Congenital erythrocytosis: a new form associated with an erythropoietin-dependent mechanism

Feline primary erythrocytosis: a multicentre case series of 18 cats

CASE SERIES SUMMARY

A retrospective multicentre case series of feline primary erythrocytosis (PE) was evaluated. The aim was to gain better understanding of disease presentation and progression to guide management and prognostication. Case records were assessed for evidence of increased packed cell volume (PCV; >48%), sufficient investigation to rule out relative and secondary erythrocytosis, and follow-up data for at least 12 months or until death. Eighteen cats were included in the case series. No significant trends in signalment were noted. Seizures and mentation changes were the most common presenting signs (both n = 10). Median PCV was 70% (median total protein concentration of 76 g/l) with no other consistent haematological changes. Sixteen cats survived to discharge. Phlebotomy was performed initially in 15/16 surviving animals and performed after discharge in 10/16. Hydroxyurea was the most common adjunctive therapy, used in 10/16 cats. Of the 16 patients surviving to discharge, 14 patients were still alive at the conclusion of the study (survival time >17 months post-discharge), with the two non-survivors having lived for 5 years or more after diagnosis. PCV, when stabilised, did not correlate with resolution of clinical signs.

RELEVANCE AND NOVEL INFORMATION

In contrast to perceptions, feline PE was generally well managed via a combination of phlebotomy and medical therapy, with evidence of prolonged survival times. The use of hydroxyurea enabled cessation or repeat phlebotomies.

Familial erythrocytosis with over-production of erythropoietin

A family is described in which the father and son had erythrocytosis associated with a normal Hb oxygen affinity. Growth of erythroid colonies in vitro (BFU-E) exhibited normal erythropoietin dependence. In the son there was an enlarged erythroid precursor compartment, while the father (who had been treated by busulphan) showed marked reduction of circulating BFU-Es. Serum erythropoietin (Epo), estimated by radio-immunoassay, was 96 miu/ml in the son and 360 miu/ml in the father (normal 25, SD 6, n = 46). We conclude that erythrocytosis in this family is due to a genetically determined hyper-production of Epo. The finding in the father of a high Hb level associated with increased Epo and decreased BFU-Es might support the hypothesis that red cell mass is regulated by Epo at the level of bone marrow CFU-Es rather than BFU-Es.

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.

Polycythemia vera: a comprehensive review and clinical recommendations

More than a century has elapsed since the appearance of the modern descriptions of polycythemia vera (PV). During this time, much has been learned regarding disease pathogenesis and PV-associated molecular aberrations. New information has allowed amendments to traditional diagnostic criteria. Phlebotomy remains the cornerstone treatment of PV, whereas myelosuppressive agents may augment the benefit of using phlebotomy for thrombosis prevention in high-risk patients. Excessive aspirin use is contraindicated in PV, although the use of lower-dose aspirin has been shown to be safe and effective in alleviating microvascular symptoms including erythromelalgia and headaches. Recent studies have shown the utility of selective serotonin receptor antagonists for treating PV-associated pruritus. Nevertheless, many questions remain unanswered. What is the specific genetic mutation or altered molecular pathway that is causally related to the disease? In the absence of a specific molecular marker, how is a working diagnosis of PV made? What evidence supports current practice in the management of PV? This article summarizes both old and new information on PV; proposes a modern diagnostic algorithm to formulate a working diagnosis; and provides recommendations for patient management, relying whenever possible on an evidence-based approach.

Congenital erythrocytosis with increased erythropoietin level

The term "absolute erythrocytosis" denotes a heterogeneous group of disorders characterized by an increased red blood cell mass. The authors describe a 20-month-old girl with absolute erythrocytosis. Erythropoietin levels were found to be extremely increased, although extensive evaluation failed to reveal a cause for such an inappropriate increase. Of interest is also the documentation of spontaneous erythroid colony formation in the patient's bone marrow cultures. Although such a finding is considered typical of polycythemia vera, the diagnostic criteria of this myeloproliferative disorder were not met.

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.

Regulation of erythropoietin production in a case of congenital erythropoietin-dependent pure erythrocytosis

In a patient with congenital erythropoietin-dependent pure erythrocytosis (EDPE) associated with hypersensitivity of erythroid progenitor cells to erythropoietin (Epo), the investigations planned to elucidate the mechanism responsible for hormone hyperproduction revealed that Epo synthesis was (1) independent of normal oxygen-mediated feedback induced by phlebotomy; (2) not modulated by adenosine as a second messenger (the treatment with the adenosine antagonist theophylline in fact left unchanged the serum Epo levels); and (3) uninfluenced by iron therapy. The Epo dose-response curve for growth of erythroid progenitor was similar to that of three age-matched thalassemia patients with increased serum Epo levels, (sEpo) suggesting that the observed erythroid progenitors hypersensitivity to Epo could represent an ex vivo artifact induced by the increased sEpo levels.

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

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

Spontaneous resolution of primary erythrocytosis in two girls

We report two girls with primary erythrocytosis in whom extensive diagnostic studies revealed no underlying cause. Normal growth of colonies derived from erythroid burst forming units (BFU-E) was observed, and serum erythropoietin concentrations were within or below the normal range. The absence of a rise in serum erythropoietin levels after isovolemic phlebotomy implicated the erythroid marrow as the site of the pathophysiologic abnormality in both patients. Spontaneous resolution of erythrocytosis occurred during the second decade of life. Our experience suggests that primary erythrocytosis may be self-limited in some children. In these cases, the proliferative abnormality may be sufficiently subtle as to not be detected by standard in vitro culture systems, which support the growth of colonies derived from erythroid progenitors.

Heterogeneity of erythropoietin-dependent erythrocytosis: case report in a child and synopsis of primary erythrocytosis syndromes

To investigate the pathogenesis of polycythaemia in a child with isolated, primary erythrocytosis, we measured serum erythropoietin activity and in vitro erythroid progenitor cell responsiveness to erythropoietin. Unstimulated erythropoietin activity was markedly elevated (1.8 IU/ml), and isovolaemic phlebotomy induced a four-fold increment above this level. In contrast to findings in our index case with this syndrome, normal erythroid colony growth patterns were present in patient marrow cultures. The primary mechanism of polycythaemia in this individual is similar to that reported in the index case: an inappropriately elevated regulatory set point for erythropoietin production. Since an additional defect of progenitor cell hypersensitivity to erythropoietin is not always present, we conclude that abnormalities at single or multiple sites of the erythropoietic regulatory axis may occur in primary erythropoietin-dependent erythrocytosis.

Pure erythrocytosis classified according to erythropoietin titers

Erythropoietin titers, measured by bioassay of plasma extracts in hypertransfused mice, were determined in 162 patients with absolute erythrocytosis, and the results were correlated with the clinical diagnosis. Fifty-two patients met the diagnostic criteria for polycythemia vera, and all had low or nonmeasurable erythropoietin titers. Of the remaining 110 patients, 62 were suspected clinically as having secondary polycythemia. However, 15 had low erythropoietin titers, casting doubt on the accuracy of the clinical diagnosis. The pathogenesis of the erythrocytosis in the last 48 patients was unknown, and they were designated clinically as having pure erythrocytosis. However, in 20, the erythropoietin titers were increased, and in 28, the titers were low, suggesting that they belonged to at least two different groups. Using erythropoietin titers in the classification of absolute erythrocytosis, the first group should be added to the category of patients with secondary polycythemia as a subgroup with disease due to idiopathic overproduction of erythropoietin (hypererythropoietinemia or essential erythrocytosis). The second group should be added as a subgroup of patients with primary polycythemia under the term erythremia.