Clinical and molecular features of subungual melanomas are site-specific and distinct from acral melanomas

Subungual melanomas (SUM) arise beneath the nails of the hands and feet, and account for 0.7-3.5% of all malignant melanomas. Most studies include SUM in the category of acral melanoma, but understanding the specific features of SUM is critical for improving patient care. In this study, we performed a site-specific comparison of the clinical and molecular features between 54 cases of SUM and 78 cases of nonsubungual acral melanoma. Compared to patients with acral melanoma, patients with SUM were younger at diagnosis, had a higher prevalence of primary melanomas on the hand, and had more frequent reports of previous trauma at the tumor site. SUM was deeper than acral melanoma at diagnosis, which correlated with an increased frequency of metastases. Analysis of common melanoma driver genes revealed KIT and KRAS mutations were predominantly found in SUM, whereas BRAF and NRAS mutations occurred almost exclusively in acral melanoma. We also discovered molecular differences in the cell cycle pathway, where CDK4/CCND1 amplifications were more frequent in SUM and CDKN2A/B loss occurred mostly in acral melanoma, and in the PI3K/mTOR pathway, where RICTOR amplification and TSC1 K587R mutations were exclusively in SUM and PTEN loss and AKT1 mutations were exclusively in acral melanoma. Comparison of hand versus foot tumors revealed more frequent ulceration of SUM foot tumors, which correlated with more distal metastases and poorer overall survival. In summary, we find SUM are both clinically and molecularly distinct from acral melanoma, and our data suggest KIT, CDK4/6, and mTOR inhibitors may be particularly relevant and effective treatments for patients with SUM.