Human erythroid colony formation in vitro: evidence for clonal origin

The JAK2 mutation

Myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell (HSC) disorders with overproduction of mature myeloid blood cells, including essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF). In 2005, several groups identified a single gain-of-function point mutation JAK2V617F in the majority of MPN patients. The JAK2V617F mutation confers cytokine independent proliferation to hematopoietic progenitor cells by constitutively activating canonical and non-canonical downstream pathways. In this chapter, we focus on (1) the regulation of JAK2, (2) the molecular mechanisms used by JAK2V617F to induce MPNs, (3) the factors that are involved in the phenotypic diversity in MPNs, and (4) the effects of JAK2V617F on hematopoietic stem cells (HSCs). The discovery of the JAK2V617F mutation led to a comprehensive understanding of MPN; however, the question still remains about how one mutation can give rise to three distinct disease entities. Various mechanisms have been proposed, including JAK2V617F allele burden, differential STAT signaling, and host genetic modifiers. In vivo modeling of JAK2V617F has dramatically enhanced the understanding of the pathophysiology of the disease and provided the pre-clinical platform. Interestingly, most of these models do not show an increased hematopoietic stem cell self-renewal and function compared to wildtype controls, raising the question of whether JAK2V617F alone is sufficient to give a clonal advantage in MPN patients. In addition, the advent of modern sequencing technologies has led to a broader understanding of the mutational landscape and detailed JAK2V617F clonal architecture in MPN patients.

Pleiotropic effects of prostaglandin E2 in hematopoiesis; prostaglandin E2 and other eicosanoids regulate hematopoietic stem and progenitor cell function

Eicosanoids have been implicated in the physiological regulation of hematopoiesis with pleiotropic effects on hematopoietic stem cells and various classes of lineage restricted progenitor cells. Herein we review the effects of eicosanoids on hematopoiesis, focusing on new findings implicating prostaglandin E(2) in enhancing hematopoietic stem cell engraftment by enhancing stem cell homing, survival and self-renewal. We also describe a role for cannabinoids in hematopoiesis. Lastly, we discuss the yin and yang of various eicosanoids in modulating hematopoietic stem and progenitor cell functions and summarize potential strategies to take advantage of these effects for therapeutic benefit for hematopoietic stem cell transplantation.

Study of two tyrosine kinase inhibitors on growth and signal transduction in polycythemia vera

OBJECTIVE

An activating somatic mutation of Janus kinase 2 V617F (JAK2V617F) is present in most polycythemia vera (PV) patients. We studied efficacy of two potent tyrosine kinase inhibitors (TKI), AEE788 and AMN107, in vitro on cells bearing this mutation.

MATERIALS AND METHODS

We employed reporter cells expressing wild-type JAK2 and mutant JAK2V617F, human erythroleukemic cells (HEL) carrying JAK2V617F)to study the efficacy of these TKIs by cell proliferation assay, Annexin-V/propidium iodide staining, and on relevant cell-signaling and apoptotic events. These data were compared to ex vivo expanded native human erythroid PV progenitor cells grown in liquid cultures.

RESULTS

AEE788 showed a time- and dose-dependent growth inhibitory effect that was greater in FDCP cells expressing JAK2V617F and HEL cells than in cells expressing wild-type JAK2. AEE788 caused dephosphorylation of Akt(S243) and signal transducer and activator of transcription 5(Y694) proteins, increase in Annexin-V binding and caspase-3 cleavage, suggesting induction of apoptosis. We also observed AEE788-mediated decrease in heat shock protein 70 and 90 antiapoptotic proteins. Similarly, native PV erythroid progenitors showed more sensitivity to AEE788 than normal erythroid progenitors. AEE788 also exerted dose-dependent inhibition of PV-specific erythroid colonies. Nilotinib (AMN107) however, lacked specificity and required high (>8 microM) concentrations to inhibit growth of JAK2V617F-carrying cells.

CONCLUSION

Our data suggest that AEE788 exerts its apoptotic activity via downregulation of proliferative and antiapoptotic regulatory proteins. To our knowledge, this is the first report demonstrating the effect of AEE788 on PV erythroid progenitors. Differential effects on PV and normal progenitor cells suggest AEE788 has potential in the treatment of PV and other JAK2V617F positive hematologic malignancies.

