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MINIREVIEW
Gene silencing at the nuclear periphery
Sigal Shaklai, Ninette Amariglio, Gideon Rechavi and Amos J. Simon
Sheba Cancer Research Center and the Institute of Hematology, The Chaim Sheba Medical Center, Tel Hashomer and the Sackler School of
Medicine, Tel Aviv University, Israel
The nuclear lamina
The nuclear envelope (NE), which separates the nucleus
from the cytoplasm, consists of the outer (ONM) and
inner (INM) nuclear membranes and nuclear pore com-
plexes (NPCs). The ONM is continuous with the endo-
plasmic reticulum (ER). The INM and ONM are
separated by a lumenal space, but join at sites that
are occupied by NPCs, which mediate bidirectional
transport of macromolecules between the cytoplasm
and the nucleus. The luminal space between the ONM
and INM is crossed by giant protein complexes
that bridge the NE and mechanically couple the cyto-
skeleton to the nucleoskeleton (reviewed in [1]). In
Keywords
epigenetics; gene silencing;
heterochromatin; histone modifications;
LAP2; laminopathies; nuclear envelope;
nuclear envelopathies; nuclear lamina;
transcription
Correspondence
A. J. Simon, Sheba Cancer Research Center
and the Institute of Hematology, The Chaim
Sheba Medical Center, Tel Hashomer and
the Sackler School of Medicine, Tel Aviv
University, Israel
Fax: 972 3 530 5351

Tel: 972 3 530 5814
E-mail:
(Received 21 August 2006, revised 4
January 2007, accepted 8 January 2007)
doi:10.1111/j.1742-4658.2007.05697.x
The nuclear envelope (NE) is composed of inner and outer nuclear mem-
branes (INM and ONM, respectively), nuclear pore complexes and an
underlying mesh like supportive structure – the lamina. It has long been
known that heterochromatin clusters at the nuclear periphery adjacent to
the nuclear lamina, hinting that proteins of the lamina may participate in
regulation of gene expression. Recent studies on the molecular mechanisms
involved show that proteins of the nuclear envelope participate in regula-
tion of transcription on several levels, from direct binding to transcription
factors to induction of epigenetic histone modifications. Three INM pro-
teins; lamin B receptor, lamina-associated polypeptide 2b and emerin, were
shown to bind chromatin modifiers and ⁄ or transcriptional repressors indu-
cing, at least in one case, histone deacetylation. Emerin and another INM
protein, MAN1, have been linked to down-regulation of specific signaling
pathways, the retino blastoma 1 ⁄ E2F MyoD and transforming growth fac-
tor beta ⁄ bone morphogenic protein, respectively. Therefore, cumulative
data suggests that proteins of the nuclear lamina regulate transcription by
recruiting chromatin modifiers and transcription factors to the nuclear per-
iphery. In this minireview we describe the recent literature concerning
mechanisms of gene repression by proteins of the NE and suggest the
hypothesis that the epigenetic ‘histone code’, dictating transcriptional
repression, is ‘written’ in part, at the NE by its proteins. Finally, as aber-
rant gene expression is one of the mechanisms speculated to underlie the
newly discovered group of genetic diseases termed nuclear envelo-
pathies ⁄ laminopathies, elucidating the repressive role of NE proteins is a
major challenge to both researchers and clinicians.

