Chronic myeloproliferative disorders--introduction.
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Alleles↗
Biomedical subjects
Josef T Prchal
Publications and source records attributed to Josef T Prchal.
Divalent metal transporter 1.
In the last few years, the field of iron metabolism has exploded with the discovery of many new proteins including ferroportin, hephaestin, hepcidin, duodenal cytochrome b and the topic of this review, divalent metal ion transporter 1 (DMT1). DMT1 functions in transport of ferrous iron, and some, but not all divalent metal ions across the plasma membrane and/or out of the endosomal compartment. DMT1 mRNA has been found in every cell type in which it has been sought and its structure is highly conserved in evolution with similar proteins expressed in plants, insects, microorganisms and vertebrate animals. Rodents with defects in iron absorption and utilization were identified long before it was determined that the defect was due to a single nucleotide mutation in DMT1. Study of these animals reveals that transport of iron and other divalent metal ions by DMT1 is pH dependent, but the exact manner in which pH exerts its effect is unknown. The structure of the DMT1 gene is complex. Alternative usage of 3' exons, results in forms with and without iron responsive elements (IREs), while alternative usage of 5' exons and less well defined products of alternative splicing results in an array of isoforms with incompletely defined function. Expression of some isoforms is tissue specific and appears to affect subcellular targeting of the protein. At least one signal for DMT1 expression appears to be intracellular iron status, however, other, as yet undefined signals may also contribute to DMT1 expression. Interestingly, DMT1 function may differ subtly between humans and other animals; the spontaneous DMT1 mutation found in mice and rats appears to limit iron uptake in the intestine and iron utilization in red cell precursors, whereas the only known human mutation has its primary effect on iron utilization by erythroid cells. The importance of DMT1 function at the level of the whole organism and the individual cell and mechanisms of its regulation on a molecular scale are only beginning to be understood; an appreciation of these process will lead to an understanding of the role of iron in various cellular processes and improved treatments for both anemia and iron-overload.
Alternative Splicing↗
Polycythemia vera and other primary polycythemias.
PURPOSE OF REVIEW: Diagnosis and therapy of polycythemia vera are controversial since the molecular basis of polycythemia vera remains unknown. Distinguishing between polycythemia vera and other polycythemic disorders can be very challenging. The purpose of this review is to discuss the recent progress in this area and critically review the published data in context of our knowledge of other polycythemic disorders. RECENT FINDINGS: Erythropoietin is the principal regulator of regulator of erythropoiesis; its production is regulated by the degree of hypoxia. Our knowledge of cellular responses to hypoxia has recently exploded and led to the elucidation of the molecular basis of a polycythemia caused by augmentation of hypoxic sensing, Chuvash polycythemia. Similar progress in understanding the molecular basis of polycythemia vera has been elusive. A simple, readily available laboratory test to establish a diagnosis of polycythemia vera would be highly desirable; however, none exists. The value of quantization of neutrophil PRV-1 mRNA, platelet c-mpl expression, in vitro assays of erythroid progenitor cells, serum erythropoietin levels, establishing clonality in female subjects using assays employing X-chromosome-based polymorphism assays, and the progress in the chromosomal location of the gene is discussed. Integration of this information underlies the complexity of the molecular biology of polycythemia vera and indicates likely interaction of multiple genetic events in the genesis of polycythemia vera. SUMMARY: The existence of family clustering of PV may facilitate the search for PV molecular basis. Only collaborative interaction of clinical researchers and laboratory scientists will lead to meaningful progress in determining the molecular basis of PV.
Diagnosis, Differential↗
Red cell enzymes.
Mutations leading to red cell enzyme deficiencies can be associated with diverse phenotypes that range from hemolytic anemia, methemoglobinemia, polycythemia, and neurological and developmental abnormalities. While most of these mutations occur sporadically, some such as common glucose-6-phosphate dehydrogenase (G6PD) mutants are endemic and rarely cause disease. Common G6PD mutants likely reached their prevalence because they provide some protection against severe malarial complications. In this review G6PD, pyruvate kinase, 5' nucleotidase, and cytochrome b5 reductase deficiencies will be discussed in greater detail. Limitations of commonly used screening tests for detection of these disorders will also be emphasized, as well as emerging knowledge about non-enzymatic function of the glycolytic enzymes.
