The identification of mammary gland stem cells (MGSC) or progenitors is important for the study of normal breast development and tumorigenesis. Based on their immunophenotype, we have isolated a population of mouse mammary gland cells that are capable of forming ''mammospheres'' in vitro. Importantly, mammospheres are enriched for cells that regenerate an entire mammary gland on implantation into a mammary fat pad. We also undertook cytogenetic analyses of mammosphere-forming cells after prolonged culture, which provided preliminary insight into the genomic stability of these cells. Our identification of new cell surface markers for enriching mammosphere-initiating cells, including endoglin and prion protein, will facilitate the elucidation of the cell biology of MGSC. [Cancer Res 2007;67(17):8131-8]

The p53 tumor suppressor is activated in response to a variety of cellular stress signals, including DNA damage. Its ability to facilitate growth arrest and/or cell death in response to such signals is believed to be the basis for its tumor suppressor function (see references 5, 16, 22, and 27 for reviews). However, specific in vivo signals that trigger tumor suppression have not been identified. Sixty to eighty percent of the spontaneous malignancies in p53-deficient mice are thymic lymphomas (13, 17), indicating that natural thymocyte events signal p53 tumor suppression. The favored hypothesis is that flawed T-cell receptor (TCR) gene recombination events signal p53-dependent elimination of damaged cells (15,17,24,28). p53 inactivation would thus facilitate the survival of cells carrying tumorigenic mutations. This hypothesis is consistent with several observations, including that (i) double-strand DNA breaks (DSBs) trigger p53 responses (29), (ii) p53 is required for DNA damage-induced thymocyte apoptosis (11, 23), (iii) thymic lymphomas induced by p53 deficiency are clonal, indicating that additional tumorigenic events are required (38), and (iv) in scid mice, which accumulate V(D)J breaks, lymphoid malignancies are accelerated by p53 deficiency (15, 28). Since V(D)J translocations are frequently associated with oncogene activation in human and mouse lymphoid tumors, it is reasonable to suspect that these events may be involved in tumorigenesis in the absence of p53-mediated surveillance.V(D)J recombination affects TCR and immunoglobulin (Ig) DNA rearrangement in developing T and B cells to generate a variety of antigen receptor specificities. Normally this process occurs during specific stages of T-and B-cell differentiation to yield a single productive rearrangement per cell for each polypeptide component of the receptor. The initiating event in V(D)J recombination, the generation of specific DSBs, requires two recombination activating genes (RAGs), RAG-1 and RAG-2 (9, 31). Mice deficient in either of these genes fail to undergo V(D)J recombination and are immunodeficient due to the lack of mature T and B cells (25,37). Thus, these mice provide an approach for assessing the role of V(D)J recombination in thymomagenesis associated with p53 deficiency. Here we examine the impact of inactivating V(D)J recombination on tumorigenesis by introducing RAG deficiencies and/or rearranged TCR transgenes into mice with a thymocyte p53 deficiency. Additionally, we analyze the chromosomes of p53-deficient thymomas for evidence of TCR translocations and other aberrations. MATERIALS AND METHODSMice. RAG-1 Ϫ/Ϫ (C57BL/6J-sv/129), scid/scid (C57BL/6J), TgN(TcrLCMV) (B6D2), and p53 Ϫ/Ϫ (C57BL/6J) mice were from Jackson Laboratory, and RAG-2 Ϫ/Ϫ mice (129, SvEv) were from Taconic Laboratory. TgT⌬N mice (B6D2), previously referred to as TgLST1135, abundantly express the dl1135 simian virus 40 (SV40) large T antigen (T-Ag) in thymocytes under the regulation of the lymphotropic papovavirus transcriptional signals (38). Although l...

Chromosome translocations involving T cell receptor (TCR) loci have been found in tumors from Ataxia telangiectasia (AT) patients and in mouse Atm−/− thymoma, suggesting the involvement of V(D)J recombination in these malignancies. By introducing a RAG-1 deficiency into Atm −/− mice in the presence of a TCR transgene, we show that V(D)J recombination is critical for thymoma development in these mice. Therefore, aberrant V(D)J recombination, normally suppressed by Atm, facilitates tumorigenic events leading to cancer. Because V(D)J recombination is dispensable for lymphomagenesis upon p53 deficiency, this study also indicates that Atm and p53 function by distinct mechanisms in suppressing thymoma. Received February 17, 1999; revised version accepted April 1, 1999. Chromosome translocations have been found in many lymphoid malignancies, resulting in activation of protooncogenes in those cells (Rabbitts 1994;Nowell 1997). In many cases, the breakpoints of translocations involve immunoglobulin or T cell receptor (TCR) loci, suggesting a predisposition by these loci to the oncogenic events. V(D)J recombination assembles immunoglobulin and TCR gene segments via double-strand break (DSB) intermediates and is active at immunoglobulin and TCR loci during B and T cell development, respectively (Lewis 1994). It has long been hypothesized that errors in this process provide a major source of chromosome translocations in lymphoid malignancies (for review, see Danska and Guidos 1997;Kirsch and Lista 1997). However, the molecules involved in regulating this process to suppress cancer proneness are not understood.Ataxia telangiectasia (AT), caused by mutations in the ATM gene, is a recessive chromosomal instability disease with pleiotropic phenotypes (Lavin and Shiloh 1997). AT patients have a high risk of cancer, particularly lymphoid malignancies. Chromosome translocations involving the TCR locus occur at a high frequency in AT T-lymphoid tumors (for review, see Taylor et al. 1996;Kirsch 1994). Furthermore, thymic lymphoma also develops in Atm-deficient mice, and evidence for TCRassociated rearrangements within tumors has been demonstrated (Barlow et al. 1996;Elson et al. 1996;. These observations suggest that the tumor suppressor function of ATM acts through a mechanism involving V(D)J recombination. To test this hypothesis, we have analyzed the effect on tumorigenesis of inactivating V(D)J recombination in Atm-deficient mice. Here, by introducing a recombination activating gene (RAG)-1 deficiency into Atm −/− mice, we show that V(D)J recombination is a critical step in thymic lymphoma development, thus identifying a critical role for Atm in suppressing such tumorigenic events in the T-cell lineage. Results Recombination-activating genes (RAG), Rag-1+/− were littermates (Materials and Methods), mouse background differences other than indicated deficiencies did not contribute to the inhibition of thymoma. Thus, prevention of V(D)J recombination by Rag-1 deficiency blocked tumorigenesis in Atm-deficient mice.As a result o...

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<div>Abstract<p>The identification of mammary gland stem cells (MGSC) or progenitors is important for the study of normal breast development and tumorigenesis. Based on their immunophenotype, we have isolated a population of mouse mammary gland cells that are capable of forming “mammospheres” <i>in vitro</i>. Importantly, mammospheres are enriched for cells that regenerate an entire mammary gland on implantation into a mammary fat pad. We also undertook cytogenetic analyses of mammosphere-forming cells after prolonged culture, which provided preliminary insight into the genomic stability of these cells. Our identification of new cell surface markers for enriching mammosphere-initiating cells, including endoglin and prion protein, will facilitate the elucidation of the cell biology of MGSC. [Cancer Res 2007;67(17):8131–8]</p></div>