This is article was written 2005 and needs updating.
CMT1A is an ideal candidate for potential new therapy.
J.D. Griffith MS
Potential Treatment for CMT1A
A recently identified, naturally occurring process, RNA interference (RNAi) or gene silencing, may hold the potential for treatment of the most common form of Charcot-Marie-Tooth (CMT) disease, CMT1A.The symptoms of most genetic diseases are caused by a problem in the DNA of a gene that codes for a product whose lack of functionality or absence results in disease. Until recently, the search for gene therapies to cure Charcot-Marie-Tooth disease and other genetic disease focused on the introduction of helpful genes (DNA) into cells to produce functional products to replace the faulty disease causing ones. CMT1A, however, is not caused by a faulty gene, but by too much of a good gene (3 copies of the gene instead of two, as a consequence of a duplication on chromosome 17), which results in an excess of the protein, peripheral myelin protein 22 (PMP22).
RNA interference (RNAi) or gene silencing, is emerging as a potential new therapy to fight disease by turning down the production of a specific protein (PMP22 in CMT1A). In 2002, the magazine, Science, named RNA interference the scientific breakthrough of the year. Harnessing this natural process may offer hope for CMT1A (the most common type). At least one major university laboratory is working on RNA interference and CMT1A and the therapy is moving up on the short list of one biotech company.
RNA interference is a variation on a theme called antisense that emerged amid much fanfare in the late 1980s. This therapy, which is still viable, can turn off or turn down the protein production of a specific gene. As a result, antisense therapy was billed as the “magic bullet” to cure many diseases. The leader in this technology, Isis Pharmaceuticals, has one antisense product on the market and seven more in clinical trials. For a review of genetics, antisense and Isis, go
RNA interference (RNAi), as the name implies, is a potent (many times more powerful than antisense) and highly specific naturally occurring cellular mechanism that interferes with the process of making a specific protein coded for by the DNA in its gene. RNAi is thought to have evolved as a defense against viruses and the replication of foreign genes integrated into the host DNA. It is highly evolutionarily conserved (hasn't’ changed much from microorganisms to humans-therefore it may be considered essential). The process can turn a specific gene off or down and involves short strings of double-stranded RNA (siRNA) with a matching code for the target gene. The most important difference between this therapy and other drug therapies is the specificity (the RNAi will theoretically only silence one gene).
Genes are strings of DNA, which code (genetic code) for a specific protein. Once the disease causing protein is discovered and the DNA sequence on the gene for that protein determined, a short double stranded RNA molecule can be produced with a complementary sequence (antisense) for part of the offending gene. This would bind to the messenger RNA and shut down or reduce the manufacture of the offending protein. Disease cured. It sounds to good to be true.
Remember what Nobel Laureate Francis Crick, who with J. D. Watson, discovered the structure of DNA, said, “DNA makes RNA, RNA makes protein and proteins makes us.”
DNA>messengerRNA>RNAi=STOP Protein Synthesis
For an easy-to-understand graphic review of the basics of DNA and genetics, click here to view: genetics.utah.edu The Nature Publishing Group also has created an animation describing the RNAi process. Click here to view their animation. This may take a while to load and requires some knowledge of genetics.
RNAi technology is an easy way to “knock out” or shut down a gene and, indeed, its first application was as a research tool. Simply turn off a specific gene in a cell or organism and study the consequences: This is a highly effective mechanism for understanding the biochemistry of life, but is particularly valuable for validating potential drug targets. Creating and supplying reagent RNAis has become big business, and the leader in this field, Dharmacon, has a library of over 4000 RNAis including human, rat and mouse PMP22.
