Block2 11. Sickle Cell Anemia And Other Hemoglobinopathies

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Sickle Cell Anemia

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Autosomal recessive disorder because of defect in HBB gene (beta chain of hemoglobin). Need two defective genes for the disorder. The defect has persisted in the population because one version of the defective gene confers enhanced resistance to malaria. Symptoms can range from mild to severe. Sickle cell crises (vasoocclusive crises) could happen just once a year or everyday, variable pain. Jaundice from breakdown of erythrocytes, damage to retina, delayed puberty, increased chance of stroke, infection, odd chest xrays

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Sickle Cell Anemia – The First “Molecular Disease”

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This study used protein electrophoresis to show that sickle cell hemoglobin differed in structure from normal hemoglobin.
•Amino acid #6 in the beta chain of Hb had been mutated from Glutamate to Val (from a negatively charged aa to a neutral one, major problem)
•In Hemoglobin C disease, the same animo acid 6 has been mutated to Lys (neg to pos, so not as bad) (mild disease that does not require therapy).

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Hemoglobin S

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Because of the mutation of the amino acid from negative to neutral, there is an additonal hydrophobic region. This causes the molecules to aggregate irregularly, favored when HemoS is in the deoxy form! Strands and even crystals form

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Hydroxyurea

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Approved as a treatment to SSA in 2006, somehow acts to promote expression of HbF. The postulated effect of hydroxyurea in inhibiting hemoglobin S-polymerization has to do with formation of HbF and HbS hybrids, which don't form strands as easily

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Human globin subunit types

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Alpha chains- two types: Alpha and Zeta
Beta chains- four types: Beta, Gamma, Delta, Epsilon
Hemoglobin A = 2alphas,2betas >95% adult hemoglobin
Hemoglobin A2 = 2alphas,2deltas 2% adults
Hemoglobin F (fetal) = 2alphas2gammas
Hemoglobin E (embryonic) = 2zetas2epsilons

Amounts of beta and gamma switch around birth (at birth they are about 50/50). Zeta decreases to be replaced by alpha very early on, and alpha stays for good.

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Methemoglobinemias (HbM)

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Oxidation of the iron from 2+ to 3+. Chemicals or drugs may cause it. Also, mutations in alpha or beta subunits near the heme can lead to increased risk. Genetic deficiencies of enzymes that reduce the levels of NADH in the cell (eg, pyruvate kinase) can also cause methemoglobinemias.

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NADH-cytochrome b5 reductase

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An RBC enzyme that is capable of reducing the iron back from 3+ to 2+. Deficiencies in this enzyme can cause HbM.

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Methemoglobinemias (HbM) symptoms

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They're fucking blue! Symptoms are related to hypoxia and can include anxiety, headache and shortness of breath. Intravenous infusion of methylene blue reduces the Fe3+ in the heme back to Fe2+

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Thalassemias - VERY IMPORTANT

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Single gene disorders, defined as mutations that diminish the production of one of the two chains of hemoglobin. Some thalassemias are mutations in coding regions
Mutations in intron/exon junctions that block correct mRNA splicing, mutations causing premature stop codons, and mutations causing a “frameshift ”, resulting in a non-functional globin will lead to β0 thalassemias, with no functional beta globin gene. Other thalassemias are mutations of control regions
•These are the Beta+ thalassemias, with reduced globin levels.
•Remember how much globin protein is needed (95% of the weight of the cell).
Beta+ diseases are often due to reduced transcription from the globin gene locus. Some are in the promoter region, just 5’ to the coding sequences.
•Many others are large deletions up to 50 kb away from the coding regions.
•Deletions obliterate the enhancer sequences in the locus control region (LCR), needed for high level transcription. Thalassemias provide information about control of gene expression
•There is control of the entire globin locus by sequences far from the coding regions.
•This LCR is essential for the high level of transcription necessary for normal blood.
•There are several sequences that serve as enhancer and promoter elements. Transcription factor proteins bind to these DNA sequences. Thalassemias are excellent candidates for gene therapy
•Rather than inject the needed globin, try to start expression of the correct globin.
•When there is a deletion, therapy would require replacement with a correct gene copy.
•Thus, try to get the correct gene into cells = gene therapy!

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beta-zero-thalassemia.

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Mutant in both genes for beta chains, make no beta subunit, severe anemia

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Summary of Thalassemias

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Gene structure is related to developmental control of gene expression.
•Some thalassemias are due to errors in the globin coding regions.
•Others are due to mutations that cause faulty regulation of globin expression.
•The globin genes have regulatory sequence that provide further control of the very high levels of protein expression required for the synthesis of the hemoglobin in RBCs.