Expression of S100A6 in the differentiation of keratinocytic tumors of skin

Objective: We studied expression of S100A6 and its pattern to see its role in differentiation of various keratinocytic tumors. Methodology: Immunohistochemistry was performed using streptavidin-biotin peroxidase technique with the polyclonal antibody for S100A6. The expression was based on percentage of tumor cells showing cytoplasmic positivity and the cytoplasmic staining intensity. Only cytoplasmic expression was considered as positive. Results: All the types of keratinocytic tumors showed positivity for S100A6 except basal cell carcinoma (BCCs). Among SCCs, higher expression of S100A6 was seen in well-differentiated squamous cell carcinoma (SCCs) as compared to poorly-differentiated ones. Only basosquamous BCCs i.e. BCCs with focal areas of squamous differentiation showed S100A6 positivity. In benign keratinocytic lesions including acanthomas and verrucas, mature squamous cells were found positive with no significant difference in pattern of staining. The precancerous lesion i.e. actinic keratosis and Bowen's disease showed non-specific S100A6 staining. Conclusions: S100A6 is consistently expressed in mature squamoid cells of keratinocytic tumors with a sensitivity of 98%, if we exclude BCCs.