[In vitro models for the evaluation of the neurotoxicity of methylmercury. Current state of knowledge]

BACKGROUND

In adults, MeHg poisoning is characterized by damage to discrete anatomical areas of the brain (visual cortex, loss of neurons from the granule layer of the cerebellum). However, the immature central nervous system (CNS), which is extremely sensitive to MeHg neurotoxicity, shows a diffuse and widespread damage disorganization of cerebral cortex cytoarchitecture, disappearance of granule cells with narrowing of the molecular layer. While adverse effects have been unequivocally demonstrated in poisoning incidents in humans (visual abnormalities, sensory impairment of the extremities, cerebellar ataxia, hearing loss, muscle weakness, tremor and mental deterioration), the implications of lower level exposures, such as those occurring in fish-eating populations, are still controversial. The high affinity of MeHg for thiol groups makes proteins and peptides bearing cysteines the predominant targets for structural and functional modification by MeHg in all subcellular compartments.

METHODS

The identification of MeHg cellular and sub-cellular targets in the CNS is complicated by the fact that it is difficult to observe the outcomes directly in vivo. In neurobiology, in vitro cell culture techniques have been successfully developed and employed to address specific questions of cell biology and nervous system functioning and provide a means to systematically study the complexity of cellular functions of the CNS elements. Moreover, they provide a convenient experimental tool for testing possible functions or postulates in vivo that otherwise might not be conducted.

RESULTS

Several mechanisms have been proposed as being implicated in the neurotoxic effects of MeHg. Examples of MeHg molecular effects which may be relevant to risk assessment are presented, including cell death mode, effects on microtubules, calcium signalling, oxidative stress, effects on neurotransmitter systems.

CONCLUSIONS

Molecular and cellular approaches permit exploration of early biological responses to chemical or physical agents and definition of the role of these early effects in altered cellular structure and function.

Time course assessment of methylmercury effects on C6 glioma cells: submicromolar concentrations induce oxidative DNA damage and apoptosis

Organic mercury is a well-known neurotoxicant although its mechanism of action has not been fully clarified. In addition to a direct effect on neurons, much experimental evidence supports an involvement of the glial component. We assessed methylmercury hydroxide (MeHgOH) toxicity in a glial model, C6 glioma cells, exposed in the 10(-5)-10(-8) M range. The time course of the effects was studied by time-lapse confocal microscopy and supplemented with biochemical data. We have monitored cell viability and proliferation rate, reactive oxygen species (ROS), mitochondrial transmembrane potential, DNA oxidation, energetic metabolism and modalities of cell death. The earliest effect was a measurable ROS generation followed by oxidative DNA damage paralleled by a partial mitochondrial depolarization. The effect on cell viability was dose dependent. TUNEL, caspase activity and real-time morphological observation of calcein-loaded cells showed that apoptosis was the only detectable mode of cell death within this concentration range. N-acetyl-cysteine (NAC) or reduced glutathione (GSH) completely prevent the apoptotic effect of MeHgOH. The lowest effective MeHgOH concentration was 10(-7) M for ROS and DNA OH-adducts generation. The effect of submicromolar concentrations of MeHgOH on C6 cells could be relevant in the developmental neurotoxicity caused by low dose, long-term exposures, such as those of food origin. In addition, we have shown that the same concentrations are effective in the induction of DNA oxidative damage, with further potential pathogenetic implications.

[The autonomic nervous system in myocardial infarction]

The occurrence of alterations in the autonomic control of cardiovascular function has long been known. Nevertheless, only in the last 15 years with the development and utilization of techniques such as heart rate variability and baroreflex analysis, unforeseeable amounts of information have been collected. The appraisal of alterations in the autonomic control mechanisms in the acute and post-acute phase of myocardial infarction has not only confirmed the presence of an increased sympathetic and of a reduced vagal modulation in most of post-myocardial infarction patients. It has also allowed the clinically relevant identification of those patients with an increased arrhythmic and total cardiac mortality. Most the high-risk patients are indeed characterized by a marked reduction in heart rate variability that can be detected using different methodologies. For example, with time-domain analysis of heart rate variability, it has been shown that an SDNN < 70 ms identifies patients at risk. By using spectral analysis, it has been possible to realize that the loss or marked attenuation of those rhythmical components that reflect autonomic modulation is also a characteristic of the high-risk patients. A decreased responsiveness of the sinus node to autonomic inputs may also explain the reduction in baroreflex sensitivity that characterizes patients with an increased arrhythmic mortality. In conclusion, the appraisal in each patient of the extent of the alterations in the autonomic control mechanism is nowadays a critical component of the clinical assessment not only for risk stratification but also in order to guide the prescription of new pharmacological and non-pharmacological therapies.