5'-Nucleotidase↗
Genetic association analysis of chronic mountain sickness in an Andean high-altitude population.
BACKGROUND AND OBJECTIVES: Millions of people live above an altitude of 2,500 m and are at risk of chronic mountain sickness (CMS), a disorder of excessive red cell and hemoglobin production. Preferential ethnic backgrounds, familial character, and heritability studies have suggested that genetic factors make a major contribution to the pathogenesis of CMS, thus our goals were to exploit a probable founder or population admixture effect in the Andean population to determine the genetic determinants of the extreme erythropoietic responses and CMS. DESIGN AND METHODS: The association of functional candidate genes with severe polycythemia was studied in Andean subjects from Cerro de Pasco, Peru (altitude 4,438 m). We used microsatellites linked to candidate genes known to be involved in hypoxia sensing and erythropoiesis: erythropoietin, erythropoietin-receptor, hypoxia-inducible factor-1a, von Hippel-Lindau, prolyl hydroxylase domain containing 1, 2, 3, and phosphatase and tensin homolog deleted on chromosome ten. Analysis of co-variance (ANCOVA) was used to test the effect of genotypes on hemoglobin values. RESULTS: Case-control comparisons revealed no significant difference in genotype and allele frequencies at any marker. Initial analysis, with age as a covariate, showed a possible association between PHD3 (marker D14S1049) and severe polycythemia (p=0.05). After the inclusion of alternative co-variates and after adjusting for multiple comparisons, no p values could be considered statistically significant. INTERPRETATION AND CONCLUSIONS: Our study does not find evidence of associations between the polymorphisms linked to the candidate genes and severe polycythemia; this does not, however, exclude that variations in these genes contribute to polycythemia and possibly CMS.
Adult↗
Congenital polycythemias/erythrocytoses.
Congenital polycythemias may result from inherited defects in hypoxia sensing, from inherited intrinsic defects in red blood cell precursors, or from inherited conditions that cause low tissue oxygen tension and secondary polycythemia. Conditions of defective hypoxia sensing feature inappropriately normal or elevated serum erythropoietin (Epo) concentrations in the setting of normoxia and erythrocytosis. They are often due to homozygous or compound heterozygous germline mutations in the von Hippel Lindau tumor suppressor gene (VHL) but without increased incidence of tumors. Affected persons have a high risk of arterial thrombosis and early mortality. The molecular biology of rare polycythemic patients with a single mutated VHL allele remains obscure. Primary congenital and familial polycythemias are characterized by low Epo levels and increased erythroid precursor responsiveness to Epo. They are often due to heterozygous gain-of function mutations in the gene for erythropoietin receptor (EPOR). Secondary congenital polycythemias have low tissue oxygen tension due to hemoglobins with high affinity for oxygen, low erythrocyte 2,3 biphosphoglycerate levels, methemoglobinemia or cyanotic heart or lung disease. Whether phlebotomy therapy reduces complications and prolongs survival in congenital polycythemia is not known.
Homeostasis↗
Congenital polycythemia with homozygous and heterozygous mutations of von Hippel-Lindau gene: five new Caucasian patients.
We report on five Caucasian patients with congenital polycythemia and mutations of the von Hipple-Lindau (VHL) gene: a compound heterozygote for the novel exon 1 (VHL 235C->T) and previously reported VHL 562C->G mutations; three homozygotes for Chuvash VHL 598C->T mutation; and a heterozygote for VHL 523->G mutation who also has ataxia-telangiectasia; a rare autosomal disease of childhood onset.
Adolescent↗
Recent advances in the molecular biology of congenital polycythemias and polycythemia vera.
This review will focus on the molecular basis of certain polycythemic disorders. Primary polycythemias are characterized by acquired somatic or inherited germ-line mutations expressed within hematopoietic progenitors that cause increased accumulation of red blood cells. Polycythemia vera (PV), an acquired condition, is the most common primary polycythemia; although some progress has been made in the understanding of PV, its molecular basis remains unknown. In contrast, recent advances in delineating the molecular defects of some inherited polycythemias have greatly furthered our knowledge of the regulation of erythropoiesis and hypoxia sensing; however, more work needs to be done.
Adult↗
Heterogeneity of the molecular biology of methemoglobinemia: a study of eight consecutive patients.