The JAK2 V617F allele burden in essential thrombocythemia, polycythemia vera and primary myelofibrosis--impact on disease phenotype

BACKGROUND AND OBJECTIVES

The JAK2 V617F tyrosine kinase mutation is present in the great majority of patients with polycythemia vera (PV), and approximately half of the patients with essential thrombocythemia (ET) and primary myelofibrosis (PMF). The three distinct disease entities may be considered as three phenotypic presentations of the same JAK2 V617F positive chronic myeloproliferative disorder. Together with physiological and genetic modifiers the phenotype may be determined by the JAK2 V617F allele burden. In the present study, we aimed to asses the JAK2 mutational load and its impact on phenotype.

METHODS

A highly sensitive real-time quantitative PCR (qPCR) assay was used for quantification of the JAK2 V617F mutational load in 165 patients with Philadelphia chromosome negative chronic myeloproliferative disorders (ET = 40, PV = 95, PMF = 30).

RESULTS

We provide evidence of increasing JAK2 V617F allele burden from ET, over PV to PMF (P = 0.001 and P < 0.00001 respectively). The present data suggests the JAK2 V617F allele burden as a key determinant of the degree of myeloproliferation and myeloid metaplasia reflected by significantly higher levels of white blood cell counts (WBC) (P = 0.03), CD34 counts (P = 0.03), lactate dehydrogenase and Polycythemia Rubra Vera gene 1 levels (P = 0.03 and P < 0.00001 respectively), as well as lower platelet counts (P = 0.02) and more cases of splenomegaly (P = 0.001) in homozygous PV patients compared to their heterozygous counterparts.

CONCLUSION

The present study support the concept of the JAK2 V617F positive chronic myeloproliferative disorders as a biological continuum with phenotypic presentation in part influenced by JAK2 V617F mutational load.

Polycythemia vera is not initiated by JAK2V617F mutation

OBJECTIVE

The somatic JAK2(V617F) mutation is seen in most polycythemia vera (PV) patients; however, it is not clear if JAK2(V617F) is the PV-initiating mutation.

METHODS

In order to examine this issue, we developed a novel real-time quantitative allele-specific PCR, in which allelic discrimination is enhanced by the synergistic effect of a mismatch in the -1 position, and a locked nucleic acid (LNA) nucleoside at the -2 position.

RESULTS

Determination of allelic frequencies was reproducible (SD = 1.5%) and sensitive--0.1% mutant allele detected in 40 ng of DNA. The JAK2(V617F) frequency in clonal granulocytes from 3 PV females was less than 50% (27.5 +/- 11) and in 7 females greater than 50% (75 +/- 10.5). We also found that wild-type JAK2 BFU-E colonies from PV patients can grow without erythropoietin. The identification of the primary genetic lesion resulting in PV is essential for the development of novel therapeutic strategies.

CONCLUSION

Our studies correlating the frequency of JAK2(V617F) mutant allele and clonality, as well as the presence of homozygous wild-type JAK2 erythropoietin-independent erythroid colonies, provide compelling evidence that the JAK2(V617F) is not the PV-initiating mutation. This supports a model wherein the JAK2(V617F) mutation arises as a secondary genetic event. Furthermore, our results indicate that an undefined molecular lesion, preceding JAK2(V617F), is responsible for clonal hematopoiesis in PV. We conclude that development of therapeutic strategies that target the JAK2(V617F) clonal cells may not be sufficient for eradication of PV.

The JAK2 V617F mutation occurs in hematopoietic stem cells in polycythemia vera and predisposes toward erythroid differentiation

Although a large proportion of patients with polycythemia vera (PV) harbor a valine-to-phenylalanine mutation at amino acid 617 (V617F) in the JAK2 signaling molecule, the stage of hematopoiesis at which the mutation arises is unknown. Here we isolated and characterized hematopoietic stem cells (HSC) and myeloid progenitors from 16 PV patient samples and 14 normal individuals, testing whether the JAK2 mutation could be found at the level of stem or progenitor cells and whether the JAK2 V617F-positive cells had altered differentiation potential. In all PV samples analyzed, there were increased numbers of cells with a HSC phenotype (CD34+CD38-CD90+Lin-) compared with normal samples. Hematopoietic progenitor assays demonstrated that the differentiation potential of PV was already skewed toward the erythroid lineage at the HSC level. The JAK2 V617F mutation was detectable within HSC and their progeny in PV. Moreover, the aberrant erythroid potential of PV HSC was potently inhibited with a JAK2 inhibitor, AG490.

