NF GEMMs Already! The Power and Promise of Mouse Tumor Models
Authors: David H. Gutmann
Journal: Cancer Cell (2014)

Plexiform neurofibromas are one of the most common tumors encountered in individuals with the neurofibromatosis type I (NF1) cancer predisposition syndrome. In this issue of Cancer Cell, Chen and colleagues define the cell of origin for murine Nf1 plexiform neurofibroma and leverage this finding to develop a platform for preclinical drug evaluation. Plexiform neurofibromas are one of the most common tumors encountered in individuals with the neurofibromatosis type I (NF1) cancer predisposition syndrome. In this issue of Cancer Cell, Chen and colleagues define the cell of origin for murine Nf1 plexiform neurofibroma and leverage this finding to develop a platform for preclinical drug evaluation. Neurofibromatosis type I (NF1) is one of the most common cancer predisposition syndromes, affecting 1 in 2,500 individuals worldwide (Lin and Gutmann, 2013Lin A.L. Gutmann D.H. Nat Rev Clin Oncol. 2013; 10: 616-624Crossref PubMed Scopus (58) Google Scholar). Among the diverse number of benign and malignant neoplasms observed in this condition, peripheral nerve sheath tumors (neurofibromas) are found in nearly all adults with NF1. One subtype of neurofibroma, the plexiform neurofibroma, is a particularly challenging tumor to manage. These extensive, highly vascular tumors typically involve multiple nerves or large segments of a nerve root, making complete surgical removal nearly impossible. In addition, they often affect adjacent structures in the body, leading to tissue compression (narrowed trachea or nerve impingement), bone erosion, or fatty replacement of muscle. Moreover, cytotoxic chemotherapy for plexiform neurofibromas has had modest effectiveness, and the use of radiation is limited by the risk of secondary malignant transformation. These benign nerve sheath tumors can arise anywhere within the body, including the eye socket (orbit; Figure 1A), neck (Figure 1B), spinal nerve roots (Figure 1C), subcutaneous tissue (Figure 1D), and extremities (Figure 1E). While histologically similar, their clinical behavior is influenced by the age of the patient and location of the tumor. First, the majority of plexiform neurofibromas grow most rapidly during the first decade of life. Second, while subcutaneous and orbital plexiform neurofibromas rarely transform into malignant peripheral nerve sheath tumors, there is a 10%–13% lifetime risk of malignant transformation for plexiform neurofibromas involving other body regions. Third, treatment with a biologically-targeted therapy (Imatinib) revealed greater responses in tumors of the neck and pelvis relative to those in other locations (Robertson et al., 2012Robertson K.A. Nalepa G. Yang F.C. Bowers D.C. Ho C.Y. Hutchins G.D. Croop J.M. Vik T.A. Denne S.C. Parada L.F. et al.Lancet Oncol. 2012; 13: 1218-1224Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar). Together, these clinical observations raise the intriguing possibility that plexiform neurofibromas arising in different areas are biologically distinct entities and that the development of effective treatments will necessitate a deeper understanding of the molecular and cellular differences that underlie this innate heterogeneity. As such, several different genetically engineered mouse models (GEMMs) of NF1-associated plexiform neurofibroma have been generated (Figure 1F). Relevant to the human condition, Nf1 GEMMs have emerged as critical experimental platforms to define the molecular and cellular pathogenesis of plexiform neurofibroma formation and growth. In this regard, these murine tumor models have been employed to demonstrate that plexiform neurofibromas can arise from some cell types (Schwann cell precursors, immature Schwann cells, and mature Schwann cells; Le et al., 2011Le L.Q. Liu C. Shipman T. Chen Z. Suter U. Parada L.F. Cancer Res. 2011; 71: 4686-4695Crossref PubMed Scopus (67) Google Scholar, Zhu et al., 2002Zhu Y. Ghosh P. Charnay P. Burns D.K. Parada L.F. Science. 