Hemoglobinopathies and Iron Metabolism

Question 1

What is a hemoglobinopathy?

Answer 1

• An inherited disorder (usually recessive) that results in moderate to severe anemia
• Can be divided into quantitiative disorders) thalassemias in which there is a reduced production of selected globin chains or:
• Structual hemoglobin disorders in which there is synthesis of defective globin chains such as sickle cell disease

Question 2

Describe the globin gene loci:

Answer 2

1. Individuals have 2 beta gobin genes (one maternal and one paternal)
   - Clustered on chromosome 11 in the order they are deveopmentally expressed)
   - Epsilon, gamma, delta then beta
2. Individuals have 4 alpha globin genes (2 maternal and 2 paternal)
   - Clustered on chromosome 16 (zeta and alpha)

Question 3

Describe the genetic cause and types of beta thalassemia:

Answer 3

1. Mutation of one beta globin gene: beta thalassemia minor
   - Mild anemia
2. Mutation of both beta globin genes so one is silenced and one is reduced: beta thalassemia intermedia
3. Mutation of both beta globin genes so they are silenced: beta thalassemia major
   - Caused transfusion dependent anemia
   - Life expectancy is 4 years untreated and 50 years if optimally treated
   - Caused anemia, bone deformity (due to production of RBCs in all bones) and iron overload (due to transfusions and extremely poor erythropoiesis)

Question 4

Describe the genetic cause and types of alpha thalassemia:

Answer 4

1. Mutation of 2 alpha globin genes: alpha trait
2. Mutation of 3 alpha globin genes: hemoglobin H disease
   - The RBCs contain multiple unpaired beta globin genes that coagulate and form inclusions
3. Mutation of all 4 alpha globin genes: hydrops fetalis
   - Severe oedema and death during fetal development due to heart failure

Question 5

How is beta thalassemia diagnosed?

Answer 5

• Microcytic and hypochromic anemia (also occurs with iron deficiency and AOCD)
• Excluding iron deficiency
• Decreased hemoglobin
• Very low MCV
• Hemoglobin electrophoresis and HPLC of globin molecules
• DNA analysis

Question 6

What is sickle cell disease?

Answer 6

• Qualitative disorder of hemoglobin production in which an adequate quantitiy is produced but it is abnormal in structure and function
• Caused by a valine to glutaminc acid mutation in the beta globin gene
• Causes RBC deformity, vessel obstruction and organ infarction
• Life expectancy: 14.3 years untreated, 42 years with optimal treatment

Question 7

How is Thalassemia treated?

Answer 7

• If untreated beta thalassemia major causes death in the first decade of life
• Treated with chronic blood transfusions
• Iron overload from chronic blood transfusions is managed with chelating agents
• New research into activation of fetal hemoglobin genes
• Bone marrow transplant
• Gene therapy

Question 8

How is sickle cell disease treated?

Answer 8

• Exchange transfusions (but can cause iron overload)
• Rapid treatment of infections
• Pain relief
• Activation of fetal hemoglobin (potential)
• Bone marrow transplant
• Gene therapy (potential)

Question 9

How is the absorption of iron regulated?

Answer 9

• The oral intake of iron (10-20mg per day) significantly outweighs the absorption (1-2mg per day)
• The body cannot store excess iron so the dietary absorption is strictly controlled

1. Non-heme iron is converted to Fe2+ by gastric acid and is transported into epithelial cells via a divalent metal transporter 1 (DMT1) and heme iron is transported into epithelial cells via the heme transporter
2. Once in epithelial cells the iron is stored as mucosal ferratin or shuttled to ferroportin 1 iorn channels on the basal surface of the enterocyte, the ferroportin channel is open only when the body requires iron (when hepcidin levels are low)
3. If iron is transported through the ferroportin channels it is is converted to Fe3+ and moved into the portal blood where it is transported by transferrin
4. Transferrin takes the bound iron to developing cells in the bone marrow and to the liver

Question 10

How does hepcidin control iron metabolism?

Answer 10

• Hepcidin is the master iron regulatory protein and is produced by the liver
• Hepcidin acts on ferroportin channels and closes them and thus prevents the absorption of iron
• Hepcidin is upregulated during inflammation, as reducing the availability of iron can fight infection- but also can result in anemia of chronic disease where the ferroportin channels remain closed and iron accumulated in eneterocytes and macrophages
• Hepcidin binds to ferroportin on villous enterocytes, macrophages and hepatocytes
• Hepcidin levels are downregulated (iron absorption upregulated) when there is:
  1. Decreased iron stores
  2. Increased erythropoietic activity
  3. Anaemia
  4. Hypoxia

Question 11

How is iron stored in the body (very limited)?

Answer 11

Ferritin:

• In an otherwise healthy patient it can be used to measure iron levels in the body (very low in iron deficiency)
• Ferritin assays cannot distinguish between apoferritin and ferritin

1. Apoferritin
   - Not complexed with iron
   - Acute phase protein that increases with non-specific inflammation
2. Ferritin:
   - Complexed with iron

Hemosiderin:

• Insoluble storage iron and iron deposition in the tissues
• A result of iron overload
• Causes damage in the tissues as iron is a key driver of reactive oxygen species which leads to chronic inflammation and fibrosis of the tissue

Question 12

What is an iron responsive element?

Answer 12

• IREs bind iron regulatory proteins and regulate the translation of some proteins
• 5’ IRES: binding of a 5’ IRE prevents the initiation of iron translation e.g. if iron levels are high, the IR-protein will not bind to the 5’ IRE of ferratin so it can be readily translated
• 3’ IRES: binding of a 3’ IRE increases the stability of mRNA and thus increases the translation of the protein e.g. when iron levels are low, IRPs will bind the 3’ IRE on transferrin mRNA and increase its synthesis

Question 13

What is the differential diagnosis for microcytic, hypochromic anaemia?

Answer 13

• B-thalassemia minor, alpha thalassemia, iron deficiency and anemia of chronic disease

Question 14

What laboratory results indicate iron deficiency anaemia?

Answer 14

• Low serum iron (not useful)
• High transferrin
• Low transferrin saturation
• Low ferritin (definitely iron deficiency if no inflammation)

Question 15

What is iron overload?

Answer 15

• When levels of iron in the body are too high, the limited iron storage capacity of the liver is overwhelmed and iron is deposited as hemosiderin in other tissues such as the heart, liver, pancreas, skin etc. which causes pigmentation, inflammation and scarring in these tissues
• Due to genetic mutations or acquired

Question 16

What is hereditary haemochromatosis?

Answer 16

• Due to muations in the HFE genes which reduce the activity of hepcidin which causes the ferrportin channels to be consituitively open
• Complex phenotypes (e.g. less serious phenotype in menstruating women and vegetarians)
• Treated with venesection to reduce ferratin levels

Question 17

How is acquired iron overload treated?

Answer 17

• Often acquires in patients with thalassemia or sickle cell disease that require chronic blood transfusions
• Iron chelation is given to patients