Acanthosis nigricans (AN) can be present in the following forms: inherited, obesity-associated, autoimmune-related, drug-induced, and malignancy-associated. Inherited AN is a heterogeneous disorder, sub-classified into non-syndromic and syndromic forms [1]. AN is characterized by papillomatous brownish lesions mainly in the intertriginous areas, such as the axillae, neck, genital and submammary regions. The non-syndromic form is present at birth or develops during childhood. Wider regions, including almost the entire skin, may be affected in some hereditary cases.

A 12-year-old Japanese woman visited us with asymptomatic aberrant skin color and structure. The abnormalities had been noticed during infancy. She had no other congenital malformations or delays in mental and physiccal development. The proband’s father and her 11-yearold brother had similar abnormal skin color and structure. A physical examination of the proband revealed asymptomatic and hyperpigmented papillomatous lesions over almost the entire body, especially in the intertriginous areas, such as the axillae and the neck (Figure 1(a)). The younger brother showed a similar clinical phenotype (Figure 1(b)). Dermoscopy of the proband’s neck showed linear crista cutis and sulcus cutis (Figure 1(c)). Focal hyperpigmented dots were present in the crista cutis. Dermoscopy of the brother’s neck revealed a similar aberrant skin structure with focal pigmented dots (Figure 1(d)). Unfortunately, we could not verify the origin of focal pigmented dots and precise structure of papillomatous projections with histopathological specimens, as the parents did not agree with biopsies.

Dermoscopic images in two siblings with non-syndromic hereditary AN identified hyperpigmented dots that could not be seen with the naked eyes. The dermoscopic features of linear crista cutis would reflect the papillomatous projections of the dermis. We could not take biopsy specimens, even though histopathologic specimens are essential to confirm the origin of the hyperpigmented dots, i.e. increased melanin granules per melanocytes, an increased number of melanin contents-containing keratinocytes, or some other non-melanin-associated pigmentation.

Syndromic acanthosis nigricans is caused by specific activating mutations in FGFR2 and FGFR3 [1]. FGFR2b, the form expressed in the epidermis, mediates FGF signals to RAS-RAF-MAPK and phosphoinositide-3 kinase signaling cascades. Epidermal nevi represent mosaic forms of syndromic AN (FGFR3) and of neuro-cardio-facial-cutaneous syndromes (HRAS, NRAS, and KRAS), a

group of heterogeneous neurodevelopmental disorders including LEOPARD syndrome and neurofibromatosis type 1 [2,3]. The dots in dermoscopic images are darker brown, suggesting that it might correspond to superficial pigmentation. We speculated that the hyperpigmented spots could develop along similar pathways to lentiginosis in LEOPARD syndrome or café-au-lait spots in neurofibromatosis type 1. The hyperpigmented dots, which could not be seen with the naked eyes, may contribute to the color of the pigmented skin.

With dermoscopy in two siblings, we confirmed hyperpigmented dots and aberrant structure of crista cutis and sulcus cutis. Dermoscopy is useful for evaluating the disorders involving structure of crista cutis and sulcus cutis with a change of color as shown in a case of generalized Dowling-Degos disease [4] or in a variant of linear atrophoderma of Moulin [5].

We reported dermoscopic features in two siblings in a family with inherited non-syndromic AN. Further study with histopathologic correlations of dermoscopic findings in AN should be addressed to recognize more precise pathogenesis.