A knock-out model of paroxysmal nocturnal hemoglobinuria: Pig-a(-) hematopoiesis is reconstituted following intercellular transfer of GPI-anchored proteins

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins II: Intercellular Transfer of Matter (Inheritance?) That Matters

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of the plasma membrane (PM) bilayer by covalent linkage to a typical glycolipid and expressed in all eukaryotic organisms so far studied. Lipolytic release from PMs into extracellular compartments and intercellular transfer are regarded as the main (patho)physiological roles exerted by GPI-APs. The intercellular transfer of GPI-APs relies on the complete GPI anchor and is mediated by extracellular vesicles such as microvesicles and exosomes and lipid-free homo- or heteromeric aggregates, and lipoprotein-like particles such as prostasomes and surfactant-like particles, or lipid-containing micelle-like complexes. In mammalian organisms, non-vesicular transfer is controlled by the distance between donor and acceptor cells/tissues; intrinsic conditions such as age, metabolic state, and stress; extrinsic factors such as GPI-binding proteins; hormones such as insulin; and drugs such as anti-diabetic sulfonylureas. It proceeds either "directly" upon close neighborhood or contact of donor and acceptor cells or "indirectly" as a consequence of the induced lipolytic release of GPI-APs from PMs. Those displace from the serum GPI-binding proteins GPI-APs, which have retained the complete anchor, and become assembled in aggregates or micelle-like complexes. Importantly, intercellular transfer of GPI-APs has been shown to induce specific phenotypes such as stimulation of lipid and glycogen synthesis, in cultured human adipocytes, blood cells, and induced pluripotent stem cells. As a consequence, intercellular transfer of GPI-APs should be regarded as non-genetic inheritance of (acquired) features between somatic cells which is based on the biogenesis and transmission of matter such as GPI-APs and "membrane landscapes", rather than the replication and transmission of information such as DNA. Its operation in mammalian organisms remains to be clarified.

Establishment of mouse model of inherited PIGO deficiency and therapeutic potential of AAV-based gene therapy

Inherited glycosylphosphatidylinositol (GPI) deficiency (IGD) is caused by mutations in GPI biosynthesis genes. The mechanisms of its systemic, especially neurological, symptoms are not clarified and fundamental therapy has not been established. Here, we report establishment of mouse models of IGD caused by PIGO mutations as well as development of effective gene therapy. As the clinical manifestations of IGD are systemic and lifelong lasting, we treated the mice with adeno-associated virus for homology-independent knock-in as well as extra-chromosomal expression of Pigo cDNA. Significant amelioration of neuronal phenotypes and growth defect was achieved, opening a new avenue for curing IGDs.

Insights Into the Emergence of Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal Nocturnal Hemoglobinuria (PNH) is a disease as simple as it is complex. PNH patients develop somatic loss-of-function mutations in phosphatidylinositol N-acetylglucosaminyltransferase subunit A gene (PIGA), required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors. Ubiquitous in eukaryotes, GPI anchors are a group of conserved glycolipid molecules responsible for attaching nearly 150 distinct proteins to the surface of cell membranes. The loss of two GPI-anchored surface proteins, CD55 and CD59, from red blood cells causes unregulated complement activation and hemolysis in classical PNH disease. In PNH patients, PIGA-mutant, GPI (-) hematopoietic cells clonally expand to make up a large portion of patients’ blood production, yet mechanisms leading to clonal expansion of GPI (-) cells remain enigmatic. Historical models of PNH in mice and the more recent PNH model in rhesus macaques showed that GPI (-) cells reconstitute near-normal hematopoiesis but have no intrinsic growth advantage and do not clonally expand over time. Landmark studies identified several potential mechanisms which can promote PNH clonal expansion. However, to what extent these contribute to PNH cell selection in patients continues to be a matter of active debate. Recent advancements in disease models and immunologic technologies, together with the growing understanding of autoimmune marrow failure, offer new opportunities to evaluate the mechanisms of clonal expansion in PNH. Here, we critically review published data on PNH cell biology and clonal expansion and highlight limitations and opportunities to further our understanding of the emergence of PNH clones.

Molecularly Imprinted Polymers Outperform Lectin Counterparts and Enable More Precise Cancer Diagnosis

Accurately analysing the particular glycosylation status of protein biomarkers is of significant importance in the precise, early diagnosis of cancer. Existing methods mainly rely on the use of antibodies and lectins. However, due to the macroscopic and microscopic heterogeneity of glycans, precise analysis of glycosylation status still remains a challenge. Molecularly imprinted polymers (MIPs), as a synthetic alternative to antibodies or lectins, may provide new solutions but have not yet been explored. Herein, we report an appealing strategy called triple MIP-based plasmonic immunosandwich assay (triMIP-PISA) for precise cancer diagnosis in terms of the relative glycosylation expression of glycoprotein biomarkers. As proof of the principle, alpha fetoprotein (AFP), which has been used as a clinical biomarker for early detection of hepatocellular carcinoma (HCC), as well as its Lens culinaris agglutinin (LCA)-reactive fraction (AFP-L3), which is mainly composed of core-fucosylated glycans, were used as two target proteoforms to test in this study. Using two MIPs that can specifically recognize the peptide sequence of AFP as well as a fucose-imprinted MIP that can specifically recognize the AFP-L3 fraction, facile simultaneous plasmon-enhanced Raman detection of AFP and AFP-L3 in serum was achieved, which allowed HCC patients to be distinguished from healthy individuals. Due to the excellent recognition properties of the MIPs that are comparable to those of antibodies and superior to those of lectins, our triMIP-PISA method exhibited improved precision as compared with an antibody plus lectin-based immunofluorescence assay. Thus, this strategy opened a new avenue towards the precise diagnosis of cancer.

A triple molecularly imprinted polymer (MIP)-based plasmonic assay was developed for precise cancer diagnosis in terms of the relative glycosylation expression of glycoprotein biomarkers.

Complement and inflammasome overactivation mediates paroxysmal nocturnal hemoglobinuria with autoinflammation

Patients with paroxysmal nocturnal hemoglobinuria (PNH) have a clonal population of blood cells deficient in glycosylphosphatidylinositol-anchored (GPI-anchored) proteins, resulting from a mutation in the X-linked gene PIGA. Here we report on a set of patients in whom PNH results instead from biallelic mutation of PIGT on chromosome 20. These PIGT-PNH patients have clinically typical PNH, but they have in addition prominent autoinflammatory features, including recurrent attacks of aseptic meningitis. In all these patients we find a germ-line point mutation in one PIGT allele, whereas the other PIGT allele is removed by somatic deletion of a 20q region comprising maternally imprinted genes implicated in myeloproliferative syndromes. Unlike in PIGA-PNH cells, GPI is synthesized in PIGT-PNH cells and, since its attachment to proteins is blocked, free GPI is expressed on the cell surface. From studies of patients' leukocytes and of PIGT-KO THP-1 cells we show that, through increased IL-1β secretion, activation of the lectin pathway of complement and generation of C5b-9 complexes, free GPI is the agent of autoinflammation. Eculizumab treatment abrogates not only intravascular hemolysis, but also autoinflammation. Thus, PIGT-PNH differs from PIGA-PNH both in the mechanism of clonal expansion and in clinical manifestations.

Membrane insertion and intercellular transfer of glycosylphosphatidylinositol-anchored proteins: potential therapeutic applications

Anchorage of a subset of cell surface proteins in eukaryotic cells is mediated by a glycosylphosphatidylinositol (GPI) moiety covalently attached to the carboxy-terminus of the protein moiety. Experimental evidence for the potential of GPI-anchored proteins (GPI-AP) of being released from cells into the extracellular environment has been accumulating, which involves either the loss or retention of the GPI anchor. Release of GPI-AP from donor cells may occur spontaneously or in response to endogenous or environmental signals. The experimental evidence for direct insertion of exogenous GPI-AP equipped with the complete anchor structure into the outer plasma membrane bilayer leaflets of acceptor cells is reviewed as well as the potential underlying molecular mechanisms. Furthermore, promiscuous transfer of certain GPI-AP between plasma membranes of different cells in vivo under certain (patho)physiological conditions has been reported. Engineering of target cell surfaces using chimeric GPI-AP with complete GPI anchor may be useful for therapeutic applications.

