Chromosomal abnormalities may be important in the pathogenesis of sporadic meningiomas and have been described on several chromosomes and on multiple sites on chromosome 22. Sporadic meningiomas were examined for loss of heterozygosity on chromosome 22 in the region of the neurofibromatosis type 2 gene, because of the almost 50% incidence of meningiomas in neurofibromatosis type 2. Sixty percent to 65% of patients had a loss of heterozygosity in at least 1 locus on chromosome 22. Somatic abnormalities of the neurofibromatosis type 2 gene on the long arm of chromosome 22 were seen in 8 tumors (Harada et al 1996; Ueki et al 1999). Loss of Merlin expression is tightly associated with the loss of heterozygosity at 22q in sporadic meningiomas (Ueki et al 1999). A second gene on chromosome 22, INI1 on exon 9, has been mutated in a small fraction of meningiomas (4/126) (Schmitz et al 2001). Loss of DAL-1, a second tumor suppressor protein and a protein 4.1 family member, from a gene on chromosome 18p11.3, occurs in approximately 75% of cases and may represent an early event in meningioma pathogenesis along with loss of Merlin expression (Gutmann et al 2000; Perry et al 2000). A third protein 4.1 family member (4.1R) was found absent in 2 meningioma cell lines by Western blotting, and in 6 of 15 sporadic meningiomas by immunohistochemistry and fluorescence in situ hybridization. Protein 4.1R membrane localization increased significantly under growth arrest in vitro, and it interacted with protein 4.1B and merlin tumor suppressors and was similar in structure. Loss of 4.1R leads to tumor cell growth (Robb et al 2003).
In addition to meningioma formation, neurofibromatosis type 2 locus inactivation also has been associated with tumor progression to higher grades. Chang and colleagues showed that in a subgroup of meningiomas with neurofibromatosis type 2, loss of heterozygosity, and concomitant P53 codon, 72 arginine-to-proline polymorphism had higher tumor grades (II, III). This study suggests that the p53 pathway is involved in progression of meningiomas, particularly those involving NF2 locus inactivation (Chang et al 2009).
In a recent genomic analysis of 300 meningiomas not associated with NF-2, new mutations were detected, suggesting there may be distinct molecular subtypes of meningiomas. Clark and colleagues found mutations in TRAF7, a proapoptotic E3 ubiquitin ligase, to be present in almost a quarter of all meningiomas analyzed. In meningiomas found to have a TRAF7 mutation, a mutation in KLF4, a transcription factor known to induce pluripotency, was often associated. SMO mutations involved in Sonic Hedgehog signaling were found in a separate subset of meningiomas analyzed, which tended to have a more benign clinical course and more often located at the skull base (Clark et al 2013). Brastianos and colleagues also recently analyzed the genomes of non-NF2 associated meningiomas, also finding mutations in AKT1 and SMO, associated with skull base location (Brastianos et al 2013). This finding suggests that perhaps medical therapy targeting these pathways could be useful in skull base tumors, which often pose therapeutic challenges
It is well known that activation of the Wnt/beta-catenin signaling pathway is involved in cancer progression. A study done at Massachusetts General Hospital considered human meningioma microRNA and its effect on tumor growth. Down regulation of miRNA in meningiomas and arachnoid cells resulted in increased expression of beta-catenin and cyclin D1 involved in cell proliferation. MiR-200a was found to directly target beta-catenin mRNA, inhibiting its translation and blocking the Wnt/beta-catenin signaling pathway. This suggests that miR-200a may act as a tumor suppressor (Saydam et al 2009).
In atypical and malignant meningiomas, the incidence of abnormalities on 22q increases and other chromosome abnormalities occur in much greater frequency than in benign meningiomas. Gene cloning of a segment of 22q 12/3-g 13.1 characterized a new member of the N-acetylglucosaminyltransferase family, a 664 kb gene called the "LARGE" gene. A 756 amino acid glycosyltransferase is encoded by the gene, and deletions in the gene may produce abnormal glycosylation of proteins and glycosphingolipids, leading to tumor formation (Peyrard et al 1999). There is also an increase in the total number of genomic alterations in each tumor (Weber et al 1997; Kim et al 1998).
