APOBEC3B Expression Research Articles

Differentiation signals induce APOBEC3A expression via GRHL3 in squamous epithelia and squamous cell carcinoma.

Two APOBEC DNA cytosine deaminase enzymes, APOBEC3A and APOBEC3B, generate somatic mutations in cancer, thereby driving tumour development and drug resistance. Here, we used single-cell RNA sequencing to study APOBEC3A and APOBEC3B expression in healthy and malignant mucosal epithelia, validating key observations with immunohistochemistry, spatial transcriptomics and functional experiments. Whereas APOBEC3B is expressed in keratinocytes entering mitosis, we show that APOBEC3A expression is confined largely to terminally differentiating cells and requires grainyhead-like transcription factor 3 (GRHL3). Thus, in normal tissue, neither deaminase appears to be expressed at high levels during DNA replication, the cell-cycle stage associated with APOBEC-mediated mutagenesis. In contrast, in squamous cell carcinoma we find that, there is expansion of GRHL3expression and activity to a subset of cells undergoing DNA replication and concomitant extension of APOBEC3A expression to proliferating cells. These findings suggest that APOBEC3A may play a functional role during keratinocyte differentiation, and offer a mechanism for acquisition of APOBEC3A mutagenic activity in tumours.

APOBEC3B expression in 293T viral producer cells drives mutations in chimeric antigen receptors and reduces CAR T cell efficacy

Chimeric antigen receptor (CAR) Tcells are a clinically approved therapy for blood cancers. To produce clinical-grade CAR Tcells, a retroviral or lentiviral vector is used to deliver the CAR and associated genes to patient Tcells. Apolipoprotein B editing enzyme, catalytic polypeptide 3 (APOBEC3) enzymes are known to be upregulated after transfection and retroviral infection and to deaminate cytidine to uracil in nucleic acids, resulting in cytidine-to-thymine mutations in DNA. Here, we hypothesized that APOBEC3 enzymes, induced during the production of CAR Tcells, impact the efficacy of the resulting CAR Tcells. We demonstrated that APOBEC3 family member APOBEC3B was upregulated at the RNA and protein levels after transfection of HEK293T cells with plasmids to make lentivirus, and that APOBEC3 signature mutations were present in the CAR construct. APOBEC3B overexpression in HEK293T cells led to further mutations in the resulting CAR Tcells, and significantly decreased CAR Tcell killing. APOBEC3B knockout in HEK293T cells led to reduced mutations in the CAR construct and significantly increased in CAR Tcell killing. These results suggest that generation of CAR-expressing viruses from producer cell lines deficient in genome-modifying proteins such as APOBEC3B could enhance the quality of CAR Tcell production.

Regulation, functional impact, and therapeutic targeting of APOBEC3A in cancer

Enzymes of the apolipoprotein B mRNA editing catalytic polypeptide like (APOBEC) family are cytosine deaminases that convert cytosine to uracil in DNA and RNA. Among these proteins, APOBEC3 sub-family members, APOBEC3A (A3A) and APOBEC3B (A3B), are prominent sources of mutagenesis in cancer cells. The aberrant expression of A3A and A3B in cancer cells leads to accumulation of mutations with specific single-base substitution (SBS) signatures, characterized by C→T and C→G changes, in a number of tumor types. In addition to fueling mutagenesis, A3A and A3B, particularly A3A, induce DNA replication stress, DNA damage, and chromosomal instability through their catalytic activities, triggering a range of cellular responses. Thus, A3A/B have emerged as key drivers of genome evolution during cancer development, contributing to tumorigenesis, tumor heterogeneity, and therapeutic resistance. Yet, the expression of A3A/B in cancer cells presents a cancer vulnerability that can be exploited therapeutically. In this review, we discuss the recent studies that shed light on the mechanisms regulating A3A expression and the impact of A3A in cancer. We also review recent advances in the development of A3A inhibitors and provide perspectives on the future directions of A3A research.

