The phosphatidyl inositol 3-kinase pathway is central to the pathogenesis of Kit-activated melanoma

Correlation Between KIT Expression and c-Kit Mutations in 2 Subtypes of Canine Oral Melanocytic Neoplasms

c-Kit mutations have been reported in 15% to 40% of certain human melanoma subtypes, including those histologically similar to canine oral malignant melanomas. Therapeutic response to tyrosine kinase inhibitors has been demonstrated in those human patients. As canine oral malignant melanomas tend to have a poor prognosis despite aggressive surgical removal, evaluation of KIT expression and identification of c-Kit mutations in canine oral melanocytic neoplasms was performed to determine if there is any indication that tyrosine kinase inhibitor drugs might effectively treat any of these cases. This study evaluated 27 canine oral malignant melanomas and 12 canine histologically well-differentiated oral melanocytic neoplasms for activating c-Kit mutations, determined differences in immunohistochemical expression of KIT and c-Kit mutation status, and determined if KIT expression could predict c-Kit mutation status. Among samples that contained intraepithelial nests of neoplastic melanocytes in the KIT-labeled sections, KIT was expressed within cells in these nests in 22/23 (96%) malignant melanomas and 5/7 histologically well-differentiated neoplasms. KIT was expressed in 10% to 30% of neoplastic melanocytes in the lamina propria in 3/24 (13%) malignant melanomas, but 0/9 (0%) histologically well-differentiated neoplasms. Next-generation sequencing identified 85 variants in c-Kit, including 9 nonsynonymous mutations that resulted in amino acid changes predicted to affect protein function. c-Kit mutations with predicted deleterious protein effects were more common in malignant melanomas (8/27 [30%] vs 1/12 [8%]). There was no apparent relationship between detected c-Kit mutations and KIT expression. These results do not support the use of therapies that target c-Kit.

Enhancer Domains in Gastrointestinal Stromal Tumor Regulate KIT Expression and Are Targetable by BET Bromodomain Inhibition

Gastrointestinal stromal tumor (GIST) is a mesenchymal neoplasm characterized by activating mutations in the related receptor tyrosine kinases KIT and PDGFRA. GIST relies on expression of these unamplified receptor tyrosine kinase (RTK) genes through a large enhancer domain, resulting in high expression levels of the oncogene required for tumor growth. Although kinase inhibition is an effective therapy for many patients with GIST, disease progression from kinase-resistant mutations is common and no other effective classes of systemic therapy exist. In this study, we identify regulatory regions of the KIT enhancer essential for KIT gene expression and GIST cell viability. Given the dependence of GIST upon enhancer-driven expression of RTKs, we hypothesized that the enhancer domains could be therapeutically targeted by a BET bromodomain inhibitor (BBI). Treatment of GIST cells with BBIs led to cell-cycle arrest, apoptosis, and cell death, with unique sensitivity in GIST cells arising from attenuation of the KIT enhancer domain and reduced KIT gene expression. BBI treatment in KIT-dependent GIST cells produced genome-wide changes in the H3K27ac enhancer landscape and gene expression program, which was also seen with direct KIT inhibition using a tyrosine kinase inhibitor (TKI). Combination treatment with BBI and TKI led to superior cytotoxic effects in vitro and in vivo, with BBI preventing tumor growth in TKI-resistant xenografts. Resistance to select BBI in GIST was attributable to drug efflux pumps. These results define a therapeutic vulnerability and clinical strategy for targeting oncogenic kinase dependency in GIST. SIGNIFICANCE: Expression and activity of mutant KIT is essential for driving the majority of GIST neoplasms, which can be therapeutically targeted using BET bromodomain inhibitors.

Enhancer Domains in Gastrointestinal Stromal Tumor Regulate KIT Expression and Are Targetable by BET Bromodomain Inhibition

Gastrointestinal stromal tumor (GIST) is a mesenchymal neoplasm characterized by activating mutations in the related receptor tyrosine kinases KIT and PDGFRA. GIST relies on expression of these unamplified receptor tyrosine kinase (RTK) genes through a large enhancer domain, resulting in high expression levels of the oncogene required for tumor growth. Although kinase inhibition is an effective therapy for many patients with GIST, disease progression from kinase-resistant mutations is common and no other effective classes of systemic therapy exist. In this study, we identify regulatory regions of the KIT enhancer essential for KIT gene expression and GIST cell viability. Given the dependence of GIST upon enhancer-driven expression of RTKs, we hypothesized that the enhancer domains could be therapeutically targeted by a BET bromodomain inhibitor (BBI). Treatment of GIST cells with BBIs led to cell-cycle arrest, apoptosis, and cell death, with unique sensitivity in GIST cells arising from attenuation of the KIT enhancer domain and reduced KIT gene expression. BBI treatment in KIT-dependent GIST cells produced genome-wide changes in the H3K27ac enhancer landscape and gene expression program, which was also seen with direct KIT inhibition using a tyrosine kinase inhibitor (TKI). Combination treatment with BBI and TKI led to superior cytotoxic effects in vitro and in vivo, with BBI preventing tumor growth in TKI-resistant xenografts. Resistance to select BBI in GIST was attributable to drug efflux pumps. These results define a therapeutic vulnerability and clinical strategy for targeting oncogenic kinase dependency in GIST. SIGNIFICANCE: Expression and activity of mutant KIT is essential for driving the majority of GIST neoplasms, which can be therapeutically targeted using BET bromodomain inhibitors.

