Crosstalk between leukemia-associated proteins MOZ and MLL regulates HOX gene expression in human cord blood CD34+ cells.

MOZ and MLL, encoding a histone acetyltransferase (HAT) and a histone methyltransferase, respectively, are targets for recurrent chromosomal translocations found in acute myeloblastic or lymphoblastic leukemia. In MOZ (MOnocytic leukemia Zinc-finger protein)/CBP- or mixed lineage leukemia (MLL)-rearranged leukemias, abnormal levels of HOX transcription factors have been found to be critical for leukemogenesis. We show that MOZ and MLL cooperate to regulate these key genes in human cord blood CD34+ cells. These chromatin-modifying enzymes interact, colocalize and functionally cooperate, and both are recruited to multiple HOX promoters. We also found that WDR5, an adaptor protein essential for lysine 4 trimethylation of histone H3 (H3K4me3) by MLL, colocalizes and interacts with MOZ. We detected the binding of the HAT MOZ to H3K4me3, thus linking histone methylation to acetylation. In CD34+ cells, depletion of MLL causes release of MOZ from HOX promoters, which is correlated to defective histone activation marks, leading to repression of HOX gene expression and alteration of commitment of CD34+ cells into myeloid progenitors. Thus, our results unveil the role of the interaction between MOZ and MLL in CD34+ cells in which both proteins have a critical role in hematopoietic cell-fate decision, suggesting a new molecular mechanism by which MOZ or MLL deregulation leads to leukemogenesis.

Epigenetic patterns of the retinoic acid receptor beta2 promoter in retinoic acid-resistant thyroid cancer cells.

Treatment with retinoic acid (RA) is effective to restore radioactive iodine uptake in metastases of a small fraction of thyroid cancer patients. In order to find predictive markers of response, we took advantage of two thyroid cancer cell lines, FTC133 and FTC238, with low RA-receptor (RAR)beta expression but differing in their response to RA. We report that in both cell lines, RA signalling pathways are functional, as transactivation of an exogenous RARbeta2 promoter is effective in the presence of pharmacological concentrations of all-trans RA, and enhanced in RA-resistant FTC238 cells after ectopical expression of RARbeta, suggesting a defective endogenous RARbeta2 promoter in these cells. Further analyses show that whereas the RARbeta2 promoter is in an unmethylated permissive status in both FTC133 and FTC238 cells, it failed to be associated with acetylated forms of histones H3 or H4 in FTC238 cells upon RA treatment. Incubation with a histone deacetylase inhibitor, alone or in combination with RA, restored histone acetylation levels and reactivated RARbeta and differentiation marker Na+/I- symporter gene expression. Thus, histone modification patterns may explain RA-refractoriness in differentiated thyroid cancer patients and suggest a potential benefit of combined transcriptional and differentiation therapies.

The novel co-activator CRABPII binds to RARalpha and RXRalpha via two nuclear receptor interacting domains and does not require the AF-2 'core'.

We identify the RARalpha, RXRalpha and CRABPII domains required for the physical interaction of these proteins. On RARalpha and RXRalpha, the sequences correspond to the DEF and DE domains, respectively, but the interaction with CRABPII does not require the AF-2AD 'core'. On CRABPII, two interacting domains are identified (NRID1 and NRID2), one of which contains the only enhancement transactivation domain of CRABPII. The interaction is ligand-independent and does not require the ligand-binding domain of CRABPII. These results further stress that interaction of CRABPII with the nuclear receptors defines a novel level of transcriptional control.

Identification of an RNA binding specificity for the potential splicing factor TLS.

The TLS/FUS gene is involved in a recurrent chromosomal translocation in human myxoid liposarcomas. We previously reported that TLS is a potential splicing regulator able to modulate the 5'-splice site selection in an E1A pre-mRNA. Using an in vitro selection procedure, we investigated whether TLS exhibits a specificity with regard to RNA recognition. The RNAs selected by TLS share a common GGUG motif. Mutation of a G or U residue within this motif abolishes the interaction of TLS with the selected RNAs. We showed that TLS can bind GGUG-containing RNAs with a 250 nm affinity. By UV cross-linking/competition and immunoprecipitation experiments, we demonstrated that TLS recognizes a GGUG-containing RNA in nuclear extracts. Each one of the RNA binding domains (the three RGG boxes and the RNA recognition motif) contributes to the specificity of the TLS.RNA interaction, whereas only RRM and RGG2-3 participate to the E1A alternative splicing in vivo. The specificity of the TLS.RNA interaction was also observed using as natural pre-mRNA, the G-rich IVSB7 intron of the beta-tropomyosin pre-mRNA. Moreover, we determined that RNA binding specificities of TLS and high nuclear ribonucleoprotein A1 were different. Hence, our results help define the role of the specific interaction of TLS with RNA during the splicing process of a pre-mRNA.

