Posted by Gene therapy is easy, in theory.
Born with a bad gene? No prob!! Just replace it with one that works! We have lots of ways of doing that! I mean, retroviruses plop new genes into our cells all the time! Half of your genome is made of leftover retroviruses! So why dont we just put a functional version of your ‘bad’ gene in a retrovirus, and let it do all the work? Yay!
Wait… no yay.
Retroviruses are still wild broncos. We cant tame them yet. Force them to insert here, but not here… so our previous efforts to use retroviruses as gene therapy vectors ended in disaster (cancer).
Luckily retroviruses arent our only hope. A potential solution to the problems retroviruses bring to the table could be a different virus– Adeno-associated virus. Wild-type AAV is completely harmless, and if we replaced the viral genome with a ‘good’ gene for gene therapy, we would know exactly where the ‘good’ gene will insert (a region on chromosome 19).
Unfortunately AAVs have their own gene therapy issues. One major problem is that up to 96% of us already have antibodies to the AAV researchers would like to use as a vector, AAV-2. And ~67.5% of us already have AAV-2 neutralizing antibodies. Inject a gene therapy AAV-2 into someone, and odds are the viruses will never reach their intended targets. Itll be creamed by the patient immune system.
Solution to this problem?
EVOLUTION, BABY!
A super sweet paper came out in Journal of Virology earlier this year, where they took the basic tenants of evolution, mutation and natural selection, and figured out a way to make a kick ass gene therapy vector:
In Vitro and In Vivo Gene Therapy Vector Evolution via Multispecies Interbreeding and Retargeting of Adeno-Associated Viruses
STEP 1: MUTATION
Previous efforts to get around all these AAV-2 antibodies was to mutate the protein these antibodies target, capsid (the structural protein). Its been kinda scatter-shot. Mutation here, mutation there, sometimes the virus cant even work anymore, meh.
Grimm et al tried something radically different. Get this. They took the capsid genes from lots of different AAVs:
AAV-2– Works great in humans
AAV-8 and AAV-9– Work super friggen great in liver cells (theyre going to try to make an ‘ideal’ vector for gene therapy for liver genes)
AAV-4, AAV-5, avian AAV, bovine AAV, caprine AAV– not prevalent in humans, we wont already have antibodies to them
They chopped up the capsid genes from each of those viruses, and smushed them back together again different combinations. So what they ended up with, is a population of viruses with lots of different kinds of capsids– each virus with different mixture than the next. One might have gotten a lot of AAV-2 and no avian. Another might have equal bits from all of the viruses, etc.
STEP 2: NATURAL SELECTION
So Grimm has a vat of viruses with different capsids. But all that diversity doesnt mean anything until you make those viruses perform a task, and let natural selection pick the variants that perform the task the best.
Grimm wanted to end up with a virus that could replicate super friggen awesome in liver cells, so he took his vat-o-virus and passaged them several times in primary liver cells, and liver cell lines. Towards the end of those passages (just five), out of that huuuuuge mix of viruses, only the viruses with a capsid combination that could replicate in liver cells the best were left. The rest had been outcompeted, and went extinct.
Well Grimm took those winner viruses and looked at their capsids– what did the mix look like? Shit. The viruses were all pretty different. Still too hard to pick ‘one’ combination that could be the best candidate for liver gene therapy.
STEP 3: MOOOOOOORE NATURAL SELECTION!!
Grimm repeated his initial experiments with another layer of selection– the viruses had to replicate really well in liver cells… AND they had to replicate really well in the presence of anti-AAV antibodies from people. Who cares if you replicate well in liver cells, if the patient will most likely neutralize the virus before it can do its job (the problem with AAV-2 in the first place).
Making the vat-o-viruses do two tricks at once narrowed the population down to one survivor. A viral variant they named AAV-DJ (I think the authors named it after themselves– Dirk and Joyce. hehe!)
Grimm replaced the wild-type AAV viral genome with the human Factor IX gene (theoretically if hFIX were given to someone with hemophilia B in a liver specific AAV, they could be ‘cured’). He then gave the viruses to mice (who were also injected with human antibodies to AAV).
Those mouse livers then made a shit load of hFIX.
The viruses got into some muscles (heart and regular muscles) and kidneys a little, but the vast majority of the new hFIX expression was confined to the liver, the place it was supposed to be.
And, AAV-DJ performed much better than the parent viruses (AAV-2, AAV-8, etc).
WHOOO!!
There is a ton more to this paper (its one of the longest I have read in a long time), but I had to write a post on their basic premise: Using evolution to make medicine better.
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