Abbreviations
BAF, barrier-to-autointegration factor; EDMD, Emery–Dreifuss muscular dystrophy; GCL, germ cell less; HDAC, histone deacetylase;
HP1, heterochromatin protein 1; IBSN, infantile bilateral striatal necrosis; INM, inner nuclear membrane; KASH, Klarsicht, ANC-1 and SYNE1
homology; LAP2b, lamina-associated polypeptide 2b; LBR, lamin B receptor; LEM domain, LAP2-emerin-MAN1 domain; ONM, outer nuclear
membrane; NE, nuclear envelope; NES1, Nesprin-1; NES2g, Nesprin-2 giant; NPC, nuclear pore complexes; pRb, retinoblastoma protein;
SREBP, sterol response element binding protein; SUN, S-phase arrest defective 1 and UNC-84 homology.; TGFb/BMP, transforming growth
factor beta ⁄ bone morphogenic protein.
FEBS Journal 274 (2007) 1383–1392 ª 2007 The Authors Journal compilation ª 2007 FEBS 1383
particular, SUN (S-phase arrest defective 1 and UNC-
84 homology) domain family of nuclear envelope pro-
teins, such as Caenorhabditis elegans UNC-84 [2] and
matefin ⁄ SUN-1 [3], interacts with various KASH
[Klarsicht, actin-noncomplementing (ANC-1) and synap-
tic nuclear envelope-1 (SYNE1) homology] domain
partners, such as ANC-1 [4], UNC-83 [5], and ZYG-12
[6], to form SUN domain-dependent ‘bridges’ across the
inner and outer nuclear membranes. In this network of
SUN–KASH interactions UNC-84 can bind either
ANC-1, which binds actin, or UNC-83, which binds
microtubules via an unidentified microtubule-dependent
motor protein. Matefin ⁄ SUN-1 binds ZYG-12 dimers,
which bind the microtubule-organizing centre. Human
proteins SUN1 and SUN2 anchor Nesprin-2 (also
known as syne-2 and NUANCE) giant (NES2g) at the
ONM. NES2g and Nesprin-1 (also known as CPG2,
syne-1, myne-1 and Enaptin) giant isoform (NES1g),
each bind actin. Nesprin-3 (NES3) binds plectin, which
links cytoplasmic intermediate filaments to actin
(reviewed in [1,7,8]). These bridges physically connect
the nucleus to component of the cytoskeleton. By ser-

ving, both as mechanical adaptors and nuclear envelope
receptors, it is proposed that SUN domain proteins con-
nect cytoplasmic and nucleoplasmic activities [1]. At the
nucleoplasm this complex is proposed to be bound by
lamins [9]. Lining the nucleoplasmic side of the NE, and
in close contact with it, is the nuclear lamina (reviewed
in [10,10a]). It is a protein meshwork composed of
lamins and a growing number of NE lamin binding pro-
teins. The nuclear lamina is proposed to have essential
roles in chromatin and NPCs architecture and organiza-
tion [11–15], nuclear positioning [4,5], NE breakdown
and reassembly during mitosis [16], DNA replication
[17], RNA polymerase II-dependent gene expression
[18], and transcriptional repression [19,20]. The number
of lamin-binding INM proteins that have been identified
in mammalian cells is growing rapidly [21]. The identifi-
cation and analysis of these proteins are essential to
understanding the diverse cellular functions attributed
to the nuclear lamina. Lamins are type-V intermediate-
filament proteins, which have a short N-terminal ‘head’
domain, a long a-helical coiled-coil ‘rod’ domain, and a
globular ‘tail’ domain (reviewed in [10,10a]). In mam-
malian cells there are two types of lamins: A- and
B-types. Lamin A ⁄ C proteins are the alternatively
spliced isoforms of LMNA gene. These lamins are
expressed in a tissue-specific manner, disperse as soluble
proteins during mitosis, and are probably incorporated
into the nuclear lamina later than B-type lamins during
postmitotic NE reassembly. B-type lamins are essential
for cell viability and are expressed in all cells during

