ERBB4 is a Driver of BRAF WT Melanomas
Type of DegreePhD Dissertation
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Approximately 50% of metastatic melanomas harbor a BRAF V600 mutation which causes elevated RAF/MEK/ERK pathway signaling. These tumors are treated using a combination of a BRAF and MEK inhibitor which targets the canonical RAF/MEK/ERK pathway. However, the other ~50% of metastatic melanomas which possess wild-type (WT) BRAF alleles are just as aggressive as BRAF V600 mutant tumors but have no targeted therapeutic available beyond immune checkpoint inhibitors. ERBB4 (HER4) is a receptor tyrosine kinase that is closely related to the epidermal growth factor receptor (EGFR/ERBB1/HER1), ERBB2 (Neu/HER2), and ERBB3 (HER3). EGFR and ERBB2 are well-established oncogenes and therapeutic targets in multiple tumor types. Our in silico analyses of BRAF-WT tumor genomes suggest that increased transcription of ERBB4 drives BRAF WT melanomas via cooperation with elevated RAS/RAF/MAPK pathway signaling via mutation in a RAS or NF1 gene. Therefore, we obtained a panel of BRAF WT melanoma cell lines that harbor RAS or NF1 mutations and showed that ERBB4 signaling is both sufficient and necessary for clonogenic proliferation and ERBB4 is necessary for anchorage-independent colony growth. Our in silico analysis found that ERBB4 mutant alleles appear to be associated with increased stimulation of the PI3K/Akt canonical pathway and also cooperate with NF1 or RAS gene mutations. Our in silico analyses have determined which ERBB4 mutants found in BRAF WT melanomas are priority candidate tumor drivers. We selected nine high-priority mutations and one known gain-of-function positive control mutation and introduced them to the MEL-JUSO BRAF WT, ERBB4-dependent melanoma cell line. We found that introduction of some of these mutations causes increased oncogenic activity greater than WT ERBB4. ERBB4 is known to be a context-dependent oncogene and tumor suppressor where EGFR-ERBB4 and ERBB2-ERBB4 heterodimers are oncogenic and ERBB4-ERBB4 homodimers are tumor suppressive. We introduced the constitutively homodimerized ERBB4 Q646C mutant to the MEL-JUSO cell line and found that indeed, MEL-JUSO cells do respond to the Q646C mutant with tumor suppressor activity. Together these experiments suggest that there exists a novel class of BRAF WT, ERBB4-dependent melanomas where ERBB4 function is tightly regulated to mitigate ERBB4 homodimer-mediated tumor suppressor activity and that these melanomas may be effectively treated with a combination MEK inhibitor and ERBB receptor inhibitor.