Detection of Pig-a Mutant Erythrocytes in the Peripheral Blood of Rats and Mice.

The endogenous X-linked phosphatidyl inositol glycan class A gene (Pig-a) can be used as a reporter of in vivo somatic cell mutation in rats and mice. Pig-a mutant cells are deficient in specific protein surface markers and can be identified and quantified by immunofluorescent staining followed by high-throughput flow cytometry. Pig-a mutation detection is commonly performed with red blood cells (RBCs) because: (1) the low volumes of blood required for determining mutant frequencies in RBCs allow multiple samplings on small laboratory animals over extended periods of time; (2) the execution of the RBC assay is easy and the interpretation of the results is straightforward; and (3) RBC Pig-a mutant frequencies are known within hours of sample collection. Two endpoints are determined in the assay: the frequency of mutant total RBCs and the frequency of mutant reticulocytes. When Pig-a mutation is used to assess the in vivo mutagenic potential of suspect hazards, the frequency of mutant reticulocytes is an early indicator of mutagenic potential, while the mutant frequency in total RBCs can be measured more rapidly and with greater precision.

The Asthma-associated PER1-like domain-containing protein 1 (PERLD1) Haplotype Influences Soluble Glycosylphosphatidylinositol Anchor Protein (sGPI-AP) Levels in Serum and Immune Cell Proliferation.

Post-glycosylphosphatidylinositol (GPI) attachment to proteins 3, also known as PGAP3 or PERLD1 (PER1-like domain-containing protein 1), participates in the lipid remodeling process of glycosylphosphatidylinositol (GPI) anchor proteins during post-translational modification. Functional defect in PERLD1 was previously hypothesized to influence this process in T-cells and their subsequent activation and proliferation. This current study aims to functionally characterize PERLD1 genetic variants and relate this with human immune cells proliferation rate upon stimulation. We first showed the association between a PERLD1 tag-single nucleotide polymorphism (tagSNP), rs2941504, and the development of asthma in our study population. This association remained significant after conditioning for the other asthma-associated SNP rs8076131 that is also located within the 17q12-21 region. Subsequent sequencing of 40 unrelated Singapore Chinese individuals identified 12 more common PERLD1 SNPs (minor allele frequency > 5%) that are in linkage disequilibrium (LD, r2 > 0.8) with rs2941504. Through in vitro studies, 7 of these SNPs were found to form a functional haplotype that influences alternative splicing of PERLD1 transcript. This result was validated in human peripheral blood mononuclear cell (PBMC), where the minor haplotype (Hap2) was shown to be associated with significantly increased PERLD1 truncated transcript. Additionally, Hap2 was found to be related to increased levels of several soluble GPI-anchored proteins (such as sCD55 and sCD59) in serum. Elevated sCD55 in the serum was demonstrated to reduce the proliferation rate of PBMCs upon phytohaemagglutinin (PHA) stimulation. Taken together, the current study has shown a functional PERLD1 haplotype, which modifies PBMC sensitivity upon stimulation and may contribute to the individual's susceptibility to allergic asthma.

Successful use of eculizumab for treatment of an acute hemolytic reaction after ABO-incompatible red blood cell transfusion.

BACKGROUND: Transfusion of ABO major-incompatible red blood cells (RBCs) can activate the complement system and can cause severe and even lethal acute hemolytic reactions. The activation of the complement system with formation of C3a and C5a (anaphylatoxins) and the release of hemoglobin from the lysed RBCs are thought to mediate clinical signs like fever, hypotension, pain, and acute renal failure. Therapeutic inhibition of the complement cascade in case of ABO-incompatible RBC transfusion would be desirable to ameliorate the signs and symptoms and to improve the outcome of the reaction. STUDY DESIGN AND METHODS: A patient with blood group B was erroneously transfused with a unit of group A2 RBCs. Within 1 hour after transfusion she received eculizumab, a monoclonal antibody that binds to the complement component C5 and blocks its cleavage. Clinical and immunohematologic observations are reported here. RESULTS: Hemoglobinemia and hemoglobinuria were present for several hours after transfusion, but she developed no hypotension, no renal failure, and no disseminated intravascular coagulation. As shown by flow cytometry, group A cells survived in the peripheral blood for more than 75 days. No immunoglobulin G was detectable by column agglutination technique on these cells. CONCLUSION: A low isoagglutinin titer and blood group A2 of the erroneously transfused cells most likely were the reason for the absence of clinical signs during and immediately after the ABO-incompatible transfusion. In the further course, eculizumab successfully protected the incompatible RBCs from hemolysis for several weeks.

