Darier disease with paired segmental manifestation of either excessive or absent involvement: a further step in the concept of twin spotting.

For the first time, we describe a case of type 2 segmental Darier disease with concomitant band-like areas of healthy skin. This clinical observation gives a further hint for the understanding of type 2 segmental manifestations in autosomal dominant diseases. We had observed a 17-year-old patient with Darier disease since the age of 13 years. On the frontal aspect of his body, the lesions were found to be diffusely and rather symmetrically disseminated. On the back, however, a band-like pattern of pronounced involvement with concomitant streaks of healthy skin, both following the lines of Blaschko, was noted. Type 2 segmental manifestation of autosomal dominant disorders can be explained by the assumption that the individual carries a germline mutation that gives rise to a diffuse, nonsegmental distribution of the disease. In addition, a postzygotic mutation occurring at an early developmental stage would result in loss of heterozygosity and give rise, in a segmental area, to a homozygous or hemizygous state of the mutation. This would explain the enhanced severity of the segmental lesions. Theoretically, an early event of mitotic recombination should give rise, simultaneously, to a clone of cells that are homozygous for the corresponding wild-type allele, and for this reason paired segmental areas of either excessive or absent involvement, in the form of twin spotting, should occur on the background of an ordinary, nonsegmental phenotype, as exemplified by Happle and König in a case of epidermolytic hyperkeratosis of Brocq. These authors stated that, in autosomal dominant skin disorders, segmental areas of healthy skin will usually be difficult to recognize. This may explain why such a twin spot phenomenon has so far not been encountered in Darier disease.