www.ScienceTranslationalMedicine.org 23 June 2010 vol 2 issue 37 37ps31 1
More than 50 years ago, Wiskott and Aldrich
separately described features of an X-linked
recessive disorder of blood cell development
during childhood that is characterized, in
affected males, by a low number of small
platelets (microthrombocytopenia), bleed-
ing, skin rash (eczema), recurrent infections,
and early death (1, 2). The mutated gene
that gives rise to this disease—known today
as Wiskott-Aldrich syndrome (WAS)—was
positionally cloned in 1994 and is referred
to as WAS (3). Clinical symptoms of WAS
vary among patients and families and, in
addition to the original phenotypical ob-
servations, now include defects in adaptive
and innate immunity that involve CD4+
T helper type 1 (TH1) cells, T regulatory
cells, antibody-producing B cells, antigen-
presenting dendritic cells, natural killer
cells, and monocytes (4–6). In the most
severe cases, autoimmunity, pre-malignant
myelodysplastic syndrome (MDS) (a bone
marrow stem cell disorder associated with
defective myeloid lineage development),
and malignancies (usually Epstein-Barr
virus–positive B cell lymphoma) may
arise. Specific types of defects in the WAS-
encoded protein WASp are often but not
invariably associated with the severity of
patient phenotypes. For example, the lack
of WASp expression causes the most severe
phenotype, termed classic WAS; whereas in-
activating WASp missense mutations cause
less severe X-linked thrombocytopenia; and
activating WASp alterations generate milder
X-linked neutropenia (5, 7). The underly-
ing molecular mechanisms for some of the
disease manifestations, including throm-
bocytopenia, autoimmunity, eczema, and
most types of immune cell dysfunction, are
incompletely characterized. In contrast, ex-
tensive studies combine to suggest a mecha-
nism for TH1 cell dysfunction via a defect in
WASp-dependent cortical actin polymer-
ization in the cytoplasm of developing TH1
cells during T cell activation (4). In the cur-
rent issue of Science Translational Medicine,
a paper by Taylor et al. challenges this purely
cytoplasmic model of WASp function in
TH1 cell differentiation. The authors show
that WASp exists in the nucleus as part of
two distinct histone-modifying complexes
at the promoters of key TH1 genes, including
the master regulator gene TBX21. This new
work suggests a nuclear mechanism for the
disruption in WASp-dependent TH1 cell de-
velopment observed in WAS patients (8).
Normally, T cell receptor–antigen inter-
actions and T cell costimulation result in
the formation of an immune synapse be-
tween a T cell and an antigen-presenting
cell. During this process, inactive WASp is
recruited to plasma membrane lipid rafts in
stimulated TH0 cells by the WASp-interact-
ing protein and the NCK adaptor protein;
this is followed by WASp activation, which
occurs as a result of WASp binding to the
Rho guanosine triphosphatase CDC42 and
subsequent WASp phosphorylation (9–11).
WASp-regulated actin cytoskeleton remod-
eling is thought to be essential for relaying
signals to the nucleus that activate the TH1
gene expression program. Defective WASp
fails to induce filamentous actin (F-actin)
formation, which inhibits expression of the
TH1 master regulator gene TBX21 (TBET),
resulting in a block of TH1 cell differentia-
tion and the absence of TH1 cytokines, such
as interferon-γ (IFN-γ) (12). WASp also reg-
ulates hematopoietic cell chemotaxis, adhe-
sion, phagocytosis, and trafficking through
its role in organizing the immune synapse
by actin cytoskeleton remodeling (13).
Antigen-stimulated CD4+ TH1 cells secrete
cytokines (IFN-γ, tumor necrosis factor–β,
and interleukin-2) that activate the prolifera-
tion and killing activities of macrophages and
CD8+ cytotoxic T cells during an effective cel-
lular immune response. In WAS, the develop-
ment of TH1 immunity is impaired by a block
in the differentiation of TH1 cells from naïve
CD4+ TH0 cells, providing a plausible expla-
nation for the recurrent infections observed
in WAS patients. WASp is a 502–amino acid,
multidomain, nonenzymatic member of a
five-member family of scaffold proteins that
includes N-WASp and three SCAR/WAVE
proteins, all of which regulate actin polym-
erization into F-actin (14). WASp is activated
by a conformational change, which unfolds
the protein to permit its interaction with
the actin-related protein complex (ARP2/3)
through a small WASp VCA (verprolin, co-
filin, and acidic region) homology domain,
thereby initiating actin filament nucleation
in the cytoplasm (15, 16).