RNAi therapy faces the same hurdles as drug and gene therapy, particularly delivery-- getting the medicine to the right place in the right cells. The use of viral and other vectors to insert a helpful gene into DNA of humans is promising, but unfortunately has had some disastrous setbacks. Because RNAIs are much smaller than genes, are not inserted into the DNA of our genome and do their work downstream from the gene they should be amenable to less risky delivery techniques and may fall into the prescription drug category
All Charcot-Marie-Tooth disease types, except CMT1A (the most common form of CMT), are genetic disorders in which a change (mutation) in the DNA of a gene (generally inherited but can be spontaneous) results in the production of a faulty protein involved with the function or structure of the peripheral nerves. CMT1A is unique in the CMT family of diseases in that the genes for PMP22 (the problem protein) are fine but there is one too many copies (three genes instead of the normal two) on chromosome 17. The mutation results in too much PMP22, a protein in the Schwann cells, which wrap in concentric layers around the nerve cell (axon) to form the myelin sheath. The myelin sheath is the fatty insulating layer that surrounds the nerve cells outside the brain and spinal cord (the peripheral nerves). Over time, the overabundance of PMP22 disrupts the normal formation, compaction and function of myelin resulting in subsequent nerve degeneration, then a lack of muscle stimulation followed by muscle wasting and the symptoms of CMT1A.
Click on CMT type 1 for an excellent review by Thomas Bird M D.
Whether gene silencing is the “magic bullet” for CMT1A will be determined at some point in the future, but a least CMT is in a few crosshairs.
Here is how it would work:
- Ready: Find the problem protein (in the case of CMT1A, PMP22). Done.
- Load: Determine the genetic code on the DNA for the gene for PMP22. Done.
- Aim: Find sections of code on the gene that are likely candidates for a short RNA segment that will block the synthesis of PMP22, test in a cell culture and create a short double stranded RNAi that does not invoke an immune response. This is not as difficult as it may appear, computer programs exist to assist this process and RNAis have been created for animal and human PMP22.
- Fire: Get the proper amount of a functional RNAi inside the correct cell -- in the case of CMT1A the Schwann cells in the peripheral nervous system -- and slow down the excess production of PMP22 without inducing an immune response. Now, this is a serious but hopefully surmountable problem.
- Kill: CMT1A disease progress is stopped and possibly reversed.
Although some issues concerning drug reaction -- in particular, immune response to RNAi exist, two biotech companies, Acuity Pharmaceuticals and Sirna Therapeutics, have recently begun RNAi phase I clinical trials for the treatment of age-related macular degeneration (AMD), the leading cause of blindness in the developing world. The RNAi is specific for the protein growth factor VEGF, which is believed to cause AMD and is also involved in cancer and other diseases. AMD was probably chosen as the first candidate because of its prevalence (money talks) and the problem of delivery is simply solved by injecting the RNAi directly into eye (ouch). For more information on the science behind this therapy and another good review of RNAi, including a video, goto the Sirna Therapeutics site:
In a recently published paper (Feb. 22) in Nature Medicine, Alnylam Pharmaceuticals and its collaborators at the Ludwig Maximilian University Munich reported that an immune response occurred in some RNAi sequences and not in others.
"Alnylam is committed to developing safe and efficacious RNAi therapeutics," said John Maraganore, Ph.D., President and Chief Executive Officer of Alnylam Pharmaceuticals. "Elucidating the mechanism of action responsible for siRNA immunostimulation will allow for efficient selection of appropriate siRNA molecules."
Interest in the nascent phenomenon of RNAi has increased exponentially. A recent search on Google Scholar for “RNA interference” found 177 citations in 1999 (most involving plants and worms) and a peak of 1970 citations in 2003.
