A 61-year-old Japanese woman presented with an erythematous eruption on the thenar region of her right palm, which had persisted for 5 years. The lesion was asymptomatic and extended gradually without any history of trauma. Physical examination revealed a 9 × 8 mm, well-circumscribed, and slightly depressed erythema on the thenar region of her right palm (figure 1A). Taking the differential diagnosis into consideration, e.g., Bowen's disease or porokeratosis, the lesion was completely excised. Histopathological examination showed depression of the epidermis with a sharp step between the normal and involved skin. The lesional epidermis was thinner and its granular layer was reduced in comparison to adjacent non-involved skin. Epidermal rete ridges were longer and the numbers of dilated capillaries in the papillary dermis were increased, but no lymphocytic infiltration was seen. The epidermal cells were otherwise normal and no cornoid lamella was identified (figure 1B). To investigate the differentiation status of the lesional keratinocytes, immunohistochemical analysis was performed in order to determine the expression of epidermal differentiation marker molecules in the lesional epidermis, including keratin 10 (K10; early differentiation marker), involucrin, filaggrin (intermediate differentiation markers), loricrin (late differentiation marker), keratin 9 (K9; palmoplantar-specific marker), and Ki67 (proliferation marker). The level of K10 expression was markedly decreased in the lesional skin (figures 1C-D). The levels of involucrin, filaggrin and loricrin expression were slightly decreased in the lesional epidermis (figures 1E-G). K9 expression showed no marked differences between lesional and normal skin (figure 1H). We performed staining for Ki-67 and counted Ki-67-positive cells at three arbitrary points at × 400 magnification. The Ki-67-positive basal cells were seen more frequently in the lesional epidermis (32 ± 4.5/visual field) than in the non-lesional epidermis (14 ± 2.3/visual field). No p53-positive keratinocytes were observed and no human papillomavirus (HPV) was detected in the specimens by immunohistochemistry (data not shown).
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