Thalassemias, feat. Dr. David Abel
/Here’s the RoshReview Question of the Week!
A 31-year-old G1P1 woman of Southeast Asian descent with a history of intrauterine fetal demise presents to your office for preconception counseling. She also reports a history of mild anemia due to alpha-thalassemia. You order DNA testing. Which of the following is most likely her genotype?
Check if you got the right answer and get a special deal on the CREOG Q-Bank at link above!
The Basics of Hemoglobin
The major oxygen carrying pigments of the body. Carries oxygen from the lungs to the tissues to meet the needs of cells for oxidative metabolism.
We carry almost two pounds of hemoglobin at any given time!
The hemoglobin molecule is a tetramer.
Typically, this tetramer is composed of two alpha chains and two non-alpha globin chains.
The molecular mass of a hemoglobin tetramer is large, approximately 64,000 daltons.
The primary structure of a particular hemoglobin is determined by its covalent bonds between the amino acids that form these polypeptide globins, and it is this primary structure that determines the behavior of a particular hemoglobin.
Hemoglobin synthesis is controlled by two multigene clusters, the alpha and beta globin genes.
The alpha genes are on chromosome 16.
Both genes for alpha globin are duplicated, thus there are four genes at the alpha globin locus, with two genes inherited from each parent.
The beta genes are on chromosome 11.
The beta globin gene consists of two genes, one inherited from each parent.
Each of these two gene clusters also contain other genes!
Common Hemoglobin Molecules and Embryology of Hemoglobin
Hemoglobin changes during fetal development.
The switch from embryonic to fetal to adult hemoglobin synthesis is a major mechanism by which the developing fetus adapts from the hypoxic intrauterine environment, as each hemoglobin has its own oxygen dissociation curve.
In the embryonic stage of development, there exists both zeta and epsilon globin chains that are synthesized by yolk sac erythroblasts.
The zeta gene is part of the alpha globin gene cluster, and the epsilon gene is part of the beta globin gene cluster.
Hb Gower-1: two zeta and two epilson chains
Hb Gower-2: two alpha and two episilon chains
Hb Portland: two zeta and two gamma chains.
After the first trimester, the zeta and epsilon globin chains are replaced by hemoglobin F, the dominant hemoglobin in-utero.
Hb F is composed of two fetal gamma globin chains and two alpha globin chains.
The gamma gene is a fetal gene that is part of the beta globin gene cluster.
Hemoglobin F declines in the third trimester of pregnancy and is slowly replaced by hemoglobin A, which consists of two alpha and two beta chains.
Also keep in mind that expression of delta globin begins near birth. The delta gene is also part of the beta globin cluster, and contributes to hemoglobin A2 (two alpha, two delta globins).
At birth, hemoglobin F accounts for approximately 75-80 percent of hemoglobin and hemoglobin A accounts for 20-25 percent.
Postnatally, hemoglobin F is slowly replaced by hemoglobin A so that infants do not rely heavily on normal amounts and function of hemoglobin A until they are between 4 and 6 months old.
In adults, hemoglobin A makes up approximately 97%, hemoglobin A2 approximately 2.5% and less than 1% consists of hemoglobin F.
The Basics of Hemoglobinopathy and Thalassemias
Hemoglobinopathies arise when a change occurs in the structure of a peptide chain or a defect compromises the ability to synthesize a specific polypeptide chain.
Can be qualitative or quantitative defects.
Thalassemias are quantitative disorders.
Thalassemia is derived from a Greek term that roughly means “the sea” (Mediterranean) in the blood.
It was first applied to the anemias frequently encountered in people from the Italian and Greek coasts and nearby islands.
Individual syndromes are named according to the globin chain whose synthesis is adversely affected.
Alpha thalassemia represents either a reduction or complete absence of production of alpha globin chains
Beta thalassemia is a reduction or complete absence of beta globin production.
Among the most common autosomal recessive disorders worldwide. More than 100 genetic forms of alpha thalassemia have been identified.
By contrast, conditions such as sickle cell anemia represent a structural hemoglobinopathy, a qualitative defect.
Beta Thalassemias
Hemoglobin electrophoresis can be used to diagnose beta thalassemia. This can reveal:
Reduction in the expression of beta globin (b+) or
Complete absence of beta globin expression (b0).
Complete absence of beta globin expression is referred to as beta thalassemia major, aka Cooley’s anemia or transfusion-dependent thalassemia.
Little to no beta globin chain production and thus minimal to absence of hemoglobin A.
Symptoms usually manifest 6-12 months of life.
Since there is no hemoglobin A due to the lack of beta globin, hemoglobin F persists.