Elevated levels of glycosylphosphatidylinositol (GPI) anchored proteins in plasma from human cancers detected by C. septicum alpha toxin.

The glycosylphosphatidylinositol (GPI) anchor is a glycan and lipid posttranslational modification added to proteins in the endoplasmic reticulum. Certain enzymes within the GPI biosynthetic pathway, particularly the subunits of the GPI transamidase, are elevated in various human cancers. Specific GPI anchored proteins, such as carcinoembryonic antigen and mesothelin, have been described as potential biomarkers for certain cancers; however, the overall levels of GPI anchored proteins present in plasma from cases of human cancers have not been evaluated. We have developed the use of a bacterial toxin known as alpha toxin from Clostridium septicum to detect GPI anchored proteins in vitro. In this study, we use alpha toxin to detect GPI anchored proteins present in plasma from cases of several types of human cancers. Our data indicate that human cancers with previously documented elevations of GPI transamidase subunits show increased alpha toxin binding to plasma from patients with these cancers, indicating increased levels of GPI anchored proteins. Furthermore, our results reveal very low levels of alpha toxin binding to plasma from patients with no malignant disease indicating few GPI anchored proteins are present. These data suggest that GPI anchored proteins present in plasma from these cancers represent biomarkers with potential use for cancer detection.

PIGO mutations in intractable epilepsy and severe developmental delay with mild elevation of alkaline phosphatase levels.

Aberrations in the glycosylphosphatidylinositol (GPI)-anchor biosynthesis pathway constitute a subclass of congenital disorders of glycosylation, and mutations in seven genes involved in this pathway have been identified. Among them, mutations in PIGV and PIGO, which are involved in the late stages of GPI-anchor synthesis, and PGAP2, which is involved in fatty-acid GPI-anchor remodeling, are all causative for hyperphosphatasia with mental retardation syndrome (HPMRS). Using whole exome sequencing, we identified novel compound heterozygous PIGO mutations (c.389C>A [p.Thr130Asn] and c.1288C>T [p.Gln430*]) in two siblings, one of them having epileptic encephalopathy. GPI-anchored proteins (CD16 and CD24) on blood granulocytes were slightly decreased compared with a control and his mother. Our patients lacked the characteristic features of HPMRS, such as facial dysmorphology (showing only a tented mouth) and hypoplasia of distal phalanges, and had only a mild elevation of serum alkaline phosphatase (ALP). Our findings therefore expand the clinical spectrum of GPI-anchor deficiencies involving PIGO mutations to include epileptic encephalopathy with mild elevation of ALP.

Laboratory tests for paroxysmal nocturnal hemoglobinuria.

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hematological disorder that is often suspected in a patient presenting with non-immune hemolytic anemia associated with pancytopenia or venous thrombosis. This disorder is a consequence of acquired somatic mutations in the phosphatidylinositol glycan class A (PIG-A) gene in the hematopoietic stem cells (HSC) of patients. The presence of these mutations leads to production of blood cells with decreased glycosyl phosphatidylinositol-anchored cell surface proteins, making red blood cells derived from the clone more sensitive to complement mediated hemolysis. The diagnosis of PNH may be difficult in some cases due a low proportion of PNH cells in the blood and occasionally due to difficulties in selecting the most appropriate diagnostic studies. The latest generation of tests allow for detection of very small populations of PNH cells, for following the natural course and response to therapy of the disease, and for helping to decide when to initiate therapy with monoclonal antibody targeting the terminal complement protein C5 (Eculizumab), anticoagulation, and in some cases allogeneic HSC transplant. In this article, we review the different diagnostic tests available to clinicians for PNH diagnosis.