Classification and molecular biology of polycythemias (erythrocytoses) and thrombocytosis

In this article, polycythemic disorders are classified based on the current understanding of biology of erythropoieses and divided into primary and secondary polycythemias. Special emphasis is given to recently uncovered molecular bases of newly described congenital polycythemic disorders. This clarification of the pathophysiology of some of the congenital polycythemic states has obvious utility for more accurate diagnosis and rational prognostic determination. The molecular basis of congenital thrombocytoses is only beginning to be uncovered. In contrast, the molecular bases of polycythemia vera and essential thrombocythemia remain unknown, thus their diagnostic criteria are imprecise and their treatment remains largely empirical. The central premise of this article is that deciphering the molecular basis of human diseases leads to improved understanding of hematopoiesis, precise diagnosis, and the potential for development of a specific therapy.

Angiotensin II stimulates proliferation of normal early erythroid progenitors

Angiotensin II exerts a mitogenic effect in several in vitro models, but a direct effect on erythroid progenitors has not been documented. Angiotensin-converting enzyme inhibitors and losartan, an angiotensin II type 1 receptor (AT1) antagonist, ameliorate posttransplant erythrocytosis, without altering serum erythropoietin levels. We studied erythroid differentiation and the effect of angiotensin II on proliferation of erythroid progenitors by culturing CD34+ hematopoietic progenitor cells in liquid serum-free medium favoring growth of erythroid precursors. Aliquots of cells were collected every third day, and were used for RNA preparation. AT1 mRNA was detected after 6 d. In these same samples, erythroid-specific mRNA (erythropoietin receptor) was also detected. AT1 protein was detected in 7-d-old burst-forming units-erythroid colonies by Western blotting. The CD34+ cell liquid cultures were used to incubate erythroid precursors with angiotensin II from days 6-9. After incubation, cells were transferred to semisolid medium and cultured with erythropoietin. Angiotensin II increased proliferation of early erythroid progenitors, defined as increased numbers of burst-forming units-erythroid colonies. Losartan completely abolished this stimulatory effect of angiotensin II. Moreover, we observed increased numbers of erythroid progenitors in the peripheral blood of posttransplant erythrocytosis patients. Thus, activation of AT1 with angiotensin II enhances erythropoietin-stimulated erythroid proliferation in vitro. A putative defect in the angiotensin II/AT1 pathway may contribute to the pathogenesis of posttransplant erythrocytosis.

Primary benign erythrocytosis with high erythropoietin levels and an early erythropoietin-sensitive population in the peripheral blood

Primary erythrocytosis diagnosed in a 10-month-old female and followed for 12 years is described. The erythrocytosis was associated with an abnormally elevated set point of erythropoietin production in which the sensitivity fluctuated independently, but corresponded to the alterations in the oxygen-carrying capacity of the blood, when the hematocrit was lowered by phlebotomies. Extensive work for secondary erythrocytoses failed to demonstrate a recognizable cause for this abnormal erythropoietin production. Erythroid cell cultures from peripheral blood mononuclear cells showed the existence of at least two populations: one consistent with dramatic expansion of the erythron in keeping with enhanced sensitivity to endogenous erythropoietin, and the other consistent with the features of typical colonies derived from burst-forming units-erythroid (BFU-Es), seen in normal peripheral blood on days 12 to 14 of culture. The expanded population was characterized by the appearance of single colonies on days 4 to 6 and enormous response to the increasing amounts of erythropoietin, which enhanced their number, size, and maturation. The combination of clinical and in vitro data as well as the absence of any abnormality in the erythropoiesis of the parents and sibling suggest that the erythrocytosis in this child represents a new form with a benign course.

