The concept of a cancer stem cell within a more differentiated tumour mass, as an aberrant form of normal differentiation, is now gaining acceptance over the current stochastic model of cancer in which all tumour cells are equivalent both in growth and tumour-initiating capacity [Hamburger AW, Salmon SE: Primary bioassay of human tumour stem cells. Science 1977, 197: 461463; Pardal R, Clarke MF, Morrison SJ: Applying the principles of stem cell biology to cancer; Nat. Rev. Cancer 2003, 3: 895902.] For example, in leukaemia, the ability to initiate new tumour growth resides in a rare phenotypically distinct subset of tumour cells [Bonnet D, Dick J.E. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat. Med. 1997, 3: 730737] which is defined by the expression of CD34+CD38 surface antigens and have been termed leukemic stem cells (LSC).
Similar tumour-initiating cells have also been found in 'solid' cancers such as breast [AIHajj M, Wicha MS, BenitoHernandez A, Morrison SJ, Clarke MF: Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci U S A 2003, 100: 39833988], brain [Singh SK, Hawkins C, Clarke ID, Squire JA, Bayani J, Hide T, Henkelman RM, Cusimano MD, Dirks PB: Identification of human brain tumour initiating cells. Nature 2004, 432: 396401 ], lung [Kim CF, Jackson EL, Woolfenden AE, Lawrence S, Babar I., Vogel S, Crowley D, Bronson RT, Jacks T: Identification of bronchioalveolar stem cells in normal lung and lung cancer. Ce// 2005, 121 : 823-835] colon [O'Brien CA, Pollett A, Gallinger S, Dick JE: A human colon cancer cell capable of initiating tumour growth in immunodeficient mice. Nature 2007, 445: 1061 10; RicciVitiani L, Lombardi DG, Pilozzi E, Biffoni M, Todaro M, Peschle C, De Maria R: Identification and expansion of human colon cancer initiating cells. Nature 2007, 445: 1 1 1 1 15]; and gastric cancers [Houghton J, Stoicov C, Nomura S, Rogers AB, Carlson J, Li H, Cai X, Fox JG, Goldenring JR, Wang TC: Gastric cancer originating from bone marrow derived cells. Science 2004, 306: 15681571 ].
A method for isolation of prostate cancer stem cells directly from lymph node and prostate glands from a series of patient samples has been developed (see licensing opportunity entitled “A method for the selective enrichment of prostate stem cells and prostate cancer stem cells”). These stem cells express markers that characterise the cells with stem cell properties. The following markers are typically expressed as prostate stem cell markers; human epithelial antigen (HEA), CD44, α2β1 integrin and CD133 and represents around 0.1 % of the total cell tumour mass.
As is apparent, the cancer stem cell comprises a small percentage of the total tumour cell mass. The analysis of the biology of the cancer stem cell is fraught with problems regarding isolation and characterization of cancer stem cells from tumour tissue. In addition the behaviour of the isolated cancer stem cell is difficult to assess when separated from bulk tumour cells. There is therefore a need to provide a validated in vivo non-human mammalian model for the analysis of cancer stem cell biology and also to test therapeutic agents specific for the cancer stem cell and/or bulk tumour.
Non-human mammalian models for the study of tumour initiation and growth are known in the art. For example, WO2008/061674 discloses a transgenic animal model that is modified in the Aph-1b gene, a cell membrane receptor that interacts with presenilin and nicastrin as a functional component of the λ secretase complex and is shown to be associated with initiation of tumour formation. WO2008/074880 discloses a transgenic animal model for the study of lung cancer initiation and progression. The model allows the site specific modification of the mouse genome to ablate genes considered involved in tumour formation. WO2008/153743 discloses a further mouse transgenic model for the study of lymphoma and the identification of genes that predispose the mouse to develop lymphoma. WO2008/021393 discloses a yet further mouse model for the study of hepatocarcinoma that uses RNA interference to identify genes involved in tumour formation. A problem associated with these models is that they do not study human tumour initiation and growth but rather the formation of murine tumours. This is unsatisfactory.
Current models used for studying prostate cancer biology and drug evaluation generally consist of xenografts in immune-deficient mice of well-established human prostate cancer cell lines that have been adapted to in vitro growth, for example LNCaP and PC-3. Such models have been useful for testing new therapeutics but they have severe shortcomings: they are highly anaplastic representing the extreme end of advanced cancers and importantly they do not reflect the hierarchies observed in solid tumours. These limitations make it impossible to predict patient's response to anticancer drugs in the clinic. In view of this we have developed more relevant models illustrated by xenografting primary prostate cancer tissue into more permissive immune-deficient mice (Rag2"/_Y C transgenics).
Using the method that we have developed, primary tissues are engrafted into Rag2" _y C _ ~ mice with an initial incidence of 19%, which is improved with increased tumour grade. Furthermore, once a xenograft has been established tumour incidence is increased to 90% from each passage of tumour tissue and an incidence rate of 70% is achieved when mouse cells are depleted, and single tumour cells are grafted, giving a robust xenograft model. These xenografts have been shown to have a consistent genotype, in relation to its origin, through routine screening, with further characterisation revealing the majority of cells have a trans-amplifying phenotype (CD44+CK18+a21 integrin ) and contain a minor population that express the luminal markers CD24 and androgen receptor. Importantly the stem cell markers CD133 and CD1 17 are also expressed. These xenograft models have been shown to express known stem cell targets and allow the measurement of tumour incidence, growth, recurrence and metastasis, making them invaluable tools in cancer research, in particular prostate cancer research.
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Last Updated Oct 2014