Purine Nucleoside Phosphorylase Deficiency

Two genetic defects of the purine salvage pathway account for two immunodeficiencies that result in severe combined immunodeficiency (SCID).[2, 3] One disorder is adenosine deaminase (ADA) deficiency, which is Online Mendelian Inheritance in Man (OMIM) subject number 102700, and the other is purine nucleoside phosphorylase (PNP) deficiency, which is OMIM subject number 164050.

ADA and PNP deficiencies are autosomal recessive disorders. ADA and PNP are ubiquitous "housekeeping genes." In both disorders, the enzyme deficiencies result in accumulation of toxic metabolites, especially in lymphocytes. In ADA deficiency, the toxic metabolites block T-cell, B-cell, and natural killer (NK)-cell development; whereas in PNP deficiency, the metabolites are especially toxic to T-lineage cells, resulting in profound T-cell deficiency and variable degree of B-cell dysfunction.

In addition, both ADA and PNP deficiencies cause developmental delays and progressive neurological deterioration if not treated. This is especially prevalent in PNP deficiency with neurologic symptoms, including mental retardation and muscle spasticity, reported in 67% of patients with PNP deficiency. In addition, PNP deficiency is associated with increased risk of autoimmune disorders, such as autoimmune hemolytic anemia, immune thrombocytopenia, neutropenia, thyroiditis, and lupus.

ADA deficiency results in absence of T, B, and NK cells, resulting in a SCID with marked lymphopenia. PNP deficiency causes profound T lymphopenia and variable numbers of B and NK cells. Serum immunoglobulin (Ig) levels are normal to near-normal, but specific antibody production is impaired.

PNP is an enzyme in the purine salvage pathway that metabolizes inosine and guanosine to hypoxanthine.[4, 5, 6, 7] In the preceding step of the pathway, ADA metabolizes adenosine to inosine. ADA deficiency causes a SCID that accounts for approximately 20% of all SCID cases. In both metabolic disorders, the enzyme deficiencies cause the accumulation of metabolites that are toxic to lymphoid lineage cells. See the image below.

In adenosine deaminase deficiency, adenosine and adenine accumulate in the plasma.[8, 9] ATP accumulates in erythrocytes, and ADP, guanosine triphosphate (GTP), and ATP accumulate in lymphocytes. Deoxy-ATP (dATP) can reach toxic levels that inhibit ribonucleotide reductase, an enzyme essential for synthesis of DNA precursors.

In purine nucleoside phosphorylase deficiency, similar changes occur, resulting in elevated deoxy-GTP (dGTP) levels. dATP and dGTP predominantly accumulates in lymphoid tissue. dGTP inhibits ribonucleotide reductase, which is needed for synthesis of deoxynucleotides. In both adenosine deaminase and purine nucleoside phosphorylase deficiencies, thymocytes are thought to be selectively destroyed because of elevated levels of dATP and dGTP.

In a further description of the mechanism of T-cell depletion in purine nucleoside phosphorylase deficiency, Arpaia et al reported increased in vivo apoptosis of T cells and increased in vitro sensitivity to gamma irradiation in a murine model.[4] The immune deficiency in purine nucleoside phosphorylase deficiency may be the result of inhibited mitochondrial DNA repair due to the accumulation of dGTP in the mitochondria. The end result is increased sensitivity of T cells and thymocytes to spontaneous mitochondrial damage, leading to T-cell depletion due to apoptosis.

With adenosine deaminase deficiency, destruction of resting T cells and B cells is increased. In comparison, purine nucleoside phosphorylase deficiency results in selective destruction of T cells, with little effect on B cells. Numerous mutations of the ADA gene (on chromosome 20) and PNP genes (on band 14q13) have been identified.[2] Purine nucleoside phosphorylase is a trimer with molecular weight of 84-94 kDa. Most identified mutations are missense mutations, but deletion is also described. All reported patients with homozygous mutations of PNP have been symptomatic. Because only small amounts of adenosine deaminase are necessary for competent immunity, some patients with ADA mutations may still have 8-42% adenosine deaminase activity and no profound immunodeficiency.[2, 3]

Frequency

Purine nucleoside phosphorylase deficiency is rare; only about 70 affected individuals have been described in the medical literature. This disorder accounts for approximately 4%  of all SCID cases.[7, 10, 11]

Mortality/Morbidity

Patients with PNP deficiency are at risk for life-threatening recurrent viral, bacterial, fungal, mycobacterial, and protozoal infections. In addition, failure to thrive eventually ensues. The risk of lymphoma is also increased in patients with PNP deficiency. Neurologic symptoms, including mental retardation and muscle spasticity, are major comorbid conditions that affect 67% of patients with PNP deficiency in one report and neurological deterioration is progressive with age if not treated.

