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Transcriptional Characterization of Glioma Neural Stem Cells Diva ...

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1.3 Primary and Secondary Glioblastomas Introduction<br />

triphosphate hydrolases (GTPase) that activate the mitogen activated protein<br />

kinase (MAPK) signaling cascade amongst other pathways [496]. Overall, at<br />

least 47 <strong>of</strong> the 206 samples harboured somatic NF1 inactivating mutations or<br />

deletions, confirming the relevance <strong>of</strong> this gene in sporadic human glioblas-<br />

toma [326].<br />

Furthermore, within the TCGA dataset it was observed that mutation rates<br />

between untreated and treated glioblastomas were markedly different, aver-<br />

aging at 1.4 and 5.8 somatic silent mutations per sample, respectively. The<br />

higher average in the treated samples was mostly due to the contributions<br />

from a cohort <strong>of</strong> seven hypermutated tumours treated with temozolomide or<br />

lomustine. The hypermutator phenotypes previously described in glioblas-<br />

toma [79,204] were known to carry mutations in MutS homolog 6 (MSH6),<br />

a component <strong>of</strong> the post-replicative DNA mismatch repair system (MMR).<br />

MSH6 heterodimerizes with MSH2 to form MutSα, which binds to DNA mis-<br />

matches thereby initiating DNA repair [6]. An analysis <strong>of</strong> the genes involved<br />

in mismatch repair within the TCGA dataset uncovered that six out <strong>of</strong> the<br />

seven hypermutated samples harbored mutations in at least one <strong>of</strong> the mis-<br />

match repair genes MLH1, MSH2, MSH6 or PMS2 [326].<br />

Recurrent focal alterations found in the TCGA samples that have already<br />

been described and are common in glioblastoma are the amplification <strong>of</strong> Epi-<br />

dermal growth factor receptor (EGFR), cyclin-dependent-kinase (CDK) CDK4<br />

and CDK6, PDGFRA, MDM2, MDM4, MET, MYCN, CCND2 and PIK2CA<br />

[104,255,262,292,374,423,429]. Interestingly, uncommon focal alterations were<br />

also found, such as the amplification <strong>of</strong> the serine/threonine protein kinase<br />

AKT3 and the homozygous deletions <strong>of</strong> NF1 and PARK2. V-akt murine thy-<br />

moma viral oncogene homolog 3 (AKT3) belongs to the AKT family <strong>of</strong> ser-<br />

ine/threonine kinases together with AKT1 and AKT2. The three AKT kinases<br />

are now known to represent central nodes in a variety <strong>of</strong> signaling cascades that<br />

regulate normal cellular process such as cell size and growth, proliferation,<br />

survival, glucose metabolism, genome stability, and neo-vascularization. It is<br />

currently less clear, however, whether AKT1, AKT2, and AKT3 are function-<br />

ally redundant or whether each carries out a specific functional role [45,46].<br />

Parkinson protein 2 (PARK2) encodes for a component <strong>of</strong> an E3 ubiquitin<br />

ligase complex that mediates the targeting <strong>of</strong> substrate proteins for protea-<br />

somal degradation. The functions carried by PARK2 are currently unknown,<br />

14

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