Download Cell lineage and cell death: Caenorhabditis elegans and

REVIEWS
MODELS OF CANCER
Cell lineage and cell death:
Caenorhabditis elegans and
cancer research
Malia B. Potts* and Scott Cameron*‡
Abstract | Cancer is a complex disease in which cells have circumvented normal restraints on
tissue growth and have acquired complex abnormalities in their genomes, posing a
considerable challenge to identifying the pathways and mechanisms that drive fundamental
aspects of the malignant phenotype. Genetic analyses of the normal development of the
nematode Caenorhabditis elegans have revealed evolutionarily conserved mechanisms
through which individual cells establish their fates, and how they make and execute the
decision to survive or undergo programmed cell death. The pathways identified through
these studies have mammalian counterparts that are co-opted by malignant cells. Effective
cancer drugs now target some of these pathways, and more are likely to be discovered.
*Departments of Pediatrics
and Molecular Biology,
Division of Pediatric
Haematology-Oncology,
University of Texas
Southwestern Medical Center
at Dallas, Dallas, Texas
75390‑9148, USA.
‡
Present address: Novartis
Institutes for Biomedical
Research, Oncology
Translational Medicine,
220 Massachusetts Avenue,
Cambridge, Massachusetts
02139, USA.
Correspondence to S.C. e‑mail: johnscott.cameron@
novartis.com
doi:10.1038/nrc2984
Published online
2 December 2010; corrected
online 25 February 2011
In multicellular organisms, a remarkable array of spe­
cific cell types must be generated to build an animal,
and the inappropriate survival or proliferation of an
individual cell can ultimately be deleterious to the organ­
ism as a whole1. Thus, mechanisms have evolved that
precisely control cell fates and kill unwanted cells2. The
breakdown of these mechanisms can contribute to
the development of cancer 3.
Although cell fate decisions have long been appreci­
ated as a part of normal development, the involvement
of cell death in this process initially seemed intellec­
tually unsatisfactory — death is on some level the break­
down of normality, so how could it be reproducible in
a way that can be defined experimentally? And what
might be the purpose of generating many cells the only
apparent function of which is to die? In this Review, we
describe how studies of cell lineage and programmed
cell death in the nematode Caenorhabditis elegans have
revealed basic mechanisms of cell death regulation that
are important in human cancer, and how this research
is contributing to the development of new approaches
to cancer therapy.
C. elegans and cell lineage
It is remarkable that studying a 1 mm-long, transparent
worm that is not susceptible to cancer has led to tremen­
dous advances in our understanding of human diseases.
A single defining aspect of C. elegans biology provided
a foundation for these advances: its simple and essen­
tially invariant cell lineage4,5. This aspect of its biology,
together with classical and modern tools for the genetic
analysis of living animals, is a unifying theme of research
with this organism. The effects of discoveries made with
this worm extend broadly in biology because many fun­
damental processes that control animal development are
evolutionarily conserved.
In C. elegans, the term cell lineage refers to the pat­
tern of divisions from the zygote to the adult, and also
to the fates adopted by individual cells at each point in
the lineage. Knowledge of this pattern allows the study
of cell fate determination with unparalleled precision.
The adult hermaphrodite has 959 somatic cells; there­
fore, mutants with an abnormal number of cells can
be readily identified and the causes of the abnormality
(lack of cell death or change in cell fate specification, for
example) defined by simply following the cell divisions
as the mutants develop on a microscope slide. Helpfully,
mutants with cell lineage defects often also have obvi­
ous defects in function or morphology that are easier
to recognize than an abnormal number of cell nuclei.
Screens for additional mutants with similar or opposite
morphological or functional defects have then been used
to discover additional genes that drive or regulate the
processes involved. Below, we discuss the results from
such screens to discover the mechanism of programmed
cell death and highlight accumulating evidence show­
ing that the genes that control the death decision in
single somatic cells of C. elegans are evolutionarily con­
served and co-opted by malignant cells to ensure their
survival.
50 | JANUARY 2011 | VOLUME 11
www.nature.com/reviews/cancer
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
At a glance
•The invariant nature of the development of cell lineages in the nematode worm
Caenorhabditis elegans has made it a useful model for understanding the mechanisms
of cell lineage control and cell death.
•Many of the genes that regulate cell death and lineage development have
homologues in mammals.
•Mapping cell lineage development in the nematode has been particularly informative
for understanding the regulation of haematopoiesis in mammals, with many of the
genes that regulate cell lineage being deregulated in leukaemia and lymphoma.
•Further understanding of the regulation of cell fate in C. elegans and mammals should
identify new targets for anticancer drug development.
Programmed cell death
Programmed cell death, or apoptosis, is crucially impor­
tant in the genesis of cancer and in the success or failure
of cancer therapy. Groundbreaking work using C. elegans
helped to expose apoptosis as a genetically controlled
process, and studies in C. elegans have continued to
illuminate the identities and functions of the molecules
involved, paving the way for the development and testing
of new cancer therapeutics that are designed to induce
apoptosis in cancer cells.
The focus on programmed cell death in cancer
began with the important work of Kerr, Wyllie and
Currie6. Their remarkable 1972 paper described the
hallmark morphological features of apoptotic cells and
the importance of apoptosis in normal and malignant
tissues. The authors recognized that tissue and tumour
growth reflected an imbalance between the generation
of new cells by mitosis and the loss of cells by apop­
tosis, and that cancer therapies such as radiation and
chemotherapy drugs induced apoptosis. They specu­
lated that apoptosis might be controlled in some man­
ner by the genome, but the challenge of identifying
the mechanisms that execute a process affecting a tiny
proportion of cells in a tissue at any given time must
have seemed daunting.
At about this time, John Sulston and Bob Horvitz in
Sydney Brenner’s laboratory had begun describing the
C. elegans cell lineage. They observed that certain cells
were born, only to die shortly after birth, with an invari­
ant morphology 4,5. Why and how do these deaths occur,
and do mammalian lineages have similar deaths?
The role of death in cell lineage. Why bother with
a cell division that generates two cells, one of which
undergoes programmed cell death, rather than just
generating the intended single cell? One possibil­
ity is that the process of division is necessary for the
proper segregation of cell fate determinants. Another
possibility is that this is an efficient mechanism that
has not been selected against during evolution (FIG. 1).
Programmed cell death is used to eliminate cells that
adopt different fates in males and hermaphrodites, or
that are needed in one part of the animal but not in
another 5,7. Using programmed cell death in these ways
allows a single cell lineage programme to operate in
both sexes or in different parts of the animal with only
minor modifications.
It is not yet possible to examine the cell lineages of
mammalian cells with the precision that is available for
C. elegans, but there are similarities in the mechanisms
that underlie cell fate decisions, and it seems likely that
mammalian lineages might use programmed cell death
in ways similar to those described in C. elegans. Indeed,
there are compelling examples of conserved mechanisms
of cell lineage control between C. elegans and haemato­
poietic lineage factors8–12, and apoptosis is a prominent
and unexplained aspect of the initial attempts to follow
mammalian haematopoietic lineages in vitro13.
From early observations of cell death to mechanisms.
The reproducible morphology and pattern of deaths in
C. elegans provided a platform for sophisticated genetic
screens that identified the molecules that are responsi­
ble for cell death. The identification and analysis of the
genes discovered in these screens revealed a genetically
encoded mechanism of cellular suicide (FIG. 2). Important
discoveries included the demonstration that CED‑3,
a protease (more specifically, a caspase), and CED‑4,
which was later discovered to be conserved in mammals
in the form of a scaffolding protein APAF1, functioned
cell autonomously and were essential for cell death2,14,15.
CED‑9 antagonized the functions of CED‑3 and CED‑4
to prevent cell death16,17. The deaths of somatic cells are
induced by egg-laying defective 1 (EGL‑1)18,19, which was
originally identified in screens for egg-laying-defective
mutant worms. As discussed below, the transcriptional
regulation of egl‑1 expression has turned out to be a
crucial step for the cell death decision, and BH3‑only
proteins serve similar functions in humans. The iden­
tification of CED‑9 as a homologue of BCL‑2 (REF. 17)
has been cited as the ‘Aha!’ moment of discovery in the
work on programmed cell death20, as it suggested that
the mechanism was evolutionarily conserved in animals.
