1. Adenosine Deaminase (ADA) Deficiency is the second most common cause of SCID (it accounts 20% of all cases). The incidence is 1 in 200,000 live births. It causes a form of autosomal recessive SCID which results in a T-B-NK- phenotype.
2. ADA deficiency leads to leads to an accumulation of intracellular adenosine and deoxyadenosine. These products are particularly toxic to developing lymphocytes, leading to a profound decrease in all lymphocyte subset numbers.
3. Patients classically present in infancy with failure to thrive, severe thrush, opportunistic infections, and chronic diarrhea. However, some patients can present with late-onset disease due to the presence of residual enzyme activity.
4. Additional clinical features include elevated liver enzymes, abnormal flaring of the anterior rib ends, prominent costochondral junctions, squared off scapula, pelvic dysplasia, and neurologic abnormalities (hypotonia, head lag, nystagmus, ataxia).
5. Lymphopenia (ALC < 2800 cells/mm3) is present with lymphocyte subset analysis showing very low T cells, B cells, and NK cells. T cell proliferation to mitogens is poor and serum immunoglobulins are decreased. Specific antibody responses to vaccine antigens are poor.
6. Measurement of ADA levels in erythrocytes as well as serum levels of toxic metabolites can be performed. Testing should be performed prior to any transfusions (red blood cell transfusions
may result in normal enzyme levels).
7. Sequencing of the ADA gene can confirm the diagnosis.
8. In addition to treatment of acute infections, the following immediate management steps must be implemented:
-Avoid all live viral vaccines
-Only irradiated, CMV negative blood products should be used (to prevent GVHD and infections
-Pneumocystis jiroveci prophylaxis with trimethoprim-sulfamethoxazole
-IVIG replacement therapy
-Start HLA-typing for the patient and any siblings for possible hematopoietic stem cell transplantation (HSCT)
9. HSCT is the definitive treatment for ADA deficiency patients. As with other types of SCID, transplantation is more successful when performed early in life (prior to development of damage from infections) and when an HLA-identical sibling donor is available.
10. ADA deficiency is the only type of SCID that can be treated with enzyme replacement therapy. Polyethylene glycol (PEG)-ADA can be infused twice weekly to correct the endogenous deficiency of this enzyme. Partial restoration of immune functions can be achieved with therapy (which can make subsequent HSCT more difficult). PEG-ADA therapy is an option for patients who do not have an HLA-matched donor or patients who fail to engraft following HSCT.
11. Gene therapy for ADA may become a viable option in the future once the safety profile can be improved.
Adenosine Deaminase (ADA) Deficiency is the second most common type of SCID after X-linked SCID. This autosomal recessive disease accounts for 20% of all SCID cases and results in a T-B-NK- phenotype. In the vast majority of affected patients, the clinical presentation occurs in early infancy with a classic SCID phenotype: FTT, diarrhea, severe thrush, PJP pneumonia, and severe viral respiratory infections. However, late-onset milder forms of disease (15% of patients) have been described in older children and even in adults. These patients have some residual ADA enzyme activity.
ADA deficiency has a number of unique clinical features in addition to the typical SCID phenotype. Liver enzymes are often elevated and may be the result of toxic metabolite accumulation [improvement can be seen with enzyme (PEG-ADA) replacement therapy]. Skeletal abnormalities including abnormal flaring of the anterior rib ends, prominent costochondral junctions, squared off scapula, and pelvic dysplasia may be present. Neurologic abnormalities have been described including hypotonia, head lag, nystagmus, and ataxia.
The absolute lymphocyte count will be low (typically below 2800 cells/mm3) in most cases. Lymphocyte subset analysis by flow cytometry will show low T, B, and NK cells. T cell proliferation to mitogens is very low.
Adenosine deaminase is an enzyme found in all cells that catalyzes the deamination of adenosine and deoxyadenosine to inosine and deoxyinosine. In ADA deficiency, accumulation of these metabolites are particularly toxic to developing lymphocytes. As a result, all lymphocyte subsets are affected and patients develop a T-B-NK- phenotype.
STEP 1: Immune Evaluation
-CBC with Differential
-Lymphocyte subset enumeration by flow cytometry (CD3, CD4, CD8, CD19, CD16/56)
-Naïve (CD45RA) and memory (CD45RO) T-cell enumeration by flow cytometry
-T-cell proliferation to Mitogens (PHA)
-IgG, IgA, IgM levels
-Specific Antibody levels (if older than 6 months)
-The absolute lymphocyte count (ALC) should be calculated from the CBC (WBC multiplied by the lymphocyte percentage). SCID presents with an ALC less than 2800 cells/mm3 in 95% of cases.
-Lymphocyte subset analysis typically shows decreased T cells, B cells, and NK cells (T-B-NK- phenotype).
-Very low naïve (CD45RA) T cell numbers can be a useful clue for lack of thymic output. Maternally derived T cells typically have a memory (CD45RO) phenotype.