The JAK-STAT3 pathway has been implicated in the pathogenesis of atypical and anaplastic meningiomas (Johnson et al 2009). In this study, 45 WHO grade I and 35 grade II and III meningiomas were evaluated for activation of MEK-1-MAPK, PI3 K-Akt-mTOR-PRAS40, and STAT3 pathways. It was found that STAT3 activation was found in 100% of grade II and III meningiomas compared to 75% of grade I meningiomas. The MEK-1-MAPK pathway was detected in all grades. STAT3 is a protein transcription factor that is involved with many cellular functions, including cell proliferation, apoptosis, and angiogenesis. It is activated by growth factors including PDGF and EGF. Its involvement in more aggressive meningiomas has not been studied previously.
SPARC (secreted protein, acidic and rich in cysteine) is a matricellular glycoprotein that is involved in cell function. The main function of SPARC is to act as a counter adhesion, regulate growth factor activity, and cell-cell inhibition. SPARC previously has been found to be overexpressed in a variety of malignancies, including melanoma, lung, glioma, esophageal, hepatocellular, renal, ovarian, prostate, colon, and bladder. A study done by Bozkurt and colleagues showed that a higher SPARC expression was significantly associated with grade of malignancy in meningiomas. Atypical and anaplastic meningiomas expressed higher levels of SPARC than benign ones. Expression of SPARC was also found to be higher in recurrent meningiomas than in nonrecurrent meningiomas. In the same study, SPARC was found to be correlated with tumor cell proliferation index. This suggests that meningiomas with higher expression of SPARC have a greater proliferative activity and perhaps and increased potential for recurrence (Bozkurt et al 2009).
Deletions of the short arm of chromosome 1 are the second most common abnormality, with loss of heterozygosity in 30% of cases (Sulman et al 1998), a site of tumor suppressor genes (Piaskowski et al 2005). Other chromosomal alterations in meningiomas were deletions of 7p that were uncommon in benign meningiomas (13%), but increased in frequency to 70% in atypical meningiomas and to 100% in 3 malignant meningiomas (Bostrom et al 1997). The same locus has been found to have chromosomal abnormalities in sporadic acoustic neurinomas. Chromosomal abnormalities and neurofibromatosis type 2 gene mutations were seen in meningotheliomatous, fibroblastic, and transitional histologic patterns. Other common sites of genetic loss were 14q, 18q, 10, and 6q and gains at 15q and 20 (Menon et al 1997; Tse et al 1997; Weber et al 1997; Korshunov et al 2007).
Telomerase activity has been shown to be important in the control of cell proliferation and regulation of cell senescence. The expression of telomerase activity may produce unlimited cell proliferation and immortality. Two groups have examined telomerase activity in meningiomas, and have found a much higher incidence of telomerase activity in malignant or atypical meningiomas than in benign meningiomas (Leuraud et al 2004). The protein product (hTERT) of telomerase messenger expression was analyzed and its presence in meningioma tissue was correlated with MRI recurrence, and its level with recurrence in a second study (Kalala et al 2005; Maes et al 2005). A significant correlation was found between telomerase activity and the Ki-67 proliferation index (Cabuy and de Ridder 2001). Tumor hypoxia has often been associated with more malignant tumor phenotypes. Carbonic anhydrase 9 is an endogenous tumor hypoxia marker, and its expression in meningiomas was correlated with higher grade meningioma histology (Yoo et al 2007).
Dysregulation of cell cycle suppression progression genes p16 (also known as ARF or CDKN2A) on chromosome 9p is found to a greater extent in anaplastic meningioma than more benign subtypes, but there was no correlation with telomerase activation (Simon et al 2001). This is felt to be a possibly critical step in progression from benign to malignant meningiomas (Chen et al 2000; Simon et al 2001). Non-neoplastic meninges and meningiomas of all malignancy grades were examined for gene expression and found to have induction of 3 components of the notch signaling pathway. Transcript and protein levels of Notch 1 and 2, 1 receptors, enhancer split and jagged 1 are induced in all meningiomas. However, 2 members of Groucho/transducin-like enhance of Split family of corepressors TLE2 and TLE3 are induced only in higher grade meningiomas (Cuevas et al 2005). Activated Notch 1 and 2 receptors induce endogenous HES1, a Notch pathway effector. HES1 is associated with tetraploid cells in meningioma cell lines. Tetraploid cells have nuclear features of chromosomal instability, increased nuclear atypia. This indicates that abnormal Notch signaling plays a role in meningioma tumor development (Baia et al 2008).
