X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency
Synonyms	X-SCID
Pronounce
Field	Immunology, Medical genetics, Pediatrics
Symptoms	Severe recurrent infections, failure to thrive, chronic diarrhea, absence of lymphoid tissue
Complications	Life-threatening infections, growth delay, death without treatment
Onset	Within the first few months of life
Duration	Lifelong without treatment
Types	X-linked (most common SCID type)
Causes	Mutation in the IL2RG gene (interleukin-2 receptor gamma chain)
Risks	Family history of SCID, male infants
Diagnosis	T-cell receptor excision circles (TREC) screening, genetic testing, flow cytometry
Differential diagnosis	Other forms of SCID, HIV/AIDS, congenital neutropenia
Prevention	Prenatal genetic testing, newborn screening
Treatment	Hematopoietic stem cell transplantation, gene therapy
Medication	Antibiotics, antifungals, immunoglobulin replacement therapy
Prognosis	Good with early treatment; fatal if untreated
Frequency	Rare; ~1 in 50,000 to 100,000 live births
Deaths	High without prompt treatment

X-linked severe combined immunodeficiency (X-linked SCID) is a genetic disorder that severely affects the immune system, leading to a lack of functional T cells and natural killer cells. This condition is part of a group of disorders known as severe combined immunodeficiency (SCID), which are characterized by a severely compromised immune system.

Genetics

X-linked SCID is caused by mutations in the IL2RG gene, which is located on the X chromosome. The IL2RG gene encodes the common gamma chain (γc), a protein that is a component of several interleukin receptors, including those for interleukin-2, interleukin-4, interleukin-7, interleukin-9, interleukin-15, and interleukin-21. These interleukins are crucial for the development and function of immune cells.

Since the disorder is X-linked, it primarily affects males, who have only one X chromosome. Females, with two X chromosomes, are typically carriers and usually do not exhibit symptoms, although they can pass the mutated gene to their offspring.

Pathophysiology

The absence of a functional common gamma chain in X-linked SCID leads to a failure in the signaling pathways necessary for the development and maturation of T cells and natural killer cells. As a result, affected individuals have a severely compromised immune system, making them highly susceptible to infections.

Clinical Presentation

Infants with X-linked SCID typically present within the first few months of life with recurrent infections, failure to thrive, and chronic diarrhea. Common infections include those caused by opportunistic organisms such as Pneumocystis jirovecii, Candida albicans, and various viruses. Without treatment, these infections can be life-threatening.

Diagnosis

Diagnosis of X-linked SCID is often made through newborn screening programs that detect low levels of T-cell receptor excision circles (TRECs), which are byproducts of T-cell development. Confirmatory testing includes genetic testing to identify mutations in the IL2RG gene.

Treatment

The primary treatment for X-linked SCID is hematopoietic stem cell transplantation (HSCT), which can restore immune function if performed early in life. Gene therapy is an emerging treatment option that involves correcting the genetic defect in the patient's own hematopoietic stem cells.

Prognosis

With early diagnosis and treatment, individuals with X-linked SCID can have a significantly improved prognosis. However, without treatment, the condition is usually fatal within the first year of life due to overwhelming infections.

Related pages

• Severe combined immunodeficiency
• Immune system
• Genetic disorder
• Hematopoietic stem cell transplantation