Although an impairment in cytosolic
actin polymerization is the current leading
hypothesis for the molecular basis of WAS,
it remains unclear how a general defect in
cortical actin polymerization causes a selec-
tive defect in TBX21 gene activation and TH1
cell differentiation. Also, mice that harbor
a form of WASp that is constitutively active
for enhanced cortical actin polymerization
still display a defect in the induction of T-bet
gene expression, which results in WAS-type
immunodeficiency (17). For these and other
reasons, including the fact that many actin-
binding proteins, such as the widely expressed
N-WASp, shuttle between the cytoplasm and
nucleus (18–20), Taylor and colleagues (8)
sought to determine whether WASp also
functions in the nucleus of T cells.
The authors showed that WASp is local-
ized in both the cytoplasm and nucleus of
primary human TH0 cells incubated in TH1-
polarizing culture conditions. In the nucle-
us, Taylor et al. (8) found that WASp associ-
ates with the histone-modifying complexes
H3K4-trimethyltransferase (RBBP5) and
H3K9-tridemethylase (JMJD2A), which are
recruited to TH1 gene promoters (TBX21,
RUNX3, and IFN-γ) but not TH2 (GATA-3)
or TH17 (RORc) gene promoters, to generate
open, transcription-competent euchroma-
IMMUNODEFICIENCY AND CHROMATIN
What’s WASp Doing in the Nucleus?
Michael A. Teitell
Published 23 June 2010; Volume 2 Issue 37 37ps31
Departments of Pathology and Pediatrics, and Jonsson
Comprehensive Cancer Center, David Geffen School of
Medicine at the University of California, Los Angeles, CA
Wiskott-Aldrich syndrome (WAS) is a rare X-linked recessive immunodeficiency disorder
of childhood that is caused by mutations in the WAS gene. WAS encodes WASp, a protein
that is known to function in the cytoplasm of hematopoietic cells and is required for the
induced differentiation of CD4+ T helper type 1 (TH1) lymphocytes. Now, a paper in Science
Translational Medicine describes another mechanism for impaired immunity in WAS by
showing that WASp localizes in the nucleus and regulates histone modifications and chro-
matin structure, thereby modulating expression of the TH1 master gene TBX21 (TBET).
www.ScienceTranslationalMedicine.org 23 June 2010 vol 2 issue 37 37ps31 2
tin during TH1 cell differentiation (Fig. 1A).
The transcription factors STAT1, T-BET,
and SP1, and general transcription pre-
initiation complexes that contain Mediator
TRAP220/MED1 and RNA polymerase II,
are also present at the TBX21 core promoter,
along with WASp and histone marks asso-
ciated with regions of active transcription
[including histone 3–lysine 4 trimethylation
(H3K4me3), histone 3–lysine 9 acetylation
(H3K9ac), and the histone 2A Z-variant,
In contrast, primary human T cells
in culture that lacked WASp expression
showed decreased RBBP5 enrichment at
the TBX21 promoter, which maintains a
TH0-like inactive, condensed heterochro-
matin state with predominantly repressive
H3K9me3 and H3K27me3 histone marks
(Fig. 1B). The inhibition of TH1 gene tran-
scription and TH1 cell differentiation in
T cells that lacked WASp expression was
ameliorated when the authors reconsti-
tuted WASp expression in a WAS patient–
derived TH cell line (human T cell leukemia
virus–immortalized by retroviral transduc-
tion) and incubated these transduced cells
with TH1-differentiating cytokines. Differ-
entiating TH1 cells also showed an enrich-
ment in F-actin at the TBX21 promoter,
which is consistent with a role for nuclear
ARP2/3 and F-actin in RNA polymerase
II–dependent gene transcription (21).
However, only two of three WASp-deficient
patient T cell samples showed reduced F-
actin at the TBX21 promoter with TH1
differentiating cytokines, suggesting that
WASp epigenetic activity may be separate
from its actin nucleation activity and more
relevant for regulating TBX21 transcrip-
tion and TH1 cell differentiation. This idea
requires further testing, although a normal
amount of F-actin at the TBX21 promoter
in one WASp-deficient T cell sample could
be generated by other actin-binding pro-
teins in the nucleus, including nonredun-
dant N-WASp (18).
Finally, WAS missense mutation–con-
taining T cells from patients showed more
chromatin-accessible histone modifications
and a more open chromatin configuration
(by a DNase hypersensitivity assay) at the
TBX21 locus with TH1 differentiation, as
compared to T cells that completely lacked
WASp. This finding suggests a link between
WAS disease severity and the extent of aber-
rant epigenetic control seen with specific de-
fects in WAS, although more samples need
to be examined to validate such an interpre-
tation. Taken together, the results of Taylor
et al. (8) suggest a crucial function for WASp
in selective TH1 chromatin regulation, TH1
gene expression, and TH1 cell differentia-
tion that may predict WAS disease severity
based on the epigenetic profile at TH1 gene
These exciting new findings raise many
interesting questions about the role of nu-
clear WASp in the regulation of TH1 gene
transcription and TH1 cell differentiation.
