CPC14 Blaschko-linear trichoepitheliomas: unveiling type 1 segmental mosaicism

Abstract Trichoepitheliomas are benign appendageal tumours with follicular differentiation. They can arise sporadically or may be inherited in an autosomal dominant manner, caused by heterozygous mutations in the CYLD gene. We describe here a 7-year-old boy with multiple clustered trichoepitheliomas over the face in a Blaschko-linear pattern. He presented with multiple firm, asymptomatic, skin-coloured papules with a slight yellowish hue, clustered over left side of the face. The lesions extended from left side of the scalp to the nasolabial fold and continued downward to the left side of the chin and neck. The lesions were present at birth and have been progressively increasing in number and size. The rest of the skin was normal. He was born to a nonconsanguineous couple and none of his relatives were affected. Histopathology of the papule showed a multinodular collection of basaloid cells with hair follicle and squamous cell differentiation with keratin pseudohorn cysts suggestive of trichoepithelioma. Based on the clinical and histological findings, a diagnosis of segmental trichoepitheliomas was made. Trichoepitheliomas are benign tumours derived from follicular germinative cells. They can present as solitary tumours or as multiple familial trichoepitheliomas (MFTs) inherited as part of Brooke–Spiegler syndrome, Rombo syndrome and Basex syndrome, in combination with other tumours. MFTs have been linked to chromosome 9p21, with an autosomal dominant pattern of inheritance and onset in early childhood or puberty. Sporadic trichoepitheliomas are linked to chromosome 9q and are present during the adult years. Blaschko-linear presentation of multiple trichoepitheliomas is rare and has been documented in only two previously reported cases (Chang YC, Colome-Grimmer M, Kelly E. Multiple trichoepitheliomas in the lines of Blaschko. Pediatr Dermatol 2006; 23: 49–151). The linear presentation, in our case, can be explained by a type 1 segmental mosaicism. Linear manifestation suggestive of genetic mosaicism is rare. There are two types of segmental mosaicism described in autosomal dominant disorders. In type 1, a de novo postzygotic somatic mutation in the absence of germline mutation causes a segmental pattern, while in type 2, it is caused by a germline mutation in combination with a postzygotic somatic mutation. Localized lesions are seen in type 1, in contrast to type 2, where segments can be seen in more severe phenotypes (Happle R. Segmental forms of autosomal dominant skin disorders: different types of severity reflect different states of zygosity. Am J Med Genet 1996; 66: 241–2). In our case, no germline or somatic mutation in the CYLD gene was identified, and we hypothesize that the patient’s phenotype represents a type 1 segmental manifestation of MFT.