A hypomorphic PIGA gene mutation causes severe defects in neuron development and susceptibility to complement-mediated toxicity in a human iPSC model

Mutations in genes involved in glycosylphosphatidylinositol (GPI) anchor biosynthesis underlie a group of congenital syndromes characterized by severe neurodevelopmental defects. GPI anchored proteins have diverse roles in cell adhesion, signaling, metabolism and complement regulation. Over 30 enzymes are required for GPI anchor biosynthesis and PIGA is involved in the first step of this process. A hypomorphic mutation in the X-linked PIGA gene (c.1234C>T) causes multiple congenital anomalies hypotonia seizure syndrome 2 (MCAHS2), indicating that even partial reduction of GPI anchored proteins dramatically impairs central nervous system development, but the mechanism is unclear. Here, we established a human induced pluripotent stem cell (hiPSC) model containing the PIGAc.1234C>T mutation to study the effects of a hypomorphic allele of PIGA on neuronal development. Neuronal differentiation from neural progenitor cells generated by EB formation in PIGAc.1234C>T is significantly impaired with decreased proliferation, aberrant synapse formation and abnormal membrane depolarization. The results provide direct evidence for a critical role of GPI anchor proteins in early neurodevelopment. Furthermore, neural progenitors derived from PIGAc.1234C>T hiPSCs demonstrate increased susceptibility to complement-mediated cytotoxicity, suggesting that defective complement regulation may contribute to neurodevelopmental disorders.

Mutation analysis with random DNA identifiers (MARDI) catalogs Pig-a mutations in heterogeneous pools of CD48-deficient T cells derived from DMBA-treated rats

Identification of mutations induced by xenotoxins is a common task in the field of genetic toxicology. Mutations are often detected by clonally expanding potential mutant cells and genotyping each viable clone by Sanger sequencing. Such a "clone-by-clone" approach requires significant time and effort, and sometimes is even impossible to implement. Alternative techniques for efficient mutation identification would greatly benefit both basic and regulatory genetic toxicology research. Here, we report the development of Mutation Analysis with Random DNA Identifiers (MARDI), a novel high-fidelity Next Generation Sequencing (NGS) approach that circumvents clonal expansion and directly catalogs mutations in pools of mutant cells. MARDI uses oligonucleotides carrying Random DNA Identifiers (RDIs) to tag progenitor DNA molecules before PCR amplification, enabling clustering of descendant DNA molecules and eliminating NGS- and PCR-induced sequencing artifacts. When applied to the Pig-a cDNA analysis of heterogeneous pools of CD48-deficient T cells derived from DMBA-treated rats, MARDI detected nearly all Pig-a mutations that were previously identified by conventional clone-by-clone analysis and discovered many additional ones consistent with DMBA exposure: mostly A to T transversions, with the mutated A located on the non-transcribed DNA strand.

Microbiota Control of Malaria Transmission

Stable mutualistic interactions between multicellular organisms and microbes are an evolutionarily conserved process with a major impact on host physiology and fitness. Humans establish such interactions with a consortium of microorganisms known as the microbiota. Despite the mutualistic nature of these interactions, some bacterial components of the human microbiota express immunogenic glycans that elicit glycan-specific antibody (Ab) responses. The ensuing circulating Abs are protective against infections by pathogens that express those glycans, as demonstrated for Plasmodium, the causative agent of malaria. Presumably, a similar protective Ab response acts against other vector-borne diseases.

A recurrent germline mutation in the PIGA gene causes Simpson-Golabi-Behmel syndrome type 2

Hypomorphic germline mutations in the PIGA (phosphatidylinositol glycan class A) gene recently were recognized as the cause of a clinically heterogeneous spectrum of X-linked disorders including (i) early onset epileptic encephalopathy with severe muscular hypotonia, dysmorphism, multiple congenital anomalies, and early death ("MCAHS2"), (ii) neurodegenerative encephalopathy with systemic iron overload (ferro-cerebro-cutaneous syndrome, "FCCS"), and (iii) intellectual disability and seizures without dysmorphism. Previous studies showed that the recurrent PIGA germline mutation c.1234C>T (p.Arg412*) leads to a clinical phenotype at the most severe end of the spectrum associated with early infantile lethality. We identified three additional individuals from two unrelated families with the same PIGA mutation. Major clinical findings include early onset intractable epileptic encephalopathy with a burst-suppression pattern on EEG, generalized muscular hypotonia, structural brain abnormalities, macrocephaly and increased birth weight, joint contractures, coarse facial features, widely spaced eyes, a short nose with anteverted nares, gingival overgrowth, a wide mouth, short limbs with short distal phalanges, and a small penis. Based on the phenotypic overlap with Simpson-Golabi-Behmel syndrome type 2 (SGBS2), we hypothesized that both disorders might have the same underlying cause. We were able to confirm the same c.1234C>T (p.Arg412*) mutation in the DNA sample from an affected fetus of the original family affected with SGBS2. We conclude that the recurrent PIGA germline mutation c.1234C>T leads to a recognizable clinical phenotype with a poor prognosis and is the cause of SGBS2.

Glycosyl phosphatidylinositol anchor biosynthesis is essential for maintaining epithelial integrity during Caenorhabditis elegans embryogenesis

Glycosylphosphatidylinositol (GPI) is a post-translational modification resulting in the attachment of modified proteins to the outer leaflet of the plasma membrane. Tissue culture experiments have shown GPI-anchored proteins (GPI-APs) to be targeted to the apical membrane of epithelial cells. However, the in vivo importance of this targeting has not been investigated since null mutations in GPI biosynthesis enzymes in mice result in very early embryonic lethality. Missense mutations in the human GPI biosynthesis enzyme pigv are associated with a multiple congenital malformation syndrome with a high frequency of Hirschsprung disease and renal anomalies. However, it is currently unknown how these phenotypes are linked to PIGV function. Here, we identify a temperature-sensitive hypomorphic allele of PIGV in Caenorhabditis elegans, pigv-1(qm34), enabling us to study the role of GPI-APs in development. At the restrictive temperature we found a 75% reduction in GPI-APs at the surface of embryonic cells. Consequently, ~80% of pigv-1(qm34) embryos arrested development during the elongation phase of morphogenesis, exhibiting internal cysts and/or surface ruptures. Closer examination of the defects revealed them all to be the result of breaches in epithelial tissues: cysts formed in the intestine and excretory canal, and ruptures occurred through epidermal cells, suggesting weakening of the epithelial membrane or membrane-cortex connection. Knockdown of piga-1, another GPI biosynthesis enzymes resulted in similar phenotypes. Importantly, fortifying the link between the apical membrane and actin cortex by overexpression of the ezrin/radixin/moesin ortholog ERM-1, significantly rescued cyst formation and ruptures in the pigv-1(qm34) mutant. In conclusion, we discovered GPI-APs play a critical role in maintaining the integrity of the epithelial tissues, allowing them to withstand the pressure and stresses of morphogenesis. Our findings may help to explain some of the phenotypes observed in human syndromes associated with pigv mutations.

Cell surface proteins, such as receptors, either integrate into the plasma membrane through a transmembrane domain or are tethered to it by an accessory glycosylated phospholipid (GPI) anchor that is attached to them after they are made. The GPI-anchor biosynthesis pathway is highly conserved from yeast to humans and null mutations in any of the key enzymes are lethal at early developmental stages. Point mutations in several genes encoding for GPI-anchor biosynthesis enzymes have been linked to human disease. Specifically, mutations in PIGV are associated with multiple congenital malformations, including renal and anorectal malformation and mental retardation. It is currently not known how the mutations in PIGV lead to these diseases. Here we describe a point mutation in the PIGV ortholog of the nematode Caenorhabditis elegans, pigv-1, which is found to cause a high degree of embryonic lethality. We documented a substantial reduction in the level of GPI-anchors in the mutant. Importantly, following its development using 4D microscopy and employing tissue-specific rescue, we identified loss of epithelial integrity as the primary cause of developmental arrest. Our results highlight the importance of GPI-anchored proteins for epithelial integrity in vivo and suggest a possible etiology for human diseases associated with PIGV mutations.

The genotypic and phenotypic spectrum of PIGA deficiency

BACKGROUND

Phosphatidylinositol glycan biosynthesis class A protein (PIGA) is one of the enzymes involved in the biosynthesis of glycosylphosphatidylinositol (GPI) anchor proteins, which function as enzymes, adhesion molecules, complement regulators and co-receptors in signal transduction pathways. Until recently, only somatic PIGA mutations had been reported in patients with paroxysmal nocturnal hemoglobinuria (PNH), while germline mutations had not been observed, and were suspected to result in lethality. However, in just two years, whole exome sequencing (WES) analyses have identified germline PIGA mutations in male patients with XLIDD (X-linked intellectual developmental disorder) with a wide spectrum of clinical presentations.