Meningiomas express both estrogen and progesterone sex hormone receptors (Bouillot et al 1994; Khalid 1994; Kuratsu et al 1994; Blaauw et al 1995; Khalid et al 1995; Black et al 1996). The expression of the progesterone receptor is seen in a greater percentage of meningiomas than the estrogen receptor, and the percentage of meningioma cells expressing the progesterone receptor is generally greater in any tumor (Bouillot et al 1994; Khalid 1994; Kuratsu et al 1994; Blaauw et al 1995; Black et al 1996). Progesterone receptor expression in benign meningiomas has been correlated with decreased recurrence (Fewings et al 2000). It is expressed to a greater extent in benign meningiomas than in atypical or malignant meningiomas (Perry et al 2000). Progesterone receptor mRNA staining was nuclear and functional using transfection techniques (Black et al 1996). Long term megestrol acetate therapy for endometrial stromal sarcoma has been associated with the development of multiple separate meningiomas in a case report (Gruber et al 2004). In a similar case report, a 63-year-old postmenopausal female on long-term megestrol acetate developed multiple intracranial meningiomas. Histologic evaluation revealed these to be of grade I type, and they tested positively for progesterone receptor. After discontinuation of hormone therapy, the patient had regression in tumor volume (Vadivelu et al 2010).
Meningiomas also express receptors for platelet-derived growth factor (Adams et al 1991; Kuratsu et al 1994; Black et al 1996; Todo et al 1996). Platelet-derived growth factor receptor-beta beta type was found in 100% of meningiomas in 1 study, and 95% of these tumors were platelet-derived growth factor positive (Kuratsu et al 1994). Conditioned medium from meningiomas grown in vitro contained proteins related to the B chain of platelet-derived growth factor and stimulated the proliferation of meningioma cells. This was blocked by a neutralizing antibody against platelet-derived growth factor receptor-beta beta (Adams et al 1991; Todo et al 1996). The intracellular transducing events of mitogenic signals from platelet-derived growth factor appear to be mediated by mitogen activated protein kinases (Johnson et al 2001). In 1 study, statistical analysis showed an inverse correlation between progesterone receptor staining and features of malignancy (Hsu et al 1997).
Epidermal growth factor receptor has been discovered in nearly 100% of meningioma specimens in tissue culture, with increased meningioma DNA synthesis after exogenous epidermal growth factor is added (Westphal and Herrmann 1986; Horsfall et al 1989; Reubi et al 1989). More recently, EGFR expression has been hypothesized to have prognostic significance. Increase in expression of EGFR was found to correlate with progression of benign meningiomas to atypical or anaplastic types, though no clear correlation was found with progression-free survival nor overall survival (Caltabiano et al 2013).
Vascular endothelial growth factor is expressed in human meningioma tumor, and the extent of peritumoral edema on T2-weighted MRI has been directly related with the vascular endothelial growth factor staining intensity (Goldman et al 1997; Provias et al 1997). This suggests that VEGF-A expression is related to histologic grade. The expression of mRNA stability factor HuR was found to be involved in upregulation of VEGF-A expression in primary meningioma cell cultures (Sakuma et al 2008).
Interleukin-6 was studied by Park and colleagues to be involved with peritumoral brain edema in meningiomas. Interleukin-6 is expressed in various tumors and is involved in induction of endothelial barrier dysfunction and increasing endothelial cell permeability. Il-6 mRNA expression was present in moderate-to-severe edema-producing tumors, suggesting a direct relationship between this inflammatory cytokine and peritumoral brain edema. Interleukin-6 can also increase peritumoral edema by stimulating VEGF and other factors known to be directly correlated (Park et al 2010).
Sixty percent of benign meningiomas are associated with peritumoral brain edema. In a study, 61 patients with benign meningiomas chosen for surgical treatment were studied for clinical, radiologic, and surgical aspects that would contribute to tumor recurrence. In this study, there was no statistically significant association between tumoral edema and clinical findings (headache, seizure, motor deficit, visual field defect), location of tumor, or patients with different degrees of resection. Size of edema did have a statistically significant positive correlation with higher recurrence rates, which suggests that edema should be used as a prognostic factor (Simis et al 2008).