Congenital methemoglobinemia can be caused by mutations involving five different genes. We studied the etiology and molecular biology of eight consecutive patients with methemoglobinemia. Four had b5R mutations; two were novel. A novel intronic mutation caused markedly reduced mRNA resulting in type II methemoglobinemia. Three patients had acquired methemoglobinemia without any b5R mutations.
Amino Acid Substitution↗
The constitutive mobilization of bone marrow-repopulating cells into the peripheral blood in idiopathic myelofibrosis.
Idiopathic myelofibrosis (IM) is characterized by the constitutive mobilization of CD34(+) cells. IM peripheral blood (PB) CD34(+) cells had a reduced cloning efficiency and a lower frequency of cobblestone areas compared with normal granulocyte colony-stimulating factor (G-CSF)-mobilized PB CD34(+) cells. IM CD34(+) cells engrafted nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, demonstrating that they contain bone marrow (BM)-repopulating cells. G-CSF-mobilized CD34(+) cells produced multiple hematopoietic lineages within the NOD/SCID mice with a predominance of CD19(+) cells. By contrast, IM CD34(+) cells produced predominantly CD33(+) cells, increased numbers of CD41(+) cells, but fewer CD19(+) cells. Transcriptional clonality assays of the engrafted human IM cells demonstrated their clonal origin. CD34(+) cells from one patient isolated prior to leukemic transformation were capable of generating acute leukemia in NOD/SCID mice. The engrafted human cells exhibited the same abnormal karyotype as primary cells in a portion of the population. These findings demonstrate that BM-repopulating cells and more differentiated progenitor cells are constitutively mobilized into the PB in IM, and that their differentiation program is abnormal. In addition, the NOD/SCID model may be useful in gaining an understanding of the events occurring during the transition of IM to acute leukemia.
Acute Disease↗
Identification of a human mutation of DMT1 in a patient with microcytic anemia and iron overload.
Divalent metal transporter 1 (DMT1) is a transmembrane protein crucial for duodenal iron absorption and erythroid iron transport. DMT1 function has been elucidated largely in studies of the mk mouse and the Belgrade rat, which have an identical single nucleotide mutation of this gene that affects protein processing, stability, and function. These animals exhibit hypochromic microcytic anemia due to impaired intestinal iron absorption, and defective iron utilization in red cell precursors. We report here the first human mutation of DMT1 identified in a female with severe hypochromic microcytic anemia and iron overload. This homozygous mutation in the ultimate nucleotide of exon 12 codes for a conservative E399D amino acid substitution; however, its pre-dominant effect is preferential skipping of exon 12 during processing of pre-messenger RNA (mRNA). The lack of full-length mRNA would predict deficient iron absorption in the intestine and deficient iron utilization in erythroid precursors; however, unlike the animal models of DMT1 mutation, the patient is iron overloaded. This does not appear to be due to up-regulation of total DMT1 mRNA. DMT1 protein is easily detectable by immunoblotting in the patient's duodenum, but it is unclear whether the protein is properly processed or targeted.
Anemia, Hypochromic↗
Regulation of ferrochelatase gene expression by hypoxia.
Ferrochelatase (FECH), the last enzyme of the heme biosynthetic pathway, catalyzes the insertion of iron into protoporphyrin to form heme. This pathway provides heme for hemoglobin and other essential hemoproteins. The regulatory role of oxygen in the pathway has not been clearly established. In this study, we examined whether FECH gene expression is upregulated during hypoxia by a mechanism which involves the hypoxia-inducible factor 1 (HIF-1). Two HIF-1 binding motifs were identified within the -150 bp FECH minimal promoter sequence. Exposure of HEL, K562, and Hep-G2 cells to hypoxia for 18 hours resulted in a significant increase in FECH mRNA expression (p < 0.05). Hypoxia also transactivated the minimal promoter for the FECH gene in the cells. Transient co-expression of wild-type HIF-1alpha or a dominant negative HIF-1alpha with the FECH minimal promoter luciferase construct stimulated or blocked FECH promoter activity, respectively. Expression of the von Hippel-Lindau (VHL) tumor suppressor factor blocked the expression of both FECH mRNA and HIF-1alpha protein during normoxic culture of renal carcinoma cell line (RCC4). The results suggest that the FECH gene is a target for HIF-1 during hypoxia.
