon-3 in chordates. Exon 7N was present in the vertebrate common ancestor, while 7C is a molecular innovation of teleost fishes. RNA-seq analysis in several species confirms expression of all these isoforms. We identify 3 blocks of amino acids in the TAD shared across deuterostomes, yet structural predictions and sequence analyses suggest an evolutionary drive for maintenance of an Intrinsically Disordered Region -IDR- within the TAD. Overall, these data help reconstruct the logic for alternative splicing of this essential eukaryotic TF.

Gene utility recapitulates chromosomal aberrancies in advanced stage neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumor in children. Although only a few recurrent somatic mutations have been identified, chromosomal abnormalities, including the loss of heterozygosity (LOH) at the chromosome 1p and gains of chromosome 17q, are often seen in the high-risk cases. The biological basis and evolutionary forces that drive such genetic abnormalities remain enigmatic. Here, we conceptualize the Gene Utility Model (GUM) that seeks to identify genes driving biological signaling via their collective gene utilities and apply it to understand the impact of those differentially utilized genes on constraining the evolution of NB karyotypes. By employing a computational process-guided flow algorithm to model gene utility in protein–protein networks that built based on transcriptomic data, we conducted several pairwise comparative analyses to uncover genes with differential utilities in stage 4 NBs with distinct classification. We then constructed a utility karyotype by mapping these differentially utilized genes to their respective chromosomal loci. Intriguingly, hotspots of the utility karyotype, to certain extent, can consistently recapitulate the major chromosomal abnormalities of NBs and also provides clues to yet identified predisposition sites. Hence, our study not only provides a new look, from a gene utility perspective, into the known chromosomal abnormalities detected by integrative genomic sequencing efforts, but also offers new insights into the etiology of NB and provides a framework to facilitate the identification of novel therapeutic targets for this devastating childhood cancer.

NF-Y subunits overexpression in gastric adenocarcinomas (STAD)

NF-Y is a pioneer transcription factor-TF-formed by the Histone-like NF-YB/NF-YC subunits and the regulatory NF-YA. It binds to the CCAAT box, an element enriched in promoters of genes overexpressed in many types of cancer. NF-YA is present in two major isoforms-NF-YAs and NF-YAl-due to alternative splicing, overexpressed in epithelial tumors. Here we analyzed NF-Y expression in stomach adenocarcinomas (STAD). We completed the partitioning of all TCGA tumor samples (450) according to molecular subtypes proposed by TCGA and ACRG, using the deep learning tool DeepCC. We analyzed differentially expressed genes-DEG-for enriched pathways and TFs binding sites in promoters. CCAAT is the predominant element only in the core group of genes upregulated in all subtypes, with cell-cycle gene signatures. NF-Y subunits are overexpressed, particularly NF-YA. NF-YAs is predominant in CIN, MSI and EBV TCGA subtypes, NF-YAl is higher in GS and in the ACRG EMT subtypes. Moreover, NF-YAlhigh tumors correlate with a discrete Claudinlow cohort. Elevated NF-YB levels are protective in MSS;TP53+ patients, whereas high NF-YAl/NF-YAs ratios correlate with worse prognosis. We conclude that NF-Y isoforms are associated to clinically relevant features of gastric cancer.

Mechanisms involved in selecting and maintaining neuroblastoma cancer stem cell populations, and perspectives for therapeutic targeting

Pediatric neuroblastomas (NBs) are heterogeneous, aggressive, therapy-resistant embryonal tumours that originate from cells of neural crest (NC) origin and in particular neuroblasts committed to the sympathoadrenal progenitor cell lineage. Therapeutic resistance, post-therapeutic relapse and subsequent metastatic NB progression are driven primarily by cancer stem cell (CSC)-like subpopulations, which through their self-renewing capacity, intermittent and slow cell cycles, drug-resistant and reversibly adaptive plastic phenotypes, represent the most important obstacle to improving therapeutic outcomes in unfavourable NBs. In this review, dedicated to NB CSCs and the prospects for their therapeutic eradication, we initiate with brief descriptions of the unique transient vertebrate embryonic NC structure and salient molecular protagonists involved NC induction, specification, epithelial to mesenchymal transition and migratory behaviour, in order to familiarise the reader with the embryonic cellular and molecular origins and background to NB. We follow this by introducing NB and the potential NC-derived stem/progenitor cell origins of NBs, before providing a comprehensive review of the salient molecules, signalling pathways, mechanisms, tumour microenvironmental and therapeutic conditions involved in promoting, selecting and maintaining NB CSC subpopulations, and that underpin their therapy-resistant, self-renewing metastatic behaviour. Finally, we review potential therapeutic strategies and future prospects for targeting and eradication of these bastions of NB therapeutic resistance, post-therapeutic relapse and metastatic progression.

