Hemoglobin C (Hb C) is one of the most common structural hemoglobin variants in the human population. Persons with hemoglobin C trait (Hb AC) are phenotypically normal, with no clinically evident limitations or symptoms, while those with hemoglobin C disease (Hb CC) may have a mild degree of hemolytic anemia, splenomegaly, and borderline anemia. Although the clinical complications of hemoglobin C disease are not severe, inheritance with other hemoglobinopathies such as hemoglobin S may have significant consequences, and genetic counseling and anticipatory guidance play an important role in providing care for these individuals.
Hemoglobin C (Hb C) is a structural variant of normal hemoglobin A (Hb A) caused by an amino acid substitution of lysine for glutamic acid at position six of the beta hemoglobin chain. Hb C is less soluble than Hb A in red cells, likely from electrostatic interactions between positively charged β6-lysyl groups and negatively charged groups on adjacent molecules. Crystal formation may result, leading to increased blood viscosity and cellular rigidity, and shortened red cell survival.
Unlike sickle cell disease, Hb C does not cause linear intracellular polymerization upon encountering intravascular areas of low oxygen tension[1] . Thus, while there is evidence for reduced red cell deformability associated with the Hb C variant (see below), vaso-occlusion does not occur. Both the heterozygous and homozygous states may induce red cell dehydration (xerocytosis), and an elevated mean corpuscular hemoglobin concentration (MCHC) may be noted on a complete blood count.
A number of studies suggest that Hb C protects against malaria, providing a selective evolutionary advantage to people who express this hemoglobin variant in regions where this disease is endemic. A large case-control study in Burkina Faso found a strong association between resistance to clinical malaria and presence of the Hb C variant in both the heterozygous and the homozygous state.[2]
A study carried out in 4 rural villages of Burkina Faso found no evidence of protection for Hb C in the heterozygous state (odds ratio [OR] for Hb AC vs Hb AA, 1.49, P = 0.31), but did find protection was higher than even that observed with Hb S in the homozygous and double heterozygous states (OR for Hb CC and Hb SC vs Hb AA, 0.04; P = 0.002).[3]
Smaller case-control and cross-sectional studies have reported no association between Hb C and reduced incidence of malaria parasitemia; however, one longitudinal study using family-based association analysis found a strong relationship between Hb C and protection against mild malaria, as well as a negative association between Hb C and parasitemia[2, 4, 5] .
In vitro studies have shown lower parasite multiplication rates in Hb CC than in Hb AA red blood cells, and it has been proposed that the inhibitory effect of Hb C on parasitemia may partially explain the protective effect against malaria[5] . Thus, it has been proposed that the Hb C variant has been evolutionarily selected for in regions across the globe where malaria is endemic, as is the case for the Hb S variant.
Given the proposed survival advantage conferred by Hb C against malaria, it is not surprising that Hb C appears to have originated on the west coast of Africa. Hb C is found in diverse populations in Africa, southern Europe, and South and Central America, though its exact allelic distribution across these varied populations remains unclear.
In a global database of population surveys, Hb C was found to reach its highest predicted frequency in the western part of Burkina Faso, with an allele frequency of 24%.[6, 7] In the United States, 2-3% of African Americans are heterozygotes for Hb C, and approximately 1 in 5000 are homozygotes.[8] Hb C has also been identified in individuals with no known African ancestry.[9]
Hemoglobin C trait (Hb AC) is clinically silent. Hemoglobin C disease (Hb CC) is a mild disorder that generally does not cause any symptoms and is associated with a normal life expectancy.[10] Affected individuals exhibit normal growth and development and will tolerate surgery and pregnancy without issues.[11, 12]
Some patients may experience a mild hemolytic anemia, though under stress conditions the hemolysis may become more brisk and come to clinical attention. Affected patients may have splenic enlargement (splenomegaly). Case reports exist of spontaneous splenic rupture, but overall splenic function is unaffected.[13] As with other diseases causing chronic hemolytic anemia, cholelithiasis from pigmented gallstones occurs with increased frequency (see gallstones).
Hemoglobin C is mainly of clinical significance when inherited in combination with Hb S (Hb SC disease), or when co-inherited with β-thalassemia (hemoglobin C–β thalassemia). Little data exist regarding individuals who have very rare genotypes (eg, hemoglobin C–hemoglobin E compound heterozygotes), though these individuals would be expected to have few clinically evident limitations or symptoms.
The clinical manifestations of Hb SC disease (hemoglobin SC disease), are generally similar to but less severe than those of Hb SS disease. Growth and development are normal or slightly decreased, vaso-occlusive episodes do occur but less frequently, and the median lifespan in the United States for male and female Hb SC patients is 60 and 68 years, respectively.[14, 15, 16, 17, 18, 19, 20] Importantly, two specific complications may occur more frequently in Hb SC than in Hb SS: vascular retinopathy see (Retinopathy) and avascular necrosis of the femoral head.
