Heterochromatin, from Chromosome to Protein
Marie-Geneviève Mattei and Judith
INSERM U 491, Faculté de Médecine, Bd Jean Moulin, 13385
(Paper co-edited with E.C.A. Newsletter)
I THE CONCEPT OF HETEROCHROMATIN
Definition of Chromatin
In prokaryotes such as Escherichia coli there is no detectable heterochromatin,
for the simple reason that there is no chromatin. The hereditary message is
carried by a circular molecule of naked DNA, and there is no separate nuclear
In eukaryotes, however, the DNA is packaged in the form of a nucleoprotein complex
called "chromatin". The hereditary message is, therefore, carried
by the chromatin. It is located in a nucleus and is organised in several separate
entities, the chromosomes.
The Concept of Heterochromatin
The concept of heterochromatin, as described by Emil HEITZ in 1928, was exclusively
based on histological observations. He defined heterochromatin (HC) as being
the chromosomal segments which appear extremely condensed and dark in colour
in the interphase nucleus. The rest of the nucleus is occupied by euchromatin,
or true chromatin, which appears diffuse and relatively light in colour.
Heitz's observations highlighted the fact that, in the interphase nucleus, chromatin
does not have a homogeneous appearance. Electron microscopy and X-ray diffraction
have since confirmed the heterogeneous structure of chromatin: it consists of
a tangle of fibres, the diameter of which not only vary during the cell cycle,
but also depend on the region of the chromosome observed. Euchromatin,
in its active form, consists of a fibre with a diameter not exceeding 10-11nm.
Its diameter corresponds to that of a nucleosome, which contains a 146 base
pair double strand DNA segment, wound around 4 homodimers of the histones H2A,
H2B, H3, and H4 .
The inactive euchromatin is enriched in linker histone H1. Histone H1 binds
two consecutive nucleosomes, which causes the 10-11nm fibre to wind itself into
a solenoid with a 30nm diameter. The 30nm fibre is further organised
through interactions with non-histone proteins which fold the chromatin fibre
in loops around an imaginary axis. These proteins include topoisomerase II,
which is located, in particular, at the base of the loops, the scaffold protein
2 and lamins, in addition to other proteins. At this stage, the diameter of
the chromatin fibre attains approximately 200nm. As regards the heterochromatin,
as defined above, its constituent fibre is more condensed and often appears
to be composed of aggregates. It involves numerous additional proteins, including
the HP1 proteins (Heterochromatin Protein 1).
II TWO TYPES OF HETEROCHROMATIN
There are two types of heterochromatin, constitutive HC and facultative
HC, which differ slightly, depending on the DNA that they contain. The
richness in satellite DNA determines the permanent or reversible nature of
the heterochromatin, its polymorphism and its staining properties (Table I).
Table I: Properties which allow to differentiate
constitutive from facultative heterochromatin.
Table I: Properties which allow to differentiate
constitutive from facultative heterochromatin.
1) Richness in satellite DNA
- Constitutive HC contains a particular
type of DNA called satellite DNA, which consists of large numbers of short
tandemly repeated sequences. There are various types of satellite DNA which
can be separated by gradient density centrifugation. The best-known type,
which is called alpha-satellite DNA, is rich in A-T and is located
in the centromeric region of the chromosomes. DNA satellite I,
which is also A-T rich, is located more specifically at the centromeres
of chromosomes 3 and 4, the short arms of the acrocentrics, and the long
arm of the Y chromosome. The DNA satellites II and III
are both A-T rich, but they are also G-C rich. DNA satellite II
is primarily located at the secondary constrictions of chromosomes 1 and
16. Satellite III is mostly present in the secondary constriction
of chromosome 9, the short arms of the acrocentrics and the Y chromosome.
The satellite DNA sequences have the distinctive feature of being able to
fold on themselves and may have an important role in the formation of the
highly compact structure of the constitutive HC.
HC is not enriched in satellite DNA. It is characterised by the presence
of G-bands, which are rich in LINE-type repeated sequences. These
sequences, dispersed throughout the genome, could promote the propagation
of a condensed chromatin structure.
