http://breast-cancer-research.com/vol1no1/02jul99/dispatch/1
`
`Commentary
`Do we now have a relevant animal model for breast cancer?
`
`Barry Gusterson, Beatrice Howard, Tim Crook and Barbara Tennent*
`
`Institute of Cancer Research, London, UK and *The Jackson Laboratory, Maine, USA
`
`Received: 4 May 1999
`Revisions requested: 28 May 1999
`Accepted: 18 June 1999
`Published: 2 July 1999
`© Current Science Ltd
`
`Important note about how to cite this article
`This article is also available online in the Breast Cancer Research
`
`website. To avoid confusion, please ensure that only the online version
`of the article is cited in any reference, as follows:
`Gustersen B, Howard B, Crook T, Tennent B: Do we now have a
`relevant animal model for breast cancer? [commentary]. http://breast-
`cancer-research.com/vol1no1/02jul99/dispatch/1
`
`Recent advances in manipulating targeted genes in a tissue-
`specific manner have opened the way to the development
`of relevant mouse models for the molecular dissection of
`
`tumours there is a high incidence of ms mutations, which
`are very rare in human breast cancers. Thus, although
`these models are valuable tools for the dissection of the
`
`the events leading to breast cancer. However, when judging
`the appropriateness of any given mouse model, it is impor-
`tant to remember that breast cancer comprises a heteroge-
`neous group of diseases characterized by different sets of
`genetic mutations, histopathological
`types and metastatic
`potentials, often within the same primary tumour mass. It is
`unlikely that any single mouse model will be able to mimic
`all these aspects of human breast cancer but this does not
`invalidate their use in studying specific aspects of the
`disease. Mouse models are particularly valuable for defining
`the molecular pathways participating in mammary epithe-
`lial cell transformation and disease progression, for identify-
`ing modifier genes
`that
`affect penetrance
`of
`the
`manipulated gene and for testing various therapeutic and
`preventative approaches. The paper by Xu et a! [1]
`in a
`recent edition of Nature Genetics describes a new model that
`
`offers promise in several respects.
`
`To put the new model in perspective, we need briefly to
`consider the historical background on mouse models of
`human cancers. From the 1950s, much effort has been put
`into describing and classifying spontaneous, viral- and
`carcinogen-induced mammary tumours in rats and mice,
`and these models have proven value in toxicology and
`drug testing. Mice infected with the mouse mammary
`tumour virus (MMTV) have played a large part in our
`understanding of insertional mutagenesis and activation of
`oncogenes
`leading to mammary tumourigenesis
`[2].
`However, only a few of the human homologues of these
`genes are mutated in human breast cancers although the
`signalling pathways through which these genes act have
`been implicated.
`In carcinogen-induced rat mammary
`
`complex signalling pathways through which these genes
`act, they do not necessarily represent the exact genetic
`events that precipitate human breast cancers.
`
`Transgenic technology has recently facilitated the develop-
`ment of an entirely new set of genetically engineered
`mouse models that can be used to define the transforming
`potential of genes implicated in human breast cancer [3,4].
`The seminal work of Stewart er a! [5] has shown that a c—myr
`transgene expressed in the mammary gland under the regu-
`lation of the MMTV long-terminal repeat (LTR) induces
`mammary tumours. Webster er a! [6] have extended this
`work, and demonstrate how sophisticated targeted muta-
`genesis can be applied to dissecting signalling pathways.
`
`The paper by Xu et a! [1] now demonstrates the power of
`conditional mutagenesis to specifically delete a gene rele-
`vant to human hereditary breast cancer (BRCAI) from the
`mouse mammary gland. BRCAI mutations are known to
`account
`for a significant proportion of familial breast
`cancers. BRCAI contains a region that
`interacts with
`RADSI, a homologue of bacterial RecA, which is involved
`in DNA repair, and is believed to be important in main-
`taining genetic stability. Homozygous loss of BRCAI in
`human tumours is thought to allow the accumulation of
`mutations in other genes, eventually resulting in tumouri-
`genesis. However, progress in studying the effects of
`BRCAI deletion or mutation on breast development and
`breast cancer has been delayed because of the lack of an
`animal model. Mice bearing homozygous null mutations
`of Brrrll die before embryonic day 9, whereas mice
`heterozygous
`for Brrrll
`deletions
`do not develop
`
`Apc = adenomatous polyposis coli; LTR = long-terminal repeat; MMTV = mouse mammary tumour virus; Wap = whey acidic protein.