Endothelin 1 is a growth factor expressed in meningiomas and correlates with vascular endothelial growth factor expression and increased microvessel count (Boldrini et al 2006). In addition, levels of platelet activating factor have been directly correlated with peritumoral edema (Hirashima et al 1998). High levels of the ratio of insulin growth factor 2 to insulin growth factor binding protein 2 have been correlated with an atypical or anaplastic pathology (Nordqvist et al 1997). Many other genes, second messengers, enzymes, growth factors, and extracellular matrix components are being assessed as possible prognostic factors for recurrence in newly diagnosed meningiomas. A high incidence of somatostatin receptors has also been found, but the significance of this is as yet unknown (Reubi et al 1989). A secreted acidic protein rich in cysteine is an extracellular matrix-associated protein concerned with cell adhesion; it was found to be present in invasive primary and recurrent meningiomas, but not in nonaggressive benign tumors (Rempel et al 1999).
Well-known proliferation markers of meningiomas include Ki-67, mitotic index, and microvessel density. It has been found that the expression of cyclooxygenase (COX2) was positively correlated with increasing tumor grade and with several known markers of cell proliferation in meningiomas. Another novel marker found to be associated with increasing tumor grade is brain-specific fatty acid binding protein. This suggests a role for fatty acid metabolism in meningioma progression and angiogenesis (Panagopoulous et al 2008).
Multiple meningiomas are uncommon, representing 1.1% of all meningiomas (Sheehy and Crockard 1983). Multiple meningiomas from each of 4 patients were studied with polymerase chain reaction for X chromosome inactivation to determine if the multiple tumors of each patient came from the same clonal population. In each patient, the same X chromosome was inactivated, suggesting the tumors originated from a single progenitor cell (Larson et al 1995). Additionally, although the presence of multiple meningiomas usually alerts the physician to suspect a possible mutation in the NF2 gene, an entity called “familial multiple meningiomas” is described when no association with NF2 is found. Recent research has found SMARCB1 to be a causative germline mutation that predisposes to multiple meningiomas (Christiaans et al 2011). Not only was this germline mutation identified, its pathogenesis was also found to follow the 2-hit hypothesis in which the mutant allele is retained and the wild-type allele is lost in these multiple meningiomas. Finally, in some cases of multiple meningioma, both mutations, that of NF2 and SMARCB1 have been found, suggesting even a 4-hit hypothesis in these subsets of patients.
Meningioma mitotic labeling index has been measured with bromodeoxyuridine, Ki-67, MIB1, and silver colloid nucleolar organizer region (Adams et al 1991; Bagni et al 1991; Shibuya et al 1992; Ohta et al 1994; Aguiar et al 2003). In all of the above studies, lower labeling activity has been associated with a decreased risk of recurrence. One study found no correlation between labeling index and recurrence in gross totally resected meningiomas (Konstantinidou et al 1998). Benign meningiomas with a proliferating cell nuclear antigen or Ki-67 labeling index of more than 2% may have a greater recurrence risk (Takeuchi et al 1997). Other investigators have correlated a high MIB-1 labeling in benign meningiomas with bcl-2 expression as an unfavorable prognostic sign (Karamitopoulou et al 1998).
Bone morphogenic proteins (BMPs) are a part of the larger transforming growth factor B superfamily. BMPs are known to be involved in cell proliferation, apoptosis, angiogenesis, and cell differentiation. BMP receptor expression on leptomeninges was studied by Johnson and colleagues (Johnson et al 2009a; 2009b). In this study, sections from 20 normal leptomeninges, 2 arachnoid cysts, and 51 meningiomas (grade I-III) were analyzed for BPM-4. Eleven of the meningiomas were tested for cell proliferation and signal activation by being treated with BMP-4. BMP was detected in 89% of grade I and 60% of grade II meningiomas by Western blot analysis. BMP receptors Ia and II were detected in leptomeninges in all meningiomas studied. Meningioma cultures treated with BMP-4 stimulated meningioma cell proliferation via activation of the SMAD1 pathway. These results indicate that BMP-4 is expressed in leptomeninges and meningiomas and may play an important role in proliferation of tumor cells.