Successful Gene Editing of Apolipoprotein E4 to E3 in Brain of Alzheimer Model MiceAfter a Single IV Dose of Synthetic Exosome-Delivered CRISPR

Abstract

BackgroundThe gene for apolipoprotein E4 (ApoE4 E4) confers an increased risk for development and lowers the age of onset of Alzheimers disease (AD), and is a highly suitable target for CRISPR-based editing because ApoE4 differs from ApoE3 by a single nucleotide polymorphism in the codon for residue 112 that codes for arginine (CGC) in E4 and cysteine (TGC) in E3. Editing of E4 to E3 could lower the risk of AD or ameliorate E4-related AD phenotypes. For AD, in order to deliver CRISPR components across the blood-brain barrier to the brain, we have developed a delivery platform termed Synthetic Exosomes (SEs) - microfluidically-synthesized deformable nanovesicles approximately the size of natural exosomes that have the ability to cross the BBB and deliver cargo to the brain. Here, we describe our use of SEs carrying CRISPR to successfully edit E4 to E3 in brain tissue of an E4-expressing mouse model. MethodsSeveral CRISPR guide RNAs (gRNA) and Cytosine Base Editor (CBE) mRNAs were synthesized by chemical and in vitro transcription syntheses, respectively. Four combinations of gRNA and CBE mRNA were tested in vitro for their relative activity to edit the E4 (cytosine) to E3 (thymine) in E4-expressing neuroblastoma (E4-N2A) and human Kelly neuroblastoma cells, to assess which combination produced the highest E4 to E3 base editing efficiency. The CRISPR RNA combination with the highest efficiency was encapsulated in SEs and injected intravenously (IV) via the tail vein into an AD model E4-expressing (E4-5XFAD) transgenic mouse; as a negative control, an E4-5XFAD mouse was injected with empty SEs. Five days after injection, mice were euthanized and brain, liver, and buffy coat (white blood cells (WBC)) collected to determine the editing of E4 to E3 measured by Next Generation Sequencing. In addition, E3 mRNA was measured in the brain and liver and compared to the %E3 gene editing. ResultsThe highest gRNA+CBE mRNA editing efficiency was [~]50% in E4-N2A cells and the same gRNA+CBE combination delivered in SEs to Kelly neuroblastoma cells showed 6.5% editing efficiency. In the E4-5XFAD mouse in vivo, five days after IV delivery of a single dose of the highest-activity SE-CRISPR gRNA+CBE mRNA, the percent of E4 edited to E3 was 0.14% in brain, 0.8% in liver, and 0.36% in WBCs. As evidence of functional editing, SE-CRISPR-treated mice had 0.03% E3 mRNA in brain and 0.09% E3 mRNA in liver. ConclusionsWhile this level of ApoE4 to E3 editing achieved five days after a single IV injection of SE-CRISPR is small, it provides initial in vivo proof-of-concept that the ApoE4 gene can be successfully edited, and editing results in functional expression of ApoE3 mRNA. The findings presented herein supports further optimization of the SE-CRISPR approach to increase the level of editing in brain as part of clinical development of SE-CRISPR as a powerful novel therapeutic approach for AD.