- Constitutive HC is stable and conserves
its heterochromatic properties during all stages of development and in all
tissues. This "heterochromatic" state is linked to the satellite
DNA it contains, and thus to the sequence of that DNA, which is clearly
HC is reversible, that is to say, it can change from the heterochromatic
state to a euchromatic state, depending on the stage of development or the
cell type examined. In this case, the reversibility of the heterochromatic
state clearly demonstrates that the DNA sequence is not involved.
In females, one of the X chromosomes is inactive and has a heterochromatic
state (Barr body) in the somatic cells. Before entering meiosis, it is reactivated
and both X chromosomes form a normal bivalent, undistinguishable from the
autosomal bivalents. The inactive X provides a classic example of facultative
Another example of facultative HC is observed at the pachytene stage of
male meiosis. At this stage, the X and Y chromosomes, joined by their telomeres,
condense to form the inactive sex vesicle (SV). The heterochromatic state
of the sex chromosomes at the pachytene stage of meiosis is transitory,
and the SV can therefore be considered as being facultative HC.
HC is highly polymorphic. This polymorphism can affect not only the size
but also the localisation of the heterochromatin, and apparently has no
phenotypic effect. Such variations are clearly observed on the secondary
constriction of chromosome 9. The frequent polymorphisms that characterise
constitutive HC are due to the instability of the satellite DNA.
- The facultative HC is not particularly
rich in satellite DNA, and is therefore not polymorphic.
4) C-band Staining :
- Constitutive HC is strongly stained by
the C-band technique. This staining could be the result of the very rapid
renaturation of the satellite DNA following denaturation.
- Facultative HC is never stained by the
III PROPERTIES OF HETEROCHROMATIN
Despite the differences described above, constitutive HC and facultative
HC have very similar properties.
1) Heterochromatin is condensed
This is in fact what defines heterochromatin, and it is applicable to both constitutive
HC and facultative
HC. This high condensation renders it strongly chromophilic,
and the intensity of HC staining is directly proportional to its degree of condensation.
The strongly condensed nature of heterochromatin has another consequence, in
that it renders it inaccessible to DNAse 1 and to other restriction enzymes
2) Heterochromatin DNA is late replicating
The incorporation of various nucleotide analogues shows that the DNA from
both constitutive and facultative HC, is late replicating.
The DNA of the inactive X is very late replicating, as it replicates at the
end of the late S phase. The incorporation of 5-Bromodeoxyuridine four hours
before harvesting allows this to be clearly seen.
As regards the heterochromatic centromere regions, their replication precedes
that of the inactive X and takes place at the beginning of the late S phase.
The centromeric DNA must, therefore, replicate sufficiently early to allow
the formation, with the centromeric proteins, of a nuclear-protein complex
that will be functional during mitosis.
The late replication of the HC results, on the one hand,
from its high degree of condensation, which prevents the replicating machinery
from easily accessing the DNA, and, on the other hand, from its location
in a peripheral nuclear domain that is poor in active elements.
The late replication of HC also leads to a less efficient
repair of its DNA, in the event of polymerase errors.
3) Heterochromatin DNA is methylated
The DNA of constitutive HC is highly methylated, with
the methylation occurring exclusively on the cytosines. An anti-5-methyl cytosine
antibody therefore strongly labels all the regions of constitutive
HC, thus showing both its richness in cytosines and their methylation.
As regards facultative HC, the methylation of the DNA is more discrete,
and cannot be detected using an antibody directed against methylated cytosines.
Nevertheless, restriction enzymes that are sensitive to methylation can be used
to reveal strong methylation of the CpG islands, which are specifically located
in the control regions of the genes.
4) In heterochromatin, histones are hypo-acetylated:
Histones may undergo post-translational modifications of their N-terminal ends
which may affect the genetic activity of the chromatin.
It has long been known that the hypo-acetylation of histone N-terminal tails
is a modification that is associated with an inactive chromatin. In contrast,
however, hyper-acetylated histones characterise the active chromatin. It is
principally the lysines ("K" in single-letter code language) of the
histones that are acetylated.