Structure and regulation of the erythroid system at the level of progenitor cells

Considerable ground has been covered since the first clonal assays for hemopoietic cells were described. The possibility of studying populations of progenitor cells and the regulatory factors that influence them has already thrown considerable light on our understanding of the structure and physiology of the normal erythroid system and its alterations in disease. The relative importance of humoral and short-range factors and of possible cell-to-cell interactions in the regulation of proliferation and differentiation in the erythroid cell lineage is now being studied actively in several laboratories. The possibility of analyzing possible regulatory networks involving such highly reactive cells as lymphocytes and monocytes-macrophages in defined in vitro conditions now exists. As these studies are being extended to the diseased state, concepts related to alterations in regulatory mechanisms in syndromes with abnormal cell proliferation can be tested. Clinical applications in the treatment of patients with hematological disease are being contemplated. The usefulness of Epo for the treatment of the anemia of renal disease has been demonstrated already.

The production of myeloid blood cells and their regulation during health and disease

The regulation of myelopoiesis in vivo most likely entails a complex set of interactions between cell-derived biomolecules and their target cells: hematopoietic stem and progenitor cells and accessory cells. Stimulating and suppressing factors have been characterized through in vitro studies, and their mechanisms of action in vitro and in vivo have begun to be elucidated. Among those factors being studied are the hematopoietic colony-stimulating factors (CSF): interleukin-3 (multi-CSF), granulocyte-macrophage-CSF, granulocyte-CSF, and macrophage-CSF; other molecules include erythropoietin, B-cell-stimulating factor-1, interleukin-1, interleukin-2, prostaglandin E, leukotrienes, acidic ferritins, lactoferrin, transferrin, the interferons-gamma, -alpha, and -beta, and the tumor necrosis factors-alpha and -beta (lymphotoxin). These factors interact to modulate blood cell production in vitro and in vivo. The proposed review characterizes these biomolecules biochemically and functionally, including receptor-ligand interactions and the secondary messengers within the cell which mediate their functional activity. The production and action of the molecules are described under conditions of hematopoietic disorders, as well as under normal conditions. Studies in vitro are correlated with studies in vivo using animal models to give an overall view of what is known about these molecules and their relevance physiologically and pathologically.

Cytogenetic evidence for involvement of erythroid and granulocyte/macrophage progenitors in an infant with monosomy 7 syndrome presenting de novo

Monosomy 7 presenting as a myelodysplastic syndrome following radiation and chemotherapy has been reported to involve a stem cell capable of both erythroid and granulocyte/macrophage differentiation. To determine if monosomy 7 presenting de novo also involves a multipotential stem cell, we examined mitoses from individual colony-forming units (CFU)-GM and burst-forming units (BFU)-E colonies derived from semisolid cultures of marrow from an infant with this disorder. Direct cytogenetic analysis of bone marrow cells disclosed the characteristic 45,XY,-7 karyotype in 32 of 35 abnormal metaphases. Metaphases were obtained from 63 (73%) of 85 CFU-GM and BFU-E colonies (median metaphases per colony = 4, range = 1-21), with well-banded analyzable chromosome spreads available for 15 of the colonies with metaphases. The monosomy 7 karyotype was present in all 14 metaphases from ten BFU-E colonies and in all seven metaphases from five CFU-GM colonies. These results indicate that the monosomy 7 karyotype can originate in haematopoietic stem cells with both erythroid and granulocyte/macrophage differentiative potential.

Clonal development, stem-cell differentiation, and clinical remissions in acute nonlymphocytic leukemia

To determine whether acute nonlymphocytic leukemia develops clonally, to study the pattern of differentiation of the involved stem cells, and to determine whether clinical remissions are true remissions, we studied 27 patients with acute nonlymphocytic leukemia who were heterozygous for the X-chromosome-linked glucose-6-phosphate dehydrogenase. In each case, leukemic blast cells manifested only one type of glucose-6-phosphate dehydrogenase, indicating that the malignant process had developed from a single cell. In six elderly patients, circulating erythrocytes, platelets, or both expressed only the glucose-6-phosphate dehydrogenase found in blast cells, indicating that these leukemias had arisen from stem cells with multipotent differentiative expression. In 16 younger adults and children, erythroid cells and platelets were predominantly derived from normal stem cells. In three other cases, the stem cell that gave rise to leukemic blasts apparently also gave rise to erythroid progenitors but not to mature erythrocytes. Heterogeneity was also found during remissions. In 8 of 13 patients, restoration of nonclonal hemopoiesis and repopulation of the marrow by normal stem cells was observed during remission. In the other five patients, marrow stem cells remained partially or completely clonal, even during remission. These data indicate that acute nonlymphocytic leukemia is a heterogeneous disease with respect to differentiation of the stem cells involved by leukemia and the nature of remissions.

