Abstract
A 17-year-old male presented with a large sebaceous naevus (SN) comprising part of his left face and scalp and a speckled lentiginous naevus (SLN) on his right trunk, hip, neck and scalp with a checkerboard pattern. His right oral hemimucosa showed extensive papillomatous lesions which were contiguous to the upper lip SN lesions. As for extracutaneous manifestations he suffered from cardiac, musculoskeletal and ocular alterations. On the other hand, he developed two primary rhabdomyosarcomas. DNA samples of the SN, SLN, the oral papillomatous hyperplasia and both rhabdomyosarcomas were analyzed by Sanger sequencing. A HRAS c.37G>C mutation was detected in all of them. Skin and blood DNA resulted wild type. PPK is characterized by the association of a sebaceous naevus with a papular naevus spilus and extracutaneous manifestations. Until not long ago, the etiopathogenetical hypothesis of didymosis was accepted. However, in 2013 Groesser et al. proved the existence of an activating HRAS mutation as the cause of this syndrome. A higher incidence of cancer has been observed in germline RASophaties. Furthermore, up to 30% of human cancers show dysregulation of the Ras-Raf-MEK-ERK pathways. In our patient, a HRAS mosaic mutation explains not only the cutaneous but also the extracutaneous manifestations. To our knowledge this is the first described case of PPK in which the existence of a HRAS mosaic mutation is the confirmed cause of rhabdomyosarcoma. Besides the HRAS 37G>C mutation has never been related to any type of rhabdomyosarcoma. Mosaicisms could be underdiagnosed causes of childhood tumours. As dermatologists we stand on a privileged position for the detection of these alterations.
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