Loss of dipeptidyl peptidase IV immunostaining discriminates malignant melanomas from deep penetrating nevi.

The deep penetrating nevus is a rare variant of benign melanocytic nevus with histologic features mimicking vertical growth phase, nodular malignant melanoma. In this study, we expand on the search for new complementary discriminating markers by analyzing a selection of both cell cycle-related factors, such as retinoblastoma protein and phospho-retinoblastoma protein Ser795 as indicators for retinoblastoma protein activation/inactivation status, and invasion-related factors, such as matrix metalloproteinase-1, matrix metalloproteinase-2, membrane-type matrix metalloproteinase-1 and integrin beta3. MIB-1/Ki-67 was analyzed as an example for a common proliferation marker. Dipeptidyl peptidase IV/CD26 was analyzed as a marker affecting both proliferation and invasion of malignant melanocytic tumors. Semiquantitative assessment of both immunolocalization and immunoreactivity of retinoblastoma protein and phospho-retinoblastoma protein Ser795, MIB-1/Ki-67, matrix metalloproteinase-1, matrix metalloproteinase-2, membrane-type matrix metalloproteinase-1 and integrin beta3 revealed no consistent differences between deep penetrating nevi (n=14) and matched cases of nodular malignant melanomas (n=10). Matrix metalloproteinase-1 and matrix metalloproteinase-2 immunostaining of some deep penetrating nevi even exceeded that of nodular malignant melanomas. Membrane-type matrix metalloproteinase-1 expression scores of nodular malignant melanomas were higher than those of deep penetrating nevi, which was, however, not significantly discriminative. In contrast, immunostaining of dipeptidyl peptidase IV was significantly discriminative due to a consistent lack of dipeptidyl peptidase IV-expression in nodular malignant melanomas. These results add evidence that among the selected markers supposed to be relevant for melanoma progression the presence of dipeptidyl peptidase IV can be used to support diagnosis of deep penetrating nevi in doubtful cases. As loss of dipeptidyl peptidase IV may also be causally linked to the transition of invasive to metastatic phenotypes, the molecular mechanisms downstream of dipeptidyl peptidase IV deserve to be studied in more detail in future investigations.