The neuronal RNA binding protein HuB as a potential tumor suppressor in glioblastoma

Post-transcriptional regulation is emerging as a fundamental step in gene expression that, when altered, can contribute to carcinogenesis. To identify potentially altered translational networks in glioblastoma multiforme (GBM), the worst of brain tumors, we correlated genomic alterations and mRNA levels of RNA-binding proteins (RBPs) in 372 publicly available GBM samples and identified 56 RBPs whose copy number alterations correlate with their altered expression levels in at least 15% of the samples. Among the genes identified using our parameters, HuB (ELAVL2) was deleted in 48% of the samples and down-regulated in more than 90% of these. Given the evidence for HuB activity as a differentiation factor in neuronal cells, we hypothesized that it may act as an oncosuppressor. However, ELAVL2 maps to the same chromosomal band (9p21.3) as CDKN2A, the most frequently inactivated oncosuppressor in gliomas, giving the possibility that ELAVL2 loss is simply a consequence to the CDKN2A deletion and therefore a passenger, albeit very frequent, mutation. To test this possibility, we analyzed the structure of the deletion spanning the two loci by qPCR analysis of 233 GBM samples, testing for the presence of the intervening region, and we obtained evidence that two independent focal deletions occurred in about 20% of the GBM samples bearing homo- and/or heterozygous deletion at CDKN2A and ELAVL2. This result highlights that, in rare but recurrent cases, ELAVL2 deletion occurs independently from CDKN2A deletion, supporting the hypothesis that loss of HuB activity is a condition contributing to tumor progression. This hypothesis was tested using U87MG cells, a commonly used glioma cell line homozygously deleted for CDKN2A and heterozygously deleted for ELAVL2 and primary glioma initiating cells (GICs) homozygously deleted for both genes. Migration, invasion and capacity to form nuerospheres were determined in U87MG cells upon HuB silencing and overexpression, and in GICs upon HuB expression. HuB expression in both cell models resulted in a decreased in migration, invasivity and the capacity to form neurospheres, supporting the hypothesis that HuB act as a tumor suppressor. We finally showed that HuB is able to determine an increase of p21 protein in normal murine neuroblast cells, providing a possible mechanism for HuB-mediated suppression of glioma cell clonality.