Acetylation/de-acetylation of histones is a mechanism that is absolutely essential
for the control of gene expression. Numerous transcription factors have been
shown to have, either an activity H
ransférase (HAT) in the case of co-activators, or H
étylases (HDAC) in the case of
5) Histones from heterochromatin are methylated on lysine
This modification of the N-terminal tail of histones has only very recently
been discovered and found to be involved in the process of heterochromatinisation
of the genome. It characterises both constitutive
HC and facultative
Methylation of the H3 histone occurs on a very specific residue, lysine 9, hence
its name, H3-K9. The H3-K9 lysine may be mono-methylated, di-methylated or tri-methylated,
and a high degree of methylation promotes the binding of the HP1 (Heterochromatin
Protein 1) proteins.
6) Heterochromatin is transcriptionally inactive:
Incorporating tritiated uridine into a cell culture does
not result in any labelling of the heterochromatin. Human constitutive
HC does not contain any genes, which explains why transcription does not take
place at these sites. In Drosophila, on the other hand, certain genes, such
as the rolled and light genes, are usually located in and expressed from the
constitutive HC. The facultative HC is relatively
poor in genes, and its genes are not usually transcribed in a heterochromatic
7) Heterochromatin does not participate in genetic recombination:
It is generally accepted that constitutive
HC does not participate
in genetic recombination. The reason for this is that there is no preliminary
pairing of the homologous heterochromatic regions, even though some aggregation
of these regions is often observed. In any case, the polymorphism that characterises
the heterochromatic regions would render such pairing difficult, if not impossible.
Moreover, this is the reason why the pericentric inversion of the secondary
constriction of chromosome 9 has no effect in terms of chromosome mechanics
and can be considered as a normal variant.
Not only does the constitutive
HC not participate in recombination,
it also acts to repress recombination in adjacent euchromatic regions.
As regards the facultative
HC, it does not participate in meiotic recombination
when it is in its inactive form.
8) Heterochromatin has a gregarious instinct:
The study of various organisms has shown that constitutive HC has
a genuine tendency to aggregate during interphase.
Thus, in the interphase nuclei of the salivar glands of Drosophila larvae,
the centromeres of polytene chromosomes, which are rich in heterochromatin,
aggregate to form the chromocentres.
In the mouse, the number of heterochromatic blocks that can be observed
in interphase nuclei is always lower than the number of heterochromatic regions
visualised on the metaphase chromosomes. The relationship between the size
and the number of HC blocks in interphase suggests a coalescence of the heterochromatic
In the human, the short arms of the acrocentric chromosomes, which are mainly
formed from heterochromatin, carry the nucleolar organiser regions. They are
frequently associated in the interphase nucleus, and this does not appear
to result solely from their common function. Indeed, numerous other chromosomes
are involved in this association, and the participation of each of them is
all the more marked because it carries a large HC block, as is the case for
chromosomes 1, 9 or 16.
This tendency of the heterochromatin to aggregate appears to be strongly
linked to the presence of satellite DNA sequences, but this is a property
which may not be exclusive and may involve other additional sequences.
IV FACTEURS INVOLVED in HETEROCHROMATINISATION
There are probably various diverse ways of organising a genomic region into
heterochromatin. Certain observations have, however, led to the identification
of various elements that have an important role in the formation of heterochromatin,
be it constitutive or facultative.
1) Large arrays of tandemly repeated sequences.
The fact that in the human genome, as well as in the genome of other organisms,
the localisation of the satellite DNA visualised by FISH corresponds exactly
to that of the constitutive heterochromatin stained by DAPI, highlights
the potential role of satellite DNA in the formation of this type of heterochromatin.
Indeed, this type of DNA sequence has the distinctive feature of bending and
folding upon itself, and this may be an important factor in determining the
extremely compact structure of the constitutive DNA.
However, this does not only concern satellite DNA. In plants, Drosophila,
and also in the mouse, certain multicopy transgenes are barely expressed,
or are not expressed at all, even when they are not subject to centromere
These different observations suggest that the tandem repetition of a DNA
sequence in a large number of copies is sufficient on its own to direct the
formation of heterochromatin. The presence of repetitive DNA, such as satellite
DNA, appears to simply allow the chromatin to be compacted to a greater extent,
as is the case for constitutive heterochromatin. The mechanism would
appear to be as follows: the large arrays of tandemly repeated DNA sequences
appear to be able to pair, thus forming characteristic structures. These structures
would then appear to be recognised by specific proteins, such as the HP1 proteins,
which in turn direct the formation of a higher-order chromatin.
