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1
Question:

A 2-day-old boy is being examined in the newborn nursery prior to discharge from the hospital.  He was born at 38 weeks gestation by vaginal delivery.  The pregnancy and delivery were uncomplicated, and the boy has been breastfeeding, stooling, and urinating normally.  The patient's mother has beta-thalassemia trait, and his father has a normal hemoglobin electrophoresis.  Vital signs and physical examination are normal.  Which of the following hemoglobin compositions is most likely predominant in this infant?

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Explanation:

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Hemoglobin

Type

Name

Components

Embryonic

Gower 1

ζ2ε2

Portland

ζ2γ2

Gower 2

α2ε2

Fetal

F*

α2γ2

Adult

A

α2β2

A2*

α2δ2

α-thalassemia intermedia

H

β4

α-thalassemia major

Barts

γ4

*Predominant in β-thalassemia.

Hemoglobin is a tetramer that consists of 2 pairs of globin chains (total of 4 chains per molecule).  During the first few weeks of embryogenesis, hemoglobin is synthesized by the yolk sac and contains zeta or epsilon globin chains (Choices G and F).  Thereafter, one pair of the globin chains should always be alpha and the other should be non-alpha.

Fetal hemoglobin (Hb F) production begins around 8 weeks gestation and replaces all embryonic hemoglobin by 14 weeks gestation, when erythropoiesis in the fetal liver and spleen is established.  Hb F consists of 2 alpha and 2 gamma protein subunits (α2γ2).  Production declines at birth, and Hb F comprises ~60-80% of all hemoglobin in a term newborn.  Hb F is gradually replaced by adult hemoglobin (Hb A, α2β2) (Choice A) during the first 6 months of life, after which Hb A composes the vast majority of adult hemoglobin.

Compared to red blood cells with Hb A, those with Hb F have a high oxygen affinity as Hb F binds to 2,3-bisphosphoglycerate poorly.  The greater affinity of Hb F facilitates transplacental oxygen delivery from the maternal circulation to that of the fetus.

(Choice C)  Hemoglobin A2 (α2δ2) is a normal hemoglobin variant that makes up 2%-3% of hemoglobin in a healthy adult and is functionally similar to Hb A.  Patients with beta-thalassemia major have impaired beta globin production, resulting in an excess of alpha globin chains (eg, Hb A2, Hb F) and no Hb A.

(Choices D and E)  Alpha-thalassemia results from a shortage of alpha globin chains.  Hemoglobin H (β4) and hemoglobin Barts (γ4) have a very high oxygen affinity and cannot release oxygen, resulting in tissue hypoxia.  Hemoglobin H disease manifests as chronic hemolytic anemia.  Hemoglobin Barts is incompatible with life (eg, hydrops fetalis) as normal fetal and adult hemoglobin cannot be produced.

Educational objective:
Hemoglobin F (Hb F) is the predominant hemoglobin type in the second and third trimesters of pregnancy and during the first few months after birth.  Hb F consists of 2 alpha and 2 gamma protein subunits (α2γ2) and has a high affinity for oxygen, which facilitates oxygen transport across the placenta to the fetus.  Hb A (α2β2) is the major hemoglobin in adults.