Research activities

RESEARCH ACTIVITIES

THE HOX TRANSCRIPTION FACTORS: FROM MAMMALIAN DEVELOPMENT TO ONCOGENESIS

Homeotic or Hox genes encode transcription factors which play crucial roles in embryonic development of bilaterian animals. Both their genomic organization and expression patterns have been highly conserved throughout evolution and they contributed to the diversification and remodelling of animal body plans.

Exploring the Hox genes functions in vertebrates revealed they critically contribute to pattern the main body axis and limbs during embryonic development. In addition, they also take part in organogenetic processes like bone, cartilage, heart, or pharyngeal glands development, for example. By adulthood, multiple homeotic genes remain active and control hematopoietic differentiation or mammary gland maturation for example.  As gatekeepers in the control of cell migration, proliferation and differentiation, Hox genes have also been associated to cancer appearance or progression under imbalance of regulatory routes.


While the biological functions of Hox proteins have been widely documented, the mode of action of these transcription factors remains largely to be investigated. In that regard, a first goal of our research group is to identify new functional determinants of such Hox proteins to unravel their mechanisms of action both in the context of embryonic development and in oncogenesis. A second important goal is to identify partner proteins that interact with Hox proteins in the context of gene regulation or of other cellular processes to unravel. Finally, a third aim is to identify and characterize  target genes and target enhancers of Hox proteins which wil alsol reveal some of their cofactors as well as the regulatory pathways they take part in.

Our research focus is about two Hox proteins, Hoxa1 and Hoxa2.

  • Activity and specificity determinants of Hoxa1


To characterize the activity and specificity determinants of Hoxa1, two parallel approaches have been followed.

A first approach was to generate site-directed Hoxa1 mutants and to evaluate their activity and functional specificity:   

  • In vitro, by transfection of model cell lines and target reporter genes activity measurements
  • In vivo, by generating knock in mice and analysing their developmental defects

A second approach was to generate a small library of random pentapeptide insertions within the Hoxa1 sequence and to assay the activity of the mutant proteins in distinct cell models on different known target genes.  A handful of gain of function and loss of function mutants are now investigated further.

Recently, Hoxa1 was revealed to play a pivotal role in mammary gland carcinogenesis occurring upon growth hormone autocrine imbalance. Our mutant screen will also be transposed in order to investigate the functional determinants of the oncogenic activity of Hoxa1.

 

  • Interactors of Hoxa1 and Hoxa2

Transcription factors like Hox proteins are supposed to integrate molecular inputs to be translated in terms of gene regulation. In that regard, their activity should be modulated if not conditioned by several partner proteins or interactors:

    • the yeast two-hybrid assay, developed in collaboration with J.C. Twizere and M. Vidal (DFCI, Harvard, USA)
    • the tandem affinity purification "TAP-tag" assay, in cell differenciation models.

Several putative interactors have been identified so far that will be validated by additionnal assays (Co-immunoprecipitation, GST pull down, ...) and studied in more detail for their involvement in Hoxa1 and Hoxa2 functions.

  • Targets genes of Hoxa2 and Hoxa1

Unravelling the mode of action of transcription factors also requires identifying target genes and target enhancers they control.  This allows searching for cofactors conditioning or modulating their activity.  This also allows searching for cofactors conditioning or modulating their activity and will pinpoints the molecular pathways they act on to exert their developmental activities.

Starting from the fact that several cross-regulatory talks have been highlighted between Hox genes, we recently identified two new Hoxa1 and/or Hoxa2 responsive regulatory sequences involved in Hox gene regulation. From that, our goal is now to further dissect the functioning of these regulatory modules. Also, wider screens for the identification of Hoxa1 and Hoxa2 target genes are under way.

| 7/12/2009 |