Using the RCAS/tv-a
system, we have recapitulated in mice some of the molecular events that mediate medulloblastoma formation in humans. Expression profile analysis of human medulloblastomas using oligonucleotide microarrays has shown that transcriptional targets of Shh (Gli transcription factors and Ptc) are frequently upregulated [33
]. This profile was found predominantly in a histological variant known as desmoplastic medulloblastoma, which is characterized by dense fibrous stroma surrounding aggregates of tumor cells. The fact that our mouse tumors did not resemble desmoplastic medulloblastomas indicates that Shh/Ptc pathway activation is not unique to the desmoplastic variant and may be a more general molecular derangement in medulloblastoma.
Overexpression of the c-Myc
oncogene occurs frequently in human medulloblastomas and correlates with poor patient prognosis [20,21
]. We showed previously that c-Myc stimulates the proliferation of undifferentiated neural progenitor cells in the forebrain of mice, and we suggested that, through this mechanism, c-Myc might promote PNET formation [17
]. In the present study, c-Myc was not sufficient to induce medulloblastoma formation. The enhanced tumorigenicity in mice injected with two RCAS vectors expressing c-Myc and Shh suggests that c-Myc cooperates with Shh to drive proliferating neural progenitors into malignant transformation. The fact that a minority of tumor cells coexpressed Shh and c-Myc suggests that the tumorigenic effect of Shh (a secreted protein) is mediated predominantly through a paracrine, rather than an autocrine (cell-automatous), mechanism.
Other mouse models of medulloblastoma have shown that these tumors can arise by subversion of cell growth pathways other than the Shh/Ptc pathway. Medulloblastomas develop in p53
-null mice after conditional inactivation of the Rb
], or in combination with homozygous loss of Lig4
, which encodes a nuclear ligase critical for DNA repair [35
]. Loss of p53 accelerates medulloblastoma formation in Ptc+/-
]. Transgenic mice expressing SV40 virus large T antigen, which functionally inactivates both Rb and p53, develop brainstem PNETs [37
]. Considering the fact that loss-of-function mutations in either RB
rarely occur in human medulloblastomas, our model has the advantage of more closely paralleling the molecular pathways relevant to human tumors.
Other investigators recently reported that ectopic expression of Shh in prenatal mouse cerebellum induces EGL thickening and medulloblastomas [16
]. They utilized a murine leukemia virus vector, injected in utero
, to infect a broad population of dividing cells. Our results confirm their observations and extend them by showing that c-Myc cooperates with Shh to enhance tumorigenicity. Furthermore, our results in Ntv-a
mice define a more narrow population of candidate cells-of-origin for medulloblastomas, namely, nestin-expressing neural progenitors.
We cannot conclude that the tumors originated from GNPs within the EGL because we did not specifically target these precursor cells. Nevertheless, several features of our model are consistent with a GNP origin for medulloblastoma. First, the EGL contains the highest concentration of mitotically active, nestin-expressing cells susceptible to infection with RCAS vectors. Second, significant numbers of cells within the EGL thickenings and in the tumors expressed βIII tubulin and NeuN—markers for early neuronal differentiation. Finally, the coexistence of EGL thickening and medulloblastoma in cerebella injected with RCAS-Shh implicates hyperproliferation of GNPs as a precursor stage in the genesis of medulloblastoma.
We could not
produce tumors in the forebrain by injecting RCAS-Shh, alone or in combination with RCAS-Myc, even though nestin-expressing progenitors are present in that part of the brain in postnatal mice. These cells are known to give rise to glioblastomas in vivo
with specific activation of the Ras and Akt signaling pathways [38
], and to oligodendrogliomas with overexpression of platelet-derived growth factor [39
]. Our observation that cerebral progenitor cells are less responsive to the same molecules that are potent mitogens for cerebellar progenitors could explain why cerebral hemisphere PNETs are rare compared with cerebellar medulloblastomas.