Screening of subclinical functional hemoglobin and red blood cell abnormalities among blood donors of Fayoum University Hospital in Egypt: Are RET-He, and IRF useful screening tools?

BACKGROUND: The effectiveness of red cell transfusion in a given blood unit that relied on both quantity and quality of donated cells undoubtedly affects prognostic outcomes. OBJECTIVE: We aimed to determine the frequency of subclinical functional hemoglobin and red cell abnormalities in donated blood of Fayoum University Hospital in Egypt. Additionally, to assess the usefulness of reticulocyte mean hemoglobin content (RET-He) and immature reticulocyte fraction (IRF) as screening measures for such abnormalities. MATERIAL AND METHODS: This cross-sectional study enrolled 200 volunteer blood donors who met the national standard criterion of blood donation. Complete blood count with reticulocyte parameters, serum ferritin, sickling test, G6PD assay, Mentzer index, and naked-eye single tube red cell osmotic fragility test were carried out. RESULTS: Functional red cell abnormalities represented 44 % of this cohort. Out of them, 4.5 % had iron deficiency, 11 % had a positive sickling test, 19 % had G6PD deficiency, and 9.5 % had suspicious thalassemia. The sensitivity and specificity test for RET-He in selective identification of functional hemoglobin abnormalities in donated blood were 83.3 % and 61.2 %, respectively at a cutoff value of 26.9. Though there was no statistically significant effect of RET-He on the selective detection of G6PD deficiency, IRF had a statistically significant high level with a p-value of 0.04. CONCLUSION: Subclinical functional red cell abnormalities seem to be prevalent among blood donors. Reticulocyte/ erythrocyte indices could be useful screening tools for red cell abnormalities. Further studies are required for assessing the impact of transfusing such abnormalities to neonates and other critical recipients.

Hereditary spherocytosis is associated with decreased pyruvate kinase activity due to impaired structural integrity of the red blood cell membrane.

Hereditary spherocytosis (HS) is characterised by increased osmotic fragility and enhanced membrane loss of red blood cells (RBC) due to defective membrane protein complexes. In our diagnostic laboratory, we observed that pyruvate kinase (PK) activity in HS was merely slightly elevated with respect to the amount of reticulocytosis. In order to evaluate whether impaired PK activity is a feature of HS, we retrospectively analysed laboratory data sets from 172 unrelated patients with HS, hereditary elliptocytosis (HE), glucose-6-phosphate dehydrogenase (G6PD) or PK deficiency, sickle cell or haemoglobin C disease, or ß-thalassaemia minor. Results from linear regression analysis provided proof that PK activity decreases with rising reticulocyte counts in HS (R2  = 0·15; slope = 9·09) and, less significantly, in HE (R2  = 0·021; slope = 8·92) when compared with other haemolytic disorders (R2  ≥ 0·65; slopes ≥ 78·6). Reticulocyte-adjusted erythrocyte PK activity levels were significantly lower in HS and even declined with increasing reticulocytes (R2  = 0·48; slope = -9·74). In this report, we describe a novel association between HS and decreased PK activity that is apparently caused by loss of membrane-bound PK due to impaired structural integrity of the RBC membrane and may aggravate severity of haemolysis in HS.

Hemolytic disease of the fetus and newborn due to alloanti-M: three Chinese case reports and a review of the literature.

BACKGROUND: Alloanti-M was once regarded as not clinically significant, with a few exceptions in extremely rare cases. However, an increasing number of cases of severe hemolytic disease of the fetus and newborn (HDFN), resulting in fetal hydrops and recurrent abortion caused by alloanti-M, have been reported mainly in the Asian population. STUDY DESIGN AND METHODS: Three pregnant Chinese women with a history of abnormal pregnancy with hydrops fetalis were encountered. During this pregnancy, a series of clinical examinations and an alloantibody identification against RBCs and platelets were conducted. Intrauterine transfusion and postnatal transfusion were then performed in the fetuses. In addition, the HDFN cases caused by alloanti-M reported in different ethnic groups as well as their clinical and serologic features are also summarized. RESULTS: Three pregnant women were identified with an M-N+ phenotype and IgM mixed with IgG alloanti-M in serum. Their fetuses were found by ultrasound examination and cord blood testing to have severe anemia. Additionally, an M+N+ phenotype and IgG alloanti-M were detected in the cord blood of the three fetuses with titers ranging from 1:1 to 1:128. Moreover, low reticulocyte counts and negative direct antiglobulin tests were also shown in two of the fetuses. After receiving intrauterine transfusions and postnatal transfusions several times, these three fetuses eventually survived and then healthfully developed in the follow-up tracking. CONCLUSION: Alloanti-M immunization can cause severe HDFN with hyporegenerative anemia, often seen in the Asian population, and suppression of erythropoiesis might account for it.

