In modern medicine, the word "gene" flashes in all sorts of contexts and conjugations. There is genetics and genomics. There is metagenomics, genetic engineering, gene prediction, and molecular genotyping. It's easy to confuse the various branches of DNA science if you don't do it yourself, but you've probably heard of one subcategory that has surfaced a lot in the media in recent years: gene therapy. What is gene therapy? This is the medical use of genetic material.
To understand what this all means and why this therapy is so powerful, you need to start with a little introduction to the genes themselves. Genes reside in the securely protected genome of the cell, a library of blueprints that allow every living thing to evolve and rebuild itself as it should. To put their code into practice, most genes must be "translated" into protein - the DNA code determines the order of amino acids that must be added to the chain, which is then folded into the shape given by the sequence. It is through this folded three-dimensional structure that the protein performs its function in the cell.
So if you want to change what is happening in the cell, you can do this by changing the DNA, which encodes the form of the protein that acts as an executor. And if there is a dosage problem, such as one copy of a gene instead of two, we can increase the protein yield by placing our own second copy. In any case, we change the genes available to the cell's regular protein-making machinery to change the behavior of the cells.
Sounds simple. But is it that easy to do? Of course not.
First, it is very difficult to fit new or changed genes into the cells that need to be corrected. Cells have been specially developed to prevent this from happening - scientists have to hack viruses that have evolved to enter the cell for this purpose. But they are still not perfect; every single cell in your body has its own copy of the genome, complete and (mostly) identical to the rest, and it is impossible to change every cell in your body. Even if we successfully edit millions of copies of your genome, billions of them will remain untouched.
Thus, the earliest and still most important applications of gene therapy involved test tubes - a bone marrow sample was removed from a patient, the gene of interest was altered, and then the corrected cells were injected back into the host. This tended to only work if the repaired cells lived longer or were in better health than their natural counterparts, so they eventually crowded out the diseased cells and dominated the population.
Only now has it become possible to edit genes in the body of a living patient. Gene therapy is now best at dealing with problems that affect a specific cell type, a limited number of them. The genetic problem that we are solving may remain in the remaining intact cells, but if it does not use them to function, then it is not considered a medical problem. Certain types of liver cells and cochlear hair cells of mammalian ears are cited as examples of current target cells.
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In both cases, re-treatment with the virus can "infect" a high enough proportion of a specific cell population with our therapeutic gene for the desired effect to occur. Some gene therapy methods simply place a medical gene in the nucleus of a host cell, where it will reside and make proteins, just as it would naturally do. However, in the long run, this only works with cells that don't divide over time, like neurons. If cells divide, like most cells, our gene can stick to the host cell's genome or lag behind whenever the cell reproduces.
The main technology for achieving this kind of splicing is called CRISPR; the abbreviation means short palindromic repeats, regularly arranged in groups. The important thing is that if our gene is inserted along with the CRISPR protein and RNA system, the gene will spliced into the genome wherever you want. The cells will divide and reproduce the embedded gene as if it was there all along.
It is important to remember that by correcting a genetic problem, we do not change anything in the heritability of the disease. Correcting deafness by editing DNA in cochlear hair cells, for example, does not reduce the likelihood of transmission of the disease to offspring.