Autosomal dominant macrothrombocytopenia in Italy is most frequently a type of heterozygous Bernard-Soulier syndrome

A form of autosomal dominant macrothrombocytopenia is characterized by mild or no clinical symptoms, normal platelet function, and normal megakaryocyte count. Because this condition has so far received little attention, patients are subject to misdiagnosis and inappropriate therapy. To identify the molecular basis of this disease, 12 Italian families were studied by linkage analysis and mutation screening. Flow cytometry evaluations of platelet membrane glycoproteins (GPs) were also performed. Linkage analysis in 2 large families localized the gene to chromosome 17p, in an interval containing an excellent candidate, the GPIbalpha gene. GPIbalpha, together with other proteins, constitutes the plasma von Willebrand factor (vWF) receptor, which is altered in Bernard-Soulier syndrome (BSS). In 6 of 12 families, a heterozygous Ala156Val missense substitution was identified. Platelet membrane GP studies were performed in 10 patients. Eight were distinguished by a reduction of GPs comparable to that found in a BSS heterozygous condition, whereas the other 2, without the Ala156Val mutation, had a normal content of platelet GPs. In conclusion, the current study provides evidence that most (10 of 12) patients with an original diagnosis of autosomal dominant macrothrombocytopenia shared clinical and molecular features with the heterozygous BSS phenotype. The remaining 2 affected subjects represented patients with "true" autosomal dominant macrothrombocytopenia; the GPIb/IX/V complex was normally distributed on the surface of their platelets. Thus, the diagnosis of heterozygous BSS must always be suspected in patients with inherited thrombocytopenia and platelet macrocytosis.

Mutations in MYH9 result in the May-Hegglin anomaly, and Fechtner and Sebastian syndromes. The May-Heggllin/Fechtner Syndrome Consortium

The autosomal dominant, giant-platelet disorders, May-Hegglin anomaly (MHA; MIM 155100), Fechtner syndrome (FTNS; MIM 153640) and Sebastian syndrome (SBS), share the triad of thrombocytopenia, large platelets and characteristic leukocyte inclusions ('Döhle-like' bodies). MHA and SBS can be differentiated by subtle ultrastructural leukocyte inclusion features, whereas FTNS is distinguished by the additional Alport-like clinical features of sensorineural deafness, cataracts and nephritis. The similarities between these platelet disorders and our recent refinement of the MHA (ref. 6) and FTNS (ref. 7) disease loci to an overlapping region of 480 kb on chromosome 22 suggested that all three disorders are allelic. Among the identified candidate genes is the gene encoding nonmuscle myosin heavy chain 9 (MYH9; refs 8-10), which is expressed in platelets and upregulated during granulocyte differentiation. We identified six MYH9 mutations (one nonsense and five missense) in seven unrelated probands from MHA, SBS and FTNS families. On the basis of molecular modelling, the two mutations affecting the myosin head were predicted to impose electrostatic and conformational changes, whereas the truncating mutation deleted the unique carboxy-terminal tailpiece. The remaining missense mutations, all affecting highly conserved coiled-coil domain positions, imparted destabilizing electrostatic and polar changes. Thus, our results suggest that mutations in MYH9 result in three megakaryocyte/platelet/leukocyte syndromes and are important in the pathogenesis of sensorineural deafness, cataracts and nephritis.

