Molecular Genetics of Hemoglobinopathies Flashcards
Hemoglobin Structure - Function Relationships
The major form of adult hemoglobin (HbA) is a α2β2 tetramer of two α- and two β-globin chains. Each globin contains one heme group with a covalently linked iron that binds oxygen, so one HbA can simultaneously bind four oxygen molecules.
All of α and α-like genes are in the α-cluster on chromosome _______ while all of β and β-like genes are in the β-cluster on chromosome _______
16
11
There are _______ copies of α in the α-cluster but only _______ of β in the β-cluster
two
only one
The genes in each cluster have the same _______. At the molecular level the _______ of these genes are very similar
- 5’-to-3’ transcriptional orientation
- promoter and enhancer regions
Pseudogene:
resembles a gene but makes no protein. ψζ, ψα, and ψβ are all pseudogenes.
The 5’-to-3’ spatial order of genes within each cluster coincides with _______. The sequential expression of these genes during development is under the regulation of the _______, which is located at the most _______ region of each cluster.
- the temporal order of their expression during development
- Locus Control Region (LCR)
- upstream
It is currently thought that the _______ and a particular globin gene affects its expression
distance between the LCR and a particular globin gene
The LCR presumably makes physical contact with the _______ via _______ to influence gene expression
- promoter and/or negative regulatory regions
- specific transcriptional factors
Deletions of the entire LCR of the beta cluster cause _______, a condition in which _______
beta-thalassemias
zero β-globin synthesis leads to precipitation of the α-globin chains.
Adults have two Hb forms (made in the bone marrow):
- major form: HbA = α2β2 (97%)
- minor form : HbA2 = α2δ2 (2%)
- Note that δ level is much lower than β because δ has a weaker promoter.
Embryonic hemoglobins (made in the yolk sac):
ζ2ε2 = Hb Gower I α2ε2 = Hb Gower II ζ2γ2 = Hb Portland
Fetal hemoglobins (made in the liver):
α2γ2 = HbF
Globin Switching
(i) turn-off of ζ and ε, turn-on of α and γ during early embryogenesis.
(ii) turn-off of γ, turn-on of β and δ around the time of birth.
Significance of Globin Switching
HbF has higher affinity for O2 at low pO2 than HbA. Thus, HbF in fetal blood is better suited to bind O2 at the placenta (lower pO2) than HbA, which binds O2 at the lung (higher pO2).
Structural Variants (qualitative hemoglobinopathies)
Mutations that alter the globin polypeptide properties without affecting its synthesis. Most structural variants are found in the β-globin chain. These mutations may alter O2 binding such as HbKemsey (too tight) and HbKansas (too weak), cause heme loss and denaturation of Hb, or make Hb less soluble such as HbS and HbC. Some can lead to serious red blood cell (RBC) diseases
Thalassemias (quantitative hemoglobinopathies)
Disorders of imbalanced globin levels resulted from markedly reduced or no synthesis of one globin type. Can be caused by virtually all types of genetic mutations including deletions, missense and nonsense mutations, and defective transcriptional control
Hereditary Persistence of Fetal Hemoglobin (HPFH) `
HPFH is a group of clinically benign conditions that impair the perinatal switch from γ- to β-globin synthesis, leading to continued high-level production of HbF in adults
Sickle cell anemia (HbSS)
Most common among people of African origin, where carrier frequency is ~10%. Single base mutation at codon#6 in the β-globin gene changes glutamate to valine. HbS is 80% less soluble than HbA when not bound to O2, and polymerizes into long fibers that distort the RBC into a characteristic sickle shape. These sickled cells become lodged in the micro-capillaries and further exacerbate the sickling crisis.
Hemoglobin C disease (HbCC)
A milder form of hemolytic anemia than sickle cell anemia. Caused by a single base mutation at codon#6 of the β-globin gene, changing glutamate to lysine. HbC is less soluble than HbA and tends to form crystals, reducing the deformability of RBC.
HbS or HbC trait
Both sickle cell anemia and hemoglobin CC disease are of autosomal recessive inheritance. Sickle cell trait (HbS trait) or hemoglobin C trait (HbC trait) describes the conditions expressed in individuals who are heterozygous of HbS/HbA and HbC/HbA, respectively. They are clinically normal except when under severe low pO2 stress.
Hemoglobin SC disease
Compound heterozygotes (βS/βC) have a milder anemia than sickle cell disease.
HbS Diagnosis Using RFLP
A recognition site (CCTNAGG) of the restriction enzyme MstII is destroyed in exon 1 by the A-to-T change in the βS mutant allele. The normal allele βA gives 1.15 kb + 0.20 kb fragments, whereas the βS mutant allele give one1.35 kb fragment.
A functional Hb tetramer is composed of _______. Homotetramers (e.g. α4, β4, γ4) are _______ O2 carriers and precipitate _______
- two α (or α-like) chains and two β (or β-like) chains
- poor
- inside the RBC
Thalassemias are caused by
an imbalance in the relative levels of the α and β globin chains, which leads to the precipitation of the globin in excess and decreases the life span of the RBC