2) Methylation of DNA
Although the presence of tandemly repeated sequences is important, it is
not the only factor, as large repetitions of transgenes do not all lead to
a transcriptional inactivation of the transgene. Most often, the silencing
induced by the tandem repeats appears to be linked to the presence of prokaryotic
DNA sequences that are rich in CpG and, therefore, likely to be methylated
(for example, the lacZ gene). The base composition of the tandem
repeat and, in particular, its ability to be methylated could therefore play
an important role in the formation of heterochromatin.
Interestingly, it has recently been shown that there is a direct relationship
between the methylation of DNA and the de-acetylation of
histones, both of which characterise heterochromatic structures. The methyl
binding protein MeCP2, which normally binds to DNA containing methylated cytosines,
has thus been shown to be able to recruit histone de-acetylases (Figure 1).
Methylation of the DNA could therefore induce a de-acetylation of histones
and thus promote heterochromatisation.
Figure 1: DNA methylation induces Histone
de-acetylation, modification which characterizes histones in both heterochromatin
and repressed euchromatin.
MeCP2 specifically binds to methylated DNA, and recruits an HDAC which de-acetylates
histones (Ac= Acetyl; Me= Methyl; MeCP2= Methyl-CpG binding Protein 2; HDAC=
However, the methylation of DNA is not indispensable for the formation of
heterochromatin. It could be an element involved in stabilisation, as has
been shown for the facultative HC of the inactive X. Indeed, in marsupials,
the inactive X is not methylated and is much less stable than in eutherian
3) Regular organisation of nucleosomes
We have seen that sequences of prokaryotic DNA inserted into eukaryotes have
the ability to be methylated and can induce the formation of heterochromatin.
Heterochromatisation may not, however, depend solely on the methylation of the
CpGs contained in the DNA.
A study of the chromatin of different transgenes digested by a micrococcal nuclease
provided interesting results. The chromatin of a transgene inserted into
HC revealed a very regular organisation of the nucleosome
that was much more regular than the organisation revealed by the same transgene
inserted into an euchromatic region. It appears that this regularity of the
structure is able either to take on a particular conformation or to be recognised
by specific proteins, and, in this way, can promote the formation of heterochromatic
4) Hypo-acetylation of Histones
We have seen that hypo-acetylation of histones is a characteristic
of silent chromatin, whether it is heterochromatin or not. In vitro,
the modification of the acetylation of the histones has a direct effect on the
stability and compaction of the nucleosomes. Thus, blocking the de-acetylation
of the histones by adding trichostatine A induces hyper-acetylation of the histones,
which causes a more open chromatin structure.
The mechanism involved is simple: in the N-terminal tail of
the histones, the basic amino-acids such as lysine are positively-charged at
cellular pH and therefore interact with the negative charges of the DNA phosphates.
Hypo-acetylation of the histones is not the only modification
of the N-terminal tails of the histones that characterises heterochromatin.
Three other modifications have been shown to be more specifically linked to
silencing: the phosphorylation of serine 10 of histone H3, the acetylation
of lysine 12 of histone H4 and the methylation of lysine 9 of histone H3 (H3-K9).
We will present the last of these modifications in more detail.
Methylation of the histone H3 on lysine 9 is an epigenetic
modification of histones that has recently been shown to be involved in the
process of heterochromatinisation. This has been demonstrated not only in constitutive
HC but also on the inactive X. The enzyme responsible for the methylation of
H3-K9 in constitutive HC is the histone methyltransferase SUV39H1.
- On lysine 9 of H3, acetylation and methylation appear to be mutually exclusive.
In Drosophila, therefore, the methyltransferase Suv39h is physically and functionally
associated with a histone de-acetylase, suggesting a single molecular
mechanism that allows the direct conversion of an acetylated lysine 9
into a methylated lysine 9.
