Genetic basis of Hemoglobinopathies, Thalassemia and Hemophilia

Question 1

Globin genes:
How many α and β-globin gene?

adult and fetal Hb proteins?

Answer 1

• there are 2 α-globin genes on chrom 16 = 4α genes in a nl individual.
• 1 β-globin gene on chrom 11 and 2 β in a nl indiv.

Adult Hb: (98%HbA “2α & 2β” + 3.5%HbA2 “2α & 2s”

Fetal Hb: (HbF “α2γ2”)

Question 2

Globin genes in early fetal dvlp. and site of EPO

Post-conceptual age “wks”:

Postnatal age “wks”:

Answer 2

α globin remains relatively constant throughout.

• γ globin high and decreases at birth. B globin low and increases at birth. EPO in the liver and spleen.
• γ globin low and B globin high postnatal. EPO in the BM.

Question 3

Hemoglobinopathies

Answer 3

Qualitatitive change: Mutation in nucleotide sequence of globin chain

Question 4

Thalassemia

Answer 4

Quantitative change: Decreased or absent globin chain synthesis

Question 5

HbS

Answer 5

• single pt mutation in B-globin gene
• Glutamic acid (-ve) substituted by Val (hydrophobic). GAG>GTG “transversion mutation”
• HbS moves slower to anode than HbA.
• homozygous HbSS causes SCA; no production of HbA.
• No manifestation of SCA during fetal life b/c B-globin production is low in early infancy. Age of presentation 6mo-1yr.

Question 6

Sickle cell crisis caused by?

Answer 6

anemia, hemolysis and vaso-occlusive ischemia round the abd and long bones.

Question 7

Treatment w/ fetal Hb inducers.

Drugs that increase expression of fetal globins (y) are

Answer 7

5-azacytidine (Decitabine; demethylating agent), Hydroxyurea, Butyrate compounds (inhibit histone deactylation)

• all these agents derepression of gamma globin gene.
• they alter epigenetic gene regulatory mechanism and may change the acetylation of chromatin proteins => more HbF formation.
• HbF has altered O2 affinity which means HbS is less likely to polymerize.

Question 8

Heterozygous carrier HbAS

Answer 8

• 1 nl B-globin gene and 1 Bs globin gene.
• benign condition but may develop sickle crisis in low O2 saturation (deep sea diving and unpressurised aircraft or extreme exercises)

Question 9

Heterozygote advantage

Answer 9

• parasite failed to propagate in peiople w/ the sickle cell trait (RBCs w/ HbA and HbS).
• explains high incidence of carriers in the area (Africa, Mediterranean basin, Middle East, India)

Question 10

HbC

Answer 10

• missense point mutation (Glu-> Lys)+ve
• HbC moves the slowest (HbA>HbS>HbC)
• homozygote for HbC have mild hemolysis
• HbC has lower solubility and crystallize in RBCs.
• pts w/ HbSC: episodes of sickling similar to sickle cell disease.

Question 11

Thalassemias:

alpha:
beta:

Answer 11

• rate of synthesis of globin chain is reduced.
• reduced alpha chain synthesis
• reduced B chain synth “more common b/c only 2 B globin gene present as opposed to 4 alpha”

Question 12

α-Thalassemia

loss of one gene (αα/α_)
loss of two genes (α_/α_) or (αα/__)
loss of three genes (α_/__)
loss of all genes (__/__)

Answer 12

• defic. in α globin chain synth.
• α globin deletion caused by unequal crossing over/misalignment during homologous recombination.
• frontal bossing, malar prominence - indicative of extramedullary HPO, “hair on end” skull radiograph.
• no clinical problems
• trans/cis => mild anemia
• cause B4 tetramers referred to as HbH, = moderate hemolytic anemia
• lethal, known as hydrops fetalis

Question 13

Hemoglobin Bart hydrops fetalis (Hb Bart) syndrome

Answer 13

• most severe form of a-thal.
• Hb Bart: aggregation of Y4 tetramers
• edema, ascites, pleural and pericardial effusions and severe hypochromic anemia.
• death in neonatal period.
• all for a-globin alleles deleted or dysfunctional.
• both parents are cis carrers and have donated chrom w/o a-globin

Question 14

HbH disease

Answer 14

• mild-to-moderate microcytic hypochromic hemolytic anemia and hepatosplenomegaly.
• 1/3rd of affected indiv have mild thalassemia like bone changes.
• compatible w/ survival into adulthood.
• aggregation of B4 tetramers
• deletion or dysfunction of 3/4 a-globin alleles.
• parent 1 is a cis carrier and parent 2 is a trans carrier.

Question 15

B-thalassemia

Answer 15

• AR w/ high carrier frequencies
• many mutations cause B-thal “allelic heterogeneity”
• absent or reduced synth of B globin gene
• B+ thal: reduced gene exp
• B0 thal: complete suppresion of gene exp “more severe”
• excess a-globin chains precipitate and results in severe hemolytic anemia
• BM compensate and expands to perform EPO = bone deformity and fractures.
• frontal bossing, malar prominence = extramedullary HPO, hair on end skull

Question 16

B-thal major (Cooley’s Anemia and Mediterranean Anemia)

Answer 16

• homozy or compound hetero for B0 or B+ genes - two severe mutations
• mutations in both B-globin
• low/absent HbA levels; High HbA2 and HbF levels
• present at ~6mo as y-globin genes are switched off.

Question 17

B-thal intermedia

Answer 17

• mostly homozy or compound hetero (diff B+ mutations on B-globin genes) one severe mu, second mut is less severe.
• Low HbA levels

Question 18

B-thal minor (B-thal carrier or B-thal trait or heterozygous B-thal)

Answer 18

• mostly heterozygous (one nl and one mutant B-globin gene)

- almost nl HbA levels; phenotypically nl

Question 19

Molecular basis/defect of B-thal

Answer 19

• excess of a-chain that don’t form tetramers. They bind to the RBC membranes, producing memb damage, and at high conc they form toxic aggregates.
• rarely results from gross gene deletion
• cure is via BM transplantation from an HLA identical sibling.

Question 20

Hemophilia A

Answer 20

• XL recessive bleeding disorder caused by defic. in activity of caogulation factor VIII
• affected indiv = hemorrhage into joints and muscles, easy bruising and prolonged bleeding from wounds.

Question 21

Hemophilia A and B

Answer 21

• both express allelic hetrogeneity
• genes for both Factor VIII and IX are located on long arm of X-chrom.
• gene of F8 is large; 40% of cases of severe FVIII from large inversion of intron “most frequent mutation”
• deletions, insertions and pt mut account for the remaining 50-60% of hemophilia A (allelic hetero).
• F9 gene is small; pt mutations and deletions most common causes of Hemophilia B.
• inheritance pattern: affected male > carrier female > affected male
• no male to male

Question 22

Hemophilia A - inversion

Answer 22

-if there’s a mis-alignment during hom. recombination where the repeats align w/ each other then part of the F8 gene is lost.

Question 23

Phenotypes of carrier females

Answer 23

• generally asymptmatic due to random X inactivation/Lyonization “skewed X” results in equal proportions of somatic cells.
• Manifesting heterozygotes: some carrier females showing a lower level of clotting factors and manifestations resemble affected males