PTEN heterozygosity promotes tumorigenesis in a Smoothened active mouse model of medulloblastoma

Recent publications have described an important role for cross talk between PI-3 kinase and sonic hedgehog signaling pathways in the pathogenesis of medulloblastoma. Our primary objective was to determine if loss of Pten promotes medulloblastoma tumor formation. When mice expressing SmoA1 were crossed with mice heterozygous for Pten, SmoA1 +; Pten +/- mice exhibited decreased survival, an increased incidence, and a shorter time to onset of symptoms, compared to SmoA1 +; Pten +/+ mice. Tumors from SmoA1 +; Pten +/+ mice exhibited classic histology. In contrast, tumors from SmoA1 +; Pten +/- mice exhibited predominantly nodular/desmoplastic histology and evidence of neuronal differentiation. Analysis by gene expression microarray revealed a clear separation of gene signatures, with upregulation of genes such as Pik3cb, Mapk8, Frap1, and multiple genes involved in angiogenesis in SmoA1+; Pten +/- tumors. Downregulated genes included Pten, Trp53, and Rb1. Western blotting and immunohistochemical analysis revealed decreased expression of Pten and increased activation of Akt in the SmoA1+; Pten +/- tumors. Consistent with these findings, we detected strong expression of PCNA throughout the Pten +/+ tumors and focal areas of intense staining for PCNA in tumors from SmoA1 +/-; Pten +/- mice. Conversely, staining for Cleaved Caspase 3 revealed multiple punctuate foci in SmoA1 +; Pten +/+ tumors and almost no staining in SmoA1 +/-; Pten +/- tumors. These results suggest that increased signaling through PI-3 kinase pathways promotes MB tumorigenesis by driving proliferation and inhibiting apoptosis of granule neuron precursor cells in the developing cerebellum. Expression of medulloblastoma tumors were compared using a mouse model with constitutive active Smoothened (SmoA+) and 4 mice with wildtype PTEN (PTEN +/+) compared to 8 mice heterozygous for PTEN (PTEN +/-)