This has been a long time coming – I approached Sangamo, a molecular therapies company in the Bay Area (www.sangamo.com) over two years ago. I had known about this company for a long time and was always impressed with their technology and the work they were conducting in other disease areas. Soon after I joined CHDI I started thinking about how, in an ideal world, I could develop a therapy that would eliminate the disease altogether. I asked myself that, if I had a magic wand and could develop an approach to delete the mutation from people, how would I do it. The answer came almost immediately, and it is a concept which could be used in a science fiction movie: I would design a drug or a therapy that would find the mutation in the DNA of patients and delete the mutation altogether. That way, if it could be done technically, the patient would be ‘cured’ , with all the implications of the word. How could we achieve this unbelievable feat?
Sangamo is a company that was formed out of technology from MIT in Cambridge, MA. The technology was derived from a concept well known in biology, that of ‘control of gene expression’. Gene expression is nothing more than the way genes are turned ‘on’ or ‘off’, ‘up’, or ‘down’ in every cell. This control, amazingly enough, is incredibly specific, and tightly regulated. There are proteins in every cell, called ‘transcription factors’ (or TFs), which can recognize specific elements in the DNA. When these motifs are selectively recognized by some TFs, they have the effect of increasing or decreasing the expression of the gene in which those ‘motifs’ are found. TFs are ancient proteins, which have evolved in very complex networks to fine tune how much a gene is expressed, at what time, and in what cells. A brain cell is a brain cell and not a heart cell, because a certain set of TFs work to turn on genes which are only expressed in a combination that is only found in neurons. They control everything in the life of a cell.
Human DNA is found in every cell of the body, and it contains approximately 3 billion bases (A, G, C or T). These 3 billion bases encode roughly 30,000 genes. Each of these genes has specific sequences (combination of the 4 letters above), and some of these sequences form the basis for the motifs which TFs can recognize. In particular, a large family of TFs called ‘zinc-fingers’ because the proteins form a structure similar to a ‘finger’ with a Zinc-molecule bound to keep the structure. These ‘zinc-fingers’ are incredibly varied and specific – each can very selectively recognize a different 3-letter sequence in DNA. You can guess where this is going. When several of these zinc fingers are found together in the same TF, they can recognize multiples of ‘3 bases’, and therefore can identify any sequence of DNA in the entire genome (genome is the entire DNA in a cell). Statistically, 17-bases is all one needs to find only one motif in the 3 billion bases of DNA found in human cells.
The MIT scientists and Sangamo exploited these naturally occurring proteins, and engineered them to encode every possible combination of DNA motifs in the genome. The consequence: they can now direct anything to a specific site in the genome.
So how does this help HD?
The unbelievable technology they built is a masterpiece of molecular biology. They took these TFs (think of this as a powerful engine with a navigation system), and added other proteins which can modify DNA. In many bacteria, other proteins involved in DNA repair, copying of the DNA, etc., can introduce changes in the DNA if they find themselves in close aposition to it. For examples, some of these proteins can direct cellular machinery to increase expression of a gene, or decrease, or introduce a mutation, or altogether delete the gene to which they are ‘driven’.
Sangamo made possible to use the zinc finger TFs to ‘drive’ these ‘machines’ to specific locations in the genome, and alter it in whichever way they want!
HD is caused by a single mutation: that is, one of the 2 copies of the Huntingtin gene contains more than 36 motifs called ‘CAG’ repeats. The normal copy contains less than 36 CAGs. Therefore, it is possible that Sangamo can develop specific TFs which can exploit the difference in the repeat number and modify it. It is almost ‘science fiction’ but I think it will work. The collaboration we just signed (the work begins on April 1st, 2011) involves generating proteins which (a) can bind to the regulatory elements of the Huntingtin gene and decrease its expression (therefore lowering expression of the mutant gene), or (b) exploit the differences in the CAG length, and the likely ‘structural’ differences between the normal and mutant genes, to direct an enzyme which will recognize the longer copy and delete it from the genome. If this is achieved, we will cure the disease in that cell. Because that cell will no longer have the mutant gene!
How will this be achieved? The therapies we will be working on are DNA-based. We will have to deliver the DNA (which makes the TF proteins) into brain cells. This can be achieved with viruses (‘gene therapy’) or through other means which we will be investigating. This is an ambitious goal, but our goal is to cure HD. And this is the only way today that can potentially achieve this.
So this weekend is a special weekend for me- after more than 2 years of trying, Sangamo and CHDI are now partners. And I know it will be a very special partnership. Lets never give up having ‘science fiction’ dreams- one never knows how far science and technology will take humanity. I am happy to be part of one step closer to this fiction being reality!
Have a great weekend