Transfusion of platelet concentrates cryopreserved with ThromboSol plus low-dose dimethylsulphoxide in patients with severe thrombocytopenia: a pilot study

We have recently reported the possibility of supporting the phase of severe thrombocytopenia after high-dose chemotherapy (HDC) and stem cell transplantation using 5% dimethylsulphoxide (DMSO)-cryopreserved autologous platelet concentrates (PCs). The aim of the present study was to evaluate the therapeutic potential of ThromboSol (a recently developed platelet storage solution) plus PCs cryopreserved in 2% DMSO in patients undergoing myeloablative chemotherapy and autologous transplantation. PCs were collected from 14 women with breast cancer by a single plateletapheresis and cryopreserved in ThromboSol/2% DMSO by either direct insertion in a -80 degrees C freezer or in liquid nitrogen after computer-controlled rate (CR) freezing. When required, PCs were thawed, centrifuged to remove the cryoprotectants and transfused. In vitro studies on thawed platelets showed loss of epitopes of surface glycoproteins and a marked reduction of functional activity compared with fresh platelets. Transfusion of CR-frozen PCs was associated with a mean 1 h corrected count increment (CCI) of 9.2 +/- 5.4 x 109/l and only one allogeneic PC was required in this group. In contrast, six out of seven patients required additional allogeneic transfusions in the -80 degrees C group (CCI = 2.7 +/- 1.4 x 109/l). ThromboSol-treated PCs have the ability to overcome thrombocytopenia if processed by a CR freezing protocol, but appear ineffective when frozen by direct placing at -80 degrees C.

In vitro and in vivo effects of desmopressin on platelet function

BACKGROUND AND OBJECTIVE

Desmopressin (DDAVP) may shorten bleeding time in patients with disorders of platelet function, but its mechanism of action in these conditions is still a matter of debate. In particular, contrasting results have been obtained concerning the ability of DDAVP to interact with platelets and to activate them directly. To gain further information on the DDAVP-platelet interaction, we studied the in vitro and ex vivo effects of DDAVP on platelet function.

DESIGN AND METHODS

Platelet responses to DDAVP both as a single agent and in conjunction with agonists of platelet activation were investigated. For in vitro experiments platelets were obtained from healthy adult volunteers, while the ex vivo effects of DDAVP were studied in 12 patients with a bleeding disorder receiving a test dose of this drug.

RESULTS

DDAVP in vitro did not induce either platelet aggregation or surface expression of the activation-dependent antigens; it did, however, greatly inhibit platelet aggregation response to vasopressin (AVP) and increased the maximal extent of platelet aggregation induced by collagen and ADP. DDAVP infusion did not promote the expression of activation antigens, but significantly enhanced ex vivo platelet aggregation stimulated by ADP and collagen. This priming effect was observed in patients with von Willebrand's disease, hemophilia A, May-Hegglin anomaly, gray platelet syndrome and Ehlers-Danlos syndrome. In all these patients bleeding time was shortened by DDAVP infusion. In contrast, neither platelet aggregation nor bleeding time was modified in two subjects with Glanzmann's thrombasthenia.

INTERPRETATION AND CONCLUSIONS

Our in vitro experiments indicate that DDAVP interacts directly with platelets and facilitates their activation via other agonists. In vivo results suggest that this effect occurs and is clinically relevant in patients with platelet dysfunction responding to DDAVP with a shortening of bleeding time.

Incompatibility for CD31 and human platelet antigens and acute graft-versus-host disease after bone marrow transplantation

Bone marrow transplantation (BMT) is often complicated by acute graft-versus-host disease (aGVHD). In patients transplanted with an HLA-matched donor the occurrence of this complication is believed to be favoured by disparities at the minor histocompatibility antigens (mHA). However, few of these polymorphic molecules have been identified. We sought to determine whether donor/recipient incompatibility for HPA-1, HPA-2, HPA-3, HPA-5 or CD31 (codon 125) antigens represented a risk factor for aGVHD and genotyped these antigens in 70 bone marrow donors and their HLA-identical recipients. All patients were children who received BMT for haematological malignancies at a single institution according to well-defined therapy protocols. Statistical analysis showed that incompatibility for CD31 (codon 125) was a risk factor for grade II-IV aGVHD in the overall patient population, whereas HPA-3 incompatibility predicted aGVHD occurrence in HLA-A2 patients only. The magnitude of the aGVHD risk was directly related to the number of HPA/CD31 incompatibilities. No correlation was found between non-identity for HPA/CD31 and aGVHD. Since incompatibility but not non-identity for CD31 or HPA-3 was a risk factor for aGVHD, we suggest that allelic variants of these molecules can serve as mHA in BMT recipients from HLA-identical donors.

