Reticular dysgenesis: international survey on clinical presentation, transplantation, and outcome

Neutropenia in Childhood-A Narrative Review and Practical Diagnostic Approach

Neutropenia refers to a decrease in the absolute neutrophil count according to age and race norms and poses a common concern in pediatric practice. Neutrophils serve as host defenders and act crucially in acute inflammation procedures. In this narrative review, we systematically present causes of neutropenia in childhood, mainly adopting the pathophysiological classification of Frater, thereby studying (1) neutropenia with reduced bone marrow reserve, (2) secondary neutropenia with reduced bone marrow reserve, and (3) neutropenia with normal bone marrow reserve. Different conditions in each category are thoroughly discussed and practically approached from the clinician's point of view. Secondary mild to moderate neutropenia is usually benign due to childhood viral infections and is expected to resolve in 2-4 weeks. Bacterial and fungal agents are also associated with transient neutropenia, although fever with severe neutropenia constitutes a medical emergency. Drug-induced and immune neutropenias should be suspected following a careful history and a detailed clinical examination. Cytotoxic chemotherapies treating malignancies are responsible for severe neutropenia and neutropenic shock. Rare genetic neutropenias usually manifest with major infections early in life. Our review of taxonomies clinical findings and associates them to specific neutropenia disorders. We consequently propose a practical diagnostic algorithm for managing neutropenic children.

UK-5099, a mitochondrial pyruvate carrier inhibitor, recovers impaired neutrophil maturation caused by AK2 deficiency in human pluripotent stem cell models

Reticular dysgenesis (RD) is a rare genetic disease caused by gene mutations in the ATP:AMP phosphotransferase, adenylate kinase 2 (AK2). Patients with RD suffer from severe combined immunodeficiency with neutrophil maturation arrest. Although hematopoietic stem cell transplantation can be a curative option, it is invasive, and complications of agranulocytosis-induced infection worsen the prognosis. Here, we report that the use of UK-5099, an inhibitor of the mitochondrial pyruvate carrier (MPC), on hemo-angiogenic progenitor cells (HAPCs) derived from AK2-deficient induced pluripotent stem cells improved neutrophil maturation. Reactive oxygen species (ROS) levels in AK2-deficient HAPCs remained unchanged throughout all experiments, implying that UK-5099 improved the phenotype without affecting ROS levels. Overall, our results suggest that the MPC is a potential therapeutic target for the treatment of neutrophil maturation defects in RD.

Minimal dose of hematopoietic stem cell transplantation without myelosuppressive conditioning for T-B+NK- severe combined immunodeficiency

We reviewed the medical records of five patients with T-B+NK- severe combined immunodeficiency (SCID) who received minimal dose allogeneic hematopoietic cell transplantation (HCT) (total nucleated cell count (TNC) lower than 1.0 × 10⁸/kg). Patients were administered a median of 5.0 mL of bone marrow or peripheral blood without conditioning (in four) or with anti-thymocyte globulin alone (in one). Three patients received HCT from a matched sibling donor, one from unrelated donor, and one from familial mismatched donor. The median TNC and CD34⁺ cells were 0.54 (0.29-0.84) × 10⁸/kg and 0.61 (0.35-0.84) × 10⁶/kg, respectively. Engraftment was achieved in all. Total T cell, CD4⁺ cell, and CD8⁺ cell recovery was obtained within a year in four, and immunoglobulin replacement was discontinued in all. All patients survived, exhibiting stable donor chimerism. We obtained sufficient therapeutic effects with minimal dose transplantation without intensive conditioning in patients with T-B+NK- SCID.

Reticular Dysgenesis: A Rare Immunodeficiency in a Neonate With Cytopenias and Bacterial Sepsis

