The Impact of Cellular Age on Leukaemic Transformation
2016
Background: Treatment of pediatric acute myeloid leukemia (AML) is largely
extrapolated from adult trials despite age related disease heterogeneity. Indeed,
during healthy ageing there are age related hemopoietic stem and progenitor
(HSPC) cell proliferative and differentiative differences. We hypothesise that
cellular age influences leukemic transformation, having implications for disease
phenotype and response to therapy.
Aims: To determine if HSPC age affects oncogene-mediated leukemic transformation.
Methods: Murine HSPCs including linsca1+cKit+
(LSK), common myeloid progenitor
(CMP) and granulocyte-macrophage progenitor (GMP) cells from fetal
liver (FL), 3 week(w), 10w and >60w adult bone marrow were isolated and
transduced with the fusion oncogenes Nup98HoxA9 (NH9), AML1ETO (AE)
and the mutant FLT3-ITD. Leukemic transformation was assessed in vitro by
serial colony forming cell (CFC) assays, by growth in liquid culture and in stromal
OP9 co-culture. In vivo leukemogenesis was assessed by transplantation
of pre-leukemic LSKs transduced with NH9 after 2 rounds of colony formation
(CFC2), into sublethally irradiated C57BL/6 mice. Gene expression was
assessed by QPCR using fluidigm technology.
Results: NH9 transformed LSKs from all 4 ages in vitro. NH9 did not result in
FL CMP and GMP transformation, while post-fetal (3w, 10w and >60w) CMPs
and GMPs all transformed. Consistent with this, AE and FLT3-ITD transformed
FL LSKs but not FL GMPs in vitro. This suggests that fetal transformation relies
on specific features of the LSK that are absent from committed myeloid progenitors,
independent from the oncogenic insult. To further assess age related
transformation differences, NH9 transformed FL, 3w, 10w and >60w LSKs were
assessed for AML in vivo. Older (10 and >60w) transformed cells led to a
shorter latency with more penetrance than young (FL and 3w) transformed
LSKs. Further, acute lymphoblastic leukemia (ALL) was observed in animals
transplanted with FL and 3w transformed LSKs, but not 10w or >60w LSKs,
suggesting young LSKs retain a lymphoid bias. As all NH9 transformed LSKs
in vitro acquired self-renewal properties, the transformation differences
observed in vivo may be cell autonomous or non-cell autonomous via the bone
marrow microenvironment. Gene expression analysis of microenvironmental
receptors and targets show the BMP pathway is upregulated in 10w and >60w
LSK transformed cells suggesting the BMP pathway may have a role in age
related transformation potential.
Summary/Conclusions: While LSKs from all 4 ages acquire self-renewal in
vitro, progression to leukemia in vivo is slower and less penetrant in young
compared to older transformed cells. This is in accordance with the lower incidence
of AML in childhood compared to older adults. Activation of the BMP
pathway has previously been associated with AML transformation, and our
data suggest that the BMP pathway may play a role in the progression to AML
specifically in older cells. The observation of ALL specifically in younger transformed
LSKs suggests that young HSPCs retain lymphoid programs after
myeloid oncogene expression, consistent with the higher incidence of ALL in
childhood. Our findings support that age defined therapies are appropriate as
HSPC age related biological differences are retained in leukemia and may
impact not only disease phenotype but response to therapy
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