Faculty Bibliography

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Cardiac metabolism affects systemic energetic balance. Previously, we showed that Krüppel-like factor (KLF)-5 regulates cardiomyocyte PPARα and fatty acid oxidation-related gene expression in diabetes. We surprisingly found that cardiomyocyte-specific KLF5 knockout mice (αMHC-KLF5^-/-) have accelerated diet-induced obesity, associated with increased white adipose tissue (WAT). Alterations in cardiac expression of the mediator complex subunit 13 (Med13) modulates obesity. αMHC-KLF5^-/- mice had reduced cardiac Med13 expression likely because KLF5 upregulates Med13 expression in cardiomyocytes. We then investigated potential mechanisms that mediate cross-talk between cardiomyocytes and WAT. High fat diet-fed αMHC-KLF5^-/- mice had increased levels of cardiac and plasma FGF21, while food intake, activity, plasma leptin, and natriuretic peptides expression were unchanged. Consistent with studies reporting that FGF21 signaling in WAT decreases sumoylation-driven PPARγ inactivation, αMHC-KLF5^-/- mice had less SUMO-PPARγ in WAT. Increased diet-induced obesity found in αMHC-KLF5^-/- mice was absent in αMHC-[KLF5^-/-;FGF21^-/-] double knockout mice, as well as in αMHC-FGF21^-/- mice that we generated. Thus, cardiomyocyte-derived FGF21 is a component of pro-adipogenic crosstalk between heart and WAT.

The BCL-2 family plays important roles in acute myeloid leukemia (AML). Venetoclax, a selective BCL-2 inhibitor, has received FDA approval for the treatment of AML. However, drug resistance ensues after prolonged treatment, highlighting the need for a greater understanding of the underlying mechanisms. Using a genome-wide CRISPR/Cas9 screen in human AML, we identified genes whose inactivation sensitizes AML blasts to Venetoclax. Genes involved in mitochondrial organization and function were significantly depleted throughout our screen, including the mitochondrial chaperonin CLPB. We demonstrated that CLPB is upregulated in human AML, it is further induced upon acquisition of Venetoclax resistance and its ablation sensitizes AML to Venetoclax. Mechanistically, CLPB maintains the mitochondrial cristae structure via its interaction with the cristae-shaping protein OPA1, whereas its loss promotes apoptosis by inducing cristae remodeling and mitochondrial stress responses. Overall, our data suggest that targeting mitochondrial architecture may provide a promising approach to circumvent Venetoclax resistance.

The bone marrow microenvironment has a key role in regulating haematopoiesis, but its molecular complexity and response to stress are incompletely understood. Here we map the transcriptional landscape of mouse bone marrow vascular, perivascular and osteoblast cell populations at single-cell resolution, both at homeostasis and under conditions of stress-induced haematopoiesis. This analysis revealed previously unappreciated levels of cellular heterogeneity within the bone marrow niche and resolved cellular sources of pro-haematopoietic growth factors, chemokines and membrane-bound ligands. Our studies demonstrate a considerable transcriptional remodelling of niche elements under stress conditions, including an adipocytic skewing of perivascular cells. Among the stress-induced changes, we observed that vascular Notch delta-like ligands (encoded by Dll1 and Dll4) were downregulated. In the absence of vascular Dll4, haematopoietic stem cells prematurely induced a myeloid transcriptional program. These findings refine our understanding of the cellular architecture of the bone marrow niche, reveal a dynamic and heterogeneous molecular landscape that is highly sensitive to stress and illustrate the utility of single-cell transcriptomic data in evaluating the regulation of haematopoiesis by discrete niche populations.

RNA-binding proteins (RBPs) are essential modulators of transcription and translation frequently dysregulated in cancer. We systematically interrogated RBP dependencies in human cancers using a comprehensive CRISPR/Cas9 domain-focused screen targeting RNA-binding domains of 490 classical RBPs. This uncovered a network of physically interacting RBPs upregulated in acute myeloid leukemia (AML) and crucial for maintaining RNA splicing and AML survival. Genetic or pharmacologic targeting of one key member of this network, RBM39, repressed cassette exon inclusion and promoted intron retention within mRNAs encoding HOXA9 targets as well as in other RBPs preferentially required in AML. The effects of RBM39 loss on splicing further resulted in preferential lethality of spliceosomal mutant AML, providing a strategy for treatment of AML bearing RBP splicing mutations.

B cell development is a highly regulated process that requires stepwise rearrangement of immunoglobulin genes to generate a functional B cell receptor (BCR). The polycomb group protein BMI1 is required for B cell development, but its function in developing B cells remains poorly defined. We demonstrate that BMI1 functions in a cell-autonomous manner at two stages during early B cell development. First, loss of BMI1 results in a differentiation block at the pro-B cell to pre-B cell transition due to the inability of BMI1-deficient cells to transcribe newly rearranged Igh genes. Accordingly, introduction of a pre-rearranged Igh allele partially restored B cell development in Bmi1^-/- mice. In addition, BMI1 is required to prevent premature p53 signaling, and as a consequence, Bmi1^-/- large pre-B cells fail to properly proliferate. Altogether, our results clarify the role of BMI1 in early B cell development and uncover an unexpected function of BMI1 during VDJ recombination.

TET2 somatic mutations occur in ~10% of DLBCLs but are of unknown significance. Herein we show that TET2 is required for the humoral immune response and is a DLBCL tumor suppressor. TET2 loss of function disrupts transit of B-cells through germinal centers (GC), causing GC hyperplasia, impaired class switch recombination, blockade of plasma cell differentiation and a pre-neoplastic phenotype. TET2 loss was linked to focal loss of enhancer hydroxymethylation and transcriptional repression of genes that mediate GC exit such as PRDM1. Notably, these enhancers and genes are also repressed in CREBBP-mutant DLBCLs. Accordingly, TET2 mutation in patients yields a CREBBP-mutant gene expression signature, CREBBP and TET2 mutations are generally mutually exclusive, and hydroxymethylation loss caused by TET2 deficiency impairs enhancer H3K27 acetylation. Hence TET2 plays a critical role in the GC reaction and its loss of function results in lymphomagenesis through failure to activate genes linked to GC exit signals.

T cell acute lymphoblastic leukemia (T-ALL) is commonly associated with activating mutations in the NOTCH1 pathway. Recent reports have shown a link between NOTCH1 signaling and intracellular Ca²⁺ homeostasis in T-ALL. Here, we investigate the role of store-operated Ca²⁺ entry (SOCE) mediated by the Ca²⁺ channel ORAI1 and its activators STIM1 and STIM2 in T-ALL. Deletion of STIM1 and STIM2 in leukemic cells abolishes SOCE and significantly prolongs the survival of mice in a NOTCH1-dependent model of T-ALL. The survival advantage is unrelated to the leukemic cell burden but is associated with the SOCE-dependent ability of malignant T lymphoblasts to cause inflammation in leukemia-infiltrated organs. Mice with STIM1/STIM2-deficient T-ALL show a markedly reduced necroinflammatory response in leukemia-infiltrated organs and downregulation of signaling pathways previously linked to cancer-induced inflammation. Our study shows that leukemic T lymphoblasts cause inflammation of leukemia-infiltrated organs that is dependent on SOCE.