Adaptive genetic changes related to haemoglobin concentration in native high-altitude Tibetans

NEW FINDINGS

What is the topic of this review? Tibetans have genetic adaptations that are hypothesized to underlie the distinct set of traits they exhibit at altitude. What advances does it highlight? Several adaptive signatures in the same genomic regions have been identified among Tibetan populations resident throughout the Qinghai-Tibetan Plateau. Many highland Tibetans exhibit a haemoglobin concentration within the range expected at sea level, and this trait is associated with putatively adaptive regions harbouring the hypoxia-inducible factor pathway genes EGLN1, EPAS1 and PPARA. Precise functional variants at adaptive loci and relationships to physiological traits, beyond haemoglobin concentration, are currently being examined in this population. Some native Tibetan, Andean and Ethiopian populations have lived at altitudes ranging from 3000 to >4000 m above sea level for hundreds of generations and exhibit distinct combinations of traits at altitude. It was long hypothesized that genetic factors contribute to adaptive differences in these populations, and recent advances in genomics provide evidence that some of the strongest signatures of positive selection in humans are those identified in Tibetans. Many of the top adaptive genomic regions highlighted thus far harbour genes related to hypoxia sensing and response. Putatively adaptive copies of three hypoxia-inducible factor pathway genes, EPAS1, EGLN1 and PPARA, are associated with sea-level range, rather than elevated, haemoglobin concentration observed in many Tibetans at high altitude, and recent studies provide insight into some of the precise adaptive variants, timing of adaptive events and functional roles. While several studies in highland Tibetans have converged on a few hypoxia-inducible factor pathway genes, additional candidates have been reported in independent studies of Tibetans located throughout the Qinghai-Tibetan Plateau. Various aspects of adaptive significance have yet to be identified, integrated, and fully explored. Given the rapid technological advances and interdisciplinary efforts in genomics, physiology and molecular biology, careful examination of Tibetans and comparisons with other distinctively adapted highland populations will provide valuable insight into evolutionary processes and models for both basic and clinical research.

Lessons to better understanding of hypoxia sensing. Acquired and congenital mutations resulting in polycythemia

Adaptation of the organism to hypoxia has profound effect on multiple tissues including regulation of erythropoiesis, vasculogenesis, a proper regulation of embryogenesis as well as other functions. The elucidation of those congenital or acquired mutations giving rise to disease states affecting physiological systems devoted to oxygen homeostasis provides not only a practical diagnostic and potential therapeutic target, but also allows to identify the essential, non-redundant physiological pathways that may be hitherto unknown. The erythropoietin gene was the first gene expression found to be upregulated by hypoxia; the mechanism of this regulation lead to our current understanding of hypoxia sensing. Thus it is appropriate that the disorders resulting from augmented erythropoiesis are subject of this review.

Genetic heterogeneity of primary familial and congenital polycythemia

Primary familial and congenital polycythemia (PFCP) is an inherited disorder of erythroid progenitor cells resulting in elevated erythrocyte mass. Several mutations of the erythropoietin receptor (EPOR) gene have been associated with PFCP, although in a few families the linkage between the EPOR gene and PFCP has been excluded. To examine the role of EPOR mutations in the pathogenesis of PFCP, we studied 43 unrelated PFCP subjects. Erythroid culture data were available in 26 subjects, and in all these subjects, we observed hypersensitivity of erythroid progenitors to erythropoietin (EPO). We screened all EPOR gene exons for mutations using ribonuclease cleavage assay and protein truncation test. We detected five mutations in exon VIII of the EPOR gene, four of which we reported earlier. A new EPOR gene mutation was found (G5959T) that changes codon 425 GAG to a termination codon, resulting in truncation of the EPOR by 84 amino acids. The G5959T mutation was found to segregate with the disease in the affected family and represents another example of a nonsense mutation associated with PFCP. We also report the first intronic mutation (A2706T) of the EPOR gene. The finding of only five disease-causing mutations in our PFCP patient pool of 43 subjects (12%) indicates that EPOR gene mutations are not the major genetic defect associated with PFCP. The hypersensitivity of erythroid progenitors to EPO seen in all examined PFCP subjects suggests a dominant lesion of an as yet unidentified gene either at the level of the EPOR signaling pathway or another erythropoiesis regulating pathway.

Non-anemic homozygous beta(o) thalassemia in an African-American family: association of high fetal hemoglobin levels with beta thalassemia alleles

We have studied a four-generation (23 subjects) African-American family with beta(o) thalassemia and high fetal hemoglobin (HbF) levels. The beta(o) thalassemia in this family is due to the splicing site mutation, beta IVS2+1G-->A, that leads to aberrant mRNA processing and the absence of beta globin. Two members of this family are homozygous for beta(o) thalassemia and are non-anemic. All family members who are heterozygous for the beta IVS2+1G-->A mutation have elevated HbF, with the exception of two individuals who also have severe alpha-globin chain deficiency. We excluded linkage with the hereditary persistence of fetal hemoglobin loci on chromosomes 6 and X. We also excluded the presence of all previously described determinants in the beta globin gene cluster associated with elevated HbF production. One thalassemia allele is in the Cameroon-like (HS2)/Benin-like beta globin gene cluster haplotype, and the other is in the Senegal-like (HS2)/Benin-like beta globin gene cluster haplotype. We speculate that in the homozygotes, those erythroid cells that express low to absent levels of gamma globin are selectively destroyed. In contrast, in the heterozygotes, the presence of the normal beta globin allele would ameliorate the globin chain imbalance and thus allow survival of erythroid cells that express the abnormal transcript, leading to a typical beta(o) thalassemia phenotype. Thus, the heterocellular gamma globin expression together with in vivo preferential survival of HbF-containing erythroid cells ameliorates Cooley's anemia in the beta(o) thalassemia homozygotes. It remains to be determined what sequences linked to each thalassemia allele and what trans-acting factors contribute to high HbF levels.

Molecular biology of polycythemias

Polycythemia is literally translated as "many cells in the blood". Only erythrocytosis (an alternative term for these disorders) produces polycythemia since leukocytes and platelets are present in blood in far smaller proportions. Polycythemia may be due to increased proliferation or decreased apoptosis of erythroid progenitors, or to delayed erythroid differentiation with an increased number of progenitor cell divisions. Prolonged red cell survival, another theoretical cause of polycythemia, has not yet been described and with intact regulatory mechanisms is unlikely to occur. Primary polycythemias result from abnormalities expressed in hematopoietic progenitors. In contrast, circulating factors cause secondary polycythemia (1). There are acquired and congenital causes of both primary and secondary polycythemia (1).