How WASp is selectively recruited by or as-
sembled into histone-modifying complexes
on TH1 target genes remains unresolved, as
is the role of nuclear WASp in regulating F-
actin. The mechanism behind the localiza-
tion of WASp in the nucleus of T cells also is
unclear. Furthermore, it is unknown wheth-
er WASp arrives in the nucleus in an active
conformation for actin polymerization or
assembly into histone-modifying complex-
es, or whether further nuclear activation of
WASp is required. Answers to these ques-
tions await additional mechanistic investi-
gations to decipher essential details behind
the nuclear role of WASp in healthy and
WAS T cells.
Early diagnosis is essential for prophylaxis
(against infections) and treatment of WAS
patients. Currently, the most effective treat-
ment is hematopoietic stem cell (HSC) trans-
plantation, which has yielded robust results
thus far when HSCs are taken from antigen-
matched family or unrelated donors, or even
from partially matched umbilical cord donors
(22–26). Early gene therapy trials are under
way that use harvested HSCs transduced
with a retroviral vector that expresses a func-
tional WAS cDNA. Furthermore, researchers
are in the process of developing a lentiviral
WAS gene replacement strategy that uses the
endogenous WAS gene promoter, which has
the potential to be safer than retroviral trans-
duction (4). However, patient management
when HSC transplantation is not an option
remains prophylaxis or targeted treatment
of specific disease symptoms. For example,
male infants with classic WAS receive pro-
phylactic protection from opportunistic
lung infections caused by Pneumocystis
carinii and intravenous immunoglobu-
lin infusions to combat additional bacte-
rial, viral, and fungal infections. Platelet
transfusions and removal of the spleen are
performed to stem active bleeding caused
by low platelet numbers and to help block
WAS platelet destruction, although sple-
nectomy is contraindicated in a young
patient who is expected to undergo poten-
tially curative HSC transplantation (27).
Steroids and other immunosuppressants
are used to treat eczema and a broad spec-
trum of WAS autoimmune manifestations.
Intriguingly, WASp nuclear activity in chro-
matin-modifying complexes suggests the
possibility that agents that stimulate
chromatin-activating histone and pos-
sibly DNA demethylating modifications
could have a previously unappreciated
role in managing WAS patients, particu-
larly with respect to TH1 cell immunode-
ficiency. Although nonspecific, the pan–
Fig. 1. Nuclear WASp opens chromatin. (A) Healthy t cells induced to undergo tH1 cell differen-
tiation contain nuclear WAsp and actively transcribe tH1-related genes, including the tH1 master
gene, TBX21 (TBET). (B) When WAsp is missing, the chromatin in tH0 cells remains compact. tH1-
related gene expression and thus tH1 cell differentiation are blocked.
WASpASp p p p p p p p p p p p
CREDIT: C. BICKEL/SCIENCE TRANSLATIONAL MEDICINE
www.ScienceTranslationalMedicine.org 23 June 2010 vol 2 issue 37 37ps31 3
histone deacetylase (HDAC) inhibitor
suberoylanilide hydroxamic acid (SAHA,
vorinostat) and the DNA methyltransferase
(DNMT) inhibitors 5-azacytidine and
5-aza-2′-deoxycytidine are U.S. Food and
Drug Administration–approved for the
treatment of MDS, which can be part of
the clinical spectrum for classic WAS. The
development of drugs that control histone
methylation could one day have an impact
on the accessibility of inactive TH1 gene pro-
moters, such as TBX21, allowing TH1 cell
differentiation and the recovery of defective
cellular immune function. Today, augment-
ing open chromatin by increasing H3K9ac
and/or DNA demethylation with HDAC
and DNMT inhibitors in WASp-deficient
T cells from mice (28) or harvested from
WAS patients could be evaluated preclini-
cally for TH1 gene activation and the correc-
tion of TH1 immune deficiency. For poorly
understood reasons, pan-HDAC inhibitors
induce responses that include cell cycle ar-
rest, apoptosis, and terminal differentiation,
with the latter outcome being desirable for
WAS TH1 cell differentiation. With more
specific epigenetic modifying drugs to be
discovered, validated, and tested for safety
and efficacy, altered chromatin from defec-
tive nuclear WASp could provide a surpris-
ing and interesting new target in the future
management of the immunodeficiency seen
in WAS patients.
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Citation: M. A. teitell, Alternative control: What’s WAsp doing
in the nucleus? Sci. Transl. Med. 2, 37ps31 (2010).