Relationship between size and thiazole orange fluorescence of platelets in patients undergoing high-dose chemotherapy

The reticulated platelet count relies upon the assumption that newly formed platelets contain a residual amount of RNA which selectively binds the dye thiazole orange (TO) and greatly enhances its fluorescence signal. It has, however, recently been shown that almost half of the platelet TO-signal is derived from the labelling of dense-granule nucleotides. It is therefore possible that the higher TO fluorescence of young platelets partially derives from the higher granule content due to their larger volume. To investigate the relationship between platelet size and TO fluorescence we studied 13 patients with high-risk breast cancer undergoing high-dose chemotherapy. Mean platelet volume, platelet distribution width, platelet-large cell ratio, membrane content of glycoprotein Ib and IIb-IIIa and platelet aggregation were significantly greater during resolution than during development of thrombocytopenia, suggesting a prevalence of young and old platelets respectively. Mean TO fluorescence per cell was higher in the platelet population enriched in young cells than in that enriched in old cells, but this difference was no longer observed when the ratio TO signal/platelet size was examined. Moreover, RNase treatment and platelet degranulation reduced TO fluorescence to a similar extent in platelet populations enriched in young or old cells. Therefore our data suggest that the higher TO signal of young platelets is derived, to a significant extent, from their larger volume and granule content.

A new variant of Bernard-Soulier syndrome characterized by dysfunctional glycoprotein (GP) Ib and severely reduced amounts of GPIX and GPV

We describe a new variant of Bernard-Soulier syndrome characterized by almost normal amounts of GPIb and severely reduced GPIX and GPV. Despite surface expression, GPIbalpha failed to support ristocetin-induced platelet agglutination and to bind two conformation-dependent monoclonal antibodies, suggesting a qualitative defect. Sequence analysis of the gene coding for GPIX revealed a T-to-C substitution at base 1811, leading to a Leu40Pro conversion, whereas no defects were found in the coding region of the GPIbalpha gene. Allele-specific restriction enzyme analysis showed that the propositus and one of his sisters. both with severe bleeding diathesis. were homozygous for the GPIX mutation: the members of the family with mild bleeding diathesis and/or giant platelets in the peripheral blood were heterozygous, whereas the healthy ones were homozygous for the normal allele. Infusion of 1-desamino-8-D-arginine vasopressin normalized bleeding time in the two severely affected patients, although it did not modify ristocetin-induced platelet agglutination or membrane expression of GPIbalpha, GPIX, GPIIb-IIIa and GMP-140. Moreover, in one patient, normalization of bleeding time and rise of von Willebrand factor plasma concentration did not seem to be directly related.

Early assessment of viable myocardium after acute myocardial infarction by low-dose echo-dobutamine

BACKGROUND

The aim of the study was to evaluate the usefulness of low-dose dobutamine echocardiographic testing performed within 48 hours from anterior AMI in order to identify the extent of viable myocardium and predict its functional outcome. The early echo-dobutamine test was also compared with a predischarge test in order to evaluate the effects of different timing on the accuracy of the test.

METHODS

Nineteen consecutive patients, aged 54 +/- 11 years, with a first anterior AMI entered the study. All patients underwent a low-dose dobutamine echocardiographic test within 48 hours from hospital admission and at predischarge. In all the patients, a rest follow-up echocardiogram was performed three months after hospital discharge. Eleven patients underwent a revascularization procedure (7 underwent PTCA and 4 CABG).

RESULTS

Of the 159 dyssynergic segments, 26% improved spontaneously at predischarge and 51% improved at the three-month follow-up. Of the 145 predischarge dyssynergic segments, 38% improved at three months. Considering the results on a segmental basis, early low-dose dobutamine echocardiography showed a sensitivity of 52%, a specificity of 87%, a positive predictive value of 81%, a negative predictive value of 64% and a diagnostic accuracy of 69% for wall-motion improvement at three months. The predischarge test showed very similar values. A slight enhancement of the sensitivity of both tests was observed considering the akinetic segments only. Finally, considering the amount of segmental reversible dysfunction inside the infarct area in the single patients, early low-dose dobutamine echocardiography showed a sensitivity of 86% and a specificity of 80%.

CONCLUSIONS

Our results indicate that: 1) recovery of regional wall motion after AMI is slow and progressive, with substantial improvement ensuing within the first days after infarction; 2) considering results on a segmental basis, low-dose dobutamine echocardiography performed within 48 hours of AMI shows a high specificity but a low sensitivity for late recovery of regional function, although it gave information similar to what was obtained performing the test at predischarge; 3) the efficiency of test can be improved by considering the amount of reversible segmental dysfunction inside the infarct area in the single patients.