Mouse model of congenital polycythemia: Homologous replacement of murine gene by mutant human erythropoietin receptor gene

Mutations causing truncations of the cytoplasmic domain of the human erythropoietin receptor (EPOR) result in a dominantly inherited disorder-primary familial congenital polycythemia. This disorder is characterized by increased numbers of erythrocytes (polycythemia) and by in vitro hypersensitivity of erythroid precursors to erythropoietin. The consequences of EPOR truncation in nonerythroid tissues are unknown. We replaced the murine EPOR gene with a wild-type human EPOR gene and a mutant human EPOR gene that we initially identified in a patient with polycythemia. This mutation leads to an EPOR truncated after the first tyrosine residue of the intracellular domain. Mice heterozygous for this mutant allele and a wild-type human EPOR allele mimicked the human disorder. Interestingly, mice that were homozygous for the mutant human allele were severely polycythemic but viable. Our results provide a model for functional studies of EPOR-triggered signaling pathways in erythropoiesis. These animals can now be used to investigate the molecular pathophysiology of this gain-of-function EPOR mutation in erythroid tissue and in those nonerythroid tissues that express EPOR.

Pathogenetic mechanisms of polycythemia vera and congenital polycythemic disorders

The absolute polycythemias--those with increased red blood cell mass--can be divided into two groups: primary, caused by acquired or inherited mutations leading to a "gain-of-function" abnormalities expressed within the erythroid progenitors; and secondary, due to circulating serum factors, typically erythropoietin, stimulating erythropoiesis. This overview concentrates on the molecular biology of polycythemia vera (PV) discussed in the context of other polycythemic disorders. Recent advances in the regulation of erythropoiesis, as they may relate to polycythemic states, are discussed as a background for those well-defined polycythemic states wherein the molecular defect has not yet been elucidated. A number of cellular abnormalities associated with PV, including the hyperresponsiveness of PV progenitors to many cytokines as well as decreased expression of the thrombopoietin receptor on platelets and increased expression of Bcl-xL, suggest that the PV defect alters a number of cellular functions and is not restricted to cytokine receptor signal transduction. The increasing number of recognized instances of familial incidence of PV suggests that in these families the predisposition for PV is inherited as a dominant trait, and that PV is acquired as a new mutation that leads to a clonal hematopoiesis and may be due to loss of heterozygosity. The existence of these families provides a unique opportunity for isolation of the mutations in the gene leading to PV. Semin Hemaol 38(suppl 2):10-20.

Development of a novel trans-lentiviral vector that affords predictable safety

Lentiviral vectors derived from human immunodeficiency virus type 1 (HIV-1) hold great promise for gene therapy. However, the possibility of generating replication-competent retrovirus (RCR) through genetic recombination raises concerns for safety. Here we describe a novel HIV-based packaging system (trans-lentiviral) that splits gag/gag-pol into two parts: One that expresses gag/gag-pro and another that expresses reverse transcriptase and integrase as fusion partners of viral protein R (Vpr). Using a sensitive assay developed to specifically detect recombinant lentiviral DNA mobilization, we demonstrated that the trans-lentiviral vector prevents the generation of recombinants that contain a functional gag-pol structure, while the lentiviral vector generates env-minus recombinant lentivirus that mobilizes recombinant genomes to other cells when pseudotyped with an exogenous envelope. Since an intact gag-pol structure is absolutely required for retroviral DNA mobilization and RCR, the trans-lentiviral vector design significantly reduces this risk. Moreover, it makes it possible to assess the risk of RCR and DNA mobilization using an in vitro assay that monitors trans-lentiviral vector stocks for the regeneration of the gag-pol structure. Therefore, the trans-lentiviral vector design will ensure the greatest predictable level of safety for the clinical application of retroviral vectors, including HIV-based vectors.

A polymorphism of the X-linked gene IDS increases the number of females informative for transcriptional clonality assays

Studies of clonality have been essential for understanding the hierarchy of hematopoiesis and the biology of malignancies. Most clonality assays are based on the X chromosome inactivation phenomenon in females; these assays detect protein polymorphisms, differences in DNA methylation, or transcripts of the active X chromosome. Assays based on protein polymorphisms or DNA methylation have significant shortcomings. The major disadvantage of transcriptional assays is their limited applicability since only approximately half of females are informative for these studies. We have developed a new transcriptional assay based on an exonic polymorphism of the X-chromosome gene IDS. This gene is located in the same X-chromosome region (Xq28) as G6PD and p55, two genes with exonic polymorphisms for which we previously developed transcriptional assays. We developed non-radioactive PCR-based assays for rapid screening of genotype and determination of clonality. We also report reaction conditions for a quantitative ligase detection assay of IDS allelic transcripts. The frequency of the IDS polymorphism is 46% in Caucasian females and 39% in African-American females; in combination with G6PD and p55, 76% of Caucasian females and 62% of African-American females are informative for these assays. While this gene is highly polymorphic in Caucasian and African-American females, it is not informative in Oriental females. We established that the IDS gene is in linkage equilibrium with G6PD and p55. Unlike methylation-based assays, this assay is suitable for studying clonality in non-nucleated cells such as platelets and reticulocytes. With the discovery of exonic polymorphisms of other X-chromosome genes, all females should eventually be suitable for X-chromosome transcriptional clonality analysis.

Effect of recombinant human erythropoietin combined with granulocyte/ macrophage colony-stimulating factor in the treatment of patients with myelodysplastic syndrome. GM/EPO MDS Study Group

This randomized, placebo-controlled trial was designed to assess the efficacy and safety of therapy with granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (epoetin alfa) in anemic, neutropenic patients with myelodysplastic syndrome. Sixty-six patients were enrolled according to the following French-American-British classification: refractory anemia (20), refractory anemia with excess blasts (35), refractory anemia with ringed sideroblasts (9), and refractory anemia with excess blasts in transformation (2). Patients were stratified by their serum erythropoietin levels (less than or equal to 500 mU/mL, n = 37; greater than 500 mU/mL, n = 29) and randomized, in a 2:1 ratio, to either GM-CSF (0.3-5.0 microg/kg.d) + epoetin alfa (150 IU/kg 3 times/wk) or GM-CSF (0.3-5.0 microg/kg.d) + placebo (3 times/wk). The mean neutrophil count rose from 948 to 3831 during treatment with GM-CSF +/- epoetin alfa. Hemoglobin response (increase greater than or equal to 2 g/dL, unrelated to transfusion) occurred in 4 of 45 (9%) patients in the GM-CSF + epoetin alfa group compared with 1 of 21 (5%) patients with GM-CSF + placebo group (P = NS). Percentages of patients in the epoetin alfa and the placebo groups requiring transfusions of red blood cells were 60% and 92%, respectively, for the low-endogenous erythropoietin patients and 95% and 89% for the high-endogenous erythropoietin patients (P = NS). Similarly, the average numbers of units of red blood cells transfused during the 12-week study in the epoetin alfa and the placebo groups were 5.9 and 9.5, respectively, in the low-endogenous erythropoietin patients and 9.7 and 8.6 in the high-endogenous erythropoietin patients (P = NS). GM-CSF +/- epoetin alfa had no effect on mean platelet count. Treatment was well tolerated in most patients, though 10 withdrew from the study for reasons related predominantly to GM-CSF toxicity. (Blood. 2000;95:1175-1179)

