Title: Systematic elucidation of transcriptional network necessary for initiation and maintenance of high-risk neuroblastoma
Description: Neuroblastoma is a heterogeneous pediatric malignancy originating from the developing sympathetic nervous system, with poor long-term survival for high-risk patients (~40%). About half of advanced neuroblastomas harbor high-level amplification of the MYCN gene, and these tumors show few, if any, additional driver lesions. Despite significant increase in the body of knowledge of genetics in neuroblastoma, all the high-risk patients follow similar therapeutic procedures and little advancement has been made on molecular target based therapies. The major challenge is to dissect the complexity and heterogeneity of these tumors to find driver genes and activated pathways that are essential for the survival of these cancer cells. We used an integrated systems biology approach to define the core regulatory machinery responsible for maintenance of an aggressive neuroblastoma phenotypic state. In the first part of the thesis, I will discuss our computational approach to decipher the tumor heterogeneity by subtype classification, followed by identification of master regulator protein modules for three distinct molecular subtypes of high-risk neuroblastomas, which were validated in a large independent cohort of cases.

We propose that such modules are responsible for integrating the effect of mutations in upstream pathways and for regulating the genetic programs and pathways necessary for tumor state implementation and maintenance. The second part of the thesis is focused on experimental validation of putative master regulators in the subtype of neuroblastomas associated with MYCN amplification. By using RNAi screening followed by experimental and computational analyses to elucidate the interdependencies between the top master regulators, we identified TEAD4-MYCN positive feedback loop as a major tumor maintenance mechanism in this subtype. While MYCN regulates TEAD4 transcriptionally, TEAD4 regulates MYCN through transcriptional and post-translational mechanisms. Jointly, MYCN and TEAD4 regulate 90% of inferred MR proteins and causally orchestrate 70% of the subtype-specific gene expression signature. TEAD4 gene expression was associated with neuroblastoma patient survival independently of age, tumor stage and MYCN status (P=2.1e-02).

In cellular assays, MYCN promoted growth and repressed differentiation, while TEAD4 activated proliferation and DNA damage repair programs, the signature hallmarks of MYCN-amplified neuroblastoma cells. Specifically, TEAD4 was shown to induce MYCN-independent proliferation by transactivating key genes implicated in high-risk neuroblastoma pathogenesis, including cyclin-dependent kinases, cyclins, E2Fs, DNA replication factors, checkpoint kinases and ubiquitin ligases. The critical role of the core master regulator module in controlling tumor cell viability, both in vitro and in vivo, and its clinical relevance as a prognostic factor highlights TEAD4 as a novel and highly effective candidate target for therapeutic intervention. In this thesis, we demonstrate that interrogation of tumor specific regulatory networks with patient-derived gene expression signatures can effectively elucidate molecular subtypes as well as the core transcriptional machinery driving subtype specific hallmarks. This approach enables identification of oncogenic and non-oncogenic dependencies of high-risk neuroblastoma and is applicable to other tumor subtypes.