Evidence linking APOBEC3B genesis and evolution of innate immune antagonism by gamma-herpesvirus ribonucleotide reductases

Meeting report: Seventh summer school on innovative approaches for identification of antiviral agents (IAAASS)

The 7th Summer School on Innovative Approaches for the Identification of Antiviral Agents (IAAASS) was held at the Sardegna Ricerche Research Park in Santa Margherita di Pula, Sardinia, Italy from September 23-27, 2024, organized by the Co.S.Me.Se, the Department of Life and Environmental Sciences of the University of Cagliari and Sardegna Ricerche in the frame of Next Generation Virology initiative and the Antiviral DiscoVery Initiatives: Educating Next-Gen Scientists (ADVISE Project) 2024. The Summer School is proposed as an informal high-level event comprehensive of the different scientific souls involved in the design and development of new antiviral drug-candidates and their validation and progression up-to the clinic, and it offers, to a limited number of early career scientists, high-level lectures, networking and mentoring opportunities by internationally recognized scientists in a highly interactive environment. The meeting was a very successful event that convened 21 senior speakers with internationally recognized experience in the field of antiviral research and 60 Early Career Scientists (graduated master students, PhD students and early post doctoral researchers) from 13 different countries (Belarus, Belgium, Brazil, Bulgaria, Chile, Denmark, Germany, Italy, Russian Federation, Spain, Switzerland, The Netherlands, and the USA).

Viral infection, APOBEC3 dysregulation, and cancer

Viral infection plays a significant role in the development and progression of many cancers. Certain viruses, such as Human Papillomavirus (HPV), Epstein-Barr Virus (EBV), and Hepatitis B and C viruses (HBV, HCV), are well-known for their oncogenic potential. These viruses can dysregulate specific molecular and cellular processes through complex interactions with host cellular mechanisms. One such interaction involves a family of DNA mutators known as APOBEC3 (Apolipoprotein B mRNA Editing Catalytic Polypeptide-like 3). The primary function of these cytidine deaminases is to provide protection against viral infections by inducing viral mutagenesis. However, induction and dysregulation of A3 enzymes, driven by viral infection, can inadvertently lead to cellular DNA tumorigenesis. This review focuses on the current knowledge regarding the interplay between viral infection, A3 dysregulation, and cancer, highlighting the molecular mechanisms underlying this relationship.

Unveiling the Connection: Viral Infections and Genes in dNTP Metabolism

Deoxynucleoside triphosphates (dNTPs) are crucial for the replication and maintenance of genomic information within cells. The balance of the dNTP pool involves several cellular enzymes, including dihydrofolate reductase (DHFR), ribonucleotide reductase (RNR), and SAM and HD domain-containing protein 1 (SAMHD1), among others. DHFR is vital for the de novo synthesis of purines and deoxythymidine monophosphate, which are necessary for DNA synthesis. SAMHD1, a ubiquitously expressed deoxynucleotide triphosphohydrolase, converts dNTPs into deoxynucleosides and inorganic triphosphates. This process counteracts the de novo dNTP synthesis primarily carried out by RNR and cellular deoxynucleoside kinases, which are most active during the S phase of the cell cycle. The intracellular levels of dNTPs can influence various viral infections. This review provides a concise summary of the interactions between different viruses and the genes involved in dNTP metabolism.

The RNA tether model for human chromosomal translocation fragile zones

One of the two chromosomal breakage events in recurring translocations in B cell neoplasms is often due to the recombination-activating gene complex (RAG complex) releasing DNA ends before end joining. The other break occurs in a fragile zone of 20-600 bp in a non-antigen receptor gene locus, with a more complex and intriguing set of mechanistic factors underlying such narrow fragile zones. These factors include activation-induced deaminase (AID), which acts only at regions of single-stranded DNA (ssDNA). Recent work leads to a model involving the tethering of AID to the nascent RNA as it emerges from the RNA polymerase. This mechanism may have relevance in class switch recombination (CSR) and somatic hypermutation (SHM), as well as broader relevance for other DNA enzymes.

APOBEC Reporter Systems for Evaluating diNucleotide Editing Levels

Precision genome editing has become a reality with the discovery of base editors. Cytosine base editor (CBE) technologies are improving rapidly but are mostly optimized for TC dinucleotide targets. Here, we report the development and implementation of APOBEC Reporter Systems for Evaluating diNucleotide Editing Levels (ARSENEL) in living cells. The ARSENEL panel is comprised of four constructs that quantitatively report editing of each of the four dinucleotide motifs (AC/CC/GC/TC) through real-time accumulation of eGFP fluorescence. Editing rates of APOBEC3Bctd and AIDΔC CBEs reflect established mechanistic preferences with intrinsic biases to TC and GC, respectively. Twelve different (new and established) base editors are tested here using this system with a full-length APOBEC3B CBE showing the greatest on-target TC specificity and an APOBEC3A construct showing the highest editing efficiency. In addition, ARSENEL enables real-time assessment of natural and synthetic APOBEC inhibitors with the most potent to-date being the large subunit of the Epstein-Barr virus ribonucleotide reductase. These reporters have the potential to play important roles in research and development as precision genome engineering technologies progress toward achieving maximal specificity and efficiency.

Human cytomegalovirus mediates APOBEC3B relocalization early during infection through a ribonucleotide reductase-independent mechanism

The APOBEC3 family of DNA cytosine deaminases comprises an important arm of the innate antiviral defense system. The gamma-herpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus and the alpha-herpesviruses herpes simplex virus (HSV)-1 and HSV-2 have evolved an efficient mechanism to avoid APOBEC3 restriction by directly binding to APOBEC3B and facilitating its exclusion from the nuclear compartment. The only viral protein required for APOBEC3B relocalization is the large subunit of the ribonucleotide reductase (RNR). Here, we ask whether this APOBEC3B relocalization mechanism is conserved with the beta-herpesvirus human cytomegalovirus (HCMV). Although HCMV infection causes APOBEC3B relocalization from the nucleus to the cytoplasm in multiple cell types, the viral RNR (UL45) is not required. APOBEC3B relocalization occurs rapidly following infection suggesting the involvement of an immediate early or early (IE/E) viral protein. In support of this possibility, genetic (IE1 mutant) and pharmacologic (cycloheximide) strategies that prevent the expression of IE/E viral proteins also block APOBEC3B relocalization. In comparison, the treatment of infected cells with phosphonoacetic acid, which interferes with viral late protein expression, still permits A3B relocalization. These results combine to indicate that the beta-herpesvirus HCMV uses an RNR-independent, yet phenotypically similar, molecular mechanism to antagonize APOBEC3B. IMPORTANCE Human cytomegalovirus (HCMV) infections can range from asymptomatic to severe, particularly in neonates and immunocompromised patients. HCMV has evolved strategies to overcome host-encoded antiviral defenses to achieve lytic viral DNA replication and dissemination and, under some conditions, latency and long-term persistence. Here, we show that HCMV infection causes the antiviral factor, APOBEC3B, to relocalize from the nuclear compartment to the cytoplasm. This overall strategy resembles that used by related herpesviruses. However, the HCMV relocalization mechanism utilizes a different viral factor(s) and available evidence suggests the involvement of at least one protein expressed at the early stages of infection. This knowledge is important because a greater understanding of this mechanism could lead to novel antiviral strategies that enable APOBEC3B to naturally restrict HCMV infection.