development. During mitosis the B-type lamins are
found in a membrane-bound form, attached to the dis-
assembled inner nuclear membranes (and their associ-
ating proteins), suggesting their complete cellular
segregation from A-type lamins when the NE is disas-
sembled [10a]. In mammalian cells lamins bind in vitro
to many known INM proteins, including emerin,
MAN1, lamin B receptor (LBR), lamina-associated
polypeptides-1 and 2 (LAP1, LAP2) isoforms and Nes-
prin-1a. In addition, lamins bind nucleoplasmic soluble
proteins, such as the chromatin histones H2A and H2B
dimers and barrier-to-autointegration factor (BAF), as
well as LAP2a, Kruppel-like protein (MOK2), actin,
retinoblastoma protein (RB), sterol response element
binding protein (SREBP), components of RNA polym-
erase II-dependent transcription complexes and DNA
replication complexes [22,23]. Mutations in lamins and
lamin-binding proteins cause a wide range of heritable
or sporadic human diseases, which are collectively
known as the ‘nuclear envelopathies’ or ‘laminopathies’
[24–26]. The majority of these disorders were linked to
mutations in A-type lamins. However, mutations in four
integral INM lamin binding proteins have also been
implicated as a cause of ‘nuclear envelopathies’. Of the
four proteins LBR, emerin, LAP2 and MAN1, the three
latter share the conserved 40 amino acid chromatin
binding LAP2-emerin-MAN1 (LEM) domain [27,28].
Heterochromatin and the nuclear
periphery
Various studies have established that a correlation

exists between positioning of genes at the nuclear per-
iphery and their silencing (reviewed in [29] and above).
Gene poor chromosomes have been shown to be more
peripherally configured than gene rich chromosomes
[30–32] and transcriptionally silent genes are located at
or translocated to the nuclear periphery upon silencing
[33,34]. Additionally, several experiments in various
model systems have shown that the translocation of
chromatin regions to the nuclear periphery results in
silencing of the genes in these regions. In Drosophila,
insertion of the gypsy insulator into a DNA sequence
caused translocation of that sequence to the nuclear
periphery correlating with changes in gene expression
[35]. In mammalian cells, the Ikaros transcriptional
regulator, which activates lymphocyte-specific expres-
sion, was found to associate with transcriptionally inac-
tive genes at centromeric loci [33]. Immunoglobulin loci
in inactivated pro-T cells preferentially colocalized with
lamin B at the nuclear periphery, while they were cen-
trally configured and active in pro-B cells [36]. Simi-
larly, dissociation of the transcriptional repressor Oct-1
from lamin B and the nuclear periphery was correlated
Gene silencing at the nuclear periphery S. Shaklai et al.
1384 FEBS Journal 274 (2007) 1383–1392 ª 2007 The Authors Journal compilation ª 2007 FEBS
with reduced inhibitory activity [37]. Several studies
have made use of the lac-operator ⁄ repressor system [38]
to demonstrate in vivo the ability of genetically engin-
eered chromosome regions to undergo decondensation
[39–41] and intranuclear repositioning [42–44] when
activated by transcription factors or acidic activation

domains. A clue to the mechanism by which intranu-
clear translocation occurs comes from the work of Chu-
ang and colleagues; they showed that migration of an
interphase chromosome locus from the nuclear periph-
ery to the nuclear center upon activation is disrupted
by specific actin or nuclear myosin mutants [45]. While
INM proteins in metazoans have been shown to func-
tion as repressors of transcription, gene regulation at
the nuclear periphery is probably a much more complex
process. Recent studies in yeast suggest that proteins of
the nuclear pore complex the nucleoporins (NUPS)
function as inhibitors of gene repression or rather as
activators of transcription. The nucleoporin Nup2p was
shown to tether chromatin to the nuclear pore complex
(NPC) blocking propagation of heterochromatin. Fur-
thermore, interaction of Nup2p with numerous genes
leads to their activation in what was coined the nucleo-
pore to-gene-promoter interaction (Nup-PI) [46]. Simi-
larly, transcriptionaly activated GAL1 genes are
preferentially found at the nuclear periphery where they
are linked to the NPC component Nup1 by SAGA
interacting factors [47]. These studies support the
notion that positioning of genes in the nuclear space
correlates to their transcriptional activity, still leaving
many unanswered questions as to the molecular mecha-
nisms by which repositioning is transacted.
Transcriptional repression by proteins
of the nuclear envelope
Peripherally located, transcriptionally silent chromatin
has distinctive structural characteristics (at the DNA

