SUMMARY Adipose tissue (AT) has previously been identified as an extra-medullary reservoir for normal hematopoietic stem cells (HSCs) and may promote tumor development. Here, we show a subpopulation of leukemic stem cells (LSCs) can utilize gonadal adipose tissue (GAT) as a niche to support their metabolism and evade chemotherapy. In a mouse model of blast crisis CML, adipose-resident LSCs exhibit a pro-inflammatory phenotype and induce lipolysis in GAT. GAT lipolysis fuels fatty acid oxidation in LSCs, especially within a subpopulation expressing the fatty acid transporter CD36. CD36+ LSCs have unique metabolic properties, are strikingly enriched in AT, and are protected from chemotherapy by the GAT microenvironment. CD36 also marks a fraction of human blast crisis CML and AML cells with similar biological properties. These findings suggest striking interplay between leukemic cells and adipose tissue to create a unique microenvironment that supports the metabolic demands and survival of a distinct LSC subpopulation.

The MLL-AF9 fusion gene is associated with aggressive leukemias of both the myeloid and lymphoid lineage in infants, whereas in adults, this translocation is mainly associated with acute myeloid leukemia. These observations suggest that differences exist between fetal and adult tissues in terms of the 'cell of origin' from which the leukemia develops. Here we show that depending on extrinsic cues, human neonatal CD34(+) cells are readily immortalized along either the myeloid or lymphoid lineage upon MLL-AF9 expression and give rise to mainly lymphoid leukemia in immunocompromised mice. In contrast, immortalization of adult bone marrow CD34(+) cells is more difficult to achieve and is myeloid-biased, even when MLL-AF9 is expressed in purified hematopoietic stem cells (HSCs). Transcriptome analysis identified enrichment of HSC but not progenitor gene signatures in MLL-AF9-expressing cells. Although not observed in adult cells, neonatal cells expressing MLL-AF9 were enriched for gene signatures associated with poor prognosis, resistance to chemotherapeutic agents and MYC signaling. These results indicate that neonatal cells are inherently more prone to MLL-AF9-mediated immortalization than adult cells and suggest that intrinsic properties of the cell of origin, in addition to extrinsic cues, dictate lineage of the immortalized cell.

In order to identify acute myeloid leukemia (AML) CD34 þ -specific gene expression profiles, mononuclear cells from AML patients (n ¼ 46) were sorted into CD34 þ and CD34 À subfractions, and genome-wide expression analysis was performed using Illumina BeadChip Arrays. AML CD34 þ and CD34 À gene expression was compared with a large group of normal CD34 þ bone marrow (BM) cells (n ¼ 31). Unsupervised hierarchical clustering analysis showed that CD34 þ AML samples belonged to a distinct cluster compared with normal BM and that in 61% of the cases the AML CD34 þ transcriptome did not cluster together with the paired CD34 À transcriptome. The top 50 of AML CD34 þ -specific genes was selected by comparing the AML CD34 þ transcriptome with the AML CD34 À and CD34 þ normal BM transcriptomes. Interestingly, for three of these genes, that is, ankyrin repeat domain 28 (ANKRD28), guanine nucleotide binding protein, alpha 15 (GNA15) and UDP-glucose pyrophosphorylase 2 (UGP2), a high transcript level was associated with a significant poorer overall survival (OS) in two independent cohorts (n ¼ 163 and n ¼ 218) of normal karyotype AML. Importantly, the prognostic value of the continuous transcript levels of ANKRD28 (OS hazard ratio (HR): 1.32, P ¼ 0.008), GNA15 (OS HR: 1.22, P ¼ 0.033) and UGP2 (OS HR: 1.86, P ¼ 0.009) was shown to be independent from the well-known risk factors FLT3-ITD, NPM1c þ and CEBPA mutation status.