A RA-dependent, tumour-growth suppressive transcription complex is the target of the PML-RARalpha and T18 oncoproteins.

PML and Tif1a are fused to RARA and Braf, respectively, resulting in the production of PML-RARalpha and Tif1alpha-B-Raf (T18) oncoproteins. Here we show that PML, Tif1alpha and RXRalpha/RARalpha function together in a transcription complex that is dependent on retinoic acid (RA). We found that PML acts as a ligand-dependent coactivator of RXRalpha/RARalpha. PML interacts with Tif1alpha and CBP. In Pml-/- cells, the RA-dependent induction of genes such as RARB2 and the ability of Tif1alpha and CBP to act as transcriptional coactivators on RA are impaired. We show that both PML and Tif1alpha are growth suppressors required for the growth-inhibitory activity of RA. T18, similar to PML-RARalpha, disrupts the RA-dependent activity of this complex in a dominant-negative manner resulting in a growth advantage. Our data define a new pathway for the control of cell growth and tumorigenesis, and provide a new model for the pathogenesis of acute promyelocytic leukaemia (APL).

Physical and functional interactions between cellular retinoic acid binding protein II and the retinoic acid-dependent nuclear complex.

Two sorts of proteins bind to, and mediate the developmental and homeostatic effects of, retinoic acid (RA): the RAR and RXR nuclear receptors, which act as ligand-dependent transcriptional regulators, and the cellular RA binding proteins (CRABPI and CRABPII). CRABPs are generally known to be implicated in the synthesis, degradation, and control of steady-state levels of RA, yet previous and recent data have indicated that they could play a role in the control of gene expression. Here we show for the first time that, both in vitro and in vivo, CRABPII is associated with RARalpha and RXRalpha in a ligand-independent manner in mammalian cells (HL-60, NB-4, and MCF-7). In the nucleus, this protein complex binds the RXR-RAR-specific response element of an RA target gene (RARE-DR5). Moreover, in the presence of retinoids that bind both the nuclear receptors and CRABPII, enhancement of transactivation by RXRalpha-RARalpha heterodimers is observed in the presence of CRABPII. Thus, CRABPII appears to be a novel transcriptional regulator involved in RA signaling.

Arsenic trioxide and melarsoprol induce programmed cell death in myeloid leukemia cell lines and function in a PML and PML-RARalpha independent manner.

Inorganic arsenic trioxide (As2O3) and the organic arsenical, melarsoprol, were recently shown to inhibit growth and induce apoptosis in NB4 acute promyelocytic leukemia (APL) and chronic B-cell leukemia cell lines, respectively. As2O3 has been proposed to principally target PML and PML-RARalpha proteins in APL cells. We investigated the activity of As2O3 and melarsoprol in a broader context encompassing various myeloid leukemia cell lines, including the APL cell line NB4-306 (a retinoic acid-resistant cell line derived from NB4 that no longer expresses the intact PML-RARalpha fusion protein), HL60, KG-1, and the myelomonocytic cell line U937. To examine the role of PML in mediating arsenical activity, we also tested these agents using murine embryonic fibroblasts (MEFs) and bone marrow (BM) progenitors in which the PML gene had been inactivated by homologous recombination. Unexpectedly, we found that both compounds inhibited cell growth, induced apoptosis, and downregulated bcl-2 protein in all cell lines tested. Melarsoprol was more potent than As2O3 at equimolar concentrations ranging from 10(-7) to 10(-5) mol/L. As2O3 relocalized PML and PML-RARalpha onto nuclear bodies, which was followed by PML degradation in NB4 as well as in HL60 and U937 cell lines. Although melarsoprol was more potent in inhibiting growth and inducing apoptosis, it did not affect PML and/or PML-RARalpha nuclear localization. Moreover, both As2O3 and melarsoprol comparably inhibited growth and induced apoptosis of PML+/+ and PML-/- MEFs, and inhibited colony-forming unit erythroid (CFU-E) and CFU granulocyte-monocyte formation in BM cultures of PML+/+ and PML-/- progenitors. Together, these results show that As2O3 and melarsoprol inhibit growth and induce apoptosis independent of both PML and PML-RARalpha expression in a variety of myeloid leukemia cell lines, and suggest that these agents may be more broadly used for treatment of leukemias other than APL.

