CHAPTER 12: Erythrocytosis

Some patients come to medical attention because of an increase in red cell mass, manifested by hematocrit and hemoglobin levels that exceed the upper limit of normal for age and sex. The terms "erythrocytosis" and "polycythemia" are often used interchangeably. The former is preferred because it covers all conditions in which the red cell mass is increased. "Polycythemia" is a more restrictive term, implying not only increased hemoglobin and hematocrit levels but also leukocytosis and thrombocytosis.

In the initial evaluation of the patient, it is important to consider whether the hematocrit and hemoglobin levels might be falsely elevated because of a reduction in plasma volume. False or "pseudo" erythrocytosis may be due to dehydration or loss of gastrointestinal fluid from vomiting or diarrhea. In addition to these acute conditions, modest elevations of hemoglobin (Hb) and hematocrit (Hct) levels may be caused by chronic contraction in plasma volume, most often encountered in hypertensive, overweight, middle-aged males, a condition known as stress erythrocytosis or Gaisböck syndrome. However, when levels exceed two standard deviations above normal (Hb >17.5 g/dL, Hct >54% in males; Hb >16.0 g/dL, Hct >46% in females), it is likely that the patient has true erythrocytosis.

Many individuals with erythrocytosis are asymptomatic. However, as the red cell mass increases, they develop a ruddy or florid facial complexion. With further increases, the patient may develop central nervous system symptoms, such as forgetfulness, lethargy, or headache, and may have signs of excess cardiac preload, such as distended neck veins and hepatic congestion. More often, the clinical presentation is dominated by symptoms and signs related to the underlying cause rather than the erythrocytosis per se.

Establishing the cause of erythrocytosis can be a diagnostic challenge that requires the application of sound pathophysiologic principles. Table 12-1 presents an etiologic framework based on different mechanisms responsible for the expansion of the red cell mass. In primary erythrocytosis, the increase is due to autonomous proliferation of erythroid progenitors in the marrow, independent of external cues. In these patients, the abundant oxygen supply from the elevated hemoglobin concentration dampens expression of the erythropoietin (Epo) gene, and, as a result, plasma Epo levels are low. In contrast, secondary erythrocytosis is caused by an increased plasma erythropoietin level, which is either an appropriate physiologic response to hypoxia or a result of inappropriate autonomous up-regulation of Epo expression.

The most commonly encountered form of primary erythrocytosis is an acquired myeloproliferative disorder called polycythemia vera. These patients have a marked expansion of the red cell mass, owing to acquisition of a mutation in the tyrosine kinase JAK2, a signaling enzyme that plays a crucial role in hematopoietic cell differentiation. The remarkable ability of this mutation to trigger autonomous erythroid proliferation is depicted in Figure 12-1B. In addition to elevations in hemoglobin and hematocrit levels, patients with polycythemia vera usually have leukocytosis, thrombocytosis, and modest splenomegaly. For more information on the molecular pathogenesis, clinical presentation, course, diagnosis, and treatment of this disorder, see Chapter 20.

Rare families have been reported in which the polycythemia vera clinical phenotype is inherited in an autosomal dominant manner. The mutation(s) responsible for the familial disease are not yet known. Other families have autosomal dominant inheritance of autonomous erythrocytosis with normal white cell and platelet counts. One affected individual was an elite cross-country skier who won several world championships. He and a large number of relatives had hemoglobin levels as high as 19 g/dL, despite low plasma erythropoietin levels. DNA sequencing revealed that those having erythrocytosis were heterozygous for a stop codon mutation that caused the truncation of the intracellular tail of the Epo receptor. As depicted in Figure 12-1C, in the normal Epo receptor, a phosphatase, binds to this site and functions as a brake, limiting the potency of signaling by the JAK2 kinase. In the absence of this negative control, receptor-mediated signaling and erythropoiesis are markedly enhanced. Subsequently, a number of other families have been identified around the world with erythrocytosis due to several other mutations causing similar truncations in the Epo receptor.

The major physiologic regulator of Epo expression is hypoxia. When low oxygen tension is sensed in the Epo-producing cells of the kidney, hypoxia-inducible transcription factor (HIF) is induced, triggering a marked induction in Epo transcription and increased levels of Epo protein in the plasma. A number of diverse inherited and acquired disorders can cause increased red cell mass owing to elevated levels of plasma erythropoietin. As shown in Table 12-1, it makes pathogenetic sense to group these disorders into those in which the elevated plasma Epo level is an appropriate response to some type of hypoxic stress and those in which the enhanced Epo production is autonomous and therefore inappropriate.

