I wanted to share a new article that just came out in "Nature" (a very well respected journal) in the field of gene therapy. The link to the abstract is below. This advance is not likely to have an impact in PWS in the near future, but it does represent a major technical advancement in the field that is likely to have far-reaching applications, so I'll try to summarize.
A primary obstacle in gene therapy is getting the 'right' gene to replace the incorrect (or mutant) gene efficiently. This is a very inefficient process in mammalian cells - typically, for every million cells that take up your therapeutic gene, only one will neatly replace the incorrect gene with the correct gene. This has been a major problem for gene therapy since the 'right' gene will insert randomly into the chromosomes, and may disrupt the function of a completely unrelated gene. This scenario has actually occurred in the clinical setting for treatment of SCID (severe combined immunodeficiency - 'boy in the plastic bubble disease). Although the gene therapy was effective in a small clinical trial, 3 of 11 children treated had the 'right' gene insert into a bad place in the chromosome, which eventually led to the development of leukemia.
Several groups have been working on how to improve the efficiency of replacing a mutated gene with the corrected gene, a process that would involve 'homologous recombination' (leading to precise replacement of gene sequences). To date, even the most efficient methods have only led to a homologous recombination rate on the order of about 1 in 1,000 cells being corrected appropriately. Here the authors report the use of a synthetic zinc finger protein to direct homologous recombination in approximately 20% of cells. That means, in 20% of the cells, this zinc finger protein is able to find the correct DNA sequence (finding and attaching to 24 out of the ~3 billion basepairs of DNA in the genome) and direct precise replacement of the sequence. Pretty remarkable. These zinc finger proteins are based on natural zinc finger proteins, which generally represent a portion of a larger protein whose job is to interact/modify/bind to DNA. The nature of the zinc finger portion of the protein determines which DNA sequences it will bind to. Synthetic zinc finger proteins have now been produced with different specificities.
So how might this apply to PWS (in the long term)? The PWS problem is a bit different. In most cases of PWS, the DNA is present and correct, but not active (on the maternal chromosome), so here the DNA not need to be replaced, but would need to be modified so as to activate it. Theoretically, zinc finger proteins may also be useful in this case - large 'libraries' of synthetically produced zinc finger proteins are being made characterized for such applications, although it is still early on in the process. Such proteins might also have application to cancer therapeutics.
But, (as always) there are significant caveats to bringing such an approach to fruition. The delivery of zinc finger proteins to the appropriate cells represents a major obstacle, as does the body's immune response to the presence of a synthetic protein. The safety of these proteins in animals (let alone humans) has not been evaluated. Nevertheless, this paper represents a significant step in the right direction.
A primary obstacle in gene therapy is getting the 'right' gene to replace the incorrect (or mutant) gene efficiently. This is a very inefficient process in mammalian cells - typically, for every million cells that take up your therapeutic gene, only one will neatly replace the incorrect gene with the correct gene. This has been a major problem for gene therapy since the 'right' gene will insert randomly into the chromosomes, and may disrupt the function of a completely unrelated gene. This scenario has actually occurred in the clinical setting for treatment of SCID (severe combined immunodeficiency - 'boy in the plastic bubble disease). Although the gene therapy was effective in a small clinical trial, 3 of 11 children treated had the 'right' gene insert into a bad place in the chromosome, which eventually led to the development of leukemia.
Several groups have been working on how to improve the efficiency of replacing a mutated gene with the corrected gene, a process that would involve 'homologous recombination' (leading to precise replacement of gene sequences). To date, even the most efficient methods have only led to a homologous recombination rate on the order of about 1 in 1,000 cells being corrected appropriately. Here the authors report the use of a synthetic zinc finger protein to direct homologous recombination in approximately 20% of cells. That means, in 20% of the cells, this zinc finger protein is able to find the correct DNA sequence (finding and attaching to 24 out of the ~3 billion basepairs of DNA in the genome) and direct precise replacement of the sequence. Pretty remarkable. These zinc finger proteins are based on natural zinc finger proteins, which generally represent a portion of a larger protein whose job is to interact/modify/bind to DNA. The nature of the zinc finger portion of the protein determines which DNA sequences it will bind to. Synthetic zinc finger proteins have now been produced with different specificities.
So how might this apply to PWS (in the long term)? The PWS problem is a bit different. In most cases of PWS, the DNA is present and correct, but not active (on the maternal chromosome), so here the DNA not need to be replaced, but would need to be modified so as to activate it. Theoretically, zinc finger proteins may also be useful in this case - large 'libraries' of synthetically produced zinc finger proteins are being made characterized for such applications, although it is still early on in the process. Such proteins might also have application to cancer therapeutics.
But, (as always) there are significant caveats to bringing such an approach to fruition. The delivery of zinc finger proteins to the appropriate cells represents a major obstacle, as does the body's immune response to the presence of a synthetic protein. The safety of these proteins in animals (let alone humans) has not been evaluated. Nevertheless, this paper represents a significant step in the right direction.
Highly efficient endogenous human gene correction using designed zinc-finger nucleases. Urnov FD, Miller JC, Lee YL, Beausejour CM, Rock JM, Augustus S, Jamieson AC, Porteus MH, Gregory PD, Holmes MC. Nature. 2005
