A 6-year-old African American boy is brought to the physician because of easy fatigability. Physical examination reveals splenomegaly, and his complete blood count shows mild anemia. Hemoglobin electrophoresis is performed at alkaline pH on a cellulose acetate strip. Findings for the patient are shown below compared to individuals with normal hemoglobin and known sickle cell disease.
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Which of the following is the most likely cause of this patient's condition?
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Hemoglobin electrophoresis is used to assess for different forms of hemoglobin in patients with suspected hemoglobinopathy. Normal hemoglobin consists primarily of hemoglobin A (HbA), which migrates rapidly toward the positive electrode (anode) because of its negative charge. Hemoglobin S (HbS) is an abnormal type of hemoglobin in which a nonpolar amino acid (valine) replaces a negatively charged amino acid (glutamate) in the beta globin chain. This amino acid replacement decreases the negative charge on the HbS molecule, which causes HbS to move more slowly toward the anode. Similarly, hemoglobin C (HbC) has a glutamate residue replaced by lysine in the beta globin chain. Because lysine is a positively charged amino acid, HbC has even less total negative charge than HbS and moves even more slowly toward the anode. Both HbC and HbS result from missense mutations, a type of mutation in which a single base substitution results in a codon that codes for a different amino acid.
Patients with sickle cell disease have HbS mutations in both beta chains; those with HbC disease have HbC mutations involving both beta chains. Patients with hemoglobin SC disease have 1 HbS allele and 1 HbC allele and will have 2 hemoglobin bands on electrophoresis. This patient's electrophoresis results show a single band that migrates less than the HbA and HbS bands, meaning that he has HbC disease. Patients with HbC disease are typically asymptomatic and often have mild hemolytic anemia and splenomegaly.
(Choice A) Deletions involving the alpha globin genes cause alpha thalassemia, which results in imbalanced beta chain production and formation of beta tetramers (hemoglobin H) that migrate further than HbA during electrophoresis.
(Choice C) Nonsense mutations introduce a stop codon within gene sequences, resulting in the formation of truncated proteins. As a result of their decreased size, these proteins tend to move further during electrophoresis.
(Choice D) Silent mutations are point mutations that have no effect on the protein formed. A mutation from UCA to UCC does not result in any change in protein structure as both codons result in the placement of serine into the growing polypeptide chain.
(Choice E) Trinucleotide expansions increase the number of trinucleotide repeats within a gene, resulting in large, unstable proteins or alterations in the epigenetic effects. These proteins would tend to move less during electrophoresis due to their increased size. However, the hemoglobinopathies are not caused by trinucleotide expansions, as the hemoglobin genes do not contain trinucleotide repeat regions.
Educational objective:
Hemoglobin C is caused by a missense mutation that results in a glutamate residue being substituted by lysine in the beta globin chain. This results in an overall decrease in negative charge for the hemoglobin molecule. The speed of hemoglobin movement during gel electrophoresis is hemoglobin A > hemoglobin S > hemoglobin C.