Identifying alternative hyper-splicing signatures in MG-thymoma by exon arrays

Background: The vast majority of human genes (>70%) are alternative spliced. Although alternative pre-mRNA processing is modified in multiple tumors, alternative hyper-splicing specific to particular tumor types are still lacking. Here we report the use of Affymetrix Human Exon Arrays to spot hyper-splicing events characteristic of myasthenia gravis (MG)-thymoma, thymic tumors which develop in patients with MG and discriminate them from colon cancer changes. Methodology/Principal Findings: We combined GO term to parent threshold-based and threshold-independent ad-hoc functional statistics with in-depth analysis of key modified transcripts to highlight vaious exon-specific changes. These denote alternative splicing in MG-thymoma tumors compared to healthy human thymus and to in-house and Affymetrix datasets from colon cancer and healthy tissues. By using both global and specific, term-to-parent Gene Oncology (GO) statistical comparisons, our functional integrative ad-hoc method allowed the detection of disease-relevant splicing events. Conclusions/Significance: Hyper-spliced transcripts spanned several categories, including the tumorogenic ERBB4 tyrosine kinase receptor and the connective tissue growth factor CTGF, as well as the immune function-related histocompatibility gene HLA-DRB1 and interleukin (IL) 19, two muscle-specific collagens and one myosin heavy chain gene; intriguingly, a putative new exon was discovered in the MG-involved acethylcholinesterase ACHE gene. Corresponding changes in splicesome composition were indicated by co-decreases in the splicing factors ASF/ SF₂ and SC35. Parallel tumor-associated changes occured in colon cancer as well, but the majority of the apparent hyper-splicing events were particular to MG-thymoma and could be validated by Flourescent In-Situ Hybridization (FISH), Reverse Transcription-Polymerase Chain Reaction (RT-PCR) and mass spectrometry (MS) followed by peptide sequencing. Our findings demonstrate a particular alternative hyper-splicing signature for transcripts over-expressed in MG-thymoma, supporting the hypothesis that alternative hyper-splicing contributes to shaping the biological functions of these and other specialized tumors and opening new venues for the development of diagnosis and treatment approaches.