and chromatin levels) and has been shown to associate
with proteins of the nuclear lamina. Whether these
associations lead to the repressive chromatin pheno-
type or are a result of it is still unknown. In a recent
study in Drosophila melanogaster 500 genes interacting
with the nuclear lamina protein B-type lamin (DmO, a
Drosophila lamin), were identified and characterized
[48]. In this study B-type lamin (DmO) was fused to
the Escherichia coli enzyme DNA adenine methyl-
transferase. The genomic DNA fragments that were
methylated on their adenine residues were identified by
cDNA microarray analysis. These genes displayed four
main features: transcriptional inactivity, lack of ‘active’
histone marks, late replication timing and presence of
long intergenic regions. Several large scale studies in
mammalian tissues have also addressed the question of
the components of nuclear envelope–chromatin associ-
ated complexes. Schirmer et al. [21], in an attempt to
identify new integral nuclear envelope proteins subjec-
ted rat liver nuclear envelopes and cofractionated
organelles to a subtractive proteomic analysis. Proteins
remaining in the nuclear fractions included histones,
chromatin associated proteins and transcription fac-
tors. Georgatus and colleagues isolated mononucleo-
somes attached to the LBR, from fractions of
peripheral heterochromatin and demonstrated that
they contain a distinct acetylatalion ⁄ methylation pat-
tern befitting heterochromatin [49]. At least three INM
proteins were shown to directly associate with chroma-
tin modifiers and transcriptional repressors: 1. Lam-

in B receptor (LBR) was found to associate with
heterochromatin protein 1 (HP1) and histones H3 ⁄ H4
under deacetylating conditions [50]; 2. lamina-associ-
ated polypeptide 2b (LAP2b) was shown by us to bind
the transcriptional repressors germ cell less (GCL) [19]
and histone deacetylase 3 (HDAC3) resulting in the
latter case in deacetylation of histone H4 [20]; 3. Emerin
was shown to associate with the death promoting fac-
tor Btf [51], the splicing associated factor YT521-B
[52] and similar to LAP2b, with the transcriptional
repressor GCL [53]. Other components of the nuclear
lamina shown to interact with transcriptional regula-
tors include LAP2a, a nucleoplasmic LAP2 isoform,
and lamin A ⁄ C [54–56]. LAP2a was shown to complex
with lamin A ⁄ C and the retinoblastoma protein (pRb).
Reduced levels of LAP2a or its aberrant localization
caused mislocalization of pRb suggesting that LAP2a
and lamin A ⁄ C serve as anchoring sites for this protein
[55]. As mentioned, lamin A ⁄ C interacts with histones,
components of the RNA II polymerase transcriptional
complex (reviewed in [10]), SREBP [57] and dephos-
phorylated pRb [55]. With regard to transcription, two
INM LEM domain proteins, MAN1 and emerin, have
been linked to specific pathways. MAN1 has been
shown to antagonize TGFb ⁄ BMP signaling through
binding to receptor-regulated Smads [sma (C. elegans)
and mothers against decapentaplegic (DPP, Droso-
phila) homologues], inhibiting downstream signaling
and preventing normal ventralization in Xenopus laevis
embryos [58–60,60a]. Emerin loss, responsible for