Blood cell parameters for screening and diagnosis of hereditary spherocytosis.

BACKGROUND: There is currently no single index for the diagnostic screening of hereditary spherocytosis (HS). However, hematology analyzers are widely used in hospital laboratories because of their highly automated performance and quality control procedure, and detection of some blood cell parameters may be useful for the early screening of HS. METHODS: We investigated the values of blood cell parameters for the screening and differential diagnosis of HS. We performed a descriptive study of 482 samples (67 cases of HS, 59 cases of G6PD deficiency, 57 cases of AIHA, 199 cases of thalassemia, and 100 cases of healthy controls) that were run on Beckman Coulter LH780 Hematology Analyzer. RESULTS: HS was characterized by increased MCHC, decreased MRV, MSCV-MCV < 0, and increased Ret with no concomitant increase in IRF. The areas under the ROC curves were MSCV-MCV (0.97; 95% CI 0.95-1.0) > MRV (0.94; 95% CI 0.91-0.97) > MCHC (0.92; 95% CI 0.88-0.97) > Ret/IRF (0.77; 95% CI 0.7-0.84). MSCV-MCV ≤ 0.6 fl was valuable parameter for the diagnostic screening of HS, with a sensitivity of 95.5% and specificity of 94.9%. CONCLUSION: These indices have high reference values for differentiating HS from thalassemia, AIHA, and G6PD deficiency.

The reason sickle reticulocytes expose PS.

In this issue of Blood, Mankelow et al link phosphatidylserine (PS) exposure in sickle erythrocytes to a physiological event in reticulocyte maturation. This discovery has implications for efforts to prevent thrombosis in sickle cell disease (SCD).

Autophagic vesicles on mature human reticulocytes explain phosphatidylserine-positive red cells in sickle cell disease.

During maturation to an erythrocyte, a reticulocyte must eliminate any residual organelles and reduce its surface area and volume. Here we show this involves a novel process whereby large, intact, inside-out phosphatidylserine (PS)-exposed autophagic vesicles are extruded. Cell surface PS is a well-characterized apoptotic signal initiating phagocytosis. In peripheral blood from patients after splenectomy or in patients with sickle cell disease (SCD), the number of circulating red cells exposing PS on their surface is elevated. We show that in these patients PS is present on the cell surface of red cells in large (∼1.4 µm) discrete areas corresponding to autophagic vesicles. The autophagic vesicles found on reticulocytes are identical to those observed on red cells from splenectomized individuals and patients with SCD. Our data suggest the increased thrombotic risk associated with splenectomy, and patients with hemoglobinopathies is a possible consequence of increased levels of circulating mature reticulocytes expressing inside-out PS-exposed autophagic vesicles because of asplenia.

Novel hematological parameters for the evaluation of patients with myeloproliferative neoplasms: the immature platelet and reticulocyte fractions.

New automated hematology analyzers have led to the availability of novel hematological parameters, including the immature platelet fraction (IPF) and the immature reticulocyte fraction (IRF), both of potential interest in patients with myeloproliferative neoplasms (MPNs). We performed a prospective analysis of 217 patients with MPN, including 32 (15%) with essential thrombocythemia (ET), 43 (20%) with polycythemia vera (PV), and 142 (65%) with myelofibrosis (MF); the IPF and IRF were measured by the Sysmex XN analyzer. As compared to patients with ET, both a higher IPF and IRF were observed among patients with PV and MF. Factors associated with high IPF among patients with PV/ET were male sex, thrombocytopenia, and diagnosis of PV; among patients with MF, they were elevated peripheral blasts, low platelet count, JAK2 V617F mutation, and previous therapy. Factors associated with high IRF among patients with PV/ET were low hemoglobin, high reticulocyte count, and PV diagnosis; among patients with MF, they were peripheral blasts and elevated reticulocytes. The IPF and IRF represent novel parameters in patients with MPN with potential relevant clinical implications. Comparison with healthy subjects and those with secondary polycythemia is needed to confirm our preliminary findings.

Residual pyruvate kinase activity in PKLR-deficient erythroid precursors of a patient suffering from severe haemolytic anaemia.

