Mosaicism—the New Plastics
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.