A molecular pathogenesis for transcription factor associated poly-alanine tract expansions

Hum Mol Genet. 2004 Oct 15;13(20):2351-9. doi: 10.1093/hmg/ddh277. Epub 2004 Aug 27.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • COS Cells
  • Cell Nucleus / chemistry
  • Chlorocebus aethiops
  • Core Binding Factor Alpha 1 Subunit
  • Cytoplasm / chemistry
  • DNA Repeat Expansion*
  • DNA-Binding Proteins / analysis
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism
  • HSP40 Heat-Shock Proteins
  • HSP70 Heat-Shock Proteins / analysis
  • HSP70 Heat-Shock Proteins / metabolism
  • Heat-Shock Proteins / analysis
  • Heat-Shock Proteins / metabolism
  • High Mobility Group Proteins / analysis
  • High Mobility Group Proteins / genetics
  • High Mobility Group Proteins / metabolism
  • Homeodomain Proteins / analysis
  • Homeodomain Proteins / genetics*
  • Homeodomain Proteins / metabolism
  • Humans
  • Mice
  • Mice, Mutant Strains
  • Mutation / genetics
  • Neoplasm Proteins / analysis
  • Neoplasm Proteins / genetics
  • Neoplasm Proteins / metabolism
  • Nuclear Proteins / genetics
  • Peptides / genetics*
  • Polydactyly / genetics*
  • Polydactyly / metabolism
  • Proteasome Endopeptidase Complex / metabolism
  • Protein Folding
  • Repetitive Sequences, Amino Acid / genetics
  • SOXB1 Transcription Factors
  • Transcription Factors / analysis
  • Transcription Factors / genetics*
  • Transcription Factors / metabolism


  • Core Binding Factor Alpha 1 Subunit
  • DNA-Binding Proteins
  • DNAJB1 protein, human
  • HOXD13 protein, human
  • HSP40 Heat-Shock Proteins
  • HSP70 Heat-Shock Proteins
  • Heat-Shock Proteins
  • High Mobility Group Proteins
  • Homeodomain Proteins
  • Hoxd13 protein, mouse
  • Neoplasm Proteins
  • Nuclear Proteins
  • Peptides
  • SOX3 protein, human
  • SOXB1 Transcription Factors
  • Sox3 protein, mouse
  • Transcription Factors
  • polyalanine
  • Proteasome Endopeptidase Complex