Adenosine deaminase (ADA)

 Adenosine deaminase (ADA)

Overview

Adenosine deaminase (ADA) deficiency is a rare, autosomal recessive metabolic disorder that impairs lymphocyte development and causes severe combined immunodeficiency (SCID). Without the ADA enzyme, toxic metabolites (such as dATP) accumulate, destroying T, B, and NK cells. Untreated ADA-SCID is typically fatal in infancy.

Symptoms

The signs and symptoms of ADA deficiency generally fall into three categories

:1. Severe Immune Deficiency (ADA-SCID)Around 80% of cases are severe and appear within the first 6 months of life. Without treatment, these are usually fatal within the first or second year.

Recurrent Infections: Frequent, persistent, or unusual cases of pneumonia, chronic diarrhea, thrush (oral yeast), and skin rashes.

Failure to Thrive: Severely slow growth in height and weight, often resulting in developmental delays.

No Functional Immunity: A near-complete lack of T-cells, B-cells, and natural killer (NK) cells.

2. Skeletal & Non-Immunological Symptoms

Because ADA is ubiquitous (found in all tissues), enzyme absence causes toxic metabolite buildup (such as deoxyadenosine) that disrupts various organ systems.

Bone Abnormalities: Deformities in the ribs, flaring of the costochondral junctions, and growth plate abnormalities.

Neurological & Behavioral: Developmental delays, motor dysfunction, seizures, and sensorineural hearing loss.

Liver & Lung Issues: Liver inflammation (transaminitis/hepatitis) and pulmonary alveolar proteinosis (a buildup of surfactant in the lungs).

3. Delayed or Late-Onset ADA Deficiency

About 15% to 20% of affected individuals experience partial ADA function, meaning symptoms do not appear until childhood, adolescence, or adulthood.

Milder Symptoms: These cases feature a less-depleted immune system. Patients experience recurrent (but less severe) sinus and lung infections, autoimmune disorders, and varying degrees of neurological problems.

Causes

The Biological Cause

The ADA gene provides instructions for making the Adenosine Deaminase enzyme, which is critical for breaking down a molecule called deoxyadenosine (generated when DNA breaks down).When the enzyme is deficient:

Toxic Buildup: Unprocessed deoxyadenosine accumulates, converting into a toxic substance called deoxyadenosine triphosphate (dATP).DNA Synthesis Block: High levels of dATP inhibit ribonucleotide reductase, an enzyme that developing cells need to synthesize DNA.

Lymphocyte Death: Developing T-cells and B-cells (the white blood cells responsible for immunity) are incredibly active and highly sensitive to this toxic environment. They are destroyed prematurely, preventing the immune system from maturing.

Diagnosis

Newborn Screening (NBS): In many regions, this is the first line of detection. It uses dried blood spots (Guthrie cards) to measure T-cell receptor excision circles (TRECs) via PCR, signaling poor T-cell development. Specific tandem mass spectrometry can also detect elevated adenosine levels in the blood.

Enzyme Activity Testing: Diagnosis is definitively established by biochemical testing showing severely reduced (< 1% of normal) ADA catalytic activity in red blood cells or dried blood spots.

Molecular Genetic Testing: Diagnostic confirmation is achieved by sequencing the ADA gene to identify biallelic pathogenic (disease-causing) mutations.

Laboratory Blood Work: Typically reveals profound lymphopenia (low levels of T, B, and Natural Killer cells) and low or absent serum immunoglobulins. 

Infants (Early-Onset): Often present with failure to thrive, persistent diarrhea, extensive dermatitis, recurrent pneumonia, and a complete lack of visible lymphoid tissues (such as tonsils).

Older Individuals (Delayed/Late-Onset): May present with unexplained, chronic sinopulmonary infections, recurrent viral infections (like HPV), or autoimmune issues.

Treatment

1. Enzyme Replacement Therapy (ERT)

What it is: Lifelong, weekly or bi-weekly injections of a modified ADA enzyme (PEG-ADA).

How it works: It acts as a metabolic sink to break down toxic metabolites, allowing the immune system to recover and helping to prevent infections.

When used: It is often used as a temporary stabilizing measure at diagnosis before a definitive cure is performed, or as a long-term option for patients who cannot undergo transplants or gene therapy.

2. Hematopoietic Stem Cell Transplantation (HSCT)

What it is: A bone marrow or stem cell transplant from a healthy donor.

How it works: Replaces the patient's defective blood stem cells with healthy ones that can produce functioning lymphocytes.

Best Outcome: It is the standard curative approach, offering the best survival and immune recovery when an HLA-matched sibling or family donor is available (often requires little to no chemotherapy conditioning).

3. Gene Therapy (GT)

What it is: A modified, "ex vivo" autologous stem cell therapy.

How it works: Doctors extract the patient's own stem cells, insert a healthy, working copy of the ADA gene in a lab, and then infuse those corrected cells back into the patient.

When used: It is a highly successful curative alternative for patients who do not have a matched family donor for a traditional transplant.

4. Supportive Care

Because the immune system is severely compromised prior to and during these treatments, patients also require lifelong or transitional supportive care, which typically includes:

Antibiotic/Antifungal prophylaxis to prevent opportunistic infections.

Intravenous immunoglobulin (IVIG) therapy to supply missing antibodies.

Strict avoidance of live vaccines and exposure to pathogens.

Type of Doctor Department :  A Pediatric Hematologist-Oncologist