Congenital and inherited polycythemia

Absolute polycythemia is a condition with increased red blood cell mass. There are a number of primary and secondary polycythemic disorders leading to absolute polycythemia. Primary polycythemias are caused by a defect intrinsic to the erythroid progenitor cells. The best characterized primary polycythemia is the autosomal dominant primary familial and congenital polycythemia (PFCP). Familial or childhood occurrence of the myeloproliferative disorder polycythemia vera are also discussed, emphasizing the importance of distinction between polycythemia vera and PFCP. Congenital or familial secondary polycythemic conditions are characterized by increased red cell mass, which is caused by circulating serum factors, typically erythropoietin.

Molecular basis for polycythemia

This overview concentrates on familial and congenital polycythemias in the context of other polycythemic disorders, with emphasis on those with established molecular lesions. Recent advances in the regulation of erythropoiesis, as they may relate to polycythemic states, are discussed as a background for those well-defined polycythemic states wherein the molecular defect has not yet been elucidated. Primary familial congenital polycythemias and congenital and familial secondary polycythemias, including hemoglobin mutants, methemoglobinemias and congenital 2,3-bisphosphoglycerate deficiency, are discussed. The most common primary polycythemia, polycythemia vera, as well as the only likely endemic congenital secondary polycythemia, known as Chuvash polycythemia, are discussed.

Haematopoietic progenitors and signal transduction in polycythaemia vera and primary thrombocythaemia

While significant progress has been made in understanding the cellular defect and molecular basis of polycythaemia vera (PV), elucidation of the primary mutation leading to PV remains elusive. While clinically useful, the PV diagnostic criteria put forward by the Polycythemia Vera Study Group are not based on the pathophysiology of this disorder and in some instances may lead to false diagnosis or may not be sufficient to diagnose an early PV. In diagnostically unclear situations, clinical and laboratory findings must take into account the acquired nature of PV, its clonality, and the presence of endogenous erythroid colony formation in serum-containing media. It is likely that other simpler assays may be developed based on the rapidly emerging knowledge of the cellular pathology of PV. Several intriguing observations of abnormalities pertaining to the erythroid signal transduction have been recently reported; these remain to be validated in other laboratories and to be proven specific for PV. The clinical concept of primary thrombocythaemia (PT) lags behind what we know about PV. While the diagnosis of PT is still based on the exclusion of other known causes of thrombocytosis, new knowledge is emerging. Recent clonality studies of a large number of PT females show that the majority are clonal. It is our belief that thrombocythaemic subjects who are not found to be clonal are those with secondary thrombocytosis. Multiple in vitro-based assays of megakaryocytic and erythroid progenitors have been developed and conflicting data published. It is likely that standardized assays of megakaryocytic progenitors will soon become available and a reproducible PT specific defect will be found. Such a specific test would be of immense diagnostic value in this most elusive of all myeloproliferative disorders.

Leukocyte transfusion-associated granulocyte responses in a patient with X-linked hyper-IgM syndrome

X-linked hyper-IgM syndrome (XHIM) is a severe congenital immunodeficiency caused by mutations in CD154 (CD40 ligand, gp39), the T cell ligand for CD40 on B cells. Chronic or cyclic neutropenia is a frequent complicating feature that heightens susceptibility to severe infections. We describe a patient with a variant of XHIM who produced elevated levels of serum IgA as well as IgM and suffered from chronic severe neutropenia. Eight of ten leukocyte transfusions with cells from a maternal aunt, performed because of mucosal infections, resulted in similar episodes of endogenous granulocyte production. Transfection studies with the mutant CD154 protein indicate that the protein is expressed at the cell surface and forms an aberrant trimer that does not interact with CD40. The data suggest that allogeneic cells from the patient's aunt, probably activated T cells bearing functional CD154, may interact with CD40+ recipient cells to produce maturation of myeloid precursors in the bone marrow.

Guinea pig serum erythropoietin (EPO) selectively stimulates guinea pig erythroid progenitors: human or mouse erythroid progenitors do not form erythroid burst-forming unit colonies in response to guinea pig serum EPO

Erythropoietin (EPO) is the primary regulator of mammalian erythropoiesis, providing a proliferative and differentiative signal to the early EPO-responsive erythroid progenitors, burst-forming unit-erythroid (BFU-E) and colony-forming unit-erythroid, as well as to later EPO-responsive erythroid progenitors. EPO is secreted by the kidney in response to hypoxia and anemia. There is an extensive biological crossreactivity between human EPO and the EPOs of other mammals. Necas et al. have reported that this crossreactivity may not include the guinea pig (Cavia porcelllus). Because the specificity of the guinea pig's erythropoietic responses may be of biological significance, we compared guinea pig hypoxic serum with mouse (m) and human (h) recombinant (r) EPOs for their ability to induce erythroid progenitor proliferation and differentiation in semisolid cultures. Guinea pig bone marrow mononuclear cells (BMMCs) formed BFU-E colonies in response to guinea pig hypoxic serum, rhEPO, or rmEPO in a dose-dependent fashion. Neither human nor mouse BMMCs responded to guinea pig hypoxic serum; however, guinea pig hypoxic serum exerted no inhibitory effect on human or mouse in vitro erythroid differentiation in the presence of rhEPO or rmEPO. The intensity of the EPO band on Western blotting analysis of guinea pig hypoxic serum was significantly greater than in nonhypoxic serum. This suggests that guinea pig erythropoiesis is mediated by EPO and stimulated by hypoxia in a fashion similar to that observed in human and mouse erythropoiesis. Furthermore, guinea pig EPO did not stimulate human or mouse erythroid differentiation in vitro, whereas guinea pig erythroid progenitors could be stimulated by human or mouse EPO, suggesting structural differences in guinea pig EPO and EPO receptor (EPOR) compared with human or mouse EPO and EPOR. These differences probably evolved after the guinea pig's ancestors diverged from myomorph rodents. Further characterization of the guinea pig EPO and EPOR should facilitate our understanding of the interaction between EPO and EPOR.

