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

Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs

Nucleoside, nucleotide, and base analogs have been in the clinic for decades to treat both viral pathogens and neoplasms. More than 20% of patients on anticancer chemotherapy have been treated with one or more of these analogs. This review focuses on the chemical synthesis and biology of anticancer nucleoside, nucleotide, and base analogs that are FDA-approved and in clinical development since 2000. We highlight the cellular biology and clinical biology of analogs, drug resistance mechanisms, and compound specificity towards different cancer types. Furthermore, we explore analog syntheses as well as improved and scale-up syntheses. We conclude with a discussion on what might lie ahead for medicinal chemists, biologists, and physicians as they try to improve analog efficacy through prodrug strategies and drug combinations.

Adenosine Deaminase Deficiency - More Than Just an Immunodeficiency

Adenosine deaminase (ADA) deficiency is best known as a form of severe combined immunodeficiency (SCID) that results from mutations in the gene encoding ADA. Affected patients present with clinical and immunological manifestations typical of a SCID. Therapies are currently available that can target these immunological disturbances and treated patients show varying degrees of clinical improvement. However, there is now a growing body of evidence that deficiency of ADA has significant impact on non-immunological organ systems. This review will outline the impact of ADA deficiency on various organ systems, starting with the well-understood immunological abnormalities. We will discuss possible pathogenic mechanisms and also highlight ways in which current treatments could be improved. In doing so, we aim to present ADA deficiency as more than an immunodeficiency and suggest that it should be recognized as a systemic metabolic disorder that affects multiple organ systems. Only by fully understanding ADA deficiency and its manifestations in all organ systems can we aim to deliver therapies that will correct all the clinical consequences.

Primary Immune Deficiency Treatment Consortium (PIDTC) update

The Primary Immune Deficiency Treatment Consortium (PIDTC) is a collaboration of 41 North American centers studying therapy for rare primary immune deficiency diseases (PIDs), including severe combined immune deficiency (SCID), Wiskott-Aldrich syndrome (WAS), and chronic granulomatous disease (CGD). An additional 3 European centers have partnered with the PIDTC to study CGD. Natural history protocols of the PIDTC analyze outcomes of treatment for rare PIDs in multicenter longitudinal retrospective, prospective, and cross-sectional studies. Since 2009, participating centers have enrolled more than 800 subjects on PIDTC protocols for SCID, and enrollment in the studies on WAS and CGD is underway. Four pilot projects have been funded, and 12 junior investigators have received fellowship awards. Important publications of the consortium describe the outcomes of hematopoietic cell transplantation for SCID during 2000-2009, diagnostic criteria for SCID, and the pilot project of newborn screening for SCID in the Navajo Nation. The PIDTC Annual Scientific Workshops provide an opportunity to strengthen collaborations with junior investigators, patient advocacy groups, and international colleagues. Funded by the National Institute of Allergy and Infectious Diseases and the Office of Rare Diseases Research, National Center for Advancing Translational Sciences, the PIDTC has recently received renewal for another 5 years. Here we review accomplishments of the group, projects underway, highlights of recent workshops, and challenges for the future.

Long-term multilineage engraftment of autologous genome-edited hematopoietic stem cells in nonhuman primates

Genome editing in hematopoietic stem and progenitor cells (HSPCs) is a promising novel technology for the treatment of many human diseases. Here, we evaluated whether the disruption of the C-C chemokine receptor 5 (CCR5) locus in pigtailed macaque HSPCs by zinc finger nucleases (ZFNs) was feasible. We show that macaque-specific CCR5 ZFNs efficiently induce CCR5 disruption at levels of up to 64% ex vivo, 40% in vivo early posttransplant, and 3% to 5% in long-term repopulating cells over 6 months following HSPC transplant. These genome-edited HSPCs support multilineage engraftment and generate progeny capable of trafficking to secondary tissues including the gut. Using deep sequencing technology, we show that these ZFNs are highly specific for the CCR5 locus in primary cells. Further, we have adapted our clonal tracking methodology to follow individual CCR5 mutant cells over time in vivo, reinforcing that CCR5 gene-edited HSPCs are capable of long-term engraftment. Together, these data demonstrate that genome-edited HSPCs engraft, and contribute to multilineage repopulation after autologous transplantation in a clinically relevant large animal model, an important step toward the development of stem cell-based genome-editing therapies for HIV and potentially other diseases as well.

