Kristina Brumme

Overcoming reprogramming resistance of Fanconi anemia cells

Fanconi anemia (FA) is a recessive syndrome characterized by progressive fatal BM failure and chromosomal instability. FA cells have inactivating mutations in a signaling pathway that is critical for maintaining genomic integrity and protecting cells from the DNA damage caused by cross-linking agents. Transgenic expression of the implicated genes corrects the phenotype of hematopoietic cells, but previous attempts at gene therapy have failed largely because of inadequate numbers of hematopoietic stem cells available for gene correction. Induced pluripotent stem cells (iPSCs) constitute an alternate source of autologous cells that are amenable to ex vivo expansion, genetic correction, and molecular characterization. In the present study, we demonstrate that reprogramming leads to activation of the FA pathway, increased DNA double-strand breaks, and senescence. We also demonstrate that defects in the FA DNA-repair pathway decrease the reprogramming efficiency of murine and human primary cells. FA pathway complementation reduces senescence and restores the reprogramming efficiency of somatic FA cells to normal levels. Disease-specific iPSCs derived in this fashion maintain a normal karyotype and are capable of hematopoietic differentiation. These data define the role of the FA pathway in reprogramming and provide a strategy for future translational applications of patient-specific FA iPSCs.

Distinct effects of concomitant Jak2V617F expression and Tet2 loss in mice promote disease progression in myeloproliferative neoplasms

Signaling mutations (eg, JAK2V617F) and mutations in genes involved in epigenetic regulation (eg, TET2) are the most common cooccurring classes of mutations in myeloproliferative neoplasms (MPNs). Clinical correlative studies have demonstrated that TET2 mutations are enriched in more advanced phases of MPNs such as myelofibrosis and leukemic transformation, suggesting that they may cooperate with JAK2V617F to promote disease progression. To dissect the effects of concomitant Jak2V617F expression and Tet2 loss within distinct hematopoietic compartments in vivo, we generated Jak2V617F/Tet2 compound mutant genetic mice. We found that the combination of Jak2V617F expression and Tet2 loss resulted in a more florid MPN phenotype than that seen with either allele alone. Concordant with this, we found that Tet2 deletion conferred a strong functional competitive advantage to Jak2V617F-mutant hematopoietic stem cells (HSCs). Transcriptional profiling revealed that both Jak2V617F expression and Tet2 loss were associated with distinct and nonoverlapping gene expression signatures within the HSC compartment. In aggregate, our findings indicate that Tet2 loss drives clonal dominance in HSCs, and Jak2V617F expression causes expansion of downstream precursor cell populations, resulting in disease progression through combinatorial effects. This work provides insight into the functional consequences of JAK2V617F-TET2 comutation in MPNs, particularly as it pertains to HSCs.

Guanine nucleotide exchange factor Vav1 regulates perivascular homing and bone marrow retention of hematopoietic stem and progenitor cells

Engraftment and maintenance of hematopoietic stem and progenitor cells (HSPC) depend on their ability to respond to extracellular signals from the bone marrow microenvironment, but the critical intracellular pathways integrating these signals remain poorly understood. Furthermore, recent studies provide contradictory evidence of the roles of vascular versus osteoblastic niche components in HSPC function. To address these questions and to dissect the complex upstream regulation of Rac GTPase activity in HSPC, we investigated the role of the hematopoietic-specific guanine nucleotide exchange factor Vav1 in HSPC localization and engraftment. Using intravital microscopy assays, we demonstrated that transplanted Vav1−/− HSPC showed impaired early localization near nestin+ perivascular mesenchymal stem cells; only 6.25% of Vav1−/− HSPC versus 45.8% of wild-type HSPC were located less than 30 μm from a nestin+ cell. Abnormal perivascular localization correlated with decreased retention of Vav1−/− HSPC in the bone marrow (44–60% reduction at 48 h posttransplant, compared with wild-type) and a very significant defect in short- and long-term engraftment in competitive and noncompetitive repopulation assays (<1.5% chimerism of Vav1−/− cells vs. 53–63% for wild-type cells). The engraftment defect of Vav1−/− HSPC was not related to alterations in proliferation, survival, or integrin-mediated adhesion. However, Vav1−/− HSPC showed impaired responses to SDF1α, including reduced in vitro migration in time-lapse microscopy assays, decreased circadian and pharmacologically induced mobilization in vivo, and dysregulated Rac/Cdc42 activation. These data suggest that Vav1 activity is required specifically for SDF1α-dependent perivascular homing of HSPC and suggest a critical role for this localization in retention and subsequent engraftment.

