Human Molecular Genetics Advance Access originally published online on August 27, 2004
Human Molecular Genetics 2004 13(20):2351-2359; doi:10.1093/hmg/ddh277
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Human Molecular Genetics, Vol. 13, No. 20 © Oxford University Press 2004; all rights reserved
A molecular pathogenesis for transcription factor associated poly-alanine tract expansions
1Max-Planck Institute for Molecular Genetics and Institute for Medical Genetics, Charité, Berlin, Germany and 2Department of Neuroproteomics, Max-Delbrück Center, Berlin, Germany
Received July 3, 2004; Accepted August 20, 2004
Poly-alanine (Ala) tract expansions in transcription factors have been shown to be associated with human birth defects such as malformations of the brain, the digits, and other structures. Expansions of a poly-Ala tract from 15 to 22 (+7)–29 (+14) Ala in Hoxd13, for example, result in the limb malformation synpolydactyly in humans and in mice [synpolydactyly homolog (spdh)]. Here, we show that an increase of the Ala repeat above a certain length (22 Ala) is associated with a shift in the localization of Hoxd13 from nuclear to cytoplasmic, where it forms large amorphous aggregates. We observed similar aggregates for expansion mutations in SOX3, RUNX2 and HOXA13, pointing to a common mechanism. Cytoplasmic aggregation of mutant Hoxd13 protein is influenced by the length of the repeat, the level of expression and the efficacy of degradation by the proteasome. Heat shock proteins Hsp70 and Hsp40 co-localize with the aggregates and activation of the chaperone system by geldanamycin leads to a reduction of aggregate formation. Furthermore, recombinant mutant Hoxd13 protein forms aggregates in vitro demonstrating spontaneous misfolding of the protein. We analyzed the mouse mutant spdh, which harbors a +7 Ala expansion in Hoxd13 similar to the human synpolydactyly mutations, as an in vivo model and were able to show a reduction of mutant Hoxd13 and, in contrast to wt Hoxd13, a primarily cytoplasmic localization of the protein. Our results provide evidence that poly-Ala repeat expansions in transcription factors result in misfolding, degradation and cytoplasmic aggregation of the mutant proteins.
* To whom correspondence should be addressed at: Max-Planck-Institute for Molecular Genetics, FG Development and Disease, Ihnestrasse 73, 14195 Berlin, Germany. Tel: +49 84131263; Fax: +49 84131385; Email: mundlos{at}molgen.mpg.de
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