August 24, 2021

Reversing the Inevitable: Using CRISPR Cas 9 to Defeat Sickle Cell Anemia

By Priya Ray

The tragedy of genetic disease lies in its frustrating inevitability, which would only please Shakespeare. A single displaced nucleotide—a mere difference in nitrogenous base—can adversely transform the human body, due to a seemingly trivial error that proceeds to haunt generations.

Sickle cell anemia, a blood disorder disproportionately affecting individuals with African, Arab, or Indian ancestry, is one of several diseases owing its presence to a misplaced nucleotide. A mutation substituting thymine for adenine in the hemoglobin gene—a mistake now residing in every nucleated cell of the human body—leads to the creation of an abnormal hemoglobin protein that proceeds to disrupt multiple body systems (1). Causing the body to produce irregularly shaped red blood cells resembling a sickle, hence the name, sickle cell anemia triggers a myriad of symptoms including blockages in the blood vessel, called vascular occlusions, as well as inadequate blood supply to certain organs, called infarctions (2). 

Already burdened with an increased mortality rate and projected life expectancy of fifty-four, sickle cell patients’ lives revolve around a painfully erratic lifestyle, balancing home life with hospitalizations during episodes of severe pain (3). While treatments exist to temporarily alleviate the pain triggered by the sickling, the sole cure involved a bone marrow transplant—until the discovery of CRISPR Cas 9, a revolutionary form of gene editing technology (4). 

When texting, autocorrect conveniently fixes a message filled with typos. Yet when autocorrect fails to identify all typos, the delete key can precisely remove mistakes. Similarly, when DNA must be edited to modify genes, CRISPR Cas 9 serves as the delete button for sequences in the DNA. Originally discovered in the bacteria’s immune system, CRISPR employs the Cas 9 enzyme to edit selected portions of the genome (5). Once placed ex vivo with the necessary cells, Cas 9, along with a single-stranded guide RNA containing twenty base pairs complementary to the DNA preceding the targeted site, detects the cleaving site (6). Here, the RNA attaches to the complementary DNA nucleotides once the strand is unwound, and an enzyme cuts the double strand, followed by a deletion or insertion at the targeted sequence (5, 6). 

CRISPR Cas 9’s heroic editing ability to defeat sickle cell anemia involves suppressing the BCL11A gene—a gene that instructs cells to switch from producing fetal hemoglobin to producing adult hemoglobin—through a gene therapy treatment called CTX001. Newborns primarily produce fetal hemoglobin, but around three months after birth, the body switches to producing adult hemoglobin—a switch directed by the BCL11A gene (7). Adult hemoglobin and fetal hemoglobin have differing compositions; in sickle cell patients, the adult hemoglobin’s structure induces sickling, unlike fetal hemoglobin (8). Therefore, if the sickle cell patients produced fetal hemoglobin rather than adult hemoglobin, risks of the blood sickling would be prevented. Treatment CTX001 incorporates CRISPR Cas 9 to inactivate BCL11A. Inactivating BCL11A will cause the cell to revert to producing fetal hemoglobin—and terminate production of adult hemoglobin—which will stop sickling since fetal hemoglobin does not trigger sickling. During treatment, after extracting patient blood, stem cells collected from the bone marrow are edited through CRISPR Cas 9 to inactivate BCL11A. Then, these cells are infused back into the patient's bloodstream. After CTX001, these edited cells will produce fetal hemoglobin rather than adult hemoglobin, allowing levels of fetal hemoglobin in the body to rise and thereby reducing threats of sickling (7). 

