L576P KIT mutation in anal melanomas correlates with KIT protein expression and is sensitive to specific kinase inhibition

Abstract

Activating mutations in either BRAF or NRAS are seen in a significant number of malignant melanomas, but their incidence appears to be dependent to ultraviolet light exposure. Thus, BRAF mutations have the highest incidence in non-chronic sun damaged (CSD), and are uncommon in acral, mucosal and CSD melanomas. More recently, activating KIT mutations have been described in rare cases of metastatic melanoma, without further reference to their clinical phenotypes. This finding is intriguing since KIT expression is downregulated in most melanomas progressing to more aggressive lesions. In this study, we investigated a group of anal melanomas for the presence of BRAF, NRAS, KIT and PDGFRA mutations. A heterozygous KIT exon 11 L576P substitution was identified in 3 of 20 cases tested. The 3 KIT mutation-carrying tumors were strongly immunopositive for KIT protein. No KIT mutations were identified in tumors with less than 41 KIT immunostaining. NRAS mutation was identified in one tumor. No BRAF or PDGFRA mutations were identified in either KIT positive or negative anal melanomas. In vitro drug testing of stable transformant Ba/F3 KIT L576P mutant cells showed sensitivity for dasatinib (previously known as BMS-354825), a dual SRC/ABL kinase inhibitor, and imatinib. However, compared to an imatinib-sensitive KIT mutant, dasatinib was potent at lower doses than imatinib in the KIT L576P mutant. These results suggest that a subset of anal melanomas show activating KIT mutations, which are susceptible for therapy with specific kinase inhibitors. ' 2007 Wiley-Liss, Inc.Key words: melanoma; anal; mucosal; KIT; NRAS; BRAF mutations The KIT receptor tyrosine kinase plays a critical role in the normal development and function of the melanocytes during embryonic stage and in the postnatal organism.1,2 Loss-of-function mutations in KIT and its ligand result in white spotting phenotype in mice and humans. [3][4][5] In contrast, activating KIT mutations have been implicated in the pathogenesis of certain human malignancies, such as gastrointestinal stromal tumors, seminomas, mastocytomas, but were not reported until recently in patients with malignant melanoma. Furthermore, KIT expression, typically found in normal melanocytes, benign nevi and in situ melanoma, appears to be downregulated in invasive and metastatic melanomas.6 This progressive decrease in KIT immunopositivity is associated with increasing dermal invasiveness of the tumor. One hypothesis is that KIT signaling may be important in regulating cell differentiation and tissue morphogenesis, and therefore loss of its expression may be a required step in the course of tumor progression. 7 In this context, isolated reports of activating KIT mutations in melanoma patients, 8,9 are intriguing and suggest an alternative mechanism, in which constitutive activation of KIT through oncogenic mutations positively regulate mitogenesis and induces neoplastic transformation.The genetic alterations identified in melanomas at different sites and with different levels of sun exposure indi...

KIT mutations in ocular melanoma: frequency and anatomic distribution

“…[6][7][8][9]14,15 Most of these reported mutations are predicted to be sensitive to tyrosine kinase inhibitors. Indeed, clinical studies using imatinib in melanoma patients with confirmed KIT-activating mutations have yielded promising results.…”

“…The PDGFRA gene, like KIT, is located at chromosome 4q12, which is a region that displays copy number variation in melanoma. 9 Although no PDGFRA mutations have been reported to date in melanoma, 7,9,26,27 no data are available for the ocular subtype. While we did not identify any PDGFRA mutations in exons 12 or 18, we did identify an intron 18-point mutation in one CD117-positive choroidal melanoma.…”

“…Our results suggest that PDGFRA mutations are extremely rare in ocular melanoma, which is in general agreement with the data from other melanoma subtypes. 7,9,26,27 The utility of CD117 staining as a screen for KIT mutational status in ocular melanoma appears to be limited. Although 6 of 7 KIT mutated melanomas were CD117 positive in our study, the correlation failed to reach statistical significance.…”

“…Subsequently, others confirmed the presence of KIT mutations in the invasive portion of primary malignant melanomas and in their metastatic deposits. [7][8][9] These findings suggested that targeted molecular therapy with tyrosine kinase inhibitors could be successful for melanoma, which is a notoriously difficult malignancy to treat. Indeed, promising results have emerged from clinical studies investigating the use of imatinib for melanoma patients with confirmed KIT-activating mutations.…”

“…Acral, mucosal, and chronic sun damaged skin melanomas have been shown to harbor KIT mutations while non-chronic sun damaged skin melanomas generally do not since they are often characterized by BRAF or NRAS mutations. [7][8][9]14,15 The frequency of KIT-activating mutations in specific melanoma subtypes is relatively low with reports of 15% for acral, 19% for mucosal, and 17% for chronic sun damaged skin melanomas. 15,16 Ocular melanoma, although a rare melanoma subtype, does account for the majority of all intraocular malignancies.…”