METHODS AND RESULTS

Here, we report on a new missense PIGA germline mutation [g.15342986C>T (p.S330N)] identified via WES followed by Sanger sequencing, in a Chinese male infant presenting with developmental arrest, infantile spasms, a pattern of lesion distribution on brain MRI resembling that typical of maple syrup urine disease, contractures, dysmorphism, elevated alkaline phosphatase, mixed hearing loss (a combination of conductive and sensorineural), liver dysfunction, mitochondrial complex I and V deficiency, and therapy-responsive dyslipidemia with confirmed lipoprotein lipase deficiency. X-inactivation studies showed skewing in the clinically unaffected carrier mother, and CD109 surface expression in patient fibroblasts was 57% of that measured in controls; together these data support pathogenicity of this mutation. Furthermore, we review all reported germline PIGA mutations (1 nonsense, 1 frameshift, 1 in-frame deletion, five missense) in 8 unrelated families.

CONCLUSIONS

Our case further delineates the heterogeneous phenotype of this condition for which we propose the term 'PIGA deficiency'. While the phenotypic spectrum is wide, it could be classified into two types (severe and less severe) with shared hallmarks of infantile spasms with hypsarrhythmia on EEG and profound XLIDD. In severe PIGA deficiency, as described in our patient, patients also present with dysmorphic facial features, multiple CNS abnormalities, such as thin corpus callosum and delayed myelination, as well as hypotonia and elevated alkaline phosphatase along with liver, renal, and cardiac involvement; its course is often fatal. The less severe form of PIGA deficiency does not involve facial dysmorphism and multiple CNS abnormalities; instead, patients present with milder IDD, treatable seizures and generally a longer lifespan.

Preparation of semisynthetic lipoproteins with fluorescent cholesterol anchor and their introduction to the cell membrane with minimal disruption of the membrane

The exogenous introduction of fluorescent lipoproteins into cell membranes is a method for visualizing the cellular traffic of membrane associated proteins, and also for altering the cell surface in a controlled manner. In order to achieve the cell membrane anchoring of proteins and their subsequent fluorescence based detection, a cholesterol derivative was designed. The headgroup of the novel cholesterol anchor contains a fluorescent reporter and a thiol reactive maleimide for protein conjugation. Protein conjugation was demonstrated by the addition of a green fluorescent maleimido anchor to the C-terminus of a Cys extended red fluorescent protein, mCherry. The resulting dual fluorescent cholesteryl lipoprotein was successfully separated from the micellar associates of the surplus fluorescent lipid anchor without denaturing the protein, and the lipoprotein containing only the covalently linked, stoichiometric fluorescent lipid was efficiently delivered to the plasma membrane of live cells. It was demonstrated that the membrane fluorescence could be directly assigned to the protein-anchor conjugate, because no excess of fluorescent lipid species were present during the imaging experiment and the protein and anchor fluorescence colocalized in the cell membrane. Molecular dynamics simulations and subsequent trajectory analysis suggest also the spontaneous and stable membrane association of the cholesterol anchor. Thus, the method could be beneficially applied for studying membrane associated proteins and for preparing mimetics of glycosylphosphatidylinositol (GPI)-anchored proteins to target cholesterol-rich membrane microdomains.

Generation of glycosylphosphatidylinositol anchor protein-deficient blood cells from human induced pluripotent stem cells

PIG-A is an X-linked gene required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors; thus, PIG-A mutant cells have a deficiency or absence of all GPI-anchored proteins (GPI-APs). Acquired mutations in hematopoietic stem cells result in the disease paroxysmal nocturnal hemoglobinuria, and hypomorphic germline PIG-A mutations lead to severe developmental abnormalities, seizures, and early death. Human induced pluripotent stem cells (iPSCs) can differentiate into cell types derived from all three germ layers, providing a novel developmental system for modeling human diseases. Using PIG-A gene targeting and an inducible PIG-A expression system, we have established, for the first time, a conditional PIG-A knockout model in human iPSCs that allows for the production of GPI-AP-deficient blood cells. PIG-A-null iPSCs were unable to generate hematopoietic cells or any cells expressing the CD34 marker and were defective in generating mesodermal cells expressing KDR/VEGFR2 (kinase insert domain receptor) and CD56 markers. In addition, PIG-A-null iPSCs had a block in embryonic development prior to mesoderm differentiation that appears to be due to defective signaling through bone morphogenetic protein 4. However, early inducible PIG-A transgene expression allowed for the generation of GPI-AP-deficient blood cells. This conditional PIG-A knockout model should be a valuable tool for studying the importance of GPI-APs in hematopoiesis and human development.

Aplastic anemia: what have we learned from animal models and from the clinic

Aplastic anemia (AA) is currently perceived as an immune-mediated disease in which aberrant effector cells recognize and destroy primitive marrow elements, resulting in pancytopenia. The immune hypothesis is based on clinical observations of responsiveness of AA to immunomodulatory agents such as anti-thymocyte globulin (ATG) and the requirement of cyclosporine to maintain response; evidence of an immune system in disarray provided by abnormal regulatory, TH1, TH17, and expanded CD8(+) T-cell populations, and animal models of immune-mediated marrow destruction, where many of the observed clinical and in vitro alterations can be confirmed and expanded. Murine models mimicking AA have used exposure to agents that result in marrow destruction through a direct toxic effect, but models that explore antigenic disparities between strains have resulted in immune-mediated destruction of the marrow, more closely modeling human AA. Many experiments in mice have helped confirm and elucidate specific mechanisms of marrow destruction. However, clinical development of regimens in AA has not relied on establishing their success in murine model. Instead, drugs and their combinations investigated in AA were those shown clinically to be active in AA, in other hematologic diseases, or in other specialties such as in rheumatology, and solid and bone marrow transplantation. In this review, the evolution of murine models and their clinical relevance in AA are discussed.

Postexit surface engineering of retroviral/lentiviral vectors

Gene delivery vectors based on retroviral or lentiviral particles are considered powerful tools for biomedicine and biotechnology applications. Such vectors require modification at the genomic level in the form of rearrangements to allow introduction of desired genes and regulatory elements (genotypic modification) as well as engineering of the physical virus particle (phenotypic modification) in order to mediate efficient and safe delivery of the genetic information to the target cell nucleus. Phenotypic modifications are typically introduced at the genomic level through genetic manipulation of the virus producing cells. However, this paper focuses on methods which allow modification of viral particle surfaces after they have exited the cell, that is, directly on the viral particles in suspension. These methods fall into three categories: (i) direct covalent chemical modification, (ii) membrane-topic reagents, and (iii) adaptor systems. Current applications of such techniques will be introduced and their advantages and disadvantages will be discussed.

Microvesicles/exosomes as potential novel biomarkers of metabolic diseases

Biomarkers are of tremendous importance for the prediction, diagnosis, and observation of the therapeutic success of common complex multifactorial metabolic diseases, such as type II diabetes and obesity. However, the predictive power of the traditional biomarkers used (eg, plasma metabolites and cytokines, body parameters) is apparently not sufficient for reliable monitoring of stage-dependent pathogenesis starting with the healthy state via its initiation and development to the established disease and further progression to late clinical outcomes. Moreover, the elucidation of putative considerable differences in the underlying pathogenetic pathways (eg, related to cellular/tissue origin, epigenetic and environmental effects) within the patient population and, consequently, the differentiation between individual options for disease prevention and therapy - hallmarks of personalized medicine - plays only a minor role in the traditional biomarker concept of metabolic diseases. In contrast, multidimensional and interdependent patterns of genetic, epigenetic, and phenotypic markers presumably will add a novel quality to predictive values, provided they can be followed routinely along the complete individual disease pathway with sufficient precision. These requirements may be fulfilled by small membrane vesicles, which are so-called exosomes and microvesicles (EMVs) that are released via two distinct molecular mechanisms from a wide variety of tissue and blood cells into the circulation in response to normal and stress/pathogenic conditions and are equipped with a multitude of transmembrane, soluble and glycosylphosphatidylinositol-anchored proteins, mRNAs, and microRNAs. Based on the currently available data, EMVs seem to reflect the diverse functional and dysfunctional states of the releasing cells and tissues along the complete individual pathogenetic pathways underlying metabolic diseases. A critical step in further validation of EMVs as biomarkers will rely on the identification of unequivocal correlations between critical disease states and specific EMV signatures, which in future may be determined in rapid and convenient fashion using nanoparticle-driven biosensors.

