PTEN/MMAC1 expression in melanoma resection specimens

Abstract

PTEN/MMAC1, a tumour suppressor gene located on chromosome 10q23.3, has been found to be deleted in several types of human malignancies. As the chromosomal region 10q22-qter commonly is affected by losses in melanomas, we addressed this gene as tumour suppressor candidate in melanomas. Investigating PTEN/MMAC1 expression at mRNA level by semi-quantitative reverse transcription-polymerase chain reaction, we did not find a statistically significant down-regulation in melanoma resection specimens in comparison to acquired melanocytic nevi from which melanomas quite often are known to arise. Upon immunohistochemistry, PTEN/MMAC1 protein expression in melanomas was not lost. Sequencing the PTEN/MMAC1 cDNAs in 26 melanoma resection specimens (21 primary melanomas, five metastases), we detected three point mutations and two nucleotide deletions which did not represent genetic polymorphisms. With respect to the predicted protein sequences, all three point mutations were silent whereas the two frame shifts at the extreme C-terminus resulted in a loss of the putative PDZ-targeting consensus sequence. As loss of this motif possibly impairs localization and function of PTEN/MMAC1 in the two corresponding primary tumours, alterations of this tumour suppressor protein may participate in some melanomas.

Figure 1

Part of the PTEN/MMAC1 cDNA sequences in melanomas in comparison to the wild type sequences. Guanine was found to be exchanged by adenine at nucleotide 1420 in a nodular melanoma, Clark's level III (left). Cytosine was found to be exchanged by thymine at nucleotide 1808 in a nodular melanoma Clark's level II (right).

Figure 2

Immunhistochemical staining of serial tissue sections of a nodular melanoma, tumour thickness according to Breslow 1.4 mm, Clark's level IV. The PTEN/MMAC1 protein was detected in melanoma cells and was not restricted to tumour associated macrophages. (a) Hematoxylin Eosin, (b) HMB45, (c) CD68, (d) PTEN/MMAC1 staining (all magnification 25×, nickel enhanced DAB reaction resulting in black signals).