Severe Combined Immunodeficiency (SCID)

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Description

Severe Combined Immunodeficiency (SCID) is a rare, life-threatening genetic disorder that affects the immune system. Individuals with SCID lack functional T cells and B cells, leaving them highly susceptible to infections. This blog will delve into the causes, symptoms, diagnosis, and management of SCID, providing insights into this complex condition.

Genes Involved

Genes Involved:

SCID is caused by mutations in various genes involved in the development and function of immune cells. Some of the most common genes include:

• IL2RG (gamma chain): This gene is involved in the development of T cells and NK cells.
• JAK3: This gene plays a critical role in the signaling pathways required for T cell development.
• RAG1 and RAG2: These genes are responsible for rearranging the DNA in immune cells, which is crucial for creating a diverse repertoire of antibodies.
• ADA (adenosine deaminase): Deficiency in this enzyme can lead to a buildup of toxic metabolites, damaging immune cells.
• Artemis: Mutations in this gene affect the repair of DNA breaks, which are essential for the proper development of B cells.

Mutations in these genes can disrupt the formation of a healthy immune system, leaving individuals susceptible to infections.

Recognizing the Signs and Symptoms

Recognizing the Signs and Symptoms:

SCID symptoms typically appear within the first few months of life. Early signs include:

• Recurrent, severe infections (e.g., pneumonia, ear infections, skin infections)
• Failure to thrive (poor weight gain, slow growth)
• Diarrhea
• Persistent thrush (fungal infection in the mouth)
• Delayed developmental milestones

If you notice these symptoms in your child, it is crucial to seek immediate medical attention.

Causes

Causes:

SCID is primarily caused by genetic mutations inherited from parents. These mutations can affect different genes involved in immune system development, leading to various types of SCID. The specific gene mutation determines the severity and type of SCID. Some common causes include:

• X-linked SCID: This is the most common type of SCID, caused by mutations in the IL2RG gene located on the X chromosome. This type primarily affects males, as they only have one X chromosome.
• Autosomal recessive SCID: This type is caused by mutations in genes other than IL2RG. It can affect both males and females equally, as the mutation must be inherited from both parents.
• ADA deficiency: This type is caused by a mutation in the ADA gene, leading to a deficiency of the enzyme adenosine deaminase. This deficiency can result in a buildup of toxic metabolites that damage immune cells.

It is important to note that SCID can also occur as a result of new mutations, meaning the mutation is not inherited from parents but occurs spontaneously during the formation of an individual‘s egg or sperm cell.

Inheritance/recurrence risk

Inheritance or Recurrence Risk:

SCID is typically inherited in an autosomal recessive or X-linked recessive pattern. This means that an individual must inherit two copies of the mutated gene (one from each parent) to develop SCID. In autosomal recessive inheritance, both males and females are equally likely to inherit the disorder. In X-linked recessive inheritance, males are more likely to be affected, as they only have one X chromosome.

If two parents carry the mutated gene, there is a 25% chance their child will inherit the disorder, a 50% chance their child will be a carrier, and a 25% chance their child will not inherit the mutated gene. The risk of having a child with SCID can be assessed through genetic counseling.