- In addition, the methylation of H3-K9 creates a high-affinity binding site
for the heterochromatin protein HP1. Co-immuno precipitation of Suvar39h with
HP1 suggests a heterochromatinisation mechanism based on the interaction of
these two proteins and lysine 9.
Nevertheless, the HP1 protein certainly acts in different ways in the formation
of heterochromatin, since it is able to bind to histone 3 and histone H1,
even when their N-terminal tails have been removed.
- Lastly, in Neurospora crassa, it has recently been shown that methylation
of H3-K9 can cause methylation of DNA. The model proposed is as follows: a
histone methyltransferase, characterised by a SET domain, such as Suvar39h,
would methylate H3-K9 and induce the binding of a specific heterochromatin
protein, such as HP1. The HP1 protein would then recruit a DNA methyl transferase
(DNMT), which would methylate the DNA, thus stabilising the inactive state
of the chromatin (Figure 2).
Figure 2: Histone H3-K9 methylation induces
DNA methylation, modification which characterizes DNA in heterochromatin
and repressive euchromatin.
SUVAR39H is a methyltransferase which specifically methylates the Lysine
9 of histone H3. Such a methylation creates a binding site for the Heterochromatin
Protein HP1 which recruits a DNA methyl transferase, capable to methylate
the CpG in DNA (Me= Methyl; Methyl H3-K9= Methyl on Lysin 9 of Histone H3;
HP1=Heterochromatin Protein 1; DNMT=DNA Methyl transferase).
6) HP1 proteins
Many different heterochromatin proteins are to be found in mammals, and at
the present time little is known about them. The HP1 proteins do appear, however,
to have a particular role in the organisation of heterochromatin. Studies
of the variegation by position effect (PEV effect) in Drosophila
and studies of transgenes in Drosophila and mouse have allowed a
better understanding of the role of HP1 proteins.
In Drosophila, the HP1 protein is coded for
by the Su(var)205 gene, which is a suppresser of variegation that can modify
the PEV effect. The variegation by position effect can be described as follows:
genes that are normally localised in active euchromatin are, following a
rearrangement of chromosomes, placed close to a centromeric region that
is heterochromatic. This change in the position of the euchromatin has three
consequences. The first is that the structure is modified to become much
more compact. The second is the association of the newly translocated chromatin
with HP1 proteins that are normally confined to centromeres. The third consequence
is the repression of the genes contained in the translocated chromatin.
In mouse, the insertion of a transgene close to the centromere
may have similar consequences, with modification of the chromatin structure,
the appearance of HP1 proteins and repression of the transgene. It is interesting
to note that even where a transgene is repressed, not as a result of a centromeric
effect but as a result of its presence in multiple copies, HP1 proteins
are also found to be associated with the repressed chromatin.
In all cases, HP1 proteins or their homologues appear to
be an essential link in the formation of heterochromatin. These heterochromatin-specific
proteins could have the role of chromatin domain organisers. The HP1
proteins thus appear to be able to recognise particular structures that are
created by the pairing and/or the association of repeated DNA sequences. In
addition, they appear to be able to establish secondary interactions with a
large number of other proteins. The HP1 proteins are perfectly adapted for such
interactions, as they have two domains of protein/protein interaction, the chromodomain
(CD) and the chromoshadow domain (CSD).
7) Nuclear RNAs
It is already well established that certain nuclear RNAs are
able to contribute to the formation of facultative HC. The transcripts of the
XIST (X inactive Specific Transcripts)
gene are nuclear RNAs that have an essential role in the facultative inactivation
of one X chromosome, which occurs in the somatic cells in female mammals. This
XIST RNA is necessary for the initiation of X inactivation, but not its maintenance.
Other nuclear RNAs, such as H19, have been shown to be involved in the regulation
of genes that are subject to genomic imprinting.
Some recent studies in mouse have suggested that nuclear transcripts
may also be involved in the formation of constitutive HC. In mouse, as in most
other species, the centromeric HC is particularly stable. It is characterised
by the presence of a high concentration of methylated H3-K9 histone and heterochromatic
HP1 proteins, which co-localise in the nuclei with regions strongly stained
with DAPI. However, incubation of permeabilised mouse cells with RNAse A causes
rapid de-localisation of the HP1 and methylated H3-K9 signals, in relation to
the heterochromatin foci. These data suggest that a nuclear RNA may be an essential
structural component of constitutive HC. The RNA may either facilitate
compaction of the centromeric HC or may serve as an additional binding site
for the proteins that associate with the chromatin.
