Abstract 3202: PTBP1-mediated aberrant splicing of a brain-specific exon in ANXA7 promotes brain tumorigenesis.
Authors: Ferrarese Roberto , Bug Eva , Maticzka Daniel , Reichardt Wilfried , Carro Maria Stella , Bredel Markus 
Published: August 2014

Abstract
Glioblastoma multiforme (GBM) is the most common and malignant type of brain tumor in adults and is nearly uniformly fatal, with a median survival of one year. Diffuse infiltration, extensive neo-angiogenesis, uncontrolled cellular proliferation, presence of necrotic regions and resistance to apoptosis are all hallmark features of these tumors. The complex nature of GBMs is worsened by an incomplete understanding of its cellular origin and of the molecular determinants of its aggressiveness. This has contributed to make this cancer one of the most difficult to treat.
Alternative splicing of pre-mRNA is an important mechanism to increase the diversity of protein functions; its dysregulation leads to the aberrant expression of specific protein isoforms, often associated with cancer. A relatively small number of splicing factors regulates the production of different splice variants of many proteins. In cancer, alterations in the expression of few splicing factors may lead to the aberrant splicing of several targets including oncogenes and tumor suppressors, which may contribute to the transformed cellular phenotype.
Here, we investigate the tumorigenic role of the heterogeneous nuclear ribonucleoprotein PTBP1, a splicing factor commonly overexpressed in GBM.
In the present study we used RNA immunoprecipitation and Exon Trap assay to describe how PTBP1 drives the aberrant splicing of a brain-specific exon in the membrane-binding tumor suppressor Annexin A7 (ANXA7). By differential overexpression of ANXA7 splice variants in tumor cell lines, we showed that expression of ANXA7 variant B, normally expressed in brain, controls EGFR pathway; conversely, the tumor variant ANXA7-vA does not stop EGFR activation cascade. Immunostaining experiments demonstrated that, compared to ANXA7-vB, ANXA7-vA reduces the endosomal targeting of EGFR and prolonging the activation of its signaling pathway. Knocking down PTBP1 in tumor cell lines restored the expression of ANXA7-vB and mimicked the effect of ANXA7-vB overexpression on EGFR trafficking and pathway activation. Furthermore, we performed in vivo experiments to show that PTBP1 increased expression has a general effect on tumor aggressiveness as it drives angiogenesis and promotes invasiveness. Finally, we assessed the association of PTBP1 expression and clinical outcome and we found that patients with high PTBP1 expression had a significantly worse survival rate.
Our study highlights the important role the splicing factors play in cancer. In fact, the mechanism we describe here could represent a common paradigm of malignant transformation which could possibly lead to the development of new therapeutic strategies for GBM treatment.
Citation Format: Roberto Ferrarese, Eva Bug, Daniel Maticzka, Wilfried Reichardt, Maria Stella Carro, Markus Bredel. PTBP1-mediated aberrant splicing of a brain-specific exon in ANXA7 promotes brain tumorigenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3202. doi:10.1158/1538-7445.AM2013-3202