How reliable are in vitro clonal cultures? Some comments based on hemopoietic cultures

Clonal cultures in semisolid medium have proved invaluable in analyzing hemopoietic subpopulations and in detecting their specific growth regulators. However, they can be subject to certain deficiencies that an investigator must take care to exclude. These include inabilities of the particular culture system to detect the true stem cells under study or to allow self-generation of clonogenic cells or a full expression of their differentiation potential. Clonal cultures, like conventional cultures, can be subject to significant cell-cell interactions, complicating attempts to characterize the action of a test regulatory molecule. Culture data need to be supplemented by a variety of other data before they can be regarded as valid evidence that a regulatory molecule detected in vitro is likely to function in a similar manner in vivo.

Human embryonic hemopoiesis. Kinetics of progenitors and precursors underlying the yolk sac----liver transition

Human embryonic development involves transition from yolk sac (YS) to liver (L) hemopoiesis. We report the identification of pluripotent, erythroid, and granulo-macrophage progenitors in YS, L, and blood from human embryos. Furthermore, comprehensive studies are presented on the number of hemopoietic progenitors and precursors, as well as of other cell types, in YS, L, and blood at precisely sequential stages in embryos and early fetuses (i.e., at 4.5-8 wk and 9-10 wk postconception, respectively). Our results provide circumstantial support to a monoclonal hypothesis for human embryonic hemopoiesis, based on migration of stem and early progenitor cells from a generation site (YS) to a colonization site (L) via circulating blood. The YS----L transition is associated with development of the differentiation program in proliferating stem cells: their erythroid progeny shows, therefore, parallel switches of multiple parameters, e.g., morphology (megaloblasts----macrocytes) and globin expression (zeta----alpha, epsilon----gamma).

Immunohistochemical and ultrastructural study on erythropoiesis in hepatoblastoma

Eighteen cases of hepatocellular carcinoma in children were examined, and it was found that erythropoiesis exclusively appeared in the well differentiated type of hepatoblastoma. In such foci large immature erythroblasts were found among the tumor cells, whereas mature forms tended to gather in the subendothelial spaces or within sinusoids. Desmosome-like attachments were frequently found between immature erythroblasts and tumor cells. The tumor cells were well differentiated and had a distinct polarity. The erythropoietic foci were never found in lymph nodes, spleens and in the non-neoplastic hepatic tissues obtained by surgery or autopsy. Erythroblastic cells did not show an increase in number in the bone marrows. These findings indicate that hepatoblastoma cells in certain stages of differentiation have the capacity to induce the differentiation of pluripotent hemopoietic stem cells into the cells of erythrocytic series, or that the microenvironment composed of one or more tumor cells offer good soil for the differentiation of erythroblastic cells. There seems to be no intimate relationship between the production of alpha-fetoprotein by tumor cells and the appearance of erythropoiesis.

Effect of pure erythropoietin on DNA-synthesis by human marrow day 15 erythroid burst forming units in short-term liquid culture

The effect of pure erythropoietin (EP) on human marrow day 15 burst-forming units-erythroid (BFU-E) was studied using a short-term liquid culture system containing 30% human serum. Non-adherent marrow cells were cultured in liquid medium for 0-48 h and then the number of BFU-E was assayed by the use of the plasma clot method. The addition of 1 U/ml of EP into the liquid culture medium resulted in maintenance of the number of BFU-E assayed after 24-48 h of incubation. The number of BFU-E recovered after 24-48 h culture was directly proportional to the concentration of EP present in the liquid medium. In addition, the proliferative status of BFU-E before and after exposure to EP was studied by 3H-thymidine and hydroxyurea suicide. It was found that EP doubles the percentage of BFU-E in DNA synthesis after 24-48 h of incubation in the liquid medium. This effect of EP on DNA synthesis by bone marrow day 15 BFU-E is detectable as early as 6 h after the onset of incubation and at EP concentrations as low as 0.2 U/ml of medium, a concentration present in the serum of moderately anaemic patients. The human marrow day 15 BFU-E is an EP-responsive cell and pure EP can induce it into DNA synthesis.

