Hemoglobin C Disease

Etiology

Hemoglobin C is caused due to mutation in the beta-globin chain in which glutamate (acidic) is replaced by lysine (basic) in the sixth position of the beta-globin chain. This mutation makes Hb C less soluble than Hb A, forming hexagonal crystals (HbC crystals as seen in the peripheral smear).

Hemoglobin C is a hemoglobinopathy that is hereditary. It is an autosomal recessive disorder that results from the biparental inheritance of the allele that codes for Hemoglobin C. If both the parents are carriers of hemoglobin C, there is a 25% chance of having hemoglobin C disease in a child, 50% chance of having a child who is a carrier, and 25% chance of having a child who is neither a carrier nor have hemoglobin C disease.

The mutation present in some of the hemoglobinopathies (Hb S, Hb C, Hb E) may be an evolutionary modification due to the effect of some selective external forces like malaria as in the heterozygous form, these mutations protect carriers from dying of malarial infection. The cohort study was done on three hundred children in Bandiagara, Mali, found out that both HbC and HbS traits provide protection from clinical falciparum malaria.[1] One in-vitro study has proposed the mechanism that the hemoglobinopathies (HbS, HbC) impair protein export in Plasmodium falciparum infected erythrocytes. This provides a mechanistic explanation for reduced disease mediating cytoadherence of parasitized hemoglobinopathic erythrocytes.[2]

A retrospective study performed on all sickle cell disease patients who underwent red cell exchange transfusions (RBCEx) at the University of Arkansas found out the presence of other Hemoglobin variants too in the recipients. This new hemoglobin variant present in the patients was acquired as a result of RBCEx. The most commonly acquired hemoglobin variant was hemoglobin C (64/66 occurrences).[3]

Epidemiology

Given the protective effect of hemoglobin C in malaria infection, it is almost understandable that this mutation has a high prevalence in Atlantic West Africa and South East Asia. Hemoglobin C is also found in a diverse population in Africa, South and Central America, and Southern Europe.

A retrospective study performed in 111 cases of hemoglobin C disease over 12 years in Morocco showed the mean age at the time of diagnosis as 38 years (ranging between 4 and 80 years). Also, the biochemical tests contributed to the diagnosis and revealed varied etiological groups in the study population: heterozygous A/C (75%), homozygous C/C (8%), double heterozygous S/C (9%), C/beta +/- Thal (6%), C/ O - Arab (2%).[4]

Pathophysiology

The substitution of lysine for glutamic acid in the sixth position of the beta-globin chain induces an electrostatic interaction between positively charged beta-6-lysyl groups and negatively charged adjacent molecules, leading to the decreased solubility of HbC in red cells. Due to this, crystal formation occurs, leading to increased blood viscosity and decreased red blood cell life span. But, unlike sickle cell disease, which is caused due to the substitution of valine for glutamic acid, Hb C does not cause intracellular polymerization during low oxygen tension. Thus, in general, the vaso-occlusive crisis is not seen with hemoglobin C disease unless it is combined with sickle cell (Hb SC).

Several heterozygous combinations of hemoglobin C with other hemoglobin variants are seen. One case study reported a young woman with compound heterozygosity for Hb-D-Ibadan and HbC, a rare Hb combination that was previously not identified. This combination was detected during prenatal screening, and HbD-Ibadan was present in excess amount than HbC in the combination (70.3% to 24.4%). The combination is clinically silent and demonstrates thalassemia-minor-like red cell indices.[5] 

Hemoglobin C-Harlem is another rare condition that has the same electrophoretic mobility as that of HbC on alkaline electrophoresis. But this molecule, similar to HbSS, can polymerize when deoxygenated and hence has a clinical course similar to sickle cell disease.[6]

History and Physical

Hemoglobin C trait (HbAC) is clinically silent. Hemoglobin C disease (HbCC) is also a mild disorder, and most people do not have any symptoms. But some patients may experience mild hemolytic anemia and hence may complain of symptoms like fatigue, lightheadedness, weakness, and paleness of the skin. On physical examination, there can be mild to moderate splenomegaly, and occasionally jaundice may occur. A retrospective study which was done in 111 cases of hemoglobin C disease also found out that the most important reason for prescription of hemoglobin analysis in these patients was splenomegaly and anemic syndrome.[4] Some patients, due to continued chronic hemolysis, may develop cholelithiasis from pigmented gall stones composed of dark-colored contents of red blood cells.

