Posts Tagged ‘gene therapy’

dsRNA: new-new therapy, or…?

1 April, 2010

We plant virologists – or almost-former plant virologists, in my case – have a slightly cynical attitude towards the all-new, all-encompassing craze among cell biologists and mainstream virologists that is siRNA, and all its purported applications.

This is because the phenomenon of RNA silencing was first discovered in the context of unexpected resistance in transgenic plants to the homologous virus, mediated by transgenes that either produced no measurable amount of protein, or mRNAs that were not translatable.  The phenomenon was known as “post-transcriptional gene silencing” back then, among plant molecular biologists and virologists – until, that is, it was taken up by the mammalian and insect cell biology folk, whereupon its origins were quickly forgotten, and the Nobels went to…well, let us just say, not to whom some folk thought they ought.

But I digress:  suffice it to say that siRNA has now been amply demonstrated to be not only the eukaryotic cell’s (and especially those of plants) adaptive nucleic acid-based immune response to virus infection, but also a widely used means of regulation of gene expression (see here).  Needless to say, its potential uses for gene and disease therapy are also multiplying daily – which is when people forget the roots of the science.   At first sight, the New Scientist issue of 23rd March – which has an excellent article on the use of siRNA-based strategies to combat insect pests – goes some way to redressing that, given that the science has found its way back to plants.  It is especially interesting that delivering siRNA-eliciting constructs in insects can be achieved by simply feeding them dsRNA of the appropriate sequence, rather than by use of chemically-altered or encapsulated material.

 However, and here’s where one can see that no-one except plant virologists reads the plant virology literature (the converse being untrue, naturally), a problem crops up when the article goes on to discuss whether or not dsRNA is safe.  In a side box in the article, this is said:

Is it safe?

Using gene silencing, or RNA interference (RNAi) to target specific pests while leaving other species unharmed sounds like an enormous step forward. But can we be sure that the key ingredient – double-stranded RNA (dsRNA) – won’t have unexpected side effects in people?

Although most RNA in cells is single-stranded, all plants and animals also produce dsRNA to regulate the activity of their own genes. “There are lots of dsRNAs in the plant and animal products that we eat every day,” says Michael Czech, a molecular biologist at the University of Massachusetts Medical School in Worcester, who is exploring ways to use RNAi to treat type II diabetes. “Those RNA molecules are rapidly chopped up by the enzymes in our gut and are non-toxic.”

There is also a second line of defence in the form of enzymes in our blood that break down dsRNA. “You could inject dsRNA into primates at moderate doses and nothing would happen, [my emphasis]” says Daniel Anderson of the David H. Koch Institute for Integrative Cancer Research at the Massachusetts Institute of Technology, who is designing drugs based on RNAi.

Ummmm…sorry, that simply isn’t true: as long ago as the early 1970s, plant and fungal virologists had hit on the idea of using antibodies specific for dsRNA to detect the molecules in extracts of hosts infected with dsRNA (and even ssRNA) viruses.  Here’s two examples of papers from then:

Detection of mycoviruses using antiserum specific for dsRNA.
Moffitt EM, Lister RM.  Virology. 1973 Mar;52(1):301-4.

Immunochemical detection of double-stranded ribonucleic acid in leaves of sugar cane infected with Fiji disease virus.
Francki RI, Jackson AO.  Virology. 1972 Apr;48(1):275-7.

And of course, using antibodies to dsRNA implies that one can raise them in the first place…which, as I recall (my career started shortly afterwards, and I read these papers), was as a consequence of raising antisera to dsRNA-containing phytoreovirus and cryptovirus virions.  It was later found that sera to synthetic ds oligoribonucleic acids also detect dsRNAs – all of which means that injection of dsRNAs is likely to elicit antibodies against them.  With who knows what corollaries…because not too many plant virologists were too worried about long-term effects in the mainly bunnies that they injected.

Of course, the route to dsRNA pesticide effects is via the insect gut – and the same New Scientist article has this to say about dosing humans thus:

“There are lots of dsRNAs in the plant and animal products that we eat every day,” says Michael Czech, a molecular biologist at the University of Massachusetts Medical School in Worcester, who is exploring ways to use RNAi to treat type II diabetes. “Those RNA molecules are rapidly chopped up by the enzymes in our gut and are non-toxic.”

