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Contents lists available at ScienceDirect

Seminars in Cell & Developmental Biology journal homepage: www.elsevier.com/locate/semcdb

Review

Cancer mouse models: Past, present and future Walid T. Khaled b,∗∗ , Pentao Liu a,∗ a b

Wellcome Trust Sanger Institute, Hinxton, Cambridge CB10 1HH, UK Department of Pharmacology, University of Cambridge, Cambridge CB2 1PD, UK

a r t i c l e

i n f o

Article history: Available online xxx Keywords: Cancer models Gene targeting Transgenics PDX Cas9

a b s t r a c t The development and advances in gene targeting technology over the past three decades has facilitated the generation of cancer mouse models that recapitulate features of human malignancies. These models have been and still remain instrumental in revealing the complexities of human cancer biology. However, they will need to evolve in the post-genomic era of cancer research. In this review we will highlight some of the key developments over the past decades and will discuss the new possibilities of cancer mouse models in the light of emerging powerful gene manipulating tools. © 2014 Published by Elsevier Ltd.

Contents 1. 2.

3. 4. 5. 6.

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The different types of mouse models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Xenograft mouse models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Transgenic mouse models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Gene targeting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. New technologies for the generation of cancer mouse models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Genetic tools for cancer gene discovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Genetic screens to identify novel tumour suppressor genes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Humanised mouse models of cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Background It is now more than 40 years since President Nixon declared war on cancer. Driven by the revolution in molecular biology and genetics in the second half of the 20th century, our understanding of the basic cancer biology has significantly improved. These developments also led to the need for a robust model organism to perform in vivo studies. Although informative at the molecular level, studies in non-mammalian model organism such as Drosophila are in general not appropriate for modelling cancer development and preclinical testing of therapeutics. Over the years the mouse (mus musculus) has emerged as the model organism of choice to study tumour development (Table 1).

00 00 00 00 00 00 00 00 00 00 00

The mouse and human are similar in many aspects such as basic physiology and genome size [1,2]. The mouse has a short reproduction cycle, large litter sizes, and is easy to maintain. The advances in transgenic mouse technologies and the extensive annotation and characterisation of the mouse genome have facilitated the prime role played by the mouse in cancer research. In this review, we will discuss the different types of cancer mouse models and the new genetic tools available, which could lead to the more rapid generation of sophisticated cancer mouse models in the future. 2. The different types of mouse models 2.1. Xenograft mouse models

∗ Corresponding author. Tel.: +44 1223834244. ∗∗ Corresponding author. E-mail addresses: [email protected] (W.T. Khaled), [email protected] (P. Liu).

Most cancer biology studies over the last 40 years have been carried out in tumour-derived cell lines cultured in vitro. However, culturing tumours in such a manner leads to the development of

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2 Table 1 The different types of cancer mouse models. Type of mouse model

Advantages

Disadvantages - Low efficiency in establishing lines.

Patient derived xenograft (PDX)

- Recapitulate molecular and cellular characteristics of the primary tumours - Could be used in preclinical trials and in drug discovery

- Long latency period which is costly. - Tends to be high grade tumours - Not easy to manipulate for cell biology and genetic studies

Transgenic Targeted

- Relatively easy to produce - Lines are quick to establish

- Relies on short exogenous promoter sequences - No control over copy numbers and where it integrates in the genome

- Uses the endogenous regulatory element (promoter and enhancers) - Could be used to delete or overexpress in a tissue specific manner