Bone marrow transplantation may cure the immunodeficiency but does not reverse neurological damage that has already been caused by toxic metabolites. Patients are at risk for autoimmune diseases, including autoimmune hemolytic anemia, immune thrombocytopenia, thyroiditis, neutropenia, and lupus.

Demographics

PNP and ADA deficiencies are autosomal recessive disorders with equal incidence in boys and girls.

Although symptoms typically appear in the first year of life in patients with PNP deficiency, gradual deterioration of the T-cell immune system may delay the onset of symptoms until the second year of life. As above, neurological deterioration is also pregressive with age.

Bone marrow transplantation (BMT) can cure immunodeficiency and prevent neurological deterioration. Since neurodevelopmental impairment is progressive with age, early diagnosis and treatment (BMT) will be the key to minimize neurodevelopmental damage and other complications. However, PNP deficiency can be missed with current newborn SCID screening which assesses native T cell output from the thymus by measuring TCR rearrangement excision circle (TREC), since progressive loss of T cells in PNP deficiency due to toxic metabolites may not be fully manifested at birth. A high index of suspicion is necessary for early diagnosis of PNP to have the best clinical outcome.

Once diagnosis was made by measuring PNP activity, a patient and his/her family members need to be educated for treatment options (mainly BMT) and expected outcomes.  Especially it needs to be clarified that immunodeficiency is curable by BMT but neurodevelopmental impairment caused by toxic metabolites may not be reversible by BMT.

Most patients with purine nucleoside phosphorylase (PNP) deficiency have a history of recurrent viral, bacterial, fungal, mycobacterial, and protozoal infections, similar to patients with severe combined immunodeficiency (SCID).[2, 3] Oral candidiasis that is recalcitrant to therapy occurs in approximately 85% of patients with severe T-cell immunodeficiency. In addition, the presenting infections are often those caused by opportunistic microorganisms, such as Pneumocystis jiroveci pneumonia.

Live-attenuated immunizations should be avoided as they may result in infection. Rotovirus immunizations resulting in infection has been reported in patients with SCID. Varivax immunization has resulted in disseminated varicella. BCG immunization may cause pulmonary or disseminated infection.[12] Progressive multifocal leukoencephalopathy due to polyomavirus infection has been reported.[13]

Neurologic problems are commonly associated with PNP and adenosine deaminase (ADA) deficiencies and have therapeutic implications. More than 50% of patients with PNP deficiency have neurologic impairments that may predate the onset of infections. Neurologic problems include developmental delay, hypertonia, spasticity, tremors, ataxia, retarded motor development, behavioral difficulties, and varying degrees of mental retardation.[7] Patients with ADA deficiency may also have neurologic problems, principally neurodevelopmental delays. Of importance, polyethylene glycol (PEG) ADA therapy does not correct the neurodevelopmental problems in ADA deficiency, although immune reconstitution does occur. Likewise, bone marrow transplantation does not correct neurological deficits in PNP or ADA deficiencies.

Autoimmune disorders are also frequent in PNP deficiency.[7] These include autoimmune hemolytic anemia (AHA), idiopathic thrombocytopenia (ITP), autoimmune neutropenia, lupus, thyroiditis, and central nervous vasculitis.

Lymphoma and lymphosarcoma has also been reported in children with PNP immunodeficiency.

Purine nucleoside phosphorylase immunodeficiency

Patients with PNP deficiency may have recurrent sinopulmonary infections that may result in a delay in diagnosis until late childhood.

Patients with PNP deficiency, similar to patients with serious T-cell immune deficiency, are susceptible to herpes infections (eg, varicella).

Patients with purine nucleoside phosphorylase deficiency are also susceptible to recurrent urinary tract infections.

