Study finds HDR template vectors frequently insert at off-target sites during Drosophila genome editing

A study published in PLOS Genetics by Emma Källstig, Evelyne Ruchti, Medha Raman, Jamshid Asadzadeh, Bernard L. Schneider, and Brian D. McCabe describes a systematic problem with a widely used genome-editing approach in Drosophila melanogaster. When CRISPR/Cas9 is paired with homology-directed repair (HDR) using a template vector to introduce precise edits — deletions, insertions, or sequence modifications — the template vector itself commonly and undesirably integrates into the genome at locations other than the intended target.

The team characterises this as insertional mutagenesis: the spurious integration of the repair template creates additional, unintended genetic changes that researchers may not detect if they screen only for the desired edit. The frequency reported is described as high, raising questions about the reliability of positive-selection screens that do not include whole-genome verification steps.

The findings are directly relevant to researchers using Drosophila as a model organism for genetics, developmental biology, and neuroscience, as well as to those developing genome-editing methodology more broadly. The authors' results suggest that experimental designs relying on conventional CRISPR/Cas9 plus HDR in Drosophila germline should incorporate verification strategies — such as whole-genome sequencing of edited lines — to identify and exclude animals carrying spurious insertions. Educators teaching CRISPR methodology should note this as a concrete illustration of off-target editing considerations.