HnRNP H drives an oncogenic splicing switch in gliomas

Tight regulation of alternative splicing patterns is essential for proper cell functioning. In tumors, aberrant alternative splicing generates splice variants that can contribute to multiple aspects of tumor establishment, progression and maintenance.Here, we investigate aberrant splicing events in gliomas through examination of the alternative splicing patterns in primary Glioblastoma Multiforme (GBM) samples, focusing on a discrete set of transcripts previously described within various systems to be directly involved in tumorigenesis. This study shows that approximately one-quarter have a change in splicing pattern, and they are consistently switched towards the more tumorigenic isoforms (from normal to high grade glioma).One of the identified changes, death domain adaptor protein IG20 is alternative spliced to promote exon 16 exclusion within 95% of the GBM samples. This alternative splicing event leads to the expression of an antagonist, anti-apoptotic isoform (MADD) which would effectively redirect TNF-alpha or TRAIL-induced death signaling to promote survival and proliferation in tumors, instead of triggering apoptosis. This event is controlled by a splicing silencer recognized by splicing factor hnRNP H, which is up-regulated in primary GBM samples and in tumor samples from glioma mouse models, suggesting a role for hnRNP H in pathogenesis and progression of malignant gliomas. Cell death induced by knock-down of hnRNP H can be rescued by specific downregulation of exon 16-containing IG20 variants.We identified a virtually identical regulatory cis-element on exon 11 of RON tyrosine kinase receptor also resulting in an hnRNP H-controlled switch to a ligand-independent, constitutively active RONDelta11 variant, which causes increased motility and invasiveness. Decreased invasiveness elicited by reduction of hnRNP H levels can also be rescued by the re-direction of RON splicing to the normal isoform, using a splicing re-direction approach.HnRNP H is therefore at the center of a splicing oncogenic switch, which might reflect stem cell patterns and mediates multiple key aspects of aggressive tumor behavior, including evasion from apoptosis and promotion of invasiveness.