- Time consuming to design targeting vectors

cell lines that are significantly different from the original tumour [3–6]. The Darwinian selection pressure on cells cultured in vitro dictates that cell lines tend to lose the heterogeneous morphological and genetic characteristics of the original tumour. This problem is important in the context of cancer drug development, which eventually progresses towards testing on patient derived tumour models. In fact, one of the reasons for the high failure rate of new agents in phase III clinical studies, is the lack of pre-clinical models that captures the heterogeneity of tumours in patients [7]. Patient derived xenografts (PDX) alleviate some of the problems raised by the use of cell lines. PDX are based on the culturing of tumours directly isolated from the patient in immune compromised mice [7]. Genetic and morphological consistency between PDX tumours and their primary counterparts has been demonstrated for breast, lung, pancreas, ovarian, colorectal and paediatric tumours [8–13]. In addition, the response to therapeutic intervention was very similar between primary and PDX tumours. This has fuelled pilot studies to test the utility of PDX models in the clinic [14]. The National Cancer Institute (NCI) has initiated a paediatric preclinical testing programme, which uses PDX models to identify and validate potential therapeutic agents for the treatment of childhood cancers [15,16]. In this study the authors have generated 87 PDX lines, which they characterised and compared to primary tumours and showed that PDX models recapitulate expression and genetic profiles of primary tumours [15,16]. In addition, the authors also showed that PDX models respond to known therapeutics in a similar fashion to primary tumour [15,16]. In another pilot study of pancreatic cancer patients, PDX models were used to test 63 drugs on PDX tumours from 14 patients [17]. Nine out of 11 patients given the prospectively identified treatments showed durable partial remission [17]. With the advances in sequencing technology, PDX models could provide a model to test the pharmacogenomics interaction of drugs with identified mutations and thus, informing the choice of drugs treatment (Fig. 1) [14]. The results of these initiatives are very encouraging and will no doubt be very useful in the future of personalised cancer treatments. So why are PDX models not more “mainstream” in cancer research? There are a number of issues that make PDX models difficult to establish and study. First, to initially establish a PDX tumour model, the latency period could be between 2 and 12 months and the rate of successful engraftment ranges between 23 and 75% depending on the tumour type [8,12,15,16]. These numbers translate to high running costs, which make it prohibitive to routinely establish PDX models. Second, PDX tumours are difficult to genetically manipulate, as they are not maintained in vitro, thus limiting the performance of any cell biology and genetic studies. Third, the majority of successfully established PDX models tend to be from high-grade tumours at late stages of tumour development, which is not ideal for studying early stages

- It could take up to 1 year to establish the line which is costly

of tumour development. Taking these issues into account, the PDX models are therefore good tools for drug screening and preclinical studies but not for basic cancer research. 2.2. Transgenic mouse models Advances in genetic manipulation of the genome via embryonic stem cells has facilitated the development of more physiologically relevant genetically engineered mouse models (GEMM) a tangible reality. Essentially, there are two broad types of GEMMs, transgenic or targeted. Transgenic GEMM’s rely on the expression of exogenous cancer genes driven by ectopic promoters. Briefly, expression constructs are introduced via pronuclear injection, which randomly integrate into the genome. One of the first transgenic mouse model of cancer was for the Simian Virus 40 (SV40), where elements of the SV40 genome, including the large T-antigen, were introduced into the mouse which then developed brain tumours [18]. The choice of promoters in the transgene constructs often allow for tissue specific expression. For example, the mouse mammary tumour virus (MMTV) promoter has been used to drive expression of the oncogenes Myc and polyomavirus middle T (PyMT) specifically in the mammary gland and to induce primary tumours and metastatic tumours [19,20,21]. Conventional transgenic technology nevertheless has inherent drawbacks. For example, the use of short exogenous promoter sequences in the expression constructs often fails to recapitulate the spatial and temporal gene expression orchestrated by endogenous promoters. In addition, this technology does not have control over the copy number and genomic location of the transgene, which can lead to expression variation among founders due to methylation of the transgenes and position effects. In recent years, several major advances have improved transgenic technology. For example, transgenes of large sizes such as of engineered artificial chromosomes (BACs) are used to ensure most, if not all, regulatory elements are included in the transgene. Moreover, transgenes can be engineered to integrate to specific genomic loci either by site-specific integrase such as phiC31 [22] or through homologous recombination as described in the following section. 2.3. Gene targeting Efficient homologous recombination in mouse embryonic stem cells, which are derived from pre-implantation embryos, has been used to modify a genomic locus. This process, also called gene targeting, permits introduction of various genetic changes and expression cassettes into a genomic region with base pair precision. This usually involves using a targeting vector [23,24], which has two homology arms for recombination and a cassette for disruption, mutation or visualisation of the endogenous locus. This

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Fig. 1. Future directions of cancer mouse models.