Absence of polycythemia in a child with a unique erythropoietin receptor mutation in a family with autosomal dominant primary polycythemia

Primary familial and congenital polycythemia (PFCP or familial erythrocytosis) is a rare proliferative disorder of erythroid progenitor cells, characterized by elevated erythrocyte mass and hemoglobin concentration, hypersensitivity of erythroid progenitors to erythropoietin (EPO), and autosomal dominant inheritance or sporadic occurrence. A number of EPO receptor (EPOR) mutations were found in subjects with PFCP; most of these mutations resulted in the truncation of the COOH-terminal of the EPOR protein. We studied a family with autosomal dominant inheritance of PFCP in which four subjects were affected in three generations. We screened the affected individuals for EPOR gene mutations using SSCP analysis and found a C5964G mutation in exon VIII that changes tyrosine codon 426 to a translation termination codon resulting in an EPOR protein truncated by 83 amino acids. The mutant C5964G-EPOR exhibited hypersensitive EPO-dependent proliferation compared to the wild-type EPOR when tested in a murine interleukin-3-dependent myeloid cell line (FDC-P1). We also examined the segregation of the mutation with PFCP in the family and found that a child in the third generation inherited the mutation without having laboratory evidence of polycythemia. Further in vitro analysis of the erythroid progenitor cells of this affected child revealed that the progenitor cells were hypersensitive to EPO (a hallmark of PFCP) suggesting the presence of the disease at the level of progenitor cells. Failure of this child to develop polycythemia suggests the existence of as yet unidentified environmental or genetic factors that may suppress disease development.

Losartan, an angiotensin II type 1 receptor antagonist, lowers hematocrit in posttransplant erythrocytosis

The mechanism by which angiotensin-converting enzyme inhibitors reduce red cell mass in renal transplant recipients with erythrocytosis is unclear. To examine the role of angiotensin II in this disorder, losartan (a competitive antagonist of the angiotensin II type 1 [AT1] receptor) was administered to 23 patients with erythrocytosis. Fourteen patients took 25 mg/d for 8 wk; nine others were treated with 50 mg/d for 8 wk. Hematocrit decreased from 0.527 +/- 0.027 to 0.487 +/- 0.045 after 8 wk (P < 0.01)--by at least 0.04 in 19 patients. Decrement in hematocrit in the initial 8 wk of therapy was significantly greater in patients administered 50 mg/d than in patients on 25 mg/d. Twelve of 14 patients initially treated with 25 mg/d showed a small change in hematocrit; the dose was increased to 50 mg/d for 8 more wk. Hematocrit decreased from 0.528 +/- 0.030 before losartan treatment to 0.483 +/- 0.055 after 16 wk (P < 0.01). After therapy, serum erythropoietin significantly decreased in eight patients with elevated baseline levels, but not in 15 patients with normal baseline levels; however, hematocrit significantly decreased in both groups. Losartan was withdrawn in 16 patients; hematocrit increased from 0.440 +/- 0.057 to 0.495 +/- 0.049 after 8.9 +/- 7.5 wk (P < 0.001), without change in serum erythropoietin. Thus, specific blockade of AT1 receptors inhibited erythropoiesis, suggesting a pathogenic role for angiotensin II in posttransplant erythrocytosis.

Human hematopoietic progenitors express erythropoietin

Erythropoietin (EPO) is a factor essential for erythroid cell proliferation, differentiation, and survival. The production of EPO by the kidneys in response to hypoxia and anemia is well documented. To determine whether EPO is also produced by hematopoietic cells, we analyzed the expression of EPO in normal human hematopoietic progenitors and in their progeny. Undifferentiated CD34(+)lin- hematopoietic progenitors do not have detectable EPO mRNA. Differentiating CD34(+) cells that are stimulated with recombinant human EPO in serum-free liquid cultures express both EPO and EPO receptor (EPOR). Because CD34(+) cells represent a heterogeneous cell population, we analyzed individual burst-forming units-erythroid (BFU-E) and nonerythroid colony-forming unit-granulocyte-macrophage colonies for EPO mRNA. Only BFU-E colonies were positive for EPO mRNA. Lysates from pooled BFU-E colonies stained positively for EPO by immunoblotting. To further confirm the intrinsic nature of erythroid EPO, we replaced extrinsic EPO in erythroid colony cultures with EPO-mimicking peptide (EMP). We show EPO expression in the EMP-stimulated BFU-Es at both mRNA and protein levels. Stimulation of bone marrow mononuclear cells (BMMCs) with EMP upregulated EPO expression. Furthermore, we found EPO and EPOR mRNAs as well as EPO protein in K562 cells, a human erythroleukemia cell line. Stimulation of K562 cells with EMP upregulated EPO expression. We suggest that EPO of erythroid origin may have a role in the regulation of erythropoiesis.

The erythropoietin receptor gene is not linked with the polycythemia phenotype in a family with autosomal dominant primary polycythemia

Primary familial and congenital polycythemia (PFCP or familial erythrocytosis) is a rare hematological disorder with either autosomal-dominant inheritance or sporadic occurrence. It is characterized by an increased proliferation of erythroid precursors that results in an elevated red blood cell mass. In some of the PFCP families, the disease phenotype is associated with mutations of the erythropoietin receptor (EPOR). Mutations in other genes are likely to cause PFCP as well, but no evidence so far has been provided to support this contention. In this study, we present a family in which 6 of 15 family members were affected in three generations. We screened exon VIII of the EPOR gene for mutations and found a C-->T substitution (C6148T) in the maternal grandmother of the propositus. The mutated allele of the affected grandmother was not passed to either of her two affected children or to her one healthy child; thus, the disease phenotype was not linked to the C6148T mutation in this family. Further examination of the inheritance of the EPOR gene alleles and sequence analysis ruled out linkage between the disease phenotype and the EPOR gene; therefore, an abnormality in another gene must be the cause of PFCP in this particular family. In three affected family members tested, erythroid progenitors were hypersensitive to EPO. This in vitro behavior of the progenitors confirms the diagnosis of PFCP in these subjects. Moreover, it suggests a dominant lesion of an as-yet unidentified gene, either at the level of the EPOR-signaling pathway or another erythropoiesis-regulating pathway that may be responsible for enhanced proliferation of the erythroid progenitors.

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.

