ATRT was first reported as a distinct clinicopathologic entity more than a decade ago in a well-documented study by Rorke et al.,7
and was subsequently adopted by the 2000 WHO classification of nervous system tumors.24,25
The initial report was followed by several series with more detailed clinical data that documented the highly malignant nature of the tumors and the very poor prognosis, even when aggressive treatment strategies were used. The development of novel, effective therapeutic approaches for ATRT has been hindered by its relative rarity and a lack of specific therapeutic targets in this polymorphic tumor. Data from histoimmunologic studies of ATRT7,26
suggest that the hallmark rhabdoid cells may constitute a highly malignant, tumor stem cell population that arises from transformation of a multipotential progenitor cell with marked proliferative and invasive features, and the predominance of rhabdoid cellularity in xenografts established from permanent cell lines or from surgical specimens is consistent with this interpretation. The sequence of molecular events that are associated with this transformation is not well understood; however, the genomic lesion that defines ATRT, and that presumptively initiates the process of malignant transformation, is homozygous inactivation of the INI1 gene. This genomic lesion is also common in malignant rhabdoid tumors arising at other body sites in children.9,27
The hSNF5/INI1 gene encodes a subunit of ATP-dependent SWI/SNF chromatin remodeling complexes28
that appear to regulate cell cycling and play critical roles in a variety of differentiation pathways. The core subunit SNF5 appears to function as a tumor suppressor by modulating the transcription of a subset of genes that regulate the balance between cell proliferation and differentiation.29
This modulation appears to be related to the strengths of the promoter and the degree of chromatin condensation at gene regulatory sites.27,30
Although the hSNF5/INI1 genomic defect defines a group of malignant rhabdoid tumors at a number of tissue sites,27
the effects of a specific alteration of hSNF5/INI1 gene combined with tissue-associated epigenomic states31
are not well understood. The complex polymorphic differentiation of MRT arising in the CNS (ie, ATRT) highlights the tissue-specific effects of the INI1 genomic lesions that impact regulation of the cell cycle and of specific differentiation pathways,29,32–36
and emphasizes the need for ATRT tumor cell–derived experimental systems. These are necessary to define the key pathways that affect the aggressive biological behavior of ATRT and would accordingly reveal potential therapeutic targets.
Studies using cultures of permanent ATRT cell lines, either derived from solid tumors (BT-12, BT-16) or from ATRT tumor cells in the CSF (KCCF),37–39
have suggested the role of the insulin/insulin growth factor pathways in tumor cell proliferation, survival, and chemosensitivity. In addition, ATRT cell lines have demonstrated in vitro chemosensitivity to inhibitors of histone deacetylase.40
Only one report has previously documented the culture and primary xenograft implantation of a pediatric ATRT,41
and this study documented the feasibility of implanting cultured tumor cells into the spinal cord of immunocompromised hosts to produce ATRT-like neoplasms. Whereas the interface between these primary xenograft implantations and the host spinal cords mimicked the aggressive invasive growth pattern of ATRT, the establishment of transplantable ATRT xenografts in nude mice from primary cell cultures was not successful in that study.
The current report compares serially transplantable ATRT xenografts, established from a primary pediatric ATRT, with xenografts established from two permanent ATRT cell lines. Orthotopic ATRT xenografts at either the cerebral or cerebellar sites, regardless of in vivo or in vitro propagation of tumor cells (ie, in vitro vs in vivo), demonstrated the key biologic features of ATRT. These included a rapid, invasive growth, as well as a propensity for neuraxis dissemination that is similar to the high incidence of ATRT dissemination in patients. Compared with orthotopic GBM xenografts,21
the ATRT xenografts more frequently spread to intraventricular spaces and subarachnoid zones after intraparenchymal implantation of cells, as documented by histopathologic analysis. Intracranial intraparenchymal injection of ATRT cells was commonly followed by development of luminescent spinal signal, in addition to a signal at the primary implantation site (Fig. ). In fact, our collective experience with BT-16 has shown that 50% BT-16 cell injections (31 of 62) result in early-neuraxis dissemination, whereas none of a total of 40 GS-2 injections have shown tumor growth outside the brain, in spite of the use of the same coordinates, same number of cells, and same volumes for all intracranial injections. Thus, our study demonstrates the utility of luciferase-modification of ATRT cells for bioluminescence monitoring of neuraxis dissemination of tumor, as well as for monitoring tumor response to therapy.
Modification of ATRT cell lines for BLI produced tumors that resembled the xenografts of primary ATRT cells with respect to histopathology, immunophenotypic diversity, and invasive growth with neuraxis dissemination. The histopathologic features of all the orthotopic xenografts were also similar to the conspicuous predominance of the polygonal rhabdoid-like cells. The histopathologic differences between the xenografts established from the short-term cultures of the primary ATRT tissue and those established from ATRT cell lines after long-term in vitro propagation were subtle and included the absence of more fusiform cells and relative loss of GFAP and SMA immunophenotypes in the tumors established from the BT-12 and BT-16 cells.
Since orthotopic ATRT xenografts in this study recapitulate the invasive growth and CNS dissemination of ATRT in patients, and because modification of these cells for BLI does not affect these properties, we examined the feasibility of an orthotopic ATRT xenograft model for therapeutic testing with correlation to biomarker analysis. For simplicity in testing this paradigm, we examined the responses of luciferase-modified ATRT cells to TMZ. The ATRT xenografts that were tested, which highly express MGMT, are resistant to TMZ treatment with respect to tumor growth and spread when compared with an orthotopic glioblastoma xenograft that is MGMT deficient and responsive to TMZ. These data suggest that this orthotopic ATRT xenograft model, in which BLI can be used for monitoring, could be used to test new therapeutic regimens with respect to tumor growth and dissemination and, potentially, to expedite the identification of effective treatments for this cancer, in relation to that which can be accomplished through clinical trial activity.