RESEARCH ARTICLE SUMMARY
◥CANCER
Mapping the cellular origin and early evolution of
leukemia in Down syndrome
Elvin Wagenblast, Joana Araújo, Olga I. Gan, Sarah K. Cutting, Alex Murison, Gabriela Krivdova,
Maria Azkanaz, Jessica L. McLeod, Sabrina A. Smith, Blaise A. Gratton, Sajid A. Marhon,
Martino Gabra, Jessie J. F. Medeiros, Sanaz Manteghi, Jian Chen, Michelle Chan-Seng-Yue,
Laura Garcia-Prat, Leonardo Salmena, Daniel D. De Carvalho, Sagi Abelson, Mohamed Abdelhaleem,
Karen Chong, Maian Roifman, Patrick Shannon, Jean C. Y. Wang, Johann K. Hitzler, David Chitayat,
John E. Dick, Eric R. Lechman*
INTRODUCTION:Leukemia is the most common
cancer in children, with the first genetic altera-
tions often occurring during fetal development.
These initiating events generate preleukemic
cells, which are the evolutionary ancestors of
leukemia that arises after birth. Because of our
inability to directly access human fetal pre-
leukemia, the identity of the cell of origin and
the steps of leukemia evolution remain largely
unknown. Down syndrome leukemogenesis
represents a disease setting to study human
preleukemia and the evolutionary steps that
lead to fully transformed leukemia. Up to 30%
of children with Down syndrome (trisomy 21)
exhibit a preleukemic transient abnormal mye-
lopoiesis (TAM) and, overall, have a 150-fold
increased risk of developing myeloid leuke-
mia within the first 5 years of life. However,
the mechanism by which an extra copy of
chromosome 21 predisposes to preleukemia
and leukemia remains unclear.
RATIONALE:Understanding Down syndrome
leukemogenesis requires a humanized model
that faithfully recapitulates the full develop-
mental spectrum of premalignant and malig-
nant stages of Down syndrome leukemia.
Using CRISPR/Cas9–mediated gene editing
in human disomic and trisomic fetal liver–
derived hematopoietic stem and progenitor
cells and xenotransplantation, we developed
a model with which to characterize the genetic
events and cellular contexts underlying the
preleukemic and leukemic phases of Down
syndrome leukemogenesis.RESULTS:Trisomy 21 hematopoietic stem and
progenitor cells (HSPCs) showed reduced pro-
liferation in vitro and generated smaller grafts
in xenotransplanted mice, with reduced serial
transplant ability, as compared with that of
disomic HSPCs. Preleukemia was initiated in
trisomy 21, but not disomic, long-term hema-
topoietic stem cells (LT-HSCs) when mutations
in the erythroid-megakaryocyte transcription
factor GATA binding protein 1 (GATA1) were
introduced, which led to exclusive expression
of the short isoform (GATA1s). Subsequentleukemic progression could occur in multiple
stem and progenitor populations, was inde-
pendent of trisomy 21, and induced through
deletion of cohesin genes, includingSTAG2
(STAG2ko). Serial engraftment in mice showed
that GATA1s-induced preleukemia underwent
spontaneous resolution, which contrasted with
the persistent ability of the GATA1s/STAG2ko–
induced leukemia to engraft serially in mice.
Leukemic progression was developmentally
restricted to fetal and early postnatal stages;
adult-derived bone marrow HSPCs were unable
to undergo GATA1s/STAG2ko-induced leuke-
mic transformation. We identified a molecular
mechanism by which three chromosome 21
microRNAs (miRNAs) contributed to the pre-
disposition toward preleukemia initiation.
Simultaneous overexpression ofmiR-99a,
miR-125b-2, andmiR-155in normal disomic
LT-HSCs recapitulated a trisomy 21–like hema-
topoietic state, as assessed through comparable
lineage differentiation, reduced self-renewal
capacity, and similar gene expression and open
chromatin accessibility profile. Removal of
these miRNAs in trisomy 21 LT-HSCs inhib-
ited GATA1s-induced preleukemia develop-
ment. Using secondary xenotransplantations
of defined cell populations, we identified
CD117+/KIT proto-oncogene (KIT) as a marker
of disease-driving cells. Pharmacological KIT
inhibition targeted preleukemic stem cells, both
in GATA1s-induced preleukemia and in primary
Down syndrome preleukemia patient samples.CONCLUSION:Collectively, our results provide
insight into how human preleukemia and leu-
kemia evolve in fetal life and early childhood.
We were able to identify distinct cellular ori-
gins and effects of trisomy 21 for preleukemia
initiation and leukemia progression. Predis-
position to preleukemia in Down syndrome
is affected by overexpression of distinct chro-
mosome 21 miRNAs, specifically in the pre-
leukemic LT-HSC cell of origin. Our study
reveals the relevance of the cellular and de-
velopmental status of the cell of origin during
leukemogenesis, which begins to explain why
genetic drivers can be distinct between pe-
diatric and adult acute myeloid leukemia. KIT
inhibitors targeted preleukemic stem cells, pro-
viding proof of principle for early prevention
strategies in childhood leukemia that may
be able to inhibit leukemia progression, and
these results encourage further preclinical and
clinical assessment.
▪RESEARCHSCIENCEsciencemag.org 9JULY2021•VOL 373 ISSUE 6551 179
The list of author affiliations is available in the full article online.
*Corresponding author. Email: elvin.wagenblast@
uhnresearch.ca (E.W.); [email protected] (J.E.D.);
[email protected] (E.R.L)
Cite this article as E. Wagenblastet al.,Science 373 ,
eabf6202 (2021). DOI: 10.1126/science.abf6202READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abf6202Chromosome 21 miRNAsCD117/KITGATA 1 sG ATA 1 sLT- H S CTrisomy 21 Preleukemia Leukemia
Cohesin FactorsStem and
ProgenitorsGATA 1 s
STAG 2
STAG 2
STAG 2
STAG 2Knock-out of
chromosome 21 miRNAs KIT inhibitionCell of origin in Down syndrome leukemogenesis.Down syndrome preleukemia originated in long-term
hematopoietic stem cells (LT-HSCs) through mutations in GATA1, leading to the expression of the short
isoform GATA1s. Progression toward leukemia occurred in various stem and progenitor cells through
mutations in cohesin factors such as STAG2. Predisposition to preleukemia was affected by chromosome 21
miRNAs, and pharmacological inhibition of KIT targeted preleukemic stem cells.