Two new EPO receptor mutations: truncated EPO receptors are most frequently associated with primary familial and congenital polycythemias

Primary polycythemias are caused by an acquired or inborn mutation affecting hematopoietic/erythroid progenitors that results in an abnormal response to hematopoietic cytokines. Primary familial and congenital polycythemia (PFCP; also known as familial erythrocytosis) is characterized by elevated red blood cell mass, low serum erythropoietin (EPO) level, normal oxygen affinity of hemoglobin, and typically autosomal dominant inheritance. In this study we screened for mutations in the cytoplasmic domain of the EPO receptor (EPOR; exons 7 and 8 of the EPOR gene) in 27 unrelated subjects with primary or unidentified polycythemia. Two new EPOR mutations were found, which lead to truncation of the EPOR similarly to previously described mutations in PFCP subjects. The first is a 7-bp deletion (del5985-5991) found in a Caucasian family from Ohio. The second mutation (5967insT) was found in a Caucasian family from the Czech Republic. In both cases the EPO dose responses of the erythroid progenitors of the affected subjects were examined to confirm the diagnosis of PFCP. In one of these families, the in vitro behavior of erythroid progenitors in serum-containing cultures without the addition of EPO mimicked the behavior of polycythemia vera progenitors; however, we show that antibodies against either EPO or the EPOR distinguish the in vitro growth abnormality of polycythemia vera erythroid progenitors from that seen in this particular PFCP family. We conclude that PFCP is a disorder that appears to be associated in some families with EPOR mutations. So far, most of the described EPOR mutations (6 out of 8) associated with PFCP result in an absence of the C-terminal negative regulatory domain of the receptor.

Trilineage extramedullary myeloid cell tumor in myelodysplastic syndrome

A patient with a 17-month history of myelodysplastic syndrome (refractory anemia with excess blasts that evolved into chronic myelomonocytic leukemia), which was treated with transfusions and erythropoietin, developed abdominal and inguinal lymphadenopathy. Biopsies of the abdominal nodes revealed virtual obliteration of the architecture by myeloid blasts admixed with maturing granulocytic, erythroid, and megakaryocytic precursors. The lymph node findings appeared to represent extramedullary dyshematopoiesis undergoing a tissue phase blast transformation. Four months later, the patient developed rising peripheral blast counts consistent with acute leukemia. Although the development of granulocytic sarcoma (also called extramedullary myeloid cell tumor) is well known to occur in patients with myelodysplastic syndromes, to our knowledge this is the first description of an extramedullary myeloid cell tumor associated with trilineage differentiation.

Rapid determination of clonality by detection of two closely-linked X chromosome exonic polymorphisms using allele-specific PCR

We reported two specific, reproducible, and quantitative clonality assays based on detection of exonic polymorphisms of the X chromosome genes p55 and G6PD using rtPCR-LDR. These assays are inconvenient for screening purposes. This study sought to develop a simple, reproducible assay, practical for screening genomic DNA samples for p55/G6PD genotypes, rapid clonality determination, and to determine the linkage relationship between these closely related loci. The salient feature of ASPCR is the performance of two PCR rounds. The first generates template; the second, using one aliquot of first-round products in two reaction tubes, each containing one allele-specific primer, detects each allele. ASPCR and rtPCR-LDR produced identical p55/G6PD results in 91 normal female genomic DNAs, and in 12 clonal hematopoietic disorder cDNAs, confirming assay validity. 209 female and 207 male genomic DNA samples were analyzed for p55/G6PD genotype by ASPCR; 60% of females were heterozygous at one or both loci. G6PD and p55 allelic frequencies were significantly different among African-American men and women, but were not significantly different among Caucasian men and women. These loci were in linkage equilibrium among African Americans, but not among Caucasians. ASPCR is a rapid, sensitive, and specific method for screening large numbers of genomic DNAs, and for rapid clonality determination.

Unusual patterns of exon skipping in Bruton tyrosine kinase are associated with mutations involving the intron 17 3' splice site

Seven individuals with the diagnosis of X-linked agammaglobulinemia were analyzed for mutations in Bruton tyrosine kinase (Btk) gene at both the cDNA transcript and genomic DNA levels. In addition, maternal carrier status was determined in six of the seven families by examining X chromosome-inactivation patterns for B cells in comparison with other types of blood cells. Three categories of mutations were identified: (1) three patients have missense mutations in either the pleckstrin or SH2 domains of Btk; (2) three patients exhibit mutations at or near intron/exon splice sites, two of which represent inherited mutations within the kinase domain; and (3) one patient has inherited a 2.5-kb deletion with the loss of a DNA segment encoding three exons of the kinase domain. Variation in the lengths of Btk transcripts was evident in two patients with splice-site mutations and in the patient with the DNA deletion. Sequences of the different cDNA transcripts from the patients with 3' splice-site mutations reveal complex patterns of exon skipping involving from one to four exons of the kinase domain. These findings implicate 3' splice sites of the penultimate exon in the recognition or processing of upstream exons.

Congenital polycythemia in Chuvashia

Familial and congenital polycythemia, not due to high oxygen affinity hemoglobin or reduced 2,3-diphosphoglycerate in erythrocytes, is common in the Chuvash population of the Russian Federation. Hundreds of individuals appear to be affected in an autosomal recessive pattern. We studied six polycythemic Chuvash patients <20 years of age from unrelated families and 12 first-degree family members. Hemoglobins were markedly elevated in the index subjects (mean +/- standard deviation [SD] of 22.6 +/- 1.4 g/dL), while platelet and white blood cell counts were normal. Although performed in only three of the index subjects, serum erythropoietin concentrations determined by both radioimmune and functional assays were significantly higher in polycythemic patients compared with first-degree family members with normal hemoglobin concentrations. Southern blot analysis of the Bgl 2 erythropoietin gene polymorphism showed that one polycythemic subject was a heterozygote, suggesting the absence of linkage of polycythemia with the erythropoietin gene, assuming autosomal recessive inheritance. Polymerase chain reaction (PCR) amplification of the GGAA and GA minisatellite polymorphic regions of the erythropoietin receptor gene showed no evidence of linkage of phenotype with this gene. We conclude that Chuvash polycythemia may represent a secondary form of familial and congenital polycythemia of as yet unknown etiology. This condition is the only endemic form of familial and congenital polycythemia described.

A high-frequency polymorphism of NADH-cytochrome b5 reductase in African-Americans

NADH-cytochrome b5 reductase (b5R) is a member of a flavoenzyme family of dehydrogenases-electron transferases that participates in the transfer of electrons from the NADH generated in glycolysis to cytochrome b5. b5R is involved in the steady-state reduction of methemoglobin to hemoglobin in erythrocytes and is also involved in lipid metabolism in other cell types. In a search for mutations of the b5R gene in two unrelated African-American families, a high-frequency polymorphism was detected in the propositi from both families, as well as unrelated normal controls, consisting of a C-to-G transversion in exon 5 that changes threonine to serine at codon 116 (T116S). This is the first polymorphism found in the b5R gene. Using allele-specific PCR on the two propositi, their family members, and unselected populations of African-American, Caucasian, Asian, Indo-Aryan, and Arabic individuals, the C/G polymorphism was found in 26 of 112. African-American chromosomes (allele frequency = 0.23), but not in 108 Caucasian, 46 Asian, 44 Indo-Aryan, or 14 Arabic chromosomes. In preliminary studies, this polymorphism did not correlate with b5R enzyme activity or cause any disease phenotype. It remains to be determined whether this African-specific polymorphism that apparently originated recently in human evolution provides any special survival advantage.