Severe combined immunodeficiency (SCID) consists of a group of disorders defined by abnormal B and T cell development that typically results in death within the first year of life if undiagnosed or untreated. Reticular dysgenesis (RD) is a rare but especially severe form of SCID that is caused by adenylate kinase 2 deficiency and is characterized not only by lymphopenia but also by profound neutropenia. RD predisposes patients to viral and fungal infections typical of SCID as well as serious bacterial infections atypical in the neonatal period in other SCID types. RD is also associated with sensorineural hearing loss not typically seen in other forms of SCID. Without rapid diagnosis and curative hematopoietic stem cell transplantation, RD is fatal within days to months due to overwhelming bacterial infection. The inclusion of the T cell receptor excision circle assay nationally in 2017 on the newborn screen has facilitated diagnosis of SCID in the neonatal period. This case reports on a male infant with RD who presented after preterm birth with severe cytopenias and a gastrointestinal anomaly and ultimately developed severe bacterial sepsis. Postmortem bone marrow evaluation and panel-based gene sequencing identifying 2 novel variants in the adenylate kinase 2 gene provided confirmation for a diagnosis of RD. This case emphasizes the importance of thorough diagnostic evaluation, including the newborn screen, in neonates and infants with persistent and unexplained cytopenias. Prompt hematology and/or immunology referral is advised for disease management and to facilitate hematopoietic stem cell transplantation to optimize long-term survival.

Reticular dysgenesis exacerbated by hemophagocytic lymphohistiocytosis and the presence of unusual histiocyte-like cells in bone marrow

We report a rare case of agranulocytosis and lymphopenia complicated with hemophagocytic lymphohistiocytosis. Diagnosis of reticular dysgenesis was made by detection of a pathogenic stop gain variant in the AK2 gene on targeted next generation sequencing and confirmed by Sanger sequencing. Parents were found to be carriers for this variant. Bone marrow aspirate and biopsy was also performed with a clinical diagnosis of severe combined immunodeficiency with HLH. However, no hemophagocytosis was noted in the bone marrow aspirate or trephine biopsy. Instead, it showed aggregates of large histiocyte-like cells, scattered erythroid precursors and megakaryocytes. These cells were confused to be some form of storage cells, but did not resemble storage cells seen in Gaucher's disease or Niemann Pick disease. Myeloid precursors were very few in number. Reticular dysgenesis was not suspected during admission due to a lack of awareness of this entity. Testing for sensorineural deafness in neonates with severe agranulocytosis and lymphopenia would facilitate an early diagnosis of reticular dysgenesis. To the best of our knowledge, hemophagocytic lymphohistiocytosis has not been previously reported in association with reticular dysgenesis.

Adenylate kinase 2 expression and addiction in T-ALL

T-cell acute lymphoblastic leukemia (T-ALL) represents the malignant expansion of immature T cells blocked in their differentiation. T-ALL is still associated with a poor prognosis, mainly related to occurrence of relapse or refractory disease. A critical medical need therefore exists for new therapies to improve the disease prognosis. Adenylate kinase 2 (AK2) is a mitochondrial kinase involved in adenine nucleotide homeostasis recently reported as essential in normal T-cell development, as defective AK2 signaling pathway results in a severe combined immunodeficiency with a complete absence of T-cell differentiation. In this study, we show that AK2 is constitutively expressed in T-ALL to varying levels, irrespective of the stage of maturation arrest or the underlying oncogenetic features. T-ALL cell lines and patient T-ALL-derived xenografts present addiction to AK2, whereas B-cell precursor ALL cells do not. Indeed, AK2 knockdown leads to early and massive apoptosis of T-ALL cells that could not be rescued by the cytosolic isoform AK1. Mechanistically, AK2 depletion results in mitochondrial dysfunction marked by early mitochondrial depolarization and reactive oxygen species production, together with the depletion of antiapoptotic molecules (BCL-2 and BCL-XL). Finally, T-ALL exposure to a BCL-2 inhibitor (ABT-199 [venetoclax]) significantly enhances the cytotoxic effects of AK2 depletion. We also show that AK2 depletion disrupts the oxidative phosphorylation pathway. Combined with pharmaceutical inhibition of glycolysis, AK2 silencing prevents T-ALL metabolic adaptation, resulting in dramatic apoptosis. Altogether, we pinpoint AK2 as a genuine and promising therapeutic target in T-ALL.

