Hemoglobinopathies

Question 1

The major form of adult hemoglobin

Answer 1

(HbA) is a α2β2 tetramer of two α- and two β-globin chains.

Question 2

Each globin contains one __________, so one HbA can simultaneously bind ______ oxygen molecules.

Answer 2

heme group with a covalently linked iron that binds oxygen,

four oxygen molecules.

Question 3

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 ____

Answer 3

16

11

Question 4

___ copies of a in an alpha cluster

Answer 4

2

Question 5

___ copies of B in a B-cluster

Answer 5

1

Question 6

a-cluster

Answer 6

α-cluster: zeta-alpha2-alpha1

ζ-α2-α1

Question 7

the genes in a cluster have the same

Answer 7

5’–3’ txn orientation

Question 8

B-cluster

Answer 8

epsilon-gammaG-gammaA-delta-beta (ε-γG-γA-δ-β)

Question 9

Pseudo gene

Answer 9

esembles a gene but makes no protein. ψζ, ψα, and ψβ are all pseudogenes.

Question 10

The 5’-to-3’ spatial order of genes within each cluster coincides with the ______

Answer 10

temporal order of their expression during development.

Question 11

The sequential expression of these genes during development is under the regulation of _____), which is located ________ of each cluster

Answer 11

of the Locus Control Region (LCR),

at the most upstream region of each cluster

Question 12

t is currently thought that the distance between the LCR and a particular globin gene affects its _____

Answer 12

expression

Question 13

The LCR makes physical contact with the _____ and/or _____ regulatory regions via specific transcriptional factors to influence gene expression.

Answer 13

promotor

negative

Question 14

Deletions of the entire LCR of the beta cluster cause _____, a condition in which________ leads to precipitation of the ______

Answer 14

beta-thalassemias,

zero β-globin synthesis

the α-globin chains.

Question 15

Adults have two Hb forms, which are made in the

Answer 15

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.

Question 16

Embryonic hemoglobins are made in the

Answer 16

yolk sac

Question 17

Embryonic hemoglobin s

Answer 17

ζ2ε2 = Hb Gower I 
α2ε2 = Hb Gower II 
ζ2γ2 = Hb Portland

Question 18

Fetal hemoglobins are made in

Answer 18

the liver

Question 19

fetal hemoglobins

Answer 19

α2γ2 = HbF

Question 20

Globin Switching

Answer 20

(i) turn-off of ζ and ε, turn-on of α and γ during early embryogenesis.
(ii) turn-offofγ,turn-on of β and δ around the time of birth.

Question 21

Significance of Globin Switching

Answer 21

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).

Question 22

Genetic Variants of Hemoglobin

Answer 22

~ 600 variants of hemoglobin. About half of them are clinically significant.

Question 23

Why do you need to study hemoglobinopathies?

Answer 23

hemoglobinopathies are among the best understood genetic disorders at molecular level. You can find almost any type of disease-causing mutations in hemoglobin disorders. Thus, they serve as great examples to illustrate the principal molecular mechanisms underlying human genetic disorders.

Question 24

Structural Variants (qualitative hemoglobinopathies)

Answer 24

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.

Question 25

genetic variants of hemoglobin:

Answer 25

1. structural varaints (qualatative)
2. thalassemia (quantative)
3. heriditary persistence of fetal hemoglobin (HPFH)

Question 26

Thalassemias (quantitative hemoglobinopathies)

Answer 26

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.

Question 27

Hereditary Persistence of Fetal Hemoglobin (HPFH)

Answer 27

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.

Question 28

Sickle cell anemia (HbSS)

Answer 28

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.

Question 29

Hemoglobin C disease (HbCC)

Answer 29

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.

Question 30

HbS or HbC trait

Answer 30

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.

Question 31

Hemoglobin SC disease

Compound

Answer 31

Compound heterozygotes (βS/βC) have a milder anemia than sickle cell disease.

Question 32

HbS Diagnosis Using RFLP

Answer 32

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.

Question 33

βA

Answer 33

-Pro-Glu-Glu- -CCTGAGGAG- MstII site present

Question 34

βS

Answer 34

-Pro-Val-Glu- -CCTGTGGAG-

MstII site gone

Question 35

βC

Answer 35

-Pro-Lys-Glu- -CCTAAGGAG- MstII site present

Question 36

Thalassemias are caused by an imbalance in the relative levels of the________, which leads to the ______

Answer 36

α and β glob in chains

precipitation of the globin in excess and decreases the life span of the RBC.

Question 37

MstII can distinguish between ______but not between _____

Answer 37

βA and βS

βA and βC.

Question 38

A functional Hb tetramer is composed of

Answer 38

two α (or α-like) chains and two β (or β-like) chains. Homotetramers (e.g. α4, β4, γ4) are poor O2 carriers and precipitate inside the RBC.

Question 39

α -Thalassemias are mostly caused by:

Answer 39

α -Thalassemias