These results demonstrate the presence and localization of a specialized nail mesenchyme containing onychofibroblasts in a well-defined area beneath the nail matrix and nail bed. Thus, we propose the term onychodermis for this specialized nail mesenchyme that is histologically and immunohistochemically distinct from the dermis of other parts of the nail unit.

The proportion of acral melanoma (AM) is much higher in Asian populations than in Caucasian populations. Although mutational profiles associated with AM have been discovered in Caucasian populations, knowledge of its genetic alterations in Asian populations is limited. To describe the molecular nature of AM in Korean patients, we performed mutational profiling of AM and matched normal tissues in patients. Fifty-one formalin-fixed paraffin-embedded AM samples and 32 matched pairs from patients' saliva DNA were analysed by next-generation sequencing. Only mutations confirmed via digital droplet PCR or in BRAF, KIT and NRAS, the most frequently altered cancer genes in cutaneous melanoma, were considered as positive. The relationship between mutational status and clinicopathological features were examined. Of the 47 AM patients screened, alteration of BRAF, NRAS and KIT genes was observed in 6.4%, 4.3% and 8.5%, respectively. We also tested matched normal tissues of patients to identify tumor-specific mutations. Examination of the mutational profile in a cohort of 28 primary melanomas and matched normal controls found BRAF mutations in two cases (7.1%), KIT mutations in three cases (10.7%) and CTNNB1 mutations in one case (3.6%). The BRAF, NRAS and KIT mutation status did not correlate with clinicopathological characteristics. Our results show that KIT, NRAS and BRAF hotspot mutations occur at a low frequency in Korean populations. We also observed a case with the CTNNB1 mutation, which raises the possibility that other pathways are associated with AM development.

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β-catenin plays an important role in hair morphogenesis. Previously, the nuclear and cytoplasmic localizations of β-catenin were identified in hair-matrix cells. To evaluate β-catenin expression in the nail matrix, we obtained human nail units. Immunohistochemistry for β-catenin was used to evaluate sections of normal nail units and of sections from a single case of onychomatricoma. In the nail unit, β-catenin was expressed in the nucleus and cytoplasm of the suprabasal nail-matrix cells. Of the other epithelial-cell types, only the cell membrane was β-catenin-positive. In the nail tissue from the onychomatricoma case, β-catenin was expressed in the nucleus and cytoplasm of the upper epithelial layers. Our result suggests that β-catenin plays an important role in nail formation. In addition, β-catenin expression in onychomatricoma supports the presence of nail-matrix cells in this condition. To our knowledge, this is the first report of β-catenin expression in the nucleus and cytoplasm of the nail matrix.