Q: What happens in BMD?
A: In BMD, the loss of genetic code involves groups of three’s, and the reading frame is not affected. The result is a shorter piece of genetic code, which still makes some sense and produces a shorter but partially functional dystrophin protein:
For example the letters ANDHISDOG are missing. This leaves us with:
This sentence is not complete but at least it makes sense and a partially functional protein is produced. You can see that the deletion in this example is larger than the example above causing DMD but because it is a multiple of three, it does not disrupt the reading frame. There is still some sense, which means that some protein can be produced.
Q: Can we turn a DMD type mutation into a BMD type mutation?
A: Yes, this is the objective of using ‘molecular patches’.
Q: What is the ‘molecular patch’ or ‘exon skipping’ technique?
A: It involves making a very small piece of genetic material (‘molecular patch’), which once inside a muscle cell, will bind to its matching sequence of genetic code. This ‘patch’ is designed so that it binds a region surrounding the genetic error. When editing takes place to remove the non-essential regions of code, the area covered by the patch is not included in the final sequence, which goes on to produce the protein. In this way the reading frame is altered so that it becomes readable. This may be clearer using the following example:
Consider the mutation we used above where the letters ND are missing. If a ‘patch’ is made to bind to the letters AHISDOG, and delivered to muscle cells, it will bind to its matching genetic sequence. During editing the AHISDOG will not be included and we are left with:
The effect of this is to turn a sentence, which could not be read, into one, which can. With the dystrophin gene, this is the difference between no protein being produced and a BMD like protein being produced.
Q: Does this really work?
A: So far scientists have shown this technique to have therapeutic effect in a mouse model of DMD (the mdx mouse), in the dystrophic dog model and in human DMD muscle cells grown in the laboratory.
The first clinical trial has been conducted in Holland testing the safety and local efficacy of administering a molecular patch built with a chemistry different from the one used by the MDEX consortium, directly in a leg muscle. The results have been published last year in the prestigious journal New England Journal of Medicine: van Deutekom JC et al, Local dystrophin restoration with antisense oligonucleotide PRO051.N Engl J Med. 2007 Dec 27;357(26):2677-86, PMID 18160687.
The MDEX Consortium completed a similar trial using a different more effective chemistry: Restoring Dystrophin Expression in Duchenne Muscular Dystrophy: A Phase I/II Clinical Trial Using AVI-4658. The results were published in 2009, Kinali et al, Lancet Neurology, 8 , 918 - 928, October 2009, PMID 19713152. In summary, the results showed that AVI-4658 is safe, does not cause any harm to humans following local administration and it is effective in restoring dystrophin production.
Recently, the MDEX Consortium completed an additional dose ranging Phase ll study using the same study drug for multiple intravenous injections in DMD patients. Details of this study is available further down this information leaflet.
Q: Do scientists understand how this works in the body?
A: The process of how the molecular patches induce exon skipping is quite well understood. However more work is required to understand precisely how the molecular patches are picked up from the muscle cells so that we can optimise the use of these ‘patches’ to make them work more efficiently and have maximum therapeutic benefit.
Q: Will it work for everyone with DMD?
A: No, but it is thought that around 60-70% of the genetic errors associated with DMD could be treated with the use of ‘molecular patches’.
Q: What other options are there if ‘molecular patches’ are not suitable for a particular type of genetic error?
A: This technique is just one of many identified by scientists as having therapeutic potential. Researchers worldwide are investigating many different approaches, which may result in other therapies. These include techniques such as transferring a working copy of the dystrophin gene and drug treatments.
Q: How do you determine whether ‘molecular patches’ will be helpful as a treatment for a particular type of mutation?
A: There are specialised tests, which enable scientists to determine the exact nature of the genetic error. In many cases the doctors already know this information, especially if you have been diagnosed in recent years.
Q: Will the same ‘molecular patch’ work for everyone?
A: No, the dystrophin gene is very large and the genetic errors associated with DMD occur in different places along this gene. There are however some common areas for mutations and initially ‘molecular patches’ will be made for these to prove that the technique works. It is thought that several different ‘patches’ will be required to cover the spectrum of genetic errors. Once the technology has been shown to be effective for a particular error it will be possible to design other ‘patches’.
Q: Will ‘molecular patches’ help the more severe forms of BMD?
A: The effectiveness of ‘molecular patches’ is not dependant on the condition. In fact it has been shown to be therapeutic in other related conditions. The key issue is whether altering the way in which the genetic code is read has a therapeutic effect. There may be certain instances where ‘molecular patches’ might be helpful in severe forms of BMD; however most individuals with BMD will probably not benefit from this approach.
