Severe Combined Immunodeficiency (SCID): Causes, Symptoms, Diagnosis, Pathophysiology & Treatment Explained

Table of Contents

• Introduction to Severe Combined Immunodeficiency (SCID)
• Causes of Severe Combined Immunodeficiency (SCID)
• Types and mechanisms of Severe Combined Immunodeficiency (SCID)
• Clinical Presentation and Diagnosis of Severe Combined Immunodeficiency (SCID)
• Treatment and Management of Severe Combined Immunodeficiency (SCID)
• Reference

READY MADE PRESENTATION

Introduction to Severe Combined Immunodeficiency (SCID)

• The immune system serves as the body’s host defense mechanism, consisting of a complex network of innate and adaptive components designed to protect against pathogenic agents.
• Its fundamental and most critical function is the ability to accurately distinguish between the body’s own healthy cells (“self”) and foreign invaders (“non-self”).
• This precise recognition is essential, as its failure leads to devastating conditions known as autoimmune diseases.
• Consequently, maintaining self-tolerance is a crucial function of a healthy immune system, enabling it to prevent autoimmune diseases rather than mistakenly attacking the body.
• Severe Combined Immunodeficiency (SCID) is a congenital disorder that leaves newborns with a profoundly deficient immune system.
• Due to a genetic defect, affected individuals are unable to produce essential immune cells such as T-cells and B-cells.
• T-cells and B-cells are the two main types of lymphocytes, which are specialized white blood cells forming the core of the adaptive immune system.
• T-cells are responsible for cellular immunity; they identify and destroy infected host cells and also help regulate the activity of other immune cells.
• B-cells are responsible for humoral immunity; they produce antibodies that bind to and neutralize pathogens, marking them for destruction by other immune cells.
• As a result, individuals with SCID are left defenceless against infections, and without a cure such as a bone marrow transplant, the condition is fatal.

Causes of Severe Combined Immunodeficiency (SCID)

• Severe Combined Immunodeficiency (SCID) is not a single disease but a spectrum of genotypic and phenotypic disorders.
• The condition is caused by specific mutations in genes that are essential for the production and function of key immune cells, particularly T-cells and, in most cases, B-cells.
• This genetic defect leaves the body unable to fight off infections, making it extremely vulnerable to pathogens that are typically harmless to healthy individuals.
• SCID is primarily caused by either biallelic or X-linked mutations.
• Biallelic mutations occur when an individual inherits a mutated copy of the gene from both parents.
• X-linked mutations refer to genetic defects located on the X chromosome, which primarily affect males.
• The fundamental issue in SCID is a genetic defect that prevents the proper development and function of immune cells, specifically lymphocytes such as T-cells and B-cells.
• This genetic block essentially prevents the body from developing a functional immune system.
• The defining feature of all SCID types is a profound deficiency of T lymphocytes.
• The absence of these “helper” T-cells also prevents B-cells from producing functional antibodies, even if B-cells are present in normal numbers.
• This is why it is termed a “combined” immunodeficiency, as both T-cell-mediated and B-cell-mediated immunity are affected.

Types and mechanisms of Severe Combined Immunodeficiency (SCID)

The classification of different SCID types is based on which specific gene is mutated and how that mutation affects the development and function of immune cells, including T-cells, B-cells, and Natural Killer (NK) cells.

X-linked SCID (SCID-X1)

• X-linked SCID is caused by mutations in the IL2RG gene, which is located on the X chromosome.
• The IL2RG gene is responsible for producing a protein known as the common gamma chain (γc), which is a vital component of receptors on the surface of immune cells.
• The common gamma chain associates with other protein subunits to form receptors for multiple cytokines, including interleukins IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21.
• These cytokines function as critical signaling molecules that regulate lymphocyte proliferation, differentiation, and survival.
• A mutation in the IL2RG gene results in the production of either a defective common gamma chain protein or no protein at all.
• Since this protein is essential for signaling pathways in T-cells and NK-cells, its absence prevents these immune cells from developing properly.
• Consequently, the body lacks a functional T-cell and NK-cell immune system, making the individual extremely vulnerable to severe and often fatal infections.