Inborn errors of immunity and metabolic disorders: current understanding, diagnosis, and treatment approaches

Inborn errors of metabolism consist of a heterogeneous group of disorders with various organ systems manifestations, and some metabolic diseases also cause immunological disorders or dysregulation. In this review, metabolic diseases that affect the immunological system and particularly lead to primary immune deficiency will be reviewed. In a patient with frequent infections and immunodeficiency, the presence of symptoms such as growth retardation, abnormal facial appearance, heart, skeletal, lung deformities, skin findings, arthritis, motor developmental retardation, seizure, deafness, hepatomegaly, splenomegaly, impairment of liver function tests, the presence of anemia, thrombocytopenia and eosinophilia in hematological examinations should suggest metabolic diseases for the underlying cause. In some patients, these phenotypic findings may appear before the immunodeficiency picture. Metabolic diseases leading to immunological disorders are likely to be rare but probably underdiagnosed. Therefore, the presence of recurrent infections or autoimmune findings in a patient with a suspected metabolic disease should suggest that immune deficiency may also accompany the picture, and diagnostic examinations in this regard should be deepened.

Primary Immunodeficiency and Thrombocytopenia

Primary immunodeficiency (PID) or Inborn errors of immunity (IEI) refers to a heterogeneous group of disorders characterized by immune system impairment. Although patients with IEI manifest highly variable symptoms, the most common clinical manifestations are recurrent infections, autoimmunity and malignancies. Some patients present hematological abnormality including thrombocytopenia due to different pathogenic mechanisms. This review focuses on primary and secondary thrombocytopenia as a complication, which can occur in IEI. Based on the International Union of Immunological Societies phenotypic classification for IEI, the several innate and adaptive immunodeficiency disorders can lead to thrombocytopenia. This review, for the first time, describes manifestation, mechanism and therapeutic modalities for thrombocytopenia in different classes of IEI.

How I investigate neutropenia

Neutropenia is a common laboratory finding in adults and children. Its underlying causes are extremely heterogeneous and include benign conditions, autoimmune disorders, infections, and malignancies. The clinical laboratory plays a central role in the diagnosis of these disorders, including data derived from hematology, microbiology, molecular biology/cytogenetics, and clinical chemistry. The purpose of this review is to (a) highlight the clinical, hematologic, and molecular genetic features of the major entities resulting in neutropenia and (b) outline an algorithm-based approach to permit the classification of neutropenias.

Frequency of pathogenic/likely pathogenic germline variants in cancer-related genes among children with acute leukemia in Saudi Arabia

BACKGROUND

The frequency of pathogenic/likely pathogenic (P/LP) germline mutations in cancer-related genes among children with cancer in highly consanguineous populations is not well studied.

METHODS

Whole-exome sequencing of germline DNA was performed in 60 children with acute leukemia. We used the St. Jude Pediatric Cancer Variant Pathogenicity Information Exchange (PeCanPIE) data portal for the classification of germline variants by the St. Jude Medal Ceremony pipeline.

RESULTS

Fifty-seven patients had acute lymphoblastic leukemia (ALL) and three patients had acute myeloid leukemia. Parental consanguinity was present in 27 (45%) patients. All patients were of Arab ancestry. Three patients (5%) had a history of cancer in their siblings. Five patients (8.3%) had P/LP germline mutations in cancer-related genes. Three patients with B-ALL had heterozygous pathogenic mutations in TP53, BRCA1, and BRCA2; one patient with B-ALL had homozygous pathogenic mutation in PMS2; and one patient with T-ALL had LP homozygous mutation in AK2 that was associated with reticular dysgenesis. Among patients who had history of parental consanguinity, three (11%) had P/LP germline mutations compared with two (8%) in the absence of parental consanguinity. Fourteen (23%) patients had gold medal variants in cancer-related genes, 13 were heterozygous, and one was homozygous. Silver medal variants were present in 35 (58%) patients; all were heterozygous except one homozygous.

CONCLUSIONS

Children with acute leukemia in Saudi Arabia had low frequency of P/LP mutations in cancer-related genes despite the high rate of consanguinity. Larger studies using whole-genome sequencing are needed to further explore the heritability of childhood leukemia.

Molecular and cellular mechanisms underlying defective antibody responses

Primary immune deficiency is caused by genetic mutations that result in immune dysfunction and subsequent susceptibility to infection. Over the last decade there has been a dramatic increase in the number of genetically defined causes of immune deficiency including those which affect B-cell function. This has not only identified critical nonredundant pathways that control the generation of protective antibody responses but also revealed that immunodeficiency and autoimmunity are often closely linked. Here we explore the molecular and cellular mechanisms of these rare monogenic conditions that disrupt antibody production, which also have implications for understanding the causes of more common polygenic immune dysfunction.