APOBEC3-related mutations in the spike protein-encoding region facilitate SARS-CoV-2 evolution

SARS-CoV-2 evolves gradually to cause COVID-19 epidemic. One of driving forces of SARS-CoV-2 evolution might be activation of apolipoprotein B mRNA editing catalytic subunit-like protein 3 (APOBEC3) by inflammatory factors. Here, we aimed to elucidate the effect of the APOBEC3-related viral mutations on the infectivity and immune evasion of SARS-CoV-2. The APOBEC3-related C > U mutations ranked as the second most common mutation types in the SARS-CoV-2 genome. mRNA expression of APOBEC3A (A3A), APOBEC3B (A3B), and APOBEC3G (A3G) in peripheral blood cells increased with disease severity. A3B, a critical member of the APOBEC3 family, was significantly upregulated in both severe and moderate COVID-19 patients and positively associated with neutrophil proportion and COVID-19 severity. We identified USP18 protein, a key molecule centralizing the protein-protein interaction network of key APOBEC3 proteins. Furthermore, mRNA expression of USP18 was significantly correlated to ACE2 and TMPRSS2 expression in the tissue of upper airways. Knockdown of USP18 mRNA significantly decreased A3B expression. Ectopic expression of A3B gene increased SARS-CoV-2 infectivity. C > U mutations at S371F, S373L, and S375F significantly conferred with the immune escape of SARS-CoV-2. Thus, APOBEC3, whose expression are upregulated by inflammatory factors, might promote SARS-CoV-2 evolution and spread via upregulating USP18 level and facilitating the immune escape. A3B and USP18 might be therapeutic targets for interfering with SARS-CoV-2 evolution.

Abstract PO5-27-03: Mutational impact of APOBEC3A and APOBEC3B in a human cell line

Abstract A major source of mutation in cancer is DNA cytosine deamination by APOBEC3 enzymes resulting in C-to-T and C-to-G mutations in TCA and TCT motifs. Here, we develop a selectable system to quantify genomic mutations and compare the mutagenic activities of three leading APOBEC3 candidates - APOBEC3A, APOBEC3B, and APOBEC3H. The human cell line, HAP1, is engineered to express the thymidine kinase (TK) gene of HSV-1, which confers sensitivity to ganciclovir. Expression of APOBEC3A and APOBEC3B, but not catalytic mutant controls or APOBEC3H, triggers increased frequencies of TK mutation and nearly indistinguishable TC-biased cytosine mutation profiles in the selectable TK reporter gene. Whole genome sequences from TK mutant clones enabled an analysis of thousands of single base substitution mutations and extraction of local sequence preferences with APOBEC3A preferring YTCW motifs over 70% of the time and APOBEC3B just under 50% of the time (Y=C/T; W=A/T). Signature comparisons with breast tumor whole genome sequences indicate that most malignancies manifest intermediate percentages of APOBEC3 signature mutations in YTCW motifs, mostly between 50 and 70%, suggesting that both enzymes are contributing in a combinatorial manner to the overall mutation landscape. These studies combine to help resolve a long-standing etiologic debate on the source of APOBEC3 signature mutations in cancer and indicate that future diagnostic and therapeutic efforts should focus on both enzymes. Citation Format: Michael Carpenter, Nuri Temiz, Mahmoud Ibrahim, Matthew Jarvis, Margaret Brown, Prokopios Argyris, William Brown, Douglas Yee, Reuben Harris. Mutational impact of APOBEC3A and APOBEC3B in a human cell line [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-27-03.

Activation of APOBEC3B in Self-Inactivating Lentiviral-Based CAR T Cell Production Increases the Mutational Load and Decreases the Killing Efficiency of CAR T Cells

Abstract Apolipoprotein B mRNA editing enzyme, catalytic polypeptides (APOBECs) are a family of cytidine deaminase enzymes well described in anti-retroviral defense which convert cytidines to uracils (C-U), leading to C-T mutations in DNA. We investigated whether activation of APOBEC3B during the process of making chimeric antigen receptor (CAR) T cells via lentiviral vector affects the overall mutational integrity of resulting CAR T cells. To investigate this question, we quantified APOBEC3B expression and APOBEC signature mutations both post transfection in 293T viral producer cells and post transduction in human T cells. We observed abundant APOBEC3B activation and APOBEC3 signature mutations after transfection of plasmids to make lentivirus, but minimal APOBEC3B activation with a few additional APOBEC3 signature mutations after transduction. To test the functional relevance of APOBEC3B activity, we overexpressed APOBEC3B in 293T cells, and produced lentivirus from the 293T cells overexpressing APOBEC3B. The CAR T cells transduced with lentivirus from the 293T cells overexpressing APOBEC3B exhibited reduced killing ability compared to CAR T cells from 293T cells with no transgene. Taken together, these data suggest that APOBEC3B activation during the process of CAR T cell production induces mutations and reduces CAR T cell killing, and that selective ablation of APOBEC enzymes in GMP-grade lentiviral producer cells may enhance the overall quality of CAR T cells for clinical use.