Adenosine Deaminase (ADA) Deficiency

• ADA deficiency is caused by mutations in the ADA gene, which is located on chromosome 20 and provides instructions for producing the enzyme adenosine deaminase (ADA).
• This enzyme plays a crucial role in breaking down a toxic byproduct known as deoxyadenosine.
• Deoxyadenosine undergoes metabolic conversion into a toxic compound called dATP (deoxyadenosine triphosphate).
• The accumulation of high levels of dATP is harmful because it inhibits the enzyme ribonucleotide reductase.
• Ribonucleotide reductase is essential for DNA synthesis, as it converts ribonucleotides into deoxyribonucleotides, which are the building blocks of DNA.
• When DNA synthesis is impaired, cell division is halted, which is particularly damaging to lymphocytes (T-cells, B-cells, and NK-cells) because they must rapidly proliferate to respond to infections.
• This disruption of the cell cycle ultimately leads to apoptosis (cell death), preventing the development of a functional immune system.
• A mutated ADA gene leads to the accumulation of toxic metabolites, which are especially lethal to developing immune cells, resulting in severe deficiencies of T-cells, B-cells, and NK-cells.

Recombinase-Activating Gene (RAG) Deficiency

• RAG deficiency is caused by mutations in the RAG1 and RAG2 genes, which together produce a protein complex known as V(D)J recombinase.
• This complex is essential for the process of V(D)J recombination.
• V(D)J recombination is the genetic mechanism through which T-cells and B-cells rearrange specific gene segments to generate unique antigen receptors.
• This process enables the immune system to create a vast diversity of receptors required to recognize and respond to a wide range of pathogens.
• Mutations in the RAG1 or RAG2 genes result in a malfunctioning V(D)J recombinase complex.
• This defect prevents T-cells and B-cells from undergoing V(D)J recombination, thereby blocking the development of functional antigen receptors.
• As a result, these immune cells fail to mature properly, leading to a severe impairment of adaptive immunity.

Clinical Presentation and Diagnosis of Severe Combined Immunodeficiency (SCID)

• Infants with SCID suffer from frequent, life-threatening infections caused by opportunistic pathogens.
• Due to constant infections and malabsorption, affected infants fail to gain weight and grow normally (failure to thrive).
• They do not show normal immune responses to vaccines or infections, and administration of live vaccines can be dangerous.
• Blood tests reveal a severely low count of lymphocytes, which is a key diagnostic indicator.
• Some infants may develop a rash due to graft-versus-host disease (GVHD) caused by maternal immune cells that crossed the placenta.
• There are extremely low or absent levels of immunoglobulins, including IgG, IgA, and IgM.

Treatment and Management of Severe Combined Immunodeficiency (SCID)

• Hematopoietic Stem Cell Transplant (HSCT) aims to replace the patient’s defective immune system with healthy stem cells from a donor, usually a matched sibling or an unrelated donor.
• Enzyme Replacement Therapy (ERT) is primarily used for ADA-SCID, in which regular injections of the missing adenosine deaminase (ADA) enzyme are administered to reduce toxic metabolite buildup and improve immune function.
• Gene therapy for SCID involves collecting the patient’s own stem cells, inserting a healthy copy of the defective gene using a viral vector, and reinfusing these corrected cells into the patient to restore a functional immune system.
• This approach is a promising alternative to bone marrow transplantation and has shown significant success in specific types of SCID, particularly ADA-SCID and X-linked SCID.

Reference

• StatPearls Publishing. (2023, August 8). Severe combined immunodeficiency. In StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK539762/
• Children’s Hospital of Philadelphia. (n.d.). Severe combined immunodeficiency (SCID). https://www.chop.edu/conditions-diseases/severe-combined-immunodeficiency-scid
• Genethon. (n.d.). Severe combined immunodeficiency (SCID). https://www.genethon.com/our-pipeline/severe-combined-immunodefiency-scid/
• Luigi D. Notarangelo., E. Mazzolari., C. Forino., & S. Giliani. (2004). Mechanisms of primary immunodeficiencies: From bedside to bench and back. Drug Discovery Today: Disease Mechanisms, 1(3), 383–390. https://doi.org/10.1016/j.ddmec.2004.10.008
• Alain Fischer. (2001). Primary immunodeficiency diseases: An experimental model for molecular medicine. The Lancet, 357(9271), 1863–1869. https://doi.org/10.1016/S0140-6736(00)04959-X
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