Preparation of intact yeast artificial chromosome DNA for transgenesis of mice

Purification of Yeast Artificial Chromosome DNA for Microinjection Using a Two-Gel Electrophoresis Procedure

This protocol describes a method to purify yeast artificial chromosome (YAC) DNA for microinjection. YAC DNA solutions cannot be concentrated by standard DNA precipitation methods and resuspended into smaller volumes. Attempts to precipitate YAC DNA solutions will, unavoidably, break and destroy the DNA molecules. Therefore YAC DNA must always be kept in a solution that maintains the integrity of the YAC DNA molecules. In the method described here, YAC DNA is extracted from pulsed-field gel electrophoresis (PFGE) agarose plugs by using two gel electrophoresis steps. The first gel electrophoresis step is the PFGE itself. The region of agarose containing the YAC DNA of interest is excised and run on a standard electrophoresis gel setup at a 90° angle to the PFGE run. The YAC DNA migrates out of the agarose slice and enters into a thicker low-melting agarose gel, thereby promoting compaction of YAC DNA molecules. The aim of this method is to convert a slice of agarose into a cube of agarose of smaller volume. The volume of this cube will determine the final concentration of the YAC DNA precipitation.

Targeted isolation and cloning of 100-kb microbial genomic sequences by Cas9-assisted targeting of chromosome segments

Cloning of long microbial genomic sequences is an essential tool in synthetic biology and genome engineering. Such long sequences are often difficult to obtain directly by traditional PCR or restriction enzyme digestion, and therefore the cloning of these sequences has remained a technical obstacle in molecular biology. Based on the in vitro application of RNA-guided Cas9 nuclease, the method of Cas9-assisted targeting of chromosome segments (CATCH) cleaves target DNA in vitro from intact bacterial chromosomes embedded in agarose plugs, which can be subsequently ligated with cloning vector through Gibson assembly. Here we describe an optimized protocol of CATCH cloning for the targeted cloning of long genomic sequences of up to 100 kb from microorganisms. The protocol uses standard laboratory equipment and takes ∼8 h of bench time over several days, and it may potentially simplify and accelerate efforts to isolate and clone large gene clusters from microorganisms.

Generation of transgenic mice with megabase-sized human yeast artificial chromosomes by yeast spheroplast-embryonic stem cell fusion

Introducing human genes into mice offers the opportunity to analyze their in vivo function or to obtain therapeutic molecules. For proper gene regulation, or in case of multigene families, megabase (Mb)-sized DNA fragments often have to be used. Yeast artificial chromosome (YAC)-mediated transgenesis is irreplaceable for this purpose, because alternative methods such as the use of bacterial artificial chromosomes (BACs) cannot introduce DNA fragments larger than 500 kb into the mouse germ line. However, YAC libraries often contain only partial gene loci. Time-consuming reconstruction of YACs, genetic instability and the difficulty in obtaining intact YAC DNA above a certain size impede the generation of humanized mice. Here we describe how to reconstruct YACs containing Mb-sized human DNA, such as the T cell receptor-α (TRA) gene locus, thus facilitating the introduction of large DNA fragments into the mouse germ line. Fusion of YAC-containing yeast and embryonic stem (ES) cells avoids the need for YAC DNA purification. These ES cells are then used to stably introduce the functional TRA gene locus into the mouse germ line. The protocol takes ∼1 year to complete, from reconstruction of the entire TRA gene locus from YACs containing partial but overlapping TRA regions to germline transmission of the YAC.

High-affinity IgG antibodies develop naturally in Ig-knockout rats carrying germline human IgH/Igκ/Igλ loci bearing the rat CH region

Mice transgenic for human Ig loci are an invaluable resource for the production of human Abs. However, such mice often do not yield human mAbs as effectively as conventional mice yield mouse mAbs. Suboptimal efficacy in delivery of human Abs might reflect imperfect interaction between the human membrane IgH chains and the mouse cellular signaling machinery. To obviate this problem, in this study we generated a humanized rat strain (OmniRat) carrying a chimeric human/rat IgH locus (comprising 22 human V(H)s, all human D and J(H) segments in natural configuration linked to the rat C(H) locus) together with fully human IgL loci (12 Vκs linked to Jκ-Cκ and 16 Vλs linked to Jλ-Cλ). The endogenous Ig loci were silenced using designer zinc finger nucleases. Breeding to homozygosity resulted in a novel transgenic rat line exclusively producing chimeric Abs with human idiotypes. B cell recovery was indistinguishable from wild-type animals, and human V(D)J transcripts were highly diverse. Following immunization, the OmniRat strain performed as efficiently as did normal rats in yielding high-affinity serum IgG. mAbs, comprising fully human variable regions with subnanomolar Ag affinity and carrying extensive somatic mutations, are readily obtainable, similarly to conventional mAbs from normal rats.

Generation of transgenic rats using YAC and BAC DNA constructs

Transgenic rats with a simple plasmid vector smaller than 20 Kb show insufficient expression and tissue specificity of the introduced transgene. Vectors derived from yeast artificial chromosome (YAC) and bacterial artificial chromosome (BAC), consisting of DNA fragments up to approximately 1 Mb (YAC) and approximately 200 Kb (BAC), respectively, and containing various endogenous regulatory sequences, were expected to work well and showed expression profiles comparable to their endogenous counterparts in transgenic animals. While attempting to make transgenic rats using YAC and BAC vectors, we faced two problems: how to prepare sufficiently concentrated intact DNA and how to reliably microinject a large DNA fragment into the fragile pronuclear ova of the rat. After solving these problems, we were able to make transgenic rats by introducing YAC/BAC gene constructs (YACs/BACs) into the pronuclear ova. And then we examined the relative transcription rates of these genes in the transgenic rats. In this chapter, we focus on this injection process.