With the recent FDA approval of larotrectinib, a selective pan-TRK inhibitor, and entrectinib, a selective pan-TRK, ROS1, and ALK inhibitor , there has been much interest in characterizing and diagnosing tumors with NTRK fusions. Both therapies have demonstrated significant treatment responses in NTRK-fused tumors [4, 13, 15,16,17, 23, 30], including CNS metastases [13, 16, 17, 23] and primary CNS tumors [1, 9, 16, 50, 57], and are generally well tolerated [4, 13, 15,16,17, 23, 30]. Our study addresses gaps in the knowledge of the clinical and molecular features of NTRK-fused gliomas. In addition, the study serves to corroborate features of NTRK-fused gliomas that have been previously described.
One of the findings of this multi-institutional study is the considerable clinicopathologic and molecular heterogeneity of NTRK-fused gliomas. NTRK fusions do not appear to define ipso facto a single glial entity but rather are a genetic feature occurring in multiple tumor types.
Clinically, we found that NTRK-fused gliomas can involve all CNS compartments but are primarily hemispheric in adults (90.9%) and infants (85.7%); the anatomic distribution of pediatric NTRK-fused gliomas is less predictable. During a median follow-up period of 23 months after diagnosis, 28.6% of patients died and 58.0% of patients showed evidence of recurrence/progression, events that were mostly associated with tumors with high-grade histology. A prior study showed a 5 year overall survival of 42.9% in young patients with hemispheric NTRK-fused gliomas . However, NTRK-fused gliomas with low-grade histology may still exhibit an aggressive clinical course . Ultimately, the prognosis of NTRK-fused gliomas may rapidly change with the more widespread use of targeted TRK inhibitors.
The diverse histology of NTRK-fused gliomas overlaps with entities such as DIGG, GG, PXA, PA (including anaplastic), DA grades 2 and 3, and GBM in our study. In keeping with the wide spectrum of NTRK-fused CNS tumor histology, NTRK rearrangements have been previously reported in GBM [9, 18, 19, 21, 28, 34, 38, 39, 41, 43, 44, 52, 53, 56], gliosarcoma , AA [9, 18, 21, 52], diffuse midline glioma / DIPG [9, 52], HGG [9, 22, 34, 57], glioneuronal tumor (including high grade) [1, 16, 29], pilocytic astrocytoma (PA) (including anaplastic) [9, 18, 21, 25, 35], low grade astrocytroma with features of PA , PXA , GG [1, 9, 36, 37], DIGG [6, 9], LGG [18, 34, 44, 50, 53], glioma, not otherwise specified [9, 18], neuroepithelial neoplasm , CNS fibroblastic tumor , primitive neuroectodermal tumor (PNET) , CNS embryonal tumor , and tumors with oligodendroglial or oligoastrocytic-like histology [12, 21, 29, 37, 55].
The present study adds to the literature by demonstrating that the histology and histologic grade of NTRK-fused gliomas vary by patient age. NTRK-fused gliomas in all infantile and most adult patients were histologically high-grade, with the majority diagnosed as GBM. In contrast, pediatric NTRK-fused gliomas were more likely to be of low-grade (46.2%) or uncertain WHO grade (38.5%), and there was no single predominant histologic diagnosis in this cohort. These features of the pediatric NTRK-fused gliomas make their diagnosis and clinical management difficult.
Gliomas with NTRK fusions have been previously reported to possess co-occurring genetic alterations such as IDH [18, 21, 39, 56], H3.3 K27M , H3F3A , EGFR amplification [21, 28, 56], EGFRvIII , PTEN [9, 21, 28], CDKN2A/2B deletion [9, 22, 28, 39, 52, 55, 57], CDKN2C deletion , TP53 mutations/inactivation [21, 39, 52, 57], and ATRX , among others . Our study matches many of these molecular findings and further demonstrates that the frequency of pathologically significant mutations in NTRK-fused gliomas appears to increase with patient age. In addition, TERT promoter mutations are observed only in histologically high-grade adult tumors, PTEN alterations are almost exclusively seen in histologically high-grade tumors, and CDKN2A/2B loss is rare in histologically low-grade tumors.
Notably, 22.7% of adult NTRK-fused gliomas in our cohort are IDH1 p.R132H mutated, raising questions about the oncogenic driver event in these specific tumors and whether they display oncogenic dependence on the NTRK fusion. In one case report of secondary IDH-mutant GBM , an NTRK fusion was detected in only a subclonal tumor population and was absent in the original AA, suggesting that the NTRK fusion was a secondary alteration. In contrast, in NTRK-fused gliomas without co-occurring pathologically significant mutations [9, 21, 52], typically arising in younger patients, NTRK fusions are almost certainly the oncogenic driver event. This is supported by multiple in vivo models that have demonstrated the capability of NTRK fusions to drive gliomagenesis/tumorgenesis [11, 28, 33, 51, 52].
In our series, gliomas with rearrangements involving the same NTRK gene and fusion partner do not necessarily have the same histology or methylation class, which suggests that other factors such as age, co-occurring genetic events, cell of origin and microenvironment potentially play an important role in tumor biology. We have also observed that the NTRK gene involved in the rearrangement differs in frequency by age, with pediatric gliomas having a high percentage of rearrangements involving NTRK2 (69.2%), and adult gliomas having a high percentage of rearrangements involving NTRK1 (68.2%).
Most NTRK-fused gliomas in our cohort were not a perfect match with known methylation entities. Only two cases matched with high confidence to methylation class families. Furthermore, cases that matched with low confidence generally had histology that was not characteristic of the methylation class family, mirroring the experience described in a prior case study of NTRK-fused glioneuronal tumor . Another study reported a proportion of NTRK-fused gliomas matching to methylation classes including IHG, diffuse midline glioma H3 K27M mutant, PXA, and GBM, IDH wildtype, subclass midline . Overall, the findings from our study and prior studies with methylation data [9, 29] suggest that better methylation profile classifier guidelines are needed to account for NTRK-fused. Our unsupervised PCA demonstrating no obvious grouping by NTRK gene involved, age, or histology highlights the heterogeneity within the NTRK-fused glioma methylome.
In summary, NTRK-fused gliomas are clinically, histologically, and molecularly diverse, with notable differences by age group and associated genetic alterations. Additional studies are needed to develop clinical guidelines for the diagnostic workup of potential NTRK-fused CNS tumors and to improve methylation classifier guidelines for NTRK-fused gliomas. Further mechanistic work is required to determine the role of NTRK fusions in driving gliomagenesis in the setting of concurrent oncogenic drivers such as IDH mutations and to demonstrate how downstream TRK signaling pathways may be mediated by different NTRK gene involved, location of NTRK fusion breakpoint, fusion partner, and cell of origin.