Adenosine Deaminase (ADA) Deficiency – SCID Causes, Symptoms & Treatment

Adenosine Deaminase (ADA) Deficiency – SCID Causes, Symptoms & Treatment

Learn about ADA-SCID, a severe immune disorder caused by ADA gene mutations. Includes causes, genetics, symptoms, diagnosis, prognosis, and treatment options such as stem cell transplant, enzyme replacement therapy, and gene therapy.

Adenosine Deaminase Severe Combined Immunodeficiency (ADA-SCID)

Adenosine Deaminase Severe Combined Immunodeficiency (ADA-SCID) is a rare hereditary primary immunodeficiency disorder in which the immune system is unable to protect the body from infections. It occurs due to mutations in the ADA gene, which lead to little or no production of the enzyme adenosine deaminase. This enzyme is essential for the development and survival of lymphocytes (including T-cells, B-cells, and NK-cells), which are responsible for fighting infections.

Without ADA enzyme activity, toxic metabolic byproducts accumulate and destroy developing immune cells, resulting in severe immune failure.

The condition typically presents within the first 6 months of life, though 10–15% of individuals develop symptoms later, leading to milder or delayed forms of combined immunodeficiency (CID).

Genetic Cause and Inheritance

• ADA-SCID is caused by mutations in the ADA gene on chromosome 20.
• It is inherited in an autosomal recessive pattern:
  • Affected persons have two mutated copies of the ADA gene.
  • Parents are usually carriers and typically do not show symptoms.
• Each pregnancy with two carrier parents has:
  • 25% (1 in 4) chance of the child having ADA-SCID
  • 50% (1 in 2) chance of the child being a carrier
  • 25% chance of no mutation

The severity and age of onset may vary depending on specific ADA gene mutation types (over 29 variants identified).

Signs and Symptoms

Typical Infant-Onset (Most Common)

Symptoms usually appear before 6 months and may include:

• Repeated bacterial, viral, and fungal infections
• Chronic diarrhea
• Pneumonia or recurring lung infections
• Persistent skin rashes
• Poor growth and failure to gain weight
• Absence or poor development of tonsils, lymph nodes, and thymus
• Developmental delays and weak muscle tone

Delayed-Onset (Childhood to Adulthood)

Symptoms may be milder initially:

• Recurrent ear and respiratory infections
• Increased susceptibility to seasonal or common infections
• Gradual decline in immune function

Systemic Features Specific to ADA-SCID

• Bone and cartilage abnormalities (chondro-osseous dysplasia)
• Neurological impairment (movement disorder, developmental delay)
• Hearing loss
• Liver dysfunction and cognitive difficulties

Diagnosis

Diagnosis may be detected through:

1. Newborn screening (TREC test for T-cell deficiency; increasingly standard in many countries)
2. Blood tests:
   • Low lymphocyte counts (T, B, and NK cells)
   • Very low ADA enzyme activity (<1%)
3. Genetic testing to confirm ADA gene mutations
4. X-ray/Imaging may show a small or absent thymus

Prenatal testing is available through CVS, amniocentesis, or cord blood testing when family history is known.

Treatment Options

Treatment aims to restore immune function and prevent infections.

1] Stem Cell / Bone Marrow Transplant (BMT/SCT)

• Most effective curative treatment
• Best outcomes when done in infancy and with a matched sibling donor

2] Enzyme Replacement Therapy (ERT)

• Uses pegylated bovine ADA enzyme (PEG-ADA)
• Restores immune function temporarily
• Used when transplant or gene therapy is not immediately possible

3] Gene Therapy (Emerging Standard)

• Autologous stem cells are collected from the patient
• A healthy ADA gene is inserted using a viral vector
• Corrected cells are returned to the body to rebuild immunity
• FDA-approved gene therapy options (like Strimvelis and newer viral-vector treatments) show high long-term survival rates with reduced risk of graft-vs-host disease

Supportive Care

• Prophylactic antibiotics, antivirals, antifungals
• Immunoglobulin replacement therapy (IVIG / SCIG)
• Protective isolation during high-risk periods

Prognosis

• Without treatment: fatal within the first 1–2 years of life
• With early diagnosis and appropriate therapy, many individuals can achieve:
  • Near-normal immune function
  • Normal growth and development
  • Long-term survival into adulthood

Outcomes are best when intervention occurs before infections develop.

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