2002; 296: 920-922Crossref PubMed Scopus (493) Google Scholar, Wu et al., 2008Wu J. Williams J.P. Rizvi T.A. Kordich J.J. Witte D. Meijer D. Stemmer-Rachamimov A.O. Cancelas J.A. Ratner N. Cancer Cell. 2008; 13: 105-116Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar, Mayes et al., 2011Mayes D.A. Rizvi T.A. Cancelas J.A. Kolasinski N.T. Ciraolo G.M. Stemmer-Rachamimov A.O. Ratner N. Cancer Res. 2011; 71: 4675-4685Crossref PubMed Scopus (49) Google Scholar), but not others (neural crest stem cells; Joseph et al., 2008Joseph N.M. Mosher J.T. Buchstaller J. Snider P. McKeever P.E. Lim M. Conway S.J. Parada L.F. Zhu Y. Morrison S.J. Cancer Cell. 2008; 13: 129-140Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar). Similarly, both the penetrance and location of plexiform neurofibroma development is impacted by the age at which somatic Nf1 gene inactivation occurs, with rare tumors arising following acquired Nf1 loss in adult mice (Le et al., 2011Le L.Q. Liu C. Shipman T. Chen Z. Suter U. Parada L.F. Cancer Res. 2011; 71: 4686-4695Crossref PubMed Scopus (67) Google Scholar). In this issue of Cancer Cell, Chen et al., 2014Chen Z. Liu C. Patel A.J. Liao C.-P. Wang Y. Le L.Q. Cancer Cell. 2014; 26 (this issue): 695-706Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar determined that Nf1 murine spinal plexiform neurofibromas arise from a small population of PLP+ Schwann cell precursors within the embryonic nerve root. Using embryonic dorsal root ganglion/nerve root neurosphere cells (DNSCs), they demonstrated that Nf1-deficient DNSCs generate plexiform neurofibromas following injection into the sciatic nerves of naive immunocompromised mice. The development of tumors in mice without a germline Nf1 gene mutation or intact immune system further adds to the controversy over the absolute requirement for a supportive Nf1+/− microenvironment in plexiform neurofibroma pathogenesis. In this regard, dependence on bone marrow-derived cells (mast cells and macrophages) and endothelial cells in the Nf1+/− tumor microenvironment has been demonstrated in some models (Joseph et al., 2008Joseph N.M. Mosher J.T. Buchstaller J. Snider P. McKeever P.E. Lim M. Conway S.J. Parada L.F. Zhu Y. Morrison S.J. Cancer Cell. 2008; 13: 129-140Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, Le et al., 2011Le L.Q. Liu C. Shipman T. Chen Z. Suter U. Parada L.F. Cancer Res. 2011; 71: 4686-4695Crossref PubMed Scopus (67) Google Scholar, Yang et al., 2008Yang F.C. Ingram D.A. Chen S. Zhu Y. Yuan J. Li X. Yang X. Knowles S. Horn W. Li Y. et al.Cell. 2008; 135: 437-448Abstract Full Text Full Text PDF PubMed Scopus (287) Google Scholar, Zhu et al., 2002Zhu Y. Ghosh P. Charnay P. Burns D.K. Parada L.F. Science. 2002; 296: 920-922Crossref PubMed Scopus (493) Google Scholar), but not in others (Mayes et al., 2011Mayes D.A. Rizvi T.A. Cancelas J.A. Kolasinski N.T. Ciraolo G.M. Stemmer-Rachamimov A.O. Ratner N. Cancer Res. 2011; 71: 4675-4685Crossref PubMed Scopus (49) Google Scholar, Wu et al., 2008Wu J. Williams J.P. Rizvi T.A. Kordich J.J. Witte D. Meijer D. Stemmer-Rachamimov A.O. Cancelas J.A. Ratner N. Cancer Cell. 2008; 13: 105-116Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar, Chen et al., 2014Chen Z. Liu C. Patel A.J. Liao C.-P. Wang Y. Le L.Q. Cancer Cell. 2014; 26 (this issue): 695-706Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar). It is also possible that the ability to form tumors in wild-type mice reflects some degree of local injury at the site of the sciatic nerve injection sufficient to provide a supportive microenvironment for tumorigenesis (Ribeiro et al., 2013Ribeiro S. Napoli I. White I.J. Parrinello S. Flanagan A.M. Suter U. Parada L.F. Lloyd A.C. Cell Reports. 2013; 5: 126-136Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). Next, they employed inducible PLP-CreER mice to define the population of DNSCs most responsible for tumor formation, revealing that PLP+ tumorigenic cells express the GAP43 Schwann cell precursor protein. In keeping with previous studies using Krox20-Cre (Zhu et al., 2002Zhu Y. Ghosh P. Charnay P. Burns D.K. Parada L.F. Science. 2002; 296: 920-922Crossref PubMed Scopus (493) Google Scholar, Yang et al., 2008Yang F.C. Ingram D.A. Chen S. Zhu Y. Yuan J. Li X. Yang X. Knowles S. Horn W. Li Y. et al.Cell. 2008; 135: 437-448Abstract Full Text Full Text PDF PubMed Scopus (287) Google Scholar) or DHH-Cre (Mayes et al., 2011Mayes D.A. Rizvi T.A. Cancelas J.A. Kolasinski N.T. Ciraolo G.M. Stemmer-Rachamimov A.O. Ratner N. Cancer Res. 2011; 71: 4675-4685Crossref PubMed Scopus (49) Google Scholar, Wu et al., 2008Wu J. Williams J.P. Rizvi T.A. Kordich J.J. Witte D. Meijer D. Stemmer-Rachamimov A.O. Cancelas J.A. Ratner N. Cancer Cell. 2008; 13: 105-116Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar) mouse strains to generate plexiform neurofibromas, they found that Krox20+ and DHH+ cells were contained within the PLP+ DNSC population. Based on these fate mapping experiments, the authors conclude that PLP+, GAP43+ embryonic Schwann cell precursors represent the cellular origin for spinal plexiform neurofibromas. Finally, because neurofibromin, the protein encoded by NF1, negatively regulates RAS/RAF/MEK activity relevant to murine Nf1 GEMM plexiform neurofibroma growth, Chen et al., 2014Chen Z. Liu C. Patel A.J. Liao C.