GPI-anchor synthesis is indispensable for the germline development of the nematode Caenorhabditis elegans

Glycosylphosphatidylinositol (GPI)-anchor attachment is one of the most common posttranslational protein modifications. Using the nematode Caenorhabditis elegans, we determined that GPI-anchored proteins are present in germline cells and distal tip cells, which are essential for the maintenance of the germline stem cell niche. We identified 24 C. elegans genes involved in GPI-anchor synthesis. Inhibition of various steps of GPI-anchor synthesis by RNA interference or gene knockout resulted in abnormal development of oocytes and early embryos, and both lethal and sterile phenotypes were observed. The piga-1 gene (orthologue of human PIGA) codes for the catalytic subunit of the phosphatidylinositol N-acetylglucosaminyltransferase complex, which catalyzes the first step of GPI-anchor synthesis. We isolated piga-1-knockout worms and found that GPI-anchor synthesis is indispensable for the maintenance of mitotic germline cell number. The knockout worms displayed 100% lethality, with decreased mitotic germline cells and abnormal eggshell formation. Using cell-specific rescue of the null allele, we showed that expression of piga-1 in somatic gonads and/or in germline is sufficient for normal embryonic development and the maintenance of the germline mitotic cells. These results clearly demonstrate that GPI-anchor synthesis is indispensable for germline formation and for normal development of oocytes and eggs.

The phenotype of a germline mutation in PIGA: the gene somatically mutated in paroxysmal nocturnal hemoglobinuria

Phosphatidylinositol glycan class A (PIGA) is involved in the first step of glycosylphosphatidylinositol (GPI) biosynthesis. Many proteins, including CD55 and CD59, are anchored to the cell by GPI. Loss of CD55 and CD59 on erythrocytes causes complement-mediated lysis in paroxysmal nocturnal hemoglobinuria (PNH), a disease that manifests after clonal expansion of hematopoietic cells with somatic PIGA mutations. Although somatic PIGA mutations have been identified in many PNH patients, it has been proposed that germline mutations are lethal. We report a family with an X-linked lethal disorder involving cleft palate, neonatal seizures, contractures, central nervous system (CNS) structural malformations, and other anomalies. An X chromosome exome next-generation sequencing screen identified a single nonsense PIGA mutation, c.1234C>T, which predicts p.Arg412(∗). This variant segregated with disease and carrier status in the family, is similar to mutations known to cause PNH as a result of PIGA dysfunction, and was absent in 409 controls. PIGA-null mutations are thought to be embryonic lethal, suggesting that p.Arg412(∗) PIGA has residual function. Transfection of a mutant p.Arg412(∗) PIGA construct into PIGA-null cells showed partial restoration of GPI-anchored proteins. The genetic data show that the c.1234C>T (p.Arg412(∗)) mutation is present in an affected child, is linked to the affected chromosome in this family, is rare in the population, and results in reduced, but not absent, biosynthesis of GPI anchors. We conclude that c.1234C>T in PIGA results in the lethal X-linked phenotype recognized in the reported family.

Dynamics of mutant cells in hierarchical organized tissues

Most tissues in multicellular organisms are maintained by continuous cell renewal processes. However, high turnover of many cells implies a large number of error-prone cell divisions. Hierarchical organized tissue structures with stem cell driven cell differentiation provide one way to prevent the accumulation of mutations, because only few stem cells are long lived. We investigate the deterministic dynamics of cells in such a hierarchical multi compartment model, where each compartment represents a certain stage of cell differentiation. The dynamics of the interacting system is described by ordinary differential equations coupled across compartments. We present analytical solutions for these equations, calculate the corresponding extinction times and compare our results to individual based stochastic simulations. Our general compartment structure can be applied to different tissues, as for example hematopoiesis, the epidermis, or colonic crypts. The solutions provide a description of the average time development of stem cell and non stem cell driven mutants and can be used to illustrate general and specific features of the dynamics of mutant cells in such hierarchically structured populations. We illustrate one possible application of this approach by discussing the origin and dynamics of PIG-A mutant clones that are found in the bloodstream of virtually every healthy adult human. From this it is apparent, that not only the occurrence of a mutant but also the compartment of origin is of importance.

We investigate the average stem cell driven dynamics of cell counts in an abstract multi compartment model. Within this framework one can represent different tissue structures, as for example hematopoiesis, the skin or the colonic crypt. Our analysis is based on an individual cell model in which cells can differentiate, reproduce or die. We give closed solutions to the corresponding system of coupled differential equations, that describe the average dynamics of all cell types. There are three cases of interest: (i) Mutations at the stem cell level, (ii) Mutations in downstream compartments associated with more mature, non stem cell types, (iii) Mutations in downstream compartments with cells acquiring stem cell like properties. The average dynamics shows for (i) and (iii) an increase of mutants towards an equilibrium, in case (ii) the average mutant cell count goes through a maximum, but mutants die out in the long run. We calculate the corresponding extinction times for every compartment. We discuss applications to hematopoietic diseases such as, PIG-A mutant cells or the classic oncogene BCR-ABL. Although the abstract model is a simplified sketch of cell differentiation, it is capable of describing many aspects of a wide variety of such tissues and associated diseases.

Transfer of the glycosylphosphatidylinositol-anchored 5'-nucleotidase CD73 from adiposomes into rat adipocytes stimulates lipid synthesis

BACKGROUND AND PURPOSE

In addition to predominant localization at detergent-insoluble, glycolipid-enriched plasma membrane microdomains (DIGs), glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-proteins) have been found associated with lipid droplets (LDs) and adiposomes. Adiposomes are vesicles that are released from adipocytes in response to anti-lipolytic and lipogenic signals, such as H(2)O(2), palmitate and the antidiabetic sulfonylurea drug, glimepiride, and harbour (c)AMP-degrading GPI-proteins, among them the 5-nucleotidase CD73. Here the role of adiposomes in GPI-protein-mediated information transfer was studied.

EXPERIMENTAL APPROACH

Adiposomes were incubated with isolated rat adipocytes under various conditions. Trafficking of CD73 and lipid synthesis were analysed.

KEY RESULTS

Upon blockade of GPI-protein trafficking, CD73 specifically associated with DIGs of small, and to a lower degree, large, adipocytes. On reversal of the blockade, CD73 appeared at cytosolic LD in time- adiposome concentration- and signal (H(2)O(2) > glimepiride > palmitate)-dependent fashion. The salt- and carbonate-resistant association of CD73 with structurally intact DIGs and LD was dependent on its intact GPI anchor. Upon incubation with small and to a lower degree, large adipocytes, adiposomes increased lipid synthesis in the absence or presence of H(2)O(2), glimepiride and palmitate and improved the sensitivity toward these signals. Upregulation of lipid synthesis by adiposomes was dependent on the translocation of CD73 with intact GPI anchors from DIGs to LD.

CONCLUSIONS

The signal-induced transfer of GPI-anchored CD73 from adiposomes via DIGs to LD of adipocytes mediates paracrine upregulation of lipid synthesis within the adipose tissue.

Inhibition of lipolysis by adiposomes containing glycosylphosphatidylinositol-anchored Gce1 protein in rat adipocytes

Small membrane vesicles released from large adipocytes and harbouring the glycosylphosphatidylinositol-anchored (GPI-) AMP-degrading protein CD73 have previously been demonstrated to stimulate the signal-induced esterification of free fatty acids into neutral lipids suggesting a role of these so-called adiposomes (ADIP) in the paracrine regulation of lipid metabolism in the adipose tissue. Here the involvement of another constituent GPI-protein of ADIP, the cAMP-degrading protein Gce1 in the signal-induced inhibition of lipolysis was investigated in primary rat adipocytes. Incubation of small, and to a lower degree, large adipocytes with ADIP inhibited lipolysis and increased its sensitivity toward inhibition by H(2)O(2), the anti-diabetic drug glimepiride and palmitate. This was accompanied by the transfer of Gce1 from the ADIP to detergent-insoluble glycolipid-enriched plasma membrane microdomains (DIGs) and its subsequent translocation to cytoplasmic lipid droplets (LD) of the acceptor adipocytes. The translocation from DIGs to LD rather than the transfer from ADIP to DIGs of Gce1 was stimulated by H(2)O(2) > glimepiride > palmitate. Both transfer and translocation led to salt- and carbonate-resistant association of Gce1 with DIGs and LD, respectively, and relied on the structural integrity of the DIGs and GPI anchor of Gce1. In conclusion, the trafficking of GPI-proteins from ADIP of donor adipocytes via DIGs to LD of acceptor adipocytes mediates paracrine regulation of lipolysis within adipose tissue.

