Molecular pathology selected abstracts

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Editors: Donna E. Hansel, MD, PhD, chair of pathology, Oregon Health and Science University, Portland; Richard D. Press, MD, PhD, professor and director of molecular pathology, OHSU; James Solomon, MD, PhD, assistant professor, Department of Pathology and Laboratory Medicine, Weill Cornell Medicine, New York; Sounak Gupta, MBBS, PhD, senior associate consultant, Mayo Clinic, Rochester, Minn.; Tauangtham Anekpuritanang, MD, molecular pathology fellow, Department of Pathology, OHSU; Fei Yang, MD, assistant professor, Department of Pathology, OHSU; and Andrés G. Madrigal, MD, PhD, molecular genetic pathology fellow, Department of Pathology, OHSU.

Assessing BCL11A gamma-globin repressor for sickle cell anemia, β-thalassemia

February 2021—Beta-thalassemia and sickle cell disease are common hereditary conditions that can have life-threatening complications. Both diseases are caused by genetic alterations affecting the beta subunit of hemoglobin. Mutations that reduce or prevent the synthesis of the beta-globin protein cause beta-thalassemia, a disease characterized by inadequate red blood cell production and, therefore, anemia. In contrast, sickle cell anemia results from a specific point mutation in the beta-globin gene that causes the resulting protein to polymerize. These protein polymers form rigid fibers that affect the stability of the red blood cell and cause its characteristic sickling deformity. Destruction of the aberrant red blood cells leads to anemia, and the sickled cells can also cause painful vaso-occlusive episodes and tissue damage. Long-term treatment for both diseases may involve blood transfusions with concomitant iron chelation therapy to prevent iron overload. Allo­geneic hematopoietic stem cell transplantation is a curative treatment, but it is performed only in severe cases because of the risk of complications and graft-versus-host disease. An alternative therapy, hydroxyurea, elevates the level of fetal hemoglobin. The latter is formed from two alpha and two gamma subunits instead of the beta subunits that are mutated in beta-thalassemia and sickle cell patients. Elevated levels of fetal hemoglobin have been shown to be protective in sickle cell patients, improving morbidity and mortality. The production of fetal hemoglobin typically is tightly regulated and decreases in infancy to the extent that adults have only a small amount of it in circulating red blood cells. BCL11A is a transcription factor gene on chromosome 2 that normally represses the expression of gamma-globin. It was recently reported in a genomewide association study that polymorphisms affecting this gene are associated with increased expression of fetal hemoglobin. Therefore, reducing or eliminating BCL11A expression might increase gamma-globin expression in hematopoietic cells. This hypothesis was examined in two trials published simultaneously in The New England Journal of Medicine. A study by Frangoul, et al., involved two patients—one with beta-thalassemia and another with sickle cell disease—who were treated with autologous CD34+ stem cells that had been genetically edited with CRISPR-Cas9 to reactivate fetal hemoglobin production. Stem cells were mobilized and collected from the patients, after which the BCL11A erythroid-specific enhancer locus was edited with CRISPR-Cas9 using a single guide RNA molecule. The patients then underwent myeloablative conditioning and their modified stem cells were infused. Once the cells engrafted, about 30 days later, high levels of fetal hemoglobin were found in almost all red blood cells, and the patients’ total hemoglobin and fetal hemoglobin levels increased rapidly. Most importantly, the patients no longer required transfusions. In the second trial, Esrick, et al., collected autologous CD34+ stem cells from six sickle cell disease patients and modified them using gene therapy with a lentiviral vector that encodes a short hairpin RNA that targets BCL11A mRNA. After myeloablative conditioning, the modified cells were infused, and the patients’ total hemoglobin and fetal hemoglobin levels increased. Furthermore, clinical manifestations of sickle cell disease vastly declined, and none of the patients had vaso-occlusive crises after infusion of the modified cells. Both studies demonstrated the feasibility of infusing genetically modified autologous hematopoietic stem cells as a potential cure for beta-thalassemia, sickle cell disease, and other hemoglobinopathies.

Esrick EB, Lehmann LE, Biffi A, et al. Post-transcriptional genetic silencing of BCL11A to treat sickle cell disease [published online ahead of print December 5, 2020]. N Engl J Med. 2020;10. doi:10.1056/NEJMoa2029392

Correspondence: Dr. David A. Williams at dawilliams@childrens.harvard.edu

Frangoul H, Altshuler D, Cappellini MD, et al. CRISPR-Cas9 gene editing for sickle cell disease and β-thalassemia [published online ahead of print December 5, 2020]. N Engl J Med. 2020;10. doi:10.1056/NEJMoa2031054