Nobel prize winning approach has been used to mutate tumour suppressor genes such as p53, Rb, Pten and Brca1 [25–28] (Table 2) in the mouse and facilitated key insights into the roles of these genes in normal development and in cancer. These earlier loss of function models, however, do not faithfully recapitulate the situation in human tumours. For example, p53 is found mutated in almost 50% of all tumours across all tissues but complete deletion of p53 is not very common [29]. Germ-line deletion of p53 in the mouse leads to a significant bias towards lymphomas. Targeted knock-in of a mutant p53 (p53R172H ) allele, which is commonly found in human cancers, into the endogenous p53 locus in the mouse, gave a more balanced repertoire of tumours [30]. The p53R172H knock-in model relies on the use of the site-specific Cre-lox recombination system (reviewed in [31]) to allow for tight temporal and spatial regulation of the mutant

Table 2 A list of some targeted and transgenic cancer mouse models. Cancer type

Mouse model

References

Breast carcinoma

Brca2;Trp53;K14-Cre MMTV-c-Neu (transgene) Brca1;Trp53;Blg-Cre Cdh1;Trp53;K14-Cre KrasG12D ;Adeno-Cre KrasG12D ;Trp53;Pdx1-Cre Nf1;Trp53 KrasV12 ;Apc;Ah-Cre Myc;Trp53

[90] [91] [28] [92] [38] [39] [93] [41] [94]

Lung adenocarcinoma Pancreas adenocarcinoma Glioblastoma Colon polypoid adenocarcinoma Hepatocellular carcinoma

p53 allele which was downstream from a lox-stop-lox (LSL) cassette [30]. In addition, the Cre-lox system could be used to delete tumour suppressors in a tissue specific manner (the conditional knockout approach). For example homozygous deletion of either of the breast cancer susceptibility genes Brca1 or Brca2 leads to embryonic lethality [32,33]. Using the Cre-lox approaches, several conditional knockout lines were then generated for both genes [34]. These alleles have been used to successfully dissect their function in mammary tumour development in conjunction with MMTV, K14 and WAP Cre-lines, which are predominantly expressed in the mammary gland [28,35]. Combined with LSL and the tissue specific Cre expression, controlled oncogene expression could be achieved [36]. Mutant KRAS is found in pancreatic, colon and non-small cell lung cancer (NSCLC) [37]. Once the mutant form of KRAS (KRASG12D ) is knocked-in to the endogenous locus of Kras with a LSL cassette upstream, tissue specific Cre expression excises the LSL cassette and induces pancreatic, colon and lung tumours that faithfully recapitulated the human cancer [38–41]. The LSL or the reporter cassettes in these knock-in mouse lines are normally targeted to the ubiquitously expressed Rosa26 locus [36]. The LSL strategy could also be used to label and track cells using fluorescent or luminescence reporters downstream of the LSL cassette [42,43]. Mouse models of cancer also benefited from the development of inducible gene expression and protein localisation systems. The most commonly used inducible systems are the: (a) tamoxifen inducible system and (b) the tetracycline inducible system. The tamoxifen inducible system is most widely used to induce Cre

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Table 3 Comparison of the different gene targeting methods. Targeting method

Advantages

Disadvantages

Electroporation (linear vector)

- Precise targeting (low off targets effects) - Ability to target large regions DNA

- Requires ES cell manipulation - Low targeting efficiency - Laborious vector design and cell screening - Heterozygous targeting - Single targeting event