Acute promyelocytic leukemia as a model for cross-talk between interferon and retinoic acid pathways: from molecular biology to clinical applications.

Acute promyelocytic leukemia (APL) has been regarded as the paradigm for therapeutic approaches utilizing differentiating agents, due to the fact that almost 95% of patients undergo complete remission when treated with all-trans retinoic acid (ATRA). However, complete clinical remission with ATRA alone is always transient, and relapse in APL is almost invariably associated with the acquisition of resistance to ATRA. Acquired resistance to ATRA in APL cell lines and in some APL clinical cases can be partially overcome by interferons (IFNs), cytokines which have well established tumor-growth suppressive activities. APL is associated in 99% of cases with a 15;17 translocation that fuses the PML and Retinoic Acid Receptor alpha (RARalpha) genes. RARalpha is one of the Retinoic Acid (RA) nuclear receptors which mediates, at the transcriptional level, ATRA differentiating and growth suppressive activity. PML is a tumor-growth suppressor whose expression is directly regulated by IFNs. Here we review the molecular mechanisms by which IFNs and RA can cooperate in controlling cell growth and differentiation of normal hemopoietic cells and leukemic cells, focusing on APL as a model system.

Role of PML in cell growth and the retinoic acid pathway.

The PML gene is fused to the retinoic acid receptor alpha (RARalpha) gene in chromosomal translocations associated with acute promyelocytic leukemia (APL). Ablation of murine PML protein by homologous recombination revealed that PML regulates hemopoietic differentiation and controls cell growth and tumorigenesis. PML function was essential for the tumor-growth-suppressive activity of retinoic acid (RA) and for its ability to induce terminal myeloid differentiation of precursor cells. PML was needed for the RA-dependent transactivation of the p21WAF1/CIP1 gene, which regulates cell cycle progression and cellular differentiation. These results indicate that PML is a critical component of the RA pathway and that disruption of its activity by the PML-RARalpha fusion protein may be important in APL pathogenesis.

Gene rearrangements in the molecular pathogenesis of acute promyelocytic leukemia.

Acute Promyelocytic Leukemia (APL) is a distinct subtype of myeloid leukemia that in the USA alone affects more than 3,000 individuals every year. APL is characterized by three distinct and unique features: i) the accumulation in the bone marrow of tumor cells with promyelocytic features; ii) the invariable association with specific translocations which always involve chromosome 17 and the Retinoic Acid Receptor alpha (RAR alpha) locus; iii) the exquisite sensitivity of APL blasts to the differentiating action of Retinoic Acid (RA). These features have led APL to become the paradigm for therapeutic approaches utilizing differentiating agents. The last 5 years have provided crucial insights into the molecular basis of APL. RAR alpha translocates in 99% of cases to a gene located on chromosome 15 that we initially named myl and subsequently has been called PML. In a few cases, RAR alpha variably translocates to chromosome 11 where it fuses to the PLZF gene or to a newly described partner, NuMA. In addition, RAR alpha is also found translocated to chromosome 5 where it fuses to the NPM gene. The cloning of variant translocations in APL and the comparative analysis of their associated products is crucial for the understanding of the molecular etiopathogenesis of the disease. The generation of animal models, i.e., transgenic mice expressing the fusion genes, will be instrumental in determining the precise contribution of these fusion genes to leukemogenesis. In fact, mice harboring a PML/RAR alpha transgene whose expression is specifically targeted to the myeloid-promyelocytic lineage develop acute myeloid leukemia with promyelocytic features. Moreover, the functional analysis of the various fusion proteins, as well as RAR alpha partners, is revealing striking common features beneath a misleading structural heterogeneity which unravels a possible unifying molecular mechanism towards APL leukemogenesis.

A cytosolic calcium transient is not necessary for degranulation or oxidative burst in immune complex-stimulated neutrophils.