Adenosine deaminase immunodeficiency

Infections typically appear in infancy. However, T-cell function can fluctuate and might not be completely absent. Therefore, a spectrum of T-cell immune deficiency is reported in patients with ADA deficiency.[14]

Live-attenuated immunizations should be avoided as they may result in infection. Rotovirus immunizations resulting in infection has been reported in patients with SCID. Varivax immunization has resulted in disseminated varicella. BCG immunization may cause pulmonary or disseminated infection.[12] Progressive multifocal leukoencephalopathy due to polyomavirus infection has been reported.[13]

Major clinical phenotypes of ADA deficiency have been described, as follows:

• Neonatal or infantile onset – This is indistinguishable from other forms of SCID; bony abnormalities are reported in 50% of patients. In ADA-deficient SCID, there are a number of metaphyseal changes that can be identified radiologically.[15] These include concavity and flaring of the anterior rib costochondral functions, squaring of the ilia, abnormal posterior costovertebral junctions, platyspondyly, growth arrest lines, trabecular paucity, and overall mild shortening of the extremities and scapular spurring. Grunebaum et al[16] reported pulmonary alveolar proteinosis (PAP) in patients with ADA deficiency. Interestingly, PAP was reversed with polyethylene glycol–ADA therapy.

• Delayed - Onset at age 0-2 years; retention of immunoglobulin (Ig) with later attrition, susceptibility to infection similar to that of ADA SCID

• Late onset - Onset at age 3-15 years; may present with recurrent bacterial sinopulmonary infections typical for antibody immunodeficiency, lymphopenia (measure adenosine deaminase and purine nucleoside phosphorylase activity), hyper-IgE, eosinophilia, autoimmunity; may be misdiagnosed as common variable immunodeficiency (CVID) or IgG2-subclass deficiency/specific antibody deficiency

• Adult onset - Same as late onset, but in adolescents or young adults, plus persistent warts, recurrent herpes zoster, idiopathic thrombocytopenic purpura, lymphopenia; may be misdiagnosed as CVID or IgG2-subclass deficiency/specific antibody deficiency

• Somatic mosaicism (de novo or revertant mutations) - May show improvement over time without treatment

Both partial ADA deficiency and somatic mosaicism have no confirmed immunodeficiency. Some individuals may have very low levels of ADA activity in lymphocytes but retain 55-80% normal ADA activity in RBCs. These children were healthy in childhood. In addition, revertant mutations of inherited ADA gene mutations have been described.[17]

Physical examination reveals a paucity of peripheral lymphoid tissue, such as lymph nodes, tonsillar tissue, and adenoids. The liver and spleen are usually normal in size but can be enlarged in patients with accompanying hemolytic anemia or lymphoma. In neonates, the thymic shadow is typically small on chest radiography. Neurologic symptoms, consisting of developmental delay, hypertonia, spasticity, and tremors, may be present. Patients may fail to thrive.

PNP deficiency is a genetic disorder caused by a deficiency of the enzyme purine nucleoside phosphorylase. The PNP gene has been localized to band 14q13. Missense mutations have been identified in some patients. The PNP protein is a trimer with a molecular weight of 84-94 kDa, with the highest levels in lymphoid tissue. The mechanism by which PNP deficiency causes neurologic disease is unknown.

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)

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.

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]

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.

Guidelines for the diagnosis and management of primary immunodeficiencies have been established.[20] The following treatment may be indicated in patients with purine nucleoside phosphorylase (PNP) and adenosine deaminase deficiencies:

Bone marrow transplantation

The treatment options discussed included allogeneic hematopoietic stem cell transplantation (HSCT) from a matched family donor (MFD), a mismatched haploidentical donor, and unrelated bone marrow donor.[21]

Hassan et al[22, 23] reported results of transplantation for the Inborn Errors Working Party of the European Group for Blood and Marrow Transplantation and European Society for Immunodeficiency. In this multicenter study, 106 patients with ADA-deficient SCID received 119 transplants from matched sibling donors (MSDs), matched family donors (MFDs), matched unrelated donors (MUDs), and haploidentical donors. Survival was 86% in MSDs, 81% in MFDs, and 66% in MUDs, compared with 43% in haploidentical donors. Survival was also greater in patients who did not receive pretransplantation conditioning compared with those that received myeloablative therapy, 81% versus 54%. T- and B-cell engraftment was achieved in all groups.