-Extremely low T-cell proliferation to mitogens is seen in SCID (<10% of control). The large blood volume required to perform mitogen proliferation is often an issue with small infants. Performing the proliferation assay with one stimulus (PHA) is acceptable and requires less blood.
-Immunoglobulin levels before 6 months of age may reflect transplacentally aquired maternal IgG). However, immunoglobulin levels can be low prior to 6 months in SCID due to accelerated consumption from recurrent infections.
-A chest X-ray may reveal absent thymic tissue or the presence of flared anterior ribs.
STEP 2: Additional Immune Evaluation.
The following tests may provide additional support for a diagnosis of SCID and can be helpful in certain clinical situations but not necessarily required.
-TCR Gene Rearrangement PCR (TCR Spectratyping)
-Maternal Engraftment Study
-TRECs (T-cell receptor excision circles) are loops of DNA excised during TCR rearrangement in the thymus. Because TRECs are not replicated with cell division, they are gradually diluted as T-cells become activated and expand. Thus, naïve T-cells that are recent thymic emigrants have high TREC numbers. SCID patients typically have very low TREC numbers.
-TCR gene rearrangement is useful for identifying oligoclonally expanded T-cells. This can be seen in maternal engraftment as well as Omenns syndrome.
-Maternal T cells can occasionally undergo clonal expansion in patients with SCID. Maternal T cells typically are CD45RO+ and proliferate poorly to mitogen stimulation. Assessing for the presence of maternal cells in circulation (maternal engraftment) is useful because it can affect the selection of a stem cell donor and it may necessitate immunosuppression prior to transplantation.
STEP 3: If lymphocyte subset analysis reveals a T-B-NK- phenotype, further evaluation for ADA deficiency should be pursued.
-Measurement of Adenosine Deaminase in Erythrocytes and Toxic Metabolites
-This can be performed at the Hershfield Laboratory at Duke University or at Baylor College of Medicine. Recent red blood cell transfusions can result in a false negative test.
-Genetic testing for ADA Deficiency (ADA)
-Genetic testing for ADA is commercially available through correlagen diagnostics. Gene sequencing typically takes up to 4-6 weeks.
Pending the completion of an immunologic evaluation for suspected ADA deficiency, it is critical to initiate certain measures to prevent life-threatening complications for patients. The following precautions should be implemented immediately:
1. Avoid all live viral vaccines (rotavirus, varicella, MMR, BCG)
Severe vaccine strain disease can occur if SCID patients receive these vaccines.
2. Only irradiated, CMV negative blood products should be used
Leukocytes from non-irradiated blood can cause graft versus host disease and CMV can cause severe infections.
3. Pneumocystis jiroveci prophylaxis with trimethoprim-sulfamethoxazole
4-6mg/kg/day of Trimethoprim component divided twice daily 3 days per week
4. IVIG replacement therapy
5. High resolution HLA-typing for the patient and any siblings
For possible Hematopoietic Stem Cell Transplantation (HSCT)
HSCT is the definitive and preferred treatment for ADA deficiency patients. As with other types of SCID, transplantation is more successful when performed early in life (prior to development of damage from infections) and when an HLA-identical sibling donor is available.
ADA deficiency is the only type of SCID that can be treated with enzyme replacement therapy. Polyethylene glycol (PEG)-ADA can be infused twice weekly to correct the endogenous deficiency of this enzyme. Partial restoration of immune functions can be achieved with therapy. It should be noted that restoration of immune function can make future stem cell transplantion more difficult. PEG-ADA therapy is an option for patients who do not have an HLA-matched donor or patients who fail to engraft following HSCT. The cost of this therapy is at least $100,000 - $300,000 per year.
Gene therapy for ADA may become a viable option in the future once the safety profile can be improved. Gene therapy in X-linked SCID has led to insertional oncogenesis in some patients resulting in the development of leukemia.
SPECIFIC TESTING FOR ADA DEFICIENCY SCID:
The following tests resources are accessible on the SCID overview diagnostic resources page: (click here to link)
1. Lymphocyte Subsets by Flow Cytometry for T-cell (CD3, CD4, CD8), B-cell (CD19),
and NK cell (CD16/56).
2. Naïve (CD45 RA) and Memory (CD45 RO) T cells by Flow Cytometry
3. T-cell proliferation to Mitogens and Specific Antigens (candida, tetanus)
4. TREC (T-cell receptor excision circle) Analysis
5. T-cell Recepror Gene Rearrangement (TCR Spectratyping)
1. Gaspar 2009
Management of ADA Deficiency
2. Gaspar 2010
HSCT and PEG-ADA for ADA Deficiency
3. Cappelli 2010
Gene therapy for ADA Deficiency
4. Nofech-Mozes 2007
Neurologic abnormalities in ADA Deficiency