Background: TMB-H in gliomas is caused by molecular alterations or alkylator treatment- induced genomic changes characterized by a large number of G:C>A:T transitions. Our study describes the molecular features of TMB-H gliomas. Methods: Gliomas were tested with NextGen sequencing (592 genes), MGMT promoter methylation (MGMT-m) fragment analysis and IHC at Caris Life Sciences. Microsatellite instability (MSI) was test by NGS, FA/IHC. The GC:AT transition rate was calculated as the prevalence of G:A and C:T changes seen in each tumor and > 80% was regarded as high transition(TR-H). TMB values were compared using Wilcoxon Rank Sum. TMB-H was defined as the top quartile of all TMB values (TMB>9). Results: TMB in the 3129 gliomas ranged from 0 to 372 mutations/MB (mean: 8.5, median: 6). TMB-H was observed in 31% of glioblastomas, 16% of astrocytomas (astro) (22% of grade III, 7% of grade I/II) and 22% of oligodendrogliomas (oligo) (32% of grade III and 15% of grade I/II). MGMT-m (58% vs. 47%; p=0.0001), pathogenic (p) or likely p (lp) EGFR (14% vs 10%, p=0.004) and PIK3CA mutations (13% vs. 9%, p=0.002), as well as p/lp in 30 other genes were more prevalent in TMB-H cases (p<0.01). In the 613 TMB-H tumors, TR-H was seen in 12% (73) and was strongly associated with increased TMB (median TMB 52 in TR-H vs. 9 in TR-L,) and MSI-H (7.3% vs. 1.1%), both p<0.0001. Tumors with both TR-H and MSI-H had a mTMB of 114 vs. 49 in TR-H /MSS tumors. MSI-H and TR-L tumors had an mTMB of 23 vs. 9 in MSS /TR-L tumors (p<0.0001). All 5 POLE-MT tumors had TMB of >100 (median 264) and TR-L; 4 of the 5 were also MSI-H. PDL1 IHC had no correlation with TMB, MSI or transition rates. In 89 paired samples taken >150 days apart (regardless of intervening treatment), acquisition of TMB-H was seen in 11 pairs: 8 glioblastomas, 2 grade II/III astro and 2 oligo. In the paired tumors that acquired TMB-H status compared to those that did not, a significantly higher prevalence of MGMT-m (82% vs. 37%, p=0.008) and IDH mutation (64% vs. 19%, p=0.004) were seen. 10 of the 11 recurrent tumors with acquisition of TMB-H had TR-H while none in the other 78 pairs. Conclusions: TMB varies significantly in gliomas and associates with POLE, TR-H and MSI-H, but not with an increase of PD-L1. POLE-mutated tumors had the highest TMB levels. TR-H, an indicator of alkylator-induced phenotype, is associated with a higher TMB than MSI-H, however, TR-H may synergize with MSI-H to further increase TMB. Tumors with an IDH mutation and MGMT-m are more prevalent in tumors with high TMB gain. Further understanding of molecular and immune profile of the TMB-H may facilitate more individualized treatment planning.