TALEN

- Precise targeting (low off target effects) - Ability to introduce double strand breaks and disrupt genes directly with no need for targeting vector

- Requires ES cell manipulation - Tedious assembly of TALE domains by cloning and need to design two constructs for each sequence - Multiple targeting is difficult

Crisper/Cas9

- Very efficient targeting

- High off target events (could be reduced by using the nickase version of Cas9 [64]

- Homozygous targeting - Easy to design using commercially available oligos - Multiple targeting possible - Direct targeting in zygote thus no need for ES manipulation - Quick to establish the line

activation. The Cre recombinase is fused to the ligand-binding domain of a mutated oestrogen receptor (ERT or ERT2) that has a diminished binding affinity to endogenous oestrogen but maintains affinity to the oestrogen analogue tamoxifen [44,45]. Only in the presence of the tamoxifen will the fusion Cre-ERT/Cre-ERT2 protein localise to the nucleus, thus allowing Cre to recombine lox sites and induce recombination [44,45]. The Cre-ERT2 has been used in combination with several tissue specific promoters to give both temporal and special regulation of Cre recombination [46]. The tetracycline inducible system on the other hand regulates transcription levels of elements downstream of the tetracycline response element (TRE) and has been used to drive Cre expression in a tissue specific manner [47]. Apart from controlling Cre expression in a spatiotemporal manner [47], the tetracycline inducible system (also known as doxycycline inducible system) has also been used to induce the expression of oncogenes and study their requirement for tumour development and maintenance. The first example of such system was to demonstrate the tumour promoting ability of HRASV12G in the skin using and doxycycline-inducible H-RASV12G [48]. Conversely, withdrawal of doxycycline from tumour bearing mice led to tumour regression thus, confirming the role of H-RASV12G in tumour maintenance [48,49]. Many types of cancer have characteristic and pathological chromosomal abnormalities such as the Philadelphia chromosome in chronic myeloid leukaemia (CML) generated by the translocation of chromosome 9 and 22 [50] which generates the gene fusion BCR-ABL [51]. The Cre-lox system has also been used to generate mouse model of chromosomal translocations [52,53]. To achieve this two lox sites are consecutively targeted to two loci in the mouse genome (in this case on different chromosomes). In presence of the Cre recombinase the two lox sites recombine to generate rearranged chromosomes. This strategy has been successfully used to generate a mouse model of translocations between AML1 and ETO found in acute myeloid leukaemia (AML) [54]. The major disadvantage to generating this type of model is the need to doubly target the ES cells which tends to have a low success rate and inefficient recombination between the two-targeted sites [53]. 2.4. New technologies for the generation of cancer mouse models The major disadvantages of generating targeted mouse model is the significant time it takes and the cost associated with it (Table 3). Indeed it can take up to 1 year to generate and establish a targeted mouse model. New gene editing technologies such as the transcription activator-like effector nucleases TALEN (reviewed in [55]) and the RNA guided Cas9 nuclease [56] systems have significantly

increased the efficiency and reduced the time and cost associated with generating new mouse models. The most promising is the CRISPR/Cas9 system which is based on the bacterial adaptive innate immune system termed clustered regularly interspaced short palindromic repeats (CRISPR) associated systems (CAS), which uses short RNA molecules to guide degradation of foreign DNA [57–59]. Recently, the Cas9 nuclease has been adapted for use in multiple eukaryotic systems including mouse and human [60,61]. The simplicity of the Cas9 system manifested in the requirement of a short guide RNA (sgRNA) to mediate genome editing has opened up a whole new set of possibilities in cancer modelling. The Cas9 mediated cleavage of the DNA could be used to directly disrupt the target gene by introducing a double strand break or could be used in conjunction with a targeting vector to introduce specific mutations/knock-in cassettes [56,60,61]. However, one draw back to the CRISPR/Cas9 system is the off target effects which are higher than that observed by TALENs (Table 3) [62,63]. This has been recently addressed by the development of the Cas9-nicakase, which overcomes some of the specificity issues and reduces off-target effects [64]. The power of the Cas9 system was demonstrated by the simultaneous introduction of mutations in multiple genes in ES cells and directly in the zygote with an efficiency of up to 80% [65]. This is a remarkable advance in the field of gene targeting as multiple transgenic mice are becoming increasingly important for the study of cancers where more than one factor tends to be involved in tumourigenesis. Generating multi-transgenic lines using classic gene targeting techniques could take years, which would be cost prohibitive and time consuming. With the increased number of candidate cancer genes identified through large-scale cancer sequencing projects [66,67], the Cas9 system indeed holds plenty of promise for the acceleration of cancer research validation in the post-genomic era.

