Sickle Cell Disease: Gene-Editing Tools Point to Possible Ultimate Cure
Posted on by Dr. Francis Collins
Scientists first described the sickle-shaped red blood cells that give sickle cell disease its name more than a century ago. By the 1950s, the precise molecular and genetic underpinnings of this painful and debilitating condition had become clear, making sickle cell the first “molecular disease” ever characterized. The cause is a single letter “typo” in the gene encoding oxygen-carrying hemoglobin. Red blood cells containing the defective hemoglobin become stiff, deformed, and prone to clumping. Individuals carrying one copy of the sickle mutation have sickle trait, and are generally fine. Those with two copies have sickle cell disease and face major medical challenges. Yet, despite all this progress in scientific understanding, nearly 70 years later, we still have no safe and reliable means for a cure.
Recent advances in CRISPR/Cas9 gene-editing tools, which the blog has highlighted in the past, have renewed hope that it might be possible to cure sickle cell disease by correcting DNA typos in just the right set of cells. Now, in a study published in Science Translational Medicine, an NIH-funded research team has taken an encouraging step toward this goal . For the first time, the scientists showed that it’s possible to correct the hemoglobin mutation in blood-forming human stem cells, taken directly from donors, at a frequency that might be sufficient to help patients. In addition, their gene-edited human stem cells persisted for 16 weeks when transplanted into mice, suggesting that the treatment might also be long lasting or possibly even curative.
CRISPR/Cas9 technology uses small RNA molecules to guide a scissor-like enzyme to a specific DNA sequence. This DNA editing tool allows researchers to target a gene in a stem cell and snip the precise spot where an error in the sequence occurs. Once the double-stranded DNA is cut, the disease-causing typo can be replaced with the correct sequence, allowing the stem cell to produce healthy normal cells that can potentially cure the condition.
Lab studies had shown that CRISPR/Cas9 could be used to correct sickle cell mutations in experimental cell lines. But modifying a sufficient number of blood-forming stem cells in hopes of treating patients with their own edited cells had proved a far greater technical challenge. While some editing was achieved in a small fraction of cells, those edited cells tended to disappear over time for reasons that aren’t yet well understood.
In the new study, Jacob Corn at the University of California, Berkeley, and colleagues sought to find a more reliable and efficient method. Using human stem cells isolated from whole blood samples, they started by using an electric shock to open up pores in the cell membrane. With those pores open, the researchers could introduce the CRISPR/Cas9 complex into the cells along with bits of guide DNA encoding the correct hemoglobin sequences.
Corn and his colleagues capitalized on the fact that the cutting enzyme hangs on to one of the two DNA strands for a time, leaving the second strand free. Using their knowledge of the sequence contained within that loose bit of DNA, they designed short and inexpensive DNA “fillers” to come in and correct the sequence to the normal state with a high rate of success.
Using this method, the researchers found that they could replace the incorrect sequence in more than a third of human stem cells. Those cells, which had started out with two copies of the sickle mutation, went on to produce red blood cells containing healthy hemoglobin. However, given the challenges of obtaining blood-forming stem cells from people with sickle cell disease, they didn’t have enough corrected human cells to transplant into mice for testing.
To get past this limitation, the researchers turned the protocol around to generate sickle-trait stem cells. They took stem cells from healthy patients and edited in the sickle cell mutation. When about a million of those CRISPR/Cas9-treated cells were transplanted into mice, a small but significant number—2 to 4 percent—of the human stem cells took up residence in the animals’ bone marrow and persisted for at least 16 weeks. That’s good news because previous studies have suggested that fixing the sickle cell gene in just 2 to 5 percent of bone marrow stem cells could be enough to benefit patients.
Genetic blood diseases are prime candidates for potentially therapeutic gene-editing approaches because a patient’s cells can be easily accessed, edited, and then replaced. Sickle cell disease is an obvious first choice, in part because the condition affects millions of people around the world—100,000 in the United States alone . But other even rarer single gene disorders of blood cells, including various types of anemia and immune deficiencies, are also fair game.
As promising as these results are, much more work is needed in the laboratory before the new approach can move forward for a possible clinical trial to treat people with sickle cell disease. In the nearer term, Corn and colleagues hope their success will inspire researchers in labs around the world to explore gene editing for other challenging health conditions.
 Selection-free genome editing of the sickle mutation in human adult hematopoietic stem/progenitor cells. DeWitt MA, Magis W, Bray NL, Wang T, Berman JR, Urbinati F, Heo SJ, Mitros T, Muñoz DP, Boffelli D, Kohn DB, Walters MC, Carroll D, Martin DI, Corn JE. Sci Transl Med. 2016 Oct 12;8(360):360ra134.
 Sickle Cell Disease: Data & Statistics. Centers for Disease Control and Prevention. August 31, 2016.
What is Sickle Cell Disease? (National Heart, Lung, and Blood Institute/NIH)
Corn Lab (Innovative Genomics Initiative)
NIH Support: National Institute of Environmental Health Sciences; National Institute on Alcohol Abuse and Alcoholism; National Institute of General Medical Sciences; National Center for Advancing Translational Sciences