“Base Editing of Haematopoietic Stem Cells Rescues Sickle Cell Disease in Mice”, Gregory A. Newby, Jonathan S. Yen, Kaitly J. Woodard, Thiyagaraj Mayuranathan, Cicera R. Lazzarotto, Yichao Li, Heather Sheppard-Tillman, Shaina N. Porter, Yu Yao, Kalin Mayberry, Kelcee A. Everette, Yoonjeong Jang, Christopher J. Podracky, Elizabeth Thaman, Christophe Lechauve, Akshay Sharma, Jordana M. Henderson, Michelle F. Richter, Kevin T. Zhao, Shannon M. Miller, Tina Wang, Luke W. Koblan, Anton P. McCaffrey, John F. Tisdale, Theodosia A. Kalfa, Shondra M. Pruett-Miller, Shengdar Q. Tsai, Mitchell J. Weiss, David R. Liu2021-06-02 (gene editing; similar)⁠:

Sickle cell disease (SCD) is caused by a mutation in the β-globin gene HBB.

We used a custom adenine base editor (ABE8e-NRCH) to convert the SCD allele (HBB^S) into Makassar β-globin (HBB^G), a non-pathogenic variant. Ex vivo delivery of mRNA encoding the base editor with a targeting guide RNA into haematopoietic stem and progenitor cells (HSPCs) from patients with SCD resulted in 80% conversion of HBB^S to HBB^G. 16 weeks after transplantation of edited human HSPCs into immunodeficient mice, the frequency of HBB^G was 68% and hypoxia-induced sickling of bone marrow reticulocytes had decreased 5×, indicating durable gene editing.

To assess the physiological effects of HBB^S base editing, we delivered ABE8e-NRCH and guide RNA into HSPCs from a humanized SCD mouse and then transplanted these cells into irradiated mice. After 16 weeks, Makassar β-globin represented 79% of β-globin protein in blood, and hypoxia-induced sickling was reduced 3×. Mice that received base-edited HSPCs showed near-normal haematological parameters and reduced splenic pathology compared to mice that received unedited cells. Secondary transplantation of edited bone marrow confirmed that the gene editing was durable in long-term haematopoietic stem cells and showed that HBB^S-to-HBB^G editing of 20% or more is sufficient for phenotypic rescue. Base editing of human HSPCs avoided the p53 activation and larger deletions that have been observed following Cas9 nuclease treatment.

These findings point towards a one-time autologous treatment for SCD that eliminates pathogenic HBB^S, generates benign HBB^G, and minimizes the undesired consequences of double-strand DNA breaks.

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