Colony assays of hematopoietic progenitor cells and correlations to clinical situations

The production of blood cells is a dynamic process that is noticeably aberrant during disease. The availability of colony assays in vitro that allow detection of hematopoietic stem and progenitor cells for the neutrophil, monocyte-macrophage, erythroid and/or megakaryocyte lineages has been of importance for the present understanding of the mechanisms controlling the proliferation, self-renewal capacity, and differentiation of morphologically nonrecognizable immature cells which give rise to the mature progeny circulating in the blood. It is through the use of these assays that the existence of potentially relevant stimulatory and inhibitory feedback interactions has been demonstrated. Abnormalities in these interactions, which may be of significance during leukemia and related disorders, have been uncovered. This communication will discuss regulatory interactions detected via the colony assays, their potential relevance physiologically and pathologically, and the use of these assays for diagnosis, prognosis, and for monitoring the clinical status of patients.

Human megakaryocytic progenitors (CFU-M) assayed in methylcellulose: physical characteristics and requirements for growth

The basic culture requirements and several physical characteristics were defined for megakaryocytic colony-forming cells (CFU-M) from normal human marrow growing in methylcellulose. Ficoll-hypaque separated mononuclear cells from human marrow gave rise to megakaryocytic colonies in the presence of normal human plasma and phytohemagglutinin-stimulated leukocyte-conditioned medium (PHA-LCM). Their identity as megakaryocytic colonies was confirmed by immunofluorescence staining with a monoclonal antibody to human factor VIII antigen and by electron microscopy of individually harvested colonies. Demonstration of the single-cell origin of the colonies was provided by analysis of the glucose-6-phosphate dehydrogenase (G-6-PD) enzyme type of individually harvested colonies grown from a G-6-PD heterozygote. The colonies grew best in heparinized or citrated plasma as opposed to serum. Detailed studies suggested that platelet-release products were responsible for this difference. Tritiated thymidine suicide studies showed that the percentage of CFU-M in DNA synthesis was 23 +/- 8% (n = 10). The modal velocity sedimentation rate of CFU-M was 4.9 +/- 0.6 mm/hr (n = 4) while that of concurrently studied granulocyte/macrophage colony-forming cells (CFU-GM) was 5.7 +/- 0.5 mm/hr. Examination of the PHA-LCM dose-response characteristics suggested the presence in the conditioned medium of an inhibitor to megakaryocyte colony growth which was partially removed by chromatography of the medium on Sephadex G-100. The resulting conditioned medium increased the cloning efficiency for CFU-M compared with that with crude PHA-LCM (15.3 +/- 7.0 and 8.2 +/- 5.3/10(5) marrow cells, respectively).

Haemopoietic colony formation (BFU-E, GM-CFC) during the development of pure red cell hypoplasia induced in the cat by feline leukaemia virus

The GM-CFC assay for granulocyte-macrophage progenitors and the BFU-E and CFU-E assay for early and late erythroid progenitors from cat bone marrow were characterized. GM-CFC gave 59 +/- 4 to 118 +/- 6 colonies per 10(5) bone marrow cells using colony stimulating factors (CSF) from cat, mouse or human sources. The CFU-E and BFU-E assays gave 114 +/- 7 and 58 +/- 7 colonies respectively with optimum doses of erythropoietin. Irradiated cat bone marrow cells were good sources of CSF and of burst promoting activity for these assays. Kittens infected with feline leukaemia virus, subgroup C (FeLV-C), which induces pure red cell hypoplasia, showed the incidence of BFU-E decreased to 25-35% of controls as early as one week postinfection, and even lower values at later times. In contrast, the incidence of GM-CFC remained normal for several weeks. No evidence of inhibitory cells or of lack of stimulatory cells in the infected marrows was seen when they were cultured together with normal marrow in the BFU-E assay. Conversely, normal marrow cells were not able to restore BFU-E growth from infected marrow. This suggests a direct action of FeLV-C on early erythroid precursors. Infection with FeLV, subgroup A, which induces only a mild transitory anaemia, produces only a moderate decrease in the incidence of BFU-E.

