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UBC Theses and Dissertations

Elucidating the mechanisms of leukemogenesis driven by FBXO11 depletion Mo, Ya-Chi Angela


We examined the AML-PMP, TCGA, TARGET pediatric, and Beat AML public datasets to identify genetic drivers of AML. We identified frequent somatic mutations in ubiquitin proteasome system genes, with recurrent inactivating mutations in FBXO11. FBXO11 encodes the substrate-recognizing component of the SKP1-CUL1-F-BOX (SCF) ubiquitin E3-ligase complex. In a mouse marrow transplant model generated by previous members of the Karsan lab, Fbxo11 knockdown cooperated with AML1-ETO expression to initiate AML. These leukemias were serially transplantable, consistent with the presence of leukemic stem cells (LSC). We hypothesized that FBXO11 depletion in hematopoietic stem and progenitor cells (HSPC) promotes self-renewal and drives leukemogenesis through dysregulated expression of its ubiquitination targets. First, we developed a serially transplantable human xenotransplant myeloid leukemia model based on FBXO11 depletion with cooperating mutations, and observed increased engraftment and expansion of primitive CD34⁺ cells with FBXO11 knockdown. Next, to identify FBXO11 targets that are dysregulated with FBXO11 depletion, we performed mass spectrometry analysis of FBXO11 co-immunoprecipitating proteins in the myeloid K562 cell line. We identified LONP1 as a top target, but LONP1 protein expression did not change with FBXO11 over-expression or loss, suggesting that LONP1 is not targeted for degradation. Knockdown of LONP1 in human CD34⁺ HSPCs resulted in similar phenotypes as FBXO11 knockdown: enriched hematopoietic stem cell, LSC, and mitochondrial defect transcriptomic signatures, increased myeloid cells, and reduced erythroid and megakaryocytic populations. In parallel, FBXO11 knockdown reduced mitochondrial membrane potential. These data suggest that FBXO11 normally positively regulates LONP1. We found that K63-linked polyubiquitination—a modification known to regulate protein activation and trafficking— of LONP1 is increased with FBXO11 overexpression, and reduced with FBXO11 loss. LONP1 overexpression alone had negligible effect on the transcriptome, and LONP1 knockdown blocked or reversed the effects of FBXO11 overexpression. This supports a model of FBXO11 regulating LONP1 mitochondrial function, presumably through K63-linked ubiquitination. These findings suggest that FBXO11 positively regulates LONP. We propose FBXO11 depletion in AML patients results in reduced LONP1 mitochondrial function, which promotes a primitive cell state. We have identified novel candidate targets of FBXO11, and demonstrated a unique link between the ubiquitin-proteosome system and the mitochondria.

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