Hemoglobinopathies Flashcards
Molecular disease
An inherited or acquired mutation causing a primary disease.
Biochemical genetics
The study of phenotype at the level of proteins, biochemistry, and metabolism.
What is necessary to understand the pathogenesis of molecular disease?
one must have an understanding of the biochemical abnormalities that result in disease pathogenesis.
What are the three regions where mutations can occur?
- Mutations in the coding region
- Mutations affecting the RNA splicing or disrupting RNA stability.
- Mutations regulating gene dosage or regulation.
Mutations in the coding region
Indicates that a gene is involved, which results in abnormal protein synthesis→Cause disease→
LOF-majority, 3º structure is lost, aa’s abnormally folded.
GOF-enhancement of a particular protein fxn or increase in protein in a region.
Novel Property-infreq. Sickle cell anemia-rbc’s develop the novel property of sickling
Ectopic or heterochronic expression-EX. Insulin gene is in every cell, but it’s only switched on in the β-cells. Mutations can lead to switching “on” of certain genes and cause cancer/disease.
Mutations disrupting RNA stability or RNA splicing
Post-trasnscriptional modification like attaching a Cap or tail so it affects the stability or splicing or RNA. Usually results in normal protein structure, but the quantity may is changed, or the there is inappropriate expression(wrong time/place)
Mutations affecting gene regulation or dosage
Enhancer/promoter region is affected→alters the amount of transcription/translation→↑/↓ of protein. So abnormal qty, but the structure is normal.
Disorders due to decreased amount of protein
α-thalassemias
Monosomies
Tumor suppressor mutations
Disorders due to increased amount of protein
Trisomies
Charcot-Marie-Tooth disease type 1a
Disorder due to inappropriate expression-wrong time/place
Hereditary Persistence of Fetal Hemoglobin
Many oncogenes
Disorders due to abnormal protein structure
Hb Hammersmith β-Thalasemmias Hb Kempsey Achondroplasia Hb S
Defect in transcription leads to
α and β thalasemmias. Genetic code is altered→altered gene expression→ can be ↑/↓ synthesis of proteins.
Def of α-globulin chain→α-thalasemmia
Def of β-globulin chain→β-thalassemia
Defect in translation leads to
β-thalassemia
Defect in folding of proteins
Thalassemias, Unstable. Depends on type of aa’s and types of domains affected. Misfolded proteins are usually degraded-PolyUbiquinated and destroyed into polysomes.
Defect in Post-translational modification
I cell disease. After protein has been synthesized, the protein needs to be targeted to various locations. Defects in post-trans mod leads to the defect in targeting of the lysosomal proteins due to mutation in Mannose-6-phosphate.
Defect in assembly of Monomers to Holomers
Osteogenesis Imperfecta-defect in the structure of collagen→inability to assemble medu-collagen→brittle bones
Defect in Sub-cellular localization of Holomer
Familial Hypercholesterolemia. Defect in “Receptor mediated endocytosis” of LDL receptors→Deficiency or defect in LDL receptors so LDLs not taken up into cells and increased amount stay in circulation→Oxidized LDL→Plaques.
Defect in Co-factor binding
Homocyateinuria-Cystathionine β-synthase.
Defective attachment site of cofactor-cofactor will not bind and enzyme efficiency will be lost.
Defect of a correctly folded protein
Hb Kempsey-LOF or a correctly folded protein. Due to some mutation, there is a LOF of a correctly folded protein.
What is the hemoglobin structure?
One pair of α-like chains
One pair of β-like chains
Each globin chain contains a single HEME moiety: a protoporphyrin ring with a complexed Fe2+.
Locus Control Region (LCR)
Responsible for the Sequential Switching on/off of gene expression. Often via methylation. i.e. Beta globin gene on chromosome 11 is partly controlled by a promoter and 2 enhancers and requires the switching on of the LCR for the β-globin chains to be synthesized.
Hemogloinopathy
An inherited mutation in globin genes leading to qualitative or quantitative abnormality of globin synthesis. Hemoglobinopathies may be: 1. Structural 2. Thalassemias 3. HPFH
Heterozygote advantage
Malarial resistance: Plasmodium falciparum grows in rbc’s. If the life cycle of rbc’s is halted due to hemolysis, then the malarial parasites cannot complete their life cycle: confers resistance.
Hemoglobinopathies have a geographic distribution.
What are the structural variants of Hemoglobinopathies due to?
Point mutations:
Silent→usually no phenotypic manifestation because the same aa is being produced.
Missense→Mutation may develop abnormal characteristics. diff aa
Nonsense→Introduce a stop codon/terminates translation sequence→abnormal proteins are synthesized and destroyed.
Hemoglobins with Novel physical properties:
HbS:Glu6Val→deoxy HbS polymerizes and form sickle cells→vascular occulusion and hemolysis. AR
HbC:Glu6Lys→Oxygenated HbC crystallizes leading to less deformable cells→mild hemolysis.
Hb S and Hb C compound heterozygotes is like a mild sickle cell disease.
Unstable Hemoglobins:
Hb Hammersmith:Phe42Ser →unstable Hb precipitates→hemolysis AD
Sickle cell disease
AR, single nucleotide substitution on β-globin gene→Glu6Val. Glutamic acid is polar which is replaced with non-polar/aliphatic valine →net loss of charge→replaced with hydrophobic aa→development of hydrophobic pockets→hydrophobic interactions →”sticky pockets”Hb starts polymerizing→insoluble congregates of Hb “heinz bodies”
Sickling occurs in tissues→not lungs where there is high O2 content and in “R” state, tissues are ↓O2 and “T” state which promotes sickling. Acidosis→sickling
Africa, Meditteranean, and Malaria endemic regions.
Homozygous Sickle cell disease
Severe hemolytic anemia present in first 2 yrs of life.
Failure to thrive
Splenomegaly→repeated infections
Painful swelling of hands/feet due to vessel/capillary occlusion.
Heterozygotes→Sicke cell Trait
Clinically normal, but RBC sickle when exposed to hypoxic conditions, low pressure cabins in high altitudes.
What is the molecular pathology of HbS?
Interaction of Valine on one chain and a HYDROPHOBIC patch formed by Phe85 and Leu88 on another deoxy Hb molecule leads to aggregation→basically hydrophobic and deoxy interaction.
Decreases net charge due to replacement of acidic glutamic acid with aliphatic Valine.
Polymerizes reversibly when deoxygenated to form gelatinous network of fibrous polymers that stiffen the erythrocyte membrane.
What is the sickle cell disease pathogenesis?
Abnormal polymers attach to the rbi membrane→lose pliabilty→sickle shaped cell attach to endothelium→vessel occlusion→ischemia and hypoxia to peripheral tissues.
GAG→CTG
