Studies on the α- and β-globin genes suggest that certain mechanisms in higher eukaryotic cells fix the stochastic expression patterns during cellular differentiation. Although there is a tight balance in the expression of α- and β-globin genes, these expression patterns are consistent with the predictions of the stochastic model [26, 27]. Analysis has revealed an imbalance between 2α- and 2β-globin gene expression in both cytoplasm and nucleus in a significant proportion of erythroid cells. Further elegant experiments have demonstrated that these stochastic expression patterns are established prior to transcriptional activation. Importantly, both active and silenced expression patterns are clonally inherited . The inherited stochastic patterns cannot be explained simply by random molecular encounters or fluctuations in the transitions between conformational states of a macromolecule, or by the amplification of feedback loops in a transcription factor network. Differentiated cells appear to have developed important mechanisms for remembering both transcriptionally active and silent states.
Previous studies have indicated that when cells enter mitosis, almost all transcription factors are peeled off the chromatin because the transcription complexes are disrupted . However, it has been found that TFIID is retained at active gene promoters during mitotic chromatin inactivation . During the process of transcription complex assembly, TFIID interacts first with the core promoter, so its retention on the mitotic chromosomes is an economical way of resuming transcription after mitosis. It has been observed that two enzyme classes, the histone acetyltransferases (HATs) and the histone deacetylases (HDACs), are spatially reorganized and displaced from the condensing chromosomes as the cells progress through mitosis . In another study  it was shown that histone acetylase (CBP, PCAF), chromatin remodeling complex component (Brg-1), SNF2H and FACT, along with RNA pol-II at the HNF-4, HNF-1 and albumin genes, are all dissociated from the chromatin during mitosis but re-recruited on mitotic exit , suggesting that they have roles in the reactivation of transcription.
By employing globin genes as a model system, we found in this study that the tissue-specific factor NF-E2p45, which is crucial for activating globin genes, is preserved on mitotic chromosomes but GATA-1 is not. This suggests that during cell division, NF-E2p45 can serve as a molecular memory marker that maintains the locally hypersensitive state of the globin gene clusters. Previous studies have shown that GATA-1 is involved in regulating transcription by undergoing modifications such as acetylation, phosphorylation and sumoylation, without the recruitment of other partners [17–19]. Chromatin remodeling complexes and histone modification enzymes, which are displaced from mitotic chromosomes, depend on other protein factors to reunite them with the chromatin . During globin gene activation, NF-E2 plays a key role in recruiting other protein factors such as TAFII130 and CBP, a coactivator of histone acetyltransferase activity . TFIID retained on the mitotic chromosomes could also recruit other basal transcription factors when transcription is activated. Hence, we predict that higher eukaryotes have evolved mechanisms for preserving certain proteins on mitotic chromosomes, which may have common functional features for recruiting other protein factors to ensure that transcription is efficiently reactivated at the onset of each cell cycle. It will therefore be worth exploring other similar protein factors and their memory functions in order to elucidate cellular memory mechanisms.
The binding to DNA of many protein factors involved in the regulation of gene expression often depends on alteration of the local chromatin structure, mediated by post-translational modifications of the terminal residues of histones. As a result, different histone modifications segregate the chromatin into different territories. Active chromatin regions are often hyperacetylated at histones H3 and H4 and hypermethylated at H3-K4 and H3-K79 . However, active histone modifications are highly dynamic. When chromatin condenses during mitosis, therefore, do these modifications function as memory 'determinants' to mark different states of gene expression? In this study, we selected transcriptionally competent globin gene clusters and genes in different transcriptional states to investigate the potential role of four active histone modifications (H3 and H4 acetylation, H3-K4 dimethylation and K79 dimethylation) as cellular memory markers during mitosis, using comparative ChIP analysis of both asynchronous and mitotic MEL cell populations. In asynchronous cells, multiple combinations of the four histone modifications are located at the distant HSs of the globin gene clusters and the gene promoter regions, and they mark different chromatin states of the genes. For example, fully activated c-myc and β-actin are hyperacetylated at both H3 and H4 and hyperdimethylated at H3-K4; incompletely activated hsp70 is hyperacetylated at both H3 and H4 and moderately dimethylated at H3-K4; potentially transcriptional α-globin and βmaj are moderately acetylated and dimethylated at H3-K4; the RNA polymerase III-transcribed gene 7SK is hyperacetylated at both H3 and H4 but not dimethylated at H3-K4. Both transcriptionally active and competent genes, including RNA polymerase II- and III-transcribed genes, are all moderately methylated at H3-K79. However, inactive genes have less of these active modifications. In mitotic cells, some active modifications, especially H3 acetylation and H3-K4 dimethylation, are well preserved on the mitotic chromosomes. These preserved active histone modifications provide epigenetic markers by which gene expression states and local chromatin states can be inherited through mitosis. For the distant regulatory elements, which play important roles in switching on many tissue- or developmental stage-specific genes and maintaining their normal transcription during development and differentiation, localized active histone modifications, together with certain transcription factors, also provide important memory markers for maintaining local chromatin states during mitosis and hence facilitating and stabilizing differentiation and development. Moreover, the preserved epigenetic markers might contribute to create a unique chromatin conformation and provide a re-activating core for the resumption of transcription in the next cell cycle. They may also have roles in distinguishing transcriptionally active from repressed regions, even in the absence of trans-acting factors, at the onset of the next cell cycle.
Moreover, active modifications such as H3-K4 methylation and H3 acetylation are enriched in the H3.3 variant of Drosophila. In contrast, H3-K9 dimethylation, the marker for repressed genes, is enriched in H3 [22, 23]. Another recent study showed that H3.3 is enriched in the promoters of active genes . Using the mouse λ5-VpreB1 locus as a model system, immuno-FISH analysis of mitotic cells revealed that H3.3 is strongly marked at the active λ5 gene on metaphase chromosomes. Moreover, this region is hyperacetylated at H3-K9 and -K14 and di- and tri-methylated at H3-K4 . These results strongly suggest that during mitosis, H3.3 might function as another epigenetic memory marker for maintaining transcriptionally active states along with active histone modification. Nevertheless, our speculation needs to be verified by testing more gene loci. It was also found that H3.3 incorporation into chromatin and its removal from chromatin depend on transcription . Therefore, it will be worth exploring how H3.3 coordinates with multiple active histone modifications to mediate transcriptional memory.