How precision health can contribute to health equity through promising applications of CRISPR-based Therapy
May 4, 2021 · IDSN 540 · Processes and Perspectives
How somatic-cell CRISPR therapy — ex vivo, autologous, targeted — could serve health equity rather than widen existing gaps.
Patients with Sickle Cell Disease (SCD), suffer from a mutation of a single amino acid within the HBB gene that replaces ß-globin with HBS1, causing an imbalance in the structural composition of hemoglobin2, a component of red blood cells that transport oxygen to organs throughout the body3. The impact of this inherited mutation4 causes red blood cells to become malformed into a sickle shape and they eventually break down as they travel through small capillaries5. This process of sickle cell degradation leads to anemia which is characterized by an absence of oxygen to organs6.
Prior therapies for this monogenic disorder have included drugs, blood transfusions, but the only known cure for SCD is a bone marrow transplant from an allogenic donor7. Allogenic bone marrow transplants can trigger a counter-productive immune response of graft versus host disease (GVHD) and patients undergoing this therapy are required to take immunosuppressant drugs to prevent this. Another emerging therapy for SCD is somatic cell editing.
The benefit of somatic cell editing through CRISPR-Cas9 involves revitalizing a patient’s own red blood cells with functionality previously hampered by the HBB gene mutation. A patient’s cells will not be rejected via GVHD and can function normally.
A process of somatic cell editing involves harvesting a patient’s own cells and then converting them into human induced pluripotent stem cells (hiPSCs) in ex vivo. These cells would then be edited by CRISPR-Cas9 to correct the mutation.
One possible CRISPR strategy would be to introduce a double strand break by HDR to knockout the amino acid valine8. In theory, an sgRNA template would be leveraged to replace the amino acid valine, with glutamic acid to restore normal ß-globin subunits in hemoglobin.
After these cells are corrected by CRISPR they then differentiate into red blood cells. The billions of revitalized cells are screened for quality before autologous transplantation back into the patient.
Scientists are currently exploring another way to leverage CRISPR to cure this disease in clinical trials. SCD does not become present until a child is 5 months old (CDC) when adult hemoglobin forms in the body9. Fetal hemoglobin has relevance in treating SCD because it is typically asymptomatic of this disease. Therefore, CRISPR has been used to alter blood cells to permanently express fetal globin10. This approach entirely bypasses the formation of mutated adult hemoglobin that would cause SCD11.
This second strategy leverages Cas9 to edit the transcription factor in hematopoietic stem cells that represses fetal (γ-globin) expression. During this procedure, sgRNA would target the enhancer region of this transcription factor with coding to reactivate the γ-globin gene12 to permanently reactivate fetal hemoglobin as a substitute for the mutated adult hemoglobin13.
Both strategies require the harvesting of hiPSCs from patients, but in the future it may be possible to utilize “universal” hiPSCs (free of GVHD) in order to scale this revolutionary therapy to a broader population of patients.
SOURCES
1. HBB Gene: Beta-Thalassemia & Sickle Cell Disease
3. NIH / NHGRI: About Sickle Cell Disease
4. CDC: Sickle Cell Disease (SCD)
5. American Society of Hematology Sickle Cell Disease and Thalassemia
6. Mayo Clinic Sickle Cell Anemia
7. Texas Children’s Hospital Bone marrow transplantation in children with sickle cell disease