Genetic science has progressed tremendously in the past three decades, with some notable milestones such as completing the Human Genome Project. Progress has been more geometric than linear, as knowledge of genetics feeds back into genetic research itself. Recently CRISPR has been getting a lot of attention, and deservedly so. The recent development of CRISPRoff adds to the power of this technology, allowing researchers to temporarily turn genes off and back on again without altering the gene itself (through an epigenetic process of methylation). With programmable technology for making genetic and epigenetic changes it seems we are on the cusp of a new age of gene therapy medicine, and I think that’s fair.
But the other half of the gene-therapy equation is just as important and deserves as much attention – we not only need to be able to engineer therapeutic genes or the mechanisms to make genetic changes, we also need a vector to get them to the target cells. This is where viral vectors come in, a technology that fortunately has been progressing alongside programmable genetic modification.
The first viral vector study was published in 1990, using an engineered retrovirus to treat two patients with Severe Combined Immunodeficiency. The results were limited and mixed, but showed the potential of viral therapy. That was over 30 years ago, which again shows the typical lag between introducing a new medical technology and working out the kinks sufficiently that it becomes mainstream therapy. In the late 1990s and early 2000s viral vector gene therapy research hit some snags due to side effects, such as viral encephalitis in one case and leukemia in another study. This meant more research was needed to make viral therapy safer before more applications could be developed.
Today we have a number of different viral vectors with good safety and different strengths and weaknesses. These include:
Adenoviruses – Delivers DNA to the nucleus but does not incorporate into the genome, so it is episomal. The advantage is that both dividing and non-dividing cells can be targeted. But the effects are limited to a single generation and do not transfer when a cell divides. The main limiting factor of adenoviruses is that they have a large immunogenic response, which limits their use in vivo.
Adeno-associated vectors (AAV) – These are similar to adenovirus in that they deliver episomal DNA. They are only mostly episomal, however, as some DNA can be incorporated into genomic and mitochondrial “hotspots”. They have a major advantage, however, because they are minimally immunogenic, and therefore safer. But they can deliver a smaller DNA payload.
Retroviruses – These were the first gene vectors, carrying RNA and reverse transcriptase so the genetic material is incorporated into the genome. The advantage here (if it is desired) is that the genetic insertion is permanent. The limiting factor, however, is that it takes cell division for this to happen, so only dividing cell populations can be targeted. There is also a potential for mutagenesis during this process – the introduction of random bits of DNA into the genome.
Lentiviruses – These are similar to retroviruses, but have the huge advantage of being able to target non-dividing cells. They can deliver a smaller RNA payload, however, and also have a potential for mutagenesis in the insertion process.
This is a good range of viral vectors that can be selected and tailored for a variety of gene therapy applications. However, none of the vectors are perfect and so there is room for improvement. But they are now safe and effective enough to be approved for clinical trials, which are now exploding. Further, there is research into combining viral vectors with CRISPR technology. AAVs appear to be the best for delivering CRISPR for the reasons stated above, mainly their great safety profile.
The potential applications for gene therapy are broad. For any tissue that can be taken out of the body and put back in, such as blood or bone marrow, the gene therapy can be done in vitro and so is much easier. This technique can be used for cancer therapy, for example by altering immune cells to target cancer cells. Gene therapy can also potentially be done in this way during in vitro fertilization (IVF), although this currently remains controversial. This would have the potential of completely curing a genetic disease prior to development.
Gene therapy in children and adults, in tissues that cannot be removed, for now requires viral vectors. The viral vector needs to be engineered to be able to infect the target cell type, and then recombinant technology is used to give it the DNA or RNA that would correct or mitigate the genetic defect. A recent example [PDF] of this approach used a recombinant adeno-associated virus serotype 5 (rAAV5) to deliver gene therapy to the retina of three subjects with a form of genetic visual impairment called Leber congenital amaurosis. The mutation results in an impaired ability to make a protein necessary to reset the photoreceptors (rods and cones) and therefore limiting sight. However, the retina can remain largely intact even into late adulthood.
The rAAV5 carries the human GUCY2D gene that makes the missing protein. In an open label preliminary study they were able to show increased photoreceptor function for all three subjects, and in one case improved visual acuity. They used a low dose to establish safety, and follow up studies will use a much higher dose in the hopes of getting a more dramatic clinical response. But the study is a good proof of principle.
It is great to see viral vector gene therapy really start to take off. This was one of the most promising new therapies introduced at the very beginning of my medical career, and it was disheartening to see clinical applications stalled for essentially two decades. But now the viral vector technology has vastly improved, and both TALEN and CRISPR offer new powerful programmable genetic modification ability. We are once again potentially at the dawn of a genuine revolution in medicine. There are potentially hundreds of diseases that can be effectively treated with gene therapy, with the promise of essentially curing diseases that previously had no hope of a cure.
But this is also a tricky time, and once again there is the potential for hype to outpace hope. This technology now works, but still it will take years and decades to develop and test specific clinical applications. I think we will see a steady drip of gene therapies in the coming years, but we have to be patient. We also have to remain vigilant for those who would exploit the hype and offer bogus gene therapies to desperate patients (similar to the explosion of fake stem cell clinics a decade ago). This can only be effectively dealt with through regulation, which often remains elusive and ineffective.
Still we can celebrate the incredible power of good reductionist science. It has given us a powerful tool, to rewrite the very basis of life itself. Now we just need to wisdom to use it optimally.