Metabolic abnormalities improved with reduction of deoxy-ATP (dATP) levels following transplant.

In PNP deficiency, human leukocyte antigen (HLA)-matched bone marrow transplantations have been successful in patients who have received pretransplantation conditioning. Haploidentical bone marrow transplantations were difficult to engraft, partly because patients did not receive a conditioning protocol before transplantation. Because residual T-cell function may be present in PNP deficiency, the transplant may have been rejected.[7, 24]

Successful immune reconstitution has been reported in a patient with PNP deficiency using transplantation of stem cells from umbilical-cord blood.[25] The conditioning regimen consisted of busulfan, cyclophosphamide, and antithymocyte globulin (ATG). The patient's neurologic impairments resolved.

A nonmyeloablative conditioning regimen of busulfan and fludarabine has resulted in successful immune reconstitution.[26] The conditioning regimen lowered the risk of vaso-occlusive disease when 2 alkylating agents (eg, busulfan and cyclophosphamide) that potentiate hepatotoxicity were used together.

One important issue in the treatment of children with severe T-cell disorders is whether to use a preparative regimen before stem cell infusion for immunosuppression to prevent rejection and myeloablation and, thus, to allow donor T-cell, B-cell, and monocytic-cell engraftment. This is an important issue with all options involving hematopoietic stem cells or umbilical cord blood.

In patients who received T-cell–depleted transplants of bone marrow grafts without prior cytoreduction, graft failures occurred in 30-50%.[27, 28]

In both murine models and in patients with severe combined immunodeficiency (SCID), normal-to-high natural killer (NK)-cell activity is associated with a higher incidence of graft failure or delayed immunologic reconstitution; this is perhaps the foremost cause for graft rejection.

The nature of the preparative regimen and the relative importance of immunosuppression versus ablation have not yet been fully defined and may depend on the nature of the hematopoietic stem cell graft. The optimal combination of ablative agents (eg, busulfan) with immunosuppressive agents (eg, ATG, cyclophosphamide), and/or newer agents (eg, fludarabine, alemtuzumab) has not been systematically studied and should be the focus of future clinical trials.

The risks of the preparative regimen are known and include sterility, liver, heart and lung toxicity, and malignancy. These risks must be balanced against morbidity and mortality associated with graft rejection and repeated transplantation, poor T-cell engraftment, and/or poor B-cell function. Children with severe T-cell dysfunction may also have serious infections that cannot be eliminated.

Ultimately, the goal of bone marrow transplantation is to provide complete T-cell, B-cell, and NK-cell function. The choice of preparative regimen is complicated by the heterogeneity of NK-cell and B-cell function, which can be expected to develop in various forms of SCID when successful T-cell engraftment occurs. This heterogeneity is not necessarily the case when T cell–depleted haploidentical bone marrow transplantation is performed.

The role of myeloablation in the preparative regimen before transplantation remains controversial. Some groups achieved stable immune reconstitution by transplanting umbilical-cord blood in patients with thymic dysplasia and SCID without ablative therapy. However, patients with some severe T-cell immunodeficiency disorders, such as reticular dysgenesis, combined immunodeficiency (CID), thymic dysplasia, and Wiskott-Aldrich syndrome, require an immunosuppression regimen for preparation.

Patients with PNP deficiency probably also require pretransplantation conditioning. Some groups use a preparative regimen in SCID with high NK-cell function. Further studies are necessary to determine whether this is a true or only theoretical advantage. In addition, patients may need posttransplantation graft versus host disease (GVHD) prophylaxis with cyclosporine and corticosteroids, which affect the function of mature T cells in the umbilical cord preparation.

Preparative regimen and GVHD prophylaxis

Transplantation groups disagree on the need for a preparative regimen for transplantations in both ADA and PNP deficiency. Groups that favor a preparative regimen disagree on what regimen should be used. Preparative regimens have included myeloablative treatments (busulfan, cyclophosphamide, ATG), nonmyeloablative treatments (busulfan, fludarabine), or busulfan alone.

