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Characterization of methylthioadenosin phosphorylase (MTAP) expression in malignant melanoma
Iris Behrmann1, Susanne Wallner, Waraporn Komyod
1Institute of Biochemistry, RWTH-Aachen, Aachen, Germany.
Abstract:
Homozygous deletions of human chromosomal region 9p21 occur frequently in malignant melanoma and are associated with the loss of the tumor suppressor genes p16(INK4a) and p15(INK4b). In the same chromosomal region the methylthioadenosine phosphorylase (MTAP) gene is localized and therefore may also serve as a tumor suppressor gene. The aim of this study was to analyze MTAP mutations and expression patterns in malignant melanomas. To examine the MTAP gene and expression of MTAP protein we screened 9 human melanoma cell lines and primary human melanocytes by reverse transcriptase-polymerase chain reaction, sequencing, and immunoblotting. Analyzing the melanoma cell lines we found significant down-regulation of MTAP mRNA expression. In only one cell line, HTZ19d, this was due to homozygous deletion of exon 2 to 8 whereas in the other cell lines promoter hypermethylation was detected. MTAP expression was further analyzed in vivo by immunohistochemical staining of 38 tissue samples of benign melanocytic nevi, melanomas, and melanoma metastases. In summary, we demonstrate significant inverse correlation between MTAP protein expression and progression of melanocytic tumors as the amount of MTAP protein staining decreases from benign melanocytic nevi to metastatic melanomas. Our results suggest an important role of MTAP inactivation in the development of melanomas. This finding may be of great clinical significance because recently an association between MTAP activity and interferon sensitivity has been suggested.
Insights
Methylthioadenosine phosphorylase (MTAP) is frequently inactivated in melanoma, with its loss correlating to tumor progression. This suggests MTAP inactivation plays a key role in melanoma development and may impact interferon sensitivity.
Area of Science:
- Oncology
- Molecular Biology
- Genetics
Background:
- Malignant melanoma frequently exhibits homozygous deletions at chromosomal region 9p21.
- This region harbors tumor suppressor genes p16(INK4a) and p15(INK4b), and also the methylthioadenosine phosphorylase (MTAP) gene.
- The MTAP gene's location suggests its potential role as a tumor suppressor in melanoma.
Purpose of the Study:
- To investigate mutations and expression patterns of the MTAP gene in malignant melanomas.
- To determine the role of MTAP inactivation in melanoma development.
Main Methods:
- Analysis of 9 human melanoma cell lines and primary melanocytes using reverse transcriptase-polymerase chain reaction (RT-PCR) and sequencing.
- Immunoblotting to assess MTAP protein expression.
- Immunohistochemical staining of 38 tissue samples (benign nevi, melanomas, metastases) to evaluate MTAP protein levels in vivo.
Main Results:
- Significant down-regulation of MTAP mRNA expression was observed in melanoma cell lines.
- Homozygous deletion of MTAP (exon 2-8) occurred in one cell line; promoter hypermethylation was found in others.
- A significant inverse correlation was found between MTAP protein expression and melanoma progression, with decreased staining from benign nevi to metastatic melanomas.
Conclusions:
- MTAP inactivation plays a significant role in the development of melanomas.
- The observed inverse correlation between MTAP expression and tumor progression highlights its potential as a tumor suppressor.
- MTAP inactivation may have clinical implications, particularly regarding interferon sensitivity in melanoma patients.