A model for reticular dysgenesis shows impaired sensory organ development and hair cell regeneration linked to cellular stress

Mutations in the gene AK2 are responsible for reticular dysgenesis (RD), a rare and severe form of primary immunodeficiency in children. RD patients have a severely shortened life expectancy and without treatment die, generally from sepsis soon after birth. The only available therapeutic option for RD is hematopoietic stem cell transplantation (HSCT). To gain insight into the pathophysiology of RD, we previously created zebrafish models for Ak2 deficiencies. One of the clinical features of RD is hearing loss, but its pathophysiology and causes have not been determined. In adult mammals, sensory hair cells of the inner ear do not regenerate; however, their regeneration has been observed in several non-mammalian vertebrates, including zebrafish. Therefore, we used our RD zebrafish models to determine whether Ak2 deficiency affects sensory organ development and/or hair cell regeneration. Our studies indicated that Ak2 is required for the correct development, survival and regeneration of sensory hair cells. Interestingly, Ak2 deficiency induces the expression of several oxidative stress markers and it triggers an increased level of cell death in the hair cells. Finally, we show that glutathione treatment can partially rescue hair cell development in the sensory organs in our RD models, pointing to the potential use of antioxidants as a therapeutic treatment supplementing HSCT to prevent or ameliorate sensorineural hearing deficits in RD patients.

Hypomorphic variants in AK2 reveal the contribution of mitochondrial function to B-cell activation

BACKGROUND

The gene AK2 encodes the phosphotransferase adenylate kinase 2 (AK2). Human variants in AK2 cause reticular dysgenesis, a severe combined immunodeficiency with agranulocytosis, lymphopenia, and sensorineural deafness that requires hematopoietic stem cell transplantation for survival.

OBJECTIVE

We investigated the mechanisms underlying recurrent sinopulmonary infections and hypogammaglobulinemia in 15 patients, ranging from 3 to 34 years of age, from 9 kindreds. Only 2 patients, both of whom had mildly impaired T-cell proliferation, each had a single clinically significant opportunistic infection.

METHODS

Patient cells were studied with next-generation DNA sequencing, tandem mass spectrometry, and assays of lymphocyte and mitochondrial function.

RESULTS

We identified 2 different homozygous variants in AK2. AK2^G100S and AK2^A182D permit residual protein expression, enzymatic activity, and normal numbers of neutrophils and lymphocytes. All but 1 patient had intact hearing. The patients' B cells had severely impaired proliferation and in vitro immunoglobulin secretion. With activation, the patients' B cells exhibited defective mitochondrial respiration and impaired regulation of mitochondrial membrane potential and quality. Although activated T cells from the patients with opportunistic infections demonstrated impaired mitochondrial function, the mitochondrial quality in T cells was preserved. Consistent with the capacity of activated T cells to utilize nonmitochondrial metabolism, these findings revealed a less strict cellular dependence of T-cell function on AK2 activity. Chemical inhibition of ATP synthesis in control T and B cells similarly demonstrated the greater dependency of B cells on mitochondrial function.

CONCLUSIONS

Our patients demonstrate the in vivo sequelae of the cell-specific requirements for the functions of AK2 and mitochondria, particularly in B-cell activation and antibody production.

Reticular Dysgenesis and Mitochondriopathy Induced by Adenylate Kinase 2 Deficiency with Atypical Presentation

Reticular dysgenesis is an autosomal recessive form of severe combined immunodeficiency (SCID) that usually manifests in newborns. It is a unique example of an immune deficiency that is linked to dysfunctional mitochondrial energy metabolism and caused by adenylate kinase 2 (AK2) deficiency. It is characterized by an early differentiation arrest in the myeloid lineage, impaired lymphoid maturation, and sensorineural hearing loss. In this study, a novel AK2 homozygous mutation, c.622 T > C [p.Ser208Pro], was identified in an Old Order Amish patient through whole exome sequencing. Functional studies showed that the patient’s cells have no detectable AK2 protein, as well as low oxygen consumption rate (OCR), extracellular acidification rate (ECAR) and proton production rate (PPR). An increased production of reactive oxygen species, mitochondrial membrane permeability, and mitochondrial mass, and decreased ATP production, were also observed. The results confirm the pathogenicity of the AK2 mutation and demonstrate that reticular dysgenesis should be considered in Amish individuals presenting with immune deficiency. We also describe other pathophysiological aspects of AK2 deficiency not previously reported.