X-linked Emery–Dreifuss muscular dystrophy, has
been shown by two recent studies to result in deregula-
tion of the Rb1 ⁄ E2F MyoD pathway involved in mus-
cle regeneration [61,62]. Stewart and his colleagues
evaluated regenerating muscle of emerin and lamin A
null mice. In addition to Rb and MyoD, lack of emer-
in resulted in up regulation of the transcriptional
repression modifiers histone deacetylase 1 (HDAC1),
S. Shaklai et al. Gene silencing at the nuclear periphery
FEBS Journal 274 (2007) 1383–1392 ª 2007 The Authors Journal compilation ª 2007 FEBS 1385
histone methyl transferase Suv39H and HP1a in what
was speculated to be a compensatory effect [61]. Both
studies are in accordance with our previous findings
[20,63], which link proteins of the nuclear envelope,
such as the LAP2 family to epigenetic gene regulation.
The epigenetic ‘histone code’ regulates
transcription
Histones undergo various types of post-translational
modifications, including acetylation and methylation of
lysines and arginines, phosphorylation of serines and
threonines, ubiquitylation and sumoylation of lysines,
as well as ribosylation. These reversible epigenetic
modifications are executed by histone modifying
enzymes, such as histone acetyl transferases and their
antagonists histone deacetylases (HDACs), histone
methyltransferases and their antagonists histone deme-
thylases, histone kinases and their antagonists histone
phosphatases and enzymes with sumoylation, ubiquity-
lation and ribosylation activities. The epigenetic ‘his-
tone code’ (or histone mark) is the pattern of these

modifications. Its complexity results from the enor-
mous number of combinations of modification type,
number and sites on which they occur in each histone.
For example, histone H3 can be acetylated on its
lysine 9 (later on written as H3 K9 acetylation), phos-
phorylated on the adjacent serine 10 residue and
methylated on its lysine 27, individually or all at the
same time. Further complexity results from the possi-
bility of single lysine and arginine residues to undergo
mono-, di- or tri- (in the case of lysine) methylation.
The histone code influences the structure of the chro-
matin fiber aiding or abating its ability to undergo
transcription at that point. Site-specific combinations
of histone modifications have been shown to correlate
with transcriptional activation or repression. For
example, the combination of H4 K8 acetylation, H3
K14 acetylation, and H3 S10 phosphorylation is often
associated with transcriptional activation. Conversely,
tri-methylation of H3 K9 and the lack of H3 and H4
acetylation correlate with transcriptional repression
(reviewed in [64,65]). Evidence points to the concentra-
tion of transcriptionally inactive heterochromatin,
lacking histone acetylation at the nuclear periphery as
opposed to acetylated, transcriptionaly competent euchro-
matin at the nuclear interior [66].
Nuclear envelopathies ⁄ laminopathies
and gene repression
Mutations in proteins of the nuclear lamina have been
shown to cause a wide array of genetic diseases termed
nuclear envelopathies ⁄ laminopathies [24–26]. Although

only few genes encoding nuclear lamina and pore
complex proteins have been identified as causing these
diseases the clinical manifestations are widely varied
[10,67]. They encompass premature ageing syndromes,
myopathies, neuropathies, lipodystrophies, dermopa-
thies and varied combinations of disease manifestations
[68]. The mutated genes underlying these disorders
include lamin A ⁄ C, which is responsible for the auto-
somal dominant form of Emery–Dreifuss muscular
dystrophy (EDMD) and various other laminopathies,
amongst them the Hutchison–Gilford progeria syn-
drome (HGPS) (reviewed in [69]); emerin, an INM
protein responsible for the X-linked cases of EDMD;
MAN1, another INM protein responsible for three
autosomal dominant diseases characterized by increased
bone density and elevated TGFb-BMP expression [70];
mutated LBR results in Pelger–Hue
¨
t anomaly and
Greenberg skeletal dysplasia, an autosomal ressesive
chondrodystrophy and LAP2a which has recently
shown to result in cardiomyopathy [71]. Two NPC pro-
teins, ALADIN (also termed Adracalin or AAAS) and
nup62, can be added to the expanding list of mutated
nucler lamina proteins causing these diseases. Muta-
tions in the WD-repeat ALADIN NPC protein cause
the Triple A syndrome, a human autosomal recessive
disorder characterized by an unusual array of tissue-
specific defects [71a]. In collaboration with Mordechai
Shohat and his colleagues we recently found that