Surprisingly, the treatment with RNAse A does not alter the methylated H3-K9 signals
at the level of the Barr body. In fact, on the inactive X chromosome, the methylation
of H3-K9 does not appear to lead to the binding of HP1 proteins. This suggests
that the facultative heterochromatinisation of the inactive X may require a different
mechanism to come into play from that of constitutive HC.
V FUNCTIONS OF HETEROCHROMATIN
The precise role of heterochromatin in the human genome long
remained a mystery, as its frequent polymorphisms did not appear to have any
functional or phenotypic effect.
1) Role of HC in the organisation of nuclear domains
Studies of the organisation of the nucleus have shown
that heterochromatin and euchromatin occupy different domains. HC is usually
localised in the periphery of the nucleus and is attached to the nuclear
membrane. In contrast, the active chromatin occupies a more central position.
The preferential localisation of HC to the periphery
of the nucleus and, in particular, against the nuclear membrane, may be
due to the characteristic properties of the protein HP1. This heterochromatin
protein interacts specifically with the B lamin receptor, which is an integral
component of the inner membrane of the nucleus.
This organisation also has functional consequences.
The peripheral localisation of HC concentrates the active elements towards
the centre of the nucleus, allowing the active euchromatin to replicate
and be transcribed with maximum efficiency.
The fact that HC is "gregarious", and that
it tends to agglutinate, may give rise to similar functional consequences.
2) Role of HC in the centromeric function
In most eukaryotes, the centromeres are loaded with a considerable mass of
heterochromatin. It has been suggested that centromeric HC is necessary for
the cohesion of sister chromatids and that it allows the normal disjunction
of mitotic chromosomes.
It is generally believed that the presence of centromeric
HC is important for centromeric function. Thus, in certain organisms which
have large blocks of centromeric HC, it has not been possible to identify
a specific DNA sequence that defines the centromere itself. Moreover, in
the yeast Schizosaccharomyces pombe, the homologue of the HP1 protein
Swi6 is absolutely essential for efficient cohesion of sister chromatids
during cell division.
However, experiments involving the deletion of satellite
DNA show that a large region of satellite DNA repeats is indispensable for
the correct functioning of the centromere.
In an attempt to synthesise all of the above observations,
a hypothesis for the function of centromeric HC has been put forward; it
may, de facto, create a compartment by increasing the local concentration
of the centromeric histone variant, CENP-A, and by promoting the incorporation
of CENP-A rather than the histone H3 during replication.
3) Does centromeric HC act as a transporter?
Many proteins have been shown to be associated with centromeric HC, in particular
on metaphase chromosomes. It can be assumed that certain proteins that must
be present and functional at the very beginning of the G1 phase will not be
able to be synthesised due to a lack of time. Following this hypothesis, binding
to the centromere would be an ideal means for such proteins to traverse cell
division unhindered, in order to be available at the beginning of G1.
4) Role of HC in gene repression (epigenetic regulation)
Gene expression may be controlled at two levels:
Firstly, at the local level, which is transcription
control. Transcription is directly controlled by the formation of local
transcription complexes. This level involves relatively small DNA sequences
(100 bp) linked to individual genes.
Transcription may also be controlled at a more global
level, in which case it is the transcriptability that is controlled.
This level involves much larger sequences that represent a large chromatin
domain. Such a domain can have one of two states: an active state, which
is sensitive to endonucleases, and an inactive state, which is insensitive.
Heterochromatin appears to be involved in controlling the
transcriptability of the genome. Genes that are usually located in
the euchromatin can, therefore, be silenced when they are placed close to a
Mechanism of inactivation in cis:
Following a chromosomal rearrangement, a euchromatic region containing active
genes may be juxtaposed with a heterochromatic region. Where the rearrangement
removes certain normal barriers that protect the euchromatin, the
heterochromatic structure is able to propagate in cis to the adjacent euchromatin,
thus inactivating the genes contained therein. This mechanism has been observed
in position effect variegation (PEV) in Drosophila and also in the inactivation
of certain transgenes in mouse. It is associated with a modification of
the structure of the newly repressed euchromatin that involves the HP1 proteins,
characteristic of heterochromatin.