Comparison of annexin V and calcein-AM as early vital markers of apoptosis in adherent cells by confocal laser microscopy

Although morphological criteria for apoptosis are in general reliable, no systematic comparison of the techniques employed thus far has yet been performed. In this study, using confocal laser microscopy, we compared the performance of annexin V-FITC and calcein-AM for early detection of apoptosis in living adherent cells. Experiments were carried out on two distinct cell lines, PC 12 and NIH3T3, endowed with different shape and adhesion properties. The apoptotic process was followed for a prolonged period in the same cells of a predetermined field by means of a special flow chamber. Our results show that both probes allowed the detection of apoptotic cells in either cell line. However, some cells that clearly exhibited apoptotic changes on calcein visualization were annexin-negative. In NIH3T3 cells, annexin negativity of apoptotic cells was correlated with the preservation of cell shape and adhesion properties. These findings show that, at least in PC12 and NIH3T3 cells, annexin might be less sensitive than calcein-AM for early apoptosis detection and, for NIH3T3 cells, suggest that phosphatidilserine exposure is in some way linked to changes in cell shape and/or adhesion to culture substrate. (J Histochem Cytochem 46:895-900, 1998)

Thrombocytopenia, giant platelets, and leukocyte inclusion bodies (May-Hegglin anomaly): clinical and laboratory findings

PURPOSE

May-Hegglin anomaly is a rare hereditary condition characterized by the triad of thrombocytopenia, giant platelets, and inclusion bodies in leukocytes. Clinical features and the pathogenesis of bleeding in this disease are poorly defined.

PATIENTS AND METHODS

From 1988 to 1996 we studied 15 new May-Hegglin anomaly patients from 7 unrelated Italian families. In addition to clinical examination and routine laboratory testing, we measured bleeding time, platelet aggregation and release reaction, and platelet staining for tubulin, and performed ultrastructural study of polymorphonuclear leukocytes.

RESULTS

Although the mean age of our patients was 33 years, May-Hegglin anomaly had not been previously recognized in any of them. Bleeding diatheses ranged from severe to absent, and platelet count from 26 to 178 x 10(9)/L. No correlation was found between bleeding tendency and platelet count. Previous therapy with corticosteroids, high-dose immunoglobulins, and splenectomy had no effect on platelet count or bleeding diathesis. Desmopressin infusion greatly shortened the bleeding time in the most severely affected patient. The in vitro function of platelets was normal except for the absence of shape change in all subjects and defective response to epinephrine in 8 of 15 patients. Platelet tubulin was distributed unevenly instead of being organized in a circumferential band at the cell periphery.

CONCLUSION

The diagnosis of May-Hegglin is easily missed, and its frequency is probably underestimated. A qualitative defect of platelets may be responsible for mild bleeding diathesis even in the absence of thrombocytopenia, while severe bleeding results from both qualitative and quantitative platelet defects. May-Hegglin anomaly should be suspected whenever a patient has a low platelet count or a bleeding diathesis of unknown origin.

Activation of the hemostatic process in patients with unruptured aortic aneurysm before and in the first week after surgical repair

BACKGROUND AND OBJECTIVE

It has been previously suggested that activation of coagulation and fibrinolysis may sometime occur in patients with unruptured aortic aneurysm. However, the incidence of this complication and the effect of surgical repair are unknown. The objective of our study was to gain further information on this topic.

METHODS

We investigated activation of the hemostatic process in 20 consecutive patients with unruptured abdominal aortic aneurysm. We then evaluated the effect of surgical repair of the vascular abnormalities.

RESULTS

Both before and in the first week after surgery, the large majority of patients showed clear signs of activation of coagulation (increased plasma levels of prothrombin fragment 1 + 2 and fibrin peptide A), and many had low levels of the natural anticoagulant antithrombin III. Platelets were activated in all cases (high levels of plasma beta-thromboglobulin), and signs of platelet consumption (thrombocytopenia and/or increased mean platelet volume) were present in most of them.

INTERPRETATION AND CONCLUSIONS

Activation of the hemostatic process occurs in nearly all patients with abdominal aortic aneurysm and could play a role in the hemorrhagic and thrombotic events that can complicate the clinical development of these subjects.