mutated nup62 causes autosomal recessive familial
infantile bilateral striatal necrosis (IBSN) severe neuro-
logical disorder [72], IBSN is characterized by symmet-
rical degeneration of the caudate nucleus, putamen,
and occasionally the globus pallidus, with little involve-
ment of the rest of the brain.
The question of how mutations in the same gene or
group of genes, which are ubiquitously expressed, cause
such a wide variety of tissue specific diseases has linked
the laminopathies to the study of transcription regula-
tion. Two major models attempt to explain how
mutated lamins and NE proteins lead to the observed
pathologies: the mechanical stress model and the gene
expression model. The mechanical stress model suggests
that nuclei that contain defective lamin or emerin pro-
teins might be mechanically more fragile than their
wildtype counterparts. This model relies on studies in
C. elegans, D. melanogaster and mice, showing dra-
matic defects in NE structure in nuclei that are deficient
in lamins, and that this fragility could ultimately lead
to nuclear damage and cell death [68]. The idea of
enhanced nuclear fragility is particularly attractive as
an explanation for the cardiac- and skeletal-muscle
pathologies, as the forces that are generated during
Gene silencing at the nuclear periphery S. Shaklai et al.
1386 FEBS Journal 274 (2007) 1383–1392 ª 2007 The Authors Journal compilation ª 2007 FEBS
muscle contraction might potentially lead to preferen-
tial breakage of nuclei that contain a defective nuclear
lamina. Nuclei in noncontractile tissues might remain
relatively unscathed, despite showing abnormal nuclear

and NE organization. The second model, that of
perturbed gene expression, is based on cumulative
evidence showing involvement of nuclear lamina pro-
teins in gene repression [29]. According to this model
mutations in lamina proteins could promote diseases
by compromising various gene regulatory pathways in
different tissues. This model is supported by several
lines of evidence: Primarily evidence to the association
of transcriptional regulators with proteins of the nuc-
lear lamina as described above, additionally morpholo-
gical studies showing disrupted heterochromatin at the
nuclear periphery of cells from laminopathy patients
and finally impaired epigenetic histone modifications in
lamin A mutated cells. Proteins of the nuclear lamina
most probably exert their effect in several nonexclusive
modes. One such example is the pRb protein which
was shown to bind both lamin A and LAP2a [55,73].
pRb, besides being involved in inhibition of prolifer-
ation, is important for skeletal muscle and adipose tis-
sue differentiation, two tissues which are frequently
affected in the ‘nuclear envelopathies’ [74]. In order to
mediate at least some of its effects pRb recruits various
histone modifying enzymes to its target promoters,
amongst them HDAC1, 2 and 3. Interestingly, a pRb–
HDAC3 complex was shown to be important for the
regulation of adipocyte differentiation by peroxisome
proliferator-activated receptor gamma [75]. Another
example of the multifaceted effects of mutated lamina
proteins are studies on cells from EDMD patients and
lamin A knockout mice showing altered organization

of heterochromatin at the nuclear periphery [76]. Simi-
larly, light and electron microscopy analyses of HGPS
fibroblasts reveal significant changes in nuclear shape,
including lobulation of the nuclear envelope, thickening
of the nuclear lamina, loss of peripheral heterochroma-
tin, and clustering of nuclear pores [77]. These struc-
tural defects worsen as HGPS cells age in culture. The
authors suggest that nuclear lamina defects in these
cells are due to the disruption of lamin-related func-
tions, ranging from the maintenance of nuclear shape
to regulation of gene expression and DNA replication.
Goldman and colleagues further analyzed the mecha-
nisms responsible for the loss of heterochromatin in
cells of HPGS patients [78]. For this purpose epigenetic
marks regulating facultative and constitutive hetero-
chromatin were examined. In cells originating from a
female HGPS patient, the transcriptionally repressive
histone H3 trimethylated on lysine 27 (H3 K27me3)
marker of facultative heterochromatin, was lost on the
inactive X chromosome (Xi). The methyltransferase
responsible for this epigenetic modification, EZH2, was
down-regulated. These alterations were detectable
before the changes in nuclear shape, reported earlier
[77]. Another transcriptionally repressive epigenetic
mark, histone H3 trimethylated on lysine 9 (H3
K9me3) which marks pericentric constitutive hetero-
chromatin, was down-regulated in these cells. This
change correlated with an altered association of the
H3K9me3 with HP1a and the calcinosis, Raynauds
phenomenon, esophageal dysmotility, sclerodactyly, tel-

