Therapeutic and Prognostic Implications of BRAF V600E in Pediatric Low-Grade Gliomas

Pediatric low-grade gliomas can be molecularly stratified for risk

Pediatric low-grade gliomas (PLGGs) consist of a number of entities with overlapping histological features. PLGGs have much better prognosis than the adult counterparts, but a significant proportion of PLGGs suffers from tumor progression and recurrence. It has been shown that pediatric and adult low-grade gliomas are molecularly distinct. Yet the clinical significance of some of newer biomarkers discovered by genomic studies has not been fully investigated. In this study, we evaluated in a large cohort of 289 PLGGs a list of biomarkers and examined their clinical relevance. TERT promoter (TERTp), H3F3A and BRAF V600E mutations were detected by direct sequencing. ATRX nuclear loss was examined by immunohistochemistry. CDKN2A deletion, KIAA1549-BRAF fusion, and MYB amplification were determined by fluorescence in situ hybridization (FISH). TERTp, H3F3A, and BRAF V600E mutations were identified in 2.5, 6.4, and 7.4% of PLGGs, respectively. ATRX loss was found in 4.9% of PLGGs. CDKN2A deletion, KIAA1549-BRAF fusion and MYB amplification were detected in 8.8, 32.0 and 10.6% of PLGGs, respectively. Survival analysis revealed that TERTp mutation, H3F3A mutation, and ATRX loss were significantly associated with poor PFS (p < 0.0001, p < 0.0001, and p = 0.0002) and OS (p < 0.0001, p < 0.0001, and p < 0.0001). BRAF V600E was associated with shorter PFS (p = 0.011) and OS (p = 0.032) in a subset of PLGGs. KIAA1549-BRAF fusion was a good prognostic marker for longer PFS (p = 0.0017) and OS (p = 0.0029). MYB amplification was also a favorable marker for a longer PFS (p = 0.040). Importantly, we showed that these molecular biomarkers can be used to stratify PLGGs into low- (KIAA1549-BRAF fusion or MYB amplification), intermediate-I (BRAF V600E and/or CDKN2A deletion), intermediate-II (no biomarker), and high-risk (TERTp or H3F3A mutation or ATRX loss) groups with distinct PFS (p < 0.0001) and OS (p < 0.0001). This scheme should aid in clinical decision-making.

BRAF Dimerization: An Underlying Resistance Mechanism in Low-Grade Pediatric Gliomas

BRAF fusions and mutations are the most frequent genetic alterations in pediatric low-grade gliomas. The work from Wang and colleagues identifies an acquired secondary BRAF mutation that confers resistance to pharmacologic BRAF inhibition in a BRAF^V600E glioma. The authors demonstrate that the mutation results in increased BRAF homodimerization, which in turn is targetable with second-generation BRAF inhibitors. Cancer Discov; 8(9); 1064-5. ©2018 AACR.See related article by Wang et al., p. 1130.

Precision Neuro-oncology: the Role of Genomic Testing in the Management of Adult and Pediatric Gliomas

OPINION STATEMENT

In recent years, large-scale genomic studies have expanded our knowledge regarding genomic drivers in tumors of the central nervous system. While histopathologic analysis of brain tumors remains the primary method for tumor classification, the clinical utility of molecular and genomic testing to support and/or complement tumor classification continues to expand. This approach enhances diagnostic accuracy and provides clinicians with objective data to facilitate discussions regarding prognosis and treatment decisions, including selection of clinical trials. Ensuring accurate diagnoses is fundamental to the management of brain tumor patients. However, given the morphologic overlap among primary brain tumors, genomic data can be used to help distinguish tumor lineage. In its clearest form, we have embraced the concept of an integrated diagnosis, which combines traditional histopathology findings with molecular and genomic data. Patient prognosis varies significantly based on a tumor's genomic profile. For neuro-oncology patients, outcome studies linking diagnoses with genomic profiles show significant differences based on tumor biomarkers such as IDH1/2, H3F3A, BRAF, and CDKN2A and TERT status. Therefore, easy access to reliable genomic data is important in understanding a patient's disease and developing a clinical strategy wherein targeted molecular or immune therapies can be incorporated into the discussion.

