CRISPR-mediated precise large fragment insertion in zygotes enables rapid generation of humanized immunoglobulin heavy-chain mice

Current CRISPR-Cas9 methods are restricted to small genomic edits. We developed a CRISPR-guided approach that enables direct insertion of large genomic sequences into mouse zygotes. We deleted the murine 2.4-Mb immunoglobulin heavy-chain (IgH) variable (V H ) locus and then precisely inserted a bacterial artificial chromosome (BAC) containing a 155-kb human V H DNA fragment flanked by 20-kb homology arms. Full-length, single-copy BAC integration occurred without ectopic recombination. Human sequences were stably transmitted and expressed V H segments that recombined with endogenous mouse sequences, and mice exhibited normal B cell development. Upon immunization, human V H -expressing B cells underwent class-switch recombination and somatic hypermutation, secreting antigen-specific antibodies. We demonstrated modular IgH humanization by replacing endogenous mouse diversity and joining (D H -J H ) segments with human sequences, producing V(D)J recombination and diverse antibodies. Unlike traditional methods requiring more than a year, this approach enables the generation and validation of mice carrying large genomic insertions within 8 weeks.