Sickle-cell disease classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Cafer Zorkun, M.D., Ph.D. [2], Aarti Narayan, M.B.B.S [3], Shyam Patel [4]

Overview

Sickle-cell disease may be classified according to the number and type of the two alleles of beta-globin. Sickle-cell disease may also be classified as an autosomal recessive genetic disorder.

Classification

Sickle-cell disease may be classified according to the number and type of the two alleles of beta-globin into the following subtypes:

• Sickle-cell anemia (HbSS)
  • These patients inherit one copy of sickle cell gene from each parent. Each parent contributes one sickle cell allele (HbS). This is the most severe form of sickle cell diseases, because there is no normal beta-globin chain. In order to acquire HbSS, both parents must either have sickle cell trait (HbAS) or sickle cell disease (HbSS). In most cases, an offspring with HbSS will have parents with the sickle cell trait.
• Sickle cell trait (HbAS)
  • These patients inherit one sickle cell allele of beta-globin and one normal allele of beta-globin. Because there is one normal allele, there can be sufficient production of hemoglobin. Patients can be asymptomatic. An offspring with sickle cell trait usually has one parent with HbA (normal alleles) and one parent with sickle cell trait (HbAS).
• Sickle-hemoglobin C disease (HbSC)
  • These patients with symptoms of sickle cell disease inherit one copy of sickle cell gene from one parent and defective hemoglobin C from another parent. HbC and HbS both involve point mutations. HbC is characterized by a point mutation that substitutes lysine for glutamic acid, as opposed to HbS in which valine is substituted for glutamic acid.
• Sickle β thalassemia (HbS β thalassemia)
  • These patients with sickle cell disease inherit one copy of sickle cell gene from one parent and another copy of β thalassaemia gene
  • There are two types of HbS β Thalassemia: '0' and '+'
    • HbS β 0 Thalassemia: more severe form of SCD
    • HbS β + Thalassemia: milder form of SCD
• HbSD, HbSE, and HbSO
  • These patients inherit one copy of sickle cell gene and another copy of abnormal hemoglobin.

The term "disease" is applied here since the inherited abnormality causes a pathological condition that can lead to death and severe complications. Not all inherited variants of hemoglobin are detrimental, a concept known as genetic polymorphisms. Hemoglobin is one of the best-characterized proteins in terms of inherited variants; some variants manifest as severe thalassemia, such as beta-zero-thalassemia, and other variants manifest as a milder thalassaemia, such as beta-plus-thalassemia.