Mosaicism—the New Plastics

1 May 2013 by Lowell Goldsmith, posted in JID Jottings

Image from Wikimedia Commons is in the public domain

by Lowell A. Goldsmith, MD

In "The Graduate" (1967), Dustin Hoffman's character learns much about sex, but more  importantly, he hears that "plastics" are the societal  trend to be followed.  Today, the new trend is mosaicism. Clinical dermatologists are inculcated with a number of diseases that have limited  and often segmental patterns in the skin, frequently following  patterns of  the lines of Blaschko. Diseases  include:  Darier disease, neurofibromatosis 1, Proteus syndrome, segmental vitiligo, segmental psoriasis, some  forms  of  ichthyosis, types of hereditary hemorrhagic telangiectasia, segmental morphea, and even acne. Another disease  is a multisystem syndrome with hypo- and hyperpigmentation and which is associated with trisomy of the short arm of chromosome 12p: Pallister-Killian syndrome.  Mosaicism allows reversion of some  areas of affected skin in some cases of junctional  epidermolysis  bullosa. A detailed review containing extensive  molecular considerations, with great diagrams of Blashko patterns, was published  in the May issue of Nature Reviews Genetics (Biesecker & Spinner, 2013).

Even more astounding is the use of the term "mosaic" for  understanding  the bones of a 1.95 million-year-old primate skeleton, Austalopithecus sediba. This woman’s skeleton is smaller than a modern human  female and slightly larger than Pan troglodytes (the chimp). Analysis of jaw bones, spine, and thorax shows it is made of combinations, or a mosaic, of  bones and strategies   from other known early human species:  lots of data from one very well preserved skeleton from South Africa.  The sorting of  the pieces in this evolutionary Lego set reveals not a hoax like Piltdown man, but instead illustrates how evolution does not work de novo but is opportunistic about using  pieces that can interact with each other and that have been successful in other species. There was no evidence, however, that Eve came from  Adam’s rib.  Perhaps detailed studies  of thousands of samples from  a single present day Homo sapiens  will allow additional axes of analysis of mosaicism under normal conditions.

Berger,LE (2013) The mosaic  nature of Australpithecus sediba. Science 340:162

Biesecker, LG and Spinner  NB (2013) A genomic view of mosaicism and human disease Nature Review Genetics 14:307-320

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