engiectasia (CREST) antigen. In contrast to the decrea-
ses in histone H3 methylation states an increase in
trimethylation of histone H4 on lysine 20, an epigenetic
mark for constitutive heterochromatin, was observed
[78]. This study is the first to define specific alterations
in histone lysine methylation as early events in disease
pathology, suggesting that either mutated lamin A or
general distortions of the nuclear lamina impair regula-
tion of epigenetic modifications. HGPS has always been
an appealing disease for researchers due to the possible
implications for the study of ageing. Recently a direct
link has been formed between heterochromatinization
defects leading to HGPS, and ageing. Scaffidi and
Misteli [79] showed that cell nuclei from old individuals
acquire similar defects to those of HGPS patients,
including changes in histone modifications and
increased DNA damage. While cells from young indi-
viduals (3–11 years old) showed robust staining of the
transcriptional repressive heterochromatin marks HP1,
LAP2s and Tri-Me-K9H3, a significant subpopulation
of nuclei in cells from old individuals displayed reduced
signals, similar to previous observations in HGPS cells
[80]. These observations implicate lamin A and nuclear
lamina-dependent epigenetic alterations as involved not
only in nuclear envelopathies but also in the physiologi-
cal process of aging.
Summary
The idea that genes are silenced at the nuclear periph-
ery is not new. In the early 1960s Mirsky and col-
leagues showed in electron micrographs of calf thymus

nuclei the peripheral localization of condensed hetero-
chromatic regions and the more centered localization
of diffused euchromatic regions. RNA synthesis was
more active in the diffused interior euchromatin than
in the condensed peripheral heterochromatin [81]. The
heterochromatic sex-chromatin body of Barr, which in
female mammalian cells is composed of a segment of
one X chromosome, was found by this group to carry
unexpressed genes. The DNA of this inactivated
X chromosome replicated later than that of other
S. Shaklai et al. Gene silencing at the nuclear periphery
FEBS Journal 274 (2007) 1383–1392 ª 2007 The Authors Journal compilation ª 2007 FEBS 1387
chromosomal segments [82]. Today we know that
the inactivated X chromosome resides at the nuclear
periphery and we use it as a compelling example of
chromosome-wide, long-range epigenetic gene silencing
in mammals (reviewed in [83]). Since the fundamental
discoveries by the group of Mirsky the development of
experimental tools, such as fluorescence in situ hybrid-
ization combined with three-dimensional microscopy,
to analyze chromosomes and proteins in living cells,
together with complementary approaches that explore
the computational biology, epigenetic modifications
and gene expression profiling along the chromosomes,
offer us today the possibility of visualizing ‘real time’
gene expression. We can follow the looping out or
‘jumping’ of loci from their gene repressed heterochro-
matic territory at the nuclear periphery to more inter-
nal gene active euchromatic territories for their
transcription [34,45,84,85]. However, still little is

