Resumen de Genomic dissection to identify pheochromocytoma’s Achiles’ heel: Novel markers of metastatic disease and therapeutic strategies
Pheochromocytomas and paragangliomas (PPGLs) are highly heterogeneous rare neuroendocrine tumors, associated with mutations in one of the at least 20 susceptibility genes described so far related to the disease. Approximately 15% of the PPGLs are metastatic with a 5-year survival rate of 40%. Although some genotype-phenotype correlations have been recognized for patients with PPGLs, disease management continues to be difficult due to their highly variable clinical behavior, and the lack of reliable predictive markers of metastatic PPGL (mPPGL). Moreover, the poor understanding of the mechanisms driving the metastatic behavior hinders the establishment of an efficient therapeutic strategy for mPPGLs.
Thus, the principal aim of this Thesis was the discovery of novel molecular markers able to predict the risk of metastasis, helping to improve patients’ clinical management and treatment options.
Firstly, we confirmed MDH2 as a novel PPGL susceptibility gene responsible of less than 1% of PPGL cases. For this purpose, we screened a large series of PPGL patients and established a workflow to classify variants of unknown significance. We also described the connection between germline mutations in MDH2 and a noradrenergic and metastatic phenotype.
Secondly, by analyzing a large series of miRNome tumor data, we established a miRNA signature associated with metastatic risk and time to progression, which could also be detected in PPGL patients’ serum. MiR21-3p and miR-183-5p were identified as the best markers to predict metastasis, and were found linked in vitro to pro-metastatic characteristics, such as a neuroendocrine-to-mesenchymal transition phenotype, and an enhanced cell migration rate. Through the integration of the miRNome, transcriptome and proteome, and the use of the in vitro models, we deciphered a miR-21-3p/TSC2/mTOR axis with therapeutic relevance for mPPGL patients.
In the third study, we aimed to widen the list of mPPGL informative miRNAs. We carried out an extensive analysis of miR-483-5p, uncovering that it is potentially released by the metastases, and consequently higher levels are detected in the circulation of mPPGL patients. Integrative analyses with transcriptomic data suggested its role in metastasis-related processes, and discovered ALCAM, a gene related to tumor immune evasion, as the best target of miR-483-5p.
Finally, by performing whole-exome-and transcriptome-sequencing in a large series of mPPGLs with extensive clinical annotated data, a higher tumor mutational burden and microsatellite instability was observed in metastatic tumors, which were both correlated with ATRX/TERT alterations and shorter progression-free survival. Although we encountered a large inter-tumoral heterogeneity at mutational level among mPPGLs, we could evince a functionally relevant enrichment of mutated genes involved in extracellular matrix organization, cell adhesion, and neuron projection morphogenesis. Moreover, this study revealed several biological processes deregulated in metastatic primary tumors at the transcriptional level that are of great significance in terms of further understanding the mechanisms leading the metastatic behavior. One of them is an immunosuppressive phenotype, further evidenced by deconvolution analysis of RNAseq data.
In summary, in this Thesis we report novel molecular markers able to assess metastatic potential at diagnosis and to guide the surveillance of those patients with high metastatic risk. Furthermore, we bring to light new data that could help to understand the mechanisms contributing to metastasis, and aid in the discovery of pheochromocytoma’s Achilles heel.