`
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`mammary tumours. The paper by Xu et a! [I] demon-
`strates one way forward that uses a conditional knockout
`approach to mutate the intact allele in the mammary
`glands of mice bearing heterozygous deletions of Brml.
`This new method uses the Cre-[oxP system to induce
`mutations in a tissue- and temporal-specific manner [7],
`and mimics human disease by producing mice in which
`one of the two Brml genes has been disabled while the
`other carries a mutation that enables it to be disabled in
`
`mammary tissue later in the life of the mouse. Specifically,
`it induces mammary tissue specific deletion of Brml exon
`11 (which encodes the region that interacts with RAD51)
`under the control of either an MMTV—Cre or a whey acidic
`protein (thp)—CM transgene. The [VA/[TV and W511) promot-
`ers are maXimally activated during pregnancy and lacta-
`tion; the Cre-[oxP system excises specific DNA sequences
`under the control of these tissue-specific promoters. The
`model mice thus lose the Brcal repair function on preg-
`nancy and lactation.
`
`The resulting Brml conditional knockout appears to
`model the molecular mechanism of BRCAI involvement
`in human breast cancer. The mice in which Brcal function
`
`has been ablated in this way develop mammary-specific
`developmental abnormalities and, after a long latency
`period, mammary tumours. The molecular pathology of
`these tumours resembles that of the carcinomas arising in
`human carriers of BRCAI mutations. A common feature of
`
`the tumours that develop in the mice is aneuploidy and
`genetic instability as indicated by chromosomal transloca-
`tions. The tumours show rearrangements or translocations
`of chromosome 11, and it is stated that rearrangements of
`other chromosomes are found. Human BRCAl-associated
`
`tumours also show frequent chromosomal aberrations
`[8—10]. It is interesting that two out of three tumours
`arising in the conditional Brml knock-out mice have
`abnormalities in the Trp53 gene. The shortened latency of
`tumour development produced by introducing a loss of
`function Trp53 allele provides further support
`for
`the
`important role of TP53 mutations in tumourigenesis in the
`BRCAI mutant background as described in human
`tumours [11]. Thus, the Xu mouse is a true breakthrough
`as it is the first model in which the mechanisms of genetic
`instability and resultant tumourigenesis in the BrmI-defi-
`cient mammary gland can be studied. These mice will
`undoubtedly be of value in elucidating the early genetic
`lesions that promote breast tumourigenesis. Furthermore,
`they should be useful for investigating the effects of an
`array of suspected agents in breast cancer because of their
`increased sensitivity to DNA-damaging insults.
`
`The comparative histopathology is an important element
`in validating a mouse model and has implications for
`deducing the stem cell of origin, and predicting future
`behaviour of
`the
`tumour. The differences
`in
`the
`
`histopathology of the tumours arising in mice compared to
`
`http://breast-cancer-research.com/vo|1no1/02jul99/dispatch/1
`
`those in women have been a major limitation of many
`mouse models, especially as they suggest a different target
`cell population for the initiating event. Comparisons have
`been complicated by the lack of an internationally
`accepted terminology for both the normal glandular struc-
`ture and the tumours that arise in the mammary glands of
`rats and mice.