OBJECTIVE: Here, we present a 7-year-old patient suffering from severe haemolytic anaemia. The most common cause of chronic hereditary non-spherocytic haemolytic anaemia is red blood cell pyruvate kinase (PK-R) deficiency. Because red blood cells rely solely on glycolysis to generate ATP, PK-R deficiency can severely impact energy supply and cause reduction in red blood cell lifespan. We determined the underlying cause of the anaemia and investigated how erythroid precursors in the patient survive. METHODS: PK activity assays, Western blot and Sanger sequencing were employed to determine the underlying cause of the anaemia. Patient erythroblasts were cultured and reticulocytes were isolated to determine PK-R and PKM2 contribution to glycolytic activity during erythrocyte development. RESULTS: We found a novel homozygous mutation (c.583G>A) in the PK-R coding gene (PKLR). Although this mutation did not influence PKLR mRNA production, no PK-R protein could be detected in the red blood cells nor in its precursors. In spite of the absence of PK-R, the reticulocytes of the patient exhibited 20% PK activity compared with control. Western blotting revealed that patient erythroid precursors, like controls, express residual PKM2. CONCLUSIONS: We conclude that PKM2 rescues glycolysis in PK-R-deficient erythroid precursors.

Dose-response relationship of temozolomide, determined by the Pig-a, comet, and micronucleus assay.

Temozolomide (TMZ), a monofunctional alkylating agent, was selected as a model compound to determine its quantitative genotoxic dose-response relationship in different tissues (blood, liver, and jejunum) and endpoints [Pig-a-, comet-, and micronucleus assay (MNT)] in male rats. TMZ was administered p.o. over 5 consecutive days (day 1-5), followed by a treatment-free period of 50 days (day 6-56) and a final administration prior to necropsy (day 57-59). TMZ showed a dose-dependent increase in DNA damage in all interrogated endpoints. A statistically significant increase in Pig-a mutant phenotypes was observed on day 44 starting at 7.5 mg/kg/day for mutant reticulocytes (for RETCD59-) and at 3.75 mg/kg/day for mutant red blood cells (RBCCD59-), respectively. In addition, a statistically significant increase in cytogenetic damage, as measured by micronucleated reticulocytes, was observed starting at 3.75 mg/kg/day on day 3 and 1.5 mg/kg/day on day 59. DNA strand breaks, as detected by the comet assay, showed a dose-dependent and statistically significant increase in liver, blood, and jejunum starting at doses of 3.75, 3.75, and 7.5 mg/kg/day, respectively. The dose-response relationships of the Pig-a, MNT, and comet data were analyzed for possible points of departure (PoD) using the benchmark-dose (BMD) software PROAST with different critical effect sizes (CES) (BMD0.1, BMD0.5, BMD1, and BMD1SD). Overall, PoD values show a high concordance between different tissues and endpoints, underlining the suitability of this experimental design to explore quantitative dose-response relationships in a variety of different tissues and endpoints, while minimizing animal use.

VLA-4 blockade by natalizumab inhibits sickle reticulocyte and leucocyte adhesion during simulated blood flow.

Very Late Antigen-4 (VLA-4, α4ß1-integrin, ITGA4) orchestrates cell-cell and cell-endothelium adhesion. Given the proposed role of VLA-4 in sickle cell disease (SCD) pathophysiology, we evaluated the ability of the VLA-4 blocking antibody natalizumab to inhibit SCD blood cell adhesion. Natalizumab recognized surface VLA-4 on leucocytes and reticulocytes in whole blood from SCD subjects. SCD reticulocytes were positive for VLA-4, while VLA-4 staining of non-SCD reticulocytes was undetectable. Titrations with natalizumab revealed the presence of saturable levels of VLA-4 on both SCD reticulocytes and leucocytes similar to healthy subject leucocytes. Under physiological flow conditions, the adhesion of SCD whole blood cells and isolated SCD leucocytes to immobilized vascular cell adhesion molecule 1 (VCAM-1) was blocked by natalizumab in a dose-dependent manner, which correlated with cell surface receptor binding. Natalizumab also inhibited >50% of whole blood cell binding to TNF-α activated human umbilical vein endothelial cell monolayers under physiological flow at clinically relevant concentrations (10 to 100 µg/ml). This indicates that VLA-4 is the dominant receptor that drives SCD reticulocyte and mononuclear cell adhesion to VCAM-1 and that the VLA-4 adhesion to VCAM-1 is a significant contributor to SCD blood cell adhesion to endothelium. Thus, VLA-4 blockade may be beneficial in sickle cell disease.

