MDS: Roadblock to differentiation

Blood (Impact Factor: 10.45). 09/2012; 120(10):1968-9. DOI: 10.1182/blood-2012-07-442749
Source: PubMed
In this issue of Blood, Will et al identify aberrant expansion of distinct stem and progenitors in myelodysplastic syndromes (MDS) and demonstrate that MDS stem cells are functionally abnormal and harbor genetic and epigenetic alterations.


Available from: Anupriya Agarwal, Nov 18, 2015
Comment on Will et al, page 2076
MDS: roadblock to differentiation
In this issue of Blood, Will et al identify aberrant expansion of distinct stem and
progenitors in myelodysplastic syndromes (MDS) and demonstrate that MDS
stem cells are functionally abnormal and harbor genetic and epigenetic alterations.
DS is thought to be a disorder of hemato-
poietic stem cells (HSCs).
abnormalities, mutations, and epigenetic changes
have been reported in MDS progenitors.
ever, the earliest stage at which these molecular
and pathogenic events occur and the functional
consequences of these events has not been clearly
established. In the present study, Will and col-
leagues functionally and molecularly character-
ize highly purified primitive stem cells and pro-
genitors using technically challenging techniques
in a variety of MDS subtypes. They show that
the primitive long-term stem cell (LT-HSC)
pool is significantly increased in high-risk
MDS subtypes compared with healthy con-
trols (see figure). Further analysis of commit-
ted progenitor populations shows that lower-
risk MDS patients have skewed expansion of
common myeloid progenitors (CMPs). In
contrast, analysis of high-risk patients reveals
expansion of the granulocyte-monocyte pro-
genitors (GMPs) with a relative decrease in the
megakaryocyte-erythrocyte progenitor (MEP)
population. Notably, this concept of differential
arrest is not unique to MDS; GMP expansion
has recently been reported in acute myeloid leu-
kemia (AML)
and block in myeloid differentia
tion is a well-characterized feature of the blastic
phase of chronic myeloid leukemia (CML-BP).
Will and colleagues further investigate the
functional and molecular signatures of primitive
MDS HSCs. Karyotypic abnormalities are iden-
tified in the majority of immature HSCs from
patients representing the various MDS sub-
types. Importantly, the authors find that these
cytogenetic abnormalities persist even in the
expanded CMP and GMP populations. Further,
they show MDS-HSCs are functionally defi-
cient in their clonogenic potential and give rise to
dysplastic colonies. The authors perform addi-
tional genome-wide methylomic and transcrip-
tomic analysis of these rare stem cells and dem-
onstrate that primitive MDS-HSCs are not
only karyotypically and functionally abnormal
but also exhibit widespread alteration in DNA
methylation and gene expression. Although
epigenetic alterations including hypermethyl-
ation have earlier been investigated in
and bone marrow cells
MDS patients, Will and colleagues are the first
to characterize such patterns in primitive stem
cells. In addition to identifying a subset of
genes that are hypermethylated, they also de-
scribe a novel signature of hypomethylated
genes in these cells, implicating a potential
previously unrecognized role for altered hy-
pomethylation in MDS pathogenesis. To vali-
date these findings functionally the authors
perform additional studies for STAT3, which
they find to be significantly hypomethylated
and overexpressed in HSCs from all tested
subsets of MDS. They demonstrate that MDS
HSCs are sensitive to pharmacologic inhibi-
tion of STAT3 in clonogenic assays, showing
reduced colony formation compared with nor-
mal HSC controls. Taken together, these find-
ings describe a list of candidate genes (includ-
ing STAT3) that are dysregulated in primitive
MDS stem cells that may serve as a target for
stem cell– directed therapies for the disease.
Clinically, the most relevant finding of this
study is that this cytogenetically abnormal
stem cell pool persisted in a patient with com-
plete morphologic remission after epigenetic
therapy with azacytidine and vorinostat. Fur-
thermore, through careful examination of se-
rial samples, the authors demonstrate that
morphologic relapse in this patient was pre-
ceded by expansion of the HSC compartment.
This is consistent with another recent report
that found persistence of rare and malignant
HSCs in MDS with the 5q abnormality in
Stage-specific aberrant expansion or differentiation block of distinct stem and progenitor populations in MDS.
(A) Normal primitive long-term hematopoietic stem cells (LT-HSCs; red circles) have the capacity for
self-renewal and give rise to a balanced number of multipotent progenitors (orange circles), common myeloid
progenitors (CMPs; blue circles), granulocyte-monocyte progenitors (GMPs; purple circles) and megakaryocyte-
erythrocyte progenitors (MEPs; green circles). (B) Low risk MDS LT-HSCs are cytogenetically abnormal (orange
bars) with expansion of CMPs but normal GMP and MEP distribution. (C) High-risk MDS LT-HSCs are
significantly expanded with increased GMP and slightly reduced MEP compartments. Status of multipotent
progenitors in low- and high-risk patients has not been evaluated in this study.
1968 6 SEPTEMBER 2012 I VOLUME 120, NUMBER 10
Page 1
remission and in subsequent progression on
lenalidomide therapy.
