Viruses make obvious targets for RNAi intervention, given that they utilize specific cellular receptors and introduce foreign genes into host cells, researchers at the Wolfson Institute for Biomedical Research at University College London wrote in an article in Current Opinions in Infectious Diseases. Hepatitis C and HIV have been very aggressively pursued thus far, but a number of other viral-associated diseases, namely cancers, make promising targets for the gene silencing technology.
In the article, Andrew Godfrey and colleagues noted that there are numerous viral-associated cancers. Among them: Burkitt’s lymphoma, Hodgkin’s disease, post-transplant lymphoproliferative disorder, and AIDS-related central nervous system lymphoma, which are all caused by Epstein-Barr virus; anogenital cancers, which are caused by humna papillomavirus; adult T-cell leukemia, which results from human T-cell leukemia virus-1; hepatocellular carcinoma, which can result from hepatitis B and C; and Kaposi’s sarcoma, which is caused by Kaposi’s sarcoma-associated herpesvirus (KSHV).
Each of these cancers has a gene target related to infection or replication that could be the focus of an RNAi-based drug. “The hope,” the researchers wrote, “is to reduce viral load or to ablate viral transcripts which promote oncogenesis.”
For example, Godfrey et al. noted that KSHV is latently present in most tumor cells, where it “persists as an episome and expresses only a small number of its genes. However, a fraction of tumor cells also undergo lytic replication, when early and late lytic genes are expressed, resulting in virion production and release from infected cells.”
There is a handful of KSHV genes that could be knocked down with RNAi, the authors stated, and several latent transcripts make particularly attractive targets since they have been directly implicated in KSHV-associated oncogenesis. “A cluster of genes in the KSHV genome (termed 'oncogenic cluster’) is expressed during latency from a tri-cistronic transcript: the latent nuclear antigen (LANA) encoded by ORF 73, a D-type cyclin homologue (vcyclin), and a FLIP homologue (vFLIP),” they wrote. “LANA maintains the viral episome by tethering it to chromatin during mitosis, ensuring delivery of viral progeny to daughter cells.”
LANA also interferes with the tumor-suppressor proteins p53 and pRb, they added.
“The vcyclin,” the scientists wrote, “functions both in exit from quiescence and in the G1/S transition of the cell cycle. Importantly, vcyclin is oncogenic in the absence of p53.” As for vFLIP, Godfrey and colleagues wrote that it “activates nuclear factor kB, which prevents apoptosis in [primary effusion lymphoma (PEL)] cells.”
Since these genes affect known signaling and tumor suppressor pathways, they added, “we reason that disrupting their functions should stop the viral signals that confer survival and proliferative capacities to infected cells.”
The researchers also noted that targeting the KSHV oncogenic cluster takes advantage of the bi-cistronic and tri-cistronic nature of the transcript. “The vcyclin and vFLIP are transcribed from a bi-cistronic transcript that is separate from the tri-cistronic transcript which includes LANA,” they wrote. “Thus, knocking out either vFLIP or vcyclin should theoretically also abrogate expression of the other, also providing possible synergistic effects.
“LANA has been the target of RNAi studies to elucidate the function of beta-catenin in PEL cells,” Godfrey et al. wrote. “LANA can be effectively inhibited in human PEL cell lines, and this inhibition abrogates the effects LANA has on beta-catenin localization.”
Another promising target for KSHV is the protein Rta, an initiator of the lytic viral gene-expression program.
According to the researchers, murine herpesvirus 68 replication can be blocked by siRNA targeted to Rta and a conserved lytic protein known as open reading frame 45 (ORF45). In the KSHV genome, the analogous protein for reactivation from latency is ORF50/Rta, they stated.
Kaposi’s sarcoma can result from suppressed immune function associated with AIDS infection. Godfrey and colleagues wrote that blocking the HIV-1 tat gene, which encodes a transactivator protein essential for viral replication, might be useful against the cancer by blocking the virus and reconstituting the immune system or by interfering with the angiogenic and possible transactivating activities of tat.
Godfrey and colleagues concluded by noting that delivery issues, as well as issues with interferon responses, still plague efforts to use RNAi-based therapeutics. But if these hurdles can be overcome, “RNAi could be a useful approach to treat virally-driven malignancies in humans.”
—DM