At the root of Huntington’s disease is a specific type of mutation, called a trinucleotide repeat expansion, in the Huntingtin (Htt) gene. The normal Htt gene contains up to 28 copies of the nucleotide sequence CAG, but this expands to more than 40 copies in the disease-causing allele. As a result of the expanded repeat, insoluble clumps of the Huntingtin protein accumulate inside neurons, causing cell death that leads to uncontrollable movements, dementia and, ultimately, death. Patients with between 28 and 35 repeats are unaffected, while those with between 36 and 40 have a form of the disease with reduced penetrance.
In animal models, reducing mutant Htt protein levels prevents disease progression and reverses some symptoms. However, most therapeutic approaches in development lower both versions of the huntingtin protein (the one produced by the normal gene, and the one made by the mutated gene). This has raised concerns about their safety for human use, because the normal protein has important, albeit as yet unknown, cellular function. To overcome this, Sangamo researchers have developed zinc finger transcriptional repressors that specifically target the mutant Htt allele and block its expression while preserving near-normal expression levels of the normal allele. Zinc fingers are naturally occurring protein segments that recognize and bind to specific DNA sequences, typically regulating the output of a given gene. Using genetic engineering, the Sangamo researchers designed zinc finger proteins containing a DNA-binding site that recognizes the prolonged tricnucleotide repeat found in the mutant Htt gene. They then fused this binding site to a protein domain that recruits other molecules that zip closed the chromosomal region containing the Htt gene with the expanded repeat—thus hindering production of mutated huntingtin protein.
In a recent experiment in a lab dish, the group added the engineered zinc fingers to fibroblast cells obtained from six people with Huntington’s disease. This lowered production of the mutant protein by more than 90%, while reducing the amount of the normal protein by just 10% or less, the researchers reported at the annual meeting of the Society for Neuroscience, held here this week. “There was very potent discrimination between the mutant and normal alleles in cells from all six patients, even though each contained mutant alleles of different lengths,” explains Phillip Gregory*, chief scientific officer at Sangamo BioSciences. “The next step is to make that sure they operate at a broad range of doses, and then we need to move into animal studies of efficacy and safety.”
This is the first attempt to apply the zinc finger approach to Huntington’s disease, and the researchers eventually aim to deliver genes for the zinc finger proteins directly into the brain using adeno-associated viral vectors*, which are already being used to successfully deliver therapeutic genes into the brains of people with Parkinson’s disease in clinical trials.
“This is very promising and exciting work,” says Sarah Tabrizi, a professor at the Institute of Neurology in London, who was not involved in the study, “but it’s still at a very early and exploratory stage, and it’s a big jump going from cells in culture to the human brain.” One challenge is that targeting viral vectors to specified brain areas and then ensuring their proper distribution is difficult, and this is further complicated by the fact that Huntington’s disease begins in deep brain structures before spreading to the cerebral cortex. “Distributing the vector will be a challenge,” Tabrizi says, “but I don’t think it’s insurmountable.”
Read more about ZFN and TALENs ("Editing the genome, here, there and everywhere"): http://tinyurl.com/ccdhao5