Receptor-mediated activation of neutrophils (PMN) initiates possibly interdependent events, including a rapid transient increase in [Ca2+]i, implicated as a second messenger. To investigate whether this transient is required for eventual degranulation, PMN were incubated with an intracellular Ca2+ chelator (BAPTA), then exposed to chemotactic peptide [N-formyl-methionyl-leucyl-phenylalanine (fMLP)l with or without cytochalasin B (CB) or to high-valency immune complexes (HIC); delta[Ca2+]i, delta(pH)i, oxidative burst, and elastase release were then evaluated (plus or minus EGTA 15 s before stimulation) after 2 and 15 min incubation in 0.9 mM Ca2+. With either fMLP plus CB or HIC stimulation, BAPTA-treated cells were unable to achieve a Ca2+ transient with a 2-min incubation, whereas a 15-min incubation allowed the BAPTA-treated cells to recover a portion of the delta[Ca2+]i. Even though BAPTA-treated cells were unable to mount a delta[Ca2+]i at 2 min, HIC-stimulated BAPTA-treated cells were able to elicit an oxidative burst (33% of control) and degranulation (67% of control). Therefore, we conclude that delta[Ca2+]i modulates but is not required for oxidative burst or degranulation.

All-trans retinoic acid rapidly decreases cathepsin G synthesis and mRNA expression in acute promyelocytic leukemia.

The cells from patients with acute promyelocytic leukemia (AML M3) undergo terminal differentiation when treated with all-trans retinoic acid (ATRA). We have analyzed the expression of the mRNA for cathepsin G, a promyelocyte stage-specific transcript, in the leukemia and in retinoic acid responsive cell lines. We showed that the transcript is perpetually synthesized in patients' cells and that it rapidly disappears when the cells are treated with ATRA. In ATRA-sensitive (HL-60, NB4) cell lines and an ATRA-resistant (HL-60R) cell line we have shown that this process is dependent on proteins synthesized during the first 6h of ATRA-triggered differentiation and may involve both pre- and post-transcriptional mechanisms. A corresponding decrease in cathepsin G protein synthesis then follows. These findings indicate that the maturation arrest in AML M3 results in cells that may constitutively continue to produce proteins whose production is temporally confined during normal hemopoiesis. This would explain the elevated plasma-free serine protease activity we have demonstrated in this disease, and has implications for both the coagulopathy and the 'retinoic acid syndrome' in AML M3.

[Nuclear receptors and ontogenesis of the hematopoietic tissue]. / Récepteurs nucléaires et ontogénèse du tissu hématopoïétique.

Differentiation of the hematopoietic tissue is controlled by growth factors which act precisely on stem cells arriving at a specific stage of differentiation. The recent identification of retinoic acid, a vitamin A metabolite, as an active differentiating agent of acute promyelocytic leukemia, has allowed to define a normal group of growth and differentiation factors of the myeloid tissue: vitamins A and B and thyroid hormones.

In vitro all-trans retinoic acid (ATRA) sensitivity and cellular retinoic acid binding protein (CRABP) levels in relapse leukemic cells after remission induction by ATRA in acute promyelocytic leukemia.

The current treatment of acute promyelocytic leukemia (APL, also called AML3 subtype) is focused on differentiating agents such as the vitamin A derivative all-trans retinoic acid (ATRA). This agent is a novel and very promising therapy for this disease characterized cytogenetically by a translocation t(15;17)(q21;q22) involving the alpha retinoic acid receptor on chromosome 17 and the PML gene on chromosome 15. Clinical trials have demonstrated that ATRA followed by or combined with conventional chemotherapy may be more beneficial than chemotherapy for inducing complete remission. Unfortunately, ATRA as a single agent, does not appear able to maintain patients in remission (median 5 months), and when relapse occurs resistance to a second induction of ATRA therapy is observed in almost all cases. Recently our laboratory investigated whether specific features of the AML3 cells at relapse could explain the in vivo resistance observed. We have demonstrated that AML3 patients' cells (from four patients) at relapse show high levels of CRABP, a cytosolic retinoic acid binding protein and this protein was not detected prior to ATRA therapy. Relapse-AML3 cells (n = 12) showed reduced differentiation induction when compared with 'virgin'-AML3 cells. Results from this study suggest that CRABP could modulate ATRA cellular concentrations reaching the nucleus. This induced ATRA hypercatabolytic state should be monitored during consolidation therapy and at relapse by evaluating CRABP and RA metabolite levels, in order to detect ATRA resistance in patients with AML3.

