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The utilisation of shRNA screens to investigate the role of phosphoinositide modulator genes in actue myeloid leukaemia
[Thesis]. Manchester, UK: The University of Manchester; 2012.
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Abstract
Phosphoinositides (PIs) are pivotal lipid molecules with both scaffolding and signalling functions regulating key aspects of cellular physiology. For example, phosphatidylinositol (3,4,5)-trisphosphate, generated by phosphoinositide 3-kinase (PI3K), is an essential mediator of the PI3K/AKT signalling pathway, which is crucial for cell proliferation, survival and apoptosis. Constitutive activation of this signalling cascade has been identified in acute myeloid leukaemia (AML), the most common haematopoietic malignancy in adults, and experimental deletion of the PI3K antagonists PTEN and SHIP cause leukaemia in mice. However, little is known regarding the role of other PI modulator proteins in AML. Thus, in this thesis, a lentivirally delivered small hairpin RNA (shRNA) library targeting 103 genes (345 pLKO knockdown constructs) with presumed or established roles in PI metabolism was utilised to screen for genes required for AML blast cell viability/proliferation and differentiation.First, knockdown constructs were tested for their impact on proliferation/viability in seven human AML cell lines by measuring fold change in fluorescence of the cell viability dye alamarBlue relative to controls (cells transduced with a non-targeting control hairpin) over three days. This identified 13 candidate genes selected with the criterion that two or more knockdown constructs per gene reduce cell viability/proliferation relative to control by ≥50 % across all cell lines. From these candidate genes, PIP4K2A, INPP5B and IMPAD1 were selected for downstream validation experiments, which reproduced the observation from the primary screen. For INPP5B and IMPAD1, knockdown constructs also reduced clonogenic potential of primary human AML samples but only showed a modest effect on normal CD34+ haematopoietic stem or progenitor cells (HSPCs) in a methylcellulose based assay. This could be recapitulated in a murine setting where knockdown constructs targeting both genes reduced clonogenic potential of murine MLL- AF9 AML cells with little effect on normal KIT+ HSPCs. In line with this, Inpp5b knockout KIT+ BM cells either failed to immortalise or weakly immortalised, following forced expression of the powerful MLL-AF9 oncogene.A further screen was performed to identify regulators of THP-1 blast cell differentiation, by seeding knockdown construct transduced cells into methylcellulose based semisolid media. After ten days of incubation the degree of macrophage differentiation was evaluated by light microscopy and an arbitrary differentiation score was given. With the criterion that ≥2 knockdown constructs per gene received the highest differentiation score, reflecting terminal macrophage differentiation of all seeded cells, SBF2 was identified as the top-scoring hit. Validation experiments have confirmed macrophage differentiation based on cytospin preparations of SBF2 knockdown THP-1 cells. Moreover, xenograft assays have shown that knockdown constructs targeting PIP4K2A and SBF2 delayed or abrogated in vivo leukaemogenesis.Thus this work has identified novel roles for PI modulator genes in human AML with possible therapeutic potential.