UC San Diego

Chromosomal translocations of neurotrophic receptor tyrosine kinases (NTRKs) lead to various pediatric cancers. While tyrosine kinase inhibitors such as Larotrectinib and Entrectinib remain a major course of treatment, relapse of tumor still occur, suggesting the need for new therapeutic targets. This work focuses on a novel translocation identified in cases of Infantile Fibrosarcoma, which contains the coiled-coil multimerization domain of Echinoderm Microtubule-like protein 4 (EML4) and the tyrosine kinase domain of Neurotrophic receptor tyrosine kinase 3 (NTRK3). Activation of EML4-NTRK3 relies on both the tyrosine kinase activity of NTRK3 and the salt bridge stabilization in the coiled-coil domain of EML4. As shown in focus formation assays, the tyrosine kinase activity of NTRK3 is essential for the biological activation of EML4-NTRK3. Furthermore, EML4-NTRK3 activates downstream signaling pathways MAPK/ERK, JAK/STAT3 and PKC/PLC. The importance of the salt bridge interactions within EML4-NTRK3 was shown as the disruption of all three salt bridge interactions blocks downstream activation, biological activity and the ability to heteromultimerize with EML4. This work also demonstrates that EML4-NTRK3 is localized in the cytoplasm and fails to associate with microtubules. Taken together, these data suggest several therapeutic strategies for Infantile Fibrosarcoma cases bearing EML4-NTRK3 fusion: inhibition of salt bridge interactions within EML4 and design of mimetic peptides of EML4 coiled coil.