`In transgenic models,
`interpretation of
`pathology is further complicated by the currently used
`promoters, which may direct recombination to cells at dif-
`ferent stages of differentiation or to cell lineages different
`from those commonly mutated in human cancers. The
`W511) promoter is eXpressed primarily during pregnancy
`and lactation throughout
`the mammary tree and one
`advantage is that the lumenal cell population only is the
`target. The MMTV-LTR, also used to generate Brml
`conditional knockouts, has the disadvantage of being
`active in many tissues. Despite these caveats, however, it
`is clear that the tumours arising in the BrmI-deficient
`animals have many of the morphological features seen in
`human breast cancers. It will now be important to evaluate
`the model in terms of invasion of these tumours, lymph
`node involvement and the pattern of dissemination, as
`mouse models of metastasis to organs other than the lung,
`such as the brain and bones, are very much needed. This
`could be difficult to ascertain in the Xu model, given the
`long latency to tumourigenesis, but will be well worth the
`effort. It will also be important to assess the patterns of
`eXpression of molecular markers such as the receptors for
`oestrogen and progesterone. The faithfulness of the Xu
`model in this respect will determine its value for biochem-
`ical analyses and treatment studies. Now that it has been
`shown that Brml-deficient mice do develop mammary
`tumours, future models may be prepared using new, more
`selective promoters as they are identified.
`
`Counsellors working with women who carry BRCAI muta-
`tions can only give them a statistical probability of the like-
`lihood of developing cancer by a given age;
`this is an
`unsatisfactory basis for making high-stake decisions regard-
`ing preventative strategies. Differences in penetrance
`among different ethnic populations are becoming appar-
`ent. These can be attributed either to specific BRCAI
`mutations or to allelic variants in modifier loci. Inbred mice
`
`are undoubtedly the most powerful eXperimental system
`for identifying modifier genes and an example of their use-
`fulness is the identification of PlaZgZd, encoding a secre-
`tory phospholipase,
`as
`a major modifier of
`intestinal
`neoplasm formation in adenomatous polyposis coli (Apt)-
`deficient mice [12]. However,
`the conditionally mutant
`Brml allele was generated using outbred mice, which
`limits immediate use for mapping modifiers. The MMTV—
`Cre and de—Cre transgenic lineages are also on segregating
`backgrounds, compounding the complications for mapping
`already presented by the cross needed to produce the
`recombined allele. This particular model is therefore not
`yet ideal for detecting additional predisposing genes.
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`13. Drebin JA, Link VC, Greene MI: Monoclonal antibodies reactive with
`distinct domains of the neu oncogene-encoded p185 molecule exert
`synergistic anti-tumor effects in vivo. Oncogene 1988, 2:273—277.
`
`Author addresses: Barry Gusterson, Beatrice Howard, Tim Crook
`(The Breakthrough Toby Robins Breast Cancer Research Centre,
`Institute of Cancer Research, London, UK) and Barbara Tennent (The
`Jackson Laboratory, Bar Harbor, Maine, USA)
`
`Correspondence: Barry Gusterson, The Breakthrough Toby Robins
`Breast Cancer Research Centre, Institute of Cancer Research,
`237 Fulham Road, London SW3 6JB, UK. E-mail: barryg@icr.ac.uk
`
`Breast Cancer Research Vol 1 No 1 Gusterson etal
`
`Finally, transgenic mice have been used successfully in
`the development of gene-based therapies such as the
`farnesyl
`transferase inhibitor approach to treating ms-
`mediated tumours. The work in Mark Greene’s labora-
`
`tory, showing that ErfiBZ-expressing mammary tumours
`could be inhibited in ciao by treatment with monoclonal
`antibodies to the receptor [13], was an important step in
`the development of Herceptin® (Genentech Inc, South
`San Francisco, California, USA). Because the Brazi-
`deficient mice appear to model the molecular mechanisms
`of human BRCA1-associated tumours,
`they should be
`valuable to the development of gene-based preventative
`or therapeutic strategies. In particular, they may be useful
`for testing therapies that induce apoptosis through Trp53-
`independent pathways.
`
`in a tissue-specific
`the ability to delete Brml
`Clearly,
`manner is a breakthrough in the development of animal
`models of the molecular mechanisms of BRCA1-associated
`
`breast cancer in humans. This alone will make it a very
`useful model for understanding how cells that have lost
`Brml become transformed, and for
`testing treatments
`aimed at blocking that process. There will be a lot of
`excitement in the scientific community as the remaining
`elements of this model are evaluated and we would
`
`encourage Xu et a! to make the mice available to the com-
`munity so that different aspects of its ability to model
`human disease are efficiently tested.
`
`2.
`
`3.
`
`95>
`
`References
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