MAPK pathway activation in pilocytic astrocytoma

Abstract

Pilocytic astrocytoma (PA) is the most common tumor of the pediatric central nervous system (CNS). A body of research over recent years has demonstrated a key role for mitogen-activated protein kinase (MAPK) pathway signaling in the development and behavior of PAs. Several mechanisms lead to activation of this pathway in PA, mostly in a mutually exclusive manner, with constitutive BRAF kinase activation subsequent to gene fusion being the most frequent. The high specificity of this fusion to PA when compared with other CNS tumors has diagnostic utility. In addition, the frequency of alteration of this key pathway provides an opportunity for molecularly targeted therapy in this tumor. Here, we review the current knowledge on mechanisms of MAPK activation in PA and some of the downstream consequences of this activation, which are now starting to be elucidated both in vitro and in vivo, as well as clinical considerations and possible future directions.

Fig. 1

Schematic representation of the genomic and protein structure of human BRAF and the fusion products detected in pilocytic astrocytoma. The gene fusions with their indicated fusion partners and break points in all cases result in a loss of the amino-terminal auto-regulatory domain. This, as well as the V600E point mutation and the Ins598T insertion in the full length protein, results in constitutive activity of the kinase domain independent of upstream Ras status. CR1-3 conserved region 1–3

Fig. 2

Distribution of oncogenic hits in the MAPK/ERK pathway by tumor location. a Representation of the MAPK/ERK signal cascade. Aberrations activating the pathway, i.e. activating mutations (star), inactivating mutation of the repressor NF1 (X), fusions proteins and unknown alterations (?), are indicated with their frequencies in cerebellar and non-cerebellar PAs. Activation of this signaling pathway can induce various cellular responses like cell growth, differentiation, and oncogene-induced senescence (OIS). b The various MAPK pathway alterations are unevenly distributed in tumors of different locations. NF1 mutation is mainly seen in optic pathway gliomas but also occasionally in other locations. Tumors in other brain regions are dominated by RAF activation, with fusions occurring primarily in cerebellar tumors and mutations in supratentorial PAs. For the more infrequent hits, no prevalent locations can be given

Fig. 3

Oncogene-induced senescence in pilocytic astrocytoma. Left Excessive MAPK activation can induce irreversible cell cycle arrest via the p16^Ink4a/Rb or the p14^Arf/p53 pathway. This state can be determined, e.g., by the appearance of senescence-associated heterochromatin foci (SAHF), immunostaining for p16 ^Ink4a and p21 ^Waf1 or by the characteristic staining for senescence-associated-β-galactosidase (SA-β-Gal) activity. Right Primary cultured pilocytic astrocytoma cells display clear SA-β-Gal activity (image kindly provided by Dr. Karine Jacob, McGill University Health Center Research Institute)

Fig. 4

BRAF-induced murine pilocytic astrocytoma. a Tumors induced by somatic gene transfer of an activated form of BRAF display histologic features of human PA, including fiber-rich tissue as well as a low proliferation index (as assessed by Ki67 immunopositivity), clear GFAP immunopositivity, and a strong activation of MAPK signaling (illustrated by ERK-phosphorylation; pErk). b Expression of activated BRAF induces proliferation in primary murine astrocytes in vitro, which can be markedly reduced by treatment with the kinase inhibitor Sorafenib