Hemoglobin C is formed by a single amino acid substitution at the same site as in sickle hemoglobin (codon 6 of the beta globin gene) but with lysine instead of valine substituted for glutamate. Hemoglobin C is nonsickling but may participate in polymer formation in association with hemoglobin S. Homozygous hemoglobin C disease produces a mild hemolytic anemia with splenomegaly, mild jaundice, and pigment (calcium bilirubinate) gallstones. The peripheral blood smear shows generalized RBC targeting and occasional cells with angular crystals of hemoglobin C (eFigure 15–15). Persons heterozygous for hemoglobin C are clinically normal.
Hemoglobin CC disease, peripheral blood smear. In this patient who is post-splenectomy, it is easy to note characteristic elliptical crystal-like precipitates of hemoglobin C as well as markedly increased frequency of target cells. The RBC inclusion in the upper right corner is a Howell Jolly body, indicative of the asplenic state. (Reproduced, with permission, from Lichtman MA, Shafer MS, Felgar RE, Wang N. Lichtman's Atlas of Hematology. McGraw-Hill, 2016.)
Patients with hemoglobin SC disease are compound heterozygotes for betaS and betaC. These patients, like those with sickle beta+-thalassemia, have a milder hemolytic anemia and milder clinical course than those with homozygous SS disease and they live longer. There are fewer vaso-occlusive events, and the spleen remains palpable in adult life. However, persons with hemoglobin SC disease have more retinopathy and more priapism than those with SS disease. The hematocrit is usually 30–38%, with 5–10% reticulocytes, and compared to SS, fewer irreversibly sickled cells on the blood smear. Target cells are more numerous than in SS disease. Hemoglobin electrophoresis will show approximately 45–50% hemoglobin C, 50% hemoglobin S, and no increase in hemoglobin F levels.