NF-Y Overexpression in Liver Hepatocellular Carcinoma (HCC)

NF-Y is a pioneer trimeric transcription factor formed by the Histone Fold Domain (HFD) NF-YB/NF-YC subunits and NF-YA. Three subunits are required for DNA binding. CCAAT-specificity resides in NF-YA and transactivation resides in Q-rich domains of NF-YA and NF-YC. They are involved in alternative splicing (AS). We recently showed that NF-YA is overexpressed in breast and lung carcinomas. We report here on the overexpression of all subunits in the liver hepatocellular carcinoma (HCC) TCGA database, specifically the short NF-YAs and NF-YC2 (37 kDa) isoforms. This is observed at all tumor stages, in viral-infected samples and independently from the inflammatory status. Up-regulation of NF-YAs and NF-YC, but not NF-YB, is associated to tumors with mutant p53. We used a deep-learning-based method (DeepCC) to extend the partitioning of the three molecular clusters to all HCC TCGA tumors. In iCluster3, CCAAT is a primary matrix found in promoters of up-regulated genes, and cell-cycle pathways are enriched. Finally, clinical data indicate that, globally, only NF-YAs, but not HFD subunits, correlate with the worst prognosis; in iCluster1 patients, however, all subunits correlate. The data show a difference with other epithelial cancers, in that global overexpression of the three subunits is reported and clinically relevant in a subset of patients; yet, they further reinstate the regulatory role of the sequence-specific subunit.

Hypoxia-induced alternative splicing: the 11th Hallmark of Cancer

Hypoxia-induced alternative splicing is a potent driving force in tumour pathogenesis and progression. In this review, we update currents concepts of hypoxia-induced alternative splicing and how it influences tumour biology. Following brief descriptions of tumour-associated hypoxia and the pre-mRNA splicing process, we review the many ways hypoxia regulates alternative splicing and how hypoxia-induced alternative splicing impacts each individual hallmark of cancer. Hypoxia-induced alternative splicing integrates chemical and cellular tumour microenvironments, underpins continuous adaptation of the tumour cellular microenvironment responsible for metastatic progression and plays clear roles in oncogene activation and autonomous tumour growth, tumor suppressor inactivation, tumour cell immortalization, angiogenesis, tumour cell evasion of programmed cell death and the anti-tumour immune response, a tumour-promoting inflammatory response, adaptive metabolic re-programming, epithelial to mesenchymal transition, invasion and genetic instability, all of which combine to promote metastatic disease. The impressive number of hypoxia-induced alternative spliced protein isoforms that characterize tumour progression, classifies hypoxia-induced alternative splicing as the 11th hallmark of cancer, and offers a fertile source of potential diagnostic/prognostic markers and therapeutic targets.

The Switch from NF-YAl to NF-YAs Isoform Impairs Myotubes Formation

NF-YA, the regulatory subunit of the trimeric transcription factor (TF) NF-Y, is regulated by alternative splicing (AS) generating two major isoforms, “long” (NF-YAl) and “short” (NF-YAs). Muscle cells express NF-YAl. We ablated exon 3 in mouse C2C12 cells by a four-guide CRISPR/Cas9n strategy, obtaining clones expressing exclusively NF-YAs (C2-YAl-KO). C2-YAl-KO cells grow normally, but are unable to differentiate. Myogenin and—to a lesser extent, MyoD— levels are substantially lower in C2-YAl-KO, before and after differentiation. Expression of the fusogenic Myomaker and Myomixer genes, crucial for the early phases of the process, is not induced. Myomaker and Myomixer promoters are bound by MyoD and Myogenin, and Myogenin overexpression induces their expression in C2-YAl-KO. NF-Y inactivation reduces MyoD and Myogenin, but not directly: the Myogenin promoter is CCAAT-less, and the canonical CCAAT of the MyoD promoter is not bound by NF-Y in vivo. We propose that NF-YAl, but not NF-YAs, maintains muscle commitment by indirectly regulating Myogenin and MyoD expression in C2C12 cells. These experiments are the first genetic evidence that the two NF-YA isoforms have functionally distinct roles.