Pilocytic Astrocytomas with Infiltrating Patterns of Growth Carry a High Rate of BRAF V600E Mutation
Gokul Kandala, Sergui Bannykh, Sean Fan, Kevin Baden, Andy Pau, Lara Baden, Patricia Fournier, Erica Thorpe, Kathy Porpora, James Mirocha, Kevin Kawachi, Amin Riley-Portuges, Jean Lopategui. Cedars-Sinai Medical Center, Los Angeles, CA
Background: Pilocytic Astrocytoma (PA), a WHO grade 1 tumor, shows two frequent alterations in BRAF oncogene.The V600E point mutation, which is the most common tumor-associated alteration in the BRAF gene and an alternative BRAF activating mechanism that involves BRAF-KIAA1549 gene fusion with corresponding duplication at chromosome band 7q34. Although the majority of PAs are well circumscribed, a subset shows an invasive pattern akin to biologically different infiltrative gliomas. We investigated whether the presence of a BRAF point mutation and/or duplication in PAs correlate with an infiltrating pattern of growth.
Design: We identified 19 cases of PAs with brain infiltration (5 from NF1 patients) and matched them with 18 localized PAs for a total of 37 cases. Infiltration was assessed by histology and MRI. 26 cases were in adults and 11 in children.
V600E mutation was identified by an allelic discrimination PCR mutational kit. Corriel Repository HBT-38 cell line was used as a positive control. FISH studies utilized Abbott Molecular kit with probes for both CEP7 (Green) and 7q34 BRAF containing region (Gold). In each case, 50 cells were evaluated. In the cells with two green signals, presence of two gold signals indicated wild type whereas three a duplication. Cases with over 20% of cells with a gain of BRAF were scored positive for duplication. Both tests were done on all cases.
Statistical analysis was performed using two group Fisher's-exact test of equal proportions. This compared infiltrative vs non-infiltrative tumors with V600E mutation.
Results: BRAF abnormality by either PCR or FISH was seen in 17/37 cases. The V600E mutation and the gene duplication were mutually exclusive except for one case. 6 cases showed a V600E mutation. Strikingly, 5/6 cases with the point mutation showed distinct brain infiltration. 5/14 infiltrating PAs (excluding patients with NF1) had the V600E mutation, whereas only 1/18 non-infiltrating tumors had this pathogenic point mutation. This difference attained a p-value of 0.064 by using Fisher's-exact test of equal proportions. Duplication of the BRAF gene was seen in 12/37 cases. In contrast to V600E mutation, the duplication was non-discriminatory in respect to infiltration of the PAs. Of 5 tumors in NF1 patients, one was positive for duplication and none for point mutation.
Conclusions: 1. Presence of BRAF V600E mutation appears to correlate with an infiltrating pattern of PAs in non-NF1 patients.
2. Duplication of the BRAF gene in PA is present in both infiltrative and localized tumors.
3. BRAF abnormalities are rare in NF1 PAs.
Tuesday, March 20, 2012 2:15 PM
Platform Session: Section F, Tuesday Afternoon