Primary polycythemias

Familial polycythemia caused by a novel mutation in the beta globin gene: essential role of P50 in evaluation of familial polycythemia

Two polycythemic subjects from a family with multiple polycythemic subjects were evaluated. Estimation of oxygen affinity of Hb from venous blood gas parameters (P50) revealed low P50 suggesting a high affinity Hb variant. Further work up, which included beta globin gene sequencing, revealed a novel mutation changing a codon to the previously reported high affinity Hb - Hb Johnstown (beta 109 Val->Leu). Polycythemic subjects with high affinity Hb variant are asymptomatic with normal life expectancy. Their differentiation from polycythemia vera (PV) is crucial to avoid therapy which is otherwise reserved for PV patients. We provide an electronic version (in Microsoft excel program) of a previously reported mathematical formula for rapid calculation of P50 from venous blood gases. Estimation of P50 is an essential initial step in the evaluation of a subject with personal and family history of polycythemia.

Familial and congenital polycythemias: a diagnostic approach

The rare absolute polycythemias with an innate and hereditary character can be grouped together under the heading "familial and congenital polycythemias" (FCPs). Primary forms, due to an intrinsic defect in the erythroid progenitor cells, and secondary forms, resulting from extrinsic factors such as an elevated erythropoietin level, have both been reported. Despite the widely divergent characteristics of the different FCPs, the range of possible diagnoses is much more restricted and the distribution of disorders markedly different compared with polycythemias in general. Therefore, in FCP, one can argue against following the algorithm of the Polycythemia Vera Study Group for the evaluation of an elevated hematocrit level, following instead a more specific algorithm. In this article the authors describe a child with primary FCP, review the different FCPs, and propose an adapted work-up scheme.

[Inherited polycythemia]

PURPOSE

The majority of polycythemias occurs sporadically without any other familial case. Very occasionally polycythemia finds a familial support. This work is a review of the actual knowledge about inherited polycythemias.

CURRENT KNOWLEDGE AND KEY POINTS

Although polycythemias linked to a anomaly of the haemoglobin affinity for oxygen are well understood (haemoglobin mutants with high oxygen affinity, 2-3 Diphosphoglycerate deficiency and methemoglobinemia), so called primary polycythemias (above all primary familial and congenital polycythemias) just begin to find an explanation for ten years (erythropoietin receptor gene mutation).

FUTURE PROSPECTS AND PROJECTS

Progressively, the part of really idiopathic polycythemias is smaller and smaller. Although most of mechanisms to explain congenital polycythemias are understood, some of them are still unresolved (Chuvash polycythemia, the majority of primary familial and congenital polycythemias).

Homozygous hemoglobin Tak causes symptomatic secondary polycythemia in a Thai boy

Secondary polycythemia caused by high-oxygen-affinity hemoglobin is rare in children. Most patients with this condition have asymptomatic erythrocytosis. In this article the authors describe a young boy from Thailand with plethora, hypoxemia, and aggravated respiratory distress following a chest infection. Hematological and molecular studies revealed that the boy is homozygous for Hb Tak, an extended beta-globin variant with high oxygen affinity. This report of a patient who is homozygous for high-oxygen-affinity hemoglobin highlights the clinical significance of this hemoglobin disorder, which has been previously reported in several unrelated families from Southeast Asia.

Mutations in the VHL gene in sporadic apparently congenital polycythemia

The congenital polycythemic disorders with elevated erythropoietin (Epo) have been until recently an enigma, and abnormality in the hypoxia-sensing pathway has been hypothesized as a possible mechanism. The tumor suppressor von Hippel-Lindau (VHL) participates in the hypoxia-sensing pathway, as it binds to the proline-hydroxylated form of the hypoxia-inducible factor 1alpha (HIF-1alpha) and mediates its ubiquitination and proteosomal degradation. The loss of VHL function may result in the accumulation of HIF-1alpha and overproduction of HIF-1 downstream target genes including Epo. VHL syndrome is an autosomal dominant disorder predisposing to the development of tumors, due to inherited mutations in the VHL gene. Some rare patients with VHL syndrome have polycythemia, which has been attributed to Epo production by a tumor. It was recently found that homozygosity for the VHL Arg200Trp mutation is the cause of Chuvash polycythemia, an autosomal recessive polycythemic disorder characterized by elevated serum Epo and hypersensitivity of erythroid cells to Epo. We evaluated the role of VHL in 8 children with a history of polycythemia and an elevated serum Epo level and found 3 different germline VHL mutations in 4 of them. One child was homozygous for the Arg200Trp VHL mutation, and another compound heterozygous for the Arg200Trp and the Val130Leu mutations. Two children (siblings) were heterozygous for an Asp126Tyr mutation, one of them fulfilling some criteria of VHL syndrome. We propose that mutations of the VHL gene represent an important cause of pediatric sporadic polycythemias with an inappropriately high serum Epo concentration.

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.

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.

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.

Erythropoietin receptor and hematological disease

This review will discuss evidence for the role of the erythropoietin (Epo) receptor in the development of erythrocytosis and other hematological disorders. The possible causative role of mutations of other genes in the pathogenesis of idiopathic erythrocytosis will be considered. Polycythemia vera (PV) is a myeloproliferative disorder that is caused by an undefined stem cell abnormality, characterized by a significant erythrocytosis, leukocytosis, and thrombocytosis. However, erythrocytosis may arise from apparent (or relative) polycythemia in which the hematocrit is raised due to a low plasma volume. In such cases the red cell mass is normal. A group of disorders with increased red cell mass caused by stimulation of erythrocyte production is known as secondary polycythemia. Investigation of such patients may reveal a congenital abnormality such as high affinity hemoglobin or an acquired abnormality caused, for example, by smoking, renal vascular impairment, or an Epo-producing tumor. Even after thorough examination there remains a cohort of patients for whom no definite cause for the erythrocytosis can be established. A careful clinical history may reveal whether this idiopathic erythrocytosis is likely to be congenital and/or familial, in which case the term "primary familial and congenital polycythemia" is sometimes applied. Access to a range of laboratory investigations may define the molecular pathophysiology. We will now discuss how this process can be investigated.

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.