The HbC p-globin mutation (p6 Glu!Lys) has similar consequences to those induced by the sickle cell disease (HbS) mutation (p6 Glu! Val); that is, a (-)-charged glutamic acid is replaced by a (+)-charged residue. Homozygosity for HbC causes severe hemo-lytic disease. Blood smears of affected individuals show frequent target cells (because of increased RBC surface area to cell volume ratio) and polychromasia. Haplotyping studies indicate that HbC mutation had a unicentric origin in sub-Saharan Africa. Recent evidence also supports the Haldane hypothesis for a protective effect of HbC heterozygosity against severe malarial infection.
The HbC band is identifiable by alkaline cellulose acetate gel electrophoresis and confirmed by acidic gel electrophoresis. Of note, HbC and HbA2 coelute on diethylaminoethyl (DEAE) cellulose column chromatog-raphy, a common procedure for quantifying HbA2. Homozygous HbC disease also can be differentiated from HbC/p-thalassemia by cation-exchange HPLC. HbC and HbS-Oman (see below) coelute on HPLC, but the presence of distinctive sickled ''Napoleon hat cells'' on a blood film will suggest HbS-Oman. DNA analysis will confirm the specific mutation(s).
Detection of HbC by DNA analysis has been more difficult than for HbS because no known restriction endonuclease site is abolished or created by the mutation. However, newer allele-specific PCR amplification techniques permit rapid identification of the HbC mutation.
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