A new method to bring the gene-modifying proteins called zinc finger nucleases (ZFNs) inside cells could make it possible to minimize the side effects of gene therapy.
In ZFN-based gene therapy, genes for ZFNs are packaged inside viral vectors to be introduced into cells. The cells then express the proteins, which modify target genes. Two vector-delivered ZFNs are currently in clinical trials—one of them modifies the gene for CCR5, a receptor HIV uses to infect immune cells. But viral vectors often lead to overproduction of the introduced genes and DNA modifications at “off-target” sites, both of which can cause side effects.
No one thought ZFN proteins could penetrate cells directly. But a team of researchers noticed that ZFNs exhibit some characteristics of cell-permeating peptides, and now they find that ZFNs can enter cells by themselves (Nat. Methods, DOI: 10.1038/nmeth.2030). However, using the proteins directly does not yet modify target genes as efficiently as some viral-vector-based techniques.
“We show we can directly knock out the human CCR5 receptor used in HIV entry by simply adding a purified protein to cells,” says ZFN specialist Carlos F. Barbas III of Scripps Research Institute, whose group carried out the study. “My lab intends to develop this as a gene-free gene therapy for HIV. The existing method, now in Phase II trials, uses viral delivery methods that are more dangerous than our new approach.”
“This is an important technical step forward” that could improve the safety of current protocols, comments gene therapy researcher Mark A. Kay of Stanford University School of Medicine.
“The fact that ZFNs work by themselves is quite unexpected,” says virus, stem cell, and gene therapy expert Paula Cannon of the University of Southern California Keck School of Medicine. “This protein delivery method will definitely give an extra level of safety because there are no viral vectors involved. The challenge will be to see if the method is as effective as the viral-vector approach.”