-P. Wang Y. Le L.Q. Cancer Cell. 2014; 26 (this issue): 695-706Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar utilized their Nf1-deficient DNSC sciatic nerve transplant model to perform proof-of-concept studies using the selective MEK inhibitor PD0325901. They found that suppression of MEK activity reduced plexiform neurofibroma growth in vivo, further establishing this preclinical model as a tractable platform for future therapeutic drug discovery and evaluation. These encouraging results provide support for two ongoing human MEK inhibitor clinical studies on NF1-associated plexiform neurofibroma (www.clinicaltrials.gov; NCT01362803, NCT02096471). Together with other Nf1 GEMM work, this study reveals that specific cell types during particular developmental windows are exquisitely vulnerable to the tumorigenic effects of somatic Nf1 gene inactivation. Similar to NF1-associated plexiform neurofibromas, not all cell types in the glial lineage are capable of generating NF1-associated low-grade brain tumors (optic gliomas). Somatic Nf1 gene loss must occur in neural stem cells originating within one specific germinal zone (third ventricle) during embryogenesis in order for mice to develop optic gliomas (Lee et al., 2012Lee Y. Gianino S.M. Gutmann D.H. Cancer Cell. 2012; 22: 131-138Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar). The requirement for somatic loss of Nf1 gene expression in a particular cell of origin during a restricted period of life may partly explain why not all people with NF1 develop particular tumors, why these tumors arise in specific locations, and why they may exhibit different growth properties or responses to biologically targeted therapies. Beyond the important insights into pathogenesis that these experimental findings provide, they additionally offer exciting opportunities to develop Nf1 GEMMs that more accurately match specific subtypes of NF1-related tumors. It is important to recognize that the Nf1 GEMM plexiform neurofibroma model reported by Chen et al., 2014Chen Z. Liu C. Patel A.J. Liao C.-P. Wang Y. Le L.Q. Cancer Cell. 2014; 26 (this issue): 695-706Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar most closely resembles the human paraspinal subtype (Figure 1C). As such, the growth control pathways deregulated in this subtype (e.g., MEK) and the relative contributions from the local microenvironment (e.g., athymic mice) may be most germane to this tumor subgroup. The creation of additional GEMMs that model other types of plexiform neurofibromas observed in people with NF1 will likely entail defining the cells of origin and vulnerable windows for each subtype using different regulatable Cre driver lines to enable somatic Nf1 loss in distinct cell types during defined periods of development (Figure 1G). The availability of subtype-specific plexiform neurofibroma mouse strains would facilitate unbiased discovery efforts aimed at determining how these tumors are molecularly distinct from each other. In this manner, the unique biologies of these different subgroups and their respective mechanisms of growth regulation might be discerned. This knowledge coupled with the ability to establish xenograft models as executed by Chen et al., 2014Chen Z. Liu C. Patel A.J. Liao C.-P. Wang Y. Le L.Q. Cancer Cell. 2014; 26 (this issue): 695-706Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar opens the door to the identification of drug therapies that might be particularly efficacious for each one of the varieties of plexiform neurofibroma. Furthermore, these future studies may lead to a greater appreciation of the factors that drive plexiform neurofibroma growth relevant to individual patient risk assessment. Continued progress in this area has the potential to allow for the development of prognostic strategies and treatments personalized to a given patient’s tumor. I thank Dr. Angela C. Hirbe for critically reading the manuscript. Our research is supported by grants from the Department of Defense and NIH. Cells of Origin in the Embryonic Nerve Roots for NF1-Associated Plexiform NeurofibromaChen et al.Cancer CellOctober 30, 2014In BriefChen et al. identify GAP43+PLP+ Schwann cell precursors in embryonic nerve roots as the cells of origin for plexiform neurofibromas and use this knowledge to develop a model of neurofibroma for preclinical drug screening to identify effective therapies. Full-Text PDF Open Archive