A MOLECULAR Trojan horse that can slip past the brain鈥檚 defences has proved to be very effective at delivering genes to the brains of primates. It could be used to treat a host of brain disorders, from Parkinson鈥檚 to epilepsy.
Treating the brain is very difficult because of the 鈥渂lood-brain barrier鈥� created by the tight junctions between the cells lining the capillaries. Only molecules recognised by the cell receptors can get in, unless they are very small. The viruses most gene therapists use to deliver genes are too big, and have to be injected directly instead. Even then, the genes are not expressed widely and evenly throughout the brain.
鈥淨uite frankly, the existing delivery systems have been woeful failures,鈥� says William Pardridge of the University of California, Los Angeles. Instead, his team has been perfecting a way to get genes into the brain hidden inside fatty spheres called liposomes.
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First the team coats the liposomes with a polymer called polyethylene glycol (PEG), without which they would be purged from the blood within minutes. Next, antibodies that latch on to some of the brain-capillary receptors are tethered to a few of the PEG strands. The antibodies trick the receptors into letting the liposomes pass, where they can deliver their cargo to brain cells.
Pardridge鈥檚 team has already shown that the technique works in rats, by delivering the gene for the luminescent protein luciferase (快猫短视频, 10 June 2000, p 10). Now the team has tested the liposomes in rhesus monkeys, using antibodies specific to primate brain receptors. Not only did it work, but the amount of luciferase produced was 50 times greater than in rats (Molecular Therapy, vol 7, p 11).
鈥淚 haven鈥檛 seen anything like this for viral or non-viral vectors,鈥� says Savio Woo, director of gene therapy at the Mount Sinai School of Medicine in New York. 鈥淭o reach the central nervous system through the blood-brain barrier in a non-human primate with this kind of efficiency 鈥� that鈥檚 absolutely fantastic.鈥�
The liposomes do not appear to have any toxic side effects, though they do deliver genes to other organs besides the brain. But the team has shown that by choosing the right switch to turn on the gene, the gene will be active only in the desired tissues.
Because the genes are not integrated into the genome, weekly or monthly injections would be needed for long-term treatment. But Pardridge sees this as an advantage, because there鈥檚 no risk of genes lodging permanently in the wrong place and triggering cancer 鈥� a worry with some gene therapy viruses.
The method shows promise for treating Parkinson鈥檚. The team gave rats a neurotoxin that causes Parkinson鈥檚-like symptoms by cutting production of the key enzyme tyrosine hydroxylase. Four weeks later, the team injected the rats with liposomes containing a gene that boosts production of the enzyme. Three days after that, the rats鈥� abnormal movements were reduced by 70 per cent.
The liposomes can also deliver cargoes other than genes, including drugs and 鈥渁ntisense鈥� RNA. The lifespans of mice with brain tumours doubled when the liposomes were used to deliver antisense RNA to block production of a growth factor. And in studies yet to be published, the team has exploited a mechanism called RNA interference (快猫短视频, 15 March, p 20), delivering fragments of double-stranded RNA that 鈥渟ilence鈥� cancer genes.