Characterization of 13 novel band 3 gene defects in hereditary spherocytosis with band 3 deficiency

Hereditary spherocytosis (HS) is a common hemolytic anemia of variable clinical expression. Pathogenesis of HS has been associated with defects of several red cell membrane proteins including erythroid band 3. We have studied erythrocyte membrane proteins in 166 families with autosomal dominant HS. We have detected relative deficiency of band 3 in 38 kindred (23%). Band 3 deficiency was invariably associated with mild autosomal dominant spherocytosis and with the presence of pincered red cells in the peripheral blood smears of unsplenectomized patients. We hypothesized that this phenotype is caused by band 3 gene defects. Therefore, we screened band 3 DNA from these 38 kindred for single strand conformational polymorphisms (SSCP). In addition to five mutations detected previously by SSCP screening of cDNA, we detected 13 new band 3 gene mutations in 14 kindred coinherited with HS. These novel mutations consisted of two distinct subsets. The first subset included seven nonsense and frameshift mutations that were all associated with the absence of the mutant mRNA allele from reticulocyte RNA, implicating decreased production and/or stability of mutant mRNA as the cause of decreased band 3 synthesis. The second group included five substitutions of highly conserved amino acids and one in-frame deletion. These six mutations were associated with the presence of comparable levels of normal and mutant band 3 mRNA. We suggest that these mutations interfere with band 3 biosynthesis leading thus to the decreased accumulation of the mutant band 3 allele in the plasma membrane.

"Benign erythrocytosis" and other familial and congenital polycythemias

The term familial and congenital polycythemia encompasses a heterogeneous group of disorders with the common characteristic of an absolute increased red cell mass since birth and/or similar phenotype also present in relatives. In the last 2 decades the differential diagnosis between primary and secondary familial polycythemias became more physiologically relevant as new sensitive techniques, such as accurate measurements of serum erythropoietin (S-EPO) concentration by radioimmunoassay (RIA) or ELISA, and assessment of growth of erythroid progenitor cells in vitro became available. Consequently, correct classification of many older previous reports of familial polycythemias is difficult. While familial secondary polycythemias due to high oxygen affinity hemoglobin mutants are not infrequent and have been well delineated in terms of molecular pathophysiology and phenotype during the last 3 decades, those secondary familial polycythemias due to 2,3 DPG deficiency are very rare. Familial and congenital polycythemias with increased EPO concentration and normal arterial oxygen saturation and oxygen dissociation kinetics represent an intriguing group of disorders wherein the molecular lesions remain obscure; however, in some instances a possibility of abnormal oxygen sensing pathway involving hypoxia inducible factor-1 (HIF-1) open an intriguing yet unexplored area of hematology and biology. In contrast the primary familial and congenital polycythemia (PFCP) has been only recently recognized (the first report published in 1977). Various designations have been used in the past to describe PFCP, a rare clinical syndrome, including: benign familial erythrocytosis, polycythemia vera of childhood, primary polycythemia, pure erythrocytosis, etc. Some of these terms stressed the relatively benign, non-progressive course of the disease with a normal lifespan of affected subjects; however, the apparent benignity of some of these disorders has been questioned. These disorders are familial and/or congenital, and the clinical and laboratory evidence of secondary polycythemias must be excluded. Only about 2 dozen familial and sporadic cases with PFCP have been reported. However, the mutations of erythropoietin receptor (EPOR) found in some of families with PFCP represent the only defined molecular defect of primary polycythemic phenotypes. All reported PFCP associated EPOR mutations result in truncation of its intracytoplasmic C-terminal domain which negatively regulates the EPO/EPOR signal transduction pathway. Subjects with these mutations have decreased or normal S-EPO and increased sensitivity of erythroid progenitor cells to low EPO concentrations in in vitro assays. Mutations of other genes involved in post EPOR signaling pathway such as JAK-2, HCP and STAT 5 may also play a causative role in pathogenesis of some of PFCP families where mutation of EPOR was not found.

A novel mutation found in the 3' domain of NADH-cytochrome B5 reductase in an African-American family with type I congenital methemoglobinemia

Congenital methemoglobinemia caused by an erythrocytic deficiency of cytochrome b5 reductase (b5R; type I) in African-American individuals was first reported by this laboratory. The rarity of this observation is possibly due to the difficulty detecting cyanosis that is masked by naturally occurring dark skin pigment. Since previous biochemical studies on the African-American family with variant enzyme b5R-Shreveport showed enzyme instability, we focused on molecular analysis of its transcript. The transcript size was the same as that of a normal control. The nucleotide sequence of both normal and variant transcripts were examined by directly sequencing reverse transcriptase-polymerase chain reaction (RT-PCR) product. The propositus was found to be homozygous for a G to A transition at codon 212 in exon 8, changing a glutamate to a lysine (E212K). In addition, a C to G transversion was found at codon 116 in exon 5, changing a threonine to a serine (T116S). Using allele-specific PCR, we determined that E212K was found only in the propositus and her heterozygous mother. Furthermore, E212K is predicted to disrupt an alpha-helix peptide structure of b5R, suggesting that this is likely the disease-causing mutation. In contrast, T116S was found to be a high-frequency polymorphism specific for the African-American population. The E212K mutation is uniquely present in the 3' end of the b5R gene (exon 8), which differs from those b5R mutations found among Japanese subjects (exons 3 and 5) and in an Italian subject (exon 4) and, thus, further contributes to our understanding of the structure/function relationship of this housekeeping enzyme.

Sickle cell acute chest syndrome associated with parvovirus B19 infection: case series and review

Acute Chest Syndrome (ACS) continues to be a major source of morbidity and mortality among patients with sickle cell disease. It is characterized by the presence of pleuritic chest pain, fever, raises on lung auscultation, and pulmonary infiltrates on chest x-ray [Castro et all: Blood 84:643-649]. The pathophysiology of this disorder remains poorly understood leading to the descriptive term "Acute Chest Syndrome" designated by Charache et al. [Arch Intern Med 139:67-69, 1979]. Typical bacterial pathogens are seldom isolated in adults, although they play a significant role in the pathogenesis of this entity in children. Until recently, the technology to accurately study viral infection as a precipitating cause of ACS has been unavailable. Parvovirus B19 is being increasingly recognized as an important human pathogen, and has been established as the cause of transient "aplastic crisis" in patients with sickle cell diseases [Saarien et al: Blood 67:-11411-11417, 1986; Young: Sem Hematol 25:159-172, 1988]. We present three patients with hemoglobin SC variant of sickle cell disease who developed ACS in association with acute parvovirus B19 infection, one of which died of respiratory failure. Parvovirus B19 infection was established by polymerase chain reaction for parvovirus B19 DNA, and the presence of parvovirus B19 specific IgM antibodies. These cases suggest that parvovirus B19 may be associated with more than self-limited illness in patients with sickle cell disease, and that this ubiquitous virus may merit further study as a precipitating cause of ACS.