Mean reticulocyte volume: a specific parameter to screen for hereditary spherocytosis.

This study assessed the value of mean reticulocyte volume (MRV) for differential diagnosis of hereditary spherocytosis (HS) so as to develop conventional and new specific screen indexes. Subjects in this study were divided into three groups: 53 cases in HS group, 217 cases in hemolytic anemia control group (109 cases of thalassemia (THAL), 56 cases of glucose-6-phosphate dehydrogenase G6PD deficiency anemia, and 52 cases of autoimmune hemolytic anemia (AIHA)), and 100 cases in healthy control group. We analyzed erythrocyte and reticulocyte parameters including MRV, mean sphered corpuscular volume, mean corpuscular hemoglobin concentration, and immature reticulocyte fraction. Results demonstrated that MRV was significantly lower in the HS group but significantly higher in the AIHA and G6PD deficiency anemia groups than that in the healthy control group (P = 0.000). MRV was not significantly different between the AIHA and G6PD deficiency anemia groups (P = 0.977) and between the healthy control and THAL groups (P = 0.168). The area under the ROC curve of MRV for diagnosis of HS was 0.942, with a standard error of 0.019, 95% confidence interval of 0.905-0.979, and optimal critical diagnosis point of 95.77 fL. When the MRV was ≤95.77 fL, the sensitivity and specificity for diagnosis of HS were 86.80% and 91.20%, respectively. Therefore, MRV is a general and specific new index for screening HS and important for differential diagnosis of different types of hemolytic anemia.

G6PD deficiency and absence of α-thalassemia increase the risk for cerebral vasculopathy in children with sickle cell anemia.

The aim of this study was to test the association between hematological/genetic factors and cerebral vasculopathy in children with sickle cell anemia (SCA). A group with cerebral vasculopathy (VASC) was composed of children who had stroke (n = 6), silent infarct (n = 11), or an abnormal transcranial Doppler (n = 5). Eighty-four patients had neither positive history of stroke or silent infarct, nor abnormal transcranial Doppler (NORM group). An intermediate group (COND; n = 15) was composed of SCA children with a conditional transcranial Doppler. Biological analyses were performed on samples obtained at steady state and before the beginning of any chronic treatment. The comparisons of the three groups demonstrated a protective effect of α-thalassemia against cerebral vasculopathy through its effects on hemoglobin and reticulocyte levels. Moreover, we observed higher frequency of G6PD deficiency in the VASC group compared with the other groups. Our study confirms the key role of α-thalassemia and G6PD status in the pathophysiology of cerebral vasculopathy in SCA children.

Stress reticulocytes lose transferrin receptors by an extrinsic process involving spleen and macrophages.

As they mature into erythrocytes during normal erythropoiesis, reticulocytes lose surface transferrin receptors before or concurrently with reticulin. Exosome release accounts for most of the loss of transferrin receptors from reticulocytes. During erythropoietic stress, reticulocytes are released early from hematopoietic tissues and have increased reticulin staining and transferrin receptors. Flow cytometry of dually stained erythrocytes of mice recovering from phlebotomy demonstrated delayed loss of reticulin and transferrin receptors during in vitro maturation compared to in vivo maturation, indicating that an in vivo process extrinsic to the reticulocytes facilitates their maturation. Splenectomy or macrophage depletion by liposomal clodronate inhibited in vivo maturation of reticulocytes and increased the numbers of reticulin-negative, transferrin receptor-positive cells during and after recovery from phlebotomy. This reticulin-negative, transferrin receptor-positive population was rarely found in normal mice. Transmission electron microscopy demonstrated that the reticulin-negative, transferrin receptor-positive cells were elongated and discoid erythrocytes, but they had intracellular and surface structures that appeared to be partially degraded organelles. The results indicate that maturation of circulating stress reticulocytes is enhanced by an extrinsic process that occurs in the spleen and involves macrophage activity. Complete loss of reticulin with incomplete loss of surface transferrin receptors in this process produces a reticulin-negative, transferrin receptor-positive erythrocyte population that has potential utility for detecting prior erythropoietic stresses including bleeding, hemolysis and erythropoietin administration, even after recovery has been completed. Am. J. Hematol. 91:875-882, 2016. © 2016 Wiley Periodicals, Inc.

In vivo micronucleus studies with 6 titanium dioxide materials (3 pigment-grade & 3 nanoscale) in orally-exposed rats.