On a hemoglobin electrophoresis, you will see at least 95% of hemoglobin F, and hemoglobin A2 will usually range between 3.5 and 7%.
The circulating red blood cells are very hypochromic, abnormal in shape, and the hemoglobin is markedly reduced, somewhere around 3-4 g/dl.
Anemia of beta thalassemia major is so severe that long-term blood transfusions are usually required for survival.
The severe anemia results in extramedullary erythropoiesis, delayed sexual development and poor growth.
Death may occur by age 10 unless treatment with periodic blood transfusions is initiated.
Beta thalassemia intermedia, now referred to as non-transfusion dependent beta thalassemia, presents as a less severe clinical phenotype.
A moderate microcytic anemia is present.
On hemoglobin electrophoresis, up to 50% of hemoglobin F will be noted and just as in beta thalassemia major, hemoglobin A2 will usually range between 3.5 and 7%.
May result from different mechanisms:
I.e., inheriting both a mild and severe beta thalassemia mutation, or
The inheritance of two mild mutations, or,
The inheritance of complex combinations of mutations.
Beta thalassemia minor, also referred to as beta thalassemia trait, is caused by the presence of a single beta-thalassemia mutation and a normal beta globin gene on the other chromosome.
Significant microcytosis with hypochromia on the blood smear but a mild anemia.
In general, thalassemia minor has no associated symptoms.
On hemoglobin electrophoresis, hemoglobin F is present up to 5%, and hemoglobin A2 at 4% or more.
Alpha Thalassemias
The alpha thalassemias are more difficult to diagnose because the typical elevations in hemoglobin F and A2 that are seen in the beta-thalassemias we have just discussed do not occur. This makes hemoglobin electrophoresis difficult to use for diagnosis.
Instead, molecular testing (DNA sequencing) is required for diagnosis.
More than 100 genetic forms of alpha thalassemia have been identified, with phenotypes ranging from asymptomatic to lethal.
The severity of this disorder is usually well correlated with the number of non-functional copies of the alpha globin genes (a one, two, three, or four-gene deletion).
Silent Carrier: one alpha globin gene deletion.
Essentially has no clinical consequences.
On the CBC, the MCV is usually normal or perhaps mildly decreased.
Alpha Thalassemia Minor: two gene deletion.
If two genes on the same chromosome are deleted, this is known as a cis deletion.
More commonly seen in those of southeast Asian ancestry.
If both parents carry a cis deletion, their offspring will have a 25% chance of having no functional alpha globin genes.
If the two deleted genes are on different chromosomes, this is trans deletion.
More common in those of African descent
If both parents have a two gene deletion in trans, their offspring will always have the same two gene deletion in trans.
Hemoglobin H: three gene deletion, which results in a moderate microcytic anemia.
When the alpha chains are reduced, the beta chains pair together and form beta globin tetramers, which is what this hemoglobin H represents.
In some cases, instead of the three gene deletion, a two gene deletion occurs with a mutant (i.e., non-functional) alpha globin mutation, such as hemoglobin Constant Spring.
This is referred to as nondeletional hemoglobin H.
Individuals with this nondeletional hemoglobiin H have a higher percentage of hemoglobin H, more splenomegaly and more advanced disease.
Most individuals with hemoglobin H don’t require regular transfusions. The anemia is typically mild; however, the phenotype is variable
With the dilutional anemia that occurs during pregnancy, the need of a transfusion may be increased.
Alpha thalassemia major, or hemoglobin Barts: Four gene deletion that results in a gamma tetramer.
Normal Hb A and Hb F are totally absent.
Hemoglobin Barts is incompatible with life and results in hydrops in-utero and stillbirth.
Its oxygen dissociation curve is markedly shifted to the left, so it holds onto oxygen and very little is released to the tissues.
Usually, the fetus or newborn will have marked anasarca and hepatosplenomegaly, with a hemoglobin level of 3-10 g/dL.
If a fetus is known to have alpha thalassemia major, multiple intrauterine transfusions can help these fetuses survive.
Prenatal diagnosis of the thalassemias can be performed using either chorionic villi from CVS or using cultured amniocytes obtained from an amniocentesis.
A study at the University of California at San Francisco is looking at the use of in utero stem cell transplantation during pregnancy to essentially cure the fetus before birth.
Take Home: When to Work Up for Thalassemia
If the MCV is decreased (<80), a hemoglobin electrophoresis is very reasonable.
Ferritin to assess for iron deficiency is also something that can be performed at the same time.
Hemoglobinopathy and iron deficiency can coexist!
If the MCV is decreased, and both a ferritin and hemoglobin electrophoresis are normal, molecular studies to assess for alpha thalassemia would be appropriate.