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.

Back at Gene Therapy

I haven't been to the Gene Therapy Advisory Committee since February. I attended the last meeting of the year yesterday, and I took some time to get back into the swim of things. There were three proposals for us to look at. The first one dealt with the problem of cytomegalovirus (CMV)in transplant recipients.

CMV is a herpes virus; most of us get it, usually subclinically, but it remains in a dormant form ready to reactivate if ever the immune system fails in its surveillance. This is likely to happen following a marrow transplant, especially when the donor stem cells come from a seronegative individual. There are drugs that are fairly effective like gancyclovir and foscarnet, but these are themselves toxic. The idea here was to transfect the donor cells with a T cell receptor that reacts with CMV antigens and infuse these into the recipient as a virus specific donor lymphocyte infusion.

The second idea was to deal with Parkinson's disease. This is another disease for which there is a treatment, but it becomes unsatisfactory as time passes. L-Dopa gets converted into dopamine which is a neurotransmitter in the substantia nigra in the brain. As the cells in the substantia nigra die there are too few to do the conversion, no matter how much L-Dopa there is. The idea here is to transfect other cells in this part of the brain with the genes to make the necessary enzymes to do the conversion.

The other submission builds on the progress made in both the UK and the USA in injecting genes directly into the retina of the eye for some forms of retinitis pigmentosa. In theory this technique should work for any single gene disorder that causes blindness. One such is choroideraemia which affects 1 in 50,000 people. The missing gene is REP-1 and this ought to work.

We have also been given the responsibility for supervising stem cell research. Thus far we have not had a proposal to consider. It may be that we will be able to see the successful and safe generation of pluripotent stem cells from the patient's own fibroblasts, before I am forced to declare my opposition to embryonic cell treatment. The one company that looked like being able to go forward with an embryonic cell line seems to have been stymied over regulatory issues.