The conservation of this crucial function in cell death
was supported by experiments in which human BCL‑2
expression rescued the cell death phenotype that is evi­
dent in ced‑9 mutants16,21. Moreover, because mutations
affecting BCL‑2 cause lymphoma by preventing the death
of lymphocytes22, it became obvious that defects in pro­
grammed cell death contributed to human disease. This
was further underpinned by discovering that mice that
are deficient for the pro-apoptotic BH3‑only proteincoding gene Bad develop lymphoma, and that mice that
are deficient in Bid develop a myeloproliferative disor­
der that can progress to leukaemia23,24. Thus, the genetic
analysis of programmed cell death in C. elegans revealed
the mechanism underlying a fundamental process
of development that is abnormal in cancer cells.
Cell death in humans: elaborating on the simple ele‑
gance of the worm. Each of the four genes that consti­
tutes the canonical, core pathway of programmed cell
death in C. elegans has one or more homologous genes in
mammals (FIG. 2). In worms and mammals, cell death can
be initiated by transcriptional activation of BH3‑only
genes18, which activate the adaptor CED‑4 or APAF1,
and this is antagonized by the pro-survival Bcl‑2 fam­
ily members19. CED‑4 or APAF1 then participate in
NATURE REVIEWS | CANCER
VOLUME 11 | JANUARY 2011 | 51
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
the activation of the caspases, which cleave multiple
targets to kill the cell25. Despite the obvious homology
between the human and nematode apoptosis path­
ways, some divergence is also apparent. In contrast to
humans, cytochrome c is not required for the activation
of nematode CED‑3 by CED‑4 in vitro, and CED‑4 lacks
the WD domain repeats that mediate interaction with
cytochrome c in the apoptosome of mammals26–28. These
and other data suggest that the mitochondrial release
of cytochrome c is not part of cell killing in C. elegans.
In a second important difference from humans, caspase
inhibition by inhibitor of apoptosis (IAP) proteins is
not apparently used as a mechanism of regulating pro­
grammed cell death in nematodes. In this regard C. ele‑
gans also differs from Drosophila melanogaster, which
relies heavily on antagonism of IAP function rather
than the generation of BH3‑only proteins to initiate cell
death29,30. Therefore, mammalian apoptotic pathways are
Figure 1 | The role of programmed cell death in cell lineage. a | Cell death is often
used to eliminate cells that are the non-essential spatial homologues of cells that are
required in other parts of the body. For example, the individual cell lineages that generate
the mature ventral nerve cord along the anterior–posterior body axis of Caenorhabditis
elegans are very similar to one another, and programmed cell death is used to kill off cells
that are not needed in certain body regions4. Specifically, six neurons generated by the
P3–P8 lineages in the midregion of the ventral nerve cord regulate the laying of eggs
through the vulva in the middle of the animal; the corresponding cells that are generated
by the anterior and posterior lineages undergo programmed cell death4,129 (arrows).
Similarly, two cells in the posterior that undergo programmed cell death are analogous to
more anterior neurons that survive and extend their axons posteriorly (arrows).
Presumably, there is no purpose served in having posterior neurons extend posterior
axons. b | Often a sex-specific cell type is initially produced in both sexes but is induced to
die specifically in the sex in which it is not needed. For example, the male-specific cephalic
(CEM) neurons survive in males and undergo programmed cell death in hermaphrodites;
the hermaphrodite-specific neurons (HSNs) survive in hermaphrodites and die in males,
and this is controlled by tra‑1 (REFS 5,7). AMso, amphid socket cell; P, twelve
postembryonic blast cells that give rise to ventral cord motorneurons; PHB, neuron type,
phasmid; W, postembryonic neuroblast.
more complex than those of C. elegans, and it is impor­
tant to note that the simple, sequential model in which
programmed cell death is initiated by the transcription
of egl‑1 in most cells of C. elegans is incomplete, as recent
data suggest important roles for additional regulation
downstream of egl‑1 transcription31–33. An important
area of current research is to understand how regulatory
pathways that might impinge on the cell death pathway
at multiple points interact to result in a consistent output
of survival or programmed cell death.
Transcription factors specify cell death
Just as the core apoptotic pathway is conserved from
worms to humans, it is also clear that the regulatory
pathways that control the cell death decision of indi­
vidual cells of C. elegans are conserved in mammals.
Moreover, these regulatory pathways are often dereg­
ulated in cancer cells to ensure their survival (FIG. 3;
TABLE 1). This raises the intriguing possibility that tran­
scriptional repression of apoptosis by lineage specific
transcription factors is a crucial function that is ena­
bled by the mutation of specific tumour suppressors and
proto-oncogenes.
ces genes and acute lymphoblastic leukaemia. In genetic
screens in C. elegans aimed at identifying genes that are
involved in the regulation of programmed cell death
in the serotonergic nervous system, some mutants
were identified that affected the deaths of only a small
number of cells. For example, ces‑1 gain-of-function
mutations and ces‑2 loss-of-function mutations
result in the survival of specific groups of cells in the
pharynx 34. Other deaths occur normally in these mutants,
and for this reason the genes involved were termed cell
death specification (ces). These data suggest that the ces
genes participate in the decision process that must pre­
cede the execution of death. Cloning of the ces genes
revealed that they encode transcription factors: Ces‑2 is
the homologue of the PAR family bZIP protein hepatic
leukaemia factor (HLF)35, and ces‑1 is a homologue of
SLUG, a zinc finger protein and Snail family member 36,37.
Given the central role of transcription in many cell fate
decisions, this finding strengthens the idea that pro­
grammed cell death is a cell fate — little different from
other cell fates. In NSM sister cells, egl‑1 transcription is
controlled by competition between CES‑1 (a repressor
of transcription) and a heterodimeric helix–loop–helix 2
(HLH‑2)–HLH‑3 basic HLH (bHLH) transcription fac­
tor complex (an activator) for specific binding sites on
the egl‑1 promoter 36,37. CES‑2 is a repressor that inhibits
ces‑1 activity in the NSM sisters; the gain-of-function
mutations in ces‑1 that cause NSM cell survival are pro­
moter mutations that probably disrupt CES‑2 binding to
the ces‑1 promoter 36.
Some children with acute lymphoblastic leukaemia
(ALL) have a chromosomal translocation that couples
the DNA binding domain of HLF to the transcrip­
tional activation domain of E2A, and brings the fusion
gene under the control of the E2A promoter, which is
expressed in lymphocytes38–40. How this fusion protein
promoted leukaemia was unclear, but the discovery that
52 | JANUARY 2011 | VOLUME 11
www.nature.com/reviews/cancer
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
HLF was homologous to CES‑2 suggested a detailed
model, which was subsequently supported by an ele­
gant series of papers38,41–43: the fusion converts HLF
from a repressor to an activator able to prevent cell
death. Consistent with a function that was predicted
by the analysis of ces‑2, the E2A–HLF fusion maintains
the transcription of the ces‑1 homologue SLUG, which
probably represses the transcription of the BH3‑only
gene PUMA (also known as BBC3), ensuring the sur­
vival of the malignant lymphoblasts38,41–43. Therefore,
genetic pathways that regulate programmed cell death
or the survival of specific cells of C. elegans can be
evolutionarily conserved and altered by mutation to
promote the survival of human cancer cells. Inhibition
of this pathway in the malignant cells can induce their
death38, suggesting that a better understanding of these
pathways might be therapeutically useful.
As noted above, many divisions in the C. elegans lin­
eage are asymmetric and generate one cell that survives
and one cell that undergoes programmed cell death.