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.

Pharmacokinetic and pharmacodynamic analysis of circulating biomarkers of anti-NRP1, a novel antiangiogenesis agent, in two phase I trials in patients with advanced solid tumors.

PURPOSE: MNRP1685A is a monoclonal antibody to neuropilin-1 (NRP1). We evaluated blood-based pharmacodynamic biomarkers of MNRP1685A in two phase I studies to assess exposure/response relationships to inform target dose and regimen selection. EXPERIMENTAL DESIGN: The phase I studies evaluated escalating doses of MNRP1685A as a single agent or in combination with bevacizumab. Plasma placental growth factor (PlGF), VEGF, and circulating NRP1 (cNRP1) were evaluated at multiple time points using meso-scale discovery (MSD) assays and ELISA, respectively. Plasma PlGF was also measured in a phase I/II trial of bevacizumab in metastatic breast cancer (AVF0776). The association between PlGF and MNRP1685A dose was described by a sigmoid E(max) model. cNRP1 and MNRP1685A PK profiles were described using a two-target quasi-steady state (QSS) model. RESULTS: A dose- and time-dependent increase in plasma PlGF and cNRP1 was observed in all patients treated with MNRP1685A. PK/PD analysis showed that bevacizumab and MNRP1685A had an additive effect in elevating PlGF. Predictions based on the two-target QSS model showed that the free drug concentration to maintain greater than 90% saturation of membrane NRP1 (mNRP1) and cNRP1 is about 8 µg/mL. CONCLUSION: These data show that MNRP1685A inhibits the VEGF pathway in humans as assessed by an increase in plasma PlGF. MNRP1685A seems to enhance bevacizumab-mediated VEGF pathway blockade, as showed by an increase in the magnitude of PlGF elevation when combined with bevacizumab. PK/PD analysis of biomarkers in the phase I population allowed identification of doses at which apparent maximal pathway modulation was observed.

Mechanism for release of alkaline phosphatase caused by glycosylphosphatidylinositol deficiency in patients with hyperphosphatasia mental retardation syndrome.

Hyperphosphatasia mental retardation syndrome (HPMR), an autosomal recessive disease characterized by mental retardation and elevated serum alkaline phosphatase (ALP) levels, is caused by mutations in the coding region of the phosphatidylinositol glycan anchor biosynthesis, class V (PIGV) gene, the product of which is a mannosyltransferase essential for glycosylphosphatidylinositol (GPI) biosynthesis. Mutations found in four families caused amino acid substitutions A341E, A341V, Q256K, and H385P, which drastically decreased expression of the PIGV protein. Hyperphosphatasia resulted from secretion of ALP, a GPI-anchored protein normally expressed on the cell surface, into serum due to PIGV deficiency. In contrast, a previously reported PIGM deficiency, in which there is a defect in the transfer of the first mannose, does not result in hyperphosphatasia. To provide insights into the mechanism of ALP secretion in HPMR patients, we took advantage of CHO cell mutants that are defective in various steps of GPI biosynthesis. Secretion of ALP requires GPI transamidase, which in normal cells, cleaves the C-terminal GPI attachment signal peptide and replaces it with GPI. The GPI-anchored protein was secreted substantially into medium from PIGV-, PIGB-, and PIGF-deficient CHO cells, in which incomplete GPI bearing mannose was accumulated. In contrast, ALP was degraded in PIGL-, DPM2-, or PIGX-deficient CHO cells, in which incomplete shorter GPIs that lacked mannose were accumulated. Our results suggest that GPI transamidase recognizes incomplete GPI bearing mannose and cleaves a hydrophobic signal peptide, resulting in secretion of soluble ALP. These results explain the molecular mechanism of hyperphosphatasia in HPMR.

Expression of CD55 and CD59 on peripheral blood cells from systemic lupus erythematosus (SLE) patients.