Phenotypic and functional characterization of a mouse model of targeted Pig-a deletion in hematopoietic cells

BACKGROUND

Somatic mutation in the X-linked phosphatidylinositol glycan class A gene (PIG-A) causes glycosyl phosphatidylinositol anchor deficiency in human patients with paroxysmal nocturnal hemoglobinuria.

DESIGN AND METHODS

We produced an animal model of paroxysmal nocturnal hemoglobinuria by conditional Pig-a gene inactivation (Pig-a(-/-)) in hematopoietic cells; mice carrying two lox sites flanking exon 6 of the Pig-a gene were bred with mice carrying the transgene Cre-recombinase under the human c-fes promoter. We characterized the phenotypic and functional properties of glycosyl phosphatidylinositol-deficient and glycosyl phosphatidylinositol-normal hematopoietic cells from these Pig-a(-/-) mice using gene expression microarray, flow cytometry, bone marrow transplantation, spectratyping, and immunoblotting.

RESULTS

In comparison to glycosyl phosphatidylinositol-normal bone marrow cells, glycosyl phosphatidylinositol-deficient bone marrow cells from the same Pig-a(-/-) animals showed up-regulation of the expression of immune function genes and contained a significantly higher proportion of CD8 T cells. Both characteristics were maintained when glycosyl phosphatidylinositol-deficient cells were transplanted into lethally-irradiated recipients. Glycosyl phosphatidylinositol-deficient T cells were inactive, showed pronounced Vbeta5.1/5.2 skewing, had fewer gamma-interferon-producing cells after lectin stimulation, and contained fewer CD4(+)CD25(+)FoxP3(+) regulatory T cells. However, the levels of T-cell receptor signaling proteins from glycosyl phosphatidylinositol-deficient cells were normal relative to glycosyl phosphatidylinositol-normal cells from wild type animals, and cells were capable of inducing target cell apoptosis in vitro.

CONCLUSIONS

Deletion of the Pig-a gene in hematopoietic cells does not cause frank marrow failure but leads to the appearance of clonally-restricted, inactive yet functionally competent CD8 T cells.

Trophoblast differentiation defect in human embryonic stem cells lacking PIG-A and GPI-anchored cell-surface proteins

Pluripotent human embryonic stem (hES) cells can differentiate into various cell types derived from the three embryonic germ layers and extraembryonic tissues such as trophoblasts. The mechanisms governing lineage choices of hES cells are largely unknown. Here, we report that we established two independent hES cell clones lacking a group of cell surface molecules, glycosyl-phosphatidyl-inositol-anchored proteins (GPI-APs). The GPI-AP deficiency in these two hES clones is due to the deficiency in the gene expression of PIG-A (phosphatidyl-inositol-glycan class A), which is required for the first step of GPI synthesis. GPI-AP-deficient hES cells were capable of forming embryoid bodies and initiating cell differentiation into the three embryonic germ layers. However, GPI-AP-deficient hES cells failed to form trophoblasts after differentiation induction by embryoid body formation or by adding exogenous BMP4. The defect in trophoblast formation was due to the lack of GPI-anchored BMP coreceptors, resulting in the impairment of full BMP4 signaling activation in the GPI-AP-deficient hES cells. These data reveal that GPI-AP-enhanced full activation of BMP signaling is required for human trophoblast formation.

Paroxysmal nocturnal hemoglobinuria: stem cells and clonality

Paroxysmal nocturnal hemoglobinuria is a clonal hematopoietic stem cell disease that manifests with intravascular hemolysis, bone marrow failure, thrombosis, and smooth muscle dystonias. The disease can arise de novo or in the setting of acquired aplastic anemia. All PNH patients to date have been shown to harbor PIG-A mutations; the product of this gene is required for the synthesis of glycosylphosphatidylinositol (GPI) anchored proteins. In PNH patients, PIG-A mutations arise from a multipotent hematopoietic stem cell. Interestingly, PIG-A mutations can also be found in the peripheral blood of most healthy controls; however, these mutations arise from progenitor cells rather than multipotent hematopoietic stem cells and do not propagate the disease. The mechanism of whereby PNH stem cells achieve clonal dominance remains unclear. The leading hypotheses to explain clonal outgrowth in PNH are: 1) PNH cells evade immune attack possibly, because of an absent cell surface GPI-AP that is the target of the immune attack; 2) The PIG-A mutation confers an intrinsic resistance to apoptosis that becomes more conspicuous when the marrow is under immune attack; and 3) A second mutation occurs in the PNH clone to give it an intrinsic survival advantage. These hypotheses may not be mutually exclusive, since data in support of all three models have been generated.

A prospective comparison of four techniques for diagnosis of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal stem cell disorder with altered expression of glycosylphosphatidylinositol (GPI)-anchored proteins, resulting in the increased susceptibility of erythrocytes to complement-mediated lysis. This study compared the available laboratory methods for detection of PNH cells and evaluated their utility in routine clinical practice. Fifty patients were evaluated by flow cytometric immunophenotyping (FCMI) using CD55 and CD59 monoclonal antibodies, PNH gel card test (GCT), Ham test and sucrose lysis test (SLT). A PNH clone was detectable in erythrocytes in 14 (28%) patients by FCMI, 13 (26%) by GCT and 10 (20%) by Ham test and SLT. The GCT and lytic tests showed 100% specificity and sensitivity was 92.8% and 71.1%, respectively. The GCT results correlated with type III cells (positive for > or =3.21% type III cells) and lytic test results correlated with CD59(-) type III cells (positive for > or =5% CD59(-) type III cells). The GCT and lytic tests were comparable in their sensitivity to detect type II cells (positive for > or =18.5% type II cells). Among the available methods, FCMI is most sensitive, can quantify and delineate PNH cells with differential expression of GPI-anchored proteins. The GCT is a useful screening tool as it is fairly sensitive, easy to perform and interpret. Well-standardized lytic tests are fairly reliable as screening tests.

Hitch-hiking between cells on lipoprotein particles

Cell surface proteins containing covalently linked lipids associate with specialized membrane domains. Morphogens like Hedgehog and Wnt use their lipid anchors to bind to lipoprotein particles and employ lipoproteins to travel through tissues. Removal of their lipid anchors or decreasing lipoprotein levels give rise to adverse Hedgehog and Wnt signaling. Some parasites can also transfer their glycosylphosphatidylinositol-anchored surface proteins to host lipoprotein particles. These antigen-loaded lipoproteins spread throughout the circulation, and probably hamper an adequate immune response by killing neutrophils. Together, these findings imply a widespread role for lipoproteins in intercellular transfer of lipid-anchored surface proteins, and may have various physiological consequences. Here, we discuss how lipid-modified proteins may be transferred to and from lipoproteins at the cellular level.

Protection of erythrocytes from human complement–mediated lysis by membrane-targeted recombinant soluble CD59: a new approach to PNH therapy

Paroxysmal nocturnal hemoglobinuria (PNH) results from the expansion of a hematopoietic clone that is deficient in glycosylphosphatidylinositol-anchored molecules. PNH is characterized by chronic hemolysis with acute exacerbations due to the uncontrolled activity of complement on PNH cells, which lack the inhibitor of homologous complement, CD59. Symptoms include severe fatigue, hemoglobinuria, esophageal spasm, erectile dysfunction, and thrombosis. We report the use of a novel synthetically modified recombinant human CD59, rhCD59-P, a soluble protein that attaches to cell membranes. In vitro treatment of PNH erythrocytes with rhCD59-P resulted in levels of CD59 equivalent to normal erythrocytes and effectively protected erythrocytes from complement-mediated hemolysis. The administration of rhCD59-P to CD1 mice resulted in levels of CD59 on erythrocytes, which protected them from complement-mediated lysis. Thus, rhCD59-P corrects the CD59 deficiency in vitro and can bind to erythrocytes in an in vivo murine model, protecting the cells from the activity of human complement, and represents a potential therapeutic strategy in PNH.

Immunoselection by natural killer cells of PIGA mutant cells missing stress-inducible ULBP

The mechanism by which paroxysmal nocturnal hemoglobinuria (PNH) clones expand is unknown. PNH clones harbor PIGA mutations and do not synthesize glycosylphosphatidylinositol (GPI), resulting in deficiency of GPI-linked membrane proteins. GPI-deficient blood cells often expand in patients with aplastic anemia who sustain immune-mediated marrow injury putatively induced by cytotoxic cells, hence suggesting that the injury allows PNH clones to expand selectively. We previously reported that leukemic K562 cells preferentially survived natural killer (NK) cell-mediated cytotoxicity in vitro when they acquired PIGA mutations. We herein show that the survival is ascribable to the deficiency of stress-inducible GPI-linked membrane proteins ULBP1 and ULBP2, which activate NK and T cells. The ULBPs were detected on GPI-expressing but not on GPI-deficient K562 cells. In the presence of antibodies to either the ULBPs or their receptor NKG2D on NK cells, GPI-expressing cells were as less NK sensitive as GPI-deficient cells. NK cells therefore spared ULBP-deficient cells in vitro. The ULBPs were identified only on GPI-expressing blood cells of a proportion of patients with PNH but none of healthy individuals. Granulocytes of the patients partly underwent killing by autologous cytotoxic cells, implying ULBP-associated blood cell injury. In this setting, the lack of ULBPs may allow immunoselection of PNH clones.

