Hematopoietic Stem Cells Are the Major Source of Multilineage Hematopoiesis in Adult Animals.

Hematopoietic stem cells (HSCs) sustain long-term reconstitution of hematopoiesis in transplantation recipients, yet their role in the endogenous steady-state hematopoiesis remains unclear. In particular, recent studies suggested that HSCs provide a relatively minor contribution to immune cell development in adults. We directed transgene expression in a fraction of HSCs that maintained reconstituting activity during serial transplantations. Inducible genetic labeling showed that transgene-expressing HSCs gave rise to other phenotypic HSCs, confirming their top position in the differentiation hierarchy. The labeled HSCs rapidly contributed to committed progenitors of all lineages and to mature myeloid cells and lymphocytes, but not to B-1a cells or tissue macrophages. Importantly, labeled HSCs gave rise to more than two-thirds of all myeloid cells and platelets in adult mice, and this contribution could be accelerated by an induced interferon response. Thus, classically defined HSCs maintain immune cell development in the steady state and during systemic cytokine responses.

Barcoding reveals complex clonal dynamics of de novo transformed human mammary cells.

Most human breast cancers have diversified genomically and biologically by the time they become clinically evident. Early events involved in their genesis and the cellular context in which these events occur have thus been difficult to characterize. Here we present the first formal evidence of the shared and independent ability of basal cells and luminal progenitors, isolated from normal human mammary tissue and transduced with a single oncogene (KRAS(G12D)), to produce serially transplantable, polyclonal, invasive ductal carcinomas within 8 weeks of being introduced either subrenally or subcutaneously into immunodeficient mice. DNA barcoding of the initial cells revealed a dramatic change in the numbers and sizes of clones generated from them within 2 weeks, and the first appearance of many 'new' clones in tumours passaged into secondary recipients. Both primary and secondary tumours were phenotypically heterogeneous and primary tumours were categorized transcriptionally as 'normal-like'. This system challenges previous concepts that carcinogenesis in normal human epithelia is necessarily a slow process requiring the acquisition of multiple driver mutations. It also presents the first description of initial events that accompany the genesis and evolution of malignant human mammary cell populations, thereby contributing new understanding of the rapidity with which heterogeneity in their properties can develop.

Disruption of IKAROS activity in primitive chronic-phase CML cells mimics myeloid disease progression.

Without effective therapy, chronic-phase chronic myeloid leukemia (CP-CML) evolves into an acute leukemia (blast crisis [BC]) that displays either myeloid or B-lymphoid characteristics. This transition is often preceded by a clinically recognized, but biologically poorly characterized, accelerated phase (AP). Here, we report that IKAROS protein is absent or reduced in bone marrow blasts from most CML patients with advanced myeloid disease (AP or BC). This contrasts with primitive CP-CML cells and BCR-ABL1-negative acute myeloid leukemia blasts, which express readily detectable IKAROS. To investigate whether loss of IKAROS contributes to myeloid disease progression in CP-CML, we examined the effects of forced expression of a dominant-negative isoform of IKAROS (IK6) in CP-CML patients' CD34(+) cells. We confirmed that IK6 disrupts IKAROS activity in transduced CP-CML cells and showed that it confers on them features of AP-CML, including a prolonged increased output in vitro and in xenografted mice of primitive cells with an enhanced ability to differentiate into basophils. Expression of IK6 in CD34(+) CP-CML cells also led to activation of signal transducer and activator of transcription 5 and transcriptional repression of its negative regulators. These findings implicate loss of IKAROS as a frequent step and potential diagnostic harbinger of progressive myeloid disease in CML patients.

Hierarchical organization of fetal and adult hematopoietic stem cells.

