Johann Meyer

OCT4-INDUCED PLURIPOTENCY IN ADULT NEURAL STEM CELLS

The four transcription factors Oct4, Sox2, Klf4, and c-Myc can induce pluripotency in mouse and human fibroblasts. We previously described direct reprogramming of adult mouse neural stem cells (NSCs) by Oct4 and either Klf4 or c-Myc. NSCs endogenously express Sox2, c-Myc, and Klf4 as well as several intermediate reprogramming markers. Here we report that exogenous expression of the germline-specific transcription factor Oct4 is sufficient to generate pluripotent stem cells from adult mouse NSCs. These one-factor induced pluripotent stem cells (1F iPS) are similar to embryonic stem cells in vitro and in vivo. Not only can these cells can be efficiently differentiated into NSCs, cardiomyocytes, and germ cells in vitro, but they are also capable of teratoma formation and germline transmission in vivo. Our results demonstrate that Oct4 is required and sufficient to directly reprogram NSCs to pluripotency.

Direct reprogramming of human neural stem cells by OCT4

Induced pluripotent stem (iPS) cells have been generated from mouse and human somatic cells by ectopic expression of four transcription factors (OCT4 (also called POU5F1), SOX2, c-Myc and KLF4). We previously reported that Oct4 alone is sufficient to reprogram directly adult mouse neural stem cells to iPS cells. Here we report the generation of one-factor human iPS cells from human fetal neural stem cells (one-factor (1F) human NiPS cells) by ectopic expression of OCT4 alone. One-factor human NiPS cells resemble human embryonic stem cells in global gene expression profiles, epigenetic status, as well as pluripotency in vitro and in vivo. These findings demonstrate that the transcription factor OCT4 is sufficient to reprogram human neural stem cells to pluripotency. One-factor iPS cell generation will advance the field further towards understanding reprogramming and generating patient-specific pluripotent stem cells.

Generation of Induced Pluripotent Stem Cells from Human Cord Blood

Induced pluripotent stem cells (iPSCs) may represent an ideal cell source for future regenerative therapies. A critical issue concerning the clinical use of patient-specific iPSCs is the accumulation of mutations in somatic (stem) cells over an organisms lifetime. Acquired somatic mutations are passed onto iPSCs during reprogramming and may be associated with loss of cellular functions and cancer formation. Here we report the generation of human iPSCs from cord blood (CB) as a juvenescent cell source. CBiPSCs show characteristics typical of embryonic stem cells and can be differentiated into derivatives of all three germ layers, including functional cardiomyocytes. For future therapeutic production of autologous and allogeneic iPSC derivatives, CB could be routinely harvested for public and commercial CB banks without any donor risk. CB could readily become available for pediatric patients and, in particular, for newborns with genetic diseases or congenital malformations.

Physiological Promoters Reduce the Genotoxic Risk of Integrating Gene Vectors

The possible activation of cellular proto-oncogenes as a result of clonal transformation is a potential limitation in a therapeutic approach involving random integration of gene vectors. Given that enhancer promiscuity represents an important mechanism of insertional transformation, we assessed the enhancer activities of various cellular and retroviral promoters in transient transfection assays, and also in a novel experimental system designed to measure the activation of a minigene cassette contained in stably integrating retroviral vectors. Retroviral enhancer-promoters showed a significantly greater potential to activate neighboring promoters than did cellular promoters derived from human genes, elongation factor-1alpha (EF1alpha) and phosphoglycerate kinase (PGK). Self-inactivating (SIN) vector design reduced but did not abolish enhancer interactions. Using a recently established cell culture assay that detects insertional transformation by serial replating of primary hematopoietic cells, we found that SIN vectors containing the EF1alpha promoter greatly decrease the risk of insertional transformation. Despite integration of multiple copies per cell, activation of the crucial proto-oncogene Evi1 was not detectable when using SIN-EF1alpha vectors. On the basis of several quantitative indicators, the decrease in transforming activity was highly significant (more than tenfold, P < 0.01) when compared with similarly designed vectors containing a retroviral enhancer-promoter with or without a well-characterized genetic insulator core element. In this manner, the insertional biosafety of therapeutic gene vectors can be greatly enhanced and proactively evaluated in sensitive cell-based assays.

Mutant IDH1 promotes leukemogenesis in vivo and can be specifically targeted in human AML

Mutations in the metabolic enzymes isocitrate dehydrogenase 1 (IDH1) and 2 (IDH2) are frequently found in glioma, acute myeloid leukemia (AML), melanoma, thyroid cancer, and chondrosarcoma patients. Mutant IDH produces 2-hydroxyglutarate (2HG), which induces histone- and DNA-hypermethylation through inhibition of epigenetic regulators. We investigated the role of mutant IDH1 using the mouse transplantation assay. Mutant IDH1 alone did not transform hematopoietic cells during 5 months of observation. However, mutant IDH1 greatly accelerated onset of myeloproliferative disease-like myeloid leukemia in mice in cooperation with HoxA9 with a mean latency of 83 days compared with cells expressing HoxA9 and wild-type IDH1 or a control vector (167 and 210 days, respectively, P = .001). Mutant IDH1 accelerated cell-cycle transition through repression of cyclin-dependent kinase inhibitors Cdkn2a and Cdkn2b, and activated mitogen-activated protein kinase signaling. By computational screening, we identified an inhibitor of mutant IDH1, which inhibited mutant IDH1 cells and lowered 2HG levels in vitro, and efficiently blocked colony formation of AML cells from IDH1-mutated patients but not of normal CD34(+) bone marrow cells. These data demonstrate that mutant IDH1 has oncogenic activity in vivo and suggest that it is a promising therapeutic target in human AML cells.

