Partial Purine Nucleoside Phosphorylase Deficiency Helps Determine Minimal Activity Required for Immune and Neurological Development.

Introduction: Complete or near complete absence of the purine nucleoside phosphorylase (PNP) enzyme causes a profound T cell immunodeficiency and neurological abnormalities that are often lethal in infancy and early childhood. We hypothesized that patients with partial PNP deficiency, characterized by a late and mild phenotype due to residual PNP enzyme, would provide important information about the minimal PNP activity needed for normal development. Methods: Three siblings with a homozygous PNP gene mutation (c.769C>G, p.His257Asp) resulting in partial PNP deficiency were investigated. PNP activity was semi-quantitively assayed by the conversion of [14C]inosine in hemolysates, mononuclear cells, and lymphoblastoid B cells. PNP protein expression was determined by Western Blotting in lymphoblastoid B cells. DNA repair was quantified by measuring viability of lymphoblastoid B cells following ionizing irradiation. Results: A 21-year-old female was referred for recurrent sino-pulmonary infections while her older male siblings, aged 25- and 28- years, did not suffer from significant infections. Two of the siblings had moderately reduced numbers of T, B, and NK cells, while the other had near normal lymphocyte subset numbers. T cell proliferations were normal in the two siblings tested. Hypogammaglobulinemia was noted in two siblings, including one that required immunoglobulin replacement. All siblings had typical (normal) neurological development. PNP activity in various cells from two patients were 8-11% of the normal level. All siblings had normal blood uric acid and increased PNP substrates in the urine. PNP protein expression in cells from the two patients examined was similar to that observed in cells from healthy controls. The survival of lymphoblastoid B cells from 2 partial PNP-deficient patients after irradiation was similar to that of PNP-proficient cells and markedly higher than the survival of cells from a patient with absent PNP activity or a patient with ataxia telangiectasia. Conclusions: Patients with partial PNP deficiency can present in the third decade of life with mild-moderate immune abnormalities and typical development. Near-normal immunity might be achieved with relatively low PNP activity.

Recent advances in understanding and managing adenosine deaminase and purine nucleoside phosphorylase deficiencies.

PURPOSE OF THE REVIEW: To review the recent advances in the understanding and management of the immune and nonimmune effects of inherited adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiencies. RECENT FINDINGS: Abnormal thymocyte development and peripheral T-cell activation in ADA-deficient and PNP-deficient patients cause increased susceptibility to infections and immune dysregulation. The impaired purine homeostasis also damages many other cell types and tissues. Animal studies suggest that defects in surfactant metabolism by alveolar macrophages cause the pulmonary alveolar proteinosis commonly seen in ADA-deficient infants, while toxicity of purine metabolites to cerebellar Purkinje cells may lead to the ataxia frequently observed in PNP deficiency. Patients' outcome with current treatments including enzyme replacement and stem cell transplantations are inferior to those achieved in most severe immunodeficiency conditions. New strategies, including intracellular enzyme replacement, gene therapy and innovative protocols for stem cell transplantations hold great promise for improved outcomes in ADA and PNP deficiency. Moreover, newborn screening and early diagnosis will allow prompt application of these novel treatment strategies, further improving survival and reducing morbidity. SUMMARY: Better understanding of the complex immune and nonimmune effects of ADA and PNP deficiency holds great promise for improved patients' outcome.

Purine nucleoside phosphorylase deficiency presenting as severe combined immune deficiency.

Purine nucleoside phosphorylase (PNP) deficiency is an autosomal recessive genetic disorder of the purine salvage pathway, associated with a variable extent of immunodeficiency. Here, we report a PNP-deficient patient who presented early in life with clinical and laboratory characteristics of severe combined immunodeficiency, including severe infections, marked T-and B-cell deficiency, lack of lymphocyte response to mitogenic stimulation, monoclonal T-cell receptors representation and the absence of T-cell receptor excision circles and Kappa-receptor excision circles. The patient carried homozygote mutation at the PNP gene that putatively led to aberrant splicing, allowing normal and abnormally spliced products from the mutant alleles. We suggest that the aberrant slice site was used preferentially over the normal slice site in some cells correlating with the severity of disease.

Immunizations for patients with metabolic disorders.

Individuals with underlying metabolic disorders are a potential high-risk group for vaccine-preventable diseases. Newborn metabolic screening has provided a means of early identification and treatment for many of these disorders, whereas childhood immunization is one of the most effective means of decreasing the morbidity and mortality resulting from communicable diseases worldwide. There are very few contraindications to the routine administration of vaccines to the healthy, immunocompetent individual. In certain high-risk groups, such as immunocompromised patients, gravid females, and those with a history of previous anaphylactic reaction to a vaccine or its components, selective withholding of immunizations must be considered to decrease potential adverse events. A detailed analysis of the medical literature revealed few specific recommendations regarding appropriate immunization techniques for patients with metabolic disorders. In this review we detail the major metabolic disorder subtypes, elaborate on the available literature on immunizations for patients with these disorders, and provide suggested vaccine recommendations.

Persistent developmental delay despite successful bone marrow transplantation for purine nucleoside phosphorylase deficiency.

A 10-month-old girl with a history of recurrent candidiasis, developmental delay, and a fulminant varicella infection is described. The diagnosis of purine nucleoside phosphorylase (PNP) deficiency was suggested by a reduced level of serum uric acid and confirmed by measurement of PNP activity. A human leukocyte antigen-matched bone marrow transplantation resulted in immune reconstitution, but poor neurodevelopmental progression.

Purine nucleoside phosphorylase deficiency.

