TREX1 restricts CRISPR-Cas9 genome editing in human cells

we found that a single exonuclease called TREX1 is an important determinant of HDR efficiency when single-stranded templates and linearized double stranded DNA templates are used as repair templates. TREX1 expression acts as a biomarker for CRISPR Cas mediated HDR, such that cell lines and primary cells expressing high levels of TREX1 have poor HDR. TREX1 removal from these cell types rescues efficient HDR. CRISPR-Cas mediated HDR can also be rescued by using single-stranded DNA templates that are chemically protected from TREX1s exonucleolytic activity. Overall, our data provide a mechanistic explanation for why some cells are easier to edit than others and suggest a route to increase CRISPR Cas mediated HDR in TREX1 expressing context.