Ye-es…they would say that, wouldn’t they?  And no-one knew you could make Abs to dsRNA before someone noticed phytoreovirus antiserum bound dsRNA – so it would be a very interesting exercise to assay sera from individuals or animals previously exposed to multiple rounds of dsRNA rotavirus infection, and see whether these contained dsRNA-specific Abs, wouldn’t it?  The NS article negates some of its own reassurances by saying:

So there is good reason to think sprays containing dsRNAs lethal to insects, or plants modified to produce them, will pass all the safety tests. However, if the RNA was altered in a way that allows it to get into human cells, perhaps as a result of changes intended to make it persist longer in the environment, it might cause problems. “If you modify the dsRNA – by encapsulating it or changing the RNA molecule – then you are imposing a new chemistry that could have toxic effects on humans,” Czech cautions.

Yeah – like encapsidating it like a virus does….

And it is all probably a bit of a sideshow, in that we are in fact exposed to dsRNAs in our diet all the time: especially if one eats organic, the fresh fruits and uncooked vegetables will be rife with dsRNA-containing fungal viruses; even material containing ss+RNA viruses contains significant amounts of replicative form dsRNA  (including insect material, BTW) – so a little more used as pesticide probably wouldn’t hurt.

We hope.

Re-engineering AAV

8 February, 2010

Adeno-associated virus (AAV) virion. Copyright Russell Kightley Media

Tweaking virus vectors used for gene therapy to change their receptor specificity is not necessarily new – but it has seldom been done (at least, to my mind) as elegantly as is reported in January’s Nature Biotechnology.  Asokan et al. report on

Reengineering a receptor footprint of adeno-associated virus enables selective and systemic gene transfer to muscle
Nature Biotechnology 28, 79 – 82 (2010)
Published online: 27 December 2009 | doi:10.1038/nbt.1599

From the abstract:

We generated a panel of synthetic AAV2 vectors by replacing a hexapeptide sequence in a previously identified heparan sulfate receptor footprint with corresponding residues from other AAV strains. This approach yielded several chimeric capsids displaying systemic tropism after intravenous administration in mice. Of particular interest, an AAV2/AAV8 chimera designated AAV2i8 displayed an altered antigenic profile, readily traversed the blood vasculature, and selectively transduced cardiac and whole-body skeletal muscle tissues with high efficiency. Unlike other AAV serotypes, which are preferentially sequestered in the liver, AAV2i8 showed markedly reduced hepatic tropism.

What impressed me most about the paper was the excellent modelling graphics: the authors were able to show, in simple 3-D atomic models, just how their mutations had changed the surface archotecture of the virus in question.  The whole-animal imaging was also very useful in showing very simply how effective their different constructs were.

(a) Three-dimensional structural model of the AAV2 capsid highlighting the 585–590 region containing basic residues implicated in heparan sulfate binding. Inset shows VP3 trimer, with residues 585-RGNRQA-590 located on the innermost surface loop highlighted in red. VP3 monomers are colored salmon, blue, and gray. Images were rendered using Pymol. (c) Representative live animal bioluminescent images of luciferase transgene expression profiles in BALB/c mice (n = 3) injected intravenously (tail vein) with AAV2, AAV8, AAV2i8 and structurally related AAV2i mutants (dose 1 × 1011 vg in 200 μl PBS) packaging the CBA (chicken beta actin)-Luc cassette. All AAV2i mutants show a systemic transduction profile similar to that of AAV8, with AAV2i8 showing enhanced transduction efficiency. Bioluminescence scale ranges from 0–3 × 106 relative light units (photons/sec/cm2). Residues within 585–590 region in each AAV2i mutant is indicated below corresponding mouse image data. (d) Comparison of AAV2, AAV2i8 and AAV8 capsid surface residues based on schematic “Roadmap” projections. A section of the asymmetric unit surface residues on the capsid crystal structures of AAV2 and AAV8, as well as a model of AAV2i8, are shown. Close-up views of the heparan sulfate binding region and residues 585–590 reveal a chimeric footprint on the AAV2i8 capsid surface. Red, acidic residues; blue, basic residues; yellow, polar residues; green, hydrophobic residues. Each residue is shown with a black boundary and labeled with VP1 numbering based on the AAV2 capsid protein sequence.

Adapted by permission from Macmillan Publishers Ltd: Nature Biotechnology 28, 79 – 82 copyright (2010)

Changing the tissue specificity of a well-characterised and often-used vector virus such as AAV in this way is an extremely useful thing to have done: it probably lowers the potential toxicity of the vector – by avoiding the liver – while preserving useful features such as the higher-level expression afforded by use of AAV2.