Pretransplantation conditioning for patients typically includes busulfan at 1 mg/kg (1.25 mg/kg if patient < 2 y) given orally every 6 hours on days -9 through -6 (transplantation is on day 0). The dose of busulfan is adjusted on the basis of first-dose kinetics (steady-state level or 400-600 ng/mL). This is followed by cyclophosphamide at 50 mg/kg intravenously (IV) on days -5 through -2 and ATG 30 at mg/kg given IV on days -3 through -1. Recently, alemtuzumab has been introduced as part of a reduced-intensity conditioning regimen in combination with busulfan and/or fludarabine for HSCT in children with malignant and nonmalignant diseases. Alemtuzumab is a humanized monoclonal antibody to CD52 antigen, and this action may help reduce GVHD by depleting T-cells. These reduced-intensity conditioning regimens appear to have improved tolerability and the success rate of HSCT for PNP deficiency.

Prophylaxis for acute GVHD includes a continuous IV infusion of cyclosporin A beginning on day -2 (target whole blood levels of 250-350 ng/mL) and methylprednisolone at 10 mg/kg/d IV on days 5-7, 5 mg/kg/d on days 8-10, and 3 mg/kg/d on days 11-13, followed by a 10% weekly reduction taper. Patients are evaluated daily for acute GVHD during hospitalization and at least weekly after their discharge home for the first 100 days after transplantation. Corticosteroids are generally discontinued by day 60 after transplantation, and cyclosporin A is discontinued between days 100 and 365, depending on clinical evidence of GVHD.

Enzyme replacement therapy

Approximately, 150-160 patients with ADA deficiency worldwide have received polyethylene glycol (PEG)–adenosine deaminase, and approximately 90 patients were receiving treatment in 2006. In the experience of these investigators, in 31 patients who received PEG–adenosine deaminase, 14 patients went on to HSCT, 14 patients continued to do well on PEG–adenosine deaminase therapy, and 3 patients died. PEG–adenosine deaminase therapy was generally well-tolerated. Immune reconstruction varied. In the first 6 months of therapy, absolute lymphocyte count (ALC), CD3+, and CD4+ T cell numbers did increase, with good lymphoproliferation responses to phytohemagglutinin (PHA), and thymopoiesis increased. However, over a prolonged time, T-cell numbers and naive T cells were reduced compared with normal controls. Metabolic abnormalities improved with PEG–adenosine deaminase but not as effective as that seen with HSCT.

The bovine-derived ADA replacement enzyme pegademase (Adagen) was approved by the FDA in 1990. In October 2018, the FDA approved elapegademase (Revcovi) for treatment of adenosine deaminase severe combined immune deficiency (ADA-SCID) in adults and children. The drug had been available as an orphan drug prior to approval. Enzyme replacement helps prevent potentially serious, life-threatening infections in this patient population.

In PNP deficiency, RBC transfusions have offered limited improvement.

Initial attempts to develop a PEG–PNP similar to PEG–ADA were disappointing in that both human and bovine PNP enzymes were too unstable at 37°C. Subsequently, the more stable hexameric Escherichia coli PNP was used and replaced 3 arginine residues in each of the 6 subunits with lysine by means of site-directed mutagenesis.[29] In murine models, this PEG–PNP was biologically active. This enzyme also had the ability to phosphorylate Ado, which may permit its use in the treatment of ADA deficiency in addition to PNP deficiency.

Recently, a novel technique to intracellularly transduce PNP protein was reported.[30] They used an 11 amino acid human immunodeficiency virus (HIV) transactor (TAT) protein transduction domain (PTD) and created a fusion protein with PNP protein. This PNP–PTD fusion protein rapidly transduced lymphocytes in vitro. Using lymphocytes from patients with PNP deficiency and PNP-/- mice, they demonstrated that the PNP–PTD fusion protein rapidly restored metabolic function and T-cell function in vitro. Furthermore, because the PNP–PTD was cellularly transduced, neutralizing antibodies had little effect.

These studies hold promise of treating patients with PNP deficiency with enzyme replacement therapy.

Live viral immunizations (eg, with oral polio vaccine) should be avoided.

Trimethoprim-sulfamethoxazole (Bactrim, Septra) is used for P carinii prophylaxis.

Fluconazole is used as prophylaxis against Candida species.

Gene therapy

Since the initial trials of ADA gene therapy performed at the National Institute of Health (NIH) and University of Southern California, gene therapy for ADA deficiency has been performed in Milan and London.[31, 32] ([33] )[34]

In these centers, autologous CD34+ human stem cells were transfected using retroviral vectors encoding the ADA gene.

