The silencing of large chromosomal regions by epigenetic mechanisms has been 
reported to occur frequently in cancer. Epigenetic marks, such as histone 
methylation and acetylation, are altered at these loci. However, the mechanisms 
of formation of such aberrant gene clusters remain largely unknown. Here, we 
show that, in cancer cells, the epigenetic remodeling of chromatin into 
hypoacetylated domains covered with histone H3K27 trimethylation is paralleled 
by changes in higher-order chromatin structures. Using fluorescence in situ 
hybridization, we demonstrate that regional epigenetic silencing corresponds to 
the establishment of compact chromatin domains. We show that gene repression is 
tightly correlated to the state of chromatin compaction and not to the levels of 
H3K27me3-its removal through the knockdown of EZH2 does not induce significant 
gene expression nor chromatin decompaction. Moreover, transcription can occur 
with intact high-H3K27me3 levels; treatment with histone deacetylase inhibitors 
can relieve chromatin compaction and gene repression, without altering H3K27me3 
levels. Our findings imply that compaction and subsequent repression of large 
chromatin domains are not direct consequences of PRC2 deregulation in cancer 
cells. By challenging the role of EZH2 in aberrant gene silencing in cancer, 
these findings have therapeutical implications, notably for the choice of 
epigenetic drugs for tumors with multiple regional epigenetic alterations.