Abstract 3401: Uncovering virus induced dysregulated processes in cancer and their impact on outcomes

Abstract The National Cancer Institute has designated several viruses such as EBV, HBV, HCV, HIV, HPV, and HTLV as agents that either cause cancer or increase the risk of developing cancer. However, extensive analyses of tumor genome and transcriptome datasets have identified numerous additional virus types, the functional implications of which remain largely unknown. Viral infection often dysregulates unique molecular processes within tumor cells and alters the composition of immune cells in the tumor microenvironment. For instance, HPV-positive tumors have been shown to exhibit a significantly elevated expression of the DNA editing enzyme APOBEC3B compared to HPV-negative tumors. Furthermore, HPV positive tumors display and increased number of C-to-T mutations induced by this protein. The genome and transcriptome of tumor tissue biopsies contain a wealth of information about these virus-associated signatures. However, advanced signal processing approaches are needed to extract these viral signatures from complex tumor datasets, and elucidate their specific roles in cancer. Here, we developed a quantitative approach to investigate the link between viral infection and dysregulation of a seven-membered A3 enzyme family (A3A/B/C/D/F/G/H). The primary role of these enzymes is to restrict viral infections by inducing C-to-U mutations in viral genomes. However, at least three members of this enzyme family (A3A/B/H) can also induce mutations in cellular DNA, and drive tumor formation and evolution. We used NMF (Nonnegative Matrix Factorization) to deconvolute the expression profiles of A3 enzymes in >10,000 bulk RNAseq datasets from diverse TCGA tumors. This analysis revealed the expression profiles of A3 enzymes within tumor and tumor microenvironment cells including various immune cells. Next, we conducted an association analysis to identify the link between the presence of viral infection and the proportion of each of these tumor and tumor microenvironment expression profiles. As expected, our analyses revealed strong associations between tumor-specific A3 expression profiles and infections by oncolytic viruses such as HPV. Notably, and somewhat unexpectedly, our analyses also revealed associations with viruses about which little is known in the field of cancer. For example, in certain tumors, we identified a robust association between bacteriophages and a tumor-specific A3 expression profile characterized by A3B upregulation. Taken together, our analyses provide evidence for the link between A3 dysregulation and infection by viruses including bacteriophages. Citation Format: Mohadeseh Soleimanpour moghadam, Azad Khosh, Diako Ebrahimi. Uncovering virus induced dysregulated processes in cancer and their impact on outcomes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3401.

Distinguishing preferences of human APOBEC3A and APOBEC3B for cytosines in hairpin loops, and reflection of these preferences in APOBEC-signature cancer genome mutations.

The APOBEC3 enzymes convert cytosines in single-stranded DNA to uracils to protect against viruses and retrotransposons but can contribute to mutations that diversify tumors. To understand the mechanism of mutagenesis, we map the uracils resulting from expression of APOBEC3B or its catalytic carboxy-terminal domain (CTD) in Escherichia coli. Like APOBEC3A, the uracilomes of A3B and A3B-CTD show a preference to deaminate cytosines near transcription start sites and the lagging-strand replication templates and in hairpin loops. Both biochemical activities of the enzymes and genomic uracil distribution show that A3A prefers 3 nt loops the best, while A3B prefers 4 nt loops. Reanalysis of hairpin loop mutations in human tumors finds intrinsic characteristics of both the enzymes, with a much stronger contribution from A3A. We apply Hairpin Signatures 1 and 2, which define A3A and A3B preferences respectively and are orthogonal to published methods, to evaluate their contribution to human tumor mutations.

