Mouse Model Reflects Small Population but Could Provide Big PayoffMouse Model Reflects Small Population but Could Provide Big Payoff https://pediatricsnationwide.org/wp-content/themes/corpus/images/empty/thumbnail.jpg 150 150 Kevin Mayhood Kevin Mayhood https://secure.gravatar.com/avatar/bd57a8b155725b653da0c499ae1bf402?s=96&d=mm&r=g
- January 05, 2016
- Kevin Mayhood
First model of exon duplication causing DMD enables testing for potential therapies
Duplication of exons, the parts of a gene that code for amino acids, cause six to 10 percent of cases of Duchenne muscular dystrophy, which is due to mutations in theDMD gene. New research suggests skipping, or excluding, one copy of a duplicated exon during assembly of the final gene message may have a large payoff: production of normal dystrophin — the missing protein that protects muscle from injury — and elimination of disease symptoms.
Researchers at Nationwide Children’s Hospital have developed the first mouse model to investigate whether this can be done. Called the Dup2 mouse, the model has two copies of exon 2. The mutation is the most common duplication found among Duchenne muscular dystrophy (DMD) patients, accounting for about 1 percent or an estimated 100 cases in the United States.
The researchers have seen positive results from treating Dup2 mice with a virus that delivers a non-coding RNA that interferes with the splicing of exon 2 during the exon splicing process that results in a mature messenger RNA (mRNA), the final blueprint for gene translation. The interference causes either one or both copies of exon 2 to be excluded.
“There are drugs in the approval process that could treat around 13 percent of patients by causing skipping of exon 51 in order to convert Duchenne muscular dystrophy to the milder Becker muscular dystrophy,” says Kevin Flanigan, MD, a neurologist and principal investigator in the Center for Gene Therapy in The Research Institute at Nationwide Children’s.”In these cases, exon 51 is adjacent to a region of deleted exons, and although the resulting deletion is one exon larger, the final mRNA encodes a partially functional protein.But if the cause of disease is a duplication and you skip one copy, you could make a DMD patient’s mRNA entirely normal, resulting in a normal dystrophin protein and potentially an even better clinical outcome.”
There are 79 exons in the dystrophin gene and most DMD patients have deletions of one or more exons. But, until recently, there was no good way to study duplications and how to treat them.
Dr. Flanigan and colleagues used homologous recombination, a well-established method, to duplicate exon 2, making use of a breakpoint region within intron 2 where duplications arecommonly located in human patients. The Dup2 mouse produces no dystrophin, and has similar muscle pathology and physiology to the standard model used in DMD research, the mdx mouse.
The new mouse will be a useful tool in which to test whether exon skipping therapies can be manipulated to result in the skipping of just one copy of a duplicated exon says Dr. Flanigan, who is also a professor of pediatrics and neurology at The Ohio State University College of Medicine. “Titrating to skip one copy may prove difficult.”
But in the case of exon 2, even skipping both copies is likely to be therapeutic. The researchers have found that the skipping of both copies activates an internal ribosome entry site (IRES) within exon 5, resulting in translation of a version of dystrophin that appears to be nearly as protective as normal dystrophin.
In testing, the rear foot grip strength and force of an untreated Dup2 mouse was significantly weaker than in a healthy control mouse. But in Dup2 mice treated with the virus-delivered non-coding RNA to skip both copies of exon 2, strength and force were comparable to the control mouse.
“We can’t say it’s as good as a normal protein,” Dr. Flanigan says, “but based upon both our experiments in the Dup2 mouse and upon our observations of patients we have identified who express this version of dystrophin, we can say that it’s pretty darn close.”
The researchers are continuing to study the virus-deliveredtreatment and are working with a company to investigate and develop other methods of skipping.
The research has been supported by the patient advocacy foundation CureDuchenne.
- Vulin A, Wein N, Simmons TR, Rutherford AM, Findlay AR, Yurkoski JA, Kaminoh Y, Flanigan KM. The first exon duplication mouse model of Duchenne muscular dystrophy: A tool for therapeutic development. Neuromuscular Disorders.2015 Nov;25(11):827-34.
- Wein N, Vulin A, Passarelli C, Falzarano MS, Szigyarto CA, Maiti B, Findlay A, Heller KN, Uhlen M, Bakthavachalu B, Messina S, Vita G, Gualandi F, Wildon SD, Rodino-Klapac LR, Yang L, Dunn DM, Schoenberg DR, Weiss RB, Howard MT, Ferlini A, Flanigann KM. Translation from a DMD exon 5 IRES results in a functional dystrophin isoform that attenuates dystrophinopathy in humans and mice. Nature Medicine.2015 Apr;21(4):414.
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