SUMMARY
1. Purine Nucleoside Phosphorylase (PNP) Deficiency is a rare autosomal recessive form of SCID (it accounts for 4% of SCID cases).
2. PNP is normally involved in the purine salvage pathway. PNP deficiency leads to an accumulation of intracellular deoxyguanosine triphosphate (dGTP). This metabolite is toxic to lymphocytes, leading to a decrease in peripheral T cell numbers.
3. The onset of disease can occur during infancy (with a classic SCID phenotype) or later in life with milder symptoms. Patients have lymphopenia with decreased T cell numbers and proliferation to mitogens. NK cell numbers are variable. Most patients have normal B cell numbers and immunoglobulin levels may be normal, or even elevated. Specific antibody responses may be normal or reduced.
4. Early onset neurological abnormalities (2/3 of patients) include ataxia, spasticity, tremor, hyperactivity, and mental retardation.
5. Autoimmune disorders (1/3 of patients) are also common and include hemolytic anemia, idiopathic thrombocytopenic purpura, autoimmune neutropenia, thyroid disease, arthritis, lupus, and cerebral vasculitis.
6. Measurement of PNP levels in erythrocytes as well as serum levels of toxic metabolites can be performed. A low serum uric acid can be a useful marker.
7. Sequencing of the PNP gene can confirm the diagnosis.
8. HSCT is the definitive treatment for PNP deficiency patients. Myeloablative conditioning is required for patients who have residual immune function. Unlike ADA deficiency, enzyme replacement therapy is not available.
OVERVIEW
Purine Nucleoside Phosphorylase (PNP) deficiency causes a rare form of autosomal recessive SCID. PNP is a key enzyme involved in the purine salvage pathway. PNP catalyzes the conversion of inosine, deoxyinosine, guanosine, and deoxyguanosine to guanine or hypoxanthine and ribose-1-phophate or 2 deoxyribose 1-phosphate. Because purine degradation is impaired, no uric acid is produced. High levels of deoxyguanosine are believed to be toxic to developing lymphocytes.
While patients can present during infancy with FTT, diarrhea, oral thrush, and Pneumocystis jiroveci pneumonia, some cases have presented later in life with only mild symptoms (this may be due to residual PNP activity). In addition to infectious complications, neurologic and autoimmune complications have been reported at high frequency in patients. Neurologic abnormalities (present in two-thirds of patients) include ataxia, spasticity, tremor, hyperactivity, and mental retardation. One-third of patients manifest autoimmune complications including hemolytic anemia, idiopathic thrombocytopenic purpura, autoimmune neutropenia, arthritis, pericarditis, and systemic lupus erythematosus.
EVALUATION
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)
-Uric Acid Level
-The absolute lymphocyte count (ALC) should be calculated from the CBC (WBC multiplied by the lymphocyte percentage). As with other types of SCID, lymphopenia is common.
-Low T cell numbers are typically seen but the B cell numbers can be normal. The NK cell numbers are variable.
-Very low naïve (CD45RA) T cell numbers can be a useful clue for lack of thymic output.
-Extremely low T-cell proliferation to mitogens can be seen (<10% of control). However, in late-onset cases, mitogen proliferation can be normal.
-Immunoglobulin levels and specific antibody responses can often be normal.
-Uric acid levels are decreased in PNP deficiency (less than 1 mg/dl)
STEP 2: Enzyme testing can be sent to confirm decreased PNP activity
-PNP Enzyme Testing
-Erythrocyte PNP enzyme levels can be measured at Duke University (Hershfield Laboratory) or Baylor College of Medicine.
Concurrent testing for Adenosine Deaminase (ADA) enzyme levels should be considered due to phenotypic overlap between these two diseases.
STEP 3: Genetic Testing
-Gene sequencing can confirm the diagnosis. Testing is now commercially available (Gene Dx, Cincinnati Children's)
MANAGEMENT
Pending the completion of an immunologic evaluation for suspected PNP deficiency SCID, 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 treatment for PNP deficiency patients. Myeloablative conditioning is required for patients who have residual immune function. Unlike ADA deficiency, enzyme replacement therapy is not available.
RESOURCES
Diagnostic Resources
SPECIFIC RESOURCES FOR PNP DEFICIENCY:
1. PNP enzyme testing Duke University
2. PNP enzyme testing Baylor College of Medicine
The following tests resources are accessible on the SCID overview diagostic resources page:
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 Receptor Gene Rearrangement (TCR Spectratyping)
Literature Resources
1. Markert 1991
PNP Deficiency Review
2. Ozkinay 2007
Neurologic Abnormalities in a PNP deficiency patient
3. Tabarki 2003
Spastic Paraplegia in a PNP deficiency patient
4. Dalal 2001
Neurologic Abnormalities in PNP Deficiency