3. Genetic tools for cancer gene discovery Apart from modelling known cancer mutations, the mouse could also be used to identify novel cancer genes. Historically, retroviral insertional mutagenesis has been performed to identify hundreds of novel cancer genes [68]. The premise of these screens is that the retrovirus can induce cancer by randomly integrating in the genome thus, deregulating oncogenes or inactivating tumour suppressors. However, the bias in both retrovirus integration sites and the repertoire of tumours (tend to be haematopoietic and mammary) induced by retroviruses has led to the development of

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different mutagenesis systems which could allow identification of mutated genes in most solid cancers. In recent years transposon-based insertional mutagenesis screens have been used as an alternative to the retrovirus based system [69]. In principle transposons have the ability to mobilise around the genome in the presence of an active transposase. Transposases recognise unique sequences in the transposon arms and catalyse their mobilisation and re-integration via a Cut and Paste mechanism [69]. Two DNA transposition systems are widely used in mutagenesis screens, Sleeping Beauty (SB) [70,71] and piggyBac (PB) [72]. In both systems, a “cargo” mutating cassette is inserted between the inverted recognition sequences which increases the rate of cancer gene discovery. The mutating transposons tend to be bi-directional with one direction carrying a splice acceptor site followed by polyA tails thus disrupting the transcription of any potential tumour suppressor gene upstream (inactivating). The other direction of the transposon is carrying a splice donor site preceded by a strong promoter, which will drive the expression of any downstream oncogenes (activating) [71]. This design was used in the SB screens with the T2/Onc2 transposon carrying the MSCV promoter, which tends to be expressed in haematopoietic cells [71], and the T2/Onc3 transposon carrying the CAGGS promoter, which is expressed strongly in multiple tissues [73]. The use of these different promoters thus allow for the discovery of more tissue specific oncogenes. A similar set of transposons was also designed for the PB screens [72]. For an even tighter control of the transposon activation, the transposase could be expressed in a tissue specific way. The SB and PB transposase lines were also designed with the LSL cassette thus requiring the activity of Cre to induce its expression. For example the Villin-Cre and Albumin-Cre have been used to activate SB transposition in intestinal epithelial cells [74] and hepatocytes [75] respectively. In addition, transposons could be used to identify cooperating mutation in models expressing known oncogenes. This approach was used to identify cooperating mutations associated with mutant Nucleophosmin (NPM1) and oncogenic KrasG12D , in acute myeloid leukaemia (AML) and pancreatic ductal carcinomas, respectively [76,77].

