ADA Deficiency (SCID): Pathophysiology, Diagnosis & Treatment

By Heath Rutledge-Jukes · Founder, Noteflix

Quick summary

• Onset: 2–6 months
• Inheritance: Autosomal recessive
• Key labs: ↑ dATP, ↓ ADA enzyme activity, T-B-NK-
• Treatment: HSCT (curative); PEG-ADA as a bridge

Memory hook

ADA is Absent, dATP is Present, and T, B, NK are all Depressed. Missing enzyme → toxic metabolite builds up → every lymphocyte lineage goes down.

Pathophysiology

Adenosine deaminase (ADA) is a purine-salvage enzyme that converts adenosine → inosine and deoxyadenosine → deoxyinosine. When ADA is deficient, deoxyadenosine accumulates and is phosphorylated to dATP. Elevated dATP causes a cascade of damage that selectively wipes out lymphocytes:

• Direct lymphocyte toxicity — dATP is preferentially cytotoxic to T, B, and NK cells.
• Inhibition of ribonucleotide reductase — depletes the dNTP pool needed for DNA replication.
• Impaired DNA synthesis — the most rapidly dividing cells (lymphocytes) are hit hardest.

The result is decreased proliferation of T, B, and NK cells — the classic SCID phenotype (T-B-NK-). Neutrophils and platelets are usually preserved, which is a useful clinical distinguisher.

Inheritance

ADA deficiency is autosomal recessive. Both parents must be carriers (Aa) for a child to be affected. Carrier × carrier crosses produce:

• 25% AA — unaffected, non-carrier.
• 50% Aa — unaffected carrier (~50% of normal ADA activity).
• 25% aa — affected (absent or very low ADA activity).

Clinical features

ADA-SCID typically presents in early infancy (~2–6 months), after maternal antibodies wane. Think SCID whenever you see severe infections + failure to thrive + lymphopenia in an infant.

• Recurrent, severe infections — Pneumocystis jirovecii pneumonia, viral (especially HSV), bacterial, fungal, opportunistic.
• Failure to thrive.
• Chronic diarrhea.
• Absent or very low T, B, and NK cells on flow cytometry.
• Usually normal neutrophils and platelets.

Diagnosis

The triad on labs:

• Lymphocyte panel: very low or absent T, B, and NK cells.
• Metabolite: elevated dATP in red blood cells or lymphocytes.
• Enzyme assay: decreased ADA activity in RBCs or lymphocytes.

| Genotype | ADA enzyme activity | | --- | --- | | Affected (aa) | Absent or very low | | Carrier (Aa) | ~50% of normal | | Normal (AA) | Normal |

Many newborn screening programs now detect SCID via low T-cell receptor excision circles (TRECs), which catches ADA-SCID before the first severe infection.

High-yield differential for SCID

• ADA deficiency — autosomal recessive, T-B-NK-.
• JAK3 deficiency — autosomal recessive, T-B+NK-.
• IL-2Rγ (common γ chain) deficiency — X-linked, T-B+NK-.
• RAG1/RAG2 deficiency — autosomal recessive, T-B-NK+ (defective V(D)J recombination).
• Artemis deficiency — autosomal recessive, T-B-NK+, radiosensitive.

Treatment & prognosis

• Hematopoietic stem cell transplant (HSCT) — curative; the definitive treatment.
• PEG-ADA — pegylated bovine ADA, used as enzyme replacement and bridge therapy.
• Gene therapy — autologous gene-corrected stem cells; an emerging option for select patients.
• Supportive care — prophylactic antimicrobials, IVIG, isolation, and avoidance of live vaccines.

Early diagnosis (often via newborn screening) and prompt HSCT lead to good long-term outcomes, with most patients achieving durable immune reconstitution.

Quick facts (board-ready)

• Onset: 2–6 months.
• Inheritance: Autosomal recessive.
• Key lab: ↑ dATP, ↓ ADA activity in RBCs/lymphocytes.
• Phenotype: T-B-NK- SCID.
• Treatment: HSCT (definitive) · PEG-ADA (bridge) · gene therapy (emerging).

Frequently asked questions

What is ADA deficiency?

ADA (adenosine deaminase) deficiency is an autosomal recessive enzyme deficiency that causes severe combined immunodeficiency (SCID). Loss of ADA leads to accumulation of deoxyadenosine and dATP, which is toxic to lymphocytes — producing absent or near-absent T, B, and NK cells.

Why does ADA deficiency cause SCID?

Without ADA, deoxyadenosine accumulates and is converted to dATP. Elevated dATP inhibits ribonucleotide reductase, impairs DNA synthesis, and is selectively toxic to lymphocytes — the most rapidly dividing immune cells. The result is a T-B-NK- SCID phenotype.

How is ADA deficiency inherited?

Autosomal recessive. Both parents are typically unaffected carriers (Aa); each child has a 25% chance of being affected (aa), 50% chance of being a carrier (Aa), and 25% chance of being unaffected (AA).

How is ADA deficiency diagnosed?

Diagnosis combines clinical features (recurrent severe infections in early infancy, failure to thrive, lymphopenia) with confirmatory labs: very low or absent T, B, and NK cell counts; elevated dATP in red blood cells or lymphocytes; and decreased ADA enzyme activity in RBCs or lymphocytes.

What is the definitive treatment for ADA deficiency?

Hematopoietic stem cell transplant (HSCT) is curative. PEG-ADA enzyme replacement is used as a bridge therapy. Supportive care includes prophylactic antimicrobials and avoidance of live vaccines. Early diagnosis and HSCT lead to good long-term outcomes.

What is the SCID T-B-NK- phenotype?

It refers to severe combined immunodeficiency where T cells, B cells, and NK cells are all absent or severely decreased. ADA deficiency, RAG1/2 deficiency, and Artemis deficiency are common causes. JAK3 and IL-2Rγ deficiency cause T-B+NK- SCID.

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Educational use only. This article is a study reference for medical students preparing for the USMLE. It is not medical advice and is not a substitute for evaluation by a qualified clinician.