Can RNAi be used as medicine?

Recently I had the occasion to take a trip to Manhattan, and visit at the Dream Downtown hotel. The designers of this hotel put a great deal of effort into creating an unusual, cutting edge modern environment, worthy of its name. You enter from the street, into this darkened…yet vast lobby. Two stories above your head, shimmery waves of day light shine through a glass bottomed, swimming pool. In the restaurant, a dramatic field of golden glass globe lights spray across the ceiling. There is an actual rowboat “floating” on a vertical koi pond wall. The hotel elevators have four fully mirrored walls. Walking around this hotel pushes you to approach spaces that suggest impossibility. You leave your comfort zone as you enter. Is this really what I am seeing? Is this safe? In the end you are rewarded with a calming experience. Calming, because your human mind has to be “present” to adjust to the challenging sensory environment.

Pushing RNAi studies towards clinical medicines has required a similar fortitude by researchers. There is a new focus on the cellular RNA environment. How extensive is the influence of RNA systems in organisms? Can RNAi be used to control gene expression precisely enough in people? Not to mention, do I have the skills to work with RNA in the lab?

RNAi is a system used by eukaryotic cells to regulate gene expression, and to defend against viral infections. While, RNAi technologies use RNAi cellular machinery to produce lab designed small interfering RNA (siRNA) to target and knock down gene expression. First, sequence coding antisense RNA is cloned into a plasmid or viral vector. There are two types of vector systems. One has Short Hairpin RNA, with both sense and anti-sense expressed by one promoter. The other is a tandem system with sense and anti-sense expressed by two separate promoters. Transfection into cells can be carried out by either electroporation or lipid formulations. Vector expressed siRNA couples with cellular RNA-Induced Silencing Complex (RISC) capable of degrading messenger RNA. The siRNA molecule recognizes and binds to its targeted cellular mRNA, producing the “red flag” dsRNA molecules. One siRNA molecule works in this fashion to eliminate multiple copies of an mRNA sequence.

A clinical goal has been to use RNAi technologies as gene therapy based medicines. For example to “knockdown” oncogene proteins, viral proteins, or mutated proteins causing diseases. Advantages and disadvantages have been discovered. Viral vector delivery systems carry the risk of causing tumors. Lipid nanotechnology is an improved, inert delivery system. However, transfection is still transient, requiring maintenance. Also, siRNA is designed to be no longer than 21-23 bp, to avoid triggering an animal host interferon system attack. “Off-target silencing” can be a problem in organisms, too. Still, there are numerous ongoing phase I and phase II clinical trials looking overcome such challenges.

Encouraging Phase I clinical trial results were reported in the New England Journal of Medicine paper, Safety and Efficacy of RNAi Therapy for Transthyretin Amyloidosis. Lipid-based nanotechnology delivered RNAi was used to knock down of mutated (and non-mutated) forms of the protein Transthyretin (TTR). Transfection was targeted to the liver, where TTR is produced. TTR functions to transport vitamin A and hormones throughout the body. In various types of Transthyretin Amyloidosis. mutated TTR forms plaques in different areas, (i.e. peripheral nervous system, autonomic nervous system, heart, kidneys, eyes, and the gastrointestinal tract). The disease is progressive and life threatening. The only current treatment is a liver transplant. Lipid-based delivery RNAi appears to be a good match for the liver. The chances of off target silencing are certainly low. The liver breaks down lipids to produce most of the body’s cholesterol. The liver is a uniquely regenerative organ, as well. In live donor transplants, the donor and recipient start out with half a liver. Six weeks later each has a full sized regrown liver. (Save that fact for a party.) In this study, their lipid nanoparticles were safe and their best formulation results had a 70% reduction at day 28 in monkeys, 90%+ reduction after 5 doses. Knockdown of TTR in human patients observed at one dosage were identical to the primates. A caveat is that long term treatments could result in vitamin A deficiency, but the authors predicted that other independent mechanisms of vitamin A transport could compensate.

Who dares nothing, need hope for nothing.

Coelho T, Adams D, Silva A, Lozeron P, Hawkins PN, Mant T, Perez J, Chiesa J, Warrington S, Tranter E, Munisamy M, Falzone R, Harrop J, Cehelsky J, Bettencourt BR, Geissler M, Butler JS, Sehgal A, Meyers RE, Chen Q, Borland T, Hutabarat RM, Clausen VA, Alvarez R, Fitzgerald K, Gamba-Vitalo C, Nochur SV, Vaishnaw AK, Sah DW, Gollob JA, & Suhr OB (2013). Safety and efficacy of RNAi therapy for transthyretin amyloidosis. The New England journal of medicine, 369 (9), 819-29 PMID: 23984729

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