4. Genetic screens to identify novel tumour suppressor genes One of the drawbacks of transposon-based screens is the significant bias in the number of oncogenes identified. It is estimated only 10% of hits tend to be putative tumour suppressors. Disruption of both alleles is needed for most tumour suppressors to be revealed – the likelihood of two transposons disrupting both copies of the same gene in one cell is very low thus explaining the low numbers of tumour suppressors identified. Therefore, the use a recessive screening strategy is required to identify tumour suppressors. Recessive screens have been predominantly performed in either haploid cells (reviewed in [78]) or by using the Bloom syndrome gene (Blm) deficient cells which have frequent loss of heterozygosity [79]. Recently, the CRISPR/CAS9 system has been adapted to perform recessive genetic screens in mammalian cells [80–82]. The screen relies on the simple principle that as long as the gRNA and the Cas9 nuclease are constitutively expressed in the cell, both copies of the target gene will eventually be cleaved. The homozygous targeting of both copies could then be exploited to perform genetic screens to identify tumour suppressor genes [80–82]. A large library of barcoded gRNAs packaged in lentivirus particles could be used to infect Cas9 expressing cells and next generation sequencing used to identify the hit genes as demonstrated in the pioneer studies [80–82]. We predict that this

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technology could soon be used to identify tumour suppressors in cells, which are amenable to ex vivo manipulation such as the mammary epithelial cells and haematopoietic cells. 5. Humanised mouse models of cancer Although cancer mouse models are very informative, there are limitations to their use as the research questions become more clinically relevant. As demonstrated by the PDX models, modelling and testing novel therapeutic intervention is more relevant on human tissue. These issues thus highlight the need for developing humanised mouse models. There are two types of humanised mouse models, (a) genetic-chimaeras; where human genes/chromosomes are introduced into the mouse genome and (b) cellular-chimaeras; where human cells are introduced into immune-compromised mice and contribute/give rise to human tissues in the mouse. Examples of the genetic-chimaera models are the Kymouse and VelocImmune mice, which carry the human immunoglobulin genes [83–85]. These mice has been used to produces fully humanised antibodies, 10 of which are clinical trials such as Nesvacumab, which binds Angiopoietin-2, and Enoticumab, which binds Dll4 [83,84,86]. However, the cellular-chimaera model is the one that is most relevant to cancer modelling. Normal human cells could be engineered to harbour genetic mutations and orthotopically transplanted into the mouse to study tumour initiation and development in vivo. These models could provide insight into the biology of early tumour development, which is not feasible with PDX models – where tumours tend to be at late stages of disease development. One of the first examples of cellular-chimaeras was in the immune system where, normal and leukemic human bone marrow cells were successfully grown in immune-compromised mice [87]. More recently, normal and malignant human breast epithelial cells have been successfully engrafted in pre-conditioned cleared fatpads of immune-compromised mice [88] thus, opening up the possibility to study cancerous and normal human mammary epithelial cells in vivo. However, genetic manipulation of primary human cells prior to transplantation into the mouse is still technically challenging and has thus far restricted the number of cellular chimaera models. We predict that in the coming years humanised mouse models of cancer will become more prevalent for the following reasons (Fig. 1). Firstly, the availability of genetically engineered mice that are even more immune-compromised and express essential human genes such as cytokines may further boost engraftment of human tissues including normal and cancer cells. Secondly, the relative ease of generating genetically stable and targetable human induced-pluripotent stem cells (iPS) [89] will facilitate the differentiation and production of multiple cell types, which could be transplanted into the mouse. Thirdly, the Cas9 system will allow us to genetically manipulate human iPS cells [65], which could be used to introduce mutations identified through the large cancer sequencing projects [66,67] and thus, test and validate their function in tumour development. These developments could lead to better understanding of the various stages of cancer development and potentially lead to the development of more relevant pre-clinical models for therapeutic development. 6. Concluding remarks Over the past 40 years mouse models of cancer have helped us understand various aspects of tumour development and progression. Is there a place for the mouse in the post-genomic era of cancer research? The answer is indeed yes; with the emergence of improved and more efficient gene targeting technologies the development of more relevant cancer mouse models will be achievable

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(Fig. 1). Thus, making the mouse a key weapon in our continuing fight against cancer.

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Please cite this article in press as: Khaled WT, Liu P. Cancer mouse models: Past, present and future. Semin Cell Dev Biol (2014), http://dx.doi.org/10.1016/j.semcdb.2014.04.003