Six pigment-grade (pg) or ultrafine (uf)/nanoscale (anatase and/or rutile) titanium dioxide (TiO2) particulates were evaluated for in vivo genotoxicity (OECD 474 Guidelines) in male and female rats by two different laboratories. All test materials were robustly characterized. The BET surface areas of the pg and uf samples ranged from 7 to 17 m(2)/g and 50 to 82 m(2)/g respectively. The materials were assessed for induction of micronuclei and toxicity in bone marrow by analyzing peripheral blood reticulocytes (RETs) by flow cytometry. Single oral gavage doses of 500, 1000 or 2000 mg/kg body weight (bw) of each material were implemented with concurrent negative (water) and positive controls (cyclophosphamide). Approximately 48 and 72 h after exposure, blood samples were collected and 20,000 RETs per animal were analyzed. For each of the six tests, there were no biologically or toxicologically relevant increases in the micronucleated RET frequency in any TiO2 exposed group at either time point at any dose level. In addition, there were a lack of biologically relevant decreases in %RETs among total erythrocytes. All six TiO2 test substances were negative for in vivo genotoxicity effects; however, it is noted that the exposure to target tissues was likely negligible. One pigment grade and one ultrafine material each were evaluated for potential systemic exposure/uptake from the gastrointestinal tract by analysis of TiO2 into blood and liver. No significant increases in TiO2 over controls were measured in blood (48 or 72 h) or liver (72 h) following exposures to 2000 mg/kg bw TiO2. These data indicate that there was no absorption of the test material from the gastrointestinal tract into the blood circulation and the lack of genotoxic effects is therefore attributed to a lack of exposure due to the inability of the test material to migrate from the gastrointestinal tract into the blood and then into target tissues.

Hydroxycarbamide decreases sickle reticulocyte adhesion to resting endothelium by inhibiting endothelial lutheran/basal cell adhesion molecule (Lu/BCAM) through phosphodiesterase 4A activation.

Vaso-occlusive crises are the main acute complication in sickle cell disease. They are initiated by abnormal adhesion of circulating blood cells to vascular endothelium of the microcirculation. Several interactions involving an intricate network of adhesion molecules have been described between sickle red blood cells and the endothelial vascular wall. We have shown previously that young sickle reticulocytes adhere to resting endothelial cells through the interaction of α4ß1 integrin with endothelial Lutheran/basal cell adhesion molecule (Lu/BCAM). In the present work, we investigated the functional impact of endothelial exposure to hydroxycarbamide (HC) on this interaction using transformed human bone marrow endothelial cells and primary human pulmonary microvascular endothelial cells. Adhesion of sickle reticulocytes to HC-treated endothelial cells was decreased despite the HC-derived increase of Lu/BCAM expression. This was associated with decreased phosphorylation of Lu/BCAM and up-regulation of the cAMP-specific phosphodiesterase 4A expression. Our study reveals a novel mechanism for HC in endothelial cells where it could modulate the function of membrane proteins through the regulation of phosphodiesterase expression and cAMP-dependent signaling pathways.

Elevated erythropoietin and cytokines levels are related to impaired reticulocyte maturation in thalassemic patients.

Serum EPO concentration is related primarily to the rate of erythrocyte production and, under the stimulation of hypoxia, increases exponentially as hemoglobin (Hb) decreased. The level of EPO was determined in 141 subjects including 43 normal, 44 thalassemic patients and 54 thalassemic trait subjects. The EPO level was significantly higher in the thalassemic patients (54.8mU/ml in HbH disease [α thal1/α thal2;], 78.1mU/ml in HbH with Hb CS [α thal 1/CS]; 95.6mU/ml in ß-thal/HbE splenectomized [BE(S)]; and 114.8mU/ml in ß-thal/HbE non-splenectomized [BE(NS)]as compared with 12.0mU/ml in normal subjects. No significant differences were detected in thalassemic trait subjects. In addition, the levels of EPO in thalassemic patients is correlated significantly with the number of reticulocytes and the reticulocyte fractions especially the fraction of immature reticulocytes. Interestingly, the highest level of EPO/% retic ratio as indicated for EPO non-responder was detected in BE(NS) patients. However, the impaired reticulocytes maturation was found to be related significantly with the levels of TNF-α,IFN-γ,IL-10, and VEGF. Since, TNF-α, IFN-γ, IL-10 and VEGF are reported as the cytokines with erythropoietic inhibitory mediators, the variation of these cytokines in thalassemic environments may be associated to the anemic crisis in these patients.

Discrimination of various thalassemia syndromes and iron deficiency and utilization of reticulocyte measurements in monitoring response to iron therapy.

