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Altering our proteome: the optionsThere are five fundamentally distinct ways to introduce a protein (or an RNA -- for brevity I will hereafter mention only proteins) into a person's cells and/or extracellular milieu. The simplest of the five is transplantation: introduce cells or organs from someone who already expresses the protein. This is clearly limited to proteins that some humans express (a relevant limitation, as discussed here, for example), and generally also carries the side-effect of an immune response which must be suppressed by lifelong drug administration, impairing the recipient's resistance to infections and possibly to cancer. The second option is ex vivo genetic modification of cells followed by their introduction into the recipient. This has many advantages over transplantation -- e.g., genes expressing non-human proteins can be added, and autologous cells (ones taken from the patient) can in theory be used and an immune response thereby avoided. However, this approach (generically termed "cell therapy" and including stem cell therapy) still has limitations: the cells that the engineered cells are designed to replace must be eliminated. Sometimes they are already gone and that is precisely the problem to be rectified, but sometimes they are present but failing. They can theoretically be eliminated in conjunction with introducing new cells, but there are profound technical difficulties in doing this smoothly enough to maintain tissue function throughout the procedure. A third option, which is in some ways a combination of the first two, goes under the general heading of "tissue engineering." Tissue engineering involves the construction of organs from cells, usually with the help of a pre-fabricated scaffold that later breaks down. The organ is then transplanted into the body in the same was as for a standard transplant. This solves some of the problems with the first two approaches and will probably play an increasingly large part in rejuvenation therapies in the future. However, tissue engineers have repeatedly discovered that cells really don't like growing into tissues in artificial environments, so tissue engineering is still lagging a fair way behind cell therapy in most applications.
Thus, a lot of the SENS interventions will require use of the other
two approaches -- somatic protein therapy and somatic gene therapy.
Several of the strategies for repairing the seven SENS targets will almost certainly require, in
some tissues, altering the genomic DNA of cells in the body -- somatic
gene therapy. This is
much harder than taking cells from the person, altering their DNA in
the laboratory and putting them back, because altering the DNA of cells
is very error-prone. If you do the alteration in the lab, you can
check whether the correct alteration happened (and that nothing else
happened) and only put back cells that pass that test. The error-prone
nature of all existing approaches to somatic gene therapy is the main
reason why it is still in its infancy: it's still dangerous.
Therefore, one of the things SENS needs most badly is improvements in
our techniques for doing what we want to do to our genomes in situ and
not accidentally doing other things at the same time. Luckily, several
methods are currently under intense investigation.
For most applications, all we need to do is get a new gene or genes
into our chromosomes, and it actually doesn't much matter where it goes
in -- unless it disrupts the genes we already have. Unfortunately, that
"unless" can't be neglected, because if we're trying to get the DNA into
all (or even most) of our cells then it's going to hit genes in a fair
proportion of them, and in a few of them it's more or less certain to
hit genes involved in cell cycle control -- which means, of course, that
it may promote cancer. The first big breakthrough in solving this
problem was when it was found that one virus, the adeno-associated
virus (AAV), preferentially inserts into a particular (safe) place on human
chromosome 19. This is good, but not good enough, because in order to
make the virus carry useful genes that we want to put into our cells
we have to take out the stuff that gives it its site-specificity. But
there are various approaches being explored for hybrid viruses that get
the best of both worlds -- enough carrying capacity to be useful, without
loss of site-specificity.
AAV doesn't always insert in this particular spot,
however, even when it still has its site-specific preference. This is
largely because it is a linear, single-stranded DNA virus, and linear
single-stranded DNA has a habit of "invading" double-stranded DNA and
sometimes undergoing recombination with it at random. Much excitement
is therefore currently surrounding a new type of virus -- actually a
bacterial virus, usually called a phage -- which is circular and
double-stranded and which therefore has only a very low tendency to
intercalate into other DNA at random. It does get into DNA, but only
when it expresses a particular enzyme called an integrase. Better yet,
it only goes into a few specific places in the genome -- and these are
not obviously as safe as the AAV site, because they tend to be in the
gaps in the middle of genes called introns. However, some success has
been achieved in "evolving" these enzymes in the lab so that they prefer
different sites, so there is strong hope that these phages will be safe
gene therapy vectors soon.