The role of APOBEC3B in lung tumor evolution and targeted cancer therapy resistance

In this study, the impact of the apolipoprotein B mRNA-editing catalytic subunit-like (APOBEC) enzyme APOBEC3B (A3B) on epidermal growth factor receptor (EGFR)-driven lung cancer was assessed. A3B expression in EGFR mutant (EGFRmut) non-small-cell lung cancer (NSCLC) mouse models constrained tumorigenesis, while A3B expression in tumors treated with EGFR-targeted cancer therapy was associated with treatment resistance. Analyses of human NSCLC models treated with EGFR-targeted therapy showed upregulation of A3B and revealed therapy-induced activation of nuclear factor kappa B (NF-κB) as an inducer of A3B expression. Significantly reduced viability was observed with A3B deficiency, and A3B was required for the enrichment of APOBEC mutation signatures, in targeted therapy-treated human NSCLC preclinical models. Upregulation of A3B was confirmed in patients with NSCLC treated with EGFR-targeted therapy. This study uncovers the multifaceted roles of A3B in NSCLC and identifies A3B as a potential target for more durable responses to targeted cancer therapy.

Mutational impact of APOBEC3A and APOBEC3B in a human cell line and comparisons to breast cancer.

A prominent source of mutation in cancer is single-stranded DNA cytosine deamination by cellular APOBEC3 enzymes, which results in signature C-to-T and C-to-G mutations in TCA and TCT motifs. Although multiple enzymes have been implicated, reports conflict and it is unclear which protein(s) are responsible. Here we report the development of a selectable system to quantify genome mutation and demonstrate its utility by comparing the mutagenic activities of three leading candidates-APOBEC3A, APOBEC3B, and APOBEC3H. The human cell line, HAP1, is engineered to express the thymidine kinase (TK) gene of HSV-1, which confers sensitivity to ganciclovir. Expression of APOBEC3A and APOBEC3B, but not catalytic mutant controls or APOBEC3H, triggers increased frequencies of TK mutation and similar TC-biased cytosine mutation profiles in the selectable TK reporter gene. Whole genome sequences from independent clones enabled an analysis of thousands of single base substitution mutations and extraction of local sequence preferences with APOBEC3A preferring YTCW motifs 70% of the time and APOBEC3B 50% of the time (Y = C/T; W = A/T). Signature comparisons with breast tumor whole genome sequences indicate that most malignancies manifest intermediate percentages of APOBEC3 signature mutations in YTCW motifs, mostly between 50 and 70%, suggesting that both enzymes contribute in a combinatorial manner to the overall mutation landscape. Although the vast majority of APOBEC3A- and APOBEC3B-induced single base substitution mutations occur outside of predicted chromosomal DNA hairpin structures, whole genome sequence analyses and supporting biochemical studies also indicate that both enzymes are capable of deaminating the single-stranded loop regions of DNA hairpins at elevated rates. These studies combine to help resolve a long-standing etiologic debate on the source of APOBEC3 signature mutations in cancer and indicate that future diagnostic and therapeutic efforts should focus on both APOBEC3A and APOBEC3B.