Mammalian hematopoiesis is a hierarchically organized process in which all types of mature blood cells are continuously generated from more primitive cells that lack any morphological evidence of differentiation. However, it is now accepted that this morphologically homogeneous precursor population consists of multiple distinct subsets of cells. The most primitive of these are defined by their ability to produce similarly undifferentiated progeny through many cell divisions, in addition to generating cells with activated differentiation programs. The term hematopoietic stem cell (HSC) is now conventionally restricted to cells with this long-term self-sustaining ability. Nevertheless, clonal tracking studies have revealed significant heterogeneity in the behavior of such stringently defined HSCs. Moreover, superimposed on the heterogeneous behavior that can be elicited from the HSCs present at any given time during development are additional differences that distinguish HSCs at different times both before and after birth. The latter include changes in the representation of HSCs that display specific differentiation programs, as well as changes in their turnover and self-renewal control. Here, we summarize recent studies characterizing these developmental changes, some of the mechanisms that control them, and their potential relevance to understanding age-associated differences in leukemia as well as normal hematopoiesis.

Placenta-specific expression of the interleukin-2 (IL-2) receptor ß subunit from an endogenous retroviral promoter.

The long terminal repeat (LTR) sequences of endogenous retroviruses and retroelements contain promoter elements and are known to form chimeric transcripts with nearby cellular genes. Here we show that an LTR of the THE1D retroelement family has been domesticated as an alternative promoter of human IL2RB, the gene encoding the ß subunit of the IL-2 receptor. The LTR promoter confers expression specifically in the placental trophoblast as opposed to its native transcription in the hematopoietic system. Rather than sequence-specific determinants, DNA methylation was found to regulate transcription initiation and splicing efficiency in a tissue-specific manner. Furthermore, we detected the cytoplasmic signaling domain of the IL-2Rß protein in the placenta, suggesting that IL-2Rß undergoes preferential proteolytic cleavage in this tissue. These findings implicate novel functions for this cytokine receptor subunit in the villous trophoblast and reveal an intriguing example of ancient LTR exaptation to drive tissue-specific gene expression.

Dominant-negative IKAROS enhances IL-3-stimulated signaling in wild-type but not BCR-ABL1(+) mouse BA/F3 cells.

Inactivating mutations in IKZF1, the gene that encodes the transcription factor IKAROS, are recurrent in poor-prognosis human B-cell leukemias, in which these mutations co-exist with BCR-ABL1 or other genetic changes that activate similar intracellular signaling pathways. However, little is known about the mechanism(s) by which loss of IKAROS activity may co-operate with BCR-ABL1 to transform lymphoid cells. To investigate this question, we used expression of a dominant-negative isoform of IKAROS (IK6) to suppress endogenous IKAROS activity in the interleukin-3 (IL-3)-dependent mouse pro-B BA/F3 cell line and in an IL-3-independent BCR-ABL1(+) derivative. We then used intracellular phospho-flow cytometry to assess the effects of BCR-ABL1 and IK6, alone and in combination, on the signaling state of the cells before and after their stimulation with IL-3. BCR-ABL1 and IK6 each produced a constitutively activated signaling phenotype and also enhanced the signaling responses of BA/F3 cells to IL-3. These effects, however, were neither equivalent nor additive, and IK6 alone was insufficient to confer the IL-3-independent growth characteristic of BCR-ABL1(+) BA/F3 cells. In addition to its effects on lymphoid cells, IK6 also induced constitutively activated signaling in a subset of myeloid leukemia cell lines. Together, these studies indicate an ability of IK6 to enhance intracellular signaling in both lymphoid and myeloid cells, but not to synergize with BCR-ABL1 in this model system.

Inhibition of the Mitochondrial Protease ClpP as a Therapeutic Strategy for Human Acute Myeloid Leukemia.

From an shRNA screen, we identified ClpP as a member of the mitochondrial proteome whose knockdown reduced the viability of K562 leukemic cells. Expression of this mitochondrial protease that has structural similarity to the cytoplasmic proteosome is increased in leukemic cells from approximately half of all patients with AML. Genetic or chemical inhibition of ClpP killed cells from both human AML cell lines and primary samples in which the cells showed elevated ClpP expression but did not affect their normal counterparts. Importantly, Clpp knockout mice were viable with normal hematopoiesis. Mechanistically, we found that ClpP interacts with mitochondrial respiratory chain proteins and metabolic enzymes, and knockdown of ClpP in leukemic cells inhibited oxidative phosphorylation and mitochondrial metabolism.

