Exon Skipping for Duchenne Muscular Dsystrophy

There was some exciting data presented at MDA’s 2014 clinical Conference held this week in Chicago regarding gene therapy for Duchenne muscular dystrophy (DMD).

What is exon skipping?

skimming_stones

Many cases of DMD are caused by small deletions in the dystrophin gene which lead to frame shifts and totally disrupt transcription:

If you imagine that the gene is made up of segments (or exons) which ultimately spliced together to make a recipe or message for producing the protein:

exon-skipping-scheme

A deletion of exon 71 would be considered “in frame” because the 70 and 72 could still joint up and allow transcription.  However, a deletion of exons 48 through 50 would be “out of frame” since 47 and 51 do not splice back together to form the message:

exon

The message would become corrupted and the gene product, in this case dystrophin, would be dysfunctional or even totally absent:

frameshift

The drug Eteplirsen will link 47 and 51 back together again, and in so doing restore the reading frame and facilitate transcription of an altered but hopefully functional gene product:

exon skiip

Does it work?

A clinical study started in August 2011

The preliminary results from this study were very encouraging – the boys who received the drug maintained strength and walking ability and there were no treatment related adverse effects.

What’s the next step?

Gene therapy trial for Duchenne Muscular Dystrophy

dmd

Duchenne and Becker muscular dystrophy are both caused by mutations in the same dystrophin gene.

How it this possible?

Well, the genetic code which is translated to from proteins “talks” in words made of three letters (base pairs).

dmd dna

A gene mutation that deletes only one or two base pairs, or worse still signals the end of the word (known a “premature stop codon”) will result it a very abnormal dysfunctional gene product, leading to complete deficiency of functioning dystrophin, and the more severe Duchenne Muscular Dystrophy.

dmd muscle bx

Normal muscle bx (a) vs Duchenne muscular dystrophy (b) with complete absence of dystrophin (d)

However a gene mutation (deletion) that removes base pairs in a multiples of three is called an in-frame mutation, and causes a (sometimes only minor) qualitative change in the dystrophin protein, leading to the milder Becker’s muscular dystrophy.

Ataluren (also known as PTC124) is a small molecule designed to overcome premature stop codons.

alturen

Put simply, the idea is that it might convert some Duchenne boys in to a milder form (more like Becker’s) of muscular dystrophy by allowing them to produce some more normal dystrophin.

The drug can only help boys affected with premature stop codons confirmed by DNA testing.

The drug is currently undergoing Phase III trialsClick here for more information.

Duchenne Muscular Dystrophy

Posted by Elliot Dubowitch from Drexel University College of Medicine Class of 2014

Duchenne muscular dystrophy (DMD) is one of the four main groups  of muscular dystrophy, a muscle disorder that affects and weakens the musculoskeletal system.

Muscular dystrophies are genetically inherited and progressive.

DMD is inherited in an x-linked manner.  This means that the mother, who is unaffected, is a carrier for the disease and has a 50% chance of passing it on to her male children.

The disease is caused by a deficiency in the Dystrophin protein, a complex that anchors the muscle to surrounding tissue.

dystrophin

This disease has a wide spectrum of symptom severity, depending on the type of genetic mutation, with Duchenne being very severe, and Becker’s muscular dystrophy being much more mild.

The symptoms in DMD are not usually present at birth.  As the child ages, however, they symptoms will gradually become worse and worse.  Most children are unable to walk by thirteen years of age and die in their twenties due to respiratory failure.

One of the earliest signs of DMD is called to as the “Gower Maneuver.”  Although not pathognomonic for DMD, this maneuver is a sign for proximal muscle weakness and is often correlated with DMD.  Below is a clip of a child performing to Gower’s maneuver to stand.  The patient must “walk” up his body using his hands from a sitting position due to weakness in his hip and thigh muscles.  Below is a video clip demonstrating this.

Another early sign is calf pseudohypertrophy.  Although the muscle looks bigger, it is not necessarily stronger, as the functional muscle is replaced by nonfunctional fibrous tissue.

Unfortunately, there is currently no cure for DMD.  However, there is symptomatic treatment available, such as respiratory support, cardiovascular monitoring and treatment and (if needed) surgery for scoliosis.

Steroids are the only current medication that has been shown to keep the boys walking longer.  A study was conducted in which one group of boys were given steroids daily, whereas the other group of boys were given steroids 10 days on and 10 days off.  The boys receiving daily steroids walked on average until the age of 14.5 year, while the boys receiving steroids intermittently walked to only 12 years of age.  The boys receiving continuous steroids also had more side effects including weight gain, mood swings, increase risk of infection, and other side effects of steroid usage.  If one is to consider steroid use, it is imperative to remember that it must be used at the time the child is still ambulating.  The boy will not regain lost function, however he may retain his current function longer.  In the future we hope that new drugs like VBP-15 will hopefully provide the benefits of corticosteroids without some of the side effects.

Genetic research is currently being done to hopefully find a cure for this disease.

VBP-15 for Duchenne Muscular Dystrophy

Prednisone has been used since the 1970s for delaying the otherwise obligatory progressive motor deterioration seen in  Duchenne Muscular Dystrophy (DMD.

A good deal of data has been acquired over the years.  In fact there are even ongoing studies looking at different dosing regimens.

The drug is typically started between ages 4-6 at a dose of 0.75 mg/Kg.  However, steroid cause may side effects to Duchenne boys, including weight gain and behavioral problems.

How does it work?  We’re not sure, but we think the medication stabilizes muscle membranes and inhibiting cytotoxic T-cells.

Side effects are mediated by binding sites in the cell nucleus which lead to initiation of metabolic pathways :
NM2

The new drug VBP-15 is molecularity very similar to Prednisone, but lacks the 11 beta hydroxy arm, which reduces the metabolic side effects, without affecting the beneficial membrane stabilizing effects:

nm3

VBP-15 has been shown to improve muscle strength and function in an animal model of DMD without the metabolic side effects of Prednisone:

nm4

Clinical trials in humans are expected to begin in 2013-14.

Watch this space for more information.

New study compares steroid regimens in boys with Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) affects 1 in 3,600 boys and is caused by a mutation in the dystrophin gene, resulting in progressive muscle weakness.

Affected male children are normal at birth, but develop signs of muscle weakness before age 6, usually first affecting the legs and pelvis, causing difficulty getting up from the floor or a sitting position, and difficulty climbing stairs. Untreated boys are usually wheelchair dependent by age 12.

Ongoing research is leading towards pre-symptomatic diagnosis of DMD, and there has been some progress in genetic therapy for affected boys identified in these early stages of the disease.

However, so far corticosteroid therapy is the only treatment that has been shown to increase muscle strength in boys already affected with symptoms of DMD.

Steroids can cause side effects, and there is wide variability whether doctors doctors prescribe prednisone or deflazacort, as well as the dosing, duration of steroid use or even whether steroids are prescribed at all.

A new study aims to answer some of these questions.

The study is looking to enroll boys aged 4-7 with genetically confirmed DMD who have not taken steroids before.

They will be included in the study for 3-5 years and receive either:
1. Prednisone 0.75mg/Kg/d, or
2. Prednisone 0.75mg/Kg/d 10days on then 10 days off, or
3. Deflazacort 0.9mg/Kg/d

They will need hospital visits every 3 months for the first 6 months, then every 6 months thereafter.

This study is available locally at Penn Sate Hershey Medical Center – parents or physicians of interested patients should contact Beth Stephens at 717-531 0003 extension 283395 or by email for more information.