Hemoglobinopathies

Question 1

Molecular disease

Answer 1

An inherited or acquired mutation causing a primary disease.

Question 2

Biochemical genetics

Answer 2

The study of phenotype at the level of proteins, biochemistry, and metabolism.

Question 3

What is necessary to understand the pathogenesis of molecular disease?

Answer 3

one must have an understanding of the biochemical abnormalities that result in disease pathogenesis.

Question 4

What are the three regions where mutations can occur?

Answer 4

1. Mutations in the coding region
2. Mutations affecting the RNA splicing or disrupting RNA stability.
3. Mutations regulating gene dosage or regulation.

Question 5

Mutations in the coding region

Answer 5

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.

Question 6

Mutations disrupting RNA stability or RNA splicing

Answer 6

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)

Question 7

Mutations affecting gene regulation or dosage

Answer 7

Enhancer/promoter region is affected→alters the amount of transcription/translation→↑/↓ of protein. So abnormal qty, but the structure is normal.

Question 8

Disorders due to decreased amount of protein

Answer 8

α-thalassemias
Monosomies
Tumor suppressor mutations

Question 9

Disorders due to increased amount of protein

Answer 9

Trisomies

Charcot-Marie-Tooth disease type 1a

Question 10

Disorder due to inappropriate expression-wrong time/place

Answer 10

Hereditary Persistence of Fetal Hemoglobin

Many oncogenes

Question 11

Disorders due to abnormal protein structure

Answer 11

Hb Hammersmith
β-Thalasemmias
Hb Kempsey
Achondroplasia
Hb S

Question 12

Defect in transcription leads to

Answer 12

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

Question 13

Defect in translation leads to

Answer 13

β-thalassemia

Question 14

Defect in folding of proteins

Answer 14

Thalassemias, Unstable. Depends on type of aa’s and types of domains affected. Misfolded proteins are usually degraded-PolyUbiquinated and destroyed into polysomes.

Question 15

Defect in Post-translational modification

Answer 15

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.

Question 16

Defect in assembly of Monomers to Holomers

Answer 16

Osteogenesis Imperfecta-defect in the structure of collagen→inability to assemble medu-collagen→brittle bones

Question 17

Defect in Sub-cellular localization of Holomer

Answer 17

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.

Question 18

Defect in Co-factor binding

Answer 18

Homocyateinuria-Cystathionine β-synthase.

Defective attachment site of cofactor-cofactor will not bind and enzyme efficiency will be lost.

Question 19

Defect of a correctly folded protein

Answer 19

Hb Kempsey-LOF or a correctly folded protein. Due to some mutation, there is a LOF of a correctly folded protein.

Question 20

What is the hemoglobin structure?

Answer 20

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+.

Question 21

Locus Control Region (LCR)

Answer 21

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.

Question 22

Hemogloinopathy

Answer 22

An inherited mutation in globin genes leading to qualitative or quantitative abnormality of globin synthesis.
Hemoglobinopathies may be:
1. Structural
2. Thalassemias
3. HPFH

Question 23

Heterozygote advantage

Answer 23

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.

Question 24

What are the structural variants of Hemoglobinopathies due to?

Answer 24

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.

Question 25

Hemoglobins with Novel physical properties:

Answer 25

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.

Question 26

Unstable Hemoglobins:

Answer 26

Hb Hammersmith:Phe42Ser →unstable Hb precipitates→hemolysis AD

Question 27

Sickle cell disease

Answer 27

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.

Question 28

Homozygous Sickle cell disease

Answer 28

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.

Question 29

Heterozygotes→Sicke cell Trait

Answer 29

Clinically normal, but RBC sickle when exposed to hypoxic conditions, low pressure cabins in high altitudes.

Question 30

What is the molecular pathology of HbS?

Answer 30

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.

Question 31

What is the sickle cell disease pathogenesis?

Answer 31

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