Differential Expression of Phospho‐S6 in Hair Follicle Tumors: Evidence of mTOR pathway activation

Background

The role of the mammalian target of rapamycin (mTOR) in hair follicle tumorigenesis is unclear. mTOR controls cell growth and can be activated through ribosomal S6 kinase. Herein, we sought to evaluate the expression of phospho‐S6 in six different benign and malignant follicular tumor types.

Methods

Seventy‐six cases were selected [17 fibrofolliculomas, 20 trichoepitheliomas, 10 tricholemmomas, 19 pilomatricomas, 1 malignant proliferating tricholemmal tumor, 8 tricholemmal carcinomas and 1 trichoblastic carcinoma] and collected over 16 years. Immunohistochemistry with monoclonal antibody for phospho‐S6 was performed and analyzed semi‐quantitatively; statistical analysis χ² test (p<0.05).

Results

All malignant neoplasms in our series [8/8 (100%) cases of tricholemmal carcinoma, 1/1 (100%) trichoblastic carcinoma and 1/1 (100%) malignant proliferating tricholemmal tumor] demonstrated a strong and diffuse pattern of staining with phospho‐S6 involving 70‐90% of tumor cells. By contrast, a minority of benign tumors were positive for phospho‐S6 and most stained in a patchy pattern including 12/17 (71%) fibrofolliculomas, 9/20 (45%) trichoepitheliomas and 1/10 (10%) tricholemmomas, involving 30‐50%, 5‐20%, and 40‐50% of tumor cells, respectively. Most pilomatricomas [17/19 (89%)] exhibited a stronger, but distinctive staining pattern, staining mostly the basaloid cells with a multifocal distribution, involving 70‐90% of tumor cell.

Conclusions

Phospho‐S6 is differentially expressed among benign and malignant hair follicle tumors (p = 0.0044). While malignant tumors show diffuse expression, only a small subset of benign neoplasms were positive, primarily in a patchy distribution.

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