Successes utilizing CRISPR Cas 9 for CTX001 have transcended lab doors, with CTX001 successfully treating seven sickle cell patients and even fifteen beta-thalassemia patients as of 2021 (9). In 2019, Dr. Haydar Frangoul treated sickle cell patient Victoria Gray of Forest, Mississippi with CRISPR Cas 9—one of CTX001’s first successes (10). After blood cells from Victoria’s bone marrow were extracted and then edited to inactivate the BCL11A gene, she underwent chemotherapy to attenuate red blood cell production, as these red blood cells present in the body produced the adult hemoglobin that induced sickling (11). Then, the extracted cells containing the edited DNA with the inactivated BCL11A gene were inserted back into Victoria’s body, so that fetal hemoglobin production started to increase (10, 11). Victoria’s hemoglobin levels rose, with reports of 99.7% of her red blood cells containing fetal hemoglobin and with the gene edit present in 81% of the cells in her bone marrow. Thanks to CRISPR Cas 9, Victoria can finally bid farewell to the distressing world of transfusions and emergency hospitalizations that once dominated her life (10). 

At last, free will prevails over the fate once sealed by genetics—a triumph led by CRISPR. 

 

 

References

  1. Inusa, B., Hsu, L., Kohli, N., Patel, A., Ominu-Evbota, K., Anie, K., & Atoyebi, W. (2019). Sickle Cell Disease—Genetics, Pathophysiology, Clinical Presentation and Treatment. International Journal of Neonatal Screening, 5(2), 20. MDPI AG. http://dx.doi.org/10.3390/ijns5020020
  2. Lonergan, G. F., Cline, D. B., & Abbondanzo, S. L. (2001). Sickle Cell Anemia. RadioGraphics, 21(4), 971-994. https://pubs.rsna.org/doi/pdf/10.1148/radiographics.21.4.g01jl23971
  3. Lubeck, D., Agodoa, I., Bhakta, N., Danese, M., Pappu, K., Howard, R., Gleeson, M., Halperin, M., & Lanzkron, S. (2019). Estimated Life Expectancy and Income of Patients With Sickle Cell Disease Compared With Those Without Sickle Cell Disease. JAMA network open, 2(11), e1915374. https://doi.org/10.1001/jamanetworkopen.2019.15374
  4. Treatments. (n.d.). Sickle Cell Disease News. https://sicklecellanemianews.com/sickle-cell-disease-treatments/
  5. CRISPR Basics. (n.d.). grcf. https://grcf.jhmi.edu/products/crisprs/crispr-basics/
  6. Li, H., Yang, Y., Hong, W., Huang, M., Wu, M., & Zhao, X. (2020). Applications of genome editing technology in the targeted therapy of human diseases: mechanisms, advances and prospects. Signal Transduction and Targeted Therapy, 5(1), 1-23. https://doi.org/10.1038/s41392-019-0089-y
  7. Cassell, D. K. (2020, July 6). First Person Treated for Sickle Cell Disease with CRISPR Is Doing Well. Healthline. https://www.healthline.com/health-news/first-person-treated-for-sickle-cell-disease-with-crispr-is-doing-well
  8. Akinsheye, I., Alsultan, A., Solovieff, N., Ngo, D., Baldwin, C. T., Sebastiani, P., Chui, D. H., & Steinberg, M. H. (2011). Fetal hemoglobin in sickle cell anemia. Blood, 118(1), 19–27. https://doi.org/10.1182/blood-2011-03-325258
  9. Vertex and CRISPR Therapeutics Present New Data in 22 Patients With Greater Than 3 Months Follow-Up Post-Treatment With Investigational CRISPR/Cas9 Gene-Editing Therapy, CTX001™ at European Hematology Association Annual Meeting. (2021, June 11). Vertex. https://investors.vrtx.com/news-releases/news-release-details/vertex-and-crispr-therapeutics-present-new-data-22-patients
  10. Stein, R. (2020, June 23). A Year In, 1st Patient To Get Gene Editing For Sickle Cell Disease Is Thriving. NPR. https://www.npr.org/sections/health-shots/2020/06/23/877543610/a-year-in-1st-patient-to-get-gene-editing-for-sickle-cell-disease-is-thriving
  11. Davis, C. (2020, December 22). Nashville doctor performs first successful gene editing procedure on Sickle Cell Anemia patient. NewsChannel5 Nashville. https://www.newschannel5.com/news/nashville-doctor-performs-first-successful-gene-editing-procedure-on-sickle-cell-anemia-patient