ERYTHROCYTOSIS DUE TO APPROPRIATE ENHANCEMENT OF EPO PRODUCTION

There are many different ways in which hypoxia elicits clinical symptoms, signs, and laboratory abnormalities. Perhaps the most commonly encountered situation involves conditions such as angina pectoris, in which compromised blood flow to an organ or tissue results in focal ischemia. However, secondary erythrocytosis is encountered only when the Epo-producing cells of the kidney sense hypoxia. The most common settings in which hypoxia leads to erythrocytosis are described in the following sections.

High Altitude

The purest form of secondary erythrocytosis is that seen in populations living at altitudes greater than 5000 feet above sea level. The pulmonary, cardiovascular, and hematologic adaptations to high altitude are complex. However, there is convincing evidence that increased red cell mass improves exercise tolerance, lessens fatigue, and enhances well-being in individuals living at high altitude. Erythrocytosis is more marked, and adverse consequences emerge, in high-altitude dwellers who are heavy smokers with obstructive lung disease and in miners who develop inhalational restrictive lung disease (pneumoconiosis).

Cardiac Hypoxemia

Patients with congenital heart disease may develop severe, prolonged hypoxemia because of shunting of blood from the right side to the left side of the heart. These patients have cyanosis owing to low oxygen saturation and therefore an increase in deoxyhemoglobin in arterial blood. The highest hemoglobin and hematocrit levels observed in clinical medicine are in patients with cyanotic heart disease. A modest increase in the red cell mass is beneficial for these patients, enabling enhanced delivery of oxygen to tissues. However, when hemoglobin levels exceed 18 g/dL, the viscosity of the blood rises sharply and is likely to compromise peripheral blood flow. Therefore, patients with cyanotic heart disease and extreme erythrocytosis are treated with phlebotomy. Congestive heart failure is a much more common problem in cardiac patients. With the development of pulmonary edema from left ventricular failure, there is a fall in arterial oxygen tension. However, the degree of hypoxemia is seldom severe enough or of sufficient duration to cause secondary erythrocytosis.

Pulmonary Hypoxemia

Although patients with chronic lung disease often have long periods of sustained hypoxemia, the erythropoietic response is less predictable and less robust. In some of these patients, low-grade pulmonary infection or inflammation may suppress red cell production. However, other patients, particularly those with chronic emphysema, develop sufficiently high hemoglobin levels that they require periodic phlebotomy. Some individuals, especially those who are obese or have upper airway obstruction, have intermittent apnea and alveolar hypoventilation during sleep. A small fraction of these individuals have sufficient periods of hypoxemia to cause mild erythrocytosis.

Increased Affinity of Hemoglobin for Oxygen

As pointed out in Chapter 3 and shown in Figure 3-3, the position of the oxygen dissociation curve is a critical determinant of oxygen delivery to tissues. An increase in oxygen affinity will result in a decrease in hemoglobin's ability to release oxygen during flow through the microcirculation. Unlike individuals at high altitude or patients with cardiac or pulmonary hypoxemia, those having hemoglobin with high oxygen affinity have normal arterial oxygen tension. Therefore, they are not hypoxemic. However, the reduction in unloading of oxygen to tissues results in cellular hypoxia. Thus the cells in the kidney that produce Epo sense hypoxia and up-regulate Epo gene expression, leading to expansion of the red cell mass. Three uncommon but highly instructive abnormalities can cause a life-long increase in hemoglobin oxygen affinity associated with erythrocytosis:

• Structural mutations of globin.

• Congenital methemoglobinemia.

• Defects in production of red cell 2,3-diphosphoglycerate (DPG).

About 25 different amino acid substitution mutations of hemoglobin have been described that cause an increase in oxygen affinity and secondary erythrocytosis. Figure 12-2A shows a pedigree in which erythrocytosis is inherited in an autosomal dominant manner. About half of the hemoglobin in red cells of affected individuals was Hb Bethesda, in which an amino acid substitution at the C-terminus of the β-globin increases oxygen affinity (Fig. 12-2B). Erythrocytosis is the only well-documented clinical finding in affected individuals with this and other high-affinity mutant hemoglobins.

