Superimposed mosaicism in tuberous sclerosis complex: a key to understanding all of the manifold manifestations?

Abstract

In patients with tuberous sclerosis, we can today distinguish between two different categories of segmental mosaicism. The well-known simple segmental mosaicism is characterized by a unilateral or otherwise localized arrangement of the ordinary lesions of the disorder, reflecting heterozygosity for an early postzygotic new mutation. By contrast, superimposed mosaicism is defined by a pronounced segmental involvement in a patient with ordinary non-segmental lesions of the same disorder, resulting in a heterozygous embryo from loss of the corresponding wild-type allele that occurred at a very early developmental stage. So far, the second category has been called 'type 2 segmental mosaicism', but here we propose the short and unambiguous term 'superimposed mosaicism'. In order to render physicians familiar with the manifold manifestations of this category as noted in tuberous sclerosis, we review the following clinical designations under which cases suggesting superimposed mosaicism have been published: forehead plaque; shagreen patch; fibrous cephalic plaque; fibromatous lesion of the scalp; folliculocystic and collagen hamartoma; segmental hypomelanosis; congenital segmental lymphedema; and segmental 'diffuse' lipomatosis. Molecular corroboration of this genetic concept has been provided in a case of forehead plaque and in a child with shagreen patch. - Extracutaneous manifestations suggesting superimposed mosaicism include columnar tuberous brain defects; 'radial migration lines' or 'cerebral white matter migration lines' as noted by brain imaging; linear hamartomatous lesions of the tongue; fibrous dysplasia of bones including macrodactyly; and unilateral overgrowth of an arm or leg. - Remarkably, superimposed mosaicism appears to occur in tuberous sclerosis far more frequently than simple segmental mosaicism.