Recent work has shown that the ces genes can cou­
ple asymmetric division and programmed cell death
during the pharyngeal neurosecretory-motor (NSM)
neuroblast divisions44. When observing cell lineages in
which a daughter cell dies, it is normally obvious dur­
ing the division which cell will die — the dying cell is
smaller than the sister cell that survives. The asymme­
try in size of the newly generated NSM neurons and the
NSM sisters is not a consequence of the death process,
because the asymmetry is preserved in egl‑1 mutants in
which death is prevented. However, in ces‑1 and ces‑2
mutants this asymmetry is disrupted44. Moreover, the
axis of NSM neuroblast division is often abnormal in
ces‑2 and ces‑1 mutants. The positioning of the mitotic
spindle and the orientation of the division axis is closely
linked to the fates established by the daughters of pro­
genitor cells in mammals and model organisms45,46, and
defects in this process are also observed in malignant
cells. Elucidating the mechanism through which the
Ces proteins influence the asymmetric division of the
NSM neuroblasts and a better understanding of how
this is coupled to programmed cell death might sug­
gest mechanisms through which malignant cells can
self-renew.
tra‑1 and GLI: cell fate and hedgehog signalling. tra‑1
encodes a transcription factor that is the terminal, glo­
bal regulator of sex determination in C. elegans, and
it promotes hermaphrodite fates7,47. Analysis of semidominant egl‑1 mutations revealed promoter muta­
tions that disrupt TRA‑1 repression of egl‑1 expression,
leading to the inappropriate activation of egl‑1 and the
death of two neurons that are essential for egg laying in
hermaphrodites, the hermaphrodite-specific neurons
(HSNs)18. tra‑1 loss-of-function mutants are transformed
to a male phenotype, with approximately one-third of
cells in the animals adopting different fates4,48. The dis­
covery that TRA‑1 directly regulates cell death as one
of its many functions poses a considerable challenge
for the recognition and analysis of genes that directly
regulate programmed cell death decisions in C. elegans
Figure 2 | The programmed cell death pathway of
C. elegans is evolutionarily conserved. The products of
four genes are required for essentially all of the
programmed cell deaths of somatic cells of Caenorhabditis
elegans, and the pathway is evolutionarily conserved.
Human cells typically have multiple family members of
each protein class. There is added complexity in the
apoptotic pathways of human cells that is not shown in the
figure: these include the release of cytochrome c from
mitochondria; the inhibitor of apoptosis proteins
functioning as an additional inhibitor of caspase activity in
addition to Bcl‑2 family members; and the extrinsic and
death receptor-mediated pathways serving as the primary
mechanism of activating apoptosis in some circumstances.
There is strong evidence that defects in apoptosis
contribute to human cancer, including the loss of BH3
domain pro-apoptotic molecules, activation of Bcl‑2 family
anti-apoptotic molecules and defects in the upstream
regulatory mechanisms that control the apoptosis cascade.
For reasons that are unclear, the evidence is substantially
weaker that mutations affecting APAF1 or the caspase
genes contribute broadly to human cancers, suggesting
that blocking apoptosis upstream of mitochondria offers a
greater advantage during the evolution of a tumour.
MOMP, mitochondrial outer membrane permeabilization.
and in human cancers, because an abnormality in cell
death may be mistakenly presumed to be a secondary
consequence of an altered cell fate.
In contrast to its pro-survival function in the HSN
neurons, TRA‑1 eliminates male-specific cephalic
(CEM) neurons in the hermaphrodite through indirect
activation of egl‑1 and ced‑3 (REFS 33,49,50) (FIG. 1). In the
absence of TRA‑1, the Bar homeodomain transcription
factor CEH‑30 is activated by UNC‑86 (a POU domain
transcription factor), LSR‑1 (a leucyl-tRNA synthetase)
NATURE REVIEWS | CANCER
VOLUME 11 | JANUARY 2011 | 53
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
and UNC‑132 (which has partial homology to PIM1 but
lacks a kinase domain)33. CEH‑30 represses egl‑1 and
ced‑3 through interaction with the general transcrip­
tional repressor UNC‑37 (a Groucho homologue) and
promotes CEM survival in males49. In hermaphrodites,
TRA‑1 represses ceh‑30, causing derepression of egl‑1
and ced‑3 and death of the CEMs33,49,50. The mechanisms
through which CEH‑30 represses egl‑1 and ced‑3 are not
yet clear.
Notably, homologues of several of the anti-apoptotic
genes identified from studies of CEM neurons in
C. elegans have oncogenic properties in mammals.
For example, the POU domain transcription fac­
tors contribute to oncogenesis in multiple cancers,
including neuroepitheliomas, Ewing’s sarcomas,
testicular germ cell tumours, melanomas and breast
cancers51–54. Overexpression of the leucyl-tRNA syn­
thetase LARS1 has been shown to promote lung can­
cer cell growth and migration55, and overexpression of
Groucho homologues has been observed in multiple
tumour types56. PIM1 is a well-established oncogene,
but whether unc‑132 shares functional similarities to
PIM1 that are relevant to tumorigenesis remains to be
determined33,57.
The human homologues of TRA‑1 are the GLI onco­
genic zinc finger transcription factors that respond to
Hedgehog (Hh) signalling 58. Consistent with its role in
normal embryonic development, recent data suggest
Figure 3 | Cell death is specified by transcription factors in C. elegans, and their
human homologues are oncogenes. The transcription factors and pathways that
determine cell death or survival are shown, as well as the locations of the cells that they
affect. Only factors that directly control the transcription of the cell death pathway
genes are included. The human homologues of the Caenorhabditis elegans genes are
shown, with mammalian oncogenes in grey. HSN, hermaphrodite-specific neurons; NSM,
pharyngeal neurosecretory-motor neurons; P, twelve postembryonic blast cells that give
rise to ventral cord motorneurons; VC, ventral cord.
that Hh functions non-cell autonomously in many
cancers59, and can promote the survival of cancer cells
partly by affecting the transcriptional regulation of a
Bcl‑2 family member in tumour cells — as TRA‑1 does
in the HSN neurons in C. elegans. Stromal Hh signalling
in the Eμ-Myc mouse model of B cell lymphoma acti­
vates GLI in the lymphoma cells and maintains expres­
sion of BCL‑2, preventing apoptosis; inhibition of Hh
signalling induces apoptosis60. This mechanism might
be conserved in human non-Hodgkin’s lymphoma and
multiple myeloma60. In primary keratinocytes, GLI
proteins directly regulate the transcription of BCL2
(REFS 61,62). Together, these data suggest that the direct
regulation of apoptosis is a conserved function of GLI
family members that promotes the survival of cancer
cells. It is not yet clear how many cancers depend on
Hh activation of BCL2 expression for their survival, or
whether the Hh pathway inhibitors that are currently
in clinical trials might kill cancer cells by interrupting
this survival pathway.
Oncoproteins and programmed cell death
Of the 13 genes that directly specify the cell death or
survival of individual cells in C. elegans, at least nine
are the homologues of human oncogenes (FIG. 3) .
These include Hox proteins; Hox cofactors, including
a Trithorax group regulator that is required for Hox
expression; a Pax family member protein, which directly
regulates ced‑9; and a Caudal homologue, which
directly regulates transcription of ced‑3. In each case
there is initial evidence that the oncogenic forms of
these transcription factors directly regulate cell survival
in mammals. Moreover, for a Six family homeodomain
protein that directly regulates egl‑1, there is good evi­
dence that human homologues are involved in cancer
and regulate apoptosis, but the mechanisms involved
that promote malignancy are unclear.
Six family homeodomain. Genetic screens using animals
in which cell type-specific reporters drive green fluores­
cent protein expression have been a powerful tool for
identifying genes that regulate the survival of specific
cell types. A recent screen using transgenic C. elegans
worms that express a reporter gene for a single neuron,
M4, identified a point mutation in a regulatory, noncoding region of egl‑1 that resulted in the survival of
the M4 neuron63. The mutation disrupted a predicted
binding site for a Six family homeodomain transcrip­
tion factor, CEH‑34, suggesting that the site was required
for the transcriptional activation of egl‑1 and the death
of the M4 neuron. Indeed, loss-of-function mutations
in ceh‑34 were also identified in the screen. CEH‑34
directly regulates egl‑1 transcription in the M4 neurons
as part of an evolutionarily conserved complex with
the eyes absent homologue EYA‑1. Several Six family
members have been studied in human cancers and, as
would be expected for transcription factors, their roles
in promoting malignancy seem to be context dependent.