A dominant-negative isoform of IKAROS expands primitive normal human hematopoietic cells.

Disrupted IKAROS activity is a recurrent feature of some human leukemias, but effects on normal human hematopoietic cells are largely unknown. Here, we used lentivirally mediated expression of a dominant-negative isoform of IKAROS (IK6) to block normal IKAROS activity in primitive human cord blood cells and their progeny. This produced a marked (10-fold) increase in serially transplantable multipotent IK6(+) cells as well as increased outputs of normally differentiating B cells and granulocytes in transplanted immunodeficient mice, without producing leukemia. Accompanying T/natural killer (NK) cell outputs were unaltered, and erythroid and platelet production was reduced. Mechanistically, IK6 specifically increased human granulopoietic progenitor sensitivity to two growth factors and activated CREB and its targets (c-FOS and Cyclin B1). In more primitive human cells, IK6 prematurely initiated a B cell transcriptional program without affecting the hematopoietic stem cell-associated gene expression profile. Some of these effects were species specific, thus identifying novel roles of IKAROS in regulating normal human hematopoietic cells.

Distinct stromal cell factor combinations can separately control hematopoietic stem cell survival, proliferation, and self-renewal.

Hematopoietic stem cells (HSCs) are identified by their ability to sustain prolonged blood cell production in vivo, although recent evidence suggests that durable self-renewal (DSR) is shared by HSC subtypes with distinct self-perpetuating differentiation programs. Net expansions of DSR-HSCs occur in vivo, but molecularly defined conditions that support similar responses in vitro are lacking. We hypothesized that this might require a combination of factors that differentially promote HSC viability, proliferation, and self-renewal. We now demonstrate that HSC survival and maintenance of DSR potential are variably supported by different Steel factor (SF)-containing cocktails with similar HSC-mitogenic activities. In addition, stromal cells produce other factors, including nerve growth factor and collagen 1, that can antagonize the apoptosis of initially quiescent adult HSCs and, in combination with SF and interleukin-11, produce >15-fold net expansions of DSR-HSCs ex vivo within 7 days. These findings point to the molecular basis of HSC control and expansion.

The Lin28b-let-7-Hmga2 axis determines the higher self-renewal potential of fetal haematopoietic stem cells.

Mouse haematopoietic stem cells (HSCs) undergo a postnatal transition in several properties, including a marked reduction in their self-renewal activity. We now show that the developmentally timed change in this key function of HSCs is associated with their decreased expression of Lin28b and an accompanying increase in their let-7 microRNA levels. Lentivirus-mediated overexpression of Lin28 in adult HSCs elevates their self-renewal activity in transplanted irradiated hosts, as does overexpression of Hmga2, a well-established let-7 target that is upregulated in fetal HSCs. Conversely, HSCs from fetal Hmga2(-/-) mice do not exhibit the heightened self-renewal activity that is characteristic of wild-type fetal HSCs. Interestingly, overexpression of Hmga2 in adult HSCs does not mimic the ability of elevated Lin28 to activate a fetal lymphoid differentiation program. Thus, Lin28b may act as a master regulator of developmentally timed changes in HSC programs with Hmga2 serving as its specific downstream modulator of HSC self-renewal potential.

Impact of the trade-related aspects of intellectual property rights (TRIPS) agreement on India as a supplier of generic antiretrovirals.

This is a commentary on how the trade-related aspects of intellectual property rights (TRIPS) agreement has impacted India as a supplier of generic antiretrovirals (ARVs). We provide a systematic review of the issues related to the TRIPS agreement that affects India. This includes discussion around (a) the legal landscape underpinning India as a supplier of generic ARVs; (b) supply of second-line ARVs; and (c) the future of generic drug production in India. The proclamation into force of TRIPS-compliant intellectual property law in India is likely to affect its position as a supplier of affordable ARVs, especially drugs brought to market after 2005. Currently, mechanisms exist for the generic production of almost all ARVs in India, including second-line drugs; however, the manufacture of these drugs by generic pharmaceutical companies may require additional market incentives. Compulsory licensing may emerge as an additional mechanism by which India can provide affordable versions of patented drugs to Least Developed Countries (LDCs).