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From the Medical Institute of the Chuvash State University, Cheboksary, Chuvashia, Russia; the Department of Medicine, The George Washington University Medical Center, Washington, D.C.; Association of Molecular Blood and Iron Overload Diseases, Moscow, Russia; McGill University, Montreal, Quebec, Canada; and the Division of Hematology, University of Alabama at Birmingham and Veterans Administration Hospital, Birmingham, AL.
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.
FAMILIAL AND CONGENITAL polycythemias refer to an uncommon group of inherited disorders that are characterized by an increase in the red blood cell mass. The term erythrocytosis is synonymous and also frequently used. These conditions may either be primary or secondary to elevated erythropoietin concentrations. Most cases of congenital polycythemia are secondary disorders caused by hemoglobin mutations that result in increased affinity for oxygen and consequently tissue hypoxia and elevated erythropoietin levels.1,2 More rarely, decreased production of 2,3-diphosphoglycerate, a molecule that facilitates the release of oxygen from hemoglobin to tissues,3,4 or inappropriate erythropoietin expression5-7 may lead to increased erythropoietin concentrations. In primary familial and congenital polycythemias, erythropoietin levels are not elevated, but erythroid progenitors have heightened sensitivity to erythropoietin.8-10 Polycythemia seems to result from mutations causing the excessive growth of erythroid progenitor cells.9 For example, mutations involving the erythropoietin receptor gene or its regulation may lead to autosomal dominant polycythemia in the presence of normal or low erythropoietin concentrations by an enhanced signal transduction, ie, gain-of-function mutation.8,11,12 Autosomal dominant mutations of genes other than the erythropoietin receptor gene affecting postreceptor responses may also lead to primary familial and congenital polycythemia.13,14 The primary and secondary congenital polycythemias described to date in the western medical literature occur sporadically, without a seeming predilection for a particular population. Not widely reported in the west, over the past 25 years Russian and Chuvash scientists have studied a form of congenital polycythemia that is endemic in the Chuvash population of the Russian Federation.15-17
The Chuvash Autonomous Republic is located on the west bank of the Volga River in the central part of European Russia. It is 18,300 square kilometers in area and has an altitude of less than 500 meters. The population was about 1,300,000 in 1979 with 70% of Chuvash ethnicity. The first reports of congenital polycythemia in Chuvashia appeared in the 1970s.15,16 One hundred and three cases of polycythemia that seemed to be familial in nature and that did not fit the classification of polycythemia vera were collected over a period of 10 years. All of these patients were Chuvash by nationality and inhabitants of the northeastern region of Chuvashia. More than two thirds of the patients were less than 20 years of age at the time of diagnosis (Table 1) and slightly more than one half were females. As shown in Table 2, most of the 103 Chuvash patients with polycythemia gave a history of fatigue and headaches and all demonstrated plethora on physical examination. Basic hematological laboratory findings are summarized in Table 3. In addition, the bone marrow, examined histologically in 62 patients, showed hypercellularity in 48 patients, normal cellularity in 13, and hypoplasia in one patient who had been treated with 32P. In all cases, there was absolute or relative erythroid hyperplasia, while megakaryocytes were not increased in number.
Over 10 years of observation, 11 of 103 Chuvash patients with polycythemia died. The causes of death in the first eight patients to die are shown in Table 4. Their ages ranged from 16 years to 58 years, and thrombosis was the leading cause of death. Genealogical studies were performed in 79 families with a total of 339 siblings. All of them were from phenotypically normal parents, and the children of the patients were also phenotypically normal. The hypothesis of autosomal recessive inheritance was tested using the "a priori" method of Hogben.18 As shown in Table 5, polycythemia appeared to follow an autosomal recessive pattern of inheritance. More recently, we have observed two pedigrees in which affected subjects are present in two generations, ie, a maternal grandmother and a grandchild in each of these families. Consanguinity is not common among the Chuvash people.
Some pathogenic features of Chuvash polycythemia have also been characterized.16,17 The plasma volume was determined in 19 patients (11 men and 8 women) by injecting albumin labeled with 131I and determining its dilution. The volume of circulating whole blood and the red blood cell mass were then estimated according to the hematocrit. The volume of plasma was normal or slightly decreased at 42.6 ± 3.2 mL/kg, while the estimated blood volume was elevated (103.3 ± 6.5 mL/kg) as was the estimated red blood cell mass (60.7 ± 5.4 mL/kg).17 Polyacrylamide gel hemoglobin electrophoresis was performed in 19 Chuvash patients with polycythemia, and an abnormal hemoglobin fraction was not identified.16 The methemoglobin concentration was measured spectrophotometrically in 10 patients and was <1% in all.16 The concentration of 2,3-diphosphoglycerate in erythrocytes was analyzed enzymatically in 11 patients and 10 healthy controls. The results for patients with Chuvash polycythemia ranged from 4.3 to 6.1 µmol 2,3-diphosphoglycerate/L erythrocytes, and the mean ± standard error (SE) of 5.2 ± 0.2 µmol/L erythrocytes did not differ significantly from healthy controls (4.7 ± 0.09).17 Analyses of arterial blood gases and O2 dissociation curves in 22 patients with polycythemia and 10 healthy controls showed that pCO2 , pO2 , and O2 saturation were similar between the patients and the controls, while the pH was lower in the patients and the O2 dissociation curve was shifted slightly to the right (Table 6).17 Erythropoietin levels were estimated in 20 Chuvash subjects with polycythemia before and after phlebotomy therapy and in 10 healthy controls. The method used was an in vivo bioassay based on the reticulocyte response in polycythemic Swiss mice.19 Erythropoietin levels tended to be in the normal to elevated range in patients with polycythemia before phlebotomy therapy, and they increased significantly with phlebotomy (Table 7).17
To summarize, the studies from Russia and Chuvashia indicate that Chuvash polycythemia is a true absolute polycythemia not due to an abnormal hemoglobin, methemoglobinemia, pulmonary disease, or reduced activity of 2,3-diphosphoglycerate. The age, sex distribution, symptoms, physical findings, certain laboratory values, and familial pattern differ from those of polycythemia vera.20,21 The serum erythropoietin levels performed with a semiquantitative biological assay are inconclusive, but may be consistent with a secondary form of familial and congenital polycythemia. The mortality of more than 10% over 10 years of observation in a very young cohort of patients indicates that this form of polycythemia is not benign.
