After decades of disappointingly slow progress, researchers have taken a substantial step towards a possible treatment for Duchenne muscular dystrophy with the help of a powerful new gene-editing technique.
Duchenne muscular dystrophy is a progressive muscle-wasting disease that affects boys, putting them in wheelchairs by age 10, followed by an early death from heart failure or breathing difficulties. The disease is caused by defects in a gene that encodes a protein called dystrophin, which is essential for proper muscle function.
Because the disease is devastating and incurable, and common for hereditary illnesses, it has long been a target for gene therapy, though without success. An alternative treatment, drugs based on chemicals known as antisense oligonucleotides, is in clinical trials.
But gene therapy - the idea of curing a genetic disease by inserting the correct gene into damaged cells - is making a comeback. A new technique, known as Crispr-Cas9, lets researchers cut the DNA of chromosomes at selected sites to remove or insert segments.
Three research groups, working independently of one another, reported in the journal Science on Thursday that they had used the Crispr-Cas9 technique to treat mice with a defective dystrophin gene. Each group loaded the DNA-cutting system onto a virus that infected the mice's muscle cells, and excised from the gene a defective stretch of DNA known as an exon.
Without the defective exon, the muscle cells made a shortened dystrophin protein that was nonetheless functional, giving all of the mice more strength.
The teams were led by Charles A Gersbach of Duke University, Eric N Olson of the University of Texas Southwestern Medical Center and Amy J Wagers of Harvard University.
"The papers are pretty significant," said Louis M Kunkel, a muscular dystrophy expert at Boston Children's Hospital who discovered the dystrophin gene in 1986.
The dystrophin protein plays a structural role, anchoring each muscle fibre to the membrane that encloses the muscle-fibre bundle. The dystrophin gene, which guides the protein's production in the cell, sprawls across about one per cent of the X chromosome and is the largest in the human genome.
That gene has 79 sections, or exons, but can evidently maintain reasonable function even if a few exons in the middle are lost. The protein works as long as its two ends are intact.
This is what happens in a milder disease known as Becker muscular dystrophy, in which mutations cause instructions from a few exons to be skipped during the protein-making process. In Duchenne muscular dystrophy, however, mutations cause muscle cells to make a truncated protein missing one end, and this protein does not work at all.
This difference suggests a possible treatment strategy: Removing damaged exons so Duchenne patients' muscle cells produce an intact, though shorter, dystrophin protein, much like that seen in Becker patients.
A laboratory strain of mice has Duchenne-type muscular dystrophy in which a major part of the dystrophin protein is lost because of a mutation in the 21st exon of the gene. In 2014, Olson's team reported that it had been able to edit out the damaged exon, enabling muscle cells to generate a functional protein.
That gene editing was done in the fertilised egg of the mouse, making an inheritable change to the mouse's genome. There is a moratorium on making such changes to the human genome, and in any case, such an intervention would come too late for muscular dystrophy patients. So Olson's next step was to see if he could produce the same result in the muscles of young mice.
In the study published Thursday, Olson's team reported that they loaded the gene-editing system into a harmless virus, along with guides that directed it to cut the two ends of the 21st exon. The virus infected muscle cells throughout the mouse's body, snipping out the exon from the dystrophin gene.
The muscle cells repaired the DNA by joining the pieces of the cut chromosome and generated an effective dystrophin protein.
©2015 The New York Times News Service