Individuals with combined sickle cell hemoglobin C disease (HbSC) present with symptoms that are generally similar to but less severe than hemoglobin SS disease. Since hemoglobin C does not polymerize like hemoglobin S, there is very little sickling in this combination. Thus, an acute vaso-occlusive crisis does occur but less frequently. A cross-sectional study that was done to compare the sickle cell anemia and hemoglobin SC disease from 2013 to 2014 in 200 patients with sickle cell disease patients (141 with SCA and 59 with HbSC) showed HbSC genotype was associated more with blood viscosity while HbSS was associated more with sickling and endothelial dysfunction. Also, the clinical events were more frequent in HbSS patients.[7] The two most important complications that may occur more frequently in HbSC are vascular retinopathy and avascular necrosis of the femoral head. A cohort study done in 461 infants with Hemoglobin SC disease showed acute splenic sequestration in 14.8%, overt stroke in 0.2%, incidence of painful vaso-occlusive episodes was 51 per 100 patient-years while that of infections was 62.2 episodes per 100 patient-years. Similarly, retinopathy was observed in 20.3% of 59 children who underwent ophthalmoscopy, and avascular necrosis was detected in seven out of twelve patients evaluated, predominantly in the left femur.[8]

Evaluation

For asymptomatic patients, routine screening laboratory studies are not necessary. Evaluation should be done for patients with a moderate to severe clinical course. The lab results depend on whether the patient is a carrier (HbAC) or has a disease (HbCC). But most importantly, hemoglobin C in both of the states, the homozygous (HbCC), and in the heterozygous (HbAC, HbSC) induces red cell dehydration resulting in red cells with increased mean corpuscular hemoglobin concentration (MCHC). A study performed on a 17-year-old male with hereditary xerocytosis-hemoglobin C trait showed that the hemoglobin C accentuates the erythrocyte dehydration of hereditary xerocytosis.[9]

The most common method used to detect hemoglobinopathies is hemoglobin electrophoresis or high-performance liquid chromatography (HPLC). Patients who are homozygous for hemoglobin C disease mostly show HbC; HbA is absent, and HbF is slightly increased while patients who are heterozygous for hemoglobin C disease may show 30% to 40% HbC, 50% to 60% HbA, and HbA2 is slightly increased.

Peripheral smear or blood film findings: P.S. reveals the presence of hexagonal or tetragonal crystals due to decreased solubility of HbC. In addition to the presence of crystals, there are red blood cells microcytosis, irregularly contracted cells, and numerous target cells, as shown in the picture.

In the patients with the hemoglobin C trait, hemoglobin concentrations are usually within the low normal to the normal range. The life span of red cells gets decreased but despite this, the reticulocyte counts are not increased. The peripheral smear may show target cells and intracellular crystals. Likewise, the patients with Hemoglobin C disease, as mentioned earlier, show the lab findings suggestive of hemolysis: increased LDH, reticulocyte count, and direct bilirubin. The peripheral smear shows microcytosis, a large number of target cells, spherocytes, and crystallized hemoglobin.

A study showed that in a diabetic patient presenting with "HbS" or "HbC," there is a considerable false decrease in the values for glycosylated hemoglobin (HbA1c) despite persistent hyperglycemia and this decrease is proportional to the percentage of abnormal hemoglobin.[10] 

There is a study describing the use of miniaturized paper-based microchip electrophoresis for point of care hemoglobin testing in low resource settings, which identifies hemoglobin variants easily and affordably with the sensitivity of 100% and an accuracy of 98.4%.[11] 

A large cohort study done in Brazil described the use of an automated pyrosequencing technique for large-scale confirmatory testing of hemoglobin mutation, which could also describe beta+ and beta- mutation present in sickle cell disease.[12]

Likewise, there is a study describing the use of a new polymerase chain reaction test (LAMP assay) that can be performed directly on dried blood cards without DNA extraction and has an accuracy of 100% for detecting HbS and HbC.[13]

Treatment / Management

In most patients, no treatment is required. As with chronic hemolysis, body stores of folic acid can get depleted; thus, folic acid is indicated, which helps produce new red cells and improve the symptoms of anemia. Despite splenomegaly, the splenic function remains normal, and hence long-term antibiotic prophylaxis is not indicated.

A study showed that in patients with sickle cell hemoglobin C disease undergoing cardiopulmonary bypass, the use of complete exchange blood transfusion makes bypass surgery relatively safe with enhanced outcomes.[14] Likewise, there is also a study demonstrating better maternal and fetal outcomes in sickle cell hemoglobin C disease patients receiving prophylactic transfusions during pregnancy.[15]

Differential Diagnosis

Following important differential diagnoses should be considered when hemoglobin C disease is suspected:

• Sickle cell anemia
• Beta thalassemia
• Other hemolytic anemias

Prognosis

Hemoglobin C is a benign hemoglobinopathy that can cause mild hemolytic anemia. Overall people with hemoglobin C have normal growth and development and a normal life expectancy.

Complications

Hemoglobin C disease, due to continued hemolysis may produce cholelithiasis due to pigmented gallstones.

A patient with combined sickle cell hemoglobin C disease (HbSC) can develop vascular retinopathy, avascular necrosis, and renal medullary microvascular thrombosis. A study performed in a patient with hemoglobin SC disease showed peritubular capillary and vasa recta thrombi and capillary basement membrane alterations mostly involving the renal medulla.[16]