These roles include the regulation of cell survival and
cell death through mechanisms that have not yet been
elucidated64.
54 | JANUARY 2011 | VOLUME 11
www.nature.com/reviews/cancer
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
Table 1 | Human oncogenes homologous to the cell death specification genes of C. elegans
C. elegans gene Cell death role
Human homologue Mutation type in cancer Cancer type
Refs
ces‑2
Inhibit
HLF
Gain of function
ALL
tra-1
Inhibit
Gli
Gain of function
Many
38,39,42,117,118
lin‑39
Inhibit
Hox
Gain of function
ALL, AML and others
99,112,121–124
ceh-20
Promote or inhibit Pbx
Gain of function
ALL
99,112,121–124
unc‑62
Inhibit
Meis
Overexpressed
Leukaemia
99,112,121–124
lin‑59
Inhibit
MLL
Gain of function
ALL and AML
99,112,121–124
egl‑38
Inhibit
Pax
Gain of function
Rhabdomyosarcoma
pal‑1
Promote
Cdx
Loss of function
Gastrointestinal cancers
126–128
pal‑1
Promote
Cdx
Gain of function
Leukaemia through Hox upregulation
126–128
119,120
125
ALL, acute lymphoblastic leukaemia; AML, acute myeloid leukaemia; Cdx, caudal-type homeobox transcription factor; C. elegans, Caenorhabditis elegans; Gli;
glioma associated oncogene homologue; HLF, hepatic leukaemia factor; Hox, homeobox; MLL, mixed lineage leukaemia; Pax, paired box gene; Pbx, pre-B cell
leukaemia transcription factor.
Hox genes. Like TRA‑1 and GLI, Hox protein complexes
have diverse roles in development that include the direct
regulation of programmed cell death. A mutation affect­
ing the Hox gene lin‑39 was the first one isolated in the
genetic screens in C. elegans that led to the elucidation
of the programmed cell death pathway 2,65. Consistent
with its classical role in animal development, the Hox
complex in C. elegans includes six genes that determine
the pattern of cell fates along the body axis65–71. The
lin‑39 gene is required for the survival of six neurons in
the mid-body of the animal65, and the more posterior
Hox gene mab‑5 is required for the deaths of two cells
in the posterior 67. In lin‑39 mutants, the six neurons in
the mid-body are homeotically transformed and adopt
the fate of their more anterior and posterior sister cells,
which undergo programmed cell death65. But what is
the mechanism responsible for this transformation?
Hox proteins often function together with TALE fam­
ily homeodomain cofactors, as this increases the DNA
binding affinity and selectivity of Hox proteins. These
TALE family members include CEH‑20 and UNC‑62 in
C. elegans and their respective mammalian homologues,
Pbx and Meis72–78. In the mid-body, LIN‑39, CEH‑20 and
UNC‑62 are all required for the repression of egl‑1 to
ensure the survival of the six neurons; a LIN39–CEH‑20
complex probably represses egl‑1 through multiple
regulatory sites79.
The death of the two cells in the posterior of the nema­
tode occurs through a similar mechanism, which involves
MAB‑5 and CEH‑20 — although with a curious twist.
In the case of one of the two cells, a MAB‑5–CEH‑20
complex binds to a single site in the egl‑1 regulatory
region to activate transcription and induce programmed
cell death74. Remarkably, the death of the second cell,
which is also dependent on MAB‑5 and CEH‑20 for
survival, is regulated indirectly through an unknown
mechanism. This indicates that the survival or death
of even very closely related cell types can be controlled
through distinct mechanisms, and is a striking demon­
stration of the kinds of discoveries made possible by the
known and essentially invariant lineage of C. elegans.
It is not clear why two distinct mechanisms are required
to regulate the death of these two similar cells.
Hox protein complexes can therefore activate or
repress the transcription of a cell death gene to ensure
the survival or programmed cell death of individual cells.
For lin‑39 and mab‑5 this role is consistent with their
specification of fates along the body axis by determining
regional patterns of programmed cell death.
Abnormal activation of Hox genes is common in
human leukaemias, and their role in normal and malig­
nant haematopoiesis has been intensively studied. The
C. elegans Trithorax group chromatin-modifying protein
LIN‑59 is required for Hox gene expression, similar to
the MLL protein in human leukaemias79 (BOX 1). Might
the core cell death pathway genes be direct targets of
Hox proteins in leukaemia cells? The C. elegans data,
and similar data from Drosophila80,81, indicate that the
regulation of programmed cell death by Hox genes is
context dependent — Hox genes are broadly expressed
but only small numbers of cells are dependent on Hox
function for their survival. There is some evidence that
this context-dependent survival role might be conserved
in human leukaemias. Inhibition of HOXA9 function in
human leukaemia cells induces apoptosis specifically in
leukaemias that harbour activating mutations in MLL82.
The mechanism underlying this dependence has not yet
been elucidated, but the model organism data suggest
that direct repression of apoptotic pathway genes such
as BH3 domain-encoding genes is a possibility.
egl‑38 and Pax‑2 regulate ced‑9 (BCL2). Two pairedbox homeodomain transcription factors directly regulate
apoptosis in C. elegans, egl‑38 and pax‑2 (REF. 32). Unlike
the ces, tra‑1 and Hox genes, egl‑38 and pax‑2 ensure the
survival of many cells in the developing animal, includ­
ing somatic cells and developing germ cells. Moreover,
egl‑38 and pax‑2 prevent programmed cell death by
stimulating the transcription of the BCL2 homologue
ced‑9, rather than by repressing egl‑1 (REF. 32).
Pax genes are subject to translocation and can
become fused to Forkhead family genes by the t(2;13)
and t(1;13) chromosomal translocations in childhood
rhabdomyosarcoma. These fusion proteins inhibit pro­
grammed cell death in rhabdomyosarcoma cells through
an unknown mechanism83. Translocations involving Pax
NATURE REVIEWS | CANCER
VOLUME 11 | JANUARY 2011 | 55
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
genes and the immunoglobulin enhancer also occur
in some non-Hodgkin’s lymphomas84–88. Pax genes
are broadly expressed in a large collection of tumour
cell lines, including the NCI60 cell lines, and inhibi­
tion of individual Pax genes in many of these cell lines
induces programmed cell death89. How Pax expression
prevents programmed cell death in these tumour lines
is unknown, although direct stimulation of BCL2L1
expression by PAX3 has been reported90; this mecha­
nism would be consistent with the pathway defined in
C. elegans.
Germline cell death and DNA damage
Germ cell development in C. elegans proceeds through
regulated but stochastic proliferation and differen­
tiation, rather than through the essentially invariant
cell lineage that is characteristic of somatic cells. In a
typical gonad, around 50% of the germ cells undergo
programmed cell death in a process that is depend­
ent on ced‑4 and ced‑3, but not egl‑1 (REF. 91) , and
that relies on the pro-apoptotic activity of the RB
pathway genes lin‑35, dpl‑1, efl‑1 and efl‑2 (REF. 92).