Genomic and Transcriptomic Landscape of Hyper-Apobec Multiple Myeloma

Introduction APOBEC mutational signatures are one of the most important mutational signatures in newly diagnosed multiple myeloma (NDMM). APOBEC mutagenesis primarily works through two enzymes, APOBEC3A (A3A) and APOBEC3B (A3B). While these genes are expressed in most NDMM patients, their influence on APOBEC mutagenesis appear to be extremely heterogeneous, with some patients having no detectable mutations and others having high contribution (hyper-APOBEC). Methods To decipher the mechanism of APOBEC mutagenesis, we interrogated 752 low coverage long insert whole genomes, 723 whole exomes (WES) and 767 RNA sequencing (RNA-Seq) from NDMM patients enrolled in the CoMMpass study. Results APOBEC mutational activity was identified in 416/723 (57.5%) patients using WES. Of these, 41 (5.8%) had MAF/MAFB translocations. Overall, 48/723 (6.6%) patients were defined as hyper-APOBEC (13 without MAF/MAFB events). We used differential expression (DE) analysis between three groups: hyper-APOBEC with MAF/MAFB (HA_TRA: n=31), hyper-APOBEC without MAF/MAFB (HA_NORM: n=8), and non-hyper-APOBEC without MAF/MAFB (WT: n=510). 534 genes were significantly associated with hyper-APOBEC independently from MAF/MAFB events. Importantly, A3A and A3B expression were higher in hyper-APOBEC compared to other cases (logFC=2.38, logFC=1.58 respectively). Using Spearman's correlation, we identified 50 significantly correlated genes (r-squared>0.18; p<0.01) with A3B. Most of the genes belonged to the APOBEC-associated group defined by the DE and were enriched for cell cycle/proliferation activity, suggesting a link between highly proliferative disease and hyper-APOBEC. To validate this finding, we evaluated the link between these APOBEC-associated genes and hyper-APOBEC among ICGC breast cancers, a tumor known to have high prevalence of APOBEC mutagenesis. Overall, 35/50 of these genes showed similar correlation between hyper-APOBEC breasts and MM. Interestingly, most of these genes appear to be negatively controlled by E2F4 (DREAM complex), and positively controlled by E2F1, FOXM1, and MYBL2 (i.e. cell cycle genes). In line with mouse models (Roelofs et al., eLife, 2020), our data confirmed that highly proliferating tumors suppress the DREAM complex leading to induction of A3B. Overall, these data suggest a strong link between proliferation and A3B expression and mutagenesis. However, not all high proliferating tumors had high APOBEC mutagenesis. To better investigate this aspect, we compared 139 known MM genomic events to identify potential differences between the hyper-APOBEC group (HA) and the rest of the cohort (WT). HA_TRA and HA_NORM showed similarities, including a highly complex genomic profile with enrichment for 1q gain/amp (p < 0.02), 13q (p < 0.05) and 16q deletions (p<0.01). Interestingly, 5/13 (38.4%) of HA_NORM samples harbor CCND1 translocation. In terms of gene expression, both groups showed enrichment for GEP70 indicating a high-risk transcriptional profile apart from MAF/MAFB overexpression. Finally, HA_TRA and HA_NORM showed similar poor outcomes compared to WT. To further validate the association between APOBEC and complex genomic profile, we explored again the ICGC breast WGS, observing a strong correlation between homologous recombination deficiency (HRD) and hyper-APOBEC (p < 0.05). To validate the relationship between the DREAM complex and APOBEC, we pharmacologically inhibited FOXM1 (FDI6, 20uM) and E2F4/6 (HLM00074, 20uM) in 8226, U266 and MM1.S multiple myeloma cell lines and showed increased A3B levels upon 72h of E2F4/6 inhibition. Simultaneously, FOXM1 inhibition confirmed A3B downregulation. Conclusion Overall, our data support a model where, APOBEC genes are variably expressed in all NDMM, but only patients with high proliferation rate and high level of complexity acquire mutations (Figure 1).

Acute expression of human APOBEC3B in mice results in RNA editing and lethality

BackgroundRNA editing has been described as promoting genetic heterogeneity, leading to the development of multiple disorders, including cancer. The cytosine deaminase APOBEC3B is implicated in tumor evolution through DNA mutation, but whether it also functions as an RNA editing enzyme has not been studied.ResultsHere, we engineer a novel doxycycline-inducible mouse model of human APOBEC3B-overexpression to understand the impact of this enzyme in tissue homeostasis and address a potential role in C-to-U RNA editing. Elevated and sustained levels of APOBEC3B lead to rapid alteration of cellular fitness, major organ dysfunction, and ultimately lethality in mice. Importantly, RNA-sequencing of mouse tissues expressing high levels of APOBEC3B identifies frequent UCC-to-UUC RNA editing events that are not evident in the corresponding genomic DNA.ConclusionsThis work identifies, for the first time, a new deaminase-dependent function for APOBEC3B in RNA editing and presents a preclinical tool to help understand the emerging role of APOBEC3B as a driver of carcinogenesis.

MiR-138–5p inhibits prostate cancer cell proliferation and chemoresistance by targeting APOBEC3B

Docetaxel is one of the most commonly used drugs in prostate cancer (PCa) chemotherapy, but its therapeutic effect in PCa is usually limited due to its drug resistance. APOBEC3B is a DNA cytosine deaminase that can alter biological processes, including chemoresistance. APOBEC3B is upregulated in various cancers. However, the biological function and underlying regulation of APOBEC3B in PCa remain unclear. In this study, we explored the role of APOBEC3B in PCa chemoresistance and the molecular mechanism of its dysregulated expression. Our results revealed that APOBEC3B was upregulated in PCa docetaxel-resistant cells, while its knockdown significantly repressed cell proliferation and docetaxel resistance of PCa cells. Bioinformatics and luciferase report analysis showed that miR-138–5p targeted APOBEC3B. In addition, miR-138–5p overexpression impeded cell proliferation and docetaxel resistance in PCa, while miR-138–5p inhibitors reversed this process. Further studies showed that upregulation of APOBEC3B expression in docetaxel-resistant cells overexpressing miR-138–5p could desensitize PCa cells to docetaxel treatment. Taken together, miR-138–5p regulates PCa cell proliferation and chemoresistance by targeting the 3′-UTR of APOBEC3B, which may provide novel insights and therapeutic targets for the treatment of PCa.

