| Summary: | 博士 === 國立陽明大學 === 微生物及免疫學研究所 === 101 === Embryonal brain tumors are a large group of malignant grade IV tumors in pediatric brain tumors. They are a group of heterogeneous tumors with characteristic of the presence of poorly differentiated stem-like cells and can be classified into medulloblastomas (MBs), Atypical Teratoid/ Rhabdoid Tumors (AT/RTs) and CNS primitive neuroectodermal tumors (CNS PNETs). AT/RTs and CNS PNETs are more malignant with even poorer prognosis, and MBs represent the largest group of embryonal brain tumors. In this dissertation, our studies on embryonal brain tumors are divided into two sections, one focuses on AT/RTs and the other focuses on MBs.
In the 1st section, the studies are focused on the genomics analysis of AT/RTs. AT/RTs are rare, highly malignant CNS tumors. Due to the similar histology features, AT/RTs are usually mis-diagnosed as MBs. However, distinguishing AT/RTs from MBs is of clinical significance, since the current 5 years survival rate of AT/RT patients is much lower than that of MB patients. Some Immunohistochemistry (IHC) markers, such as lack of INI1/ SMARCB1 protein expression, are applied in AT/RTs clinical diagnosis. Deletions or mutations of INI1 gene, which occur in ~75% of AT/RTs, are also used for AT/RTs diagnosis. However, some recent studies showed that retained INI1 IHC staining and/or lacked genetic alterations of INI1 gene revealed in certain AT/RTs. Therefore, we re-evaluated the genetic alterations and INI1 immunostaining in Taiwanese AT/RT cases. In our gene expression microarray analysis, we unexpectedly found a subgroup of AT/RTs still express INI1 mRNA, even though INI1 IHC is negative in those cases. Direct DNA sequencing showed no INI1 sequence alternation in 3/4 AT/RTs. Global array CGH analysis further confirmed no aberration around INI1 gene at 22q11.2. We hypothesize that a novel yet unidentified post-transcriptional regulatory mechanism(s) for INI1 protein synthesis exists in these AT/RT tumors. Further, IHC staining showed that INI1 protein was expressed in 16/35 of AT/RT cases. Global array CGH analysis and INI1 gene sequencing showed that there is no differential chromosomal aberration marker between INI(+) and INI1(-) cases. Nevertheless, transcriptome profiling revealed INI1(+) and INI1(-) AT/RT cases expressed distinct mRNA profiles. INI1(+) and INI1(-) AT/RTs shared the closest transcriptome profile to neural stem cells (NSCs) and embryonic stem cell (ESCs), respectively; while MBs were closer to fetal brain. All AT/RTs had worse prognosis than MBs. However, survival rate of INI1(+) and INI1(-) subgroups is similar. Novel biomarkers also were identified to distinguish these three embryonal brain tumors. In the 1st section of this dissertation, our studies disclosed a novel yet unidentified post-transcriptional regulatory mechanism(s) for INI1 protein synthesis exists in AT/RT tumor cells; and we also identified a INI1(+) ATRT subtype among Taiwanese pediatric cases. New diagnostic biomarkers, as well as new therapeutic tactics, can be developed according to the transcriptome information unveiled in this work.
In the 2nd section of this dissertation, the studies are focused on the molecular mechanisms of MBs. MBs are common pediatric brain tumors which mainly arise from cerebellum, accounting for around 13 to 15% of all pediatric brain tumors. Though the detailed mechanisms of MBs are not completed understanding, it’s recognized that “cancer stem cell” hypothesis may partially explain these pathologies. Recent studies showed that brain tumor stem cells (BTSCs) from gliomas possess the mesenchymal-lineage differentiation potential and the gene expression profile of the mesenchymal lineage correlate with clinical outcome. Here we identified that such cross lineage differentiation occurred not only in gliomas, but also in MBs. mRNA array profiles of brain tumors, including MBs and GBMs, were analyzed together with several normal somatic stem cells to trap the tumor de-differentiation trail. Principle component assay (PCA) and Average Linkage Distance Analysis both showed a cross-lineage transition during the process of de-differentiation: the adherent cells shared the closet transcriptome with mesenchymal stem cells (MSCs), but recaptured the neuron stem cells (NSCs)-like transcriptome after expanding the BTSCs-like cells in the process of sphere formation. N-myc, a member of myc family, was up-regulated in the process of sphere formation. In MB and astrocytoma cells, N-myc induced tumorigenesis and NSCs-like de-differentiation, but inhibit mesenchymal-like transition. BMP7, another oncogene in MBs, is up-regulated by N-myc in MB cells. Depleted BMP7 inhibit the stemness and tumorigenisis induced by N-myc. Further, BMP7 also participated in the process of mesenchymal-like to neuronal-like transition. This study supports the concept that BTSCs have the potential to differentiate to mesenchymal-like cells and it’s the first time to observe such phenomenon in MB cells. We also identified BMP7 as a novel candidate drug target for BTSCs elimination therapy in MB treatment.
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