| Medulloblastomas represent a heterogeneous group of cerebellar tumors that constitute the most frequent primary pediatric solid malignancy. Molecular characterization of these tumors have led to the understanding that distinct subtypes possess characteristic properties such as gene expression profile, histological classification, and degree of dissemination that are predictive of disease progression and prognosis. Fractionation of primary medulloblastomas has led to the appreciation of brain tumor stem cells (BTSC) that may be driving the more aggressive and malignant disease. However, the developmental origins of these cells as well as the influences of early mutations in tumor suppressors on development and tumorigenesis remain unclear.;My work is geared towards understanding the impact of mutations in the key tumor suppressor genes Ptc1 and p53 on medulloblastoma formation. I first identified key differences in neural stem cell marker expression that distinguish between Ptc1 and Ptc1;p53 medulloblastomas, demonstrating that the Ptc1;p53 genotype may pre-dispose to a more malignant, stem-like tumor. Through the use of a somatic mosaic model, we describe a synergistic interaction between Ptc1 haploinsufficiency and p53 deficiency leading to developmental seeding of the cerebellar field by pre-malignant cells and term this phenomenon "developmental field cancerization." Interestingly, we observed this pre-malignant colonization in the cerebellar stem cell compartment as well, resulting in an aberrant population of self-renewing cells. Upon loss-of-heterozygosity at the Ptc1 locus, the Ptc;p53 animals alone develop robust cerebellar tumors that possess a definable stem-like population of cells that can re-initiate metastatic secondary tumors. These findings demonstrate how early mutations in the tumor suppressor genes, such as Ptc1 and p53, may lead to stem cell field cancerization and play an important role in determining future tumor character and prognosis.;Finally, bisulfite-based mass spectrometry methylation assays were utilized to define differences in methylation status at various imprinted loci between BTSC and bulk tumor. We observed methylation differences at the KCNQ1OT1/Lit1 and CDKN1C/p57 and corroborated these findings with mRNA expression and RNA-FISH demonstrating bi-allelic expression of Lit1 in the BTSC. Through lentiviral-mediated shRNA knockdown of Lit1 in BTSC, I ascribe a putative role for the long non-coding RNA in maintenance of BTSC self-renewal. |