Shedding and uptake of gangliosides and glycosylphosphatidylinositol-anchored proteins

Gangliosides and glycosylphosphatidylinositol (GPI)-anchored proteins have very different biosynthetic origin, but they have one thing in common: they are both comprised of a relatively large hydrophilic moiety tethered to a membrane by a relatively small lipid tail. Both gangliosides and GPI-anchored proteins can be actively shed from the membrane of one cell and taken up by other cells by insertion of their lipid anchors into the cell membrane. The process of shedding and uptake of gangliosides and GPI-anchored proteins has been independently discovered in several disciplines during the last few decades, but these discoveries were largely ignored by people working in other areas of science. By bringing together results from these, sometimes very distant disciplines, in this review, we give an overview of current knowledge about shedding and uptake of gangliosides and GPI-anchored proteins. Tumor cells and some pathogens apparently misuse this process for their own advantage, but its real physiological functions remain to be discovered.

Differential gene expression in hematopoietic progenitors from paroxysmal nocturnal hemoglobinuria patients reveals an apoptosis/immune response in 'normal' phenotype cells

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired stem cell disorder characterized clinically by intravascular hemolysis, venous thrombosis, and bone marrow failure. Despite elucidation of the biochemical and molecular defects in PNH, the pathophysiology of clonal expansion of glycosylphosphatidylinositol-anchored protein (GPI-AP)-deficient cells remains unexplained. In pursuit of evidence of differences between GPI-AP-normal and -deficient CD34 cells, we determined gene expression profiles of isolated marrow CD34 cells of each phenotype from PNH patients and healthy donors, using DNA microarrays. Pooled and individual patient samples revealed consistent gene expression patterns relative to normal controls. GPI-AP-normal cells from PNH patients showed upregulation of genes involved in apoptosis and the immune response. Conversely, genes associated with antiapoptotic function and hematopoietic cell proliferation and differentiation were downregulated in these cells. In contrast, the PNH clone of GPI-AP-deficient cells appeared more similar to CD34 cells of healthy individuals. Gene chip data were confirmed by other methods. Similar gene expression patterns were present in PNH that was predominantly hemolytic as in PNH associated with aplastic anemia. Our results implicate an environmental influence on hematopoietic cell proliferation, in which the PNH clone evades immune attack and destruction, while normal cells suffer a stress response followed by programmed cell death.

Lipoprotein particles are required for Hedgehog and Wingless signalling

Wnt and Hedgehog family proteins are secreted signalling molecules (morphogens) that act at both long and short range to control growth and patterning during development. Both proteins are covalently modified by lipid, and the mechanism by which such hydrophobic molecules might spread over long distances is unknown. Here we show that Wingless, Hedgehog and glycophosphatidylinositol-linked proteins copurify with lipoprotein particles, and co-localize with them in the developing wing epithelium of Drosophila. In larvae with reduced lipoprotein levels, Hedgehog accumulates near its site of production, and fails to signal over its normal range. Similarly, the range of Wingless signalling is narrowed. We propose a novel function for lipoprotein particles, in which they act as vehicles for the movement of lipid-linked morphogens and glycophosphatidylinositol-linked proteins.

Transfer of glycosylphosphatidylinositol-anchored proteins to deficient cells after erythrocyte transfusion in paroxysmal nocturnal hemoglobinuria

In paroxysmal nocturnal hemoglobinuria (PNH), an acquired mutation of the PIGA gene results in the absence of glycosylphosphatidylinositol (GPI)-anchored cell surface membrane proteins in affected hematopoietic cells. Absence of GPI-anchored proteins on erythrocytes is responsible for their increased sensitivity to complement-mediated lysis, resulting in hemolytic anemia. Cell-to-cell transfer of CD55 and CD59, 2 GPI-anchored proteins, by red cell microvesicles has been demonstrated in vitro, with retention of their function. Because red cell units stored for transfusion contain many erythrocyte microvesicles, transfused blood could potentially serve as a source of CD55 and CD59. We examined whether GPI-anchored proteins could be transferred in vivo to deficient cells following transfusions given to 6 patients with PNH. All patients were group A(1) blood type. Each was given transfusions of 3 U of compatible, washed group O blood. Patient group A(1) cells were distinguished from the transfused group O cells by flow cytometry and staining with a labeled lectin, Dolichos biflorus, which specifically binds to group A(1) erythrocytes. Increased surface CD59 was measured on recipient red cells and granulocytes 1, 3, and 7 days following transfusion in all 6 patients. Our data suggest a potential therapeutic role for GPI-anchored protein transfer for severe PNH.

Frequent HPRT mutations in paroxysmal nocturnal haemoglobinuria reflect T cell clonal expansion, not genomic instability

Paroxysmal nocturnal haemoglobinuria (PNH) results from acquired mutations in the PIG-A gene of an haematopoietic stem cell, leading to defective biosynthesis of glycosylphosphatidylinositol (GPI) anchors and deficient expression of GPI-anchored proteins on the surface of the cell's progeny. Some laboratory and clinical findings have suggested genomic instability to be intrinsic in PNH; this possibility has been supported by mutation analysis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene abnormalities. However, the HPRT assay examines lymphocytes in peripheral blood (PB), and T cells may be related to the pathophysiology of PNH. We analysed the molecular and functional features of HPRT mutants in PB mononuclear cells from eleven PNH patients. CD8 T cells predominated in these samples; approximately half of the CD8 cells lacked GPI-anchored protein expression, while only a small proportion of CD4 cells appeared to derive from the PNH clone. The HPRT mutant frequency (Mf) in T lymphocytes from PNH patients was significantly higher than in healthy controls. The majority of the mutant T lymphocyte clones were of CD4 phenotype, and they had phenotypically normal GPI-anchored protein expression. In PNH patients, the majority of HPRT mutant clones were contained within the Vbeta2 T cell receptor (TCR) subfamily, which was oligoclonal by complementarity-determining region three (CDR3) size analysis. Our results are more consistent with detection of uniform populations of expanded T cell clones, which presumably acquired HPRT mutations during antigen-driven cell proliferation, and not due to an increased Mf in PNH. HPRT mutant analysis does not support underlying genomic instability in PNH.

A new aspect of the molecular pathogenesis of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal hematologic disorder which is manifest by complement-mediated hemolysis, venous thrombosis, and bone marrow failure. Complement-mediated hemolysis in PNH is explained by the deficiency of glycosylphosphatidylinositol (GPI)-anchored proteins, CD55 and CD59 on erythrocyte surfaces. All the PNH patients had phosphatidylinositol glycan-class A (PIG-A) gene abnormalities in various cell types, indicating that PIG-A gene mutations cause the defects in GPI-anchored proteins that are essential for the pathogenesis of PNH. In addition, a PIG-A gene abnormality results in a PNH clone. Bone marrow failure causes cytopenias associated with a proliferative decrease of its hematopoietic stem cells and appears to be related to a pre-leukemic state. Although it is unclear how a PNH clone expands in bone marrow, it is considered that the most important hypothesis implicates negative selection of a PNH clone, but it does not explain the changes in the clinical features at the terminal stage of PNH. Recently, it has been suggested that an immune mechanism, in an HLA-restricted manner, plays an important role in the occurrence or selection of a PNH clone and GPI may be a target for cytotoxic-T lymphocytes. Also, it has been indicated that the Wilms' tumor gene (WT1) product is related to a PNH clone, but the significance of WT1 expression is not clear because of the functional diversity of the gene. To elucidate this problem, it is important to know the pathophysiology of bone marrow failure in detail and how bone marrow failure affects hematopoietic stem cells and immune mechanisms in bone marrow failure syndromes.

