Over-expression of Sec61-gamma fosters glioblastoma growth and survival in culture

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Sec61-gamma (Sec61G) is a subunit of the mammalian endoplasmic reticulum (ER) translocon, an evolutionarily conserved transmembrane protein that moves newly synthesized proteins across the ER. Crystal structures of the bacterial and yeast Sec61 translocon suggest that Sec61 a,b and g subunits form a complex to transport protein from the ribosome into golgi, plasma membrane or cytoplasm. Sec61g is located on the short arm of chromosome 7, next to the epidermal growth factor receptor (EGFR). Since EGFR is frequently amplified in GBM, with a large amplicon encompassing megabases, Sec61g is likely amplified and over-expressed in most of GBM with EGFR amplification. The effect of SEC61g over-expression on GBM behavior is unknown. We used 3 sets of published Affymatrix GBM gene expression data from the University of California, San Francisco (Nigro et al, 2005), M.D.Anderson Cancer Center (Phillips et al, 2006) and the University of California, Los Angeles (Freije et al, 2004), and an array of traditional and novel statistical methods, bioinformatics, structural modeling, and biological assays to investigate effects of Sec61g overexpression. There was extremely high correlation between Sec61g expression and EGFR expression (p=2.2e-16) in this set of 195 GBM. Almost all EGFR amplification cases had Sec61g amplification and overexpression. Sec61g was over-expressed in the poorly surviving mesenchymal and proliferative subgroups (Phillips et al, 2006), but not in the longer surviving proneural subgroup. Furthermore, Sec61g was associated with poor survival (p=0.005), and was a strong negative predictor of >1 year survival (p=7.6e-6). Structural modeling of the human SEC61 complex indicates similarity to lower species. Human Sec61g is composed of two alpha helices of unequal length forming a structure that resembles the number “7” with an evolutionarily conserved proline residue at the junction of the arms. Over-expression of Sec61g in U251NCI cells in culture increased growth rate (p<0.05) and survival (p<0.01) but had no discernable effect on morphology, adhesion or migration. Our analysis of gene expression profiles in 195 GBM indicated strong correlations between Sec61g and several cell cycle regulation genes, suggesting that Sec61g may increase cell growth via these genes. Similar analyses indicate that Sec61g may exert its survival effects via key genes in the AKT and NFkb pathways. Protein-protein interaction database search suggested that Sec61g interacts with 107 known and hypothetical proteins. Pathway Enrichment analyses indicate associations between Sec61g and 26 biological pathways (e.g., calcium signaling, cell communication etc.). We are experimentally validating bioinformatics results to investigate how Sec61g exerts its effects on growth and survival. This work was supported by NIH (NS42927 and CA85799), National Brain Tumor Foundation, and the Barrow Neurological Foundation.