Arthropod-borne flaviviruses and RNA interference: seeking new approaches for antiviral therapy

Abstract

Flaviviruses are the most prevalent arthropod-borne viruses worldwide, and nearly half of the 70 Flavivirus members identified are human pathogens. Despite the huge clinical impact of flaviviruses, there is no specific human antiviral therapy available to treat infection with any of the flaviviruses. Therefore, there is a continued search for novel therapies, and this review describes the current knowledge on the usage of RNA interference (RNAi) in combating flavivirus infections. RNAi is a process of sequence-specific gene silencing triggered by double-stranded RNA. Antiviral RNAi strategies against arthropod-borne flaviviruses have been reported and although several hurdles must be overcome to employ this technology in clinical applications, they potentially represent a new therapeutic tool.

Figure 4.1

RNA interference (RNAi). RNAi is a post‐transcriptional gene silencing mechanism that regulates gene expression and mediates resistance against pathogenic nucleic acids. The main effector molecules of RNAi are miRNAs and siRNAs. MiRNAs are encoded in the genome and transcribed into primary miRNA transcripts (pri-miRNAs). Pri-miRNAs are processed to miRNA precursors (pre-miRNAs) in the nucleus by the RNase III-like enzyme Drosha and the DiGeorge syndrome critical region 8 (DGCR8). Pre-miRNAs are transported to the cytoplasm by exportin 5, where they are cleaved by Dicer to yield ∼ 19–23 nt miRNA duplexes. One strand is then selected and loaded in the RNA-induced silencing complex (RISC). The key components of RISC are proteins of the Argonaute (AGO) family which mediate translational repression or cleavage of target mRNAs. Furthermore, dsRNA molecules are targets for the RNAi pathway. These dsRNA molecules are artificially introduced in the cell cytoplasm or are virus RNAs. Like miRNA precursors, long dsRNA are processed by Dicer into ∼ 21 nt siRNA duplexes. One strand of the siRNA is selected and loaded into RISC. The binding of an siRNA to its target mRNA typically induces degradation.

Figure 4.2

Flavivirus life cycle. Flaviviruses bind to cellular receptors on the surface of susceptible host cells (1). Virions are internalized by endocytosis (2). The acidic environment within the endosomes induces fusion (3) between the viral and cell membranes resulting in the release of the RNA genome (4). The RNA genome of flaviviruses contains a single open reading frame and highly structured 3′ and 5′UTRs ends. The RNA is translated into a polyprotein precursor that is processed into three structural proteins and seven nonstructural proteins (5). The complementary sequences CS (cyclization sequence) and UAR (upstream AUG regions) at both ends of the genome allow its circularization a required step for replication. Replication is mediated by the nonstructural proteins. Virus assembly occurs in the endoplasmic reticulum (ER) (6). The resultant immature particles are transported through the Golgi network where furin-mediated cleavage of prM to M generates mature infectious particles (7) that are released by exocytosis (8).