In the present report, we provide results based on studies of several polycythemic subjects and nonpolycythemic family members from Chuvashia that bear on the pathophysiology of Chuvash polycythemia. Additionally we suggest the appropriate classification of this polycythemic disorder.
Clinical Studies of Chuvash Patients With Polycythemia and Their Family Members
Assessment of Hemoglobin and Hemoglobin Association
Determination of Serum Erythropoietin Concentrations Erythropoietin concentrations were determined in the sera of three patients with polycythemia by a radioimmunoassay performed by Dr J. Goldwasser's laboratory (University of Chicago, Chicago, IL)24 and by a functional assay involving antierythropoietin antibodies bound to magnetic beads (Dr Gerald Krystal, The Terry Fox Laboratory, Vancouver, British Columbia).25,26 Erythropoietin concentrations were measured in the sera from six first-degree family members by the radioimmunoassay and from eight first-degree family members by the functional assay.DNA Studies Genomic DNA was isolated from peripheral blood leukocytes by a standard procedure.27Statistical Analyses The Student's t-test was used to compare continuous variables between index subjects and first-degree family members. Fisher's exact test was used to compare proportions. Serum erythropoietin concentrations were compared after log transformation.
Clinical Studies The six index subjects who form the basis of the present study reported intermittent symptoms of headache and malaise and one man gave a history of seizures. In all of these subjects, there was historical evidence of plethora and/or elevated hemoglobin concentrations within the first year of life. In two individuals, elevated hemoglobin concentrations had been noted in the neonatal period before discharge from the hospital. On physical examination, all of the patients had plethora, while none of them had hepatomegaly or splenomegaly. There were no signs of cardiac or pulmonary disease. Among the four index subjects who underwent echocardiography and abdominal ultrasonography, no evidence of valvular or septal heart defects was found, and there were no hepatic or renal masses. The demographic and laboratory features of the six Chuvash patients with polycythemia and the 12 first-degree family members we studied are shown in Table 8.
Laboratory Studies The values for hemoglobin, hematocrit, red blood cells, and red blood cell distribution width were significantly higher in the index subjects than the first-degree relatives, but there were no significant differences in white blood cells or platelets (Table 8). The frequency distributions of hemoglobin concentration and red blood cell distribution width for the 18 subjects included in this study are shown in Fig 1. There were distinct bimodal distributions to these measures with the index subjects all falling in the upper modes and the first-degree relatives in the lower modes.
Assessment of Hemoglobin and Hemoglobin Association No electrophoretically distinguishable hemoglobin variant was detected. In two index subjects, the p50s were 26.8 and 27.4 mm Hg (normal, 24 to 30) and the Hill dissociation constant was 2.81 in both cases (normal, 2.72 to 2.91). The erythrocyte 2,3-diphosphoglycerate concentration was 11.9 µmol/g hemoglobin in one index subject (normal, 12.3 ± 1.9).Erythropoietin Levels Geometric mean values for radioimmunoassay and functional measurements of erythropoietin are shown in Table 9. Erythropoietin levels were significantly higher in the index subjects than in the first-degree family members, even though the index subjects had much higher hematocrits.
DNA Studies Erythropoietin gene. The Hinf I and Bgl II polymorphisms of the erythropoietin gene were examined in six Chuvash pedigrees comprising six patients with polycythemia and 11 first-degree family members. The Bgl II polymorphism was found to be informative in these studies. Five patients were homozygotes for this polymorphism, but one was a heterozygote. The heterozygous polycythemic patient's mother, an obligate carrier assuming autosomal recessive inheritance, was a homozygote for a different allele than the patient's father and the five other unrelated polycythemic patients (Fig 2).
Polycythemia was detected in the Chuvash population of the mid-Volga River region of Russia in the 1960s15 and by 1977, 103 cases from 81 families had been described.16 Since then, more cases have come to light, and we estimate that hundreds of children may suffer from this condition. Virtually all of the individuals with this form of polycythemia live in the Chuvash Autonomous Region and other nearby geographical areas in the mid-Volga River region of the Russian Federation. The condition causes thrombotic and hemorrhagic vascular complications, which lead to early mortality, and survival beyond the age of 40 years is uncommon.16 Thus, this disease has substantial morbidity that may be even higher than that observed in the primary familial and congenital polycythemias that were initially considered to be benign conditions.32 As reviewed above, affected patients do not fit the picture of polycythemia vera and they tend to have normal blood gases. Children of both sexes are born with the disease to normal parents, and the occurrence of the condition within sibships is consistent with autosomal recessive inheritance. Thus, Chuvash polycythemia appears to fall in the category of a familial and congenital polycythemia, and it is probably the most common congenital polycythemia in the world.
Submitted June 14, 1996;
accepted October 22, 1996.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hearly marked
``advertisment'' in accordance with 18 U.S.C. section 1734 solely to
indicate this fact. We are grateful to Drs Goldwasser and Krystal for performing the erythropoietin determinations and to Dr Fu-Kuen Lin for providing the probes for erthropoietin gene polymorphisms.
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