The Georgia Tech-led Nanomedicine Center for Nucleoprotein Machines has received an award of $16.1 million for five years as part of its renewal by the National Institutes of Health (NIH).
The eight-institution research team plans to pursue development of a clinically viable gene correction technology for single-gene disorders and demonstrate the technology's efficacy with sickle cell disease.
Sickle cell disease is a genetic condition present at birth that affects more than 70,000 Americans. It involves a single altered gene that produces abnormal hemoglobin — the protein that carries oxygen in the blood. In sickle cell disease, red blood cells become hard, sticky and "C" shaped. Sickle cells die early, which causes a constant shortage of red blood cells. The abnormal cells also clog the flow in small blood vessels, causing chronic pain and other serious problems such as infections and acute chest syndrome.
"Even though researchers know sickle cell disease is caused by a single A to T mutation in the beta-globin gene, there is no widely available cure," said center director Gang Bao, the Robert A. Milton Chair in Biomedical Engineering in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University. "By directly and precisely fixing the single mutation, we hope to reduce or eliminate the sickle cell population in an individual's blood stream and replace the sickle cells with healthy red blood cells."
The center is one of eight NIH Nanomedicine Development Centers established in 2005 and 2006, a key initiative of the NIH's long-term nanomedicine research goals. The centers have highly multidisciplinary scientific teams that include biologists, physicians, mathematicians, engineers and computer scientists. Through an intense competition, the NIH selected four centers for second phase funding, including the one led by Georgia Tech.
In addition to experts in the Coulter Department of Biomedical Engineering at Georgia Tech and Emory University and the School of Chemical & Biomolecular Engineering at Georgia Tech, researchers from Medical College of Georgia, Cold Spring Harbor Laboratory, New York University Medical Center, Massachusetts Institute of Technology, Stanford University and Harvard University are also members of the center.
The gene correction approach proposed by the research team to treat sickle cell disease involves delivering engineered zinc finger nucleases (ZFNs) -- genetic scissors that cut DNA at a specific site -- and DNA correction templates into the nuclei of hematopoietic stem cells isolated from the bone marrow of individuals with sickle cell disease. The researchers chose hematopoietic stem cells because they are the precursors of all blood cells, including the cells rendered dysfunctional in sickle cell patients. Hematopoietic stem cells possess such potent regenerative potential that transplantation of even a single hematopoietic stem cell is sufficient to rebuild the entire blood system of an organism.
The researchers plan to engineer and optimize the ZFN proteins so they will induce a double-strand break in the DNA near the sickle cell disease mutation, thereby activating the gene for correction. The broken DNA ends will enter the homologous recombination repair pathway, which will use the genetic information provided by the donor template -- rather than the original flawed information -- to correct the mutation. When the gene-corrected hematopoietic stem cells are injected back in the body, they will produce healthy red blood cells to replace the sickle cells.
"This approach represents a significant paradigm shift in current gene targeting and gene therapy technology in that no viral-based vector or foreign DNA is used," explained Bao, who is also a Georgia Tech College of Engineering Distinguished Professor. "We think it's a promising approach because we do not need to fix all of the mutations in all cells; we only need to greatly reduce the sickle cell population by replacing those cells with healthy red blood cells."
There are significant challenges in achieving the goals of the center, including the need to dramatically increase the rate of homologous recombination-mediated gene correction, improve the activity and specificity of ZFNs to maximize gene correction efficiency and minimize potentially harmful off-target effects, deliver the components necessary for gene correction to hematopoietic stem cells with high efficiency and throughput, avoid unwanted genomic rearrangements and optimize the engraftment of ZFN-modified hematopoietic stem cells.
To increase the efficiency of gene correction in the hematopoietic stem cells, the proposed gene correction approach will require a shift in repair pathway choice from non-homologous end joining toward homologous recombination. To accomplish this, the researchers plan to use methods they developed in the last four years to visualize the assembly of repair complexes at double-strand break sites and develop interventions to shift pathway choice toward homologous recombination.
To control ZFN activity so that unwanted off-target effects or gene rearrangements can be minimized or avoided, the researchers plan to refine and optimize the design and production of the proteins and develop photoactivatable proteins for better temporal control of ZFN activity. In addition, by investigating the fate and dynamics of the engineered proteins and donor template in living cells, and the incidence and biological effects of undesired mutations and gene rearrangements, the research team will further improve the process.
With novel imaging probes and methods already developed in the Nanomedicine Center for Nucleoprotein Machines, the researchers will be able to observe and systematically optimize each step in the gene correction process. Once that is accomplished, the research team will demonstrate the gene correction approach in a mouse model of sickle cell disease. Their goal is to demonstrate that gene-corrected cells can reconstitute the mouse hematopoietic system and reverse the sickle cell disease phenotype, according to Bao.
"We want to focus on sickle cell disease to demonstrate this approach, but if we are successful, the same approach can be adopted to treat some of the other 6,000 estimated single gene disorders in the world today, such as cystic fibrosis and Tay-Sachs," noted Bao.