DNA damage increases germ cell death in a dosedependent manner and induces cell cycle arrest in the
proliferating germ line, whereas somatic cells are pro­
foundly resistant 93. Damage-induced germ cell death,
in contrast to normal homeostatic germ cell death, is
controlled by egl‑1, a second BH3 domain-encoding
gene ced‑13 and ced‑9, and it also requires ced‑4 and
ced‑3. Furthermore, the TP53 homologue cep‑1 is
required for damage-induced apoptosis, but not cell
cycle arrest, in the germ line, as are lin‑35, dpl‑1 and
efl‑2 (REF. 92). Recent work has also revealed a new and
surprising mechanism through which HIF1A, which
encodes the α-subunit of the hypoxia-inducible factor,
can inhibit apoptosis. The HIF1A homologue activates
the expression of a tyrosinase in two head neurons,
which act on the C. elegans germ line through cep‑1
to inhibit apoptosis94. Thus, the C. elegans germ line
is a simple but powerful genetic context in which
conserved aspects of the DNA-damage response and
Box 1 | Hox genes and cancer
Hox function can be activated by chromosomal translocations, including those that
directly affect individual Hox genes, Pbx family cofactors, or the Trithorax homologue
MLL. MLL encodes a clinically important component of a chromatin-modifying complex
that directly binds and activates Hox gene expression96–100. Ten percent of adults and
70% of infants with acute myeloid leukaemia (AML) or acute lymphocytic leukaemia
(ALL) have translocations affecting MLL101–103, and 2–5% of children or adults with ALL
have translocations affecting PBX1 (REFS 102,104); direct activation of Hox genes by
translocation also occurs but is less common. Leukaemias with increased Hox gene
activity are considerably more difficult to cure101,105. Individual Hox genes have defined
roles in normal haematopoiesis, as revealed by gene disruptions in mice, although
these experiments probably underestimate their importance because of compensatory
changes in Hox gene expression in the knockout animals106–110. Individual Hox genes
also have defined roles in malignant haematopoiesis, and HOXA9 in particular
mediates some of the essential properties of the leukaemic stem cell in MLL-dependent
AML111–115. The Pbx and Meis cofactors are probably essential for leukaemogenesis, as is
the motif in the Hox proteins that mediates interaction with Pbx proteins112,116,
suggesting that complexes between these proteins regulate at least some genes that
are crucial for the development of leukaemia.
the TP53 and RB tumour suppressors can be studied,
topics that are greatly important in cancer biology
but that cannot be as easily addressed in the somatic
tissues of C. elegans 95.
Looking forwards to improve cancer therapy
Elucidating the programmed cell death pathways of
C. elegans has immensely improved our understand­
ing of how current cancer therapy works and has paved
the way for developing new therapeutic strategies.
Unexpectedly, many of the lineage-specific regulatory
pathways controlling cell viability in C. elegans are evo­
lutionarily conserved in mammalian cell lineages and
can be mutated in cells, resulting in aberrant survival.
There seems to be a remarkable correlation between the
transcriptional regulators of cell death in C. elegans and
haematopoietic malignancies, including leukaemia. This
may reflect the fairly extensive knowledge of the factors
that control haematopoietic cell lineages, which are the
best understood of the mammalian lineage programmes.
Alternatively, transcriptional programmes directed by
oncogenic transcription factors that are the products of
chromosomal translocations often drive haemato­poietic
malignancies, and suppression of apoptosis by the fusion
proteins may have a more important role in the develop­
ment of blood cancers. The comparatively much more
complex genomes of solid tumours has made it difficult
to identify chromosomal translocations that contribute
to malignancy, but new genomics methods are now
identifying an increasing number of recurrent fusion
proteins. As our knowledge of mammalian lineage pro­
grammes develops, it will be interesting to see whether
they too have transcription factors that directly regulate
apoptosis as one aspect of the control of cell fate, which
we predict will be the case.
We are now in an exciting new era in which we are
beginning to apply our knowledge of the biological
pathways that control cell fate and programmed cell
death to develop new cancer drugs. Looking forwards,
we can anticipate the approval of therapies that directly
target the apoptotic and cell fate pathways in human
cancers. To effectively apply these new drugs we need to
develop a comprehensive understanding of the mecha­
nisms that cancer cells depend on to avoid apoptosis
and to promote self renewal and proliferation; develop
strategies for identifying tumours and patients with
specific classes of defects in the pathways involved; and
generate molecules that target pathway components to
kill cancer cells. The sophisticated genomics technolo­
gies that are rapidly being developed indicate that we
will soon know the mutations in each patient’s tumour
that alter cell fate and apoptosis to drive cancer forma­
tion and progression. Understanding which pathways
are crucial for cancer cell survival will be aided by a
clear understanding of how the corresponding path­
ways operate in model organisms, which offer a level
of resolution not typically achievable in mammalian
models, and are often the source of the most remark­
able discoveries. Bringing these approaches together in
well-designed clinical trials should begin to ‘turn the
tide’ against cancer.
56 | JANUARY 2011 | VOLUME 11
www.nature.com/reviews/cancer
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
White, K. et al. Genetic control of programmed cell
death in Drosophila. Science 264, 677–683 (1994).
Ellis, H. M. & Horvitz, H. R. Genetic control of
programmed cell death in the nematode C. elegans.
Cell 44, 817–829 (1986).
The discovery of ced‑3 and ced‑4 revealed for the
first time the genetic control of programmed cell
death.
Yin, C., Knudson, C. M., Korsmeyer, S. J. & Van Dyke, T.
Bax suppresses tumorigenesis and stimulates
apoptosis in vivo. Nature 385, 637–640 (1997).
Sulston, J. E. & Horvitz, H. R. Post-embryonic cell
lineages of the nematode, Caenorhabditis elegans.
Dev. Biol. 56, 110–156 (1977).
Sulston, J. E., Schierenberg, E., White, J. G. &
Thomson, J. N. The embryonic cell lineage of the
nematode Caenorhabditis elegans. Dev. Biol. 100,
64–119 (1983).
This and reference 4 began the technical and
scientific milestone of defining the C. elegans
lineages.
Kerr, J. F., Wyllie, A. H. & Currie, A. R. Apoptosis: a
basic biological phenomenon with wide-ranging
implications in tissue kinetics. Br. J. Cancer 26,
239–257 (1972).
Zarkower, D. Somatic sex determination. WormBook
[online], http://www.wormbook.org/chapters/www_
somaticsexdeterm/somaticsexdeterm.html (2007).
Cameron, S., Clark, S. G., McDermott, J. B., Aamodt,
E. & Horvitz, H. R. PAG‑3, a Zn-finger transcription
factor, determines neuroblast fate in C. elegans.
Development 129, 1763–1774 (2002).
Jia, Y., Xie, G., McDermott, J. B. & Aamodt, E. The
C. elegans gene pag‑3 is homologous to the zinc finger
proto-oncogene gfi‑1. Development 124, 2063–2073
(1997).
Hock, H. et al. Gfi‑1 restricts proliferation and
preserves functional integrity of haematopoietic stem
cells. Nature 431, 1002–1007 (2004).
Hock, H. et al. Intrinsic requirement for zinc finger
transcription factor Gfi‑1 in neutrophil differentiation.
Immunity 18, 109–120 (2003).
Saleque, S., Cameron, S. & Orkin, S. H. The zinc-finger
proto-oncogene Gfi‑1b is essential for development of
the erythroid and megakaryocytic lineages. Genes Dev.
16, 301–306 (2002).
Wu, M. et al. Imaging hematopoietic precursor division
in real time. Cell Stem Cell 1, 541–554 (2007).
Yuan, J., Shaham, S., Ledoux, S., Ellis, H. M. &
Horvitz, H. R. The C. elegans cell death gene ced‑3
encodes a protein similar to mammalian interleukin‑1
β-converting enzyme. Cell 75, 641–652 (1993).
The discovery that proteases now known as
caspases were required for programmed cell death.
Yuan, J. Y. & Horvitz, H. R. The Caenorhabditis
elegans genes ced‑3 and ced‑4 act cell autonomously
to cause programmed cell death. Dev. Biol. 138,
33–41 (1990).
Hengartner, M. O., Ellis, R. E. & Horvitz, H. R.
Caenorhabditis elegans gene ced‑9 protects cells from
programmed cell death. Nature 356, 494–499
(1992).
Hengartner, M. O. & Horvitz, H. R. C. elegans cell
survival gene ced‑9 encodes a functional homolog of
the mammalian proto-oncogene bcl‑2. Cell 76,
665–676 (1994).
Conradt, B. & Horvitz, H. R. The TRA‑1A sex
determination protein of C. elegans regulates sexually
dimorphic cell deaths by repressing the egl‑1 cell
death activator gene. Cell 98, 317–327 (1999).
The discovery of the mechanism through which cell
death decisions are controlled for somatic cells.
Conradt, B. & Horvitz, H. R. The C. elegans protein
EGL‑1 is required for programmed cell death and
interacts with the Bcl‑2‑like protein CED‑9. Cell 93,
519–529 (1998).