In vitro all-trans retinoic acid (ATRA) sensitivity and cellular retinoic acid binding protein (CRABP) levels in relapse leukemic cells after remission induction by ATRA in acute promyelocytic leukemia.

The current treatment of acute promyelocytic leukemia (APL, also called AML3 subtype) is focused on differentiating agents such as the vitamin A derivative all-trans retinoic acid (ATRA). This agent is a novel and very promising therapy for this disease characterized cytogenetically by a translocation t(15;17)(q21;q22) involving the alpha retinoic acid receptor on chromosome 17 and the PML gene on chromosome 15. Clinical trials have demonstrated that ATRA followed by or combined with conventional chemotherapy may be more beneficial than chemotherapy for inducing complete remission. Unfortunately, ATRA as a single agent, does not appear able to maintain patients in remission (median 5 months), and when relapse occurs resistance to a second induction of ATRA therapy is observed in almost all cases. Recently our laboratory investigated whether specific features of the AML3 cells at relapse could explain the in vivo resistance observed. We have demonstrated that AML3 patients' cells (from four patients) at relapse show high levels of CRABP, a cytosolic retinoic acid binding protein and this protein was not detected prior to ATRA therapy. Relapse-AML3 cells (n = 12) showed reduced differentiation induction when compared with 'virgin'-AML3 cells. Results from this study suggest that CRABP could modulate ATRA cellular concentrations reaching the nucleus. This induced ATRA hypercatabolytic state should be monitored during consolidation therapy and at relapse by evaluating CRABP and RA metabolite levels, in order to detect ATRA resistance in patients with AML3.

Resistance to all-trans retinoic acid (ATRA) therapy in relapsing acute promyelocytic leukemia: study of in vitro ATRA sensitivity and cellular retinoic acid binding protein levels in leukemic cells.

All-trans retinoic acid (ATRA) induces leukemic cell differentiation and complete remission (CR) in a high proportion of patients with acute promyelocytic leukemia (AML3 subtype). However, relapses occur when ATRA is prescribed as maintenance therapy, and resistance to a second ATRA-induction therapy is frequently observed. An induced hypercatabolism of ATRA has been suggested as a possible mechanism leading to reduced ATRA sensitivity and resistance. CRABPII, an RA cytoplasmic binding protein linked to RA's metabolization pathway, is induced by ATRA in different cell systems. To investigate whether specific features of the AML3 cells at relapse could explain the in vivo resistance observed, we studied the CRABP levels and in vitro sensitivity to ATRA of AML3 cells before and at relapse from ATRA. Relapse-AML3 cells (n = 12) showed reduced differentiation induction when compared with "virgin"-AML3 cells (n = 31; P < .05). Dose-response studies were performed in 2 cases at relapse and showed decreased sensitivity to low ATRA concentrations. CRABPII levels and in vitro differentiation characteristics of AML3 cells before and at relapse from ATRA therapy were studied concomittantly in 4 patients. High levels of CRABPII (median, 20 fmol/mg of protein) were detected in the cells of the 4 patients at relapse but were not detected before ATRA therapy. Three of these patients showed a decrease in differentiation induction of their leukemic cells, and a failure to achieve CR with a second induction therapy of ATRA 45 mg/m2/day was noted in all patients treated (n = 3). Results from this study provide evidence to support the hypothesis of induced-ATRA metabolism as one of the major mechanisms responsible for ATRA resistance. Monitoring CRABPII levels after ATRA withdrawal may help to determine when to administer ATRA in the maintenance or relapse therapy of AML3 patients.

Non-age related differences in thrombin responses by platelets from male patients with advanced Alzheimer's disease.

Alzheimer's Disease(AD), characterized by a deposition of beta-amyloid peptide (beta/A4) in the brain and in the cerebral microvasculature of affected individuals, is derived from its precursor protein (beta APP) via proteolytic processing by enzyme(s) which have not yet been characterized or localized. Since platelets carry APP in one of their granules, they have been implicated as a source of the beta/A4 deposits in the microvasculature of AD patients, attributable to either an abnormality in the platelets' stimulus response, in the quantity or nature of the APP they release upon activation and/or in the processing of that protein. We show here that platelets from patients with severe AD have abnormal stimulus responses to alpha-thrombin. Specifically, these cells hyperacidify. While it is not clear why this abnormality occurs, it may contribute to aberrant granule secretion since we have demonstrated earlier that release of platelet granule contents is partially controlled by the cytoplasmic pH.

