Atypical melanocytic nevi with segmental distribution resembling seborrheic keratoses

A 66-year-old Japanese man had skin lesions on the left side of his trunk and the left upper extremity for approximately 10 years. The skin lesions were asymptomatic but increased gradually, which brought the patient to our hospital. On the initial examination, we noticed 5 mm x 5 mm to 15 mm x 5 mm, round or oval, light to dark brown keratotic papules on the left side of chest and abdomen and the left upper extremity. The papules were aligned in an S-shaped line on the trunk and in a straight line on the upper extremity. Clinical and histopathological findings led to a diagnosis of seborrheic keratosis that followed Blaschko's lines. To our knowledge, no such a case has been reported previously. Our case supports the hypothesis that seborrheic keratosis can be associated with genetic mosaicism.

We previously reported a single case of agminated acquired melanocytic nevi, consisting of a localized clustering of banal and atypical moles. We now report 4 more cases, confirming that the initial case was not an isolated finding. We examined the lesions clinically, with a dermoscope, with a Wood's light, and in 3 cases with UV photography so as to exclude nevus spilus from the differential diagnosis. The presence of an underlying dysplastic nevus syndrome phenotype in 4 of the 5 cases raises the possibility that agminated nevi arise as a consequence of postzygotic loss of heterozygosity and, thus, may represent a type 2 segmental manifestation of the atypical mole syndrome phenotype. Further studies of similar cases using microdissection techniques for analysis of loss of heterozygosity pattern are warranted.