Horvitz, H. R. Worms, life, and death (Nobel lecture).
Chembiochem 4, 697–711 (2003).
Vaux, D. L., Weissman, I. L. & Kim, S. K. Prevention of
programmed cell death in Caenorhabditis elegans by
human bcl‑2. Science 258, 1955–1957 (1992).
Vaux, D. L., Cory, S. & Adams, J. M. Bcl‑2 gene
promotes haemopoietic cell survival and cooperates
with c‑myc to immortalize pre‑B cells. Nature 335,
440–442 (1988).
Zinkel, S. S. et al. Proapoptotic BID is required for
myeloid homeostasis and tumor suppression. Genes
Dev. 17, 229–239 (2003).
Ranger, A. M. et al. Bad-deficient mice develop diffuse
large B cell lymphoma. Proc. Natl Acad. Sci. USA 100,
9324–9329 (2003).
25. Yan, N. et al. Structure of the CED‑4‑CED‑9 complex
provides insights into programmed cell death in
Caenorhabditis elegans. Nature 437, 831–837
(2005).
26. Zou, H., Henzel, W. J., Liu, X., Lutschg, A. & Wang, X.
Apaf‑1, a human protein homologous to C. elegans
CED‑4, participates in cytochrome c-dependent
activation of caspase‑3. Cell 90, 405–413 (1997).
27. Zou, H., Li, Y., Liu, X. & Wang, X. An APAF‑1.
cytochrome c multimeric complex is a functional
apoptosome that activates procaspase‑9. J. Biol.
Chem. 274, 11549–11556 (1999).
28. Oberst, A., Bender, C. & Green, D. R. Living with
death: the evolution of the mitochondrial pathway of
apoptosis in animals. Cell Death Differ. 15, 1139–
1146 (2008).
29. Orme, M. & Meier, P. Inhibitor of apoptosis proteins in
Drosophila: gatekeepers of death. Apoptosis 14,
950–960 (2009).
30. Goyal, L. Cell death inhibition: keeping caspases in
check. Cell 104, 805–808 (2001).
31. Maurer, C. W., Chiorazzi, M. & Shaham, S. Timing of
the onset of a developmental cell death is controlled
by transcriptional induction of the C. elegans ced‑3
caspase-encoding gene. Development 134,
1357–1368 (2007).
32. Park, D., Jia, H., Rajakumar, V. & Chamberlin, H. M.
Pax2/5/8 proteins promote cell survival in C. elegans.
Development 133, 4193–4202 (2006).
33. Nehme, R. et al. Transcriptional upregulation of both
egl‑1 BH3‑only and ced‑3 caspase is required for the
death of the male-specific CEM neurons. Cell Death
Differ. 17, 1266–1276 (2010).
34. Ellis, R. E. & Horvitz, H. R. Two C. elegans genes
control the programmed deaths of specific cells in the
pharynx. Development 112, 591–603 (1991).
35. Metzstein, M. M., Hengartner, M. O., Tsung, N., Ellis,
R. E. & Horvitz, H. R. Transcriptional regulator of
programmed cell death encoded by Caenorhabditis
elegans gene ces‑2. Nature 382, 545–547 (1996).
36. Metzstein, M. M. & Horvitz, H. R. The C. elegans
cell death specification gene ces‑1 encodes a snail
family zinc finger protein. Mol. Cell 4, 309–319
(1999).
37. Thellmann, M., Hatzold, J. & Conradt, B. The Snail-like
CES‑1 protein of C. elegans can block the expression
of the BH3‑only cell-death activator gene egl‑1 by
antagonizing the function of bHLH proteins.
Development 130, 4057–4071 (2003).
38. Inaba, T. et al. Reversal of apoptosis by the leukaemiaassociated E2A‑HLF chimaeric transcription factor.
Nature 382, 541–544 (1996).
Together with reference 35, this paper
demonstrated that the mechanisms that decide
whether individual cells survive or die are
conserved and abnormal in human cancers.
39. Inaba, T. et al. Fusion of the leucine zipper gene HLF
to the E2A gene in human acute B‑lineage leukemia.
Science 257, 531–534 (1992).
40. Hunger, S. P. et al. The t(1;19)(q23;p13) results in
consistent fusion of E2A and PBX1 coding sequences
in acute lymphoblastic leukemias. Blood 77,
687–693 (1991).
41. Hemavathy, K., Guru, S. C., Harris, J., Chen, J. D. & Ip,
Y. T. Human Slug is a repressor that localizes to sites of
active transcription. Mol. Cell. Biol. 20, 5087–5095
(2000).
42. Inukai, T. et al. SLUG, a ces‑1‑related zinc finger
transcription factor gene with antiapoptotic activity, is
a downstream target of the E2A‑HLF oncoprotein.
Mol. Cell 4, 343–352 (1999).
43. Wu, W. S. et al. Slug antagonizes p53‑mediated
apoptosis of hematopoietic progenitors by repressing
puma. Cell 123, 641–653 (2005).
44. Hatzold, J. & Conradt, B. Control of apoptosis by
asymmetric cell division. PLoS Biol. 6, e84 (2008).
45. Buchman, J. J. & Tsai, L. H. Spindle regulation in
neural precursors of flies and mammals. Nature Rev.
Neurosci. 8, 89–100 (2007).
46. Neumuller, R. A. & Knoblich, J. A. Dividing cellular
asymmetry: asymmetric cell division and its
implications for stem cells and cancer. Genes Dev. 23,
2675–2699 (2009).
47. Ellis, R. & Schedl, T. Sex determination in the germ
line. WormBook [online], http://www.wormbook.org/
chapters/www_sexgermline/sexgermline.html
(2007).
48. Hodgkin, J. A. & Brenner, S. Mutations causing
transformation of sexual phenotype in the nematode
Caenorhabditis elegans. Genetics 86, 275–287
(1977).
NATURE REVIEWS | CANCER
49. Peden, E., Kimberly, E., Gengyo-Ando, K., Mitani, S. &
Xue, D. Control of sex-specific apoptosis in C. elegans
by the BarH homeodomain protein CEH‑30 and the
transcriptional repressor UNC‑37/Groucho. Genes
Dev. 21, 3195–3207 (2007).
50. Schwartz, H. T. & Horvitz, H. R. The C. elegans protein
CEH‑30 protects male-specific neurons from apoptosis
independently of the Bcl‑2 homolog CED‑9. Genes
Dev. 21, 3181–3194 (2007).
51. Collum, R. G. et al. A novel POU homeodomain gene
specifically expressed in cells of the developing
mammalian nervous system. Nucleic Acids Res. 20,
4919–4925 (1992).
52. Gidekel, S., Pizov, G., Bergman, Y. & Pikarsky, E.
Oct‑3/4 is a dose-dependent oncogenic fate
determinant. Cancer Cell 4, 361–370 (2003).
53. Budhram-Mahadeo, V. S. & Latchman, D. S. Targeting
Brn‑3b in breast cancer therapy. Expert Opin. Ther.
Targets 10, 15–25 (2006).
54. Eisen, T., Easty, D. J., Bennett, D. C. & Goding, C. R.
The POU domain transcription factor Brn‑2: elevated
expression in malignant melanoma and regulation of
melanocyte-specific gene expression. Oncogene 11,
2157–2164 (1995).
55. Shin, S. H. et al. Implication of leucyl-tRNA synthetase
1 (LARS1) over-expression in growth and migration of
lung cancer cells detected by siRNA targeted knockdown analysis. Exp. Mol. Med. 40, 229–236 (2008).
56. Buscarlet, M. & Stifani, S. The ‘Marx’ of Groucho on
development and disease. Trends Cell Biol. 17,
353–361 (2007).
57. Shah, N. et al. Potential roles for the PIM1 kinase in
human cancer - a molecular and therapeutic appraisal.
Eur. J. Cancer 44, 2144–2151 (2008).
58. Zarkower, D. & Hodgkin, J. Molecular analysis of the
C. elegans sex-determining gene tra‑1: a gene
encoding two zinc finger proteins. Cell 70, 237–249
(1992).