Gastrointestinal stromal tumor enhancers support a transcription factor network predictive of clinical outcome

Activating mutations in the KIT or PDGFRA receptor tyrosine kinases are hallmarks of gastrointestinal stromal tumor (GIST). The biological underpinnings of recurrence following resection or disease progression beyond kinase mutation are poorly understood. Utilizing chromatin immunoprecipitation with sequencing of tumor samples and cell lines, we describe the enhancer landscape of GIST, highlighting genes that reinforce and extend our understanding of these neoplasms. A group of core transcription factors can be distinguished from others unique to localized and metastatic disease. The transcription factor HAND1 emerges in metastatic disease, binds to established GIST-associated enhancers, and facilitates GIST cell proliferation and KIT gene expression. The pattern of transcription factor expression in primary tumors is predictive of metastasis-free survival in GIST patients. These results provide insight into the enhancer landscape and transcription factor network underlying GIST, and define a unique strategy for predicting clinical behavior of this disease.

Hypoxia and MITF regulate KIT oncogenic properties in melanocytes

KIT mutations are frequent in acral, mucosal and chronic sun-damage (CSD) melanoma, but little is known about the mechanisms driving the transformation of KIT-mutated melanocytes into melanoma cells. We showed that exposition of melanocytes harboring the (L576P)KIT mutation to a hypoxic environment induced their transformation into malignant cells. Transformed (L576P)KIT melanocytes showed downregulation of MITF expression characteristic of melanoma initiating cells (MICs). In agreement, these cells were able to form spheres in neural crest cell medium and low-adherence conditions, also a characteristic of MICs. Downregulation of MITF by RNA interference induced transformation of KIT-mutated melanocytes in normoxia and acquisition of a MIC phenotype by these cells. Hence, low level of MITF cooperates with oncogenic KIT to transform melanocytes. Activation of the cAMP pathway in transformed (L576P)KIT melanocytes stimulated MITF expression, and reduced cellular proliferation and sphere formation. These findings highlight the essential role of MITF in revealing the oncogenic activity of KIT in melanocytes and suggest that the cAMP pathway is a therapeutic target in KIT-mutated melanoma.

Navigating the therapeutic complexity of PI3K pathway inhibition in melanoma

Melanoma is entering into an era of combinatorial approaches to build upon recent clinical breakthroughs achieved by novel single-agent therapies. One of the leading targets to emerge from the growing understanding of the molecular pathogenesis, heterogeneity, and resistance mechanisms of melanomas is the phosphoinositide 3-kinase (PI3K)-AKT pathway. Multiple genetic and epigenetic aberrations that activate this pathway have been identified in melanomas de novo and in acquired resistance models. These developments have been paralleled by the establishment of models for preclinical testing and the availability of compounds that target various effectors in the pathway. Thus, in addition to having a strong rationale for targeting, the PI3K-AKT pathway presents an immediate clinical opportunity. However, the development of effective strategies against this pathway must overcome several key challenges, including optimizing patient selection, overcoming feedback loops, and pathway cross-talk that can mediate resistance. This review discusses the current understanding and ongoing research about the PI3K-AKT pathway in melanoma and emerging strategies to achieve clinical benefit in patients by targeting it.

Obesity and melanoma: exploring molecular links

Obesity is now a major health problem due to its rapidly increasing incidence worldwide and severe consequences. Among many conditions associated with obesity are some cancers including melanoma. Both genetic defects and environmental risk factors are involved in the carcinogenesis of melanoma. Activation of multiple signal pathways such as the PI3K/Akt and MAPK pathways are necessary for the initiation of melanoma. Activation of the MAPK pathway as a result of activating mutations in BRAF is commonly seen in melanoma though it alone is not sufficient to cause malignant transformation of melanocytes. Obesity can result in the activation of many signal pathways including PI3K/Akt, MAPK, and STAT3. The activation of these pathways may have a synergistic effect with the genetic defects thereby increasing the incidence of melanoma.

The MAPK pathway functions as a redundant survival signal that reinforces the PI3K cascade in c-Kit mutant melanoma

Stimulation of the c-Kit receptor tyrosine kinase has a critical role in the development and migration of melanocytes, and oncogenic c-Kit mutants contribute to the progression of some melanomas. c-Kit signalling activates the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways and their relative contribution to the activities of oncogenic and ligand-dependent c-Kit remains uncertain. We show that PI3K is a major regulator of MAPK activation in response to c-Kit activity and the dominant effector of c-Kit-driven melanocyte proliferation and melanoma survival. Nevertheless, inhibition of the PI3K pathway in c-Kit mutant melanoma cells did not replicate the apoptotic efficacy of the c-Kit inhibitor, imatinib mesylate. Instead, the simultaneous suppression of the PI3K and MAPK pathways promoted a strong synergistic apoptotic effect. These data indicate that MAPK functions as a redundant survival signal that reinforces the PI3K cascade in c-Kit mutant melanoma. Thus, the concurrent inhibition of PI3K and MAPK signalling is required to suppress oncogenic c-Kit activity and may provide an effective therapeutic strategy in c-Kit mutant melanomas.