Ratones Humanizados

PROGRESS

Genomically humanized mice: technologies and promises
Anny Devoy, Rosie K. A. Bunton-Stasyshyn, Victor L. J. Tybulewicz, Andrew J. H. Smith and Elizabeth M. C. Fisher

Abstract | Mouse models have become an invaluable tool for understanding human health and disease owing to our ability to manipulate the mouse genome exquisitely. Recent progress in genomic analysis has led to anincrease in the number and type of disease-causing mutations detected and has also highlighted the importance of non-coding regions. As a result, there is increasing interest in creating ‘genomically’ humanized mouse models, in which entire human genomic loci are transferred into the mouse genome. The technical challenges towards achieving this aim are large but are starting to be tackled withsuccess.
The mouse is the model of choice for recapitulating genetic changes that give rise to human disease. Over the past 30 years, mouse molecular genetics has been refined to allow production of an impressive range of mutants. These include additive transgenic, knockout mice and knock-in mice (all of which can have conditional mutations or inducible mutations), strains containing chromosomalrearrangements or megabase-sized deletions and duplications and even transchromosomic mouse strains1. It is a fast developing area: new technologies are arising all of the time, and they include methods for modelling ‘sporadic’ disease, such as cancer 2. Almost all human disease models have been made to study changes in the coding genome. Typically, this has been done by pronuclear injection to generatetransgenics that ectopically express a mutant protein or by gene targeting in embryonic stem cells (ESCs): for example, by creating a gene knock-in. As proteins with a human amino acid sequence can have different biochemical characteristics from their mouse orthologues, transgenics have often been made with human cDNAs, and targeting has involved placing human coding sequences into theorthologous mouse gene. This genetic ‘humanizing’ strategy using coding sequences can result in a more accurate mouse model of disease than working with a mutant mouse protein. However, recent progress in genomic analysis has highlighted the importance of the non-coding genome (both transcribed and non-transcribed), making it clear that this category of sequence also needs to be taken into account whenmodelling disease. In particular, projects such as the ENCylopedia Of DNA Elements
14 | JANUARY 2012 | VOLUME 13 © 2011 Macmillan Publishers Limited. All rights reserved

(ENCODE)3 have discovered extensive transcription of the non-coding genome, and human genome-wide association studies (GWASs) demonstrate that variation (including copy number variation) in noncoding regions confers susceptibilityand resistance to disease in ways that we do not comprehend. As we learn more about the complexity of the genome, it is apparent that understanding human biology, particularly with respect to disease models, will require humanized mouse models that address the potential roles of both coding and non-coding genomic sequence (BOX 1). Laboratories worldwide are developing the technology for creatingsuch ‘genomically’ humanized mice, which are generated by transferring entire human genomic loci (including coding and non-coding regions) into the mouse genome. This is achieved either by the addition of human genomic sequences or by replacing regions of the mouse genome with equivalent human genomic sequences. However, the technical challenges remain daunting, and although current approaches foroptimizing different strategies are proving successful, they are far from routine. Here we look at the different approaches that have been developed for creating genomically humanized mice, why genomic humanization remains a challenge and which new technologies are the most promising. We also speculate on the direction of future advances in this field.
YAC and BAC transgenics The first...