In contrast to the US trials, these patients received mild conditioning regimens prior to infusion of the gene-modified human stem cells, consisting of either busulfan (Milan) or melphalan (London). PEG–adenosine deaminase was discontinued prior to gene therapy.

In the Milan experience, in 6 of 8 children who were monitored more than 6 months after gene therapy, vector-adenosine deaminase+ cells progressively became most of the T cells, B cells, and NK cells. Stable engraftment of gene corrected cells was seen in 0.1-10% of the myeloid cells. Thymopoiesis, T-cell number and function, and B-cell antibody responses improved. Metabolic abnormalities with decreased deoxyribonucleotides levels were seen. No adverse events or toxicity related to gene therapy were observed.

In London, one patient received ADA gene therapy. This patient experienced similar immune reconstitution and improvement of deoxyribonucleotides toxic metabolites as that observed in the Milan experience.

Despite immunologic reconstruction and decreased deoxyribonucleotides toxic metabolites, significant cognitive and behavioral abnormalities have persisted following HSCT and PEG–adenosine deaminase therapy. Specifically, the London group documented significant reduction in both verbal and performance intelligence quotient (IQ) levels. Metabolic detoxification is not complete with both HSCT and PEG–adenosine deaminase therapies, especially in erythrocytes. However, with gene therapy, erythrocyte ADA enzyme activity does improve, leading to decreased levels of deoxyribonucleotides metabolites in erythrocytes. Clinically, these patients have normal development to date.

PNP deficiency, similar to ADA deficiency, may be amenable to correction with gene therapy.[35] In reports of in vitro studies, gene therapy corrected PNP-deficient cells. In a murine model, Liao P et al reported successful gene therapy using a lentiviral vector containing the human PNP gene (lentiPNP).[36] Lymphocytes from a PNP-deficient patient and PNP deficient (-/-) were transduced with the lentiPNP gene and then transplanted into PNP (-/-) mice. The lentiPNP transduction corrected the abnormalities associated with PNP deficiency.

A proposed alternative to gene therapy is in situ repair of the defective gene. The principle is to synthesize a short oligodeoxyribonucleotide complementary to the section of the defective gene containing the error (except for the site corresponding to the error). Here, an oligomer contains the nucleotide complementary to that of the normal gene. The oligomer is transfected into the cells by using liposome vectors and binds to its complementary sequence in the defective gene. DNA repair enzymes then delete the defective sequence and insert the correct sequence.

Supportive care

All the patients are hospitalized in single rooms with high-efficiency particulate air-filtration systems.

All blood product transfusions are irradiated with 25 Gy before their administration to prevent GVHD.

Live viral immunizations (eg, with oral polio vaccine) should be avoided.

Trimethoprim-sulfamethoxazole (Bactrim, Septra) is used for P jiroveci prophylaxis.

Antifungal prophylaxis (fluconazole) is used as prophylaxis against infection with Candida species.

Consult a hematologist or an immunologist skilled in bone marrow transplantation.

Because patients with purine nucleoside phosphorylase are susceptible to viral, fungal, and bacterial infections, limit these patients' exposure to other persons.

The bovine-derived ADA replacement enzyme (Adagen) was approved by the FDA in 1990. In October 2018, the FDA approved elapegademase for treatment of adenosine deaminase severe combined immune deficiency (ADA-SCID) in adults and children. The drug had been available as an orphan drug prior to approval. Enzyme replacement helps prevent potentially serious, life-threatening infections in this patient population.

Regarding replacement therapy with intravenous immunoglobulin (IVIG) in patients with primary immune deficiencies, the overall consensus among clinical immunologists is that a dose of IVIG at 400-600 mg/kg/mo or a dose that maintains trough serum IgG levels of more than 500 mg/dL is desirable.[37, 38] Patients with meningoencephalitis (X-linked agammaglobulinemia) require higher doses (eg, 1 g/kg) and perhaps intrathecal therapy.

Preinfusion, or trough, serum IgG levels are measured every 3 months until a steady state is achieved and then every 6 months if the patient is stable. These levels may be helpful in adjusting the dose of IVIG to achieve adequate serum levels. For persons with high catabolism of infused IgG, more frequent infusions (eg, every 2-3 wk) of smaller doses may maintain the serum level in the reference range. The rate of IgG elimination may be increased during active infection; therefore, measuring serum IgG levels and adjusting to higher doses or shorter intervals may be required.

