Stem cell markers immunoexpression in canine cutaneous melanocytic tumours

Nestin and SOX9 were identified as specific markers of melanocytic stem cells. Nestin is a class IV intermediate filament, mainly expressed in the cytoplasm of neuroepithelial stem cells and in developing yet not differentiated endothelial cells of blood vessels. Nestin expression was increased in numerous cancers. In many tumors, such as melanoma, nestin has been identified as a prognostic factor. Transcription factors belonging to the SOX family have been shown to have a role in the survival and migration of oligodendrocyte precursors. A SOX family protein, SOX9, has been demonstrated to interact with SOX10 and BRN2 in melanocytic differentiation and to be strongly expressed in pigmented cells of cultured melanomas. Nestin, SOX9, BRN2 and SOX10 were found to be strongly expressed in primary and metastatic melanomas in humans, while the levels of expression of these molecules in melanocytic nevi were much lower. In particular nestin and SOX9, respectively, were associated with the presence of ulcerations in primary tumors and with a more advanced stage of disease progression and therefore considered as negative prognostic markers. In oure study, with few exceptions, almost all melanocytic neoplasms investigated showed an absent or very low reactivity of neoplastic cells for both markers used. However the positivity of both external and internal positive controls used (hair follicles) confirmed the validity of these markers as putative stem cell markers in the dog, similar to what is described in humans. In particular, SOX-9 positivity was present in <5% of neoplastic cells in only three cases, while nestin immunoreactivity was noted in 5 cases. Three of these five cases, all represented by melanomas, were characterized by a percentage of positive neoplastic cells ranging from 40 to 60%. In general, the nestin-positive cells were located at the periphery (the invasive front) of the tumor. In one of these cases (metastatic melanoma), the reactivity was present both in the primary tumor as well as its metastases (lung, pancreas, adrenal). None of melanocytomas was positive for nestin. In conclusion, the results of this study in the dog, different from what is described in human medicine, dampen the use of nestin and SOX9 as valid prognostic negative markers in canine melanocytic tumors. However, the positivity for nestin in rare cases of melanoma, in the face of a total negativity for this marker in melanocytomas, would lead to speculate that this molecule may be involved in the process of malignant transformation of melanocytic cells. Further studies are needed to investigate this hypothesis.