Quantifying CRISPR off-target effects

Recent advances in the era of genetic engineering have significantly improved our ability to make precise changes in the genomes of human cells. Throughout the years, clinical trials based on gene therapies have led to the cure of diseases such as X-linked severe combined immunodeficiency (SCID-X1), adenosine deaminase deficiency (ADA-SCID) and Wiskott–Aldrich syndrome. Despite the success gene therapy has had, there is still the risk of genotoxicity due to the potential oncogenesis introduced by utilising viral vectors. Research has focused on alternative strategies like genome editing without viral vectors as a means to reduce genotoxicity introduced by the viral vectors. Although there is an extensive use of RNA-guided genome editing via the clustered regularly interspaced short palindromic repeats (CRISPR) and associated protein-9 (Cas9) technology for biomedical research, its genome-wide target specificity and its genotoxic side effects remain controversial. There have been reports of on- and off-target effects created by CRISPR–Cas9 that can include small and large indels and inversions, highlighting the potential risk of insertional mutagenesis. In the last few years, a plethora of in silico, in vitro and in vivo genome-wide assays have been introduced with the sole purpose of profiling these effects. Here, we are going to discuss the genotoxic obstacles in gene therapies and give an up-to-date overview of methodologies for quantifying CRISPR–Cas9 effects

Base-edited CAR T cells for combinational therapy against T cell malignancies

Targeting T cell malignancies using chimeric antigen receptor (CAR) T cells is hindered by ‘T v T’ fratricide against shared antigens such as CD3 and CD7. Base editing offers the possibility of seamless disruption of gene expression of problematic antigens through creation of stop codons or elimination of splice sites. We describe the generation of fratricide-resistant T cells by orderly removal of TCR/CD3 and CD7 ahead of lentiviral-mediated expression of CARs specific for CD3 or CD7. Molecular interrogation of base-edited cells confirmed elimination of chromosomal translocations detected in conventional Cas9 treated cells. Interestingly, 3CAR/7CAR co-culture resulted in ‘self-enrichment’ yielding populations 99.6% TCR−/CD3−/CD7−. 3CAR or 7CAR cells were able to exert specific cytotoxicity against leukaemia lines with defined CD3 and/or CD7 expression as well as primary T-ALL cells. Co-cultured 3CAR/7CAR cells exhibited highest cytotoxicity against CD3 + CD7 + T-ALL targets in vitro and an in vivo human:murine chimeric model. While APOBEC editors can reportedly exhibit guide-independent deamination of both DNA and RNA, we found no problematic ‘off-target’ activity or promiscuous base conversion affecting CAR antigen-specific binding regions, which may otherwise redirect T cell specificity. Combinational infusion of fratricide-resistant anti-T CAR T cells may enable enhanced molecular remission ahead of allo-HSCT for T cell malignancies

Cytosine deaminase base editing to restore COL7A1 in dystrophic epidermolysis bullosa human:murine skin model

Recessive dystrophic epidermolysis bullosa (RDEB) is a debilitating blistering skin disorder caused by loss-of-function mutations in COL7A1 encoding type VII collagen (C7), the main component of anchoring fibrils (AFs) at the dermal-epidermal junction (DEJ). Although conventional gene therapy approaches through viral vectors have been tested in pre-clinical and clinical trials, they are limited by transgene size constraints and only support unregulated gene expression. Genome editing could potentially overcome some of these limitations, and CRISPR/Cas9 has already been applied in research studies to restore COL7A1 expression. Delivery of suitable repair templates for repair of DNA cleaved by Cas9 is still major challenge, and alternative base editing strategies may offer corrective solutions for certain mutations. We demonstrate highly targeted and efficient cytidine deamination and molecular correction of a defined RDEB mutation (c.425A>G) leading to restoration of full-length C7 protein expression in primary human fibroblasts and iPSCs. C7 basement membrane expression and skin architecture were restored with de novo AFs identified by electron microscopy in base edited human RDEB grafts recovered from immunodeficient mice. The results demonstrate the potential and promise of emerging base editing technologies in tackling inherited disorders with well-defined single nucleotide mutations

Pathogenic Huntingtin Repeat Expansions in Patients with Frontotemporal Dementia and Amyotrophic Lateral Sclerosis.

We examined the role of repeat expansions in the pathogenesis of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) by analyzing whole-genome sequence data from 2,442 FTD/ALS patients, 2,599 Lewy body dementia (LBD) patients, and 3,158 neurologically healthy subjects. Pathogenic expansions (range, 40-64 CAG repeats) in the huntingtin (HTT) gene were found in three (0.12%) patients diagnosed with pure FTD/ALS syndromes but were not present in the LBD or healthy cohorts. We replicated our findings in an independent collection of 3,674 FTD/ALS patients. Postmortem evaluations of two patients revealed the classical TDP-43 pathology of FTD/ALS, as well as huntingtin-positive, ubiquitin-positive aggregates in the frontal cortex. The neostriatal atrophy that pathologically defines Huntington's disease was absent in both cases. Our findings reveal an etiological relationship between HTT repeat expansions and FTD/ALS syndromes and indicate that genetic screening of FTD/ALS patients for HTT repeat expansions should be considered

'Mini' U6 Pol III promoter exhibits nucleosome redundancy and supports multiplexed coupling of CRISPR/Cas9 effects

RNA polymerase III (Pol III) promoters express short non-coding RNAs and have been adopted for expression of microRNA, interference RNA, and CRISPR single guide RNA (sgRNA). Vectors incorporating H1 and U6 Pol III promoters are being applied for therapeutic genome editing, including multiplexed CRISPR/Cas9 effects. We report a nucleosome-depleted, minimal U6 promoter, which when embedded within lentiviral long terminal repeat (LTR) regions, supports high level transcriptional activity. Furthermore, duplex minimal H1 & U6 promoters transcribed dual sgRNAs for simultaneous disruption of T cell receptor (TCR) and human leukocyte antigen (HLA) molecules, supporting efficient generation of ‘universal’ CAR T cells

CRISPR mediated base conversion allows discriminatory depletion of endogenous T cell receptors for enhanced synthetic immunity

Emerging base editing technology exploits CRISPR RNA-guided DNA modification effects for highly specific C>T conversion which has been used to efficiently disrupt gene expression. These tools can enhance synthetic T cell immunity by restricting specificity, addressing HLA barriers and promoting persistence. We report lentiviral delivery of a Hepatitis B virus (HBV) specific recombinant TCR (rTCR) and a linked CRISPR single-guide RNA for simultaneous disruption of endogenous TCRs (eTCR) when combined with transient cytosine deamination. Discriminatory depletion of eTCR and coupled expression of rTCR resulted in enrichment of HBV specific populations from 55% (SEM ± 2.4%) to 95% (SEM ± 0.5%). Intensity of rTCR expression increased 1.8-2.9 fold compared to cells retaining their competing eTCR and increased cytokine production and killing of HBV antigen-expressing hepatoma cells in a 3D microfluidic model was exhibited. Molecular signatures confirmed seamless conversion of C>T (G>A) had created a premature stop codon in TCR beta constant 1/2 loci, with no notable activity at predicted off-target sites. Thus, targeted disruption of eTCR by cytosine deamination and discriminatory enrichment of antigen-specific T cells offers the prospect of enhanced, more specific T cell therapies against HBV associated hepatocellular carcinoma (HCC) as well as other viral and tumour antigens