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The molecular pathogenesis of pédiatrie astrocytomas is still poorly understood. To further understand the genetic abnormalities associated with these tumors, we performed a genome-wide analysis of DNA copy number aberrations in pédiatrie low-grade astrocytomas by using array-based comparative genomic hybridization. Duplication of the BRAF protooncogene was the most frequent genomic aberration, and tumors with BRAF duplication showed significantly increased mRNA levels of BRAF and a downstream target, CCND1, as compared with tumors without duplication. Furthermore, denaturing HPLC showed that activating BRAF mutations were detected in some of the tumors without BRAF duplication. Similarly, a marked proportion of low-grade astrocytomas from adult patients also had BRAF duplication. Both the stable silencing of BRAF through shRNA lentiviral transduction and pharmacological inhibition of MEK1/2, the immediate downstream phosphorylation target of BRAF, blocked the proliferation and arrested the growth of cultured tumor cells derived from low-grade gliomas. Our findings implicate aberrant activation of the MAPK pathway due to gene duplication or mutation of BRAF as a molecular mechanism of pathogenesis in low-grade astrocytomas and suggest inhibition of the MAPK pathway as a potential treatment.
Nonstandard abbreviations used: array-CGH, array-based CGH; CGH, comparative genomic hybridization; DHPLC, denaturing HPLC; MTT, 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide; NFl, neurofibromatosis type 1;TMA, tissue microarray.
Introduction
Pilocytic astrocytomas of WHO grade I are the most common primary brain tumors in children and are usually associated with a favorable prognosis, as indicated by a 10-year survival rate of 96% in the population-based study of Ohgaki and Kleihues (1). Diffuse astrocytomas of WHO grade II constitute a biologically and clinically distinct group of "low-grade" astrocytomas that are far less frequent in the pediatric age group. In contrast to pilocytic astrocytomas, diffuse astrocytomas are more common in the cerebral hemispheres than in the cerebellum, demonstrate a diffusely infiltrative growth behavior, and have a higher risk for recurrence and malignant progression. Therefore, the overall prognosis of diffuse astrocytomas is less favorable, with a median survival time of 5.6 years (1). Interestingly, diffuse astrocytomas in children show malignant transformation in only around 10% of cases (2), while the majority (60%-70%) of these tumors in adults eventually progress to anaplastic astrocytoma or secondary glioblastoma (1, 3). In addition to the WHO grade, the prognosis of low-grade astrocytoma patients depends on the tumor...