Oligonucleotide Synthesis Errors Are a Source of Untoward Variation in HDR-Mediated Gene Editing.

Background/

Objectives

Single-stranded oligonucleotides (ssODNs) are used as donor templates for therapeutic gene editing by CRISPR-Cas9 cleavage and homology-directed repair (HDR). Although ssODN sequence fidelity is critical to the safety and efficacy of editing, standard quality control methods cannot resolve individual nucleotide errors.

Methods

We performed deep sequencing of ssODNs from three manufacturers and amplicons from edited hematopoietic stem/progenitor cells.

Results

We find that synthesis errors are present in all ssODNs tested at rates that vary more than two-fold among manufacturers, at positions that are dependent on sequence context. These synthesis errors are propagated into the genome by HDR at frequencies proportional to their abundance in the ssODN. In our sickle cell mutation correction protocol, the most prevalent SNEs are predicted to produce benign β-globin variants, while the less frequent frameshift deletions are predicted to generate β-thalassemia-like alleles.

Conclusions

Current quality control standards are insufficient to detect these errors, and deep sequencing of ssODNs should be incorporated into regulatory submissions for clinical gene editing programs.