Purine nucleoside phosphorylase (PNP) deficiency is a rare inherited disease accounting for approximately 4% of patients with severe combined immunodeficiency. Thirty-three patients have been reported. PNP-deficient patients suffer from recurrent infections, usually beginning in the first year of life. Two thirds of patients have evidence of neurologic disorders. Findings range from spasticity to developmental delay, to mental retardation. One third of patients develop autoimmune disease. The most common manifestation of this is autoimmune hemolytic anemia. Idiopathic thrombocytopenic purpura and systemic lupus erythematosis have also been reported. Patients usually present with infections but approximately one fourth have come to medical care initially for neurological problems. In PNP deficiency, T- and B-cell immunity are affected. T-cell function may be profoundly deficient, may be normal at birth and then decrease with time, or may fluctuate repeatedly between low and normal. B-cell function can be normal but is deficient in approximately one third of patients. PNP protein is a trimer of approximately 90,000 daltons. It is found in most tissues of the body but is at highest levels in lymphoid tissues. This tissue distribution explains why the lymphoid system is predominantly affected in PNP deficiency. Many mechanisms have been proposed to explain the metabolic toxicity in PNP deficiency. The elevated dGTP found in PNP deficiency is thought to inhibit ribonucleotide reductase and, thus, impede cell division. Depressed GTP levels may correlate with neurologic dysfunction. The gene for PNP has been cloned; it is located on the long arm of chromosome 14. Studies of a mutant PNP gene isolated from one patient showed that a point mutation resulting in an amino acid substitution was responsible for PNP deficiency. PNP deficiency has a grave prognosis. No patient has reached the third decade of life. Twenty-nine of the 33 reported patients have died from their disease. Prenatal diagnosis is currently available. Many different therapies have been utilized for PNP deficiency including bone marrow transplantation, red cell transfusions, and supplementation of the diet with purines and pyrimidines. None of these therapies has been consistently successful. In light of the poor prognosis for PNP deficiency, bone marrow transplantation should be considered for all patients. In the future, improved forms of therapy such as gene therapy may become available.

Activities of purine catabolism related enzymes in zinc deficiency: relationship to T-lymphocyte dysfunction and hyperammonemia.

The activities of liver, muscle, and red blood cell enzymes involved in the degradation of purine were assayed in zinc deficient (ZD), pair-fed control (PF) and ad libitum-fed control (Ad lib) rats. Diets (1.5 ppm of zinc in ZD diet and 110 ppm in control diet) were fed to rats for 6 weeks after which they were sacrificed and tissues isolated for several assays. Results indicated a significant increase in the concentration of plasma NH3 (P less than .0005) and uric acid (P less than .05), and a decrease in the concentration of blood urea nitrogen (BUN) (P less than .025) in ZD rats compared to both control groups. The activities of liver and red blood cell (RBC) nucleoside phosphorylase (NPase) were lower (P less than .025) in ZD rats. Activity of 5'adenosine monophosphate deaminase (AMPDA) was higher in liver (P less than .0001) and muscle (P less than .01) in ZD groups vs. controls. Activity of hypoxanthine polyribosyl transferase (HPRT) in liver was higher (P less than .005) in ZD group compared to controls. The present findings could be viewed from two aspects: first, they provide an insight into the specific role of zinc on T-lymphocyte function which is believed to be mediated by the regulatory effect of zinc on the enzyme nucleoside phosphorylase and, second they provide a possible mechanism for hyperammonemia observed in zinc deficient human subjects and experimental animals.

Adenosine deaminase and purine nucleoside phosphorylase deficiencies: evaluation of therapeutic interventions in eight patients.

The courses of six patients with adenosine deaminase (ADA) and two with purine nucleoside phosphorylase (PNP) deficiencies were evaluated before and after therapy. The heterogeneity of immunologic and clinical parameters was striking in each enzyme deficiency. In both PNP and ADA deficiency, some patients had very low immunoglobulin levels, while others had normal levels. T-cell function was always low in patients with ADA deficiency. In the two patients with PNP deficiency, contrary to the classical descriptions of this disorder, T-cell function fluctuated with time. Five ADA-deficient patients were treated with irradiated normal red-cell transfusions as a form of enzyme replacement and showed no lasting benefit. Three of the ADA-deficient patients and one of the PNP-deficient patients were given transplants of haploidentical parental bone marrow stem cells without pretransplant immunosuppression. In the PNP-deficient patient, chimerism has not been documented on enzymatic testing. One ADA-deficient patient has demonstrated long-term engraftment with good B- and T-cell function. Haploidentical bone marrow transplantation is currently the preferred therapy for enzyme-deficient patients with absent T-cell function who do not have an HLA-identical donor, as it may result in a lasting reconstitution of immune function. In those patients with unsatisfactory responses to transplantation, however, specific enzyme replacement or gene therapy may be considered in the future.

Immunodeficiencies associated with errors in purine metabolism.

The genetic deficiencies of adenosine deaminase and purine nucleoside phosphorylase lead to blocks in the purine pathway. The intracellular accumulation of deoxynucleosides and deoxynucleotides is toxic to both dividing and nondividing lymphocytes via multiple mechanisms. T-lymphocytes are uniquely sensitive to purine-mediated cytotoxicity because of a functional imbalance of phosphorylating and dephosphorylating enzymatic activities. These inborn errors or purine metabolism are rare disorders. The study of these conditions, however, has uncovered unique enzymatic properties of lymphocytes and lymphocyte subclasses. A better understanding of the mechanisms of lymphocytotoxicity in these two purine enzyme defects may lead to better modes of therapeutic manipulation of the immune system.