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About Sickle Cell

What is Sickle Cell Disease?

​Sickle cell disease (SCD) encompasses a collection of inherited disorders affecting red blood cells. Red blood cells contain hemoglobin, a protein responsible for transporting oxygen throughout the body. In individuals with SCD, their hemoglobin is abnormal, leading to the deformation of red blood cells into rigid, C-shaped structures resembling a farm tool known as a "sickle." These sickle-shaped cells have a shorter lifespan, resulting in a chronic shortage of red blood cells. Additionally, they tend to obstruct small blood vessels during their circulation, causing blockages in blood flow. This obstruction can give rise to pain and various severe complications, including infections, acute chest syndrome, and strokes.

Causes of Sickle Cell Disease

Sickle cell disease (SCD) is a genetic disorder that primarily affects the structure and function of red blood cells. It is caused by a mutation in the HBB gene, which encodes the beta-globin subunit of hemoglobin, the protein responsible for carrying oxygen in red blood cells. This genetic mutation results in the production of abnormal hemoglobin, known as hemoglobin S (HbS), instead of the normal hemoglobin A (HbA). When individuals inherit two copies of the mutated gene (one from each parent), they develop SCD. If they inherit one normal gene and one mutated gene, they are considered carriers (sickle cell trait) and typically do not show symptoms of the disease.

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primary symptoms and complications associated with sickle cell disease include:

  1. Pain Crises: The hallmark symptom of SCD is the sickle cell crisis, which is characterized by sudden and severe pain. This occurs when the sickle-shaped red blood cells block small blood vessels, restricting blood flow and oxygen delivery to tissues and organs.

  2. Anemia: SCD leads to chronic anemia, as the sickle cells have a shorter lifespan than normal red blood cells. Anemia can result in fatigue, weakness, and pallor.

  3. Infections: People with SCD are more susceptible to infections, as the misshapen cells can impair the immune system's ability to function effectively.

  4. Acute Chest Syndrome: This is a serious complication characterized by chest pain, fever, and difficulty breathing. It can be life-threatening and often requires hospitalization.

  5. Stroke: The blockage of blood vessels in the brain can lead to stroke, which can cause permanent neurological damage.

  6. Organ Damage: Over time, the repeated blockages of blood vessels can result in damage to various organs, including the spleen, liver, lungs, and kidneys.

  7. Growth Delay: Children with SCD may experience delayed growth and development.

  8. Jaundice: The rapid breakdown of sickle cells can lead to an accumulation of bilirubin, causing yellowing of the skin and eyes (jaundice).

  9. Leg Ulcers: SCD can cause painful ulcers on the lower legs.

  10. Eye Problems: Damage to the blood vessels in the eyes can lead to vision problems, including retinopathy.

  11. Priapism: In males, SCD can cause prolonged and painful erections.

  12. Gallstones: The breakdown of red blood cells can lead to the formation of gallstones.

Types of Sickle Cell

Sickle cell disease (SCD) is an umbrella term for a group of inherited blood disorders that primarily affect the structure and function of red blood cells. There are several different types and variations of sickle cell disease, with the two most common being Sickle Cell Anemia and Sickle Cell Trait. These conditions are caused by specific genetic mutations in the HBB gene, which encodes the beta-globin subunit of hemoglobin.

Let's delve into these types of sickle cell in more detail:

Sickle Cell Anemia (HbSS):

  • Sickle Cell Anemia is the most severe and well-known form of SCD. It occurs when an individual inherits two copies of the mutated hemoglobin S (HbS) gene, one from each parent (HbSS genotype).

  • In this type, the red blood cells are primarily composed of abnormal hemoglobin S (HbS), which causes them to take on a characteristic sickle or crescent shape when they release oxygen, becoming rigid and prone to clumping together.

  • These sickle-shaped cells can block small blood vessels, leading to pain crises, anemia, organ damage, and a range of complications, including strokes, infections, acute chest syndrome, and more.

  • Sickle Cell Anemia typically presents with more frequent and severe symptoms than other sickle cell variations.

Sickle Cell Trait (HbAS):

  • Sickle Cell Trait occurs when an individual inherits one copy of the mutated HbS gene and one copy of the normal hemoglobin A (HbA) gene (HbAS genotype).

  • People with Sickle Cell Trait generally do not show symptoms of the disease and live normal lives. They often have a mix of normal and sickle-shaped red blood cells, but the majority of their red blood cells function normally.

  • While they don't experience the severe symptoms associated with Sickle Cell Anemia, carriers of the trait can pass it on to their offspring. If both parents have Sickle Cell Trait, there is a 25% chance their child may have Sickle Cell Anemia.

​Information on how sickle cell is inherited and its genetic basis

Sickle cell disease is caused by a mutation in the HBB gene, which is located on chromosome 11. This gene encodes the beta-globin subunit of hemoglobin, a protein found in red blood cells that is responsible for transporting oxygen from the lungs to the rest of the body.

Normal hemoglobin, referred to as hemoglobin A (HbA), is composed of two alpha-globin and two beta-globin subunits. In individuals without the SCD mutation, both beta-globin subunits are normal, resulting in the production of HbA.

In contrast, individuals with SCD have a mutation in one or both of their HBB genes. The most common SCD mutation is called hemoglobin S (HbS). People with SCD inherit one or two copies of the HbS gene along with one or two normal beta-globin genes, depending on the specific genetic makeup. The combination of normal and mutated beta-globin subunits gives rise to various forms of sickle cell disease, as explained in the previous response.

Inheritance

The inheritance of sickle cell disease follows an autosomal recessive pattern, meaning that both copies of the HBB gene must be mutated (homozygous) to develop the full-blown disease. Let's explore how this inheritance pattern works:

  1. Homozygous SCD (Sickle Cell Anemia, HbSS):

    • If both parents carry one mutated HbS gene (HbS) and one normal HbA gene (HbA), their child has a 25% chance of inheriting two mutated HbS genes (HbSS). This results in the development of Sickle Cell Anemia.

  2. Heterozygous SCD (Sickle Cell Trait, HbAS):

    • If one parent carries a mutated HbS gene (HbS) and the other parent carries a normal HbA gene (HbA), their child has a 50% chance of inheriting one mutated HbS gene and one normal HbA gene (HbAS). This leads to the condition known as Sickle Cell Trait, where the individual typically does not exhibit symptoms but is a carrier of the disease.

  3. Other Variations (HbSC, HbSβ-thalassemia, etc.):

    • Various other combinations of beta-globin gene mutations can result in different forms of sickle cell disease, depending on the specific mutations inherited from each parent.

It's essential to recognize that an individual with Sickle Cell Trait (HbAS) can still pass on the mutated HbS gene to their offspring, potentially resulting in SCD if both parents are carriers. Genetic counseling and testing can help individuals and couples assess their risk and make informed decisions about family planning.

In summary, the inheritance of sickle cell disease is linked to the presence of mutated and normal HBB genes in an individual's genetic makeup. The genetic basis of SCD provides valuable insights into its inheritance patterns and the risk of passing the disease on to future generations.

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