Hemoglobin Barts

Hemoglobin Barts, abbreviated Hb Barts, is an abnormal type of hemoglobin that consists of four gamma globins. It is moderately insoluble, and therefore accumulates in the red blood cells. It has an extremely high affinity for oxygen, resulting in almost no oxygen delivery to the tissues. As an embryo develops, it begins to produce alpha-globins at weeks 5-6 of development. When both HBA1 and HBA2, the two genes that code for alpha globins, are non-functional, only gamma globins are produced. These gamma globins bind to form hemoglobin Barts.[1] It is produced in the disease alpha-thalassemia and in the most severe of cases, it is the only form of hemoglobin in circulation. In this situation, a fetus will develop hydrops fetalis and normally die before or shortly after birth, unless intrauterine blood transfusion is performed.

Since hemoglobin Barts is elevated in alpha thalassemia, it can be measured, providing a useful screening test for this disease in some populations.[2]

The ability to measure hemoglobin Barts makes it useful in newborn screening tests. If hemoglobin Barts is detected on a newborn screen, the patient is usually referred for further evaluation since detection of hemoglobin Barts can indicate either one alpha globin gene deletion, making the baby a silent alpha thalassemia carrier, two alpha globin gene deletions (alpha thalassemia), or hemoglobin H disease (three alpha globin gene deletions). Deletion of four alpha globin genes was previously felt to be incompatible with life, but there are currently 69 patients who have survived past infancy[3]

The chance of a fetus developing Hemoglobin Bart's hydrops fetalis is dependent upon if one or both parent carries the alpha-thalassemia trait. Due to this disease being incompatible with life, diagnosis for it is done prenatally. Moreover, the prenatal diagnosis also provides the ability to prevent detrimental pregnancy complications. [4]

Anemia is a factor in fetuses with Hemoglobin Bart's disease as there is an "increased cardiac output" and hypovolemia as the tissues of the fetus require oxygen because of the gamma globulin's high affinity for oxygen. This deprives the tissues of receiving oxygen to function well. The symptoms of anemia occur within the first trimester.[5]

This variant of hemoglobin is so called as it was discovered at St Bartholomew's Hospital in London, often abbreviated to Barts.

Notes
  1. "Pathophysiology of alpha thalassemia". www.uptodate.com. Retrieved 2016-08-30.
  2. F. P. van der Dijs, M. Volmer, D. G. van Gijssel-Wiersma, J. W. Smit, R. van Veen & F. A. Muskiet (September 1999). "Predictive value of cord blood hematological indices and hemoglobin Barts for the detection of heterozygous alpha-thalassemia-2 in an African-Caribbean population". Clinical chemistry. 45 (9): 1495–1500. PMID 10471652.CS1 maint: multiple names: authors list (link)
  3. Songdej, Duantida; Babbs, Christian; Higgs, Douglas R. (9 March 2017). "An international registry of survivors with Hb Bart's hydrops fetalis syndrome". Blood. 129 (10): 1251–1259. doi:10.1182/blood-2016-08-697110. PMC 5345731. PMID 28057638.
  4. Wu, M. , Xie, X. , Li, J. and Li, D. (2015). "Neonatal screening for α‐thalassemia by cord hemoglobin Barts: how effective is it?. Int. Jnl. Lab. Hem". International Journal of Laboratory Hematology. 37 (5): 647–653.CS1 maint: multiple names: authors list (link)
  5. Suchaya Luewan,FuangladaTongprasert, Kasemsri Srisupundit, Theera Tongsong (1 April 2015). "Fetal cardiac Doppler indices in fetuses with hemoglobin Bart's disease at 12–14 weeks of gestation". International Journal of Cardiology. 184: 614–616. doi:10.1016/j.ijcard.2015.02.053. PMID 25770840.CS1 maint: multiple names: authors list (link)
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