Synthesis and Potent Anti-HCMV Activity of 2',5',5'-trifluoro-3'-hydroxy-apiosyl Nucleoside Phosphonic Acid Analogues

As antiviral nucleosides containing a fluorine atom at 2'-position are endowed with increased stabilization of glycosyl bond, it was of interest to investigate the influence of three fluorine atoms at 2'- and 5'-positions of apiosyl nucleoside phosphonate analogues. Various pyrimidine and purine 2',5',5'-trifluoro-3'-hydroxy-apiose nucleoside phosphonic acid analogues were synthesized from 1,3-dihydroxyacetone. Electrophilic fluorination of lactone was performed using N-fluorodibenzenesulfonimide. Difluorophosphonation was performed by direct displacement of triflate intermediate with diethyl(lithiodifluoromethyl) phosphonate to give the corresponding (α,α-difluoroalkyl) phosphonate. Condensation successfully proceeded from a glycosyl donor with persilylated bases to yield nucleoside phosphonate analogues. Deprotection of diethyl phosphonates provided the final phosphonic acid sodium salts. The synthesized nucleoside analogues were subjected to antiviral screening against various viruses.

Clinical applications of gene therapy for primary immunodeficiencies

Primary immunodeficiencies (PIDs) have represented a paradigmatic model for successes and pitfalls of hematopoietic stem cells gene therapy. First clinical trials performed with gamma retroviral vectors (γ-RV) for adenosine deaminase severe combined immunodeficiency (ADA-SCID), X-linked SCID (SCID-X1), and Wiskott-Aldrich syndrome (WAS) showed that gene therapy is a valid therapeutic option in patients lacking an HLA-identical donor. No insertional mutagenesis events have been observed in more than 40 ADA-SCID patients treated so far in the context of different clinical trials worldwide, suggesting a favorable risk-benefit ratio for this disease. On the other hand, the occurrence of insertional oncogenesis in SCID-X1, WAS, and chronic granulomatous disease (CGD) RV clinical trials prompted the development of safer vector construct based on self-inactivating (SIN) retroviral or lentiviral vectors (LVs). Here we present the recent results of LV-mediated gene therapy for WAS showing stable multilineage engraftment leading to hematological and immunological improvement, and discuss the differences with respect to the WAS RV trial. We also describe recent clinical results of SCID-X1 gene therapy with SIN γ-RV and the perspectives of targeted genome editing techniques, following early preclinical studies showing promising results in terms of specificity of gene correction. Finally, we provide an overview of the gene therapy approaches for other PIDs and discuss its prospects in relation to the evolving arena of allogeneic transplant.

The First Report of a Pregnancy in a Patient with Purine Nucleoside Phosphorylase Deficiency

BACKGROUND

The cause of primary immunodeficiency has expanded to nearly 200 distinct disorders. An improved understanding of these disorders has resulted in decreased morbidity and mortality with reciprocal improved life expectancy. Obstetricians should have knowledge of primary immunodeficiency, as more women with these disorders will reach reproductive age.

CASE

21-year-old G1P0 with purine nucleoside phosphorylase (PNP) deficiency delivered a viable infant vaginally at 37 weeks. Although the patient's diagnosis and pregnancy placed her at increased risk for infection, she remained asymptomatic and infection-free throughout pregnancy.

CONCLUSION

The management of pregnancy complicated by PNP deficiency requires strict immune surveillance and regimented immunoglobulin replacement.

Site-Selective Ribosylation of Fluorescent Nucleobase Analogs Using Purine-Nucleoside Phosphorylase as a Catalyst: Effects of Point Mutations

Enzymatic ribosylation of fluorescent 8-azapurine derivatives, like 8-azaguanine and 2,6-diamino-8-azapurine, with purine-nucleoside phosphorylase (PNP) as a catalyst, leads to N9, N8, and N7-ribosides. The final proportion of the products may be modulated by point mutations in the enzyme active site. As an example, ribosylation of the latter substrate by wild-type calf PNP gives N7- and N8-ribosides, while the N243D mutant directs the ribosyl substitution at N9- and N7-positions. The same mutant allows synthesis of the fluorescent N7-β-d-ribosyl-8-azaguanine. The mutated form of the E. coli PNP, D204N, can be utilized to obtain non-typical ribosides of 8-azaadenine and 2,6-diamino-8-azapurine as well. The N7- and N8-ribosides of the 8-azapurines can be analytically useful, as illustrated by N7-β-d-ribosyl-2,6-diamino-8-azapurine, which is a good fluorogenic substrate for mammalian forms of PNP, including human blood PNP, while the N8-riboside is selective to the E. coli enzyme.

New medications in development for the treatment of hyperuricemia of gout

PURPOSE OF REVIEW

To update recent developments in medications targeting hyperuricemia, but not including medications recently labelled in the European Union and the United States.

RECENT FINDINGS

A new xanthine oxidase inhibitor, Topiloric (Fujiyakuhin Co., Ltd. Japan) Uriadec (Sanwa Kagaku Kenkyusho Co., Ltd. Japan), has been developed and labelled in Japan. An inhibitor of purine nucleoside phosphorylase, Ulodesine, is in development in combination with allopurinol. The rest of the medications in the pipeline for hyperuricemia are targeting renal transporters of uric acid, mainly URAT1 and OAT4, acting as uricosuric agents. Most of them, such as lesinurad and arhalofenate, are being tested in trials in combination with allopurinol and febuxostat. The most potent RDEA3170 is being tested in monotherapy, but also associated with febuxostat. Recently, medications showing dual activity, inhibiting both xanthine oxidoreductase and URAT1, have been communicated or started exploratory clinical trials. There is no report of medications targeting other transporters such as Glut9 or ABCG2.

