PubMed · 4145271
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E H Zackai, W R Breg. 1973. Ring chromosome 7 with variable phenotypic expression.. https://doi.org/10.1159/000130436
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The concept of autosomal lethal genes surviving only in a mosaic state was proposed by Happle to explain the genetic basis of several syndromes characterized by (almost always) sporadic occurrence, distribution of lesions in a scattered or asymmetrical pattern, variable extent of involvement, lack of diffuse involvement of entire organs, and equal sex ratio. The mosaic may either arise from a gametic half-chromatid mutation or from an early postzygotic mutation. The purpose of this article is to review current knowledge of the genetics and cutaneous manifestations of some of the birth defects to which the lethal gene concept is thought to apply: the Schimmelpenning (Feuerstein-Mims) syndrome, Proteus syndrome, encephalocraniocutaneous lipomatosis, Sturge-Weber and Klippel-Trenaunay syndrome, cutis marmorata teleangiectatica congenita (van Lohuizen syndrome), and neurocutaneous melanosis.
A diagnosis of IFAP (ichthyosis follicularis with atrichia and photophobia) syndrome was established in a 1-year-old boy with congenital hairlessness, generalized ichthyotic skin changes with follicular hyperkeratoses, and photophobia. IFAP syndrome is considered to be an X-linked recessive trait. The phenotype present in female carriers has so far not been delineated. A 2-year-old sister had atrophoderma and ichthyotic skin lesions arranged in a linear pattern and a large noncicatrical bald patch on her scalp. Similarly, the mother had linear lesions of scaling and atrophy as well as circumscribed hairless areas involving the scalp, the axillary region, and the lower legs. Sweat testing by means of iodine starch-reaction visualized hypohidrotic linear lesions corresponding to the areas of hyperkeratosis and atrophy. In both mother and daughter the lesions followed the lines of Blaschko, whereas the boy was diffusely affected. Family history showed that the boy's maternal uncle who had died at age 1 year was likewise affected with the same disorder. Moreover, the maternal grandmother had reportedly bald patches on her scalp and very dry skin. This is the first report to document linear skin lesions visualizing lyonization in women heterozygous for IFAP syndrome.
A 10-month-old girl with psychomotor retardation, microcephaly, bilateral microphthalmia, and postaxial polydactyly of the feet was karyotyped using banding techniques and (single or dual color) fluorescent in situ hybridization (FISH) with four probes: D13Z1/D21Z1, pancentromeric, pantelomeric, and a mix of 13q subtelomeric and 13/21 alphoid repeats. She was found to have a 47-chromosome karyotype in which a normal 13 was replaced by two stable markers derived from a breakpoint at 13q21.1, namely a del(13)(q21.1) and an isofragment(13) (qter-->q21.1::q21.1-->qter). The latter had a single C-negative but Cd-positive primary constriction at 13q32 which, however, was not obvious in about 12% of the cells. FISH studies showed that the small 13q- had the 13-centromere and a 13q telomere (as shown for a specific 13q subtelomeric signal) onto the broken end whereas the isofragment lacked alphoid signals but had 13q subtelomeric sequences on both ends. Parental karyotypes were normal. The patient's rearrangement represents the eighth chromosome-13-derived marker with a nonalphoid neocentromere located at 13q. All in all, such neocentromeres have been described in 29 markers derived from chromosomes 2, 3, 8-11, 13-15, 20, and Y, and plausibly result from the epigenetic activation of a latent centromere, which may even be a telomere with neocentric activity. The 13q telomere found in the del(13q) was probably captured from the homologous chromosome.