Clonal expression of the Tn antigen in erythroid and granulocyte colonies and its application to determination of the clonality of the human megakaryocyte colony assay

To evaluate whether exposure of Tn determinants at the surface of human erythrocytes, platelets, and granulocytes could arise from a somatic mutation in a hemopoietic stem cell, burst-forming unit erythroid (BFU-E) colonies, colony-forming unit granulocyte-macrophage (CFU-GM), and colony-forming unit-eosinophil (CFU-Eo) were grown from a blood group O patient with a typical Tn syndrome displaying two distinct populations (Tn(+) and Tn(-)) of platelets, granulocytes, and erythrocytes. A large number of colonies was observed. Individual colonies were studied with a fluorescent conjugate of Helix pomatia agglutinin (HPA). A sizeable fraction of each of the erythroid and granulocytic colonies appeared to consist exclusively of either HPA-positive or HPA-negative cells, thereby demonstrating the clonal origin of those exhibiting the Tn marker. Similar results were obtained from a second patient. These findings establish that the HPA labeling of Tn cells is an accurate marker permitting assessment of the clonality of the human megakaryocyte (MK) colony assay. For the study of MK cultures a double-staining procedure using the HPA lectin and a monoclonal antiplatelet antibody (J-15) was applied in situ to identify all MK constituting a colony. Our results, obtained in studies of 133 MK colonies, provide definitive evidence that the human MK colony assay is clonal because all MK colonies were exclusively composed of Tn(+) and Tn(-) MK. Furthermore, the distribution of MK within a single colony was shown to be seminormal with a mean at 6 MK, isolated MK typically being absent in culture. Comparison of the proportion of mature Tn(+) cells in blood with their respective Tn(+) progenitors has also shown that no proliferative advantage occurs after the commitment; because Tn polyagglutinability is an acquired disorder, then the expansion of the Tn(+) clone must occur either during the proliferative stage of the pluripotent stem cell or during the commitment itself. This study therefore affords evidence that a blood group antigen plays a role in the differentiation of a pluripotent stem cell.

Human hemoglobin switching: insights from studies of erythroid cultures

Clonal erythroid cultures have been used in the investigation of the cellular mechanisms underlying the switch from fetal to adult Hb formation during ontogeny and the expression of fetal hemoglobin in adult life. It has been shown that adult BFUes have the potential to form F cells and A cells in culture. The cellular segregation in the expression of Hb F within a clone leads to the formation of characteristic bursts displaying a bimorphic F+/F-pattern of Hb F expression. Analysis of distribution of F-expressing and not F-expressing subclones and sectors among bursts of various sizes is compatible with the assumption that progenitor cells exercise options on whether they will or they will not form progeny expressing the Hb F phenotype. Distributions of Hb F expressing subclones among bursts fit the expectations of a stochastic model; probabilities for F expression in the adult erythroid cells may be influenced by the environment in which the cells differentiate and mature. Experimental data from cultures of progenitors from earlier stages of human development indicate that the change in globin expression during ontogeny is controlled at the level of progenitor cells. Observations of Hb F levels produced by single erythroid bursts of the neonate make it unlikely that hemoglobin switching during ontogeny is accomplished through replacement of stem cell lines. The findings in erythroid cultures are compatible with the hypothesis that hemoglobin switching during ontogeny is accomplished through intrinsic or interactive changes in the programs of differentiation of progenitors.

A method for obtaining high quality chromosome preparations from single hemopoietic colonies on a routine basis

The need for improved methodology to facilitate cytogenetic analysis of hemopoietic stem cell populations, particularly in studies of the hemopoietic malignancies, has been recognized for a number of years. Since primitive hemopoietic cells can be stimulated to form colonies of identifiable progeny under appropriate conditions in vitro, it should in theory, be possible to obtain such information. However, hemopoietic colonies of human origin rarely contain more than 1000 cells, and in handling such small samples, cell loss has historically been a major problem. We now describe a method that has allowed from two up to more than 50 metaphases per colony to be obtained from most erythroid and granulopoietic colonies harvested individually from standard methylcellulose assay cultures. Of key importance is the selection of large but still immature colonies and the use of polylysine coated slides, which ensures recovery of 80 - 90% of the sample. The method yields reproducibly high quality metaphases suitable for analysis after G, Q, and fluorescent-reverse banding. Cytogenetic analysis of 58 colonies removed individually from cultures initiated with a mixture of male and female cells showed both male and female colonies to be present as expected. In all instances only one type of metaphase was ever found in a single colony. A procedure for measuring the incidence of sister chromatid exchange (SCE) in the progeny of primitive hemopoietic progenitors has also been established. SCE values obtained for 1-2-week-old colonies derived from normal progenitors were similar to previously published normal SCE values. Preliminary analysis of erythroid and granulocytic colonies from one patient with polycythemia vera (PV) and two patients with chronic myelogenous leukemia (CML) indicate that preparations of equal quality are obtained from such individuals and thus, for the first time, the Ph1 chromosome could be readily demonstrated in erythroid colonies. Further application of this method to the investigation of patients with myeloproliferative disease is now underway.