Quality assurance in radiotherapy: the importance of medical physics staffing levels. Recommendations from an ESTRO/EFOMP joint task group

The safe application of ionising radiation for diagnosis and therapy requires a high level of knowledge of the underlying processes and of quality assurance. Sophisticated modern equipment can be used effectively for complicated diagnostic and therapeutic techniques only with adequate physics support. In the light of recent analyses and recommendations by national and international societies a joint working group of representatives from ESTRO (European Society for Therapeutic Radiology and Oncology) and from EFOMP (European Federation of Organisations for Medical Physics) was set up to assess the necessary staffing levels for physics support to radiotherapy. The method used to assess the staffing levels, the resulting recommendations and examples of their practical application are described.

The stimulation of Na,K,Cl cotransport and of system A for neutral amino acid transport is a mechanism for cell volume increase during the cell cycle

It has been known for several years that the triggering of cell proliferation is associated with an increase of the activity of Na,K,Cl cotransport and of transport system A for neutral amino acids. These systems are also enhanced during the volume recovery of hypertonically shrunk cells. We demonstrate here that during the cell cycle of NIH3T3 cells, an increase in cell volume is associated with an enhanced cell content of potassium and amino acids. Bumetanide delays cell cycle progression and hampers volume increase. The nonmetabolizable analog 2-methylamino-isobutyric acid, a specific substrate of system A, can partially substitute natural amino acids accumulated during the cell cycle as intracellular osmolytes. It is therefore proposed that the stimulation of Na,K,Cl cotransport and of system A, observed in proliferating cells, causes an expansion of cell volume through an enhanced intracellular accumulation of both inorganic and organic osmolytes and the concurrent, osmotically obliged uptake of water.

Platelet composition and function in patients undergoing cardiopulmonary bypass for heart surgery

BACKGROUND

Previous studies showed severe biochemical and functional damage to platelets in patients undergoing cardiopulmonary bypass for cardiac surgery, and suggested that this derived from the proteolytic action of plasmin on the platelet surface.

METHODS

A double-blind study was carried out to compare platelet function and composition in patients randomized to receive the protease inhibitor aprotinin or placebo during reoperation for valvular prosthesis replacement or coronary artery bypass grafting.

RESULTS

Flow cytometry with specific monoclonal antibodies and polyacrylamide gel electrophoresis did not show any significant proteolysis of platelet glycoprotein Ib and IIb-IIIa either in the placebo or the aprotinin group. Functional studies were consistent with these results, since ristocetin-induced platelet agglutination was unchanged and platelet aggregation and ATP release induced by collagen and ADP were only slightly reduced by cardiopulmonary bypass. These mild defects in platelet function were partially prevented by aprotinin infusion.

CONCLUSIONS

On the basis of our data and those from literature, we suggest that platelets may be affected very little or severely damaged during cardiopulmonary bypass for cardiac surgery, probably depending on some aspects of the technical procedure which remain to be identified. Aprotinin infusion significantly protects platelets in the latter condition, while its role is obviously slight in the former.

Characterization of apoptotic phenomena induced by treatment with L-asparaginase in NIH3T3 cells

The treatment of NIH3T3 cells with L-asparaginase causes a complete and reversible growth arrest with a decrease of cell number in the first 2 days. The enzyme induces impressive morphological changes that have been studied exploiting eosin in fixed cells and calcein in intact cells as sources of fluorescence for confocal microscopy. The first changes are observed after 12 h of treatment and the process is complete after 48 h. Both nucleus and cytoplasm shrink, while cells round and lose processes. Eventually most cells break; several debris include strongly hematoxylinic bodies negative for eosin fluorescence. Some cells neither round nor break in fragments. Throughout the process cells and fragments retain calcein fluorescence, thus indicating the integrity of the cell membrane. A rapid depletion of the intracellular pools of both glutamine and glutamate occurs in treated cells, followed by a decrease in DNA and protein syntheses, while the cell content of ATP, the transmembrane gradient of sodium, and the active transport of amino acids are scarcely affected. It is concluded that (i) L-asparaginase induces an apoptotic process in NIH3T3 cells that is forerun by a marked intracellular depletion of glutamate and glutamine; and (ii) although the enzyme completely suppresses cell proliferation, only a subset of cells undergoes apoptosis upon treatment. These findings provide a model for the characterization of factors that determine cell sensitivity to the effects of L-asparaginase.