CD55 and CD59 are glycosylphosphatidylinositol-anchored proteins with complement inhibitory properties. CD55 inhibits the formation of C3 convertases, and CD59 prevents the terminal polymerisation of the membrane attack complex. It has been reported that SLE patients seems to have an acquired deficiency of these proteins associated with secondary autoimmune haemolytic anaemia and lymphopenia. The aim of this study was to evaluate the presence of altered CD55 and CD59 expression on peripheral blood cells from SLE patients. Flow cytometric analyses were performed on red and white blood cells from 23 SLE patients and 23 healthy controls. We observed more CD55- and CD59-lymphocytes (p=0.005 and p=0.019, respectively), and CD59-granulocytes (p=0.045) in SLE patients than in controls. These results suggest there is an altered pattern of CD55 and CD59 expression on the peripheral blood cells of SLE patients, and it may play a role in the cytopenias in these patients.

Origin and fate of blood cells deficient in glycosylphosphatidylinositol-anchored protein among patients with bone marrow failure.

Peripheral blood from 489 recently diagnosed patients with aplastic anaemia (AA) and 316 with refractory anaemia (RA) of myelodysplastic syndrome was evaluated to characterize CD55(-)CD59(-) [paroxysmal nocturnal haemoglobinuria (PNH)]-type blood cells associated with bone marrow (BM) failure. PNH-type cells were detected in 57% and 20% of patients with AA and RA, respectively. The percentages of PNH-type granulocytes ranged from 0.003% to 94.2% and the distribution was log-normal with a median of 0.178%. Serial analyses of 75 patients with PNH-type cells over 5 years revealed that the percentage of PNH-type cells constantly increased in 13 (17%), persisted in 44 (59%), disappeared in the remaining 18 (24%) although even in the 'Disappearance' group, PNH-type granulocytes persisted for at least 6 months. A scattergram profile of PNH-type cells unique to each patient persisted regardless of the response to immunosuppressive therapy and only single PIGA mutations were detected in PNH-type granulocytes sorted from four patients. These findings suggest that the PNH-type cells in patients with BM failure are derived from single PIGA mutant haematopoietic stem cells even when their percentages are <1% and their fate depends on the proliferation and self-maintenance properties of the individual PIGA mutants.

C3b deposition on human erythrocytes induces the formation of a membrane skeleton-linked protein complex.

Decay-accelerating factor (DAF, also known as CD55), a glycosylphosphatidylinositol-linked (GPI-linked) plasma membrane protein, protects autologous cells from complement-mediated damage by inhibiting complement component 3 (C3) activation. An important physical property of GPI-anchored complement regulatory proteins such as DAF is their ability to translate laterally in the plasma membrane. Here, we used single-particle tracking and tether-pulling experiments to measure DAF lateral diffusion, lateral confinement, and membrane skeletal associations in human erythrocyte membranes. In native membranes, most DAF molecules exhibited Brownian lateral diffusion. Fluid-phase complement activation caused deposition of C3b, one of the products of C3 cleavage, onto erythrocyte glycophorin A (GPA). We then determined that DAF, C3b, GPA, and band 3 molecules were laterally immobilized in the membranes of complement-treated cells, and GPA was physically associated with the membrane skeleton. Mass spectrometry analysis further showed that band 3, alpha-spectrin, beta-spectrin, and ankyrin were present in a complex with C3b and GPA in complement-treated cells. C3b deposition was also associated with a substantial increase in erythrocyte membrane stiffness and/or viscosity. We therefore suggest that complement activation stimulates the formation of a membrane skeleton-linked DAF-C3b-GPA-band 3 complex on the erythrocyte surface. This complex may promote the removal of senescent erythrocytes from the circulation.

Streptococcus agalactiae CAMP factor binds to GPI-anchored proteins.

CAMP factor (protein B) is a pore-forming protein secreted by Streptococcus agalactiae. It causes lysis of sheep red blood cells when these have been sensitized with staphylococcal sphingomyelinase. We here show that CAMP factor binds to GPI-anchored proteins, and that this interaction involves the carbohydrate core of the GPI-anchor. Enzymatic cleavage of GPI-anchors with phosphatidylinositol-specific phospholipase C strongly reduces the sensitivity of erythrocytes to CAMP factor. Incorporation of alkaline phosphatase, a model GPI-anchored protein, into liposome membranes renders the latter susceptible to permeabilization by CAMP factor. GPI-anchored proteins therefore function as cellular receptors for CAMP factor.