Nearly everything that we would like to do with gene therapy, whether
for aging or for any other condition, can probably be done pretty well
by introducing new genes into cells in a safe place. Sometimes we want
to stop a gene from expressing its product, because the product is
toxic (such as the mutation that causes Huntington's disease), but even
then we can probably achieve the desired effect just by putting in a
gene, because we can use the amazing phenomenon of RNA interference to
cause the gene's transcript to be destroyed before it is translated.
But there is one case where we probably will really need somatic gene
therapy targeted at a specific place in the genome, and that's for my
preferred anti-cancer therapy, WILT. It's not
going to be good enough to use RNAi against telomerase -- that's just
as easy to "escape" as pharmacological telomerase inhibitors. What we
need to do for WILT is actually delete (or at least seriously disrupt)
the telomerase genes. At present, there are several approaches to
targeted gene disruption (or gene targeting, as it's usually called)
that can be customised to attack anywhere in the genome, but they're
all highly error-prone, disrupting other locations a great deal. The
best bet at the moment is probably the zinc finger nuclease strategy
being pioneered by the Californian company Sangamo.
Another thing to remember about gene therapy (including gene targeting)
is that we can get plenty of benefit in many aspects of SENS even if
our delivery technology doesn't get to all cells. For example, if we
succeeded in giving half our muscle fibres nuclear
versions of the motochondrial DNA, the number of fibre segments that
were toxic to the rest of the body would be halved, allowing all manner
of other aspects of SENS to be more effective.
Many components of SENS entail alteration of the genome in many different cell types. For cells that are constantly renewed from stem cells, this is relatively easy, because we can extract cells from the individual, do what we want to those cells in the laboratory, check that we've done exactly what we wanted to do, and put them back in. This is not easy, let me stress -- especially hard is doing all this without the cells losing their "stemness" -- but it'll probably be a lot easier than the alternative of somatic gene therapy. But tissues that do not have continuous renewal can't be altered in this way, so at first it might seem that somatic gene therapy is the only option there. There is in fact another possibility, however. The basic reason we want to change the genome of cells is so that those cells will make different proteins than they used to. For most SENS purposes (and indeed for most biomedical purposes generally), we want the cell to have proteins that it didn't have before, as opposed to lacking ones that it used to have. In principle, therefore, this can be done by introducing the proteins themselves, rather than the genes encoding them The obvious problem with such an approach is scale. Most proteins are rather short-lived, so the cell needs to make them again and again in order to have them around in the required abundance. Thus, it would be impractical to introduce enough protein. When Roscoe Brady first sought to explore this approach he was soundly dismissed for this reason. It turns out, however, that there are plenty of cases where this is not a showstopper. The class of proteins that Brady was (and still is) interested in are enzymes which break things down in lysosomes; these enzymes are congenitally absent in sufferers of lysosomal storage diseases. Brady eventually succeeded in developing methods to make enough enzyme and target it to the right cells to be able to give many such people a normal life when they would otherwise certainly have died in childhood. One of the most important SENS strands, lysosomal enhancement, may well be able to work this way for many tissues. Another way out of the protein scale problem is to introduce the genes for the desired proteins into one tissue and arrange for them to be exported from the cells that make them and imported by the ones that need them. This makes sense because genes can be introduced safely in stem cells in vitro much more easily than somatically, as noted above. It is quite easy to modify genes so that their encoded protein will be secreted, and there are also techniques for targeting proteins in the circulation to particular organs. An especially important one is the brain, which is protected from the circulation by a special system that stitches the cells of the blood vessel lining together much more tightly than elsewhere in the body -- this is called the blood-brain barrier. Some proteins need to be transported into the brain from the blood, and we now have a moderate understanding of how this happens and how we could exploit that system to get our chosen proteins across. Finally I should say a word about germline gene therapy. This means changing the genome of either a gamete (sperm or egg) or a zygote (a single cell formed either by fertilisation or by somatic cell nuclear transfer, a.k.a. cloning) so that people are born with a designed genetic alteration. Some people think this would always be far too dangerous to be useful, but others have argued persuasively that these dangers can be overcome. However, the attractiveness of this approach is limited by the timescales involved (the fact that aging only starts being bad for us after we reach 50 or so). The point here is that even though somatic gene therapy (putting new genes into the cells of an adult) is technically much harder than germline gene therapy, 50 years is such a long time in science that we are virtually certain to be able to do a lot more for someone in year N+50 by somatic gene therapy than we could do in year N by germline therapy. So I think germline gene therapy is quite likely to become an important biomedical procedure in the future, but not in combating aging.
Talks on this topic at SENS2: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||