The Hb SC genotype is a well-known risk factor for proliferative sickle cell retinopathy (PSCR).[21, 22] . Stages of PSCR have been described by peripheral retinal ischemia, peripheral neovascularization, intravitreal hemorrhage, and tractional or mixed retinal detachment that can lead to vision loss. A longitudinal analysis showed that Hb SC patients were significantly more likely to develop severe (stage III-IV) PSCR than Hb SS patients.[23]
Avascular necrosis of the femoral head has also been reported with increased frequency in Hb SC than Hb SS. This phenomenon usually occurs during the final months of pregnancy in women with Hb SC and is attributed to fat emboli after bone marrow infarction.[24] A longitudinal study in patients with Hb SC reported advanced retinopathy in 22.9% and osteonecrosis in 14.8%,.re-emphasizing the prominence of these complicaitons in this population.[10]
Individuals may be compound heterozygotes for beta thalassemia and hemoglobin C, which could result in mild to moderate chronic hemolytic anemia. These patients are more likely to be symptomatically anemic due to alpha chain/beta chain expression imbalance, though generally their clinical phenotype closely resembles those with beta-thalassemia trait.
The differential diagnosis of hemoglobin C disease includes the following conditions:
Patients can usually be evaluated in an outpatient setting. Routine screening laboratory studies are not necessary for asymptomatic patients; those with a moderate to severe clinical course should be evaluated for other hemoglobinopathies or chronic conditions.
The most commonly used method for the detection of hemoglobinopathies is hemoglobin electrophoresis or high-performance liquid chromatography (HPLC). Cellulose acetate screening will distinguish between the normal hemoglobins HbA, HbF, and HbA2, as well as the common variants, Hb S and Hb C, by charge. In the United States, newborn screening is the most common method of detecting individuals with Hb C disease. For those not diagnosed at birth, hemoglobin variant analysis by HPLC can provide the diagnosis.
Hemoglobin analysis results are as follows:
Hemoglobin variants that have the same mobility as Hb C include Hb E and Hb O-Arab. If co-migration is suspected, usually a second electrophoretic procedure is performed on citrate agar (acid electrophoresis), which is capable of distinguishing the respective variants.
In pregnancy, if prenatal screening suggests an increased risk of hemoglobinopathy, hemoglobin analysis is indicated for the mother. If maternal hemoglobin analysis shows that the mother is either homozygous or a heterozygous carrier for Hb C, paternal evaluation may be indicated to assess fetal risk of Hb SC disease. DNA-based testing for hemoglobin C can be performed during the first trimester of pregnancy from chorionic villus sampling at 10-12 weeks gestation, or by amniocentesis after 15 weeks' gestation.
Patients’ hemoglobin concentrations are usually within normal range, but the red cell mass and red cell survival may both be decreased[1] . Despite decreased survival, the reticulocyte count will not typically be increased; this is thought to be secondary to the lower oxygen affinity of Hb C, which facilitates sufficient oxygen delivery to tissues despite a lower red cell mass. Peripheral blood may show moderate amounts of target cells and intracellular crystals (5-30%)[24, 25] .
As stated before, hemolytic anemia may be mild to moderate in severity. Markers of hemolysis include increased LDH, reticulocyte count, and direct bilirubin[1] . Despite the anemia, reticulocyte counts are only slightly elevated in homozygous individuals, again due to the reduced oxygen affinity of this hemoglobin variant. Erythrocyte morphology is markedly abnormal, with prevalent microcytosis, target cells (>90%), spherocytes, and crystallized hemoglobin[26, 25, 27] .
As mentioned above, the reticulocyte count in these patients may be normal despite the presence of a chronic hemolytic anemia; this is proposed to be due to a reduced affinity of Hb C for oxygen. The lower affinity facilitates sufficient delivery of oxygen to tissues in the face of mild anemia.
The electrostatic interactions between the positively charged β-6 amino groups and the negatively charged groups on adjacent molecules lead to decreased solubility of deoxy-Hb C. This decreased solubility likely leads to shortened red cell survival. In settings of hypertonicity or deoxygenation, Hb CC cells form intracellular crystals of hemoglobin. These crystals reduce the red cells’ internal viscosity, leading to reduced deformability and a predisposition for fragmentation, spherocyte formation and splenic sequestration[27] .
Dental radiographs may show infarction. If the patient has right upper quadrant pain, abdominal ultrasonography may show gallstones. Fluorescein angiography detects neovascularization present at the equatorial region of the eye, which may be missed with funduscopic examinations.
In an oxygenized state, the hemoglobin C cell forms circulating intraerythrocytic crystals (tactoids) and has reduced solubility. In a deoxygenated state, virtually all hemoglobin C cells have crystalloid inclusions. Deoxygenation further reduces cell solubility and increases blood viscosity. Addition of 3% salt solution to a drop of blood will result in the appearance of the crystals, which are visible on a smear.
As with any chronic hemolytic anemia, body stores of folic acid may be depleted rapidly from high red cell turnover, and therefore folic acid at a dosage of 1 mg/day orally is indicated. Long-term antibiotic prophylaxis is not indicated, as these patients have normal splenic function. No special diet is required, and physical activities are not restricted.
Consultations that may be useful include the following:
Patients may benefit from genetic counseling, as it is important to discuss the possibility of potential co-inheritance with other hemoglobinopathies.