Cytogenetic abnormalities in paroxysmal nocturnal haemoglobinuria usually occur in haematopoietic cells that are glycosylphosphatidylinositol-anchored protein (GPI-AP) positive

Some patients with paroxysmal nocturnal haemoglobinuria (PNH) have bone marrow findings characteristic of myelodysplastic syndrome. We studied nine PNH patients to determine whether these karyotypic abnormalities were more likely to occur in glycosylphosphatidylinositol-anchored protein (GPI-AP)-negative cells. Abnormal chromosome patterns were evident only in the GPI-AP-positive populations of the PNH clone in 8 of 9 cases studied. Purified GPI-AP-negative CD34 cells gave rise only to cells of normal karyotype, whereas the progeny of the GPI-AP-positive CD34 cells showed the karyotypic abnormality. These findings suggest that environmental factors, but not genetic instability of the GPI-AP-deficient clone, foster development or survival of haematopoietic cells with chromosomal abnormalities.

Hematopoietic stem cells in aplastic anemia

Profound cytopenia involving all blood lineages, a hallmark of aplastic anemia (AA), can result in devastating morbidity and high mortality. Although various etiologies and distinct pathophysiologic mechanisms may be involved, a profound defect in the stem cell compartment is a unifying feature in most patients with AA. As a stem cell disease, AA is very instructive and provides insights into the function and quantity of normal hematopoietic stem cells and their ability to regenerate. Pathophysiologically, understanding of AA may reveal mechanisms as to the evolution of other related bone marrow failure syndromes such as paroxysmal nocturnal hemoglobinuria and myelodysplasia-clonal diseases of hematopoiesis associated with defective stem cells. Conversely, constitutional forms of AA occurring in association with Fanconi anemia and dyskeratosis congenita demonstrate the role of specific genes and pathways in the dysfunction of the stem cells leading to the failure of the stem cell compartment. The acquired mechanisms resulting in depletion of stem cells in AA may involve fundamental pathways such as apoptosis and senescence as well as exhaustion of proliferative capacity or excessive differentiation. Inherent in the paucity of the bone marrow in AA, the study of the stem cells in AA has been very difficult due to their natural rarity and disease-specific contraction of the stem cell pool. Despite these scientific challenges, laboratory studies and systematic clinical observation provide valuable information of significance beyond its specific application to AA.

Differential apoptosis and Fas expression on GPI-negative and GPI-positive stem cells: a mechanism for the evolution of paroxysmal nocturnal haemoglobinuria*

Paroxysmal nocturnal haemoglobinuria (PNH) has a dual pathogenesis. PIG-A mutations generate clones of haemopoietic stem cells (HSC) lacking glycosylphosphatidylinositol (GPI)-anchored proteins and, secondly, these clones expand because of a selective advantage related to bone marrow failure. The first aspect has been elucidated in detail, but the mechanisms leading to clonal expansion are not well understood. We have previously shown that apoptosis and Fas expression in HSC play a role in bone marrow failure during aplastic anaemia. We have now investigated apoptosis in PNH. Ten patients were studied. Apoptosis, measured by flow cytometry, was significantly higher among CD34+ cells from patients compared with healthy controls. Fas expression was also increased. Cells that were stained for CD34, CD59 and apoptosis showed a significantly lower apoptosis in CD34+/CD59- compared with CD34+/CD59+ cells from the same patient. In three patients, staining for CD34, CD59 and Fas revealed lower Fas expression on CD34+/CD59- cells. Differential apoptosis of CD34+/CD59- HSC may be sufficient in itself to explain the expansion of PNH clones in the context of aplastic anaemia. In addition to demonstrating a basic mechanism underlying PNH clonal expansion, these results suggest further hypotheses for the evolution of PNH, based on the direct or indirect resistance of GPI-negative HSC to pro-inflammatory cytokines.

In vitro and in vivo evidence of PNH cell sensitivity to immune attack after nonmyeloablative allogeneic hematopoietic cell transplantation

It has been proposed that paroxysmal nocturnal hemoglobinuria (PNH) cells may proliferate through their intrinsic resistance to immune attack. To evaluate this hypothesis, we examined the impact of alloimmune pressure on PNH and normal cells in the clinical setting of nonmyeloablative allogeneic hematopoietic cell transplantation (HCT). Five patients with severe PNH underwent HCT from an HLA-matched family donor after conditioning with cyclophosphamide and fludarabine. PNH neutrophils (CD15(+)/CD66b(-)/CD16(-)) were detected in all patients at engraftment, but they subsequently declined to undetectable levels in all cases by 4 months after transplantation. To test for differences in susceptibility to immune pressure, minor histocompatibility antigen (mHa)-specific T-cell lines or clones were targeted against glycosylphosphatidylinositol (GPI)-negative and GPI-positive monocyte and B-cell fractions purified by flow cytometry sorting. Equivalent amounts of interferon-gamma (IFN-gamma) were secreted following coculture with GPI-negative and GPI-positive targets. Furthermore, mHa-specific T-cell lines and CD8(+) T-cell clones showed similar cytotoxicity against both GPI-positive and GPI-negative B cells. Presently, all 5 patients survive without evidence of PNH 5 to 39 months after transplantation. These in vitro and in vivo studies show PNH cells can be immunologically eradicated following nonmyeloablative HCT. Relative to normal cells, no evidence for a decreased sensitivity of PNH cells to T-cell-mediated immunity was observed.

Somatic gene mutation and human disease other than cancer

While the focus of much mutation research is on germ-line mutation, somatic mutation is being found to be important in human disease. Neurofibromatosis 1 and McCune-Albright are disorders which are detected in the skin and other systems. The skin manifestations were essential for the demonstration of somatic mosaicism in neurofibromatosis 1, while analysis of blood DNA demonstrated somatic mutation in neurofibromatosis 2. Incontinentia pigmenti is also a disorder seen in skin and other tissues, but here it is the rare variant of the disorder in males, where it is usually lethal, that involves somatic mosaicism. Paroxysmal nocturnal hemoglobinuria is a disorder of the blood and cell separation of blood elements allows the demonstration of the somatic mosaicism. This review also discusses disorders in which somatic mosaicism, for mutations probably incompatible with life if the mutation had been germ-line, are likely to be involved. These include the Proteus syndrome, which involves both vascular tissues and bones, and two disorders which might be thought of as representing two subtypes of Proteus: Klippel-Trenaunay, which involves vascular tissues, and Maffuci, which involves bones. Embryonic mutations, which create mosaicism for both the soma and germ-line, are being increasingly found in a number of disorders and are discussed more briefly. Finally, reverse mutations involving the soma have been recently found in several disorders and such revertant mutations are also examined. While the review focuses on the clinical importance of somatic mutations, many of the mutations found to date are tabulated. It is too early to see if there is a different pattern of somatic mutation as compared to germ-line mutation. Although the parameters to allow careful quantitation are not yet available, it seems that the frequency of gene mutation in embryonic cells is not markedly different than that in the germ-line.

CD34+ cells from paroxysmal nocturnal hemoglobinuria (PNH) patients are deficient in surface expression of cellular prion protein (PrPc)

Cellular prion protein (PrP(c)) is a glycosylphosphatidylinositol (GPI)-anchored protein (GPI-AP) constitutively expressed by neurons but also in hematopoietic cells. In trasmissible spongiform encephalopathies, the protease-resistant form of prion (PrP (s c)) converts the host PrP(c) into the pathologic form. We have investigated PrP(c) expression in hematopoietic cells from paroxysmal nocturnal hemoglobinuria (PNH). In this disease, due to somatic mutations in PIG-A gene, biosynthesis of the (GPI)-anchor is impaired and affected cells lack membrane expression of all GPI-AP. Normal and PNH hematopoietic progenitors and paired wild-type (WT) and PIG-A mutant cell lines were used for analysis of intracellular and surface PrP(c) expression using flow cytometry and Western blot.By flow cytometry, PrP(c) was constitutively present on normal CD34(+) cells, including more immature CD38(dim) cells, as well as hematopoietic cell lines. Similar results were obtained in purified CD34(+). Phospholipase C treatment confirmed that PrP(c) was expressed on the membrane via the GPI-anchor. In PNH patients, GPI-AP-deficient CD34(+) cells lacked PrP(c) membrane expression. PIG-A-mutated cell lines (Jurkat, K562, C(EBV), A(EBV)), in contrast to their normal counterparts, did not express surface PrP(c). However, we detected intracellular PrP(c) at approximately equivalent levels in both normal and PIG-A-mutated cells using intracellular flow cytometry and Western blotting. Cells and cell lines with PNH phenotype together with their normal counterparts may be a suitable system to explore the function of membrane PrP(c) in the hematopoietic system. Conversely, PrP(c) is a good model to elucidate the fate of GPI-AP in PIG-A-deficient cells.

