ADA therapy refers to the treatment of adenosine deaminase deficiency, a rare genetic condition where the body lacks a critical enzyme needed for immune cell survival. Without a functioning ADA enzyme, toxic byproducts build up and destroy lymphocytes, the white blood cells responsible for fighting infection. The most severe form, ADA-SCID (severe combined immunodeficiency), appears near birth and leaves infants with virtually no immune defense. Three main treatment approaches exist: enzyme replacement therapy, stem cell transplant, and gene therapy.
How ADA Deficiency Damages the Immune System
The ADA enzyme is part of the body’s system for recycling old DNA building blocks called purines. When this enzyme is missing or barely functional (less than 1% of normal activity), adenosine and deoxyadenosine accumulate to dangerous levels. These substances are toxic to rapidly dividing cells, and lymphocytes divide faster than almost any other cell type in the body. The result is a severe shortage of T cells, B cells, and NK cells, the three pillars of the adaptive and innate immune system.
The most complete form of ADA deficiency causes SCID, which typically presents within the first months of life. Infants develop recurrent, life-threatening infections because they essentially have no functioning immune system. Less complete deficiency can appear later in childhood or even adulthood with milder but still significant immune dysfunction. ADA-SCID accounts for roughly 15% of all SCID cases and is inherited in an autosomal recessive pattern, meaning both parents must carry a copy of the faulty gene.
Enzyme Replacement Therapy
Enzyme replacement therapy (ERT) is typically the first treatment started after diagnosis. It works by injecting a lab-made version of the ADA enzyme directly into the body, which breaks down the toxic metabolites that would otherwise kill immune cells. The current FDA-approved product, approved in 2019, is a modified bovine ADA enzyme coated with polyethylene glycol to help it last longer in the bloodstream. It’s given as an intramuscular injection, usually once or twice per week.
ERT is effective at reducing toxic adenosine levels and allowing some immune recovery, but it is not considered a cure. Most treatment guidelines recommend starting ERT immediately after diagnosis to stabilize the child while a more definitive treatment, either transplant or gene therapy, is planned. About 70% of ADA-SCID patients in the U.S. receive enzyme replacement at some point during their care. The therapy must continue indefinitely unless the patient undergoes a successful transplant or gene therapy, and over time some patients develop antibodies against the injected enzyme that can reduce its effectiveness.
Stem Cell Transplant
Hematopoietic stem cell transplant (HSCT) is the preferred curative option when a matched sibling or family donor is available. The procedure replaces the patient’s defective bone marrow with healthy donor stem cells capable of producing functioning immune cells with normal ADA activity. Survival rates with a matched sibling donor reach 100% in modern series, making it the gold standard when a suitable donor exists.
The challenge is that only about 20% of ADA-SCID patients have a matched sibling or family donor. For the remaining 80%, other donor types carry higher risks. Historical data showed survival rates of 66% with matched unrelated donors and just 43% with partially matched (haploidentical) donors. However, outcomes have improved substantially in recent years. Data from transplants performed after 2007 show 100% survival with matched unrelated donors, reflecting advances in donor selection, conditioning regimens, and supportive care.
Gene Therapy
ADA-SCID holds a unique place in medicine: it was the first disease ever treated with gene therapy, in a landmark trial in the early 1990s. The approach has evolved significantly since then. Modern gene therapy uses the patient’s own stem cells, which are removed, modified in a lab to carry a working copy of the ADA gene, and then infused back. Because the cells come from the patient, there is no risk of graft rejection or graft-versus-host disease.
Long-term results have been striking. In the largest study to date, published in the New England Journal of Medicine, overall survival was 100% among 62 treated patients, and 95% achieved sustained engraftment of the corrected cells without needing to restart enzyme replacement. Of those successfully engrafted patients, 98% were eventually able to stop immunoglobulin replacement therapy entirely and showed robust responses to standard vaccinations, a sign of genuine immune recovery.
Immune reconstitution after gene therapy follows a predictable pattern. The therapeutic effects seen in the first one to two years, including enzyme activity, metabolic detoxification, and immune cell recovery, remain stable for years afterward. Age at treatment matters: infants treated by three months reached normal lymphocyte counts above 1,000 per cubic millimeter, while those treated between 4 and 20 months reached more modest levels above 500, and older children and teenagers recovered to lower counts around 200. This pattern underscores the importance of early diagnosis and treatment.
A gene therapy product called Strimvelis was approved by European regulators in 2016, making it the first approved gene therapy for this condition. Its marketing authorization has since been transferred from the pharmaceutical company that developed it to Fondazione Telethon, an Italian biomedical charity, which now manages manufacturing and access for eligible patients in the European Union. In the United States, gene therapy for ADA-SCID remains available primarily through clinical trials.
How Diagnosis Happens
Most cases of ADA-SCID are now caught through newborn screening programs. The screening test measures T-cell receptor excision circles (TRECs), small DNA fragments that serve as a marker of new T-cell production. Healthy newborns produce abundant TRECs, while babies with SCID have very low levels, typically below 25 per microliter. When screening flags a low result, confirmatory testing measures ADA enzyme activity directly and checks for elevated toxic metabolites in the blood.
Newborn screening has transformed outcomes for ADA-SCID because it allows treatment to begin before severe infections occur. Babies diagnosed and treated early have significantly better immune recovery than those diagnosed after their first serious illness.
The Treatment Decision
The current treatment pathway follows a clear sequence. After diagnosis, enzyme replacement starts immediately to stabilize the immune system and prevent infections. The medical team then evaluates whether a matched sibling or family donor is available for transplant. If a matched family donor exists, transplant is the first choice. If not, gene therapy is considered where it’s accessible. Patients who don’t have access to either option continue on long-term enzyme replacement.
Each approach involves tradeoffs. Enzyme replacement is the safest to start but requires lifelong injections and doesn’t fully restore immunity. Transplant from a matched sibling is curative with excellent survival but depends entirely on donor availability. Gene therapy offers a potential one-time cure using the patient’s own cells, with outstanding long-term data, but access remains limited by geography and clinical trial enrollment. For families navigating an ADA-SCID diagnosis, the choice depends on donor availability, the child’s clinical condition, and which therapies are accessible at their treatment center.