Normal patterns of expression of glycosylphosphatidylinositol-anchored proteins on different subsets of peripheral blood cells: a frame of reference for the diagnosis of paroxysmal nocturnal hemoglobinuria.

BACKGROUND: Evaluation of the expression of glycosylphosphatidylinositol-anchored membrane proteins (GPI-AP) is currently used for the diagnosis of paroxysmal nocturnal hemoglobinuria (PNH). In this study, we analyzed the amount of expression of a wide variety of GPI-AP in different subsets of hematopoietic cells present in normal peripheral blood (PB), to establish their normal patterns of expression and provide a frame of reference for the definition of the best combination of GPI-AP and PB cell subsets to be applied in the diagnosis and monitoring of PNH. RESULTS: Our results show variable patterns of expression of different GPI-AP in distinct subsets of normal PB cells. Combined use of CD55 and CD59 represented the most useful dual-marker combination; however, its utility remained suboptimal for several subsets of leukocytes and for platelets. CONCLUSIONS: For some cell subsets such as the neutrophils additional useful markers could be selected from a relatively broad panel (CD16/CD24/CD55/CD59/CD66b/CD157), whereas for other cell subsets the number of useful antigens was either restricted (monocytes: CD14/CD55/CD157; B cells: CD24/CD48/CD52/CD55; CD4+ T cells: CD48/CD52/CD55; eosinophils: CD55/CD59; CD8+ T cells: CD48/CD55) or limited to a single marker (CD48 on CD56low NK cells, CD55 on BDCA3- dendritic cells and CD56high NK cells, and CD59 for red cells), from all antigens analyzed.

Accelerated telomere shortening in glycosylphosphatidylinositol (GPI)-negative compared with GPI-positive granulocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH) detected by proaerolysin flow-FISH.

Telomere length has been linked to disease stage and degree of (pan-)cytopenia in patients with bone marrow failure syndromes. The aim of the current study was to analyze the impact of replicative stress on telomere length in residual glycosylphosphatidylinositol-positive (GPI+) versus GPI- hematopoiesis in patients with paroxysmal nocturnal hemoglobinuria (PNH). Peripheral blood granulocytes from 16 patients and 22 healthy individuals were analyzed. For this purpose, we developed proaerolysin flow-FISH, a novel methodology that combines proaerolysin staining (for GPI expression) with flow-FISH (for telomere length measurement). We found significantly shortened telomeres in GPI- granulocytes (mean +/- SE: 6.26 +/- 0.27 telomere fluorescence units [TFU]), both compared with their GPI+ counterparts (6.88 +/- 0.38 TFU; P = .03) as well as with age-matched healthy individuals (7.73 +/- 0.23 TFU; P < .001). Our findings are in support of a selective growth advantage model of PNH assuming that damage to the GPI+ hematopoietic stem-cell (HSC) compartment leads to compensatory hyperproliferation of residual GPI- HSCs.

G-CSF induced progenitor mobilization in mice with PIGA- blood cells.

OBJECTIVE: In patients with paroxysmal nocturnal hemoglobinuria (PNH) a proportion of blood cells are deficient in glycosyl phosphatidylinositol (GPI) anchored proteins due to a mutation in the PIGA gene. Previous studies showed that in PNH the majority of circulating early progenitor cells were normal but after G-CSF were mainly, of the PNH phenotype. This suggested that GPI-linked proteins contribute to the regulation of progenitor trafficking from bone marrow to peripheral blood. METHODS: To test this hypothesis we studied progenitor cells in bone marrow, spleen, and peripheral blood in response to G-CSF in mice genetically engineered to have a proportion of blood cells deficient in GPI-linked proteins (LF mice). RESULTS: In contrast to humans, LF and wild-type mice have comparable numbers of progenitor cells in bone marrow, spleen, and peripheral blood. Similarly, in LF mice the proportion of PIGA- progenitor cells in peripheral blood corresponds the proportion of PIGA- progenitor cells measured in bone marrow and spleen. After G-CSF the number of circulating progenitors significantly increased but the proportion of PIGA- cells remained the same in peripheral blood,bone marrow, and spleen. CONCLUSIONS: Our data indicate that under basal laboratory conditions the lack of GPI-linked protein does not cause a retention of progenitor cells in the bone marrow. This implies that the preferential circulation of normal progenitor cells in patients with PNH requires an additional component that most likely is provided by the altered microenvironment of the underlying bone marrow failure.

