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

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.

Hornstein-Knickenberg syndrome vs. Birt-Hogg-Dubé syndrome: a critical review of an unjustified designation.

The disorder that is presently called 'Birt-Hogg-Dubé syndrome' was in reality delineated in 1975 by Otto P. Hornstein and Monika Knickenberg from Erlangen (Germany) who emphasized that the occurrence of multiple 'perifollicular fibromas' represented a distinct autosomal dominant trait heralding extracutaneous cancer proneness. By contrast, Arthur R. Birt, Georgina R. Hogg and W. James Dubé from Winnipeg, Manitoba, Canada, claimed in 1977 that they had discovered 'a previously unrecognized hereditary pilar hamartoma' for which they proposed the name 'fibrofolliculoma', to be distinguished from the perifollicular fibromas as reported by Hornstein and Knickenberg. Today, many authors believe that 'fibrofolliculoma' is identical with 'perifollicular fibroma', but for the purpose of the present article this question can be left open. More importantly, the Canadian authors did not mention any association with extracutaneous cancer proneness within the large family examined in Winnipeg, nor when discussing the report from Erlangen, which means that they have neither described nor redescribed the syndrome that presently bears their names. Hence, the autosomal dominant disorder of multiple perifollicular fibromas heralding proclivity to extracutaneous cancer should be called after the original authors, Hornstein-Knickenberg syndrome.

Mosaicism due to postzygotic mutations in women with focal dermal hypoplasia.

Focal dermal hypoplasia (FDH, Goltz syndrome, MIM #305600) constitutes a rare multisystem genetic disorder of the skin, skeleton, teeth and eyes with considerable variation in the clinical features. FDH is transmitted as an X-linked dominant trait and is caused by mutations in PORCN. In male children, hemizygous PORCN mutations are lethal in utero. Around 300 cases have been reported in the literature to date. About 10% of them are male patients presenting with either Klinefelter syndrome (karyotype 47, XXY) or mosaicism of a postzygotic mutation. Here we describe four cases of women with typical features of FDH, in whom a PORCN mutation was found in DNA from affected cutaneous tissue but not in DNA from peripheral blood. This study suggests that mosaicism caused by a postzygotic mutation occurs more often than assumed to date in female patients with FDH. A negative analysis performed on peripheral blood DNA does not exclude the diagnosis of FDH and it is therefore of practical importance to analyse DNA from the affected skin in order to identify low-level mosaicism and thus to improve diagnostic precision. In total, we found two missense variants, one novel indel and one novel splice-site variant. Individuals harbouring postzygotic mosaicism run a risk of transmitting the disorder to their daughters, because the maternal mosaic could also affect the gonads.

The concept of type 2 segmental mosaicism, expanding from dermatology to general medicine.

In autosomal dominant skin disorders, the well-known type 1 segmental mosaicism reflects heterozygosity for a postzygotic new mutation. By contrast, type 2 segmental mosaicism originates in a heterozygous embryo from an early postzygotic mutational event giving rise to loss of the corresponding wild-type allele, which results in a pronounced segmental involvement being superimposed on the ordinary, non-segmental phenotype. Today, this concept has been proven by molecular analysis in many cutaneous traits. The purpose of this review was to seek publications of cases suggesting an extracutaneous manifestation of type 2 segmental mosaicism. Case reports documenting a pronounced extracutaneous segmental involvement were collected from the literature available in PubMed and from personal communications to the author. Pertinent cases are compared to the description of cutaneous segmental mosaicism of type 1 or type 2 as reported in a given trait. In total, reports suggesting extracutaneous type 2 segmental mosaicism were found in 14 different autosomal dominant skin disorders. In this way, clinical evidence is accumulated that extracutaneous type 2 segmental mosaicism does likewise occur in many autosomal dominant skin disorders. So far, however, molecular proof of this particular form of mosaicism is lacking. The present review may stimulate readers to inform colleagues of other specialties on this new concept, in order to initiate further research in this particular field of knowledge that has important implications for diagnosis, treatment and genetic counselling.