Neonatal cyanosis with Hb St Mandé, a low O₂ affinity hemoglobin variant affecting β‐globin

Abstract

Globin gene mutations affecting hemoglobin (Hb) are present within 7% of the world population. More than 1000 Hb variants are known and most of them are not health compromising. Some of these variants may alter affinity for oxygen (O 2 ), through switching from the deoxygenated low-affinity "T" (for tense) to the oxygenated high-affinity "R" (for relaxed) Hb state.Low O 2 affinity variants are caused by globin amino acid substitution that destabilize the R state and/or reinforce the T state. This favors Hb-O 2 release in tissue capillaries, but may impede full Hb saturation with Hb-O 2 in the alveolar PO 2 , reflecting by an elevated P50 and resulting in chronic cyanosis. Hb Saint Mandé, an autosomal dominant disorder described in 1981, 1 is characterized by isolated asymptomatic moderate anemia and cyanosis with no respiratory distress. It is due to a mutation leading to an Asn-Tyr substitution, prone to modify both α1β2 and α2β1 contacts through disturbance of several hydrogen bonds. [2][3][4] Oxygen-affinity curve shifts to the right with P50 around 50 mmHg. Functional studies and structural modeling reveal an altered