Phacomatosis Pigmentokeratotica: A Mosaic RASopathy with Malignant Potential.

Phacomatosis pigmentokeratotica (PPK) is a rare epidermal nevus syndrome characterized by the co-occurrence of a nevus sebaceous arranged along the lines of Blaschko with a speckled lentiginous nevus (SLN). We report a novel KRAS mutation in a patient with a large nevus sebaceous and an SLN who subsequently developed a vaginal botryoid rhabdomyosarcoma, an association not previously reported in the literature. This case expands our knowledge of the genetic basis for phacomatosis, in which mutations in HRAS have been previously described, although this report provides evidence that activating mutations in KRAS or HRAS may cause PPK. This report confirms that PPK is a mosaic RASopathy with malignant potential and raises the question of whether screening for other RAS-associated malignancies should be performed for all children with PPK.