The role of constitutive BRAF
activation in a large proportion of pediatric low-grade gliomas is of interest both from diagnostic/prognostic and therapeutic perspectives—the former because it may help differentiate indolent tumors from more aggressive tumors, and the latter because BRAF is a “druggable” target for improved adjuvant treatment of residual or recurrent tumors. Trials for anti-BRAF drugs in low-grade pediatric gliomas are in progress, following the favorable results with such agents in melanomas.30,31
From a prognostic perspective it is still unclear whether rearrangement or V600E mutation have differing impacts on outcome or whether other molecular markers can affect the prognostic power of BRAF
The strong trend toward improved PFS in BRAF
-rearranged low-grade gliomas (P
= .06, not shown), plus its preponderance in PAs relative to most other tumors (Table ), could be taken at first glance to be evidence that BRAF
rearrangement is indicative of a PA even if tumor morphology does not quite match. However, this trend might not have been because BRAF
rearrangement was more common in PAs per se, but rather because BRAF
rearrangement is more common in the cerebellum, where tumors are usually more resectable. Multivariate analysis suggested as much (Table ), and non-PA tumors with BRAF
rearrangement are not rare, either in this cohort or in another recently studied cohort.7
On the other hand, even though neither PFS nor OS was significantly different by BRAF
rearrangement when adjusting for tumor location (Fig. ), it is noteworthy that in the high-risk midline tumors, the only deaths were from tumors lacking rearrangement (Fig. F). Still, caution is suggested before equating BRAF
rearrangement with PA, especially in locations where non-PA low-grade gliomas are more frequent.
Prior work showed that BRAF
rearrangement was an independently favorable prognostic factor for a cohort of 70 incompletely resected low-grade pediatric gliomas of the optic pathway, brainstem, and spinal cord.7
In the current cohort there were no non-NF1 optic gliomas and only 20 cases of midbrain, brainstem, and spinal cord tumors, though this cohort had more diencephalic, cerebral, and cerebellar tumors (Table ). The closest comparison to that prior work is the DMBS subset in this study, where rearrangement did not significantly stratify PFS but did, as mentioned above, show fewer deaths compared with BRAF
-intact midline tumors (Fig. E and F). Thus, these results do not preclude a favorable effect of rearrangement on PFS in certain contexts but that such an effect may be outweighed by other variables like midline location and p16 status (Table ).
The p16 protein is located on the CDKN2A
locus on 9p21 and is a major checkpoint in the cell cycle. Loss of p16 is well known to correlate with increased WHO glioma grade32
and has been seen in BRAF
-driven pediatric low-grade gliomas that behaved more aggressively.13,16,19
Our data support those studies but further suggest that p16 deletion is an adverse marker independent of BRAF
status (Figs and , Table ). Interestingly, nuclear atypia also tended to be higher in p16-deleted gliomas (see Results). Likewise, elevated MIB1 generally corresponds to higher glioma grade and was recently reported to connote a higher risk of recurrence in partially resected PAs,26
though its general significance in PAs is unclear.26,33,34
The current data suggest that very high MIB1 indices do trend toward shorter PFS and possibly a higher risk of resistance to radiotherapy but may not specifically modify the prognosis of BRAF
-driven gliomas (Figs and , Tables and ).
Regarding p53, its increased expression is more characteristic of grade II astrocytomas rather than grade I PAs,35
which might account for the shorter PFS seen in DMBS tumors with high p53 expression (Fig. B). However, the data suggest that high p53 does not modify BRAF
-driven outcomes and probably does not add as much clinical value as p16 and MIB1.
Univariate analysis did not show consistently worse PFS in BRAF V600E
tumors (Fig. ), but V600E trended toward worse PFS on multivariate analysis, with an HR opposite to that of BRAF
rearrangement. While the latter did not reach significance on multivariate analysis, the more favorable HR associated with rearrangements was consistent with prior work.7,34
These results seem counterintuitive, considering that both rearrangement and V600E produce constitutive BRAF activity and that V600E can induce PA growth but eventual senescence in vivo.16,18
One hypothesis is that although both types of BRAF
alterations have the same basic activity, other molecular lesions may preferentially be found in BRAF V600E
-rearranged tumors and thus drive the tumors toward different outcomes. In our cohort there was no difference in p53 expression, p16 deletion, or MIB1 proliferation between BRAF
-rearranged and BRAF V600E
gliomas (not shown), so none of these variables are likely to account for the divergent HRs. BRAF V600E
mutation may therefore be indicative of a tumor that is at higher risk of worse behavior than a grade I PA. Along those lines, a recent study showed that 10% of malignant pediatric astrocytomas had the BRAF V600E
which is a far higher percentage than those that had BRAF
rearrangement. Clearly, more cases with dual V600E and rearrangement testing are needed to firmly establish whether there is a true difference in behavior. Until then, caution is recommended when comparing tumors with rearrangement or V600E or when combining them in outcome-based studies like clinical trials.
Except for our prior work focused on key molecular and cell cycle variables, including BRAF
rearrangement, p53, p16, and MIB1, in the outcome of pediatric PAs,8
to our knowledge this is the first study to weigh the prognostic and predictive significance of both BRAF
rearrangement and BRAF
V600E, plus the aforementioned cell cycle markers, in a large cohort of PA and non-PA pediatric low-grade gliomas. Some limitations include the retrospective nature of this study, the predominance of PAs versus other tumor subtypes, and the fact that substratification by region, then by BRAF status (especially V600E), then by p53/MIB1/p16 was often difficult from a statistical perspective. And other than knowing that the vast majority of midline tumors had subtotal resections while most cerebellar and cerebral cases had gross total resections, the intraoperative impression of the degree of resection was not available in most cases. This precludes separating out the individual contributions of resection extent and tumor location, though it is likely that the midline location is an adverse factor due in large part to the difficulty in achieving total tumor resection. Furthermore, specific information on doses and durations of radiotherapy and chemotherapy were often not retrievable in this cohort, thus limiting that aspect of the analysis.
Nevertheless, our data suggest that p16 deletion independently shortens PFS in these gliomas, including in BRAF-altered tumors; that tumors with very high MIB1 proliferation indices may be at higher risk of adjuvant radiotherapy resistance and/or worse outcome, though none of these markers is a strong predictor of response; that the behavior of tumors with BRAF rearrangement might be different from those with V600E mutations; and that despite all of these molecular biomarker discoveries, tumor location (and by extension, tumor resectability) remains by far the most important prognostic variable.