THE INTERNATIONAL JOURNAL OF Int J Dev Biol 53 259 268 2009 DEVELOPMENTAL BIOLOGY doi 10 1387 ijdb 082673ae www intjdevbiol com The secret message of heterochromatin new insights into the mechanisms and function of centromeric and pericentric repeat sequence transcription ANGELINE EYMERY MARY CALLANAN and CLAIRE VOURC H Universit Joseph Fourier INSERM Institut Albert Bonniot U823 F 38706 Grenoble France ABSTRACT In the fission yeast S Pombe small dsRNA generated by RNAi dependent mechanisms are involved in the establishment and maintenance of heterochromatic regions The existence of conserved features within the general organization of centromeric and pericentromeric repeats in yeast mouse and human argues in favor of a conserved role for centromeric and pericentromeric derived transcripts across these species In support of this evidence is accumulating that centromeric and pericentromeric sequences are transcriptionally competent in diverse biological contexts in mammalian cells Given the importance of centromeric and pericentromeric regions not only with respect to centromere function but also to gene regulation this review examines the biological contexts in which mouse and human centromeric and pericentromericspecific transcripts have been observed The structure of the transcripts generated the molecular mechanisms underlying their expression and their supposed functions will be discussed KEY WORDS heterochromatin satellite sequence transcription epigenetics RNA processing Introduction It has recently been discovered that RNA is an essential component of constitutive heterochromatin Maison et al 2002 Muchardt et al 2002 This has spurred several groups to search for specific transcripts likely to participate in heterochromatin structure Fission yeast is the only organism in which a specific RNA fullfiling this function has been identified Indeed in S pombe small dsRNA generated by RNAi dependent mechanisms are involved in the establishment and maintenance of heterochromatic regions Fig 1 Strikingly these RNAs are encoded by PCT regions which have long been thought to be transcriptionally silent Grewal and Elgin 2007 This exciting discovery adds a new dimension to our current view of how CT and PCT sequences participate to the formation maintenance and function of specific constitutive heterochromatin structures such as centromeres By highly conserved mechanisms CT repeats are known to load the histone variant cenH3 together with specific centromeric proteins required for kinetochore formation to centromeres Likewise PCT regions are enriched in epigenetic repressive marks such as histone H3 lysine 9 trimethylation H3K9me3 and heterochromatin factors such as HP1 Heterochromatin Protein 1 that are involved in ensuring sister chromatid cohesion until anaphase onset Amor et al 2004 The existence of conserved features within the general organization of CT and PCT repeats in yeast mouse and humans Fig 1 argues in favor of a conserved role for CT and PCT derived transcripts across these species In support of this evidence is accumulating that CT and PCT sequences are transcriptionally competent in diverse biological contexts in mammalian cells CT and PCT derived transcripts have been detected throughout different stages of mammalian development and during cellular differentiation proliferation transformation senescence and adaptation to environmental stress What is less clear is whether these situations necessarily correspond to the need to form or maintain heterochromatin Indeed CT and PCT regions have been shown to represent centers for transcriptional repression of genes located in their vicinity not only in cis but also in trans Fisher and Merkenschlager 2002 and it is possible that a transient transcriptional derepression of these regions is used by the cell as a way to modify the functional organization of the cell Abbreviations used in this paper CT centromere PCT pericentomere Address correspondence to Dr Claire Vourch or Dr Mary Callanan University Joseph Fourier INSERM Grenoble F 38041 France e mails claire vourch ujf grenoble fr or mary callanan ujf grenoble fr Published online 20 April 2009 ISSN Online 1696 3547 Print 0214 6282 2009 UBC Press Printed in Spain 260 A Eymery et al nucleus Given the importance of CT and PCT regions not only with respect to centromere function but also to gene regulation this review examines the biological contexts in which mouse and human CT and PCT specific transcripts have been observed The structure of the transcripts generated the molecular mechanisms underlying their expression and their supposed functions will be discussed Cellular response to stress One of the most dramatic examples of transcriptional activation of centromeric specific sequences is that which occurs in response to cell stress In numerous normal primary and cancer human cell lines heat shock has been shown to induce satellite III sequence sat III transcription primarily from the 9q12 locus although transcription from other PCT regions has been observed particularly in tumours cells independent of the cell cycle Jolly et al 1997 Denegri et al 2002 Controversy exists concerning the size of these polyadenylated transcripts Indeed sat III transcripts are either detected as very long transcripts Jolly et al 2004 or as a continuum of transcripts ranging in size from more than 5 kb to less than 2 kb Rizzi et al 2004 thereby raising the possibility that the latter could be generated from long precursors through post transcriptional mechanisms involving splicing see below In heat shocked cells sat III sequences are mainly transcribed in the sense orientation although low level of antisense transcription has been detected Fig 2 Interestingly low levels of sense and antisense sat III transcripts have also been detected in non heat shocked cells Furthermore in both heat shocked and non heatshocked cells sense transcripts are more abundant than antisense transcripts thus suggesting that the transcriptional orientation is tightly controlled Valgardsdottir et al 2008 Although originally described upon heat shock induction of sat III expression in a sense orientation has recently been shown to be triggered by a wide range of stress conditions including cellular exposure to DNA damaging agents and oxidative stress Fig 3 Interestingly transcript levels vary according to the nature of the stress signal MMS etoposide aphidilcolin and oxidative stress are low inducers of sat III