Self-inactivating retroviral vectors with improved RNA processing

Three RNA features have been identified that elevate retroviral transgene expression: an intron in the 5′ untranslated region (5′UTR), the absence of aberrant translational start codons and the presence of the post-transcriptional regulatory element (PRE) of the woodchuck hepatitis virus in the 3′UTR. To include such elements into self-inactivating (SIN) vectors with potentially improved safety, we excised the strong retroviral promoter from the U3 region of the 3′ long terminal repeat (LTR) and inserted it either downstream or upstream of the retroviral RNA packaging signal (Ψ). The latter concept is new and allows the use of an intron in the 5′UTR, taking advantage of retroviral splice sites surrounding Ψ. Three LTR and four SIN vectors were compared to address the impact of RNA elements on titer, splice regulation and transgene expression. Although titers of SIN vectors were about 20-fold lower than those of their LTR counterparts, inclusion of the PRE allowed production of more than 106 infectious units per ml without further vector optimizations. In comparison with state-of-the-art LTR vectors, the intron-containing SIN vectors showed greatly improved splicing. With regard to transgene expression, the intron-containing SIN vectors largely matched or even exceeded the LTR counterparts in all cell types investigated (embryonic carcinoma cells, fibroblasts, primary T cells and hematopoietic progenitor cells).

Induction of pluripotency in human cord blood unrestricted somatic stem cells

OBJECTIVE Generation of induced pluripotent stem (iPS) cells from human cord blood (CB)-derived unrestricted somatic stem cells and evaluation of their molecular signature and differentiation potential in comparison to human embryonic stem cells. MATERIALS AND METHODS Unrestricted somatic stem cells isolated from human CB were reprogrammed to iPS cells using retroviral expression of the transcription factors OCT4, SOX2, KLF4, and C-MYC. The reprogrammed cells were analyzed morphologically, by quantitative reverse transcription polymerase chain reaction, genome-wide microRNA and methylation profiling, and gene expression microarrays, as well as in their pluripotency potential by in vivo teratoma formation in severe combined immunodeficient mice and in vitro differentiation. RESULTS CB iPS cells are very similar to human embryonic stem cells morphologically, at their molecular signature, and in their differentiation potential. CONCLUSIONS Human CB-derived unrestricted somatic stem cells offer an attractive source of cells for generation of iPS cells. Our findings open novel perspectives to generate human leukocyte antigen-matched pluripotent stem cell banks based on existing CB banks. Besides the obvious relevance of a second-generation CB iPS cell bank for pharmacological and toxicological testing, its application for autologous or allogenic regenerative cell transplantation appears feasible.

Efficient expansion and gene transduction of mouse neural stem/progenitor cells on recombinant fibronectin

Neural stem/progenitor cells (NSCs) are commonly grown as floating neurospheres in medium containing basic fibroblast growth factor and epidermal growth factor. Under these conditions, about 1% of the cells retain multipotentiality. We developed a protocol based on culture of NSCs in adherence on recombinant fibronectin (rFN) to transduce up to 90% NSCs at a multiplicity of infection of 2 with no need for viral concentration or production of serum-free retroviral supernatants. NSCs grew faster on rFN than as neurospheres on tissue culture plastic and did not lose their stem cell nature or multipotentiality. Furthermore, retroviral-mediated transgene expression was sustained with time in culture and upon differentiation into neurons and astrocytes. These experimental conditions may be utilized to study the function of various genes in NSCs, and to manipulate NSCs for gene and cell therapy of several neurological diseases.

High-affinity neurotrophin receptors and ligands promote leukemogenesis.

Neurotrophins (NTs) and their receptors play a key role in neurogenesis and survival. The TRK (tropomyosin-related kinase) receptor protein tyrosine kinases (TRKA, TRKB, TRKC) are high-affinity NT receptors that are expressed in a variety of human tissues. Their role in normal and malignant hematopoiesis is poorly understood. In a prospective study involving 94 adult patients we demonstrate for the first time cell-surface expression of the 3 TRKs and constitutive activation in blasts from patients with de novo or secondary acute leukemia. At least one TRK was expressed in 55% of the analyzed cases. We establish a clear correlation between the TRK expression pattern and FAB classification. Although only few point mutations were found in TRK sequences by reverse-transcriptase-polymerase chain reaction (RT-PCR), we observed coexpression of BDNF (ligand for TRKB) in more than 50% of TRKB(+) cases (16/30). Activation of TRKA or TRKB by NGF and BDNF, respectively, efficiently rescued murine myeloid cells from irradiation-induced apoptosis. Coexpression of TRKB/BDNF or TRKA/NGF in murine hematopoietic cells induced leukemia. Moreover, activation of TRKs was important for survival of both human and murine leukemic cells. Our findings suggest that TRKs play an important role in leukemogenesis and may serve as a new drug target.

Efficient Designer Nuclease-Based Homologous Recombination Enables Direct PCR Screening for Footprintless Targeted Human Pluripotent Stem Cells

Summary Genetic engineering of human induced pluripotent stem cells (hiPSCs) via customized designer nucleases has been shown to be significantly more efficient than conventional gene targeting, but still typically depends on the introduction of additional genetic selection elements. In our study, we demonstrate the efficient nonviral and selection-independent gene targeting in human pluripotent stem cells (hPSCs). Our high efficiencies of up to 1.6% of gene-targeted hiPSCs, accompanied by a low background of randomly inserted transgenes, eliminated the need for antibiotic or fluorescence-activated cell sorting selection, and allowed the use of short donor oligonucleotides for footprintless gene editing. Gene-targeted hiPSC clones were established simply by direct PCR screening. This optimized approach allows targeted transgene integration into safe harbor sites for more predictable and robust expression and enables the straightforward generation of disease-corrected, patient-derived iPSC lines for research purposes and, ultimately, for future clinical applications.