These observations
suggest that such analysis of the HSC com-
partment could be performed on MDS pa-
tients as both a metric for monitoring response
and as a biomarker for development of tar-
geted therapeutics. This further raises the
possibility of whether the reason current treat-
ment strategies are unable to cure these pa-
tients is the direct result of their inability to
eliminate the residual, clonally abnormal
HSC pool.
The big remaining question is whether
MDS LT-HSCs, CMPs, and GMPs have
capability to self-renew in vitro and in vivo,
and if so, by what molecular mechanisms. A
few studies to date suggest that phenotypic
stem and progenitor cell compartments con-
tain leukemia initiating cells in MDS.
It will
also be worth evaluating whether MDS GMPs
may serve as a reservoir for disease progression
to AML, because a recent study found that
AML can arise from GMP-like stem cells.
Lastly, interrogation of the precise molecular
switches that control this lineage-dependent
differentiation block in low- versus high-risk
MDS patients will be of great interest and
could represent a turning point in therapeutic
strategies for the disease. It would also be in-
teresting to know whether such changes occur
before disease onset, potentially predisposing
to low- versus high-risk MDS, or arises as part
of disease progression.
The observation that a small pool of
cancer-initiating stem cells cannot be readily
eliminated by conventional cytotoxic therapies
appears to be something of a common theme
among a variety of cancers. In this regard,
MDS is another in the queue where efforts are
needed for targeting stem cells. Consequently,
ongoing efforts to better understand the mo-
lecular pathways that regulate disease-
initiating cells will potentially have further
implications for the development of future
targeted therapies in a variety of cancers.
Overall, the study by Will and colleagues pro-
vides the impetus for defining the genetic and
epigenetic events governing HSC and
progenitor-cell resistance to therapy and their
role in disease progression.
Conflict-of-interest disclosure: The author
declares no competing financial interests.
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Comment on Gallagher et al, page 2098
Mutation associations in RA-defiant APL
One-quarter of acute promyelocytic leukemia (APL) patients develop resistance to
all-trans retinoic acid (ATRA)/chemotherapy (CT). In this issue of Blood,
Gallagher et al report the associations of PML-RAR mutations, FLT3 mutations,
and additional chromosome abnormalities (ACAs) in relapsed APL.
ntrinsic or acquired resistance to anticancer
drugs can arise from a variety of factors in-
cluding the development of mutations in drug
targets and additional genetic abnormalities.
In APL, which is driven by the t(15;17)–
generating PML-RAR, ATRA in combina-
tion with CT achieves a complete remission
rate of 90% and a 5-year disease-free survival
of 74%.
However, ATRA resistance has
been reported for 2 decades and was shown to
be associated with increased catabolism and
decreased delivery to cell nucleus of ATRA, as
well as mutations in the ligand-binding do-
main (LBD) of the RAR portion of the fusion
In this study, Gallagher et al further
dissect the potential association of PML-
RAR LBD mutations (PR/LBD
), FLT3
mutations, and ACAs in relapsed APL on
ATRA/CT (see figure).
The authors show that among 45 relapsed
patients from the ATRA/CT treatment
group, 18 cases harbor PR/LBD
7 of whom (39%) relapsed more than 30 days
after last ATRA dose (off ATRA) selection
pressure, suggesting a possible active role of
. Indeed, Gallagher et al observed
in 2 cases the selection of a pre-existing mutant
subclone by ATRA that lead to relapse on or
off ATRA treatment.
Unlike PR/LBD
the incidence and quantitation of FLT3-
are not increased during relapse with
out any evidence of influence of ATRA treat-
ment. The fact that all FLT3-ITD
who relapsed off ATRA lacked PR/LBD
and most FLT3-ITD
patients (83%) who
relapsed on ATRA had a coincident PR/
, suggests that ATRA could not elimi
nate the double-mutant subclone. Exclusive
ACAs are identified at diagnosis, and are sig-
nificantly increased (2-fold) at relapse (29% to
62%). Structural chromosome changes are
predominantly newly present at relapse and
differ from ACA at diagnosis. Interestingly,
despite the heterogeneity of ACAs, they are
associated with a phenotype of pWBC
L-isoform, if relapse occurred off ATRA.