Stable transduction of recombinant adeno-associated virus into hematopoietic stem cells from normal and sickle cell patients

Stable introduction of genes into human hematopoietic stem cells with self-renewing potential is a necessary requirement for gene therapy strategies. We have developed an adeno-associated virus (AAV) vector and a partial packaging cell line that produces recombinant AAV at a titer of 10(8) transducing particles per milliliter. A high-titer viral stock containing the CMV/lacZ gene was used to transfer lacZ sequences into CD34+ Lin-Thy+ hematopoietic stem cells purified from normal and homozygous sickle cell patients. After infection, the cells were cultured in two ways. In the first set of experiments, the cell were expanded 300-fold in liquid culture for 21 days and plated in methylcellulose. Burst-forming units-erythroid (BFU-E) and colony-forming units-granulocyte/macrophage (CFU-GM) were then analyzed for lacZ sequences. In the second set of experiments, infected cells were cultured for 6 weeks under conditions that maintain long-term culture-initiating cells (LTC-IC). Progenitors were plated in methylcellulose, and BFU-E were analyzed for lacZ DNA. Stable transduction of lacZ sequences was observed in 25% of the colonies in both sets of experiments. These results demonstrate for the first time that LTC-IC can be transduced stably with a recombinant AAV vector. The results suggest that AAV may be a useful vector for genetic therapy of sickle cell disease and other hematopoietic disorders.

Clonal stability of blood cell lineages indicated by X-chromosomal transcriptional polymorphism

The idea that stem cells oscillate between a state of activity and dormancy, thereby giving rise to differentiating progeny either randomly or in orderly clonal succession, has important implications for understanding normal hematopoiesis and blood cell dyscrasias. The degree of clonal stability in individuals also has practical implications for the evaluation of clonal lymphomyeloproliferative diseases. To evaluate the clonality pattern of the different types of blood cells as a function of time we have validated the applicability, sensitivity, and reproducibility of a thermostable ligase reaction to detect transcripts of the G6PD allele on the active X-chromosome in normal heterozygous females. While the ratio of the two X-chromosome-derived allelic transcripts varied widely among hemopoietic tissues in a given individual, this allelic ratio was virtually identical in all types of mature myeloid and lymphoid cells. Longitudinal studies indicated constancy of the G6PD allelic ratio in blood cells over a 912-d period of observation in healthy females. The individual variability observed in this allelic ratio suggests that the progeny of a relatively small number of original embryonic hemopoietic stem cells, approximately eight, contribute to the sustained production of all types of blood cells in healthy individuals.

Plasma/serum levels of flt3 ligand are low in normal individuals and highly elevated in patients with Fanconi anemia and acquired aplastic anemia

The flt3 ligand is a growth factor that stimulates the proliferation of hematopoietic progenitor and stem cells. We established a sensitive enzyme-linked immunosorbent assay (ELISA) to measure the concentration of flt3 ligand in plasma or serum from normal individuals, as well as in patients with hematopoietic disorders. Concentrations of flt3 ligand in plasma or serum from normal individuals were quite low: only 12% (7 of 60) of normal individuals had flt3 ligand levels above 100 pg/mL (the limit of detection). In contrast, 86% (19 of 22) of samples from patients with Fanconi anemia and 100% (eight of eight) of samples from patients with acquired aplastic anemia had plasma or serum levels above 100 pg/mL. Mean plasma or serum concentrations (calculated by assigning a value of 0 pg/mL to any sample reading below the level of detection) were as follows: normal volunteers, 14 pg/mL; patients with Fanconi anemia, 1,331 pg/mL; and patients with acquired aplastic anemia, 460 pg/mL. Concentrations of flt3 ligand in blood are, therefore, specifically elevated to a level that may be physiologically relevant in hematopoietic disorders with a suspected stem cell component. The elevated flt3 ligand concentrations in these individuals may be part of a compensatory hematopoietic response to boost the level of progenitor cells.

A novel clonality assay based on transcriptional polymorphism of X chromosome gene p55

The clonal nature of cell populations in malignant and myeloproliferative disorders can be determined in female subjects by random-inactivation assays of the X chromosome. Assays utilizing either expression of the G6PD isozymes or DNA-methylation differences between the active and inactive X chromosomes have significant short-comings. We developed a test based on nucleotide #1311 exonic polymorphism of G6PD that allows detection of clonality by determining the transcriptional polymorphism of the active X chromosome using a reverse transcription-polymerase chain reaction-ligase detection reaction (rtPCR-LDR). Since only 18% of females in the United States are informative (heterozygous) for this chromosome with this assay, we searched for other exonic X chromosome polymorphisms. Concentrating on a discrepancy (G or T at cDNA #358) of published sequences of ubiquitously expressed gene, palmitoylated membrane protein for p55, we confirmed this conservative polymorphism at the cDNA level. To detect the genotype of this polymorphism, we established the intron/exon boundary of the first 5' exons and determined the whole sequence of the second intron. We found that the polymorphic site is at the third exon, nine nucleotides downstream from the 294-bp second intron. This close proximity to the intron necessitated the development of a separate PCR-LDR reaction with oligonucleotides for cDNA and genomic DNA. Furthermore, we determined that the p55 gene is subject to X chromosome inactivation. Based on these observations, we developed a novel p55 clonality assay that is reproducible, quantitative, and very sensitive. Screening of 37 randomly selected healthy females of Caucasian, African-American, and Asian origin revealed that 38% of females are informative when this assay is used. We demonstrate a multiple crossover between the G6PD and the p55 polymorphisms (separated by approximately 200 kb), suggesting that these two polymorphisms are in linkage disequilibrium; thus, approximately 50% of female subjects are informative for clonality studies using the two assays.

