Purine Nucleoside Phosphorylase Deficiency Workup

Updated: Aug 07, 2019
  • Author: Alan P Knutsen, MD; Chief Editor: Harumi Jyonouchi, MD  more...
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Workup

Laboratory Studies

Newborn screening of severe T-cell lymphopenia

Newborn screening for severe T-cell immunodeficiency disorders has been recommended in the United States using polymerase chain reaction (PCR) quantitation and the measurement of T-cell receptor rearrangement excision circles (TRECs) as a validated assay. [18] TRECs are small episomal pieces of DNA that are formed during rearrangement of the T-cell receptor genes of thymocytes undergoing differentiation in the thymus. Quantitation of TRECs in peripheral blood T cells is a measure of recent emigrants from the thymus of naïve T cells, a surrogate marker for thymopoiesis.

In newborn screening, the TREC assay is performed on DNA isolated from the Guthrie card blood spots. Decreased TRECs as a measure of decreased thymopoiesis are seen in infants with congenital T cell defects, such as severe combined immunodeficiency (SCID). Both ADA and ADA deficiencies causing SCID can be identified through newborn SCID screening. Further studies are needed to identify the specific genetic disorder of SCID (see below).

Patients often have autoimmune cytopenias, such as autoimmune hemolytic anemia, idiopathic thrombocytopenia, or autoimmune neutropenia. Patients with PNP deficiency may develop other autoimmune diseases, such as systemic lupus erythematosus (SLE) and thyroiditis.

Immunoglobulin (Ig)G autoantibodies should be measured when warranted.

Autoantibodies (eg, antinuclear antibodies [ANA], antibodies to double-stranded DNA [dsDNA], thyroid antibodies) should be measured when clinically indicated.

Purine nucleoside phosphorylase deficiency

In PNP deficiency, immunologic evaluation reveals lymphopenia and markedly decreased CD3+ T cells (< 15%), but the percentages and number of B cells are variable and often normal. T-cell function may be normal at birth but progressively decreases with age. T-cell function may also fluctuate.

Serum Ig levels may be decreased but are often normal. Antibody responses to immunizations and infectious pathogens are impaired.

Diagnosis is confirmed by low PNP activity in erythrocytes, lymphocytes, and fibroblasts. Low levels of serum uric acid suggest PNP deficiency but not ADA deficiency.

Adenosine deaminase deficiency

In infantile-onset ADA deficiency, lymphopenia and attrition of T-cell, B-cell, and natural killer (NK)-cell function occurs (see Table 1). Profound lymphopenia of less than 500 cells/mcL, is typical of ADA severe combined immunodeficiency (SCID) and distinguishes it from other genetic causes of SCID. Percentages of T cells and numbers of CD3+, CD4+, and CD8+ T cells are markedly decreased. Percentages of CD19+ B-cells and CD16+/CD56+ NK-cells vary, but absolute numbers of B and NK cells are markedly decreased, resulting in a T-, B-, NK- phenotype of SCID. T-cell function as measured by lymphoproliferative responses to mitogens, antigens, and alloantigens are absent. Hypogammaglobulinemia and antibody deficiency complete the immune profile of SCID.

In late-onset ADA deficiency, serum Ig levels are low or absent with decreased antibody responses. Lymphopenia and reduced CD3+ and CD4+ T cells are present. Although T-cell responses may be decreased, they are not so suppressed as to predispose patients to intracellular and opportunistic infections. This form may be misdiagnosed as common variable immunodeficiency (CVID). Lymphopenia in a patient with CVID warrants consideration of possible ADA deficiency. Eosinophilia and elevated serum IgE levels are often present.

In adult-onset ADA deficiency, IgG2-subclass deficiency with decreased antibody responses to polysaccharide antigens may be present, predisposing patients to sinopulmonary infection by encapsulated bacteria. Lymphopenia, decreased numbers of CD3+ and CD4+ T cells, elevated serum IgE levels, and eosinophilia are present, as is seen in late-onset ADA deficiency. Recurrent varicella-zoster, herpes simplex, and Candida infections may be present.

Several immunologic studies may be helpful in assessing ADA deficiency, including those seen in the following table:

Table 1. Immunologic Studies and Findings in Adenosine Deaminase Deficiency (Open Table in a new window)

Study

Infantile Onset

Late Onset

Adult Onset

Lymphopenia

Markedly decreased

Decreased

Decreased

CD3+ cells

Absent or trace

Markedly reduced

Markedly reduced

CD4/CD8 ratio

Too few to test

< 1

< 1

Phytohemagglutinin response

Absent

Reduced

Reduced

Antigen response

Absent

Trace

Trace

Mixed lymphocyte culture response

Reduced

...

...

Ig response

Absent

Low to absent

Normal (low IgG2)

IgE

Low

Elevated

Elevated

Antibody response

Absent

Absent to low

Low to polysaccharides antigens

Eosinophilia

Rare

Common

Common

Infections

Predominantly viral, fungal, opportunistic, bacterial

Bacterial sinopulmonary

Bacterial sinopulmonary, varicella-zoster, herpes simplex, candidal

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Imaging Studies

The thymic shadow is absent on chest radiography. Adenoid tissue is absent on lateral airway radiographs.

In ADA deficiency, the characteristic radiographic finding of bony structures are sometimes observed and correlate with bony histologic abnormalities.

These findings include cupping or flaring of the ribs, similar to the appearance seen in rickets.

In addition, abnormalities of the vertebral transverse processes and scapula may be observed.

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Other Tests

Genetic studies to examine mutations of genes that encode for ADA and PNP are readily available and should be performed.

In PNP deficiency, Grunebaum et al identified “hot spots” at codons 58 and 234 with increased frequency of mutations in the gene that encodes PNP. [19, 2]

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Histologic Findings

In ADA deficiency, if thymic biopsy is performed (which is usually not necessary), the results demonstrate marked loss of corticomedullary differentiation; absence of Hassall corpuscles; and depletion of thymocytes, especially in the thymic cortex and medulla.

In PNP deficiency, histopathology of lymphoid tissue reveals abnormalities, predominantly in T-cell dependent areas. The thymus is markedly reduced in size, with depleted thymocytes. Hassall corpuscles are present but poorly formed. By comparison, Hassall corpuscles are usually absent in patients with classical SCID. In the lymph nodes and spleen, paracortical regions are reduced or absent. Germinal centers are reduced; however, plasma cells can be identified.

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