TY - JOUR
T1 - BRAF activation induces transformation and then senescence in human neural stem cells
T2 - A pilocytic astrocytoma model
AU - Raabe, Eric H.
AU - Lim, Kah Suan
AU - Kim, Julia M.
AU - Meeker, Alan
AU - Mao, Xing Gang
AU - Nikkhah, Guido
AU - Maciaczyk, Jarek
AU - Kahlert, Ulf
AU - Jain, Deepali
AU - Bar, Eli
AU - Cohen, Kenneth J.
AU - Eberhart, Charles G.
PY - 2011/6/1
Y1 - 2011/6/1
N2 - Purpose: BRAF is frequently activated by gene fusion or point mutation in pilocytic astrocytoma, the most common pediatric brain tumor. We investigated the functional effect of constitutive BRAF activation in normal human neural stem and progenitor cells to determine its role in tumor induction in the brain. Experimental Design: The constitutively active BRAF V600E allele was introduced into human neurospheres, and its effects on MAPK (mitogen-activated protein kinase) signaling, proliferation, soft agarose colony formation, stem cell phenotype, and induction of cellular senescence were assayed. Immunohistochemistry was used to examine p16 INK4a levels in pilocytic astrocytoma. Results: BRAF V600E expression initially strongly promoted colony formation but did not lead to significantly increased proliferation. BRAF V600E-expressing cells subsequently stopped proliferating and induced markers of oncogene-induced senescence including acidic β-galactosidase, PAI-1, and p16 INK4a whereas controls did not. Onset of senescence was associated with decreased expression of neural stem cell markers including SOX2. Primary pilocytic astrocytoma cultures also showed induction of acidic β-galactosidase activity. Immunohistochemical examination of 66 pilocytic astrocytomas revealed p16 INK4a immunoreactivity in the majority of cases, but patients with tumors negative for p16 INK4a had significantly shorter overall survival. Conclusions: BRAF activation in human neural stem and progenitor cells initially promotes clonogenic growth in soft agarose, suggesting partial cellular transformation, but oncogene-induced senescence subsequently limits proliferation. Induction of senescence by BRAF may help explain the low-grade pathobiology of pilocytic astrocytoma, whereas worse clinical outcomes associated with tumors lacking p16 INK4a expression could reflect failure to induce senescence or an escape from oncogene-induced senescence.
AB - Purpose: BRAF is frequently activated by gene fusion or point mutation in pilocytic astrocytoma, the most common pediatric brain tumor. We investigated the functional effect of constitutive BRAF activation in normal human neural stem and progenitor cells to determine its role in tumor induction in the brain. Experimental Design: The constitutively active BRAF V600E allele was introduced into human neurospheres, and its effects on MAPK (mitogen-activated protein kinase) signaling, proliferation, soft agarose colony formation, stem cell phenotype, and induction of cellular senescence were assayed. Immunohistochemistry was used to examine p16 INK4a levels in pilocytic astrocytoma. Results: BRAF V600E expression initially strongly promoted colony formation but did not lead to significantly increased proliferation. BRAF V600E-expressing cells subsequently stopped proliferating and induced markers of oncogene-induced senescence including acidic β-galactosidase, PAI-1, and p16 INK4a whereas controls did not. Onset of senescence was associated with decreased expression of neural stem cell markers including SOX2. Primary pilocytic astrocytoma cultures also showed induction of acidic β-galactosidase activity. Immunohistochemical examination of 66 pilocytic astrocytomas revealed p16 INK4a immunoreactivity in the majority of cases, but patients with tumors negative for p16 INK4a had significantly shorter overall survival. Conclusions: BRAF activation in human neural stem and progenitor cells initially promotes clonogenic growth in soft agarose, suggesting partial cellular transformation, but oncogene-induced senescence subsequently limits proliferation. Induction of senescence by BRAF may help explain the low-grade pathobiology of pilocytic astrocytoma, whereas worse clinical outcomes associated with tumors lacking p16 INK4a expression could reflect failure to induce senescence or an escape from oncogene-induced senescence.
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U2 - 10.1158/1078-0432.CCR-10-3349
DO - 10.1158/1078-0432.CCR-10-3349
M3 - Article
C2 - 21636552
AN - SCOPUS:79957880389
SN - 1078-0432
VL - 17
SP - 3590
EP - 3599
JO - Clinical Cancer Research
JF - Clinical Cancer Research
IS - 11
ER -