Stearoyl-CoA Desaturase (SCD) Enhances Central Nervous System Leukemia

Background: Central nervous system (CNS) involvement by acute lymphoblastic leukemia (ALL) is a major clinical concern. Leukemic cells can be found in the CNS at diagnosis (1-2%) or, more frequently, at relapse (30%). Very little is known about the pathogenesis and therefore there are no targeted therapies. Prophylactic CNS-directed conventional intrathecal chemotherapy or irradiation are required for relapse-free survival. However, they are associated with substantial rates of short and long term toxicity. Therefore, elucidation of molecular mechanisms and pathways mediating leukemia-cell entry and survival in the CNS is needed to develop alternative CNS-directed treatment strategies. Previous studies showed an increased expression of Stearoyl-CoA desaturase (SCD), a key enzyme of the de novo fatty acid synthesis pathway, in B cell precursor (BCP) ALL cells isolated from cerebrospinal fluid (CSF) of patients at the time of CNS relapse. A small SCD positive population was detected in the bone marrow (BM) at leukemia diagnosis in patients who later developed isolated CNS relapse, defining a potential biomarker for CNS relapse. It is unknown, however, if SCD has a functional role in CNS leukemia.

Aim: To examine the hypothesis that increased expression of SCD enhances trafficking and survival of human B-ALL cells in the CNS

Methods: We analyzed leukemia-cell entry into the CNS using xenografts of human BCP-ALL cell lines. Microarray profile of cells isolated from CNS and BM of transplanted mice was performed. Cell lines were transduced to overexpress human SCD and evaluated in vitro for proliferation kinetics and metabolic SCD activity. In vivo, SCD overexpressing cells were transplanted in NSG mice,sacrificed upon the first symptoms of CNS involvement, e.g. hind limb paralysis. BM, spleen and meninges were collected and analyzed to check human engraftment by FACS. The tumor load was expressed as total amount of leukemic cells in each organ. Competition assays were performed by transplanting SCD overexpressing and WT cells in the same mouse in a 1:1 ratio.

Results: BCP-ALL cells transplanted into NSG mice faithfully recapitulated pathological features of meningeal infiltration seen in patients with ALL. Gene expression analysis of cells collected from BM and meninges of leukemic mice revealed up-regulation of the genes belonging to the signaling pathway of sterol regulatory element binding proteins (SREBPs) in ALL cells isolated from the CNS. SCD, whose transcription is controlled by the SREBP family, was significantly upregulated. SCD overexpression did not alter proliferation in vitro. Since SCD introduces a double bond in Stearoyl-CoA, its activity was measured as the ratio of unsaturated/saturated fatty acids in the cells. That ratio was increased in SCD overexpressing cells in vitro, confirming the functionality of the enzyme. In vivo, mice transplanted with SCD overexpressing cells led to a faster onset of CNS disease manifested by a clinical phenotype of earlier hind limb paralysis compared to control and significant increased number of leukemic cells in the CNS (Figure 1A).SCD overexpression also induced CNS engraftment of another B-ALL cell line, REH, which is not naturally prone to invade the central nervous system. Mice transplanted with SCD overexpressing REH cells showed the same phenotype of earlier hind limb paralysis and accumulation of leukemic cells in the CNS as the CNS-prone 018z cells, while WT REH did not show any CNS engraftment but comparable tumor load in BM and spleen (Figure1B). To reproduce the clonal heterogeneity in SCD expression observed previously in patients' BM, we performed a competition assay transplanting SCD overexpressing cells and control cells, expressing different fluorochromes, in the same mouse in a 1:1 ratio. In the CNS, the ratio between SCD overexpressing and WT cells ranged from 2 to 20 fold. This effect was unique to the CNS and not reproducible in the other hematopoietic organs where the 1:1 ratio was maintained (Figure 1C). Moreover, SCD overexpression sensitized leukemic cells to mTOR inhibitors, suggesting a potential therapeutic option

Conclusion: SCD has a role in homing and survival of leukemic cells in the CNS and may be used as early predictor of CNS relapse. This study reveals a role for SCD and fatty acid metabolism in the pathogenesis of CNS leukemia suggesting that this pathway maybe targeted for specific therapy of this devastating disease.

Figure 1.

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Figure 1.

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