Several genes encode different types of globin proteins, and their various tetrameric combinations generate multiple types of Hb, which are normally expressed at different stages of life - embryonic, fetal and adult. Hb is expressed by red blood cells, both reticulocytes (immature red blood cells) and erythrocytes (mature red blood cells). Haemoglobin (Hb) is a tetrameric protein composed of different combinations of globin subunits each globin subunit is associated with the cofactor haem, which can carry a molecule of oxygen. Sickle cell disease (SCD) is an umbrella term that defines a group of inherited diseases (including sickle cell anaemia (SCA), HbSC and HbSβ-thalassaemia, see below) characterized by mutations in the gene encoding the haemoglobin subunit β ( HBB) ( Fig. Early diagnosis is crucial to improve survival, and universal newborn screening programmes have been implemented in some countries but are challenging in low-income, high-burden settings.
Hydroxycarbamide, blood transfusions and haematopoietic stem cell transplantation can reduce the severity of the disease. Common acute complications are acute pain events, acute chest syndrome and stroke chronic complications (including chronic kidney disease) can damage all organs. SCD is characterized by a remarkable phenotypic complexity. Other pathophysiological mechanisms that contribute to the SCD phenotype are vaso-occlusion and activation of the immune system. Haemoglobin molecules that include mutant sickle β-globin subunits can polymerize erythrocytes that contain mostly haemoglobin polymers assume a sickled form and are prone to haemolysis. The incidence is estimated to be between 300,000 and 400,000 neonates globally each year, the majority in sub-Saharan Africa. Sickle cell disease (SCD) is a group of inherited disorders caused by mutations in HBB, which encodes haemoglobin subunit β.
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