Precautions and adverse reactions to intravenous immunoglobulin therapy

The US Food and Drug Administration (FDA) advises against exceeding the recommended doses and infusion rates and suggest the use of minimal practical concentrations in patients at risk for renal failure (eg, those with preexisting renal insufficiency, diabetes, volume depletion, sepsis, paraproteinemia; those older than 65 y; and those using nephrotoxic drugs).

Initial treatment should be administered under the close supervision of experienced personnel. The risk of adverse reactions with initial treatments is high, especially in patients with infections and in those who form immune complexes. In patients with active infection, infusion rates may need to be reduced and the dose halved (ie, 200-300 mg/kg), with the remaining dose given the next day to achieve a full dose. Treatment should not be discontinued. After normal serum IgG levels are achieved, adverse reactions are uncommon unless patients have active infections.

With the new generation of IVIG products, adverse effects are reduced. Adverse effects include tachycardia, chest tightness, back pain, arthralgia, myalgia, hypertension or hypotension, headache, pruritus, rash, and low-grade fever. More serious reactions are dyspnea, nausea, vomiting, circulatory collapse, and loss of consciousness. Patients with profound immunodeficiency or active infections have reactions more severe than those of other patients.

The adverse reactions are thought to be related to the anticomplementary activity of IgG aggregates in the IVIG and the formation of immune complexes. The formation of oligomeric or polymeric IgG complexes that interact with Fc receptors and that trigger the release of inflammatory mediators is another cause. Most adverse reactions are rate related. Slowing the infusion rate or discontinuing therapy until symptoms subside may diminish the reaction. Pretreatment with ibuprofen (5-10 mg/kg every 6-8 h), acetaminophen (15 mg/kg/dose), diphenhydramine (1 mg/kg/dose), and/or hydrocortisone (6 mg/kg/dose, maximum 100 mg) 1 hour before the infusion may prevent adverse reactions. In some patients with a history of severe adverse effects, doses of analgesic and antihistamine may be repeated.

Acute renal failure is a rare but significant complication of IVIG treatment. Reports suggest that IVIG products that contain sucrose as a stabilizer are associated with an increased risk for this renal complication. Acute tubular necrosis, vacuolar degeneration, and osmotic nephrosis suggest osmotic injury to the proximal renal tubules. The infusion rate for sucrose-containing IVIG should not exceed 3 mg/kg/min based on sucrose content. Risk factors for this adverse reaction are preexisting renal insufficiency, diabetes mellitus, dehydration, age older than 65 years, sepsis, paraproteinemia, and concomitant use of nephrotoxic agents. For patients at increased risk, BUN and creatinine levels should be monitored before the start of treatment and before each infusion. If renal function deteriorates, the product should be discontinued.

IgE antibodies to immunoglobulin A (IgA) have been reported to cause severe transfusion reactions in patients with IgA deficiency. The literature has a few reports of true anaphylaxis in patients with selective IgA deficiency and common variable immunodeficiency (CVID) who developed IgE antibodies to IgA after treatment with Ig. In actual experience, however, this is rare. In addition, this is not a problem for patients with X-linked agammaglobulinemia (Bruton disease) or severe combined immunodeficiency (SCID). Caution should be exercised in patients with IgA deficiency (< 7 mg/dL) who need IVIG because of IgG-subclass deficiencies. IVIG preparations with low concentrations of contaminating IgA are advised (see Table 2).

Comparison of intravenous immunoglobulin products

For replacement therapy in patients with primary immune deficiency, all brands of IVIG are probably equivalent, although their viral inactivation processes (eg, solvent detergent vs pasteurization and liquid vs lyophilized) may differ (see Table 2). The choice may depend on the formulary, local availability, and/or cost. The dose, manufacturer, and lot number should be recorded for each infusion to review for adverse events or other consequences. Recording all adverse effects that occur during the infusion is crucial. Periodic monitoring of liver and renal function about 3-4 times yearly, is also recommended.

Table 2. Intravenous Immunoglobulin ^{[39, 40, 41]} (Open Table in a new window)

Class Summary

Prophylactic treatment for P carinii pneumonia is TMP-SMZ. Administer IVIG therapy to provide functional antibodies.

