5 Takeaways: Genetic association studies improve red blood cell production in vitro
Access to lifesaving blood transfusions can be limited due to supply. And even when matched donor blood is available, it can still be rejected by the patient’s immune system. A more effective means of generating red blood cells (RBCs) from stem cells could be game-changing for a number of different patient groups.
Researchers around the world have been working to optimize protocols for producing RBCs in culture, but there is an upper limit to what such efforts can achieve. New work from the lab of Broad Institute associate member Vijay Sankaran extends that limit by leveraging results from previous genome wide association studies (GWAS), which demonstrated a significant association between the gene SH2B3 and RBC production. In a paper published this week in the journal Cell Stem Cell, Sankaran and his team show that SH2B3 suppression increases and improves RBC production both in vivo and in vitro and could bring the cost of a unit of culture-grown RBCs down to less than one-fifth of its current $8,000 to $15,000 pricetag. Here are five things you need to know about this exciting new research:
1. A common SH2B3 variant is present in approximately 40 percent of humans, but the team focused on a rare loss-of-function variant to develop the new protocol.
GWAS has been around long enough to have generated thousands of associations between genes and biological observations. But for a vast majority of these, the underlying biology is still unknown. This is the story of tracing one such association — a common coding variant of SH2B3 — to its roots. This variant, known as rs184504, had been shown to be present at 40 percent among the general population, “but because it’s so common, it has a very subtle effect on every individual,” says Sankaran. “So it was hard to figure out what it was doing.”
He and his team worked with the lab of institute member Joel Hirschorn to identify rare variants among a cohort of 4,678 individuals who had already undergone whole-exome sequencing. They discovered multiple rare variants with substitutions in the SH2B3 gene associated with increased hemoglobin levels in those individuals. It turns out that SH2B3 encodes a protein that negatively regulates signalling among blood progenitor cells, also known as hematopoietic stem cells (HSCs). “We were able to learn a lot not just about the biology, but ultimately how to use it in a way that we can take advantage of for regenerative medicine,” Sankaran says.
2. Silencing SH2B3 in vitro yields red blood cells at higher quantity and quality than those produced via the current state of the art.
Sankaran’s team, which includes Broad researchers and co-first authors Felix Giani, Claudia Fiorini, Aoi Wakabayashi, suppressed SH2B3 in HSCs using a technique called RNA interference and then let them differentiate in culture into mature RBCs. The resulting cells matured earlier, contained more hemoglobin (the oxygen-toting component of RBCs), and lost their nuclei more readily — an essential step for production of mature RBCs. The team also replicated these results using a variety of different starting cell populations including embryonic stem cells and induced pluripotent stem cells, demonstrating the generalizability of the protocol. “There are limitations to what types of red blood cells are produced from pluripotent stem cells right now,” Sankaran says, “but I think this provides a proof of principle. And hopefully as we improve upon these methods for differentiation, we can get better over time.”
3. The new method could improve in vitro production of RBCs for use in the clinic.
In addition to his research, Sankaran is also a practicing pediatric hematologist. “One of the problems we have, particularly with our patients who are regularly transfused every month or so, is that often times they develop antibodies against donor red blood cells,” he says. One of the advantages of the new method is its ability to produce blood that is a direct match with the patient, by using their own cells as starting points.
4. It could also provide a more effective and efficient method for drug delivery.
Because the RBCs produced by the new method readily eliminate their nuclei, they may be an optimal vehicle for drug delivery. When injecting other cell types into a host, there is always a concern that genetic material found in the nucleus can mutate and cause cancer. With the nucleus eliminated, that is no longer an issue — and the resulting structure makes the new cells an ideal package for drug delivery; the culture-grown cells can be loaded with a drug in the lab before being transfused into a patient. Chemical modifications to the surface of the cells act like mailing addresses, sending them to specific locations in the body. Since the drug is only released in this targeted fashion, the side effects that arise from systemic therapies are limited.
Several companies are already working on using RBCs for this purpose (“it sounds like science fiction, but it’s actually working pretty well,” says Sankaran). With more efficient and cost-effective means of producing “enucleated” RBCs, the new protocol stands to improve those efforts significantly. One example offered by Sankaran is Asparaginase, a highly effective drug for treating leukemia. Unfortunately, the severe allergic reactions that many patients suffer upon injection limit its use. Promising clinical trials are already ongoing to test whether RBCs can be used to effectively deliver Asparaginase without eliciting a reaction.
5. RBCs are only the beginning: the method holds promise for regenerative medicine in general.
“The genetics have allowed us to shine a flashlight on one particular factor for RBC production, but I think this is going to be a very generalizable approach,” Sankaran says. Similar approaches could be used to expand limited quantity progenitor cells from other sources, such as cord blood, which is used in hematopoietic stem cell transplantation. “When you have cord blood you can usually get a match for most bone marrow recipients,” he says. “But unfortunately you just don’t have enough cells per single unit of cord blood from adults. And so if we could expand those stem cells, that would really help medicine in general.”
Paper Cited: Giani, Fiorini, Wakabayashi, et al. Targeted Application of Human Genetic Variation Can Improve Red Blood Cell Production from Stem Cells. Cell Stem Cell. DOI: TKTK.
Other Broad Researchers: Felix C. Giani, Claudia Fiorini, Aoi Wakabayashi, Leif S. Ludwig, Rany M. Salem, Stephanie N. Regan, Jacob C. Ulirsch, Michael H. Guo, Tõnu Esko, , Joel N. Hirschhorn, Leonard I. Zon, Kiran Musunuru.