A Secondary Mutation in BRAF Confers Resistance to RAF Inhibition in a BRAFV600E-Mutant Brain Tumor

BRAF^V600E hyperactivates ERK and signals as a RAF inhibitor-sensitive monomer. Although RAF inhibitors can produce impressive clinical responses in patients with mutant BRAF tumors, the mechanisms of resistance to these drugs are incompletely characterized. Here, we report a complete response followed by clinical progression in a patient with a BRAF^V600E-mutant brain tumor treated with dabrafenib. Whole-exome sequencing revealed a secondary BRAF^L514V mutation at progression that was not present in the pretreatment tumor. Expressing BRAF^V600E/L514V induces ERK signaling, promotes RAF dimer formation, and is sufficient to confer resistance to dabrafenib. Newer RAF dimer inhibitors and an ERK inhibitor are effective against BRAF^L514V-mediated resistance. Collectively, our results validate a novel biochemical mechanism of RAF inhibitor resistance mediated by a secondary mutation, emphasizing that, like driver mutations in cancer, the spectrum of mutations that drive resistance to targeted therapy are heterogeneous and perhaps emerge with a lineage-specific prevalence.Significance: In contrast to receptor tyrosine kinases, in which secondary mutations are often responsible for acquired resistance, second-site mutations in BRAF have not been validated in clinically acquired resistance to RAF inhibitors. We demonstrate a secondary mutation in BRAF (V600E/L514V) following progression on dabrafenib and confirm functionally that this mutation is responsible for resistance. Cancer Discov; 8(9); 1130-41. ©2018 AACR.See related commentary by Romano and Kwong, p. 1064This article is highlighted in the In This Issue feature, p. 1047.

Precision Medicine for Primary Central Nervous System Tumors: Are We There Yet?

In recent years, technologic advances have increased tremendously our understanding of the molecular characteristics and genetic drivers of a variety of brain tumors. These discoveries have led to paradigm shifts in the treatment of these tumor entities and may therefore have a considerable impact on the outcome of affected patients in the near future. Here, we provide a broad overview of recently discovered clinically actionable mutations that have been identified in three different primary brain tumors: gliomas, meningiomas, and craniopharyngiomas. We furthermore highlight the diagnostic and therapeutic implications of these findings and summarize recently published and ongoing trials.

Clinicopathological and genetic association between epithelioid glioblastoma and pleomorphic xanthoastrocytoma

Epithelioid glioblastoma (eGBM) is a rare variant of GBM which was adopted in the 2016 WHO classification. eGBM and pleomorphic xanthoastrocytoma (PXA) sometimes show overlapping features histologically and genetically, such as epithelioid pattern and a highly frequent V600E mutation in the gene for vRAF murine sarcoma viral oncogene homolog B1 (BRAF), respectively. Accurate diagnosis of these rare tumors is challenging according to the new criteria in the revised 2016 WHO classification. It is an urgent task to elucidate the biological properties of the tumors and to select appropriate treatment. Twenty consecutive cases diagnosed as PXA or eGBM histologically were investigated. Twelve of the 20 cases were PXAs and eight were eGBMs. Morphologically, mitotic activity, necrosis and degenerative changes such as intracellular lipid accumulation, eosinophilic granular bodies and reticulin fiber deposits were scored. Immunohistochemical and molecular biological assessment for isocitrate dehydrogenases 1 and 2 (IDH1/2), α-thalassemia/mental-retardation-syndrome-X-linked gene (ATRX), p53, BRAF, telomere reverse transcriptase promoter (TERT-p), H3F3A, and integrase interactor 1 (INI1) were performed. eGBM tended to lack the degenerative changes characteristic for PXA. Of the 20 cases tested, Sanger technique showed no mutation in IDH1/2. BRAF mutation at T1799 > A (V600E) was detected in 4/12 (33.3%) PXA and 4/8 (50.0%) eGBM, while TERT-p mutation was detected at C228 > T in 2/12 (16.7%) PXA and at C250 > T in 1/8 (12.5%) eGBM. Retained nuclear ATRX was observed in 12/12 (100%) PXA and 6/7 (85.7%) eGBM while p53 mutation was observed in 2/10 (20%) PXA and 7/7 (100%) eGBM. All tumors retained INI1 expression in their nuclei. None of the tumors harbored H3F3A mutation. One PXA without BRAF mutation acquired TERT-p mutation at recurrence and one eGBM harbored both BRAF and TERT-p mutation. Molecular biological similarity between eGBM and PXA was suggested in our series, while degenerative changes reflected the features of PXA. It was speculated that the common genetic alterations for development and progression of eGBM and PXA might include BRAF and TERT-p mutations.