SUMMARY

There are a number of medications in the pipeline targeting hyperuricemia, mostly uricosurics in combination with xanthine oxidase inhibitors, but some targeting both xanthine oxidoreductase and URAT1. Increasing the number of available medications will ensure proper control of hyperuricemia to target serum urate levels in the near future for most, if not all, patients with hyperuricemia.

Novel Genetic Mutations in the First Swedish Patient with Purine Nucleoside Phosphorylase Deficiency and Clinical Outcome After Hematopoietic Stem Cell Transplantation with HLA-Matched Unrelated Donor

Purine nucleoside phosphorylase (PNP) is an enzyme active in the purine salvage pathway. PNP deficiency caused by autosomal recessive mutations in the PNP gene leads to severe combined immunodeficiency (SCID) and in two thirds of cases also to neurological effects such as developmental delay, ataxia, and motor impairment.PNP deficiency has a poor outcome, and the only curative treatment is allogenic hematopoietic stem cell transplantation (HSCT). We present the first Swedish patient with PNP deficiency with novel mutations in the PNP gene and the immunological results of the HSCT and evaluate the impact of HSCT on the neurological symptoms. The patient presented early in life with neurological symptoms and suffered later from repeated serious respiratory tract infections. Biochemical tests showed severe reduction in PNP activity (1% residual activity). Genetic testing revealed two new mutations in the PNP gene: c.729C>G (p.Asn243Lys) and c.746A>C (p.Tyr249Cys). HSCT was performed with an unrelated donor, resulting in prompt and sustained engraftment and complete donor chimerism. There was no further aggravation of the patient's neurological symptoms at 21 months post HSCT, and appropriate developmental milestones were achieved. HSCT is curative for the immunological defect caused by PNP deficiency, and our case strengthens earlier reports that HSCT is effective as a treatment even for neurological symptoms in PNP deficiency.

A successful unrelated peripheral blood stem cell transplantation with reduced intensity-conditioning regimen in a patient with late-onset purine nucleoside phosphorylase deficiency

PNP deficiency is a rare combined immunodeficiency with autosomal recessive mode of inheritance. The immunodeficiency is progressive with normal immune functions at birth, but then, T-cell deficiency with variable B-cell functions usually presents by the age of two yr. The only curative treatment for PNP deficiency is hematopoietic stem cell transplantation. Here, we present a 13-yr-old girl with late-onset PNP deficiency. Despite many complications of infections, she was successfully transplanted with a reduced intensity-conditioning regimen from an HLA-identical unrelated donor.

Hemophagocytic lymphohistiocytosis and primary immune deficiency disorders

Hemophagocytic lymphohistiocytosis (HLH) is characterized by uncontrolled immune activation and is traditionally associated with inherited gene defects or acquired causes. In addition to abnormalities in cytotoxic granules and lysosomes, various primary immune deficiency disorders (PID) have been identified among patients suffering from HLH. Our purpose was twofold: to better characterize and detail the association between PID and HLH. We found that HLH occurs infrequently among patients with PID, particularly those suffering from abnormalities that impair T cell function. The prognosis of patients suffering from PID and HLH is poor, emphasizing the need for rapid clinical and genetic diagnosis of the PID as well as initiation of appropriate management of the HLH, including allogeneic hematopoietic stem cell transplantations. The association of HLH and PID implicates abnormal T cell function as an important factor in HLH development. It also suggests that the partition of HLH into genetic versus acquired forms might be misleading.

Inosine in DNA and RNA

Deamination of the nucleobases in DNA and RNA is a result of spontaneous hydrolysis, endogenous or environmental factors as well as deaminase enzymes. Adenosine is deaminated to inosine which is miscoding and preferentially base pairs with cytosine. In the case of DNA, this is a premutagenic event that is counteracted by DNA repair enzymes specifically engaged in recognition and removal of inosine. However, in RNA, inosine is an essential modification introduced by specialized enzymes in a highly regulated manner to generate transcriptome diversity. Defect editing is seen in various human disease including cancer, viral infections and neurological and psychiatric disorders. Enzymes catalyzing the deaminase reaction are well characterized and recently an unexpected function of Endonuclease V in RNA processing was revealed. Whereas bacterial Endonuclease V enzymes are classified as DNA repair enzymes, it appears that the mammalian enzymes are involved in processing of inosine in RNA. This yields an interesting yet unexplored, link between DNA and RNA processing. Further work is needed to gain understanding of the impact of inosine in DNA and RNA under normal physiology and disease progression.