Melanoma drug discovery: Prioritization of candidate melanoma driver mutations in the ErbB4 receptor tyrosine kinase gene and identification of novel molecules that disrupt ErbB4 signaling

Abstract

BRAF V600E mutations are found in roughly 50% of metastatic melanomas. While these tumors typically initially respond to BRAF and/or MEK inhibitors, they frequently develop resistance to BRAF and/or MEK inhibition. Fortunately, the five-year survival for metastatic melanoma has improved to approximately 50%, in part, due to the contribution of anti-CTLA-4, -PD-1, and -PD-L1 immunomodulators. However, there are still no effective targeted therapies against metastatic melanomas that do not harbor the BRAF V600E driver mutation. Thus, there is a pressing need for additional biomarkers and actionable targets in metastatic melanoma. In this work, analyses of The Cancer Genome Atlas (TCGA) Skin Cutaneous Melanoma (SKCM) clinical melanoma genomic data set revealed that a significant fraction of melanomas (14%) harbor at least one mutation in the gene that encodes for the ErbB4 receptor tyrosine kinase. However, unlike the validated melanoma oncogene BRAF, no single ERBB4 mutation was predominant among the melanoma genomes. Instead, there were 71 unique ERBB4 nonsynonymous missense mutations. Here, various in silico approaches were used to prioritize ERBB4 nonsynonymous missense mutations in melanoma on their likelihood to function as melanoma drivers. The results of these analyses suggest that many of the ERBB4 mutations in the TCGA-SKCM data set are likely to contribute to the malignant phenotype of melanoma. Therefore, there remains a need to determine which ERBB4 mutations are bona fide drivers of melanoma cell proliferation. The results of this work will serve as a platform for the characterization of putative ERBB4 driver mutations in melanoma. Since it appears that a significant fraction of melanoma patients likely harbor driver mutations in ERBB4, there is a need for the development of therapeutics that disrupt ErbB4 signaling. As a result, in this work, a drug discovery strategy was developed based on the observation that the Q43L mutant of the naturally occurring ErbB4 agonist NRG2β functions as a partial agonist at ErbB4. NRG2β/Q43L stimulates tyrosine phosphorylation, fails to stimulate ErbB4-dependent cell proliferation, and inhibits agonist-induced ErbB4-dependent cell proliferation. Molecules that exhibit these characteristics are likely to function as ErbB4 partial agonists, and as such hold promise as therapies for the treatment of ErbB4-dependent melanomas. Consequently, three highly sensitive and reproducible (Z’ > 0.5) semi-automated screening assays were developed and deployed for the identification of small-molecules that function as partial agonists at ErbB4. As a result, six small-molecules were identified that stimulate ErbB4-phosphorylation, fail to stimulate ErbB4-dependent cell proliferation, and appear to selectively inhibit ErbB4-dependent cell proliferation. Therefore, these molecules could potentially function as ErbB4 partial agonists and have potential as therapies against ErbB4-dependent melanomas. While further characterization and optimization is required to evaluate the full therapeutic potential of these molecules, the platform on which they were identified certainly demonstrates reliable and scalable approaches for the discovery of ErbB4 inhibitors.