**3. Epigenetics**

Through whole-genome analysis, global DNA methylation profiling has demonstrated that higher methylation levels are associated with increased tumor aggressiveness and risk of recurrence. DNA methylation is an epigenetic change hypothesized to contribute to genomic instability by silencing genes involved with DNA repair and control of cell cycling. Evidence suggests that methylation status may predict tumor behavior more accurately than the current WHO classification, thus, DNA methylation status has been proposed as an alternative classification system for MNs [47]. The most important genes involved in the DNA methylation of MNs are tissue inhibitors of metalloproteinase 3 (TIMP3), cyclin-dependent kinase inhibitor 2A (CDKN2A), and tumor protein 73 (TP73), which are hypermethylated in at least 10% of cases [48]. TIMP3 hypermethylation results in transcriptional downregulation and inhibits its tumor suppressor properties [49]. In addition, TIMP3 is frequently hypermethylated in higher-grade MNs (40–60%) and is related to a decrease in relapse-free time and increased biological aggressiveness [50]. Notably, TIMP3 is found on chromosome 22q12, and almost all cases with gene hypermethylation had a concurrent allelic loss of 22q. About 60–80% of high-grade meningiomas carry TP73 promoter methylation, a queue event not common in grade I tumors, suggesting its potential use as a marker for high-grade lesions [51].

Recently, several studies highlighted the importance of global methylation profiles in the molecular subclassification of meningiomas [52], Olar et al. demonstrated that unsupervised clustering of DNA methylation data classified meningiomas into two distinct subgroups associated with recurrence-free survival. A statistically significant association between DNA methylation subclasses and tumor recurrence was maintained after adjusting for clinical factors, such as WHO grade and Simpson grade [41]. Similarly, Sahm et al. identified two major groups and six subgroups of meningiomas based on unsupervised clustering of DNA methylation data, with significantly different genomic makeup and clinical behaviors. Interestingly, most non-NF2 meningiomas clustered together into a single benign subgroup [53]. These initial efforts suggest that epigenetic signatures may have solid clinical associations with tumor recurrence, to a more significant extent than can be correlated with mutational genetic analysis and could be used clinically to stratify patients. An additional manifestation of the importance of epigenetic changes in meningioma clinical behavior was recently shown, describing an increased risk of recurrence in tumors that show a loss of histone H3K27 trimethylation [54].

#### **3.1 Protein expression**

Classically, the identification of meningiomas using immunohistochemistry has been done using the expression of the progesterone receptor (PR) and the epithelial membrane antigen (EMA). However, over the last few years, it has been found that the specificity of RP for the diagnosis of high-grade meningiomas is low, especially when trying to differentiate between clear cell, fibrous, and microcystic subtypes. Likewise, EMA expression correctly identifies ~90% of grade I meningiomas, but only 75% of grade III, with even lower specificity rates for secretory and microcystic subtypes [55]. Due to these markers' poor performance, the expression of somatostatin receptor 2A (SSTR2A) in combination with EMA was included, a profile

#### *High Grade Meningiomas: Current Therapy Based on Tumor Biology DOI: http://dx.doi.org/10.5772/intechopen.100432*

that provides a sensitivity of 100% and specificity of 95%, regardless of tumor grade. Likewise, recent work suggests that the absence of Sox10 and STAT6 [56, 57] are superior approaches to distinguishing meningioma from schwannoma, solitary fibrous tumor, and synovial sarcoma.

In addition, marking for lymphocyte infiltration can contribute to the grading of meningiomas and the prediction of response to some interventions. Most low-grade meningiomas possess a high percentage of CD-3+ T-lymphocytes but relatively few CD20+ B cells; however, across tumor grades, these populations are greatly enriched compared to those seen in peripheral blood mononuclear cells (PBMC) [58]. Flow cytometry analysis reveals evidence of class switching in B cells, an increased percentage of CD8+ cells compared to CD4+ T cells, and a prevalence of CD45RO+/CD45RA− effector cells compared to naive T cells [59]. This information allows predicting that tumor-infiltrating immune cells have had exposure to various tumor antigens despite low BMR. Among high-grade meningiomas and particularly anaplastic tumors, there is a reduction in the count of CD4+, CD8+, and PD-1+ T cells, and an increase in the number of FoxP3+ T-regulatory cells (Tregs) [60]. This immune cell phenotype, also observed in other tumor types, is associated with tumor-mediated evasion of the immune system.

Du et al. report high levels of PD-L1 mRNA, which correlated to protein expression levels, in ~40% of grade I, 60% of grade II, and 77–88% of grade III meningiomas [59]. Nevertheless, Everson et al. only identified PD-L1 expression in 25% of grade III cases, with no expression detected in grade I or II cases [25]. The controversy has been amplified since PD-L1 does not predict outcomes. However, in the future, the expression of TIM-3 and LAG-3 could be helpful to consider the use of agonist monoclonal antibodies [58]. Another potential biomarkers that could

predict the response to targeted therapies are EGFR expression, which is present in up to 90% of meningiomas [25]. Furthermore, the expression of TOP2A (35% of the samples) is associated with a higher tumor grade and could be useful to assess the usefulness of anthracyclines or trabectedin. Likewise, TOP1 over-expression is observed in 29% of meningiomas and correlates with sensitivity to irinotecan and topotecan, while elevated levels of PDGFR and c-MET are observed in more than 20% of cases [25] (**Figure 2**).