Signaling pathways and mesenchymal transition in pediatric high-grade glioma

Pediatric high-grade gliomas (pHGG), including diffuse intrinsic pontine gliomas (DIPG), are the most lethal types of cancer in children. In recent years, it has become evident that these tumors are driven by epigenetic events, mainly mutations involving genes encoding Histone 3, setting them apart from their adult counterparts. These tumors are exceptionally resistant to chemotherapy and respond only temporarily to radiotherapy. Moreover, their delicate location and diffuse growth pattern make complete surgical resection impossible. In many other forms of cancer, chemo- and radioresistance, in combination with a diffuse, invasive phenotype, are associated with a transcriptional program termed the epithelial-to-mesenchymal transition (EMT). Activation of this program allows cancer cells to survive individually, invade surrounding tissues and metastasize. It also enables them to survive exposure to cytotoxic therapy, including chemotherapeutic drugs and radiation. We here suggest that EMT plays an important, yet poorly understood role in the biology and therapy resistance of pHGG and DIPG. This review summarizes the current knowledge on the major signal transduction pathways and transcription factors involved in the epithelial-to-mesenchymal transition in cancer in general and in pediatric HGG and DIPG in particular. Despite the fact that the mesenchymal transition has not yet been specifically studied in pHGG and DIPG, activation of pathways and high levels of transcription factors involved in EMT have been described. We conclude that the mesenchymal transition is likely to be an important element of the biology of pHGG and DIPG and warrants further investigation for the development of novel therapeutics.

Adolescents and young adults with brain tumors in the context of molecular advances in neuro-oncology

Adolescents and young adults (AYA) comprise a specific group of oncology patients with a distinct biological and epidemiological spectrum of central nervous system neoplasms. It has been well documented that they differ clinically, especially in relation to prognosis and chemotherapy tolerance; however, the underlying reasons for this are unclear. Recent advances in the genomics of both childhood and adult brain tumors have provided new explanations and insights into the previously described age-dependent heterogeneity. Herein, we summarize the current state of the AYA population in neuro-oncology, specifically how biological advances can help personalize therapy for this unique group of patients.

Review of molecular classification and treatment implications of pediatric brain tumors

PURPOSE OF REVIEW

Brain tumors are the most common solid tumors and leading cause of cancer-related death in children. The advent of large-scale genomics has resulted in a plethora of profiling studies that have mapped the genetic and epigenetic landscapes of pediatric brain tumors, ringing in a new era of precision diagnostics and targeted therapies. In this review, we highlight the most recent findings, focusing on studies published after 2015, and discuss how new evidence is changing the care of children with brain tumors.

RECENT FINDINGS

Genome-wide and epigenome-wide profiling data have revealed distinct tumor entities within, virtually, all pediatric brain tumor groups including medulloblastoma; ependymoma; high-grade and low-grade gliomas; atypical teratoid/rhabdoid tumors; and other embryonal tumors, previously called CNS primitive neuroectodermal tumors. Whenever integrated with clinical information, many molecular alterations emerge as powerful prognostic markers and should thus be used to stratify patients and tailor therapies.

SUMMARY

Optimal integration of this newly emerging knowledge in a timely and meaningful way into clinical care is a remarkable task and a matter of active debate. The historical morphology-based classification of tumors is being replaced by a genetic-based classification, and the first generation of molecularly informed clinical trials is underway.