Nevus sebaceous is a common congenital cutaneous malformation. Affected individuals may develop benign and malignant secondary tumors in the nevi during life. Schimmelpenning syndrome is characterized by the association of nevus sebaceous with extracutaneous abnormalities. We report that of 65 sebaceous nevi studied, 62 (95%) had mutations in the HRAS gene and 3 (5%) had mutations in the KRAS gene. The HRAS c.37G>C mutation, which results in a p.Gly13Arg substitution, was present in 91% of lesions. Nonlesional tissues from 18 individuals had a wild-type sequence, confirming genetic mosaicism. The HRAS c.37G>C mutation was also found in 8 of 8 associated secondary tumors. Mosaicism for HRAS c.37G>C and KRAS c.35G>A mutations was found in two individuals with Schimmelpenning syndrome. Functional analysis of HRAS c.37G>C mutant cells showed constitutive activation of the MAPK and PI3K-Akt signaling pathways. Our results indicate that nevus sebaceous and Schimmelpenning syndrome are caused by postzygotic HRAS and KRAS mutations. These mutations may predispose individuals to the development of secondary tumors in nevus sebaceous.

Phacomatosis pigmentokeratotica (PPK) is a rare epidermal nevus syndrome characterized by the co-occurrence of a sebaceous nevus and a speckled lentiginous nevus. The coexistence of an epidermal and a melanocytic nevus has been explained by two homozygous recessive mutations, according to the twin spot hypothesis, of which PPK has become a putative paradigm in humans. However, the underlying gene mutations remained unknown. Multiple tissues of six patients with PPK were analyzed for the presence of RAS, FGFR3, PIK3CA, and BRAF mutations using SNaPshot assays and Sanger sequencing. We identified a heterozygous HRAS c.37G>C (p.Gly13Arg) mutation in four patients and a heterozygous HRAS c.182A>G (p.Gln61Arg) mutation in two patients. In each case, the mutations were present in both the sebaceous and the melanocytic nevus. In the latter lesion, melanocytes were identified to carry the HRAS mutation. Analysis of various nonlesional tissues showed a wild-type sequence of HRAS, consistent with mosaicism. Our data provide no genetic evidence for the previously proposed twin spot hypothesis. In contrast, PPK is best explained by a postzygotic-activating HRAS mutation in a multipotent progenitor cell that gives rise to both a sebaceous and a melanocytic nevus. Therefore, PPK is a mosaic RASopathy.

Our results indicate that FGFR3, PIK3CA and RAS mutations are present in approximately 50% of BLKs. These findings support the concept on the molecular genetic level that at least a proportion of BLKs represents regressive variants resulting from former benign epidermal tumours such as seborrhoeic keratosis and solar lentigo.

Epidermal growth factor receptor (EGFR) is highly expressed in squamous cell carcinoma (SCC). The response of patients with lung cancer to EGFR inhibitors is significantly associated with the presence of EGFR mutations. Although these drugs have already been used for the treatment of advanced cutaneous SCC, the knowledge about EGFR mutations in this cancer is limited to one previous study in the US population. We analysed the presence of EGFR and concomitant HRAS mutations in a German cohort of 31 patients with cutaneous SCC by direct sequencing of EGFR and SNaPshot analysis of concomitant RAS mutations. We found a low prevalence of EGFR mutations (1 ⁄ 31; 3%) and HRAS mutations (1 ⁄ 31; 3%). The detected P741L EGFR mutation was proven to be somatic. Our results indicate that both EGFR and HRAS mutations are rare events in the carcinogenesis of cutaneous SCC, and therefore, only a small subgroup of patients will benefit from the screening for EGFR mutations in the run-up to targeted therapies with EGFR inhibitors.

Seborrheic keratosis (SK) represents a frequent epidermal skin tumor. Although lacking a malignant potential, these tumors reveal multiple oncogenic mutations. A previous study identified activating mutations in 89% of SK, particularly in FGFR3 and PIK3CA genes. The aim of this study was to identify further oncogenic mutations in human SK. Therefore, we screened for mutations in EGFR, FGFR2, PIK3R1, HRAS, KRAS, and NRAS genes using both Sanger sequencing of selected exons and a multiplex SNaPshot assay in 58 SK of 14 patients. We identified a somatic EGFR p.L858R mutation in 1 SK. Furthermore, the HRAS mutations p.G13R (2/58 SK) and p.Q61L (2/58 SK) were found. These mutations have not been described in human SK yet. In addition, 1 SK revealed the KRAS p.G12V mutation, which has already been reported in SK. No mutations were detected in FGFR2, PIK3R1, and NRAS genes. The results of this study suggest that activating mutations of EGFR, HRAS, and KRAS contribute to the pathogenesis of human SK, although at a lower frequency than FGFR3 and PIK3CA mutations. FGFR2, PIK3R1, and NRAS mutations obviously do not have a significant role in the development of SK.