Trimethoprim and sulfamethoxazole (Bactrim, Bactrim DS, Sulfatrim Pediatric)

Inhibits bacterial growth by inhibiting synthesis of dihydrofolic acid; for prophylaxis of P carinii pneumonia.

Fluconazole (Diflucan)

Synthetic PO antifungal (broad-spectrum bistriazole) that selectively inhibits fungal cytochrome P-450 and sterol C-14 alpha-demethylation, which prevents conversion of lanosterol to ergosterol, thereby disrupting cellular membranes. Has little affinity for mammalian cytochromes, which is believed to explain its low toxicity. Available as tablets for PO administration, as a powder for PO suspension, and as a sterile solution for IV use. Indicated for fungal prophylaxis during immunosuppression

Class Summary

The optimal combination of an ablative agent (ie, busulfan) with immunosuppressive agents (eg, antithymocyte globulin, cyclophosphamide) has not been systematically studied and should be the focus of future clinical trials. The risks of this preparative regimen are sterility, liver, heart and lung toxicity, and malignancy.

Busulfan (Myleran)

Potent cytotoxic drug; causes profound myelosuppression at recommended dose. As alkylating agent, mechanism of action of active metabolites may involve cross-linking of DNA, which may interfere with growth of normal and neoplastic cells.

Cyclophosphamide

Chemically related to nitrogen mustards. As alkylating agent, mechanism of action of active metabolites may involve cross-linking of DNA, which may interfere with growth of normal and neoplastic cells.

Lymphocyte immune globulin (Atgam)

May modify T-cell function and might eliminate antigen-reactive T-lymphocytes in peripheral blood.

Class Summary

Cyclosporine and corticosteroids are administered to prevent acute GVHD.

Cyclosporine (Sandimmune, Neoral)

Cyclic polypeptide; suppresses some humoral immunity and more so cell-mediated immune reactions (eg, delayed hypersensitivity, allograft rejection, experimental allergic encephalomyelitis, GVHD) in many organs. Base dose on ideal body weight.

Methylprednisolone (Solu-Medrol, Depo-Medrol, Medrol)

Decreases inflammation by suppressing migration of polymorphonuclear leukocytes and reversing increased capillary permeability.

Class Summary

Adenosine deaminase (ADA) enzyme replacement can reduce potentially serious, life-threatening infections in ADA deficient patients.

Elapegademase (Revcovi, elapegademase-lvlr)

Elapegademase a recombinant adenosine deaminase based on bovine amino acid sequence and conjugated to PEG. It is indicated for treatment of ADA severe combined immune deficiency (ADA-SCID) in pediatric and adult patients.

Pegademase (Adagen)

Bovine-derived adenosine deaminase (ADA) that is conjugated with PEG. It is indicated for enzyme replacement therapy for ADA deficiency in patients with SCID who are not suitable candidates for or who have failed bone marrow transplantation.

See the list below:

• Intravenous immunoglobulin (IVIG) therapy is typically continued for 6-12 months after bone marrow transplantation. Reimmunization of the patient begins approximately 1 year after transplantation. Live viral vaccines should be avoided.

• Bone marrow transplantation does not correct the neurologic problems associated with purine nucleoside phosphorylase deficiency. Ongoing therapy for these problems should continue.

See the list below:

• Patients with adenosine deaminase (ADA) deficiency or purine nucleoside phosphorylase (PNP) deficiency who have acute infections may require hospitalization for diagnostic studies to identify opportunistic pathogens.

• For stem cell reconstitution, patients are typically hospitalized in single-occupancy protective reverse-isolation rooms with high-efficiency particulate air-filtration systems. Patients should remain in isolation until engraftment is evident.

See the list below:

• The patient's prognosis depends on the success of immune reconstitution of the T-cell and B-cell systems.

• If immune reconstitution is successful, the patient's prognosis is good. However, bone marrow transplantation does not correct the neurologic disease.

See the list below:

• Genetic counseling: Purine nucleoside phosphorylase deficiency is an autosomal recessive inherited immunodeficiency. If purine nucleoside phosphorylase deficiency is diagnosed in a child, the parents have a 25% risk of having affected children in subsequent pregnancies.

• Prenatal diagnosis: Prenatal diagnosis can be performed (see Special Concerns below).