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The SWI/SNF complex is a multiprotein complex essential for chromatin remodelling. As such, it plays a key role in the epigenetic regulation of genome expression. This complex is composed of a dozen of proteins, some of which are constant and ubiquitous, especially SMARCB1 and SMARCA4. Mutations in these genes are now described in an increasing number of tumors. Mutations in SMARCB1 characterize the majority of rhabdoid tumors, an aggressive malignancy that exquisitely depends on this single genetic event. Rare rhabdoid tumors have mutation in SMARCA4, a genetic abnormality also found in some medulloblastomas. Many other tumor types, of variable aggressiveness, show an abnormal loss of expression of SMARCB1, but the genetic underlying cause most often remains elusive. The recent sequencings of whole exomes have described frequent mutations in other genes of the SWI/SNF complex: mutations in ARID1A in liver, gastric or bladder carcinomas, and PBRM1 mutations in renal cancers. These data establish the wide role of SWI/SNF complex in cancers and justify that major efforts should now be devoted to this common mechanism of human oncogenesis.

Rhabdoid tumours are rare aggressive tumours of infancy. The definition classically relies on a characteristic morphology and the inactivation of the hSNF5/INI1 tumour suppressor gene. This entity includes central nervous system tumours (ATRT), renal tumours (RTK) and soft-part tumours. Their rarity and morphological pleomorphism make the diagnosis often challenging. However, the recently introduced immunohistochemistry with anti-INI1 (anti-SMARCB1) antibody is a very useful diagnostic tool. Deletions at the 22q11.2 locus and mutations in hSNF5/INI1 sequence must be investigated in order to confirm the diagnosis and to give insights on a presumable germline mutation. Indeed, a predisposition may be found in up to 30% of cases. The treatment is based on aggressive chemotherapy, surgery and irradiation. The prognosis remains poor and the survival rate is below 30%, whatever the anatomic location. Understanding the role of hSNF5/INI1 within the SWI-SNF complex for the epigenetic regulation of transcription might drive the future targeted therapies.

The cytogenetic observation of the nucleus of Arabidopsis thaliana, a plant member of the brassicaceae family, reveals a simple organization of the nuclear content. Indeed, the nuclear volume is occupied by two distinct and easily distinguishable forms of chromatin: a large fraction of relatively decondensed and transcriptionnally active euchromatin surrounds about ten conspicuous regions, the chromocenters, which contain most repeated and highly condensed heterochromatic sequences. Remarkably, during the development of A. thaliana or when the plant is exposed to certain environmental variations, dramatic changes in the appearance, the size or the presence of the chromocenters occur. A number of cytogenetic studies have not only characterized the genomic sequences accommodated in the chromocenters, but have also established the dynamics of their assembly and disruption. Moreover, various endogenous and exogenous factors involved in the presence and the size of chromocenters were recently identified. Taken together, these studies carried out in A. thaliana suggest that heterochromatin is a truly "malleable" fraction of the genome whose dynamic organization is not controlled only by epigenetic marks and whose importance in nuclear function goes beyond merely grouping together non-coding genomic sequences., (© Société de Biologie, 2010.)

Gene expression is in great part under the control of proteins associated with DNA: histones which can be phosphorylated and acetylated, non-histone proteins which include enzymes, hormone receptors and other regulatory proteins. Phosphorylation seems to play an important role in chromatin regulation.