known about the molecular mechanisms responsible
for nuclear lamina-dependent gene regulation.
In recent years great advancement has been achieved
in understanding the role of the nuclear lamina ⁄ envel-
ope in regulation of transcription. A major boost to
the subject arrived from the unexpected finding that
mutations in nuclear lamina ⁄ envelope proteins are the
cause of a large family of diseases with varied expres-
sion. Involvement of INM proteins in transcription
occurs on different levels. By binding transcription fac-
tors they inhibit gene expression on the basic linear
DNA level and by binding chromatin modifiers they
influence gene expression at the epigenetic level.
Our model (Fig. 1) suggests that under certain, yet
unknown, physiological conditions, NE proteins, such
as LAP2b and LBR are triggered to modify the chro-
matin in their vicinity in order to induce gene silencing
there. The transition from nucleoplasm decondensed
gene active euchromatin to condensed gene silenced
heterochromatin requires the creation of transcrip-
tional repressive environment at the nuclear periphery.
This can be achieved by forming repressive complexes
by the NE proteins as illustrated in Fig. 1. By serving
as docking sites at the INM, LAP2b, LBR, and poss-
ibly other INM proteins, can anchor DNA, chromatin
and chromatin modifiers in order to execute reversible
epigenetic modifications on DNA, histone tails and
transcription factors. We propose that the outcome of
these energy-free or energy-dependent enzymatic
reactions is the remodeling of the INM-attached chro-

matin, such that specific loci and genes are transcrip-
tionally inhibited. Our model, based on LAP2b and
Fig. 1. Gene silencing at the nuclear periphery. The ONM is a continuation of the endoplasmic reticulum. It joins the INM at the NPCs.
Lamins A ⁄ C (black line) and B (red line) are shown as filaments at the nuclear periphery and across the nucleoplasm (lamin A ⁄ C). Associa-
tions of the INM proteins LAP2b, LBR, emerin and MAN1 with lamins, chromatin and their specific partners are shown: LAP2b binds
BAF, HDAC3, GCL and HA95; LBR binds HP1, histones H3 and H4; MAN1 binds BAF and GCL, emerin binds BAF and GCL competitively,
YT521-B and actin. The question marks indicate, yet unidentified, LAP2b-associating proteins catalyzing gene silencing through epigenetic
modifications. The two chromatin states, of gene-active unwrapped euchromatin at the nucleoplasm, and of gene-silenced condensed
heterochromatin at the vicinity of the INM are circled. In the latter state, epigenetic modifications on histones and DNA are illustrated. The
intranuclear complex containing LAP2a, lamin A, Rb and BAF proteins is shown. C, cytosol; NL, nuclear lamina; NP, nucleoplasm; NPC, nuc-
lear pore complex; ONM, outer nuclear membrane; INM, inner nuclear membrane; H-Chr (GR), heterochromatin (gene repression); Ec-Chr
(GA), euchromatin (gene activation); Me, methylation; deAC, deacetylation; Ri, ribosylation; Ub, ubiquitination; P, phosphorylation.
Gene silencing at the nuclear periphery S. Shaklai et al.
1388 FEBS Journal 274 (2007) 1383–1392 ª 2007 The Authors Journal compilation ª 2007 FEBS
LBR studies, suggests that two collaborative under-
acetylated chromatin complexes are formed at and
anchored to the NE. In one complex, LAP2b recruits
the enzyme (HDAC3) while in the other complex LBR
recruits the substrates (histones H3 ⁄ H4) [20,50]. In
both cases, acetylation conditions, alleviated the
LAP2b ⁄ HDAC3 dependent transcriptional repression
[20] or dissociated the LBR–HP1–histones repressive
complex [50].
The proposed concept places proteins of the nuclear
lamina as high hierarchical transcriptional regulators.
This may have implications in the study of cancer dis-
eases, where a strong link was established in recent
years between gene inactivation and tumorigenesis,
mainly in hematological malignancies [63], and
NE ⁄ lamina associated diseases and ageing in which

perturbed gene regulation and peripheral heterochro-
matin formation have been shown to exist [79,86].
Acknowledgements
Our research is supported by the Israel Science Fund
grant no. 804. We thank ‘PA’AMEI TIKVA’ founda-
tion for their support of our research. GR holds the
Djerasi Chair for Oncology (Sackler School of Medi-
cine, Tel Aviv University, Israel).
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