A search of CRISP, http://crisp.cit.nih.gov/, a database for federally funded biomedical research projects for “RNA interference,” returned no citations for RNA interference and CMT, but returned three citations for RNAi and Multiple Sclerosis. On another sad note related to the CRISP search, although CMT is nearly as common as MS and can be as serious, a CRISP search for the total grants for MS in 2004 returned 390 citations and a pitiful, but new, high of 31 grants for CMT. Hopefully, this inequity will be corrected soon – due, in part, to the efforts of people such as Gary Gasper, a recent addition to the CMT Association board. Gasper is responsible for inserting the language of CMT into the Congressional Record to direct the National Institute of Health to fund research on the disease. Alnylam Pharmaceuticals announced, in a March 16, 2005 press release, a collaborative effort with the Cystic Fibrosis Foundation Therapeutics, Inc. (CFFT), the drug discovery and development affiliate of the Cystic Fibrosis Foundation (CFF), to discover RNAi therapeutics for the treatment of Cystic Fibrosis (CF). The CFFT is expected to provide $1.5 million in funding for the discovery effort. CF, a horrible disease, is caused by a genetic defect that would appear to present a more formidable challenge to treat than CMT. Zachary Zimmerman, Ph.D., Director, External Alliances for Alnylam Pharmaceuticals in an e-mail comments, “Briefly, targeting CF with RNAi may be difficult but we already know that we can get siRNAs into lung cells (from our RSV [Respiratory Syncytial Virus] program). On the other hand, targeting PMP22 with siRNAs should be easy – but therapeutic delivery is the major issue.”
The hype surrounding the RNAi has recently reached a level of absurdity. A Los Angeles cat cloning company, Geneticas, recently announced --to much skepticism in the scientific community -- that by 2007 it will create a hypoallergenic cat using RNAi to turn off the offending kitty allergen (incidentally the company reports accepting hundreds of non refundable $250.00 deposits for these hypoallergenic tabbies). The company is also working on transgenic glow-in-the-dark deer. The rationale for this is unclear. While it may prevent some Bambi road carnage, such a critter would certainly be a boon to the deer hunter with a day job and, as an added bonus, eliminate any necessity for camouflage.
Lou Hawthorne, CEO of pet cloning competitor Genetic Savings and Clone (yes, that is the correct name) of Sausalito, California however predicts, “They'll be out of business by summer," and indeed, their website geneticas.com no longer exists. Genetic Savings and Clone, incidentally, has cloned two cats commercially, Little Gizmo and Little Nicky. Go to
Fascinating details of the cellular activities of RNA are emerging daily . In “RNAi-Mediated Pathways in the Nucleus,” published in the January Nature Reviews Genetics 6, 24-35 (2005), the authors, Matsky, M. AND Birchler, J. say, “Recent developments have revealed that there is also extensive involvement of RNAi-related processes in regulation at the genome level.” This subject is beyond the scope of this article but for a review of this latest info click: Nature Genetics. The paper is quite technical but may be of interest to some readers.
Whether an RNAi therapy for CMT1A will ever halt the progression and even cure the disease remains to be seen, but at least there exist a faint glimmer of hope.Halting the progression of this insidious disease would be a godsend, particularly to our children, but the possibility of a reversal of the symptoms would be incredible. Indeed, some evidence exists that to an extent CMT is reversible in humans. In a small study, presented by Dr. Zarife Sahenk, of Ohio State University, a pilot trial of genetically engineered human neurotrophin-3 (NT-3), a nerve growth factor, yielded statistically significant clinical improvements and demonstrated nerve regeneration in four patients CMT-1A.
Peripheral nerve regeneration and correction of the symptoms of CMT1A has been demonstrated in a major French study in CMT1A mouse models using ascorbic acid (vitamin C) by Michal Fontes, et al., published in Nature Medicine, March 2004.
A comprehensive review in the March 16, 2005 Journal of the American Medical
Association entitled, “The Prospect of Silencing Disease Using RNA Interference,” examines the mechanism, potential obstacles and possibilities for RNAi. The article covers RNAi and inherited neurological diseases, but fails to mention CMT. The authors conclude, “The therapeutic potential for RNAi is enormous, with applications for a wide spectrum of diseases, including some that have thus far proven intractable. Whether the obstacles now being considered or others not foreseen prove formidable will remain uncertain until results from pilot clinical studies are available….the next few years should be an important time for testing this new therapeutic approach.” Stay tuned.