Congenital dyserythropoietic anaemia type I: absence of clonal expression in the nuclear abnormalities of cultured erythroblasts

Erythroid colonies derived from the circulating early erythroid precursor (BFU-E) of a patient with congenital dyserythropoietic anaemia type I (CDA I) have been grown in plasma clot and studied by electron microscopy. The number of circulating BFU-E was in the normal range with a roughly normal appearance at the light microscopic level. However, investigation of individual colonies by electron microscopy has always shown a mixture of normal and abnormal erythroblasts exhibiting the typical nuclear aberrations found in vivo. The proportion of normal erythroblasts varied from one colony to another. After the release of the cells from the clot in order to permit new cellular interactions, macrophages were observed to phagocytose abnormal erythroblasts but also a few erythroblasts with normal nuclei. These findings demonstrate that CDA I is a disorder which results from a defective erythroid stem cell but that the progeny of each BFU-E may vary considerably in the extent to which they express the morphological defects. Based on studies of cultures of BFU-E, similar conclusions were previously made for CDA II.

Aplastic anemia: recent advances in pathogenesis and treatment

Successful bone marrow transplantation and bone marrow culturing techniques have generated a large body of research into the pathogenesis and treatment of aplastic anemia. Most prominent has been the emphasis on autoimmune mechanisms. Several etiologic types and diagnostic criteria are discussed, the evidence supporting immune and other mechanisms of pathogenesis is examined, and results of current therapeutic trials are addressed.

Polycythemia vera. The in vitro response of normal and abnormal stem cell lines to erythropoietin

Bone marrow cells from two glucose-6-phosphate dehydrogenase (G-6-PD) heterozygotes with polycythemia vera were cultured to determine whether progenitors which wre not of the polycythemia vera clone were present, and, if present, which cell lines contributed to the increase in erythroid colonies observed in response to added erythropoietin (ESF). To accomplish this, the G-6-PD isoenzyme activity of individual erythroid colonies was determined. All of the erythroid colonies analyzed in cultures without added ESF, contained the G-6-PD isoenzyme type characteristic of the abnormal clone. With higher ESF concentrations in the culture, however, there was an increase in the colonies that were not of the polycythemia vera clone. Analysis of the ratio of the various types of colonies indicated that normal and polycythemia vera cells are capable of responding to ESF in vitro. In selected patients, this technique permits analysis of the ratios of normal to abnormal cells during the course of the disease, in response to therapy and during late complications, such as myelofibrosis or leukemic transformation.

Regulation of hemoglobin synthesis during the development of the red cell (third of three parts)

In this article we have surveyed the current state of knowledge regarding the accumulation of globin mRNA and hemoglobin in red cells. We have attempted to examine the interplay of numerous processes that seem to be necessary to achieve this highly differentiated state. Finally, we have made an effort to formulate some of the mechanisms whereby individual red cells may come to contain varying proportions of specific hemoglobins. The past several years have been characterized by a veritable explosion of knowledge concerning the globin structure genes, and the structure, transcription, processing and function of globin mRNA in erythroid cells. It now seems possible to analyze the earlier stages of erythropoiesis by cultivation and examination of erythroid colonies in vitro. The primary differentiation events leading to the production of specific globins, especially for hemoglobin F production in man, are now experimentally accessible. There is good reason to hope that these advances will soon permit achievement of the long desired therapeutic goal of enhancing hemoglobin F synthesis in patients with severe beta-chain hemoglobinopathies. Our aim has been to review the scientific information that might provide the rationable for amelioration of the clinical phenotypes in patients inheriting abnormal globin genes.