The relationship of aplastic anemia and PNH

Bone marrow failure has been regarded as one of the triad of clinical manifestations of paroxysmal noctumal hemoglobinuria (PNH), and PNH in turn has been described as a late clonal disease evolving in patients recovering from aplastic anemia. Better understanding of the pathophysiology of both diseases and improved tests for cell surface glycosylphosphatidylinositol (GPI)-linked proteins has radically altered this view. Flow cytometry of granulocytes shows evidence of an expanded PNH clone in a large proportion of marrow failure patients at the time of presentation: in our large NIH series, about 1/3 of over 200 aplastic anemia cases and almost 20% of more than 100 myelodysplasia cases. Clonal PNH expansion (rather than bone marrow failure) is strongly linked to the histocompatability antigen HLA.-DR2 in all clinical varieties of the disease, suggesting an immune component to its pathophysiology. An extrinsic mechanism of clonal expansion is also more consistent with knock-out mouse models and culture experiments with primary cells and cell lines, which have failed to demonstrate an intrinsic proliferative advantage for PNH cells. DNA chip analysis of multiple paired normal and PIG-A mutant cell lines and lymphoblastoid cells do not show any consistent differences in levels of gene expression. In aplastic anemia/PNH there is surprisingly limited utilization of the V-beta chain of the T cell receptor, and patients' dominant T cell clones, which are functionally inhibitory of autologous hematopoiesis, use identical CDR3 regions for antigen binding. Phenotypically normal cells from PNH patients proliferate more poorly in culture than do the same patient's PNH cells, and the normal cells are damaged as a result of apoptosis and overexpress Fas. Differences in protein degradation might play a dual role in pathophysiology, as GPI-linked proteins lacking an anchor would be predicted to be processed by the proteasome machinery and displayed in a class I H.A. context, in contrast to the normal pathway of cell surface membrane recycling, lysosomal degradation, and presentation by class II HLA. The strong relationship between a chronic, organ-specific immune destructive process and the expansion of a single mutant stem cell clone remains frustratingly enigmatic but likely to be the result of interesting biologic processes, with mechanisms that potentially can be extended to the role of inflammation in producing premalignant syndromes.

Decreased susceptibility of leukemic cells with PIG-A mutation to natural killer cells in vitro

The cloning of the PIG-A gene has facilitated the unraveling of the complex pathophysiology of paroxysmal nocturnal hemoglobinuria (PNH). Of current major concern is the mechanism by which a PNH clone expands. Many reports have suggested that an immune mechanism operates to cause bone marrow failure in some patients with PNH, aplastic anemia, and myelodysplastic syndromes. Because blood cells of PNH phenotype are often found in patients with these marrow diseases, one hypothesis is that the PNH clone escapes immune attack, producing a survival advantage by immunoselection. To test this hypothesis, we examined the sensitivity of blood cells, with or without PIG-A mutations, to killing by natural killer (NK) cells, using 51Cr-release assay in vitro. To both peripheral blood and cultured NK cells, PIG-A mutant cells prepared from myeloid and lymphoid leukemic cell lines were less susceptible than their control counterparts (reverted from the mutant cells by transfection with a PIG-A cDNA). NK activity was completely abolished with concanamycin A and by calcium chelation, indicating that killing was perforin-dependent. There were no differences in major histocompatibility (MHC) class I expression or sensitivity to either purified perforin or to interleukin-2-activated NK cells between PIG-A mutant and control cells. From these results, we infer that PIG-A mutant cells lack molecules needed for NK activation or to trigger perforin-mediated killing. Our experiments suggest that PIG-A mutations confer a relative survival advantage to a PNH clone, contributing to selective expansion of these cells in the setting of marrow injury by cytotoxic lymphocytes.

Dual roles of Cripto as a ligand and coreceptor in the nodal signaling pathway

The EGF-CFC gene Cripto encodes an extracellular protein that has been implicated in the signaling pathway for the transforming growth factor beta (TGF beta) ligand Nodal. Although recent findings in frog and fish embryos have suggested that EGF-CFC proteins function as coreceptors for Nodal, studies in cell culture have implicated Cripto as a growth factor-like signaling molecule. Here we reconcile these apparently disparate models of Cripto function by using a mammalian cell culture assay to investigate the signaling activities of Nodal and EGF-CFC proteins. Using a luciferase reporter assay, we found that Cripto has activities consistent with its being a coreceptor for Nodal. However, Cripto can also function as a secreted signaling factor in cell coculture assays, suggesting that it may also act as a coligand for Nodal. Furthermore, we found that the ability of Cripto to bind to Nodal and mediate Nodal signaling requires the addition of an O-linked fucose monosaccharide to a conserved site within EGF-CFC proteins. We propose a model in which Cripto has dual roles as a coreceptor as well as a coligand for Nodal and that this signaling interaction with Nodal is regulated by an unusual form of glycosylation. Our findings highlight the significance of extracellular modulation of ligand activity as an important means of regulating TGF beta signaling pathways during vertebrate development.

Superior growth of glycophosphatidy linositol-anchored protein-deficient progenitor cells in vitro is due to the higher apoptotic rate of progenitors with normal phenotype in vivo

OBJECTIVE

Recently, phenotypically normal CD34 cells from the marrow of patients with paroxysmal nocturnal hemoglobinuria (PNH) were reported to show impaired growth and elevated Fas receptor expression as compared to glycophosphatidylinositol-anchored protein (GPI-AP)-deficient CD34 cells and CD34 cells from normal individuals. These results are consistent with the theory that PNH cells have an intrinsic growth advantage, but their superior expansion in vitro could also be the outcome of selective extrinsic pressure in vivo.

MATERIAL AND METHODS

Growth characteristics, competitive features, and susceptibility to apoptosis of sorted normal or GPI-AP-deficient CD34(+) cells derived from PNH patients were assessed in suspension and methylcellulose cultures.

RESULTS

When we directly compared the growth of patients' CD34 cells, separated based on expression of GPI-AP CD55 and CD59, in most of the patients studied, mutant CD34 cells showed higher progeny production and outgrew phenotypically normal CD34 cells derived from PNH patients in mixing experiments. However, their proliferation rate did not exceed that of control CD34 cells. To determine whether deficient growth of phenotypically normal CD34 cells in PNH was secondary to a pre-existing in vivo insult, we determined the fraction of apoptotic cells within fresh normal and PNH CD34 cells. Normal CD34 cells from PNH patients showed a high proportion of apoptotic cells and higher Fas expression, while GPI-AP-deficient and control CD34 cells showed similar, low rates of apoptosis. After correction for pre-existing apoptosis, the proliferation potential of normal and PNH CD34 cells was similar.

CONCLUSIONS

These results strongly suggest that clonal expansion of GPI-AP-deficient progenitor cells from PNH patients is due to their selection in the hostile marrow environment of the patient.

HLA class II haplotype and quantitation of WT1 RNA in Japanese patients with paroxysmal nocturnal hemoglobinuria

It is unclear how a paroxysmal nocturnal hemoglobinuria (PNH) clone expands in bone marrow, although immune mechanisms involving cytotoxic T lymphocytes, autosomal proliferation, and apoptosis resistance have been hypothesized. To clarify aspects of immune mechanisms and proliferation of PNH cells, we investigated HLA-DRB1, -DQA1, and -DQB1 alleles by polymerase chain reaction (PCR)-based genotyping and expression of the Wilms' tumor gene, WT1, by real-time reverse transcriptase-PCR (RT-PCR) in 21 PNH and 21 aplastic anemia (AA) patients. HLA genotyping indicated that the frequency of DRB1*1501, DQA1*0102, and DQB1*0602 alleles in PNH patients and of DQB1*0602 allele in AA patients was significantly higher than in 916 Japanese controls, and that the HLA-DRB1*1501-DQA1*0102-DQB1*0602 haplotype, found in 13 of 21 PNH patients, 5 of 7 AA-PNH syndrome patients, and 7 of 21 AA patients showed significant differences compared with healthy individuals. RT-PCR analysis showed that the mean values of WT1 RNA were 3413, 712, and 334 copies/microg RNA in PNH, AA, and healthy individuals, respectively. The values for PNH patients were significantly higher than for AA patients and healthy volunteers and were correlated with the proportion of CD16b(-) granulocytes. The high frequency of HLA-DRB1*1501-DQA1*0102-DQB1*0602 haplotype in PNH, including AA-PNH syndrome, and AA patients suggests that linkage exists between the disorders and that immune mechanisms in an HLA-restricted manner play an important role in the pathogenesis of these disorders. In addition, high expression of WT1 RNA in PNH patients is related to a PNH clone, but it remains unclear whether this causes expansion of a PNH clone.