Duplication of 7q34 is specific to juvenile pilocytic astrocytomas and a hallmark of cerebellar and optic pathway tumours

Abstract

BACKGROUND: Juvenile pilocytic astrocytomas (JPA), a subgroup of low-grade astrocytomas (LGA), are common, heterogeneous and poorly understood subset of brain tumours in children. Chromosomal 7q34 duplication leading to fusion genes formed between KIAA1549 and BRAF and subsequent constitutive activation of BRAF was recently identified in a proportion of LGA, and may be involved in their pathogenesis. Our aim was to investigate additional chromosomal unbalances in LGA and whether incidence of 7q34 duplication is associated with tumour type or location. METHODS AND RESULTS: Using Illumina-Human-Hap300-Duo and 610-Quad high-resolution-SNP-based arrays and quantitative PCR on genes of interest, we investigated 84 paediatric LGA. We demonstrate that 7q34 duplication is specific to sporadic JPA (35 of 53 -66%) and does not occur in other LGA subtypes (0 of 27) or NF1-associated-JPA (0 of 4). We also establish that it is site specific as it occurs in the majority of cerebellar JPA (24 of 30 -80%) followed by brainstem, hypothalamic/optic pathway JPA (10 of 16 -62.5%) and is rare in hemispheric JPA (1 of 7 -14%). The MAP-kinase pathway, assessed through ERK phosphorylation, was active in all tumours regardless of 7q34 duplication. Gain of function studies performed on hTERT-immortalised astrocytes show that overexpression of wild-type BRAF does not increase cell proliferation or baseline MAPK signalling even if it sensitises cells to EGFR stimulation. CONCLUSIONS AND INTERPRETATION: Our results suggest that variants of JPA might arise from a unique site-restricted progenitor cell where 7q34 duplication, a hallmark of this tumour-type in association to MAPK-kinase pathway activation, potentially plays a sitespecific role in their pathogenesis. Importantly, gain of function abnormalities in components of MAP-Kinase signalling are potentially present in all JPA making this tumour amenable to therapeutic targeting of this pathway.

An activated mutant BRAF kinase domain is sufficient to induce pilocytic astrocytoma in mice

“…Previously, we identified tandem duplications or activating mutations (V600E) of the proto-oncogene BRAF at 7q34 as by far the most prevalent genetic mechanism leading to constitutive MAPK pathway activation, occurring in more than 50% of sporadic PAs (4). Subsequent studies confirmed similar or even higher frequencies of BRAF tandem duplications and activating mutations in PAs, with V600E being the most frequent, and showed that the duplication leads to formation of fusion genes of BRAF (5)(6)(7)(8)(9). The unifying molecular feature of all RAF fusion products identified to date is that the RAF kinase domain alone, upon loss of the autoinhibitory N terminus, has the potential to drive proliferation via constitutive MAPK activation.…”

BRAF-KIAA1549 Fusion Predicts Better Clinical Outcome in Pediatric Low-Grade Astrocytoma

“…Later the same year, a comprehensive study by the Collins laboratory showed that this gain is a result of a tandem duplication between BRAF and KIAA1549 (B-K), producing a novel fusion oncogene (11). Several groups have subsequently confirmed the findings and extended the spectrum of RAS-MAPK activation in pediatric gliomas (12)(13)(14)(15)(16)(17).…”

“…To test this hypothesis, we overexpressed BRAF in hTERT-immortalized human astrocytes, as previously published by our group (14), and reversed the hTERT immortalization by telomerase inhibition (ref. 24, Fig.…”

“…BRAF overexpressing human telomerase reverse transcriptase (hTERT)-immortalized human astrocytes were previously well characterized and published by our group (14). Additional data are available in Supplementary Methods.…”

Pilocytic astrocytomas of the optic nerve and their relation to pilocytic astrocytomas elsewhere in the central nervous system

“…[8][9][10][11][12][13] The biologic significance of BRAF duplication lies in the activation of the MAPK pathway, which can drive tumor proliferation. 9 In addition to KIAA1549:BRAF fusion product, other molecular alterations have been reported in pilocytic astrocytoma, including other BRAF fusion products, 14,15 rare BRAF V600E mutations, 16 BRAF insertions, 17 and KRAS mutations.…”