Decreased hemoglobinization of red cells resulting in hypochromia and microcytosis are the main features of thalassemia syndromes, and also of iron deficiency anemia (IDA). A simple and reliable method is required to distinguish the two conditions in the routine laboratories. In this study we analyzed the red cell and reticulocyte parameters from 414 samples of various types of thalassemias and IDA and discovered a variety of discriminating criteria including a discrimination index (DI) which should be useful for differential diagnosis. Slightly decreased MCV and CH are suggestive of α-thalassemia 2, Hb CS, and Hb E heterozygotes whereas the increased Rbc counts are obvious in α-thalassemia 1 and ß-thalassemia. In Hb E, the number of microcytic red cells was greater than the number of hypochromic red cells resulting in an increased M/H ratio. Hb H diseases are characterized by a higher number of hypochromic red cells and decreased CHCM, while broadening of hemoglobin concentration histogram results in increased HDW in ß-thalassemia diseases. Iron deficiency anemia results in hypochromic-microcytic red cells and increased RDW. The number of reticulocyte with %High Retic and CHr value were increased in the first month of iron supplementation indicating the response to iron therapy.

Severity of Brazilian sickle cell disease patients: severity scores and feasibility of the Bayesian network model use.

The integration of the several clinical and laboratory dimensions and the influence of each parameter on the sickle cell disease (SCD)-related mortality is useful for predicting the phenotype of an individual. This study evaluated the feasibility of the SCD severity calculator use to measure disease severity in Brazilian patients. The study group was composed of 500 SCD patients (440 HbSS and 60 HbSC) diagnosed by molecular biology. We observed a decrease in severity scores in 72 SCD patients assessed before and after the hydroxyurea (HU) use. Furthermore, the HU influenced the increase of mean corpuscular volume (MCV) and HbF concentration, and the decrease of leukocytes and total bilirubin. We found 180 (36.0%) patients with intermediate phenotype, 170 (34.0%) mild phenotype and 150 (30.0%) with severe phenotype. Patients with ages >40 years had higher mean score (0.778±0.177) than patients between 18 and 40 years (0.562±0.152) and patients between 5 and 17 years (0.322±0.145). We observe that there is a tendency of individuals with leg ulcers, avascular necrosis and cardiac complications with increasing age. Correlation analysis showed relations between severity scores with leukocytes, reticulocytes, bilirubin, lactate dehydrogenase, HbS, hemoglobin and hematocrit (p<0.05). Several comparisons involving age groups, SCD genotype and phenotypic classification had satisfactory results and this classification will be used for future studies involving genetic polymorphisms, response to treatment with HU and oxidative stress markers in SCD.

Rat Pig-a mutation assay responds to the genotoxic carcinogen ethyl carbamate but not the non-genotoxic carcinogen methyl carbamate.

Determination of the mode of action of carcinogenic agents is an important factor in risk assessment and regulatory practice. To assess the ability of the erythrocyte-based Pig-a mutation assay to discriminate between genotoxic and non-genotoxic modes of action, the mutagenic response of Sprague Dawley rats exposed to methyl carbamate (MC) or ethyl carbamate (EC) was investigated. EC, a potent carcinogen, is believed to induce DNA damage through the formation of a DNA-reactive epoxide group, whereas the closely structurally related compound, MC, cannot form this epoxide and its weaker carcinogenic activity is thought to be secondary to inflammation and promotion of cell proliferation. The frequency of Pig-a mutant phenotype cells was monitored before, during, and after 28 consecutive days of oral gavage exposure to either MC (doses ranging from 125 to 500 mg/kg/day) or EC (250 mg/kg/day). Significant increases in the frequency of mutant reticulocytes were observed from Days 15 through 43, with a peak mean frequency of 19.9×10(-6) on Day 29 (i.e. 24.9-fold increase relative to mean vehicle control across all four sampling times). As expected, mutant erythrocyte responses lagged behind mutant reticulocyte responses, with a maximal mean frequency of 8.2×10(-6) on Day 43 (i.e. 16.4-fold increase). No mutagenic effects were observed with MC. A second indicator of in vivo genotoxicity, peripheral blood micronucleated reticulocytes, was also studied. This endpoint was responsive to EC (3.3-fold mean increase), but not to MC. These results support the hypothesis that genotoxicity contributes to the carcinogenicity of EC but not of MC, and illustrates the value of the Pig-a assay for discriminating between genotoxic and non-genotoxic modes of action.