The
Townes-Brocks syndrome (TBS) is an autosomal dominantly inherited malformation syndrome presenting as an association of
imperforate anus, triphalangeal and supernumerary thumbs, malformed ears and
sensorineural hearing loss. Mutations in SALL1, a gene mapping to 16q12.1, were identified as a cause for TBS. To elucidate how SALL1 mutations lead to TBS, we have performed a series of functional studies with the SALL1
protein. Using epifluorescence and confocal microscopy it could be shown that a GFP-SALL1 fusion
protein localizes to chromocenters and smaller
heterochromatin foci in transiently transfected NIH-3T3 cells. Chromocenters consist of clustered pericentromeric
heterochromatin and contain telomere sequences. Indirect immunofluorescence revealed a partial colocalization of GFP-SALL1 with M31, the mouse homolog of the Drosophila heterochromatic
protein HP1. It was further demonstrated that SALL1 acts as a strong transcriptional repressor in mammalian cells. Transcriptional repression could not be relieved by the addition of the
histone deacetylase inhibitor Trichostatin-A. In a yeast two-hybrid screen we identified PIN2, an
isoform of
telomere-repeat-binding factor 1 (TRF1), as an interaction partner of SALL1, and showed that the N-terminus of SALL1 is not necessary for the interaction with PIN2/TRF1. The interaction was confirmed in vitro in a GST-pulldown assay. The association of the developmental regulator SALL1 with
heterochromatin is striking and unexpected. Our results propose an involvement of SALL1 in the regulation of higher order
chromatin structures and indicate that the
protein might be a component of a distinct
heterochromatin-dependent silencing process. We have also provided new evidence that there is a close functional link between the centromeric and telomeric
heterochromatin domains not only in Drosophila and yeast, but also in mammalian cells.