Pronounced linear calcinosis in a boy with mild dermatomyositis. A further possible example of superimposed segmental manifestation of a polygenic disorder.

A pronounced linear eruption with ulcerations and calcium extrusion present in a boy with a mild generalized rash clinically consistent with juvenile dermatomyositis or overlap syndrome is reported. Loss of heterozygosity (LOH) is a postzygotic mechanism by which a heterozygous somatic cell may become homozygous or hemizygous at a given gene locus. Such a mechanism can be suspected when a pronounced segmental manifestation of an acquired skin condition with a polygenic background is found to be superimposed on more or less symmetrically distributed nonsegmental lesions of the same disorder. Alternatively, such a segmental manifestation may reflect heterozygosity for a postzygotic mutation involving an additional gene locus. The severe linear lesions in our patient showed a Blaschko-linear arrangement and were superimposed on mild nonsegmental lesions of either amyopathic dermatomyositis or overlap syndrome. Either LOH or a postzygotic mutation at an additional gene locus may explain the pronounced linear involvement.