The Silencing Mediator of retinoid and Thyroid hormone (SMRT) is a transcription co-repressor whose association with nuclear receptors both in solution and bound to DNA response elements is destabilized by ligand. SMRT and a related co-repressor N-CoR are recruiting a transcriptional repression complex, that contains sin3A/B protein and histone deacetylases (HDAC1/2) to nuclear receptors. It is now well established that SMRT can interact with non-receptor proteins such as BCL6, for its repression activities. The ability of the “silencing complex” to deacetylate histones results in a condensed chromatin state that is inhibitory to transcription. We have previously shown that SMRT was localized at chromosome 12q24. In addition, down-regulation of the SMRT protein due to 12q24 rearrangements was recurrently associated with NHL transformation, supporting a tumor suppressor function for SMRT (

Cancer Res
,
65
(11):
4554
–4561,
2005
). We further investigated the reasons why the 12q24 region and SMRT was targeted by chromosomal rearrangements. Genetic breakage is one mechanism by which functional loss of tumor suppressor gene activity may occur. Chromosomal locations in which genetic breakage may be induced are known as fragile sites. Fragile sites have been shown to be involved in some malignancies in which the fragile site lies within known genes, such as the FHIT gene (chromosome 3p) in lung cancer, and where small deletions are consistently observed on chromosome 3. Two fragile sites exist on the long arm of chromosome 12. FRA12B is located at 12q24.13 and FRA12E has been located at 12q24.2–3. Interestingly the FRA12E region corresponds to the site of SMRT (12q24.2). To assess whether the FRA12E fragile site is localized within the SMRT gene, we studied normal lymphocytes cultured in the presence of aphidicolin (an inducer of fragile sites) and metaphase chromosomes were subsequently prepared following classical cytogenetics protocols. These preparations were then subjected to fluorescence in situ hybridization (FISH) using a set of SMRT-specific RPCI BAC clones. Using this approach, we were able to demonstrate that the FRA12E fragile site is localized within the SMRT gene. We further characterize several lymphoblastoid cell lines that were either carrier or not of the FRA12E fragile site. This provides us with a mechanistic explanation for recurrent 12q24 rearrangements seen in transformed NHLs. The FRA12E-carrier lymphoblastoid cell lines will provide us the opportunity to characterize the structure of this fragile site.

Topics:
aphidicolin, cancer, chromatin, chromosome rearrangements, dna, fluorescent in situ hybridization, genes, histone deacetylase, histones, ligands

Disclosure: No relevant conflicts of interest to declare.

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2006, The American Society of Hematology
2006
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