Research Update from Peter Harris, Ph.D., Mayo Clinic
Advances in Gene Editing; Implications for PKD
Presently, treatment options in ADPKD focus on addressing changes in cellular pathways, such as increased proliferation or altered secretion that are thought to promote cystic expansion. However, theoretically in a simple genetic disease, such as ADPKD, an alternative approach is to correct the gene mutation itself. Gene therapy methods, where a correct version of the gene is expressed in cells in the disease organ, have been around for a couple decades but have had limited success. In the last few years the focus has shifted to gene editing methods that target the defect in the patient’s gene directly. A number of methods to edit genes have been identified, with CRISPR-Cas9 present showing most promise. Using this method the alteration in the disease-associated gene can be targeted and the DNA cut and repaired by addition of the correct sequence. While these methods are presently being widely used as a research tool to generate disease models in cells and experimental animals such as mice, rat and pigs, they have not been applied in patients. This is because of low efficiency of accurate correction and high levels of off target changes to the DNA; changes that could contribute to other health problems such as cancer.
A recent development of this methodology, published in Nature and highlighted by the New York Times, illustrates promise but also spotlights ethic issues in this area. Part of the ethic issue is that the study was performed in human embryos (the first time the CRISPR-Cas9 method was successfully employed in human embryos in the US). A normal oocyte was fertilized in vitro with sperm from a patient with a dominant heart disease – so half of the sperm had a disease causing gene mutation (similar to the situation in an ADPKD male) – with CRISPR-Cas9 reagents added to induce repair. The investigators optimized the cell cycle period when the DNA break and repair was induced resulting in more efficient repair than previously described. In addition, surprisingly, the repair used the corresponding DNA region from the maternal oocyte, rather than an added template. The fertilized embryos were able to develop to the blastocyst stage (early embryonic stage, ~day 5) and global sequencing did not find other off target changes, suggesting quite precise repair. The authors highlighted that this gene repair could increase the number of usable embryos for preimplantation genetic diagnosis (PGD). PGD involves mutation screening of embryos derived by in vitro fertilization to select ones for implantation that do not have the gene mutation. This method is used quite widely in families at risk of having an infant with a severe genetic disease, but is presently not widely employed in ADPKD.
The major issue with germline gene editing is that theoretically it could be employed to generate “designer babies”, with for instance, enhanced intelligence or athletic ability. These concerns have led to the recent publication of a Position Statement by the American Society of Human Genetics (in combination with other organizations) urging caution in performing human germline gene-editing, and that altered embryos should not culminate in a pregnancy. Despite the controversy about employing gene editing in human embryos, advances in the efficiency of the method and reduction of off target effects may improve the prospects of gene targeting in somatic cells, such as kidney epithelial cells in ADPKD. However, since the kidney is a difficult organ to target and it is not known if correcting the gene defect in cells in an already cystic kidney will be of benefit to slow the progression of disease, this form of gene editing is still some way from clinical trials for ADPKD.