Friday, August 12, 2011

More gene therapy

The CLL "cure" is not the only gene therapy paper published this week. In today's Lancet is another 'miracle'. It is a study that I gave authority to proceed when I was on the Gene Therapy Advisory Committee.

Duchenne muscular dystrophy is a progressive, severely disabling neuromuscular disease that affects one in 3500 newborn boys and causes premature death in late teens of twenties. In Duchenne muscular dystrophy, the open reading frame of the X-linked dystrophin gene (DMD) is disrupted by deletions (roughly 65%), duplications (10%), point mutations (10%), or other smaller rearrangements.

That means that this is a very severe genetic disease of young boys that is universally fatal at an early age. We know the genetic lesions that cause it, though only two thirds have the commonest mechanism, the other mechanisms have the same effect in producing (or failing to produce) an abnormal muscle protein. These boys get weaker and weaker until they are completely paralyzed.

Dystrophin is located underneath the sarcolemma and assembles with sarcolemmal proteins such as dystroglycan, α-sarcoglycan, and neuronal nitric oxide synthase
(NOS) to form the dystrophin-associated glyco protein complex. The essential function of dystrophin in muscle is to connect the subsarcolemmal cytoskeleton to the
sarcolemma by binding N-terminally to F-actin and C-terminally to β-dystroglycan. Loss of dystrophin results in inflammation, muscle degeneration, and replacement
of muscle with fibroadipose tissue.

That bit is really for those with a technical background.

In the milder allelic Becker muscular dystrophy, dystrophin mutations do not disrupt the open reading frame; a shortened but functional dystrophin protein is produced, and most patients (male and female) are able to walk into late adulthood and have a normal lifespan. Therefore, induction of exon skipping to restore the open reading frame is an attractive therapeutic strategy in Duchenne muscular dystrophy that can be achieved with splice switching oligomers.

This means that they are going to try by genetic engineering to replace the faulty gene with one that at least gets read by the cells even though it does not produce the proper protein. It will mean substituting a fatal disease by a less severe one, though one which will mean being unable to walk by the time they are 60.

These oligomers are typically 20–30 nucleotides in length and are complementary in
sequence to regions of the pre-mRNA transcript relevant for targeted DMD exon skipping. Splice switching oligomers targeting dystrophin exons have been successfully used to restore dystrophin expression in vitro and in various animal models of Duchenne muscular dystrophy. In the mdx mouse, administration of 2´O-methyl-ribooligonucleosidephosphorothioate (2´OMe) and phosphorodiamidate morpholino oligomers (PMOs) identified PMOs as more effective for induction of exon skipping and restoration of long-lasting dystrophin production after intramuscular or intravenous administration. In the X-linked muscular dystrophy dog, PMO administration was followed by dystrophin restoration and clinical benefit without adverse reactions.

I know it sounds terribly complicated, but this is what scientists do all day. In a former life I made a film for television about these boys and made a plea that animal experiments should be allowed to continue in order to find a cure.

In this paper, the authors show for the first time that repeated systemic
administration of a PMO splice switching oligomer (AVI-4658) induces targeted exon skipping in skeletal muscle in patients with Duchenne muscular dystrophy, restoring correctly localised dystrophin at the sarcolemma. The administration of AVI-4658 was very well tolerated, without clear drug-induced adverse events with single doses of up to 900 mg and cumulative exposure exceeding 10,000 mg. The absence of drug-related adverse events after 12 weeks is encouraging, but caution is still needed because any splice switching oligomer would need to be given lifelong.

A clear and significant dose response was recorded in terms of dystrophin protein expression, leading to seven patients who responded to treatment at the higher doses.
This finding was accompanied by a significant reduction of inflammatory infiltrates in patients in the two highest dose cohorts. Patients with the highest levels of
dystrophin also had increased sarcolemmal expression of proteins of the dystrophin-associated glycoprotein complex.

The safety profile that they have noted with AVI-4658 at doses of 20 mg/kg, supported by animal testing at up to human equivalent doses of 100 mg/kg, is encouraging and bodes well for longer administration periods and higher clinical dose. Preclinical data suggest that repeated administration of even small doses over an extended time achieves more homogeneous restoration of dystrophin than does the
same cumulative dose administered as a bolus injection of PMO. This finding suggests that a long period of administration will be necessary to achieve homogeneous
dystrophin expression.


Jon Moulton said...

"This means that they are going to try by genetic engineering to replace the faulty gene..." That's an interesting characterization, and one I sometimes consider using but typically avoid. Genetic engineering implies a change to the genetic material, but these oligo-based splice-modification therapies make no change to DNA, instead affecting the splicing of pre-mRNA to make a modified mRNA. Is this genetic engineering? Is it a gene therapy? I generally disagree with the former but accept the latter. It is a hazy area of semantics.

Deb Light said...

Thanks for your valued opinion on this subject Dr. Terry...Dr. Keating was wanting to try something similar to this with my blood in 2004 but decided against it because there wasn't enough positive results back then.Maybe now there soon will be..maybe perhaps my next treatment but not unless I have a rich uncle somewhere or my Insurance would cover it!
God Bless,

Deb Light

Terry Hamblin said...


I was being innacurate. There is a splice variant that produces Besker type dydtrophe. This procedure excludes th translation of the Duchenne damaged Exon and instead allows the splice variant to be used.

Jon Moulton said...

Typically it is an exon adjacent to the Duchenne damaged exon that is targeted, at least in the exon-51 oligos now in clinical trials. The mutations entering the clinical trials cause a frameshift of downstream sequence. The frameshift can be corrected by skipping a second exon which has a number of bases appropriate to restore the reading frame of the dystrophin mRNA. In the simple case, this results in translation of a dystrohin mRNA that is missing two exons (the mutation-skipped exon and the therapeutically-skipped exon). Some mutations, not yet to trials, may require skipping several exons to restore the reading frame.

In the case of the well-studied mdx mouse, the oligo is targeted to remove a non-frameshifting exon that contains a mutant stop codon from the mRNA (this is the sort of situation you describe in your reply of 13 August 2011).