Segmental manifestation of Darier disease. What is the genetic background in type 1 and type 2 mosaic phenotypes?

Darier disease is an autosomal dominant disorder which may occasionally become manifest in a segmental form. Two clinical phenotypes with a different genetic background have been elaborated in recent years. More than 50 patients with isolated linear disease expression have been documented. In this phenotype the skin outside the segmental affection is absolutely normal. Such a phenotype is explained by a postzygotic mutation with somatic mosaicism which was labeled as type 1 manifestation of segmental forms in autosomal skin disorders. A patient with classical type 1 segmental Darier disease is presented. On the other hand, only 3 patients with Darier disease showing a segmental manifestation in combination with a diffuse distribution have so far been observed. These cases correspond to the recently described type 2 manifestation of segmental forms of autosomal dominant disorders. We describe a fourth patient with type 2 segmental Darier disease. The genetic explanation of such a phenotype is possible with the assumption that a germline mutation for the disease exists but, in addition, a postzygotic mutation is needed resulting in loss of heterozygosity. Hence, in a circumscribed region a homozygous or hemizygous state of the mutation is apparent which can explain the enhanced severity of the segmental manifestation.