59. Yauch, R. L. et al. A paracrine requirement for
hedgehog signalling in cancer. Nature 455, 406–410
(2008).
60. Dierks, C. et al. Essential role of stromally induced
hedgehog signaling in B‑cell malignancies. Nature
Med. 13, 944–951 (2007).
61. Regl, G. et al. Activation of the BCL2 promoter in
response to Hedgehog/GLI signal transduction is
predominantly mediated by GLI2. Cancer Res. 64,
7724–7731 (2004).
62. Bigelow, R. L. et al. Transcriptional regulation of bcl‑2
mediated by the sonic hedgehog signaling pathway
through gli‑1. J. Biol. Chem. 279, 1197–1205 (2004).
63. Hirose, T., Galvin, B. D. & Horvitz, H. R. Six and Eya
promote apoptosis through direct transcriptional
activation of the proapoptotic BH3‑only gene egl‑1 in
Caenorhabditis elegans. Proc. Natl Acad. Sci. USA
107, 15479–15484 (2010).
64. Christensen, K. L., Patrick, A. N., McCoy, E. L. & Ford,
H. L. The six family of homeobox genes in development
and cancer. Adv. Cancer Res. 101, 93–126 (2008).
65. Clark, S. G., Chisholm, A. D. & Horvitz, H. R. Control
of cell fates in the central body region of C. elegans by
the homeobox gene lin‑39. Cell 74, 43–55 (1993).
66. Salser, S. J., Loer, C. M. & Kenyon, C. Multiple HOM‑C
gene interactions specify cell fates in the nematode
central nervous system. Genes Dev. 7, 1714–1724
(1993).
67. Kenyon, C. A gene involved in the development of the
posterior body region of C. elegans. Cell 46, 477–487
(1986).
68. Ferreira, H. B., Zhang, Y., Zhao, C. & Emmons, S. W.
Patterning of Caenorhabditis elegans posterior
structures by the Abdominal‑B homolog, egl‑5. Dev.
Biol. 207, 215–228 (1999).
69. Chisholm, A. Control of cell fate in the tail region of
C. elegans by the gene egl‑5. Development 111,
921–932 (1991).
70. Brunschwig, K. et al. Anterior organization of the
Caenorhabditis elegans embryo by the labial-like Hox
gene ceh‑13. Development 126, 1537–1546 (1999).
71. Aboobaker, A. & Blaxter, M. Hox gene evolution in
nematodes: novelty conserved. Curr. Opin. Genet. Dev.
13, 593–598 (2003).
72. Chan, S. K., Jaffe, L., Capovilla, M., Botas, J. & Mann,
R. S. The DNA binding specificity of ultrabithorax is
modulated by cooperative interactions with
extradenticle, another homeoprotein. Cell 78,
603–615 (1994).
73. Liu, J. & Fire, A. Overlapping roles of two Hox genes
and the exd ortholog ceh‑20 in diversification of the
C. elegans postembryonic mesoderm. Development
127, 5179–5190 (2000).
VOLUME 11 | JANUARY 2011 | 57
© 2011 Macmillan Publishers Limited. All rights reserved
REVIEWS
74. Liu, H., Strauss, T. J., Potts, M. B. & Cameron, S.
Direct regulation of egl‑1 and of programmed cell
death by the Hox protein MAB‑5 and by CEH‑20, a
C. elegans homolog of Pbx1. Development 133,
641–650 (2006).
75. Yang, L., Sym, M. & Kenyon, C. The roles of two
C. elegans HOX co-factor orthologs in cell migration
and vulva development. Development 132,
1413–1428 (2005).
76. Streit, A. et al. Conserved regulation of the
Caenorhabditis elegans labial/Hox1 gene ceh‑13. Dev.
Biol. 242, 96–108 (2002).
77. Van Auken, K. et al. Roles of the Homothorax/Meis/
Prep homolog UNC‑62 and the Exd/Pbx homologs
CEH‑20 and CEH‑40 in C. elegans embryogenesis.
Development 129, 5255–5268 (2002).
78. Moens, C. B. & Selleri, L. Hox cofactors in
vertebrate development. Dev. Biol. 291, 193–206
(2006).
79. Potts, M. B., Wang, D. P. & Cameron, S. Trithorax,
Hox, and TALE-class homeodomain proteins ensure
cell survival through repression of the BH3‑only gene
egl‑1. Dev. Biol. 329, 374–385 (2009).
This paper and references 65 and 74 describe the
biology and mechanism of Hox regulation of cell
death.
80. Lohmann, I., McGinnis, N., Bodmer, M. & McGinnis,
W. The Drosophila Hox gene deformed sculpts head
morphology via direct regulation of the apoptosis
activator reaper. Cell 110, 457–466 (2002).
81. Rogulja-Ortmann, A., Renner, S. & Technau, G. M.
Antagonistic roles for Ultrabithorax and Antennapedia
in regulating segment-specific apoptosis of
differentiated motoneurons in the Drosophila
embryonic central nervous system. Development 135,
3435–3445 (2008).
82. Faber, J. et al. HOXA9 is required for survival in
human MLL-rearranged acute leukemias. Blood 113,
2375–2385 (2008).
83. Bernasconi, M., Remppis, A., Fredericks, W. J.,
Rauscher, F. J. & Schafer, B. W. Induction of apoptosis
in rhabdomyosarcoma cells through down-regulation
of PAX proteins. Proc. Natl Acad. Sci. USA 93,
13164–13169 (1996).
84. Busslinger, M., Klix, N., Pfeffer, P., Graninger, P. G. &
Kozmik, Z. Deregulation of PAX‑5 by translocation of
the Emu enhancer of the IgH locus adjacent to two
alternative PAX‑5 promoters in a diffuse large-cell
lymphoma. Proc. Natl Acad. Sci. USA 93, 6129–6134
(1996).
85. Davis, R. J. et al. Structural characterization of the
FKHR gene and its rearrangement in alveolar
rhabdomyosarcoma. Hum. Mol. Genet. 4,
2355–2362 (1995).
86. Epstein, J., Cai, J., Glaser, T., Jepeal, L. & Maas, R.
Identification of a Pax paired domain recognition
sequence and evidence for DNA-dependent
conformational changes. J. Biol. Chem. 269,
8355–8361 (1994).
87. Fredericks, W. J. et al. The PAX3‑FKHR fusion protein
created by the t(2;13) translocation in alveolar
rhabdomyosarcomas is a more potent transcriptional
activator than PAX3. Mol. Cell. Biol. 15, 1522–1535
(1995).
88. Shapiro, D. N., Sublett, J. E., Li, B., Downing, J. R. &
Naeve, C. W. Fusion of PAX3 to a member of the
forkhead family of transcription factors in human
alveolar rhabdomyosarcoma. Cancer Res. 53,
5108–5112 (1993).
89. Muratovska, A., Zhou, C., He, S., Goodyer, P. & Eccles,
M. R. Paired-Box genes are frequently expressed in
cancer and often required for cancer cell survival.
Oncogene 22, 7989–7997 (2003).
90. Margue, C. M., Bernasconi, M., Barr, F. G. &
Schafer, B. W. Transcriptional modulation of the antiapoptotic protein BCL-XL by the paired box
transcription factors PAX3 and PAX3/FKHR. Oncogene
19, 2921–2929 (2000).
91. Gumienny, T. L., Lambie, E., Hartwieg, E., Horvitz, H. R.
& Hengartner, M. O. Genetic control of programmed
cell death in the Caenorhabditis elegans hermaphrodite
germline. Development 126, 1011–1022 (1999).
92. Schertel, C. & Conradt, B. C. elegans orthologs of
components of the RB tumor suppressor complex
have distinct pro-apoptotic functions. Development
134, 3691–3701 (2007).
93. Stergiou, L. & Hengartner, M. O. Death and more:
DNA damage response pathways in the nematode
C. elegans. Cell Death Differ. 11, 21–28 (2004).
94. Sendoel, A., Kohler, I., Fellmann, C., Lowe, S. W. &
Hengartner, M. O. HIF‑1 antagonizes p53‑mediated
apoptosis through a secreted neuronal tyrosinase.
