Clustered regulatory interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 nuclease (Cas9), the third-generation genome editing tool, has been favored because of its high efficiency and clear system composition. In this technology, the introduced double-strand breaks (DSBs) are mainly repaired by non-homologous end joining (NHEJ) or homology-directed repair (HDR) pathways. The high-fidelity HDR pathway is used for genome modification, which can introduce artificially controllable insertions, deletions, or substitutions carried by the donor templates. Although high-level knock-out can be easily achieved by NHEJ, accurate HDR-mediated knock-in remains a technical challenge. In most circumstances, although both alleles are broken by endonucleases, only one can be repaired by HDR, and the other one is usually recombined by NHEJ. For gene function studies or disease model establishment, biallelic editing to generate homozygous cell lines and homozygotes is needed to ensure consistent phenotypes. Thus, there is an urgent need for an efficient biallelic editing system. Here, we developed three pairs of integrated selection systems, where each of the two selection cassettes contained one drug-screening gene and one fluorescent marker. Flanked by homologous arms containing the mutated sequences, the selection cassettes were integrated into the target site, mediated by CRISPR/Cas9-induced HDR. Positively targeted cell clones were massively enriched by fluorescent microscopy after screening for drug resistance. We tested this novel method on the amyloid precursor protein (APP) and presenilin 1 (PSEN1) loci and demonstrated up to 82.0％ biallelic editing efficiency after optimization. Our results indicate that this strategy can provide a new efficient approach for biallelic editing and lay a foundation for establishment of an easier and more efficient disease model.

Xinyi LI;Bing SUN;Hongrun QIAN;Jinrong MA;Magdalena PAOLINO;Zhiying ZHANG

College of Animal Science and Technology,Northwest A&F University,Yangling 712100,China;Karolinska Institute,Department of Medicine Solna,Center for Molecular Medicine,Karolinska University Hospital Solna,17176 Stockholm,Sweden

浙江大学学报（英文版）（B辑：生物医学和生物技术）

[1]Xinyi LI;Bing SUN;Hongrun QIAN;Jinrong MA;Magdalena PAOLINO;Zhiying ZHANG-.A high-efficiency and versatile CRISPR/Cas9-mediated HDR-based biallelic editing system)[J].浙江大学学报（英文版）（B辑：生物医学和生物技术）,2022(02):141-152,中插1-中插2

biallelic,endonucleases,homozygotes,cassettes,Flanked

high,efficiency,versatile,CRISPR,Cas9,mediated,HDR,editing,Clustered,regulatory,interspaced,short,palindromic,repeats,associated,protein,third,generation,genome,tool,has,been,favored,because,its,clear,composition,In,this,technology,introduced,double,strand,breaks,DSBs,are,mainly,repaired,by,homologous,joining,NHEJ,homology,directed,pathways,fidelity,used,modification,which,can,artificially,controllable,insertions,deletions,substitutions,carried,donor,templates,Although,level,knock,out,easily,achieved,accurate,remains,technical,challenge,most,circumstances,although,both,alleles,broken,only,other,usually,recombined,For,function,studies,disease,model,establishment,generate,homozygous,cell,lines,needed,ensure,consistent,phenotypes,Thus,there,urgent,efficient,Here,developed,three,pairs,integrated,selection,systems,where,each,two,contained,drug,screening,fluorescent,marker,arms,containing,mutated,sequences,were,into,site,induced,Positively,targeted,clones,massively,enriched,microscopy,after,resistance,We,tested,novel,method,amyloid,precursor,presenilin,PSEN1,loci,demonstrated,up,optimization,Our,results,indicate,that,strategy,provide,new,approach,lay,foundation,easier,more

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