as a mechanism leading to off-
ATRA disease progression may be proved by
the observation that ACA-PR/LBD
negatively associated with FLT3-ITD
which presents an opposite phenotype of pW-
and S-isoform. However, in on-ATRA
relapsed patients, the above associations are
not apparent, suggesting a distinct different
mechanism of resistance and progression. In
prognostic analysis, only the presence of ACA
at relapse is associated with reduced postre-
lapse outcome. In the patients with PR/
6 SEPTEMBER 2012 I VOLUME 120, NUMBER 10 1969
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    • "Our model does not distinguish between different hematopoietic lineages (such as lymphoid and myeloid differentiation ). This is a simplification which assumes that all the lineages have similar proliferation and maturation structure and dynamics – most likely this does not represent physiological conditions, particularly with regard to the different MDS-subtypes, which may affect one or several lineages [43]. However, consideration of all hematopoietic lineages (or of specific MDS-subtypes) would entail very complex models which would not facilitate such clear conclusions. "
    [Show abstract] [Hide abstract] ABSTRACT: Myelodysplastic syndromes (MDS) are triggered by an aberrant hematopoietic stem cell (HSC). It is, however, unclear how this clone interferes with physiologic blood formation. In this study, we followed the hypothesis that the MDS clone impinges on feedback signals for self-renewal and differentiation and thereby suppresses normal hematopoiesis. Based on the theory that the MDS clone affects feedback signals for self-renewal and differentiation and hence suppresses normal hematopoiesis, we have developed a mathematical model to simulate different modifications in MDS-initiating cells and systemic feedback signals during disease development. These simulations revealed that the disease initiating cells must have higher self-renewal rates than normal HSCs to outcompete normal hematopoiesis. We assumed that self-renewal is the default pathway of stem and progenitor cells which is down-regulated by an increasing number of primitive cells in the bone marrow niche - including the premature MDS cells. Furthermore, the proliferative signal is up-regulated by cytopenia. Overall, our model is compatible with clinically observed MDS development, even though a single mutation scenario is unlikely for real disease progression which is usually associated with complex clonal hierarchy. For experimental validation of systemic feedback signals, we analyzed the impact of MDS patient derived serum on hematopoietic progenitor cells in vitro: in fact, MDS serum slightly increased proliferation, whereas maintenance of primitive phenotype was reduced. However, MDS serum did not significantly affect colony forming unit (CFU) frequencies indicating that regulation of self-renewal may involve local signals from the niche. Taken together, we suggest that initial mutations in MDS particularly favor aberrant high self-renewal rates. Accumulation of primitive MDS cells in the bone marrow then interferes with feedback signals for normal hematopoiesis - which then results in cytopenia.
    Full-text · Article · Apr 2014 · PLoS Computational Biology
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    • "There are probably several main mechanisms contributed to MDS development: the genetic and epigenetic alterations in haematopoietic stem/progenitor cells and the changes of microenvironment including immune system456789. These clonal and heritable alterations in haematopoietic stem cells of MDS patients are responsible for its tendency of malignant myeloid progression and the progenitor cells differentiation blockage as well as the dysplastic features of bone marrow hypohaemia [10]. Another most common malignant haematological disease chronic myeloid leukaemia (CML), in most of which has BCR-ABL fusion protein, also has a high risk of myeloid progression even after treatment with tyrosine kinase inhibitors11121314 . "
    [Show abstract] [Hide abstract] ABSTRACT: The polycomb group BMI1 is proved to be crucial in malignant myeloid progression. However, the underlying mechanism of the action of BMI1 in myeloid malignant progression was not well characterized. In this study, we found that the patients of both myelodysplastic syndromes and chronic myeloid leukaemia with BMI1 overexpression had a higher risk in malignant myeloid progression. In vitro gene transfection studies showed that BMI1 inhibited cell myeloid and erythroid differentiation induced by 12-O-tetradecanoyl phorbol-13-acetate (TPA) and histone deacetylase inhibitor sodium butyrate respectively. BMI1 also resisted apoptosis induced by arsenic trioxide. Moreover, the transcript levels of Runx1 and Pten were down-regulated in Bmi1-transfected cells in company with histone deacetylation modification. By using chromatin immunoprecipitation (ChIP) collaborated with secondary generation sequencing and verified by ChIP-PCR, we found that BMI1 directly bound to the promoter region of Zmym3, which encodes a component of histone deacetylase-containing complexes. In addition, as one of the downstream target genes of this complex, c-fos was activated with increasing histone acetylation when ZMYM3 was suppressed in the Bmi1-transfected cells. These results suggested that BMI1 may reprogramme the histone acetylation profile in multiple genes through either indirect or direct binding effects which probably contributes to the malignant progression of myeloid progenitor cells.
    Full-text · Article · Feb 2014 · Journal of Cellular and Molecular Medicine
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    [Show abstract] [Hide abstract] ABSTRACT: Evidence for distinct human cancer stem cells (CSCs) remains contentious and the degree to which different cancer cells contribute to propagating malignancies in patients remains unexplored. In low- to intermediate-risk myelodysplastic syndromes (MDS), we establish the existence of rare multipotent MDS stem cells (MDS-SCs), and their hierarchical relationship to lineage-restricted MDS progenitors. All identified somatically acquired genetic lesions were backtracked to distinct MDS-SCs, establishing their distinct MDS-propagating function in vivo. In isolated del(5q)-MDS, acquisition of del(5q) preceded diverse recurrent driver mutations. Sequential analysis in del(5q)-MDS revealed genetic evolution in MDS-SCs and MDS-progenitors prior to leukemic transformation. These findings provide definitive evidence for rare human MDS-SCs in vivo, with extensive implications for the targeting of the cells required and sufficient for MDS-propagation.
    Full-text · Article · Jun 2014 · Cancer Cell