- Mechanism of inactivation in trans:
During cell differentiation, certain active genes are likely to be transposed
into a heterochromatic nuclear domain, thus causing them to become inactive.
Such a mechanism has been proposed as an explanation for the co-localisation
in lymphocyte nuclei of the protein IKAROS and the target genes of which
it controls the expression with blocks of centromeric heterochromatin. Thus,
target genes could be repressed by a mechanism of inactivation in trans
that propagates from the heterochromatin towards the adjacent euchromatin.
Another hypothesis is that the protein IKAROS first inactivates the target
gene by binding to its promoter and then transposes it into a heterochromatic
nuclear domain in order to stabilise the inactivation.
VI HETEROCHROMATIN DISEASES
1) Diseases of the constitutive heterochromatin
These diseases are generally the result of an alteration in the process of
They may be constitutional, as in the case
of the ICF and Roberts syndromes:
The ICF syndrome associates Immunodeficiency, Centromeric
instability and Facial anomalies. It is a rare recessive
disease that is linked to mutations of the gene DNMT3B, a DNA methyl transferase
localised on the long arm of chromosome 20 (Xu et al 1999).
The chromosomal anomalies mimic the anomalies obtained with 5-Azacytidine,
which is a demethylating agent. Naturally, it is the satellite DNAs that
are rich in G-C that are demethylated, that is to say, DNA satellites II
and III, and, to a lesser extent, satellite I. Consequently, it is mainly
the secondary constrictions of chromosomes 1 and 16 that present an instability.
Decondensation of the secondary constrictions alters the normal segregation
of the sister chromatids, which explains the formation of multiradial figures,
deletions, micronuclei, etc.
The centromeric HC that is rich in alpha-satellite DNA, then rich in A-T
bases is not affected by this instability.
- They may be acquired, then associated with various types of cancer.
Anomalies of the constitutive heterochromatin, involving either the DNA or
the heterochromatin proteins, have been found in many types of cancer.
In particular, non-Hodgkin's lymphoma and multiple
myeloma have been shown to be associated with anomalies of the secondary
constriction of chromosome 1, these anomalies being similar to those observed
in the ICF syndrome. This observation strongly suggests that an anomaly
of methylation could affect the satellite DNA in these acquired pathologies.
Indeed, it has been shown that there is a global hypomethylation of the
genome, associated, in particular, with a hypomethylation of DNA satellite
II. The hypomethylation is generally correlated with a worsening of the
phenotype. It may be that the tumour and oncogenesis progression are linked
to an imbalance of genes that results from rearrangements involving the
long arm of chromosome 1 or chromosome 16.
In metastatic breast cancer, it has been shown that
there is a decrease in the HP1 alpha protein, which is a protein that
is usually localised in the heterochromatic regions of the chromosomes.
2) Diseases of the facultative heterochromatin
They can result from a defect in the inactivation of
an X chromosome in female somatic cells.
Such a defect may, in particular, result from a mutation in the XIST gene
that is essential for initiating the process of inactivation on the X chromosome.
It may lead to the expression of an X-linked recessive disease in females.
In conclusion, although heterochromatin is apparently amorphous
and isolated at the periphery of the nucleus, it appears to have an absolutely
essential role in the organisation and function of the genome.
Throughout this review we have mainly presented the characteristics linked with
heterochromatin, be it constitutive or facultative. We have
shown that the properties of constitutive HC, despite the presence
of satellite DNA, are not fundamentally different from those of facultative
HC. It therefore seems clear that the mechanisms involved in facultative heterochromatinisation,
which are epigenetic mechanisms, are the same mechanisms that intervene in the
repression of euchromatin in general.
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|Written||01-2003||Marie-Geneviève Mattei and Judith Luciani|
|Inserm U 491, faculte de medecine, boulevard jean moulin, 13385 marseille, France|
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