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Background: Eccrine porocarcinoma (EPC) is a rare skin cancer arising from the eccrine sweat glands. Due to the lack of effective therapies, metastasis is associated with a high mortality rate. Objectives: To investigate the drivers of EPC progression. Methods: We carried out genomic and transcriptomic profiling of metastatic EPC (mEPC), validation of the observed alterations in an EPC patient-derived cell line, confirmation of relevant observations in a large patient cohort of 30 tumour tissues, and successful treatment of a patient with mEPC under the identified treatment regimens. Results: mEPC was characterized by a high tumour mutational burden (TMB) with an ultraviolet signature, widespread copy number alterations and gene expression changes that affected cancer-relevant cellular processes such as cell cycle regulation and proliferation, including a pathogenic TP53 (tumour protein 53) mutation, a copy number deletion in the CDKN2A (cyclin dependent kinase inhibitor 2A) region and a CTNND1/PAK1 [catenin delta 1/p21 (RAC1) activated kinase 1] gene fusion. The overexpression of EGFR (epidermal growth factor receptor), PAK1 and MAP2K1 (mitogen-activated protein kinase kinase 1; also known as MEK1) genes translated into strong protein expression and respective pathway activation in the tumour tissue. Furthermore, a patient-derived cell line was sensitive to EGFR and MEK inhibition, confirming the functional relevance of the pathway activation. Immunohistochemistry analyses in a large patient cohort showed the relevance of the observed changes to the pathogenesis of EPC. Our results indicate that mEPC should respond to immune or kinase inhibitor therapy. Indeed, the advanced disease of our index patient was controlled by EGFR-directed therapy and immune checkpoint inhibition for more than 2 years. Conclusions: Molecular profiling demonstrated high TMB and EGFR/MAPK pathway activation to be novel therapeutic targets in mEPC.

Summary Background Galli–Galli disease (GGD) is a rare genodermatosis. Its clinical presentation is identical to that of Dowling–Degos disease (DDD), but the presence of the histopathological feature of acantholysis in GGD is thought to distinguish the two disorders. Mutations in the keratin 5 gene (KRT5) have been identified in the majority of patients with DDD and in a small number of patients with GGD. Objectives To provide further support for the hypothesis that GGD is merely a variant of DDD, and to examine whether acantholysis is genuinely rare in DDD or rather a common but under-reported histological feature of DDD. Methods We conducted the first systematic mutational investigation of patients with GGD and re-examined the histopathology of patients previously assigned a diagnosis of DDD. For the mutational investigation, KRT5 was sequenced in seven unrelated patients with clinically and histopathologically confirmed GGD. In addition, the histopathological findings of six patients with DDD were re-evaluated. Results The mutation c.418dupA was found in five patients with GGD. The typical histopathological features of GGD were identified in six patients who had previously been assigned a diagnosis of DDD. Conclusions We found further evidence to suggest that GGD is indeed a variant of DDD and not a distinct disease entity. Two facts in particular support this conclusion: the same KRT5 mutation was found in patients with GGD and in patients with DDD, and acantholysis seems to be present in a large number of patients who had previously been assigned a diagnosis of DDD.

The histopathological differentiation between well-differentiated carcinomas and atypical adenomas of sweat gland origin may be difficult, even if immunohistochemical methods are used. Therefore, additional techniques may be helpful. We previously demonstrated that DNA image cytometry (ICM-DNA) can be useful in distinguishing between malignant and benign clear cell hidradenoma. In the present study, a larger series of sweat gland tumours, with a clear-cut diagnosis as malignant or benign on histopathological criteria, was examined by ICM-DNA. Enzymatic cell separation specimens were prepared from paraffin-embedded tissues of 18 sweat gland carcinomas (14 porocarcinomas, one classic eccrine adenocarcinoma, two microcystic adnexal carcinomas and one mostly ductal apocrine carcinoma) and 47 benign sweat gland tumours (three syringocystadenomas, five spiradenomas, 14 cylindromas, three syringomas, seven nodular hidradenomas, 10 cutaneous mixed tumours, four poromas and one apocrine hidrocystoma). Specimens were examined by ICM-DNA according to the current recommendations of the European Society for Analytical Cellular Pathology with the AutoCyte QUIC-DNA workstation using mesenchymal cells as an internal reference. DNA aneuploidy was detected by the stemline interpretation according to Bocking and/or at least three 5[c]-exceeding events. DNA aneuploidy was detected in 16 of 18 (89%) of the sweat gland carcinomas, but in none of the 47 adenomas. These results suggest that the detection of DNA aneuploidy in sweat gland tumours using ICM-DNA is a clear and specific indicator of prospective malignancy.