[Expression of three kinds of GPI-anchor proteins in paroxysmal nocturnal hemoglobinuria, aplastic anemia and myelodysplastic syndromes patients and their clinical implications].

OBJECTIVE: To investigate the expressions of three kinds of glycosyl-phosphatidylinositol anchor proteins (GPI-AP), the CD(55), CD(59) and CD(87) on the peripheral granulocytes and the soluble u-PAR (su-PAR) level in serum from patients with paroxysmal nocturnal hemoglobinuria (PNH), aplastic anemia (AA), and myelodysplastic syndromes (MDS), and to analyse their clinical implications. METHODS: Twenty-two PNH patients, including 4 complicated with thrombotic diseases and 5 with AA-PNH syndrome, 30 AA patients, including 9 being severe AA (SAA) and 11 chronic AA (CAA), 27 MDS-RA patients, and 20 healthy individuals were tested. The expressions of CD(55), CD(59) and CD(87) on peripheral granulocytes were analyzed with flow cytometry. Serum su-PAR was assayed by ELISA. RESULTS: The CD(55)(+), CD(59)(+) and CD(87)(+) granulocytes in peripheral blood of 20 normal controls were all more than 90%. The expressions of three kinds of GPI-APs in 22 PNH patients were significantly decreased as compared with that in normal controls, AA patients and MDS-RA patients. The serum level of su-PAR in PNH group was higher than that of the other three groups. The expression of CD(87) was significantly decreased in thrombotic PNH patients as compared with that in non-thrombotic PNH patients. The expression of CD(87) was significantly decreased in AA patients than in normal controls. The expressions of three kinds of GPI-APs in 5 AA-PNH syndrome patients were remarkably reduced as compared with AA patients, but no significant difference was found for these indexes between AA-PNH syndrome and PNH patients and between 27 MDS-RA patients and 20 normal controls. CONCLUSION: Measurement of CD(55), CD(59) and CD(87) expressions levels on the peripheral granulocytes and su-PAR in serum would be alternative approaches for diagnosis and differential diagnosis of PNH. Serum level of su-PAR may be helpful to monitor thrombosis in PNH patients.

[Spontaneous recovery from pancytopenia in a young female patient with paroxysmal nocturnal hemoglobinuria(PNH): changes in the GPI-anchor expression on peripheral blood cells].

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic disorder, caused by impaired cell surface expression of GPI-anchors in hematopoietic cells as a result of somatic mutation in the PIG-A gene, and often progresses into bone marrow aplasia. We experienced a girl diagnosed as having PNH with spontaneous recovery from pancytopenia, and analyzed the GPI-anchor expression on peripheral blood cells. Thrombocytopenia was first determined when she was 5-years old, and Ham and sugar water tests were negative at the age of 6 years. Subsequently, the pancytopenia slowly progressed, and the diagnosis of PNH was made at the age of 8 years based on the positive Ham test. Frame-shift of the PIG-A gene in exon 2 was confirmed in the peripheral granulocytes at the age of 11 years. Pancytopenia spontaneously subsided over two years after diagnosis, and an almost normal hematogram except for mild thrombocytopenia has been maintained for the subsequent 4 years. In periodical flow cytometric analysis of the CD55 expression on granulocytes and erythrocytes and the CD48 expression on lymphocytes, the population of the PNH clone was almost unchanged during the first two years of spontaneous recovery, but that of CD55-negative erythrocytes gradually decreased, suggesting that regression of PNH clone might be unnecessary in transient improvement of pancytopenia.

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.