Q: I have heard about a therapeutic trial funded by the Medical Research Council and AVI Biopharma on “molecular patches”. What does this involve?
A: A group of scientists (MDEX Consortium) who work very closely with the three charities involved in DMD in the UK, Action Duchenne, Duchenne Family Support Group (DFSG) and the Muscular Dystrophy Campaign (MDC), secured funds towards a further genetic therapy trial in DMD using “molecular patches” from the Medical Research Council (MRC) and AVI-Biopharma. The aims of this project were to perform a dose ranging study of intravenously administered patches in a group of 18 DMD children to test the safety and the efficacy of this approach. Efficacy will be measured by checking at the end of the study if functional dystrophin is produced in these boys after treatment.
Q: Who participated in this dose finding safety and efficacy trial?
A: Participants were selected using specific inclusion criteria. The following are some of the main inclusion criteria:
Boys with a diagnosis of DMD between ≥ 5 years and ≤ 15 years of age at the time of entrance in the study and being able to walk more than 25m by themselves.
Eligible deletions of the dystrophin gene are those that can be rescued by the skipping of exon 51 [45-50; 47-50; 48-50; 49-50; 50; 52; 52-63].
A number of other clinical parameters were taken into account such as the ability of the patients and the family to consent and collaborate with the assessment protocol. Patients with severe cardiomyopathy or respiratory insufficiency (patients ventilated at night) were excluded from the study.
The vast majority of individuals are seen by specialists who keep detailed records and these were used to select suitable candidates for the trial. It was stressed that this was only a safety trial and there will be no therapeutic benefit for those participating.
For more detailed information please consult the following web page:
http://www.mdex.org.uk/index.php
Q: How did this study look like?
A: Six groups of 2-4 children each were recruited into this study, between London and Newcastle. Each group of 2-4 children received a different dose of AVI-4658, the same molecular patch that the MDEX consortium has previously used in the intramuscular study. The molecular patch was administered directly into the veins, each week, for a period of 12 weeks in each participant.
Q: How long will it be before it is known whether ‘molecular patches’ can be used as a treatment for DMD?
A: This research project is funded for three years. We recruited all children into this study in 2009, and the clinical part of the study was completed in early 2010. The MDEX Consortium and AVI BioPharma are in the process of analysing the data for submission to the UK regulatory bodies.
Preliminary data from this study was presented as ‘Late Breaking News’ at the International Congress on Neuromuscular Diseases, Naples, Italy 17-22 July 2010.
In summary - AVI-4658 was well tolerated and no drug related severe or serious adverse events were recorded with either single doses or cumulative exposure. AVI-4658 induced dystrophin protein expression in a generally dose dependent manner in boys from cohort 3 onwards. A total of 7 patients had displayed a clear increase of dystrophin expression in the post treatment biopsy.
Q: Are ‘molecular patches’ a cure?
A: No, this type of therapy is not a cure because the faulty dystrophin gene is still present. This means that if proven to be effective, this treatment would need to be repeated and how often this would need to be done will become apparent during this project.
GLOSSARY:
Animal models: Animals having conditions comparable to humans, which can be used to study disease processes. Animals represent a simpler system than humans for studying the roles of different proteins and for testing potential therapies. Results can then be applied to humans; the use of a simpler system often allows quicker progression of research.
Becker muscular dystrophy (BMD): A milder variant of Duchenne muscular dystrophy. It is X-linked, slowly progressive, causes muscle weakness and usually only affects boys.
Deletion: The loss of a bit of genetic material from a chromosome or gene.
DNA: Deoxyribonucleic acid, the chemical composition of genes. It contains coded information, arranged in a linear sequence. Each cell’s chromosomes contain about two metres of DNA, yet it is so thin that it is barely visible even with the most powerful microscope. If all the DNA in a human body were stretched end to end it would be long enough to reach the moon and back about 10,000 times.
Duchenne muscular dystrophy (DMD): A genetic disorder, which causes progressive muscle weakness as the muscle cells break down and are eventually lost. Usually it affects only boys and is caused by a lack of dystrophin protein.
Dystrophin: The protein, which is missing in boys with Duchenne muscular dystrophy and reduced in boys with Becker muscular dystrophy. Dystrophin binds to other proteins in the dystrophinglycoprotein complex (DGC), absence of these components are implicated in different forms of muscular dystrophy.
Genes: The coded instruction that govern the make-up of every human being. Genes are made of DNA. Each gene carries instructions for the production of a specific protein. Genes usually come in pairs, one copy inherited from each parent. They are passed on from one generation to the next, and are the basic units of inheritance. Alterations in genes (mutations) can cause inherited disorders.