Aberrant APOBEC3B Expression in Breast Cancer Is Linked to Proliferation and Cell Cycle Phase.

APOBEC3B (A3B) is aberrantly overexpressed in a subset of breast cancers, where it associates with advanced disease, poor prognosis, and treatment resistance, yet the causes of A3B dysregulation in breast cancer remain unclear. Here, A3B mRNA and protein expression levels were quantified in different cell lines and breast tumors and related to cell cycle markers using RT-qPCR and multiplex immunofluorescence imaging. The inducibility of A3B expression during the cell cycle was additionally addressed after cell cycle synchronization with multiple methods. First, we found that A3B protein levels within cell lines and tumors are heterogeneous and associate strongly with the proliferation marker Cyclin B1 characteristic of the G2/M phase of the cell cycle. Second, in multiple breast cancer cell lines with high A3B, expression levels were observed to oscillate throughout the cell cycle and again associate with Cyclin B1. Third, induction of A3B expression is potently repressed throughout G0/early G1, likely by RB/E2F pathway effector proteins. Fourth, in cells with low A3B, induction of A3B through the PKC/ncNF-κB pathway occurs predominantly in actively proliferating cells and is largely absent in cells arrested in G0. Altogether, these results support a model in which dysregulated A3B overexpression in breast cancer is the cumulative result of proliferation-associated relief from repression with concomitant pathway activation during the G2/M phase of the cell cycle.

Abstract 3723: Identification and functional characterization of long noncoding RNAs that regulate TGF-<&946;>-Smad pathway as Smad co-factors

Abstract TGF-β-Smad pathway participates in various biological processes such as development, differentiation, growth regulation, and cancer progression including epithelial-mesenchymal transition (EMT). Long non-coding RNAs (lncRNAs) also involved in carcinogenesis and cancer malignancies. To identify lncRNAs regulating TGF-β-Smad pathway, we comprehensively identified lncRNAs induced by TGF-β and found and reported a lncRNA ELIT-1 (EMT-associated lncRNA induced by TGF-β-1) which was induced by TGF-β and promoted EMT by facilitating TGF-β-Smad signaling. ELIT-1 bound to Smad and participated in expression of TGF-β target genes including Snail. Prognosis of lung adenocarcinoma and gastric cancer patients with high expression of ELIT-1 was poor suggesting that ELIT-1 may be useful as a novel prognostic and therapeutic target. Recently, we found that lincNMR (long intergenic noncoding RNA-nucleotide metabolism regulation) was induced via TGF-β-Smad pathway in various cell lines. Moreover, RNA-seq analysis using lincNMRs-depleted cells indicated that lincNMRs was involved in expression of APOBEC3B, a cytidine deaminase, promoting C to U mutation and highly expressed in various human cancers. Although, regulatory mechanisms of APOBEC3B expression have not been fully elucidated, we proved that lincNMRs bound to Smad and promoted the activity of APOBEC3B promoter. These data suggest that lincNMRs participate in APOBEC3B expression by collaborating with TGF-β-Smad pathway. Additionally, high expression of lincNMRs was positively correlated with high expression of APOBEC3B in various cancer cell lines. The high expression of APOBEC3B as well as lincNMR was significantly associated with their poor prognosis in liver and lung cancer patients, suggesting that lincNMR may contribute to tumor malignancy by enhanced expression of APOBEC3B via TGF-β-Smad pathway. Citation Format: Masatoshi Kitagawa, Kosuke Ota, Tatsuya Ohhata, Satoshi Sakai. Identification and functional characterization of long noncoding RNAs that regulate TGF-<&946;>-Smad pathway as Smad co-factors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3723.

