Oncogenic RAF1 rearrangement and a novel BRAF mutation as alternatives to KIAA1549:BRAF fusion in activating the MAPK pathway in pilocytic astrocytoma.
Thus, combined molecular analysis of BRAF and IDH1 is a sensitive and highly specific approach to separate pilocytic astrocytoma from diffuse astrocytoma.
This review summarizes recent studies on diagnostic and prognostic markers in gliomas such as the BRAF fusion gene in pilocytic astrocytomas and 1p/19q codeletion, O-6-methylguanine-DNA methyltransferase status and isocitrate dehydrogenase 1 (IDH1)/IDH2 mutations in diffuse gliomas.
Although oncogenic rearrangements of the BRAF gene have recently been described in PAs, it is not clear whether such alterations have an impact on outcome.
The latest discovery involving the tandem duplication and fusion BRAF-KIAA1549 on chromosome 7q34 in pilocytic astrocytoma has drawn attention to the MAPK-ERK pathway and its potential chemotherapeutic manipulation.
Recent studies of genetic abnormalities in pediatric low-grade gliomas (LGGs) have focused on activation of the ERK/MAPK pathway by KIAA1549-BRAF gene fusions in the majority of pilocytic astrocytomas (PAs) and by rare mutations in elements of the pathway across histopathologically diverse LGGs.
For the first time, we report concomitant presence of a somatic BRAF(V600E) mutation in an NF1 patient indicating that more than one Ras/ERK pathway component can be affected in PA.
These findings were confirmed by immunohistochemistry for neuronal markers, as well as combined phospho-S6/ phospho-p70S6K immunoreactivity in 4 (of 4) LGSI vs. 5 (of 13) NF1-associated PA (p=0.02), and 13 (of 39) sporadic PA. Phospho-ERK immunoreactivity was uniformly present in PA and LGSI groups, while BRAF duplication was absent by FISH in 8 NF1-associated low grade astrocytomas.
The high mutation frequencies in pleomorphic xanthoastrocytomas, gangliogliomas and extra-cerebellar pilocytic astrocytomas implicate BRAF (V600E) mutation as a valuable diagnostic marker for these rare tumor entities.
Our in vivo model for PA shows that the activated BRAF kinase domain is sufficient to induce PA and highlights its role as a potential therapeutic target.
In addition, our study confirmed previously reported BRAF and RAF1 fusion variants in 72% (90/125) of PA. Mutations in BRAF (8/125), KRAS (2/125) and NF1 (4/125) and the rare RAF1 gene fusions (2/125) were mutually exclusive with BRAF rearrangements, with the exception of two cases in our series that concomitantly harbored more than one hit in the MAPK pathway.
Further studies in larger RGNT case series are needed in order to demonstrate the possible presence of KIAA1549-BRAF fusion and better delineate its relationship with pilocytic astrocytomas.
This article provides an overview of the most common molecular markers in neurooncology, including 1p/19q codeletion in oligodendroglial tumors, mutations in the isocitrate dehydrogenase 1 and 2 genes in diffuse gliomas, hypermethylation of the O(6)-methylguanine-DNA methyltransferase gene promoter in glioblastomas and anaplastic gliomas, alterations in the epidermal growth factor receptor and phosphatase and tensin homolog genes in high-grade gliomas, as well as BRAF alterations in pilocytic astrocytomas.
We tested expression of established senescence markers in three independent cohorts of sporadic PA. We also assessed for OIS in vitro, using forced expression of wild-type and V600E-mutant BRAF in two astrocytic cell lines: human telomerase reverse transcriptase (hTERT)-immortalized astrocytes and fetal astrocytes.
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.
The frequency of BRAF-KIAA1549 fusion transcripts is significantly lower in adult patients with pilocytic astrocytoma, weakening the sensitivity of this specific diagnostic marker in that age group.
The data further support previous observations that these two alterations of the BRAF, KIAA1549 fusions and V600E point mutations, are associated primarily with pilocytic astrocytomas and nonpilocytic gliomas, respectively.
The emergence of genomic arrays, including comparative genomic hybridization and SNP arrays, have helped in the discovery of novel critically important genes and novel genomic biomarkers involved in PCNST oncogenesis (e.g., BRAF duplication in pilocytic astrocytoma).