Viral-Mediated miRNA Therapeutics
Along with investigating miRNAs as biomarkers and diagnostic tools, there have also been a number of advances in the field of miRNA therapeutics. Benjamin tenOever, Ph.D., Fishberg professor of medicine at Mount Sinai School of Medicine, has been investigating how miRNAs can be utilized in engineered RNA-based vectors.
Dr. tenOever’s lab has developed a method for exploiting endogenous miRNAs to regulate tissue tropism of viral vectors. “Because viruses lack a mechanism for antagonizing miRNA function, we can exploit miRNA expression to control viruses,” he says. “We have shown that if you incorporate targets for a cell-specific miRNA into an RNA virus genome, you can create a virus that looks identical at a protein level but would be selectively blocked from infecting these particular cells.”
The other side of Dr. tenOever’s research focuses on engineering viral vectors to produce functional miRNAs, which can be used as a therapeutic platform to deliver miRNAs (or other small RNAs) to any tissue in the body. Dr. tenOever states that, while it was known that RNA viruses do not produce miRNAs, it was unclear whether viruses lack the capacity to do so, or whether this activity was perhaps detrimental to the viral life cycle.
His lab addressed this question by incorporating primary miRNAs (pri-miRNA) into various RNA viral vectors that localize to the nucleus or the cytoplasm in different tissues. In all cases, mature miRNAs were properly processed and loaded into the RISC complex. In fact, with cytoplasmic viruses, Drosha, an RNase, was exported out of the nucleus to process the artificial pri-miRNA.
It appears that most RNA viruses are capable of producing functional miRNAs, but do not do so naturally, presumably to prevent some degree of self-attenuation. “What we can capitalize on now is to convert an miRNA that behaves like a tailor-made, sequence-specific siRNA. We can then adapt any RNA virus, regardless of tropism, and use it to generate siRNAs to silence a desired host gene,” Dr. tenOever reported.
The RNA viruses only produce miRNAs for 7–10 days, but transient cytoplasmic RNA viruses have become very useful in Dr. tenOever’s lab for a variety of applications including the reprogramming of fibroblast cells into iPSCs by introducing a few miRNAs.
“The advantage of our system is that we are not going into the nucleus with our vectors and we’re not integrating into the genome; we’re only there for 7–10 days and then our vectors disappear, but by then we have reprogrammed our cells to become pluripotent stem cells.”