Induction of retinoic acid-binding protein in normal and malignant human myeloid cells by retinoic acid in acute promyelocytic leukemia patients.

Retinoic acid has striking effects on development and cell differentiation. Its biological effect is a highly regulated process that is controlled by specific proteins. In the nucleus, different retinoic acid receptors have been identified and their genes cloned. In the cytosol, retinoid binding proteins, cellular retinoic acid-binding protein and cellular retinol-binding protein, have been correlated with normal and malignant tissue differentiation. Recently, differentiation therapy of acute promyelocytic leukemias (AML3 subtype) with all-trans-retinoic acid has been shown to be an efficient alternative to chemotherapy. The retinoic acid receptor alpha gene has been shown to be specifically rearranged in AML3 through the t(15;17) translocation. The molecular basis of the effect to reverse the leukemic phenotype of all-trans-retinoic acid is not yet elucidated. To further study retinoic acid efficacy in AML3 leukemia, retinoic acid-binding proteins were studied in the cytosol extracts of hematopoietic cells. No retinoic acid binding activity was detected in normal or malignant hematopoietic cells whether sensitive or not to retinoic acid. However, detectable binding to a cytosolic protein corresponding to cellular retinoic acid-binding protein (M(r) 15,000, Kd 3 nM) was observed in the bone marrow cells of AML3 patients undergoing all-trans-retinoic acid therapy. We suggest that both the induction and subsequent presence of cellular retinoic acid-binding protein may influence the therapeutic efficacy of retinoic acid and must be taken into account when studying its effect in acute promyelocytic patients.

[Mechanism of action of retinoids in a new therapeutic approach to acute promyelocytic leukemia]. / Mode d'action des rétinoïdes dans une nouvelle approche thérapeutique des leucémies aiguës promyélocytaires.

Vitamin A (retinol) and retinoic acid, its natural derivative, play an important role in the growth, differentiation and development of known normal tissues. Retinoids have recently become of interest to research in areas as diverse as dermatology, embryonal development and cancer research. Retinol is the major retinoid transported in the blood and tissues by its specific carrier retinol binding protein (RBP). The normal level of retinol in plasma is regulated very precisely by retinol homeostasis. RBP-retinol circulation supplies target cells, which then activate retinol into retinoic acid (RA) if they possess the NAD-dependent enzymatic oxidation system. RA, which is one of the most active metabolites of retinol, is also present in low concentration in the blood and the RA rate formation varies from tissues depending on specific need of the cell. The cellular transport and biological activity of retinoids may be mediated by their specific cytoplasmic binding proteins cellular retinol binding protein (CRBP) and the cellular retinoic acid binding protein (CRABP) which may function as shuttles targetting RA to nucleosol fraction and/or as regulator of cellular concentration of RA. The nuclear proteins RARs (retinoic acid receptors), which are members of the nuclear receptor superfamily are likely to be the final transducers of the RA signal at the gene expression. All-trans retinoic acid (ATRA) is able to specifically differentiate the malignant cells from leukemic patients with APL in short-term culture. For this reason, APL patients were successfully treated with ATRA (Chinese and French results). Acute promyelocytic leukemia M3 (French-American-British FAB classification) is a rare disease (10% of AML), characterized by a reciprocal chromosome 15-17 translocation. It has been shown that the chromosome 17 breakpoint of the translocation is localized within the RAR alpha gene. Due to the t(15;17) RAR alpha gene translocated to a gene PML on chromosome 15 resulting in synthesis of PML/RAR alpha fusion messenger RNA. Detection of PML/RAR alpha transcript is now a molecular marker of the disease. The abnormal PML/RAR alpha protein exhibits altered transcription activation properties when compared with RAR alpha. Clinical trials have demonstrated that ATRA is extremely efficient in inducing complete remission in APL patients. The morphologic finding of maturing elements in the bone marrow and peripheral blood during retinoic acid treatment indicates that the remission is obtained without hypoplasia and suggests that a differentiating mechanism is involved.(ABSTRACT TRUNCATED AT 400 WORDS)