Is Gene Therapy the Answer?
I am often asked why we just can’t use gene therapy and put back that missing piece of chromosome or take out that extra chromosome. Currently that idea is technically impossible because of the sheer size of even the short arm of chromosome 18. Chromosome 18p is 3000 times too big to be packaged into the commonly used viral vectors for gene therapy. However, I hate to say that such an approach is impossible because science is all about making the impossible possible. So instead of saying it is impossible I would rather say it is impractical. It is impractical, less because the technology does not exist to accomplish it but more because there are so much easier more currently feasible approaches to making the chromosome 18 conditions treatable.
The most important fact is that it is not necessary to delete a whole extra chromosome or insert a missing chromosome arm. This is because only a few of the many genes in these large multi-gene pieces of chromosome are in actual need of correction. In fact only a very small proportion of the genes cause problems when there are too few or too many copies. We call these rare genes “dosage sensitive” genes because their functions are sensitive to the “dose” or the number of copies of the gene. In the case of the chromosome 18 deletions (18p-, 18q- and Ring 18) 90% of the 263 genes on chromosome 18 code for sufficient protein to sustain normal function when present in one copy (i.e. a half dose). This majority of genes are dosage insensitive.
In the case of duplications it is not yet clear exactly how many genes are responsible and in need of some dosage correction but it is likely to be a similarly small number. It might even be the same genes in need of down regulation in duplications are the same ones in need up upregulation in deletions. For this reason there is no need to make the impossible possible by replacing a whole chromosome arm or removing an extra copy of the entire chromosome. What we need to do is to identify the dosage sensitive genes, learn in which cells, tissues and organs they are active and essential and then determine how to upregulate or down regulate their activity. This is hardly a trivial task, but it is most definitely a doable task using established scientific methods.
After reading this you are probably thinking that gene therapy is not the answer. However, according to the Food and Drug Administration, “gene therapy seeks to modify or manipulate the expression of a gene or to alter the biological properties of living cells for therapeutic use.” This is a broader definition than I think most people were expecting. Gene therapy is more than just replacing a defective gene with one that is functional. It is essentially any therapy that seeks to correct a condition by changing gene action. This action could be through gene replacement. But it could also include gene regulation, be it upregulation or down regulation.
The task before us therefore is to get one gene to be upregulated to produce the same amount of protein as two genes (18q-, 18p- and Ring 18). Or, to get 3 (Trisomy 18) or 4 (Tetrasomy 18p) copies of the genes to be downregulated so they only produce the amount of protein typically made by two genes. We need to be able to entice perfectly normal genes to either do more or do less. There are no defective genes that must be replaced or repaired which is the most common approach for gene therapy. Instead, for the chromosome 18 conditions there are merely too few or too many copies of perfectly normal genes. The gene therapy task therefore is to be able to change gene activity by changing their regulation.
Upregulating and downregulating genes is not a completely unnatural process. In fact, you do it to some degree every day in the foods you eat and the activities you participate in. This is an underlying basis for how your body responds and adjusts to life. However, the science on how to accomplish this for therapeutic purposes by manipulating the genes themselves has not progressed to include treatments used in people. There was a recent publication in the journal Science that described a major advance toward making therapeutic gene regulation a reality. The authors demonstrated the use of the relatively new CRISPR gene editing technology, but in this case not to edit a gene but to use it to flag a specific gene in order to upregulate it and thereby rescue an abnormality in mice. These investigators showed two separate examples demonstrating the validity of their strategy. There are two different genes that cause obesity when mice (and people) have one copy instead of the normal two copies. In these experiments the team used mice engineered to have only one copy instead of two of one of the obesity causing genes. Then using the CRISPR method to flag the specific gene with what is essentially an on switch in a mouse embryo they were able to show that the altered single copy of the gene was able to be upregulated and make more of the protein than is typically made from just one copy of the gene. The mice that developed from these treated embryos did not grow up to be obese. And importantly, the now supercharged single copy of the gene was only active in the specific types of cells in which it is normally active. The CRISPR supercharged gene was not active all across the body in cells and tissues in which it is not supposed to be active. It is important to point out that this strategy did not alter the actual gene. It did not change the mouse genome. It merely flagged the gene for activation.
This is pretty exciting stuff. It begins to open the door to possible methods to upregulate genes and correct the effects of a chromosomal deletion. Although these experiments were focused on upregulating genes, the concept might also be applied to down-regulating genes in the cases of trisomy or tetrasomy
And as straight forward as these experiments sound, the practical application of this strategy has some significant challenges yet to be overcome. First these investigators actually employed two different methods to administer the treatment. One was to inject the CRISPR components directly into mouse embryos. The second was to inject it into a young mouse in the specific part of the brain in that utilized the obesity gene. Both of these methods were successful. Neither of these methods of administration are feasible for use in people. However, getting therapeutic agents to the brain could potentially be accomplished by spinal canal injection. Other lines of research are actively testing ways to create compounds that are able to cross the blood brain barrier. This means that therapeutic compounds could be injected into the bloodstream and still get to the brain in order have an effect. Additionally, the method of direct delivery to an organ does hold some promise for use in some other organs for some aspects of the chromosome 18 conditions.
All in all these are exciting times with new developments in many areas that can help to advance our goal of making the chromosome 18 conditions fully treatable.