Human genome--chromosome no. 19

Chromosome 19 is short but has higher relative density of genes than other chromosomes. Increasing number of the genes coding for proteins implicated in the pathogenesis of various human diseases have been mapped on chromosome 19. Mutations of low density lipoprotein receptor (LDL-R) result in one of the most frequent mendelian inherited disorder-familial hypercholesterolemia. Mutations of insulin receptor (INSR) are causative for rare syndromes of insulin resistance and some of non insulin dependent diabetes mellitus (NIDDM). Erythropoietin receptor (EPOR) mutations are causative for rare primary familial and congenital polycythemias (PFCP). Defects of one of the largest gene in the human genome RYR 1 (ryanodine receptor gene) (> 240 kb in size) accounts for majority of malignant hyperthermia (MH) and central core disease (CCD). All these disorders represent group of receptor diseases. The amplification of GCT trinucleotide repeats in myotonic dystrophy protein kinase (DMPK) gene is causative for myotonic dystrophy (DM) and represents a new class of human gene mutations: trinucleotide repeat mutations. Apolipoprotein E (APOE) locus plays a role in pathogenesis of the late onset familial Alzheimer's disease. Translocation of EA2 gene which encodes two helix-loop-helix (HLH) transcription proteins and its fusion with PBXI or hepatic leukemia factor (HLF) leads to the leukemogenesis in subgroup of ALL. Interestingly adeno-associated virus (AAV), currently widely used as vector for gene therapy has unique capability of specific integration into human chromosome 19q.

Primary familial polycythemia: a frameshift mutation in the erythropoietin receptor gene and increased sensitivity of erythroid progenitors to erythropoietin

Primary familial and congenital polycythemia (PFCP) is characterized by erythrocytosis with normal arterial PO2, blood P50, and serum erythropoietin (EPO) levels. In two PFCP families EPO receptor (EPOR) polymorphisms cosegregated with PFCP. A heterozygous insertion of G at EPOR nucleotide 5975 was identified in genomic DNA from polycythemic members of family no. 2. 5974insG shifts the reading frame at codon 430, predicting amino acid substitutions and truncation of the last 64 amino acids. Wild-type and mutant EPOR transcripts were detected in erythroid progenitors from affected individuals. Burst-forming units-erythroid from patients exhibited increased colony size and sensitivity to EPO. Transfected Ba/F3 cells expressing EPOR 5974insG exhibited increased EPO sensitivity compared with cells expressing wild-type EPOR. The functional effect of this EPOR mutation was directly compared with the other C-terminal mutations reported in unrelated PFCP families by expression in Ba/F3 cells. The transfected cells with another primary polycythemia associated EPOR mutant construct (G6002A) also exhibited increased sensitivity to EPO.

Hb Bibba or alpha 2 136(H19)Leu-->Pro beta 2 in a Caucasian family from Alabama

Several members of a large Caucasian family who presented with a congenital Heinz body hemolytic anemia were found to be carriers of the unstable Hb Bibba or alpha 2 136(H19)Leu-->Pro beta 2. Identification by protein analysis was hampered by the instability of the variant which complicated its isolation from shipped blood samples. Moreover, the detection of the CTG-->CCG mutation at codon 136 of the alpha 2 gene required the substitution of dGTP by dITP during the DNA sequencing process to prevent the occurrence of secondary structures and compressions in the sequencing gel. The first Hb Bibba heterozygote, characterized in 1968 (1), is believed to be a member of this family. The clinical expression of the disease is surprisingly variable.

Primary polycythemias

In this review, primary polycythemic states are discussed in the context of other polycythemic disorders. Primary polycythemias result from an acquired or inborn mutation affecting hematopoietic and erythroid cells. The best-known type of primary polycythemia is polycythemia vera, which is caused by an acquired somatic mutation of a hematopoietic stem cell with exaggerated myeloid proliferation; the molecular events leading to this disease are not understood. In contrast, primary familial and congenital polycythemias result from inborn mutation affecting hematopoietic and erythroid cells. The molecular mechanisms causing primary familial and congenital polycythemias may be different in different families; some have already been defined. Progress in defining the molecular defect of primary polycythemias and the better understanding of altered signal transduction leading to excessive erythrocyte production should provide important insights into the pathogenesis of primary polycythemias, other leukemic disorders, as well as normal hematopoiesis. Such information should contribute to better understanding of and more effective therapeutic interventions in myeloproliferative and leukemic disorders.

A TaqI polymorphism in the human erythroid beta spectrin gene

Human erythroid spectrin consists of an alpha beta heterodimer. Abnormalities of spectrin are a common cause of hereditary haemolytic anaemias such as hereditary elliptocytosis (HE) and hereditary spherocytosis (HS). To identify the spectrin gene mutation one needs initially to establish which of the spectrin subunits is defective. For this purpose, the beta spectrin restriction fragment length polymorphism (RFLP) we describe here will be useful in linkage analysis. The elucidation of an Ala-->Gly beta spectrin gene mutation in a family with HE, highlights the importance of this TaqI polymorphism in establishing linkage.

Clonality in juvenile chronic myelogenous leukemia

Juvenile chronic myelogenous leukemia (JCML) is a myeloproliferative disease in which morbidity and mortality are primarily caused by nonhematopoietic organ failure from myelomonocytic infiltration or by failure of the normal bone marrow. Morphologic evidence of maturation arrest, karyotypic abnormalities, and progression to blast crisis are infrequent events. Viral infections and other reactive processes can initially mimic the clinical course of JCML, creating diagnostic problems. Because of the rarity of JCML and technical limitations, formal clonality studies have not been reported previously. Nine female JCML patients were identified by clinical criteria, characteristic 'spontaneous' in vitro cell growth, and negative cultures and titers for various viral agents. Peripheral blood and bone marrow samples were obtained at the time of diagnosis for cell separation and RNA and DNA isolation. To assess clonality, X-chromosome inactivation patterns were evaluated using three different, recently developed polymerase chain reaction-based clonality assays. All nine female JCML patients showed evidence for monoclonal origin of mononuclear cells at the time of diagnosis. Cell separation studies further traced the monoclonal origin back to at least the most primitive myeloid progenitor cell. Reversion to a polyclonal state was demonstrated after bone marrow transplant and also in one patient following treatment with 13-cis retinoic acid. This demonstration of clonality in JCML delineates it from the reactive processes and provides a basis for molecular genetic strategies to identify causally associated mutations.

Clonal hematopoiesis and acquired thalassemia in common variable immunodeficiency

BACKGROUND

Common variable immunodeficiency (CVID) is defined by hypogammaglobulinemia and increased susceptibility to infections. The gene defect responsible for CVID remains unknown.

METHODS

During the course of their CVID disease, a female and three male patients developed microcytic anemia. The investigation of this anemia forms the basis for this report.

RESULTS

Reticulocyte globin chain synthesis studies revealed the abnormal alpha/beta ratios that are pathognomonic of thalassemia. Through transcriptional analysis of the glucose-6-phosphate-dehydrogenase (G6PD) locus of the active X-chromosome in blood cells, we determined that the female patient has clonal reticulocytes, platelets, granulocytes, and B and T lymphocytes.

CONCLUSIONS

The simultaneous presence of globin synthesis abnormalities and panhypogammaglobulinemia suggests that a common insult at the stem cell level could contribute to the development of CVID and acquired thalassemia.