Modeling hematopoietic disorders in zebrafish

Zebrafish offer a powerful vertebrate model for studies of development and disease. The major advantages of this model include the possibilities of conducting reverse and forward genetic screens and of observing cellular processes by in vivo imaging of single cells. Moreover, pathways regulating blood development are highly conserved between zebrafish and mammals, and several discoveries made in fish were later translated to murine and human models. This review and accompanying poster provide an overview of zebrafish hematopoiesis and discuss the existing zebrafish models of blood disorders, such as myeloid and lymphoid malignancies, bone marrow failure syndromes and immunodeficiencies, with a focus on how these models were generated and how they can be applied for translational research.

Summary: This At A Glance article and poster summarize the last 20 years of research in zebrafish models for hematopoietic disorders, highlighting how these models were created and are being applied for translational research.

Now Is the Time to Use Molecular Gene Testing for the Diagnosis of Primary Immune Deficiencies

The discovery of chromosomes, genes, and DNA in the early 20th century paved the way for the development of techniques to examine the role of these elements in disease pathogenesis. Since the start of the 21st century, genetic testing and particularly next-generation sequencing has allowed for a rapid rate of gene disease associations for a broad range of primary immunodeficiency patients. At the same time, biologic and small molecule-based therapies targeting specific molecular pathways have been developed and are being applied clinically and in research settings to treat genetically defined immunodeficiencies. In recent years, both the American Academy of Allergy Asthma and Immunology and the Clinical Immunology Society have recommended the use of genetic testing for diagnosis, therapy guidance, and genetic counseling in patients with clinical symptoms of primary immunodeficiency.

Hematopoietic Stem Cell Transplantation in Primary Immunodeficiency Diseases: Current Status and Future Perspectives

Primary immunodeficiencies (PID) are disorders that for the most part result from mutations in genes involved in immune host defense and immunoregulation. These conditions are characterized by various combinations of recurrent infections, autoimmunity, lymphoproliferation, inflammatory manifestations, atopy, and malignancy. Most PID are due to genetic defects that are intrinsic to hematopoietic cells. Therefore, replacement of mutant cells by healthy donor hematopoietic stem cells (HSC) represents a rational therapeutic approach. Full or partial ablation of the recipient's marrow with chemotherapy is often used to allow stable engraftment of donor-derived HSCs, and serotherapy may be added to the conditioning regimen to reduce the risks of graft rejection and graft versus host disease (GVHD). Initially, hematopoietic stem cell transplantation (HSCT) was attempted in patients with severe combined immunodeficiency (SCID) as the only available curative treatment. It was a challenging procedure, associated with elevated rates of morbidity and mortality. Overtime, outcome of HSCT for PID has significantly improved due to availability of high-resolution HLA typing, increased use of alternative donors and new stem cell sources, development of less toxic, reduced-intensity conditioning (RIC) regimens, and cellular engineering techniques for graft manipulation. Early identification of infants affected by SCID, prior to infectious complication, through newborn screening (NBS) programs and prompt genetic diagnosis with Next Generation Sequencing (NGS) techniques, have also ameliorated the outcome of HSCT. In addition, HSCT has been applied to treat a broader range of PID, including disorders of immune dysregulation. Yet, the broad spectrum of clinical and immunological phenotypes associated with PID makes it difficult to define a universal transplant regimen. As such, integration of knowledge between immunologists and transplant specialists is necessary for the development of innovative transplant protocols and to monitor their results during follow-up. Despite the improved outcome observed after HSCT, patients with severe forms of PID still face significant challenges of short and long-term transplant-related complications. To address this issue, novel HSCT strategies are being implemented aiming to improve both survival and long-term quality of life. This article will discuss the current status and latest developments in HSCT for PID, and present data regarding approach and outcome of HSCT in recently described PID, including disorders associated with immune dysregulation.

