Stem cells harvested from the blood of sickle cell patients have been subjected to gene editing, which succeeded in correcting the mutation responsible for sickle cell disease (SCD). In fact, stem cells were corrected in such large numbers that they have the potential to alleviate symptoms of the disease, which primarily afflicts those of African descent and leads to anemia, painful blood blockages, and early death.
If corrected stem cells were to be reinfused into patients, they might generate sound hemoglobin and avoid the harms of defective, sticky hemoglobin, which deforms blood cells into the sickle shape. In a test of this notion, corrected stem cells were deployed in a mouse model of SCD. The stem cells remained in circulation for months and produced normal hemoglobin at levels that were deemed likely to be clinically beneficial.
These promising findings were reported by a team of scientists and clinicians from the University of California, Berkeley, UC San Francisco Benioff Children's Hospital Oakland Research Institute (CHORI) and the University of Utah School of Medicine. In a paper (“Selection-Free Genome Editing of the Sickle Mutation in Human Adult Hematopoietic Stem/Progenitor Cells”) that appeared October 12 in Science Translational Medicine, the team members described their gene-editing approach, which focused on CD34+ hematopoietic stem/progenitors (HSPCs), as well as their work with a mouse model of SCD.
“We optimize design and delivery parameters of a ribonucleoprotein (RNP) complex comprising Cas9 protein and unmodified single guide RNA, together with a single-stranded DNA oligonucleotide donor (ssODN), to enable efficient replacement of the SCD mutation in human HSPCs,” the paper’s authors wrote. “Corrected HSPC cells from SCD patients produced less sickle hemoglobin RNA and protein and correspondingly increased wild-type hemoglobin when differentiated into erythroblasts.”
Essentially, the scientists confirmed that their CRISPR/Cas9 approach succeeded in correcting stem cells so that they produced healthy hemoglobin. Subsequently, the scientists engrafted the corrected cells into immunocompromised mice.
“Ex vivo treated human HSPCs maintain SCD gene edits throughout 16 weeks at a level likely to have clinical benefit,” the authors noted. “These results demonstrate that an accessible approach combining Cas9 RNP with an ssODN can mediate efficient HSPC genome editing, enables investigator-led exploration of gene editing reagents in primary hematopoietic stem cells, and suggests a path toward the development of new gene editing treatments for SCD and other hematopoietic diseases.”
Future preclinical work will require additional optimization, large-scale mouse studies and rigorous safety analysis, the researchers emphasized. Contributors to the current study are already collaborating to initiate an early-phase clinical trial to test their new approach within the next 5 years.
Research groups might be able to apply the approach described in this study to develop treatments for other blood diseases such as β-thalassemia, severe combined immunodeficiency (SCID), chronic granulomatous disease, rare disorders like Wiskott-Aldrich syndrome, and Fanconi anemia, and even HIV infection.
“Sickle cell disease is just one of many blood disorders caused by a single mutation in the genome,” said Jacob Corn, Ph.D., a senior author of the current study and scientific director of the Innovative Genomics Initiative at UC Berkeley. “It's very possible that other researchers and clinicians could use this type of gene editing to explore ways to cure a large number of diseases.”