A functional role for the histone demethylase UTX in normal and malignant hematopoietic cells

Ubiquitously transcribed tetratricopeptide repeat, X chromosome (UTX), an H3K27Me2/3 demethylase, has been implicated in development, self-renewal, and differentiation of various organs and embryonic stem cells through chromatin modifications and transcriptional regulation of important developmentally related genes, such as Hox genes. However, the function of UTX in hematopoiesis is not well understood. To study the role of UTX in the mammalian hematopoietic system, we used lentiviral short hairpin RNA constructs to knockdown UTX in the murine hematopoietic progenitor cell line EML, in primary murine bone marrow cells and in leukemic cell lines. We report that Utx is highly expressed in the hematopoietic compartment and that it plays an important role in cell proliferation and homeostasis of hematopoietic cells in vitro. Knockdown of UTX in EML and primary murine bone marrow cells impairs their colony-forming ability. Moreover, knockdown of UTX affects expression of key genes that regulate hematopoietic differentiation such as Mll1, Runx1, and Scl in primary murine bone marrow cells. And we further demonstrate that UTX directly associates with the promoters of the Mll1, Runx1, and Scl genes and modulate their transcription by controlling H3K27me3 marks on respective promoter regions. In addition, UTX depletion severely impaired proliferation of several human leukemia cell lines. Together, these data demonstrate a functional role for UTX in normal and malignant hematopoiesis.

Myeloproliferative Neoplasm Animal Models

Myeloproliferative neoplasm (MPN) animal models accurately re-capitulate human disease in mice and have been an important tool for the study of MPN biology and therapy. Transplantation of BCR-ABL transduced bone marrow into irradiated syngeneic mice established the field of MPN animal modeling. Genetically engineered MPN animal models have enabled detailed characterization of the effects of specific MPN-associated genetic abnormalities on hematopoietic stem and progenitor cells (HSPCs). Xenograft models have allowed the study of primary human MPN-propagating cells in vivo. JAK2V617F, the most common molecular abnormality in BCR-ABL negative MPN, has been extensively studied using retroviral, transgenic, knock-in and xenograft models.

Cdx4 is dispensable for murine adult hematopoietic stem cells but promotes MLL-AF9-mediated leukemogenesis

Background Cdx4 is a homeobox gene essential for normal blood formation during embryonic development in the zebrafish, through activation of posterior Hox genes. However, its role in adult mammalian hematopoiesis has not been extensively studied and its requirement in leukemia associated with Hox gene expression alteration is unclear. Design and Methods We inactivated Cdx4 in mice through either a germline or conditional knockout approach and analyzed requirement for Cdx4 in both normal adult hematopoiesis and leukemogenesis initiated by the MLL-AF9 fusion oncogene. Results Here, we report that loss of Cdx4 had a minimal effect on adult hematopoiesis. Indeed, although an increase in white blood cell counts was observed, no significant differences in the distribution of mature blood cells, progenitors or stem cells were observed in Cdx4-deficient animals. In addition, long-term repopulating activity in competitive transplantation assays was not significantly altered. In vitro, B-cell progenitor clonogenic potential was reduced in Cdx4-deficient animals but no significant alteration of mature B cells was detected in vivo. Finally, induction of acute myeloid leukemia in mice by MLL-AF9 was significantly delayed in the absence of Cdx4 in a retroviral transduction/bone marrow transplant model. Conclusions These observations indicate that Cdx4 is dispensable for the establishment and maintenance of normal hematopoiesis in adult mammals. These results, therefore, outline substantial differences in the Cdx-Hox axis between mammals and zebrafish and support the hypothesis that Cdx factors are functionally redundant during mammalian hematopoietic development under homeostatic conditions. In addition, our results suggest that Cdx4 participates in MLL-AF9-mediated leukemogenesis supporting a role for Cdx factors in the pathogenesis of myeloid leukemia.