Methemoglobin is incapable of binding to oxygen because the heme iron has been oxidized to the ferric form. In partially oxidized hemoglobin, the oxygen affinity of the remaining normal (ferrous) hemes is increased. Accordingly, individuals with congenital methemoglobinemia, due to a deficiency in the enzyme that catalyzes the reduction of heme iron, often have a modest increase in hemoglobin and hematocrit levels.

Secondary erythrocytosis is also seen in rare families with low levels of red cell 2,3-DPG owing to a deficiency of 2,3-DPG mutase, the red cell enzyme required for the synthesis of this glycolytic intermediate. As shown in Chapter 3, Figure 3-3, 2,3-DPG is an important regulator of the oxygen affinity of hemoglobin in red cells.

ERYTHROCYTOSIS DUE TO INAPPROPRIATE EPO PRODUCTION

In other settings, erythrocytosis is a consequence of hypoxia-independent overproduction of Epo. These forms of erythrocytosis are usually caused by tumors but also occasionally by genetic defects in the oxygen-sensing mechanism that regulates Epo production.

Erythrocytosis-Associated Tumors

Although most tumors are associated with mild to moderate normocytic anemia (Chapter 7), occasionally erythrocytosis appears as a paraneoplastic phenomenon. Interestingly, the tumors that most often cause erythrocytosis are carcinomas of the kidney and liver, the two organs in which Epo is expressed. Other tumors associated with secondary erythrocytosis are cerebellar hemangioblastoma, pheochromocytoma (adrenal medulla), Cushing adenoma (adrenal cortex), and uterine myomata (fibroid tumors). Many of these tumors are highly vascular, and several occur at increased frequency in patients with von Hippel-Lindau syndrome (described in the next paragraph), which is caused by inherited mutations in the VHL gene. Epo secretion by tumor cells is believed to be the cause of paraneoplastic erythrocytosis, although this has been proven in only a few cases.

Some families have an increased risk of developing cancer due to the autosomal dominant inheritance of a mutated tumor suppressor gene, one of a large class of genes that normally protect cells from malignant transformation. In such patients, if a cell anywhere in the body suffers a somatic inactivating mutation in the single normal allele, tumor suppressor function is completely lost. One prominent and widely studied tumor suppressor gene is VHL, which encodes a protein that plays a crucial role in oxygen sensing and the regulation of the HIF transcription factor, mentioned earlier in this chapter. As shown in Figure 12-3, in well-oxygenated cells, the alpha (α) subunit of HIF undergoes oxygen-dependent hydroxylation of specific proline residues. This prolyl hydroxylase functions as the oxygen sensor for HIF regulation. Proline hydroxylation of HIF-α results in specific binding of HIF-α to a protein complex containing the VHL protein enabling the polyubiquitination and proteasomal degradation of HIF-α. Thanks to this elegant regulatory mechanism, HIF-dependent transcription is turned on only in hypoxic cells, in which HIF fulfills a critical role by up-regulating genes whose products facilitate adaptation to hypoxia.

The tumors that occur in von Hippel-Lindau syndrome arise from cells that have completely lost VHL function. As a result, the tumor cells exhibit constitutive or ongoing HIF activity, irrespective of oxygen tension. The resultant overexpression of HIF-dependent genes is probably the primary mechanism underlying the malignant transformation and, indeed, helps to explain the characteristic vascularity of von Hippel-Lindau syndrome–associated tumors, which secrete HIF-dependent proangiogenic factors such as vascular endothelial growth factor in large amounts.

Tumors most frequently encountered in von Hippel-Lindau syndrome are renal cell carcinoma, retinoblastoma, cerebellar hemangioblastoma, and tumors of the adrenal medulla and pancreas. About 15% of tumors occurring in those with von Hippel-Lindau syndrome are associated with secondary erythrocytosis. Of note, most sporadic renal cell carcinomas are also associated with somatic loss-of-function mutations in both copies of the VHL gene.

Inherited Defects of the Oxygen Sensing HIF Pathway

In an isolated enclave in central Russia, there is a high frequency of a particular mutation in the VHL gene that is not associated with development of tumors. However, although heterozygotes have no apparent clinical phenotype, homozygotes develop severe erythrocytosis due to elevated levels of Epo. Despite the absence of increased cancer risk, life expectancy is shortened due to complications of erythrocytosis. In other locales, rare families have been reported with erythrocytosis and high plasma Epo levels linked to mutations in the HIF2-a gene or a prolyl hydroxylase gene, which, as shown in Figure 12-3, regulates HIF-α. Thus, genetic defects in the system for oxygen sensing and HIF regulation can cause secondary erythrocytosis.