Abstract 1166: APOBEC deaminases compete with tobacco smoking mutagenesis and affect age at onset of lung cancer

Abstract APOBEC enzymes are part of the innate immunity and are responsible for restricting viruses and retroelements by deaminating cytosine residues. Most solid tumors harbor different levels of somatic mutations attributed to the off-target activities of APOBEC3A (A3A) and/or APOBEC3B (A3B). However, how APOBEC3A/B interact with exogenous mutagenic processes in shaping tumor development is largely unknown. Here, by combining deep whole-genome sequencing with multi-omics profiling of 309 lung cancers from smokers with detailed tobacco smoking information, we identify two subtypes defined by low (LAS) and high (HAS) APOBEC mutagenesis. LAS are enriched for A3B-like mutagenesis and KRAS mutations, whereas HAS for A3A-like mutagenesis and TP53 mutations. Unlike APOBEC3A, APOBEC3B expression is strongly associated with an upregulation of the base excision repair pathway. Hypermutation by unrepaired A3A and tobacco smoking mutagenesis combined with TP53-induced genomic instability can trigger senescence, apoptosis, and cell regeneration, as indicated by telomere shortening, high expression of pulmonary healing signaling pathway and stemness markers in HAS. The expected association of tobacco smoking exposure with genomic/epigenomic changes are not observed in HAS, a plausible consequence of frequent cell senescence or apoptosis. HAS tumors have slower clonal expansion and older age at onset compared to LAS, particularly in heavy smokers, consistent with high proportions of newly generated, unmutated cells in HAS. These findings show how heterogeneity in mutational burden across competing mutational processes and cell types contributes to tumor development, with important clinical implications. Citation Format: Tongwu Zhang, Jian Sang, Phuc H. Hoang, Wei Zhao, Jennifer Rosenbaum, Leszek J. Klimczak, John McElderry, Alyssa Klein, Christopher Wirth, Erik N. Bergstrom, Marcos Díaz-Gay, Raviteja Vangara, Amy Hutchinson, Scott M. Lawrence, Nathan Cole, Bin Zhu, Teresa M. Przytycka, Jianxin Shi, Neil E. Caporaso, Robert Homer, Angela C. Pesatori, Dario Consonni, Stephen J. Chanock, David C. Wedge, Dmitry A. Gordenin, Ludmil B. Alexandrov, Reuben S. Harris, Maria Teresa Landi. APOBEC deaminases compete with tobacco smoking mutagenesis and affect age at onset of lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1166.

Abstract P2-17-01: Targeted CRISPR screen to identify synthetic lethal combinations between APOBEC3B and DNA repair

Abstract APOBEC-catalyzed deamination of cytosine bases is the largest enzymatic and second largest overall source of mutation in cancer. One member from the APOBEC family of enzymes, APOBEC3B (A3B) is overexpressed and dysregulated in many different cancer types. In addition to hallmark C-to-T transitions and C-to-G transversions, APOBEC-catalyzed uracil lesions can be processed into single- and double-strand DNA breaks. Therefore, A3B-positive tumors are under continual stress to repair DNA breaks and may be vulnerable to DNA repair inhibition. Previous results from the Harris lab have identified UNG2, DNA uracil glycosylase 2 (UNG2), initiator of the base excision repair pathway, as synthetic lethal pair with A3B. The genetic disruption of UNG2 combined with high expression of A3B, causes cell death. This provides the rational to hypothesize that also other DNA repair proteins could be putative synthetic lethal pairs with A3B, when its expression and activity are high. To test this hypothesis, CRISPR guide RNA library targeting 237 DNA damage repair and response genes was used in doxycycline-inducible TREX-293-A3Bi-eGFP cell line. Cells expressing or not A3B were harvested for DNA extraction and sequencing at different time points. Comparison of guide RNA abundance between doxycycline and H2O treated cells revealed the dropout guides that disrupt genes and create putative synthetic lethal combinations with A3B. The screen design and overall results will be presented Citation Format: Bojana Stefanovska, Kevin Lin, Benjamin Troness, Chad Myers, Reuben Harris. Targeted CRISPR screen to identify synthetic lethal combinations between APOBEC3B and DNA repair [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-17-01.

APOBEC3B Is Co-Expressed with PKCα/NF-κB in Oral and Oropharyngeal Squamous Cell Carcinomas.