Base Sequence↗
Dysregulation of ferroportin 1 interferes with spleen organogenesis in polycythaemia mice.
Regulatory interferences at the iron transporter ferroportin 1 (Fpn1) cause transient defects in iron homeostasis and erythropoiesis in polycythaemia (Pcm) mutant mice. The present study identified decreased Fpn1 expression in placental syncytiotrophoblast cells at late gestation as the mechanism of neonatal iron deficiency in Pcm mutants. Tissue specificity of embryonic Fpn1 dysregulation was evident from concomitant decreases in Fpn1 mRNA and protein expression in placenta and liver, as opposed to upregulation of Fpn1 protein despite decreased transcript levels in spleen, implicating post-transcriptional regulation of Fpn1. Dysregulation of Fpn1 and decreased iron levels in Pcm mutant spleens correlated with apoptotic cell death in the stroma, resulting in a semidominant spleen regression. At 7 weeks of age, a transient increase in spleen size in Pcm heterozygotes reflected a transient erythropoietin-mediated polycythemia. Structurally, Pcm mutant spleens displayed a severe defect in red pulp formation, including disruption of the sinusoidal endothelium, as well as discrete defects in white pulp organization during postnatal development. Reduced functional competence of the Pcm mutant spleen was manifested by an impaired response to chemically induced hemolytic anemia. Thus, aberrant Fpn1 regulation and iron homeostasis interferes with development of the spleen stroma during embryogenesis, resulting in a novel defect in spleen architecture postnatally.
Animals↗
Severe hypochromic microcytic anemia caused by a congenital defect of the iron transport pathway in erythroid cells.
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Adult↗
Congenital disorder of oxygen sensing: association of the homozygous Chuvash polycythemia VHL mutation with thrombosis and vascular abnormalities but not tumors.
Adaptation to hypoxia is critical for survival and regulates multiple processes, including erythropoiesis and vasculogenesis. Chuvash polycythemia is a hypoxia-sensing disorder characterized by homozygous mutation (598C>T) of von Hippel-Lindau gene (VHL), a negative regulator of hypoxia sensing. Although endemic to the Chuvash population of Russia, this mutation occurs worldwide and originates from a single ancient event. That VHL 598C>T homozygosity causes elevated normoxic levels of the transcription factor hypoxia inducible factor-1alpha (HIF-1alpha), serum erythropoietin and hemoglobin is known, but the disease phenotype has not been documented in a controlled manner. In this matched cohort study, VHL 598C>T homozygosity was associated with vertebral hemangiomas, varicose veins, lower blood pressures, and elevated serum vascular endothelial growth factor (VEGF) concentrations (P <.0005), as well as premature mortality related to cerebral vascular events and peripheral thrombosis. Spinocerebellar hemangioblastomas, renal carcinomas, and pheochromocytomas typical of classical VHL syndrome were not found, suggesting that overexpression of HIF-1alpha and VEGF is not sufficient for tumorigenesis. Although hemoglobin-adjusted serum erythropoietin concentrations were approximately 10-fold higher in VHL 598C>T homozygotes than in controls, erythropoietin response to hypoxia was identical. Thus, Chuvash polycythemia is a distinct VHL syndrome manifested by thrombosis, vascular abnormalities, and intact hypoxic regulation despite increased basal expression of hypoxia-regulated genes.
Adaptation, Physiological↗
Angiotensin II receptor type 1 expression in erythroid progenitors: Implications for the pathogenesis of postrenal transplant erythrocytosis.
Under normal physiological conditions red blood cell production is controlled primarily by erythropoietin, although multiple additional stimulatory factors are likely to be involved. One of these factors, angiotensin II, can modulate erythropoiesis directly via its type 1 receptor, as well as indirectly through multiple secondary mediators. We propose that angiotensin II exerts its stimulatory effect during the early stages of erythropoiesis, and that this effect serves as an important compensatory mechanism if erythropoietin production is chronically inadequate. We speculate that if this compensatory stimulation continues to be abnormally high after restoration of erythropoietin production following renal transplantation, erythrocytosis ensues.
Erythroid Precursor Cells↗
Oxygen-dependent regulation of erythropoiesis.
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Erythroid Precursor Cells↗
Erythropoietin and erythropoiesis: polycythemias due to disruption of oxygen homeostasis.
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2,3-Diphosphoglycerate↗