URPP Fellow: Sandra Varum

Overall survival rates for metastatic melanoma have increased dramatically in the last few years as a result of the latest therapeutic advances. However, a significant number of patients are non-responsive or do not exhibit long-term responses to current therapeutic options. In this URPP subgroup, we tackle this problem by focusing our research on the etiology of melanoma. Melanoma originates from melanocytes that derive during embryonic development from neural crest stem cells (NCSCs). Increasing evidence from our lab and others suggests that melanoma cells re-use aspects of neural crest (NC) developmental programs to mediate melanoma initiation, progression, and/or acquired resistance to targeted therapies. Building on these findings, the main goal of our previous URPP terms was to identify novel NCSC regulators and determine whether some of these play a role in melanoma initiation or progression. Among several factors, we identified the transcription factor Yin Yang (YY1) as a potential modulator of NCSC biology. Conditional ablation of Yy1 in the NC lineage resulted in agenesis or hypoplasia of various NC derivatives, suggesting that Yy1 is required for NCSC development. Moreover, conditional ablation of Yy1 in the melanoma mouse model Tyr::Nras^Q61K;Cdkn2a^-/- revealed that depletion of Yy1 impaired skin melanoma initiation. Mechanistically, we observed that YY1 regulates human melanoma cell proliferation by regulating fundamental metabolic processes and protein synthesis rates (Varum et al., 2019 in revision). Cancer cells display high rates of protein synthesis and disruption of protein synthesis frequently impairs tumorigenesis. In melanoma, inhibition of the translation initiation factor eIF4G1 results in diminished tumor incidence and growth. Moreover, the RNA binding protein URN promotes melanoma metastases by promoting the translation of specific transcripts involved in invasion. Taking these observations in consideration and given the fact that Yy1 is a crucial translation regulator, the main goal of our next URPP term is to determine the contribution of translation reprogramming to melanoma initiation and progression.

Aims

1. Characterizing translation reprogramming that accompanies transformation of human melanocytes.
   The majority of studies addressing translation in cancer have been executed in already established tumors and changes in protein synthesis that actually accompany tumor initiation remain largely unknown. To address this point, we plan to take advantage of human embryonic stem cell (hESC)-derived melanocytes and induce cellular transformation of these cells by introducing a series of alterations commonly found in human melanoma such as NRAS^Q61K and BRAF^V600E mutations and loss of the tumor suppressors CDKN2A and PTEN. We will perform ribosome profiling and proteomics shortly after mutation induction to determine the translational landscape.
2. Characterizing translation reprogramming that accompanies melanoma initiation in vivo. In the next stage of the project, we aim to validate whether putative changes in translation observed in Aim 1 actually take place during melanoma initiation in an in vivo melanoma genetic mouse model. To this end, we will cross the RiboTag mouse (that harbors a modified ribosomal protein L22) with a genetic melanoma mouse model carrying Braf^V600E and loss of the tumor suppressor Pten. We will use ribosome profiling and proteomics to determine translation profile of tumor-initiating cells.
3. Determining whether translation alterations observed upon melanoma initiation are simply bystanders or driver of melanomagenesis. To address this point, we plan to modulate the expression of the differentially translated transcripts (identified in Aims 1 and 2) and determine whether a subset of those play a role in melanoma initiation and/or invasion. We plan to execute these experiments not only in mouse models, but also in human melanoma cell lines derived from the URPP Biobank.
4. Determining whether translation reprogramming observed upon melanoma initiation reflects a developmental process conserved during human NCSC specification. For this purpose, we will analyze translational landscape of hESCs undergoing NCSC specification (by using the same methodology mentioned in Aim 1) and compare the changes in protein synthesis with those observed in Aims 1 and 2.
5. Determining whether translation reprogramming is involved in therapy resistance formation. Given that drug-tolerant melanoma cells acquire NCSC properties in response to targeted therapies, we plan to determine –if time allows– whether translation reprogramming as observed in NC development and melanoma initiation also takes place and is functionally implicated in resistance formation.