The enzymatic activity of APOBEC3B (A3B) has been implicated as a prime source of mutagenesis in head and neck squamous cell carcinoma (HNSCC). The expression of Protein Kinase C α (PKCα) and Nuclear Factor-κΒ p65 (NF-κΒ p65) has been linked to the activation of the classical and the non-canonical NF-κB signaling pathways, respectively, both of which have been shown to lead to the upregulation of A3B. Accordingly, the aim of the present study was to evaluate the expression of PKCα, NF-κΒ p65 and A3B in non-HPV related oral and oropharyngeal squamous cell carcinomas (SCC), by means of immunohistochemistry and in silico methods. PKCα was expressed in 29/36 (80%) cases of oral and oropharyngeal SCCs, with 25 (69%) cases showing a PKCα+/A3B+ phenotype and only 6/36 (17%) cases showing a PKCα-/A3B+ phenotype. Εxpression of NF-κB p65 was seen in 33/35 (94%) cases of oral and oropharyngeal SCCs, with 30/35 (86%) cases showing an NF-κB p65+/A3B+ phenotype and only 2/35 (6%) cases showing an NF-κB p65-/A3B+ phenotype. In addition, mRNA expression analysis, using the UALCAN database, revealed strong expression of all three genes. These findings indicate that the expression of A3B is associated with PKCα/NF-κB p65 expression and suggest a potential role for the PKC/NF-κB signaling pathway in the development of oral and oropharyngeal cancer.

High APOBEC3B mRNA Expression Is Associated with Human Papillomavirus Type 18 Infection in Cervical Cancer.

The APOBEC3 (A3) proteins are cytidine deaminases that exhibit the ability to insert mutations in DNA and/or RNA sequences. APOBEC3B (A3B) has been evidenced as a DNA mutagen with consistent high expression in several cancer types. Data concerning the A3B influence on HPV infection and cervical cancer are limited and controversial. We investigated the role of A3B expression levels in cervical cancer in affected women positive for infection by different HPV types. Tumor biopsies from cancerous uterine cervix were collected from 216 women registered at Hospital do Câncer II of Instituto Nacional de Câncer, and infecting HPV was typed. A3B expression levels were quantified from RNA samples extracted from cervical biopsies using real-time quantitative PCR. Median A3B expression levels were higher among HPV18+ samples when compared to HPV16+ counterparts and were also increased compared to samples positive for other HPV types. In squamous cell carcinoma, HPV18+ samples also showed increased median A3B expression when compared to HPV Alpha-9 species or only to HPV16+ samples. Our findings suggest that A3B expression is differentially upregulated in cervical cancer samples infected with HPV18. A3B could be potentially used as a biomarker for HPV infection and as a prognostic tool for clinical outcomes in the context of cervical cancer.

APOBEC3B expression is promoted by lincNMR collaborating with TGF-β-Smad pathway.

Long non-coding RNAs (lncRNAs) participate in carcinogenesis and cancer malignancies. Transforming growth factor-β (TGF-β) is involved in various cellular processes including cancer progression. We performed comprehensive RNA sequencing analyses to identify lncRNAs regulated by TGF-β and found that lincNMR (long intergenic noncoding RNA-nucleotide metabolism regulator, also identified as MAP3K9-DT) was induced by TGF-β in various cell lines. There are several variants of lincNMR (hereafter lincNMRs) in the lincNMR/MAP3K9-DT locus, and their expression was increased by TGF-β. TGF-β-mediated induction of lincNMRs was decreased by depletion of Smad2/3 in Huh7, suggesting that the TGF-β-Smad pathway is involved in lincNMRs expression. We also found that APOBEC3B but not other APOBEC family members were a target gene of lincNMRs. APOBEC3B, a cytidine deaminase, promotes C to U mutation and highly expressed in various human cancers. Although it is associated with cancer progression, regulatory mechanisms of APOBEC3B expression have not been fully elucidated. We performed RNA immunoprecipitation assays and proved that lincNMRs bound to endogenous Smad2 in Huh7 cells. The increased activity of the promoter of APOBEC3B induced by overexpression of Smad2/3 was inhibited by depletion of lincNMRs. These data suggest that lincNMRs participate in APOBEC3B expression by collaborating with TGF-β-Smad pathway. High expression of lincNMRs was positively correlated with high expression of APOBEC3B in various cancer cell lines. Overexpression of APOBEC3B as well as lincNMR was found in human cancers such as hepatic and lung cancers and was associated with their poor prognosis, suggesting that lincNMR may contribute to tumor malignancy via enhanced expression of APOBEC3B.