THE GENOME GATEKEEPER: TREX1 RESTRICTS CRISPR-CAS9 GENOME EDITING, PUBLISHED IN NATURE BIOTECHNOLOGY

CRISPR-Cas9 gene editing is widely used to introduce targeted mutations in cells and organisms. During the gene editing process, Cas enzymes induces a double-strand break at a target genomic site that is subsequently repaired by on of two mechanisms: error-prone nonhomologous end joining (NHEJ) that results in genomic insertions and deletions (indels), or templated homology-directed repair (HDR) to precisely insert, delete, or replace a genomic sequence.  Have you ever wondered why CRISPR-Cas mediated HDR editing is so efficient in some cells but terribly inefficient in others? Struggling with gene editing in your cells? We’ve got the solution you’ve been looking for!

We are excited to announce a significant advancement in our understanding of CRISPR-Cas9-mediated HDR through genome-wide screening conducted in Fanconi anemia (FA) patient lymphoblastic cell lines. Our research led by Postdoc Erman Karasu uncovered a single suppressor of CRISPR-Cas9 mediated HDR, revealing that exonuclease TREX1 plays a critical role in reducing HDR efficiency when the repair template is either single-stranded or linearized double-stranded DNA. TREX1 expression serves as a biomarker for CRISPR-Cas9-mediated HDR, and high levels of TREX1, observed in various cell types including U2OS, Jurkat, MDA-MB-231, primary T cells, and hematopoietic stem and progenitor cells (HSPCs), are predictive of poor HDR outcomes. Moreover, we have demonstrated that HDR efficiency can be significantly improved, by 2- to 8-fold, through either knockout of TREX1 or the use of chemically protected single-stranded DNA templates that are resistant to TREX1 activity. Namely, phosphorothioate 3’ end protection is sufficient for fast inexpensive improvements to HDR in contexts with appreciable TREX1 expression. These strategies offer promising avenues for enhancing CRISPR-Cas9–mediated HDR, particularly in cell types with high TREX1 expression.

Overall, our data sheds mechanistic light on why donor template protection increases HDR, provide a concrete biomarker for the targeted use of template protection, and resolve long-standing confusion around why editing works like a breeze in some cells, but fails miserably in others. This breakthrough holds substantial potential for advancing research and therapeutic applications.

For more, check out our paper, it is now out in Nature Biotechnology!

Don’t miss the explainer video highlighting Erman’s work!