Rare Genetic Blood Disease Modeling in Zebrafish

Hematopoiesis results in the correct formation of all the different blood cell types. In mammals, it starts from specific hematopoietic stem and precursor cells residing in the bone marrow. Mature blood cells are responsible for supplying oxygen to every cell of the organism and for the protection against pathogens. Therefore, inherited or de novo genetic mutations affecting blood cell formation or the regulation of their activity are responsible for numerous diseases including anemia, immunodeficiency, autoimmunity, hyper- or hypo-inflammation, and cancer. By definition, an animal disease model is an analogous version of a specific clinical condition developed by researchers to gain information about its pathophysiology. Among all the model species used in comparative medicine, mice continue to be the most common and accepted model for biomedical research. However, because of the complexity of human diseases and the intrinsic differences between humans and other species, the use of several models (possibly in distinct species) can often be more helpful and informative than the use of a single model. In recent decades, the zebrafish (Danio rerio) has become increasingly popular among researchers, because it represents an inexpensive alternative compared to mammalian models, such as mice. Numerous advantages make it an excellent animal model to be used in genetic studies and in particular in modeling human blood diseases. Comparing zebrafish hematopoiesis to mammals, it is highly conserved with few, significant differences. In addition, the zebrafish model has a high-quality, complete genomic sequence available that shows a high level of evolutionary conservation with the human genome, empowering genetic and genomic approaches. Moreover, the external fertilization, the high fecundity and the transparency of their embryos facilitate rapid, in vivo analysis of phenotypes. In addition, the ability to manipulate its genome using the last genome editing technologies, provides powerful tools for developing new disease models and understanding the pathophysiology of human disorders. This review provides an overview of the different approaches and techniques that can be used to model genetic diseases in zebrafish, discussing how this animal model has contributed to the understanding of genetic diseases, with a specific focus on the blood disorders.

Discovery of genetic variants of the kinases that activate tenofovir among individuals in the United States, Thailand, and South Africa: HPTN067

Tenofovir (TFV), a nucleotide reverse transcriptase inhibitor, requires two phosphorylation steps to form a competitive inhibitor of HIV reverse transcriptase. Adenylate kinase 2 (AK2) has been previously demonstrated to phosphorylate tenofovir to tenofovir-monophosphate, while creatine kinase, muscle (CKM), pyruvate kinase, muscle (PKM) and pyruvate kinase, liver and red blood cell (PKLR) each have been found to phosphorylate tenofovir-monophosphate to the pharmacologically active tenofovir-diphosphate. In the present study, genomic DNA isolated from dried blood spots collected from 505 participants from Bangkok, Thailand; Cape Town, South Africa; and New York City, USA were examined for variants in AK2, CKM, PKM, and PKLR using next-generation sequencing. The bioinformatics tools SIFT and PolyPhen predicted that 19 of the 505 individuals (3.7% frequency) carried variants in at least one kinase that would result in a decrease or loss of enzymatic activity. To functionally test these predictions, AK2 and AK2 variants were expressed in and purified from E. coli, followed by investigation of their activities towards tenofovir. Interestingly, we found that purified AK2 had the ability to phosphorylate tenofovir-monophosphate to tenofovir-diphosphate in addition to phosphorylating tenofovir to tenofovir-monophosphate. Further, four of the six AK2 variants predicted to result in a loss or decrease of enzyme function exhibited a ≥30% decrease in activity towards tenofovir in our in vitro assays. Of note, an AK2 K28R variant resulted in a 72% and 81% decrease in the formation of tenofovir-monophosphate and tenofovir-diphosphate, respectively. These data suggest that there are naturally occurring genetic variants that could potentially impact TFV activation.

Recent advances in understanding the pathogenesis and management of reticular dysgenesis

Reticular Dysgenesis is a rare immunodeficiency which is clinically characterized by the combination of Severe Combined Immunodeficiency (SCID) with agranulocytosis and sensorineural deafness. Mutations in the gene encoding adenylate kinase 2 (AK2) were identified to cause this phenotype. In this review, we will demonstrate important clinical differences between reticular dysgenesis and other SCID entities and summarize recent concepts in the understanding of the pathophysiology of the disease and the management strategies for this difficult condition.