CPR 52 - Hemoglobinopathies

Question 1

Where are the α-like and β-like globin genes located and how many of them are there in a normal person?

Answer 1

There are two α-globin genes (α₁, α₂) on chromosome 16 which means there are four in a normal person.

There is one β-globin gene on chromosome 11 which means there are two in a normal person.

Question 2

Differentiate a hemoglobinopathy from a thalassemia.

Answer 2

Hemoglobinopathies are qualitative changes to Hb that result from a mutation in the globin genes.

Thalassemias are quantitative changes to Hb that result from a mutation that leads to decreased or absent globin chain synthesis.

Question 3

What is the most most prominent method used to treat sickle cell disease? How does it work?

Answer 3

HbF is normally suppressed after birth by methylation of CpG islands in the γ-globin gene promoter. 5-azacytidine will increase γ-globin gene expression via demethylation. Hydroxyurea Butyrate compounds will increase γ-globin gene expression by inhibiting histone deacetylation.

This will lead to more HbF formation which will decrease the degree of sickling.

Question 4

What is the disease called when an individual is heterozygous for the sickle cell mutation? What is the shorthand name for this disease? Is this disease symptomatic?

Answer 4

Sickle Cell Trait

HbAS

Usually not symptomatic except during

Question 5

What causes HbC to form? What is detrimental about HbC production? What are the symptoms of a person heterozygous and homozygous for the production of HbC?

Answer 5

HbC production is caused by a point mutation at the 6th position of the β-globin gene, which changes glutamate to lysine. HbC is less soluble than HbA and tends to crystallize in RBCs. Persons that are homozygous for the HbC mutation have mild hemolysis. Persons that are heterozygous for the mutation are generally asymptomatic.

Question 6

What causes the formation of HbSC? What are the symptoms of someone who has these mutations?

Answer 6

HbSC occurs when someone has the HbS mutation in one β-globin allele and the HbC mutation in the other allele. Patient’s that have HbSC may have episodes of sickling that are similar to sickle cell disease but much more mild.

Question 7

How do HbA, HbS, HbAS, HbC, HbSC run on a Hb electrophoresis gel?

Answer 7

Top - HbC (glu to lys)

HbS (glu to val)

Bottom - HbA

HbAS has both HbA and HbS bands

HbSC has both HbS and HbC bands

Question 8

How will an RFLP gel appear for HbA, HbS, and HbAS? Explain.

Answer 8

MstII is a restriction enzyme that cuts the normal β-globin gene in three places and the sickle-cell β-globin gene in two. Refer to image for how gel appears.

Question 9

What is the normal concentration ratio for the α and β globin chains? How does this change with α and β thalassemias?

Answer 9

Normally the ratio is 1:1. With β-thalassemia the α-chain predominates. The opposite is true with α-thalassemia.

Question 10

What is the most common cause of α-thalassemia?

Answer 10

α-globin gene deletion caused by unequal crossing over during homologous recombination.

Question 11

List the different levels of α-thalassemia that exist and their symptoms.

Answer 11

1. (αα/α–) is an asymptomatic silent carrier
2. (α–/α–) or (αα/– –) results in mild anemia
3. (α–/– –) causes β4 tetramers referred to as HbH, which causes moderate hemolytic anemia.
4. (– –/– –) is lethal and is know as hydrops fetalis or Hb Bart (an accumulation of fluid in at least two fetal compartments)

Question 12

What are some telltale symptoms of HbH disease? Why do these symptoms develop?

Answer 12

Frontal bossing - protuberance of the forehead

Malar (zygoma/cheek bone) prominence

This occurs because extramedullary hematopoiesis takes place in this disease to try to compensate for the lack of α-globin.

Question 13

Differentiate allelic heterogeneity from locus heterogeneity. Which type is β-thalassemia?

Answer 13

Allelic heterogeneity - different mutations at the same locus produce the same phenotype

Locus heterogeneity - different mutations at differnt loci produce the same phenotype

β-thalassemia is allelic heterogeneity

Question 14

What are the two general types of β-thalassemia mutations?

Answer 14

β+ mutation - reduced gene expression

β⁰ mutation - complete suppression of gene expression

Question 15

What do more severe forms of β-thalassemia lead to?

Answer 15

A decrease in functional HbA and an increase in α-globin precipitates which leads to severe hemolytic anemia.

Because of the decrease in functional HbA, bone marrow tries to compensate and it expands to perform more erythropoiesis. This leads to the bone deformities also seen in HbH disease (frontal bossing and malar prominence).

Question 16

What is a compound heterozygote?

Answer 16

A gene that has a different mutation in each allele

Question 17

What causes β-thalassemia major and which Hb levels are high/low in this disease?

Answer 17

Caused by severe mutations in both β-globin genes. This is usually caused by a homozygote or compound heterozygote for β⁰ or β+ mutations.

Very low or absent HbA levels

High HbA2 and HbF levels

Question 18

What causes β-thalassemia intermedia and which Hb levels are high/low in this disease?

Answer 18

Caused by one severe mutation and one less severe or by two less severe mutations. Mostly homozygotes or compound heterozygotes for different β+ mutations.

Low HbA levels

High HbA2 and HbF levels

Question 19

What causes β-thalassemia minor and which Hb levels are high/low in this disease?

Answer 19

Caused by one normal and one mutant β globin gene. Mostly heterozygotes.

Almost normal HbA levels

Question 20

How many different types of mutations have been identified that cause β-thalassemias?

Answer 20

>200

Question 21

When does β-thalassemia major normally present and why?

Answer 21

At six months because that is typically when γ-globin genes are switched off.

Question 22

How is β-thalassemia usually treated?

Answer 22

Regular transfusions will correct the anemia and suppress erythropoiesis but lead to iron deposition. Iron chelation and dietary control can manage these complications.

The only available long term cure is a bone marrow transplant from a HLA (human leukocyte antibody) identical sibling.

Question 23

How do β-thalassemias inherit?

Answer 23

Autosomal Recessive

Question 24

How does Hemophilia A & B inherit? Why?

Answer 24

X-linked recessive

The factor 8 and 9 genes are located on the long arm of the X chromosome

Question 25

What types of mutations typically lead Hemophilia A and why? Hemophilia B?

Answer 25

The factor 8 gene is unusually large. Because of this about 40% of Hemophilia A cases are caused by large inversions. The remaining 50-60% of cases are caused by deletions, insertions, and point mutations.

The factor 9 gene is small. Because of this almost all Hemophilia B causing mutations are point mutations and deletions.

Question 26

Why is intron inversion such a common mutation for Hemophilia A?

Answer 26

Exons 1 to 22 and Exons 23 to 26 are very similar in sequence and length. This causes abberant recombination to occur leading to an inversion.

Question 27

Why are most women not affected by Hemophilia A & B? What is the exception?

Answer 27

Random X-inactivation results in all somatic tissues having half the cells with a normal X-chromosome. This is sufficient for normal hemostasis to occur. However, in some females skewed X-inactivation occurs leading to a higher percentage of inactivation of the normal X-chromosome.