Week 8: Cytosine Base Editing and Beam-201

Hello! I’m Sidharth, and this week I’ll be discussing cytosine base editing and a newly published “Off-the-Shelf” CAR T-cell method.

Cytosine Base Editing

Gene editing techniques are required to create allogeneic CAR T-cells. However, many of these techniques have flaws that can unintentionally disrupt the order of the DNA. Transcription activator-like effector nucleases (TALENS) are artificial enzymes (proteins) that can cut DNA strands at any desired sequence. CRISPR-Cas9 is another editing system that has been widely used. Since there are so few clinical trials using allogeneic CAR T-cells, it’s unclear what the actual effect of these gene editing techniques are.

A new editing technique that uses cytosine base editors (CBE) may be a good alternative to the current editing methods. DNA strands consist of adenosine (A), thymine (T), cytosine (C), and guanine (G) nucleic acids. CBEs utilize enzymes to install A.T or C.G point mutations in the DNA. They also inhibit the cell’s mechanisms of base repair to ensure that the point mutations are properly added to the DNA. A recent report, published by scientists at the Philadelphia Children’s Hospital and Beam Therapeutics on May 13th, 2022, details a method to create allogeneic CAR T-cells using CBE editing. (Diorio et al., 2022)

CD7 CAR T-cells

T cells and NK cells express CD7; however, CD7 is also highly expressed in cancer cells, specifically in >95% of lymphoblastic T cell leukemias and lymphomas. Therefore, how can researchers develop CARs that target CD7 without causing cell fratricide? Dr. Mamonkin and his team at the Baylor College of Medicine hypothesized that knocking out the CD7 gene in the engineered CAR T-cells would prevent cell fratricide. (Note: CD7 CAR T-cells target the CD7 antigen on other T cells but do not need to contain the gene for CD7.) (Gomes-Silva et al., 2017) The mechanisms behind why this approach is successful are unclear to me; however, I will delve deeper into this technique. Nevertheless, HLA-matched donor-derived CAR T-cells targeting CD7 in T-ALL have caused complete remission in 18 out of 20 patients. (Pan et al., 2021)

Beam-201: A New Method

Researchers at Beam Therapeutics and the Philadelphia Children’s hospital recently developed allogeneic CD7-targeted CAR T-cells (called Beam-201). They demonstrated that cytokine base editors can be used to safely eliminate the T cell receptor complex, PD-1, and Beta 2 Microglobulin. This marks a major improvement in allogeneic CAR T-cell development as the CBEs did not cause any DNA damage or hamper T cell expression. (Diorio et al., 2022)

Conclusion

This week, I (and many other scientists) discovered a new alternative to gene editing techniques like TALEN and CRISPR-Cas9: Cytosine Base Editors. While one study is not enough to draw any conclusions, I will keep CBEs on my radar as I complete my final paper. I also learnt about CD7 CAR T-cells and plan to delve deeper into why they may not cause cell fratricide. I hope you’ll follow along as I continue my quest to “Off-the-Shelf” CAR T-cells.

Sidharth