Mitf-Mdel, a novel melanocyte/melanoma-specific isoform of microphthalmia-associated transcription factor-M, as a candidate biomarker for melanoma

BACKGROUND Melanoma incidence is on the rise and advanced melanoma carries an extremely poor prognosis. Treatment options, including chemotherapy and immunotherapy, are limited and offer low response rates and transient efficacy. Thus, identification of new melanocyte/melanoma antigens that serve as potential novel candidate biomarkers in melanoma is an important area for investigation. METHODS Full length MITF-M and its splice variant cDNA were cloned from human melanoma cell line 624 mel by reverse transcription polymerase chain reaction (RT-PCR). Expression was investigated using regular and quantitative RT-PCR in three normal melanocytes (NHEM), 31 melanoma cell lines, 21 frozen melanoma tissue samples, 18 blood samples (peripheral blood mononuclear cell; PBMC) from healthy donors and 12 non-melanoma cancer cell lines, including three breast, five glioma, one sarcoma, two kidney and one ovarian cancer cell lines. RESULTS A novel splice variant of MITF-M, which we named MITF-Mdel, was identified. The predicted MITF-Mdel protein contains two in frame deletions, 56- and 6- amino acid deletions in exon 2 (from V32 to E87) and exon 6 (from A187 to T192), respectively. MITF-Mdel was widely expressed in melanocytes, melanoma cell lines and tissues, but almost undetectable in non-melanoma cell lines or PBMC from healthy donors. Both isoforms were expressed significantly higher in melanoma tissues than in cell lines. Two of 31 melanoma cell lines expressed only one isoform or the other. CONCLUSION MITF-Mdel, a novel melanocyte/melanoma-specific isoform of MITF-M, may serve as a potential candidate biomarker for diagnostic and follow-up purposes in melanoma.