Sickle cell disease


Welcome back to another post on my blog, this week we are discussing Sickle cell disease!

(Credit: Sickle Cell Foundation of Minnesota)

Sickle cell disease is the name for a group of inherited conditions that causes red blood cells (RBC) to become sickle-shaped, as seen in the image above. It most commonly affects people of African, Asian and Caribbean descent causing many complications due to a shorter lifespan and reduced flexibility of affected RBCs. This disease is caused by abnormal haemoglobin (Hb).

Hb is the protein contained within RBCs and is responsible for transporting oxygen to cells in the body. Hb is composed of two alpha and two beta subunits (see below), which are encoded by two alpha-globin genes located on chromosome 16 and one beta-globin gene found on chromosome 11. 

(Credit: Microbe notes)

Sickle cell disease is the most common hemoglobinopathy, a single gene disorder that causes a defect in the globin chain structure. Normally there is no interaction between Hb molecules, but when there is a mutation in the beta chain, the substitution fits perfectly into a pocket on the adjacent beta chain and many Hb form connections (see below).


(Credit: Biologyonline)

The normal Hb gene is HbA, in which everyone should have two copies of this. Occasionally a single point mutation on the beta-globin chain can cause the HbS gene. When an individual acquires one HbA and one HbS gene, they are recognised as a 'carrier' of Sickle cell disease; these people will be generally asymptomatic and have protection against malaria. There is a correlation between areas with high numbers of malarial cases and sickle cell carriers. Theories for this potential protection include:

  • Parasite-infected RBCs will sickle quickly and be destroyed by the spleen.
  • Sickled RBCs have a smaller oxygen-carrying capability, so will starve the parasite faster.
  • Slower replication of the parasite allows more time for the immune system to respond. 
The most common form of Sickle cell disease is Sickle cell anaemia, which occurs when an individual possesses two HbS genes. This is a severe form and reduces life expectancy due to a low percentage of normal RBCs, and increased RBC destruction. Individuals will experience anaemia, frequent infections, leg ulcers, severe pain, spleen damage, and an increased risk of strokes. How Sickle cell anaemia is passed on can be seen in the image below. 

(Credit: National institute of health)

Another form of Sickle cell disease is HbSC, which has less sickling than Sickle cell anaemia. Individuals with this variation acquire one HbS from one parent and one HbC from the other parent. Symptoms include fatigue, pain, and death of bone tissue.

A rare form of Sickle cell disease is HbSD which occurs when people inherit one HbS from one parent and one HbD from the other parent. This is another severe sickling disorder as the HbD enhances the sickling of HbS. This form is characterised by moderate anaemia, and infrequent pain, occurring more in Asian populations. 

(Credit: Sickle cell foundation of Minnesota)

There are many Sickle cell disease variants, with a range of severities. Being a carrier of these diseases is thought to be an adaption made to evade severe malarial infections, but when people inherit two abnormal Hb genes, the symptoms and risks outweigh the benefits of this protection. 

Thank you for reading to the end! Next, we will be exploring Prader-Willi syndrome which is a complex genetic condition characterised by learning difficulties, dangerous weight gain and behavioural challenges.

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