Erythrocytosis due to presumed polycythemia vera
OUR PATIENT’S FURTHER WORKUP
Our patient’s erythropoietin level is 34.2 mIU/mL (reference range 4.7–28.6). Her oxygen saturation is 96%, and her carboxyhemoglobin level is 1.1% (0–5).
She undergoes bone marrow biopsy. Analysis finds that the marrow is normocellular (60%) with trilineage hematopoiesis and decreased stainable iron.
Cytogenetic analysis shows a 46,XX[20] karyotype. Chromosomal microarray analysis shows no pathogenic copy-number changes. There is no detectable JAK2 V617F or exon 12-to-15 mutation.
The patient’s erythrocytosis and abnormal hemoglobin electrophoresis study raise suspicion for a variant type of hemoglobin that has a higher affinity for oxygen than normal.
3. What is the next best step to evaluate this patient?
- Red-cell oxygen equilibrium curve to calculate the P50 (the partial pressure of oxygen that is required to saturate 50% of the hemoglobin.)
- High-performance liquid chromatography
- Globin gene DNA sequencing
- Testing 2,3-bisphosphoglycerate mutase (BPGM) activity
Nearly 200 mutational variants in alpha and beta globin chains that lead to an increased affinity of hemoglobin for oxygen have been reported.12 While not all mutations are clinically significant, increased oxygen affinity variants can lead to impaired oxygen delivery to tissues, especially the kidneys, resulting in a physiologic increase in erythropoietin and erythrocytosis.
In patients being evaluated for a high-oxygen-affinity hemoglobinopathy, a two-step approach has been outlined.13 The first involves measuring the oxygen-binding properties of a freshly collected sample of blood by directly measuring the oxygen saturation of the hemoglobin and pO2 using a co-oximeter. This information is used to create a red cell oxygen equilibrium curve and to calculate the P50. A low P50 correlates with an abnormally high affinity of hemoglobin for oxygen.
The second step is to identify the abnormal hemoglobin. High-performance liquid chromatography is now widely available as a screening test but does not detect all variants. For many years, sequencing of globin chain DNA has been a gold standard for identifying specific mutations. Subsequent to analyzing a catalog of known hemoglobin variants, mass spectrometry can serve as a screening and identification technique. Mass spectroscopy can also detect known rare variants with posttranslational modifications14 that are not recognized by DNA analysis. Mass spectroscopy and DNA sequencing are complementary techniques available only in specialized reference laboratories.
Erythrocytosis due to BPGM deficiency is very rare. Clinical and laboratory features mimic those of high-oxygen-affinity hemoglobin, but patients do not have a demonstrable mutation in alpha or beta globin genes. The level of BPGM is low, and the diagnosis is established by measuring BPGM levels and sequencing the BPGM gene.15
RESULTS OF THE ADDITIONAL WORKUP
In our patient, hemoglobin electrophoresis reveals an abnormal hemoglobin variant. High-performance liquid chromatography reveals an abnormal peak that comprises approximately 23.7% of the total hemoglobin, consistent with an alpha globin variant. Further characterization (using a sample of venous blood) shows an oxygen dissociation P50 of 22 mm Hg (normal 24–30 mm Hg) (Figure 1).
Mass spectrometry identifies the variant as hemoglobin Tarrant. This variant is characterized by a substitution of asparagine for aspartic acid at position 126 of the alpha globin chain, a known site of contact between the alpha 1 and beta 1 chains.16 It has been seen in patients of Hispanic heritage and clinically correlates with mild erythrocytosis. Indeed, this woman’s mother was from Mexico.
EDUCATING PATIENTS
4. What should patients know about their high-oxygen-affinity hemoglobinopathy?
- High altitudes and air travel can be risky
- Pregnancy may have adverse outcomes
- Systemic anticoagulation may lower the risk of venous thromboembolism
- Periodic phlebotomy may help control symptoms
Most patients with high-oxygen-affinity hemoglobin do not require specific clinical management but only counseling and education about their condition. Establishing an accurate diagnosis is important in order to avoid further inappropriate, invasive, and expensive testing.
Although exposure to high altitudes may be associated with decreased ambient oxygen levels, hypoxia is usually not a problem because of hemoglobin’s high affinity for oxygen.
Impaired delivery of oxygen across the placenta may be anticipated in a mother with high-oxygen-affinity hemoglobin, but this has not been observed clinically.17
Compared with patients with polycythemia vera, patients with high-oxygen-affinity hemoglobin have fewer complications from hyperviscosity and thrombosis, even with comparable degrees of erythrocytosis.
Although patients usually do not require treatment, phlebotomy may be helpful for symptoms that can be attributed to the higher hemoglobin concentration.
Our patient continues to be seen in clinic for periodic blood counts and phlebotomy for her headaches, as required.
HEMOGLOBIN: RELAXED OR TENSE
Normal adult hemoglobin is a tetramer composed of two pairs of globin polypeptide chains: alpha and beta (Figure 2). The intrinsic properties of the constituent globin chains and their allosteric conformation—as well as extrinsic factors including temperature, pH, and the binding of hydrogen ion and 2,3-BPG—play important roles in modifying the affinity of hemoglobin for oxygen. The major modulator of hemoglobin-oxygen affinity in human erythrocytes is 2,3-BPG.
The hemoglobin tetramer, consisting of two identical halves, alpha 1-beta 1 and alpha 2-beta 2, oscillates between two quaternary conformations, “relaxed” (fully oxygenated) and “tense” (fully deoxygenated).18 High-oxygen-affinity hemoglobins can result from factors that enhance the relaxed state, either by stabilizing the relaxed state or by destabilizing the tense state. Structural modifications in hemoglobin typically affect the main contacts involved in the transition from the deoxygenated to the oxygenated state, the 2,3-BPG binding sites, the heme pocket, or elongation of globin chains by various mutations. In hemoglobin Tarrant, the mutation prevents formation of noncovalent salt bridges in the alpha 1-beta 1 contact that normally stabilize the deoxygenated conformation of hemoglobin. As a result, the deoxygenated (tense) state is destabilized, shifting the allosteric equilibrium in favor of the oxygenated (relaxed) state with consequent high oxygen affinity.16
MORE ABOUT HIGH-OXYGEN-AFFINITY HEMOGLOBINS
The first case of erythrocytosis due to an abnormal hemoglobin was identified in 1966. This was an alpha chain variant with an arginine-to-leucine substitution at position 92, named hemoglobin Chesapeake.19
High-oxygen-affinity hemoglobin variants are usually transmitted as autosomal dominant traits. Patients are most often identified because of unexplained erythrocytosis detected on a routine blood cell count, as in our patient.
Not all high-oxygen-affinity hemoglobinopathies are associated with erythrocytosis. The degree of increased oxygen affinity may only be mild or the abnormal hemoglobin may be slightly unstable, thereby masking the usual clinical signs and symptoms.
Therapeutic phlebotomy should be used cautiously since it can decrease delivery of oxygen to tissues. A subset of patients whose symptoms are related to an elevated red cell mass may experience some relief, as did our patient.