Nature 465, 577–583 (2010).
95. Reddien, P. W., Andersen, E. C., Huang, M. C. &
Horvitz, H. R. DPL‑1 DP, LIN‑35 Rb and EFL‑1 E2F act
with the MCD‑1 zinc-finger protein to promote
programmed cell death in Caenorhabditis elegans.
Genetics 175, 1719–1733 (2007).
96. Milne, T. A. et al. MLL targets SET domain
methyltransferase activity to Hox gene promoters.
Mol. Cell 10, 1107–1117 (2002).
97. Rowley, J. D. The role of chromosome translocations
in leukemogenesis. Semin. Hematol. 36, 59–72
(1999).
98. Ernst, P., Wang, J. & Korsmeyer, S. J. The role of MLL
in hematopoiesis and leukemia. Curr. Opin. Hematol.
9, 282–287 (2002).
99. Krivtsov, A. V. & Armstrong, S. A. MLL translocations,
histone modifications and leukaemia stem-cell
development. Nature Rev. Cancer 7, 823–833 (2007).
100.Armstrong, S. A. et al. MLL translocations specify a
distinct gene expression profile that distinguishes a
unique leukemia. Nature Genet. 30, 41–47 (2002).
101.Grimwade, D. et al. The importance of diagnostic
cytogenetics on outcome in AML: analysis of 1,612
patients entered into the MRC AML 10 trial. The
Medical Research Council Adult and Children’s
Leukaemia Working Parties. Blood 92, 2322–2333
(1998).
102.Moorman, A. V. et al. Karyotype is an independent
prognostic factor in adult acute lymphoblastic
leukemia (ALL): analysis of cytogenetic data from
patients treated on the Medical Research Council
(MRC) UKALLXII/Eastern Cooperative Oncology Group
(ECOG) 2993 trial. Blood 109, 3189–3197 (2007).
103.Biondi, A., Cimino, G., Pieters, R. & Pui, C. H.
Biological and therapeutic aspects of infant leukemia.
Blood 96, 24–33 (2000).
104.Pui, C. H., Relling, M. V. & Downing, J. R. Acute
lymphoblastic leukemia. N. Engl. J. Med. 350,
1535–1548 (2004).
105.Mancini, M. et al. A comprehensive genetic
classification of adult acute lymphoblastic leukemia
(ALL): analysis of the GIMEMA 0496 protocol. Blood
105, 3434–3441 (2005).
106.Bijl, J. et al. Analysis of HSC activity and compensatory
Hox gene expression profile in Hoxb cluster mutant
fetal liver cells. Blood 108, 116–122 (2006).
107.Lawrence, H. J. et al. Mice bearing a targeted
interruption of the homeobox gene HOXA9 have
defects in myeloid, erythroid, and lymphoid
hematopoiesis. Blood 89, 1922–1930 (1997).
108.Ko, K. H. et al. Hoxb3 deficiency impairs B
lymphopoiesis in mouse bone marrow. Exp. Hematol.
35, 465–475 (2007).
109.Magnusson, M., Brun, A. C., Lawrence, H. J. &
Karlsson, S. Hoxa9/hoxb3/hoxb4 compound null mice
display severe hematopoietic defects. Exp. Hematol.
35, 1421–1428 (2007).
110. Brun, A. C. et al. Hoxb4‑deficient mice undergo
normal hematopoietic development but exhibit a mild
proliferation defect in hematopoietic stem cells. Blood
103, 4126–4133 (2004).
111. Krivtsov, A. V. et al. Transformation from committed
progenitor to leukaemia stem cell initiated by
MLL‑AF9. Nature 442, 818–822 (2006).
58 | JANUARY 2011 | VOLUME 11
112. Wong, P., Iwasaki, M., Somervaille, T. C., So, C. W. &
Cleary, M. L. Meis1 is an essential and rate-limiting
regulator of MLL leukemia stem cell potential. Genes
Dev. 21, 2762–2774 (2007).
113. Somervaille, T. C. & Cleary, M. L. Identification and
characterization of leukemia stem cells in murine
MLL‑AF9 acute myeloid leukemia. Cancer Cell 10,
257–268 (2006).
114. Kumar, A. R. et al. Hoxa9 influences the phenotype
but not the incidence of Mll‑AF9 fusion gene leukemia.
Blood 103, 1823–1828 (2004).
115. Ayton, P. M. & Cleary, M. L. Transformation of myeloid
progenitors by MLL oncoproteins is dependent on
Hoxa7 and Hoxa9. Genes Dev. 17, 2298–2307 (2003).
116. Schnabel, C. A., Jacobs, Y. & Cleary, M. L.
HoxA9‑mediated immortalization of myeloid progenitors
requires functional interactions with TALE cofactors Pbx
and Meis. Oncogene 19, 608–616 (2000).
117. Hunger, S. P., Ohyashiki, K., Toyama, K. & Cleary,
M. L. Hlf, a novel hepatic bZIP protein, shows altered
DNA-binding properties following fusion to E2A in
t(17;19) acute lymphoblastic leukemia. Genes Dev. 6,
1608–1620 (1992).
118. Seidel, M. G. & Look, A. T. E2A‑HLF usurps control of
evolutionarily conserved survival pathways. Oncogene
20, 5718–5725 (2001).
119. Beachy, P. A., Karhadkar, S. S. & Berman, D. M. Tissue
repair and stem cell renewal in carcinogenesis. Nature
432, 324–331 (2004).
120.Curran, T. & Ng, J. M. Cancer: Hedgehog’s other great
trick. Nature 455, 293–294 (2008).
121. Eklund, E. A. The role of HOX genes in malignant myeloid
disease. Curr. Opin. Hematol. 14, 85–89 (2007).
122.Argiropoulos, B., Yung, E. & Humphries, R. K.
Unraveling the crucial roles of Meis1 in
leukemogenesis and normal hematopoiesis. Genes
Dev. 21, 2845–2849 (2007).
123.Abramovich, C. & Humphries, R. K. Hox regulation of
normal and leukemic hematopoietic stem cells. Curr.
Opin. Hematol. 12, 210–216 (2005).
124.Grier, D. G. et al. The pathophysiology of HOX genes
and their role in cancer. J. Pathol. 205, 154–171
(2005).
125.Mercado, G. E. & Barr, F. G. Fusions involving PAX and
FOX genes in the molecular pathogenesis of alveolar
rhabdomyosarcoma: recent advances. Curr. Mol. Med.
7, 47–61 (2007).
126.Colleypriest, B. J., Palmer, R. M., Ward, S. G. & Tosh, D.
Cdx genes, inflammation and the pathogenesis of
Barrett’s metaplasia. Trends Mol. Med. 15, 313–322
(2009).
127.Bansal, D. et al. Cdx4 dysregulates Hox gene
expression and generates acute myeloid leukemia alone
and in cooperation with Meis1a in a murine model.
Proc. Natl Acad. Sci. USA 103, 16924–16929 (2006).
128.Guo, R. J., Suh, E. R. & Lynch, J. P. The role of Cdx
proteins in intestinal development and cancer. Cancer
Biol. Ther. 3, 593–601 (2004).
129.Bany, I. A., Dong, M. Q. & Koelle, M. R. Genetic and
cellular basis for acetylcholine inhibition of
Caenorhabditis elegans egg-laying behavior.
J. Neurosci. 23, 8060–8069 (2003).
Acknowledgements
The authors would like to acknowledge Matt Porteus, Jim
Amatruda, John Abrams and Bob Horvitz for their contributions to the manuscript. S.C. would also like to particularly
thank Matt and Jim for their friendship.
Competing interests statement
The authors declare no competing financial interests.
FURTHER INFORMATION
WormAtlas: http://www.wormatlas.org/
Wormbase: http://www.wormbase.org/
ALL LINKS ARE ACTIVE IN THE ONLINE PDF
www.nature.com/reviews/cancer
© 2011 Macmillan Publishers Limited. All rights reserved