Effects of assisted reproductive technologies on transposon regulation in the mouse pre-implanted embryo.

STUDY QUESTION: Do assisted reproductive technologies (ARTs) impact on the expression of transposable elements (TEs) in preimplantation embryos? SUMMARY ANSWER: The expression of all TE families is globally increased with mouse embryo culture with differences according to culture medium composition. WHAT IS KNOWN ALREADY: Mammalian genomes are subject to global epigenetic reprogramming during early embryogenesis. Whether ARTs could have consequences on this period of acute epigenetic sensitivity is the matter of intense research. So far, most studies have examined the impact of ARTs on the regulation of imprinted genes. However, very little attention has been given to the control of TEs, which exceed by far the number of genes and account for half of the mammalian genomic mass. This is of particular interest as TEs have the ability to modulate gene structure and expression, and show unique regulatory dynamics during the preimplantation period. STUDY DESIGN, SIZE, DURATION: Here, we evaluated for the first time the impact of ART procedures (superovulation, in-vitro fertilisation and embryo culture) on the control of different TE types throughout preimplantation development of mouse embryos. We also made use of a mouse model carrying a LINE-1 retrotransposition-reporter transgene to follow parental patterns of transmission and mobilisation. PARTICIPANTS/MATERIALS, SETTING, METHODS: Hybrid B6CBA/F1 mice were used for the expression analyses. Relative TE expression was evaluated by using the nCounter quantification methodology (Nanostring®). This quantitative method allowed us to simultaneously follow 15 TE targets. Another technique of quantification (RTqPCR) was also used.A mouse model carrying a LINE-1 retrotransposition-reporter transgene (LINE-1 GF21) was used to follow parental patterns of transmission and mobilisation. MAIN RESULTS AND THE ROLE OF CHANCE: We found that the superovulation step did not modify the dynamics nor the level of TE transcription across the preimplantation period. However, upon in-vitro culture, TE expression was globally increased at the blastocyst stage in comparison with in-vivo development. Finally, by monitoring the transmission and mobilisation of a transgenic LINE-1 transposon, we found that in-vitro fertilisation may alter the mendelian rate of paternal inheritance. LARGE SCALE DATA: N/A. LIMITATIONS, REASONS FOR CAUTION: Even though the Nanostring results concerning the dynamics of transcription throughout preimplantation development were based on pools of embryos originating from several females, only two pools were analysed per developmental stage. However, at the blastocyst stage, consistent expressional results were found between the Nanostring technology and the other technique of quantification used, RTqPCR. WIDER IMPLICATIONS OF THE FINDINGS: Our findings highlight the sensitivity of TEs to the ART environment and their great potential as biomarkers of culture medium-based effects. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by funding from the 'Agence de la Biomedecine', 'Conseil Régional de Bourgogne' and 'RCT grant from INSERM-DGOS'. The authors have no conflicts of interest to declare.

Tif1γ regulates the TGF-ß1 receptor and promotes physiological aging of hematopoietic stem cells.

The hematopoietic system declines with age. Myeloid-biased differentiation and increased incidence of myeloid malignancies feature aging of hematopoietic stem cells (HSCs), but the mechanisms involved remain uncertain. Here, we report that 4-mo-old mice deleted for transcription intermediary factor 1γ (Tif1γ) in HSCs developed an accelerated aging phenotype. To reinforce this result, we also show that Tif1γ is down-regulated in HSCs during aging in 20-mo-old wild-type mice. We established that Tif1γ controls TGF-ß1 receptor (Tgfbr1) turnover. Compared with young HSCs, Tif1γ(-/-) and old HSCs are more sensitive to TGF-ß signaling. Importantly, we identified two populations of HSCs specifically discriminated by Tgfbr1 expression level and provided evidence of the capture of myeloid-biased (Tgfbr1(hi)) and myeloid-lymphoid-balanced (Tgfbr1(lo)) HSCs. In conclusion, our data provide a new paradigm for Tif1γ in regulating the balance between lymphoid- and myeloid-derived HSCs through TGF-ß signaling, leading to HSC aging.

Symplekin, a polyadenylation factor, prevents MOZ and MLL activity on HOXA9 in hematopoietic cells.

MOZ and MLL encoding a histone acetyltransferase and a histone methyltransferase, respectively, are targets for recurrent chromosomal translocations found in acute myeloblastic or lymphoblastic leukemia. We have previously shown that MOZ and MLL cooperate to activate HOXA9 gene expression in hematopoietic stem/progenitors cells. To dissect the mechanism of action of this complex, we decided to identify new proteins interacting with MOZ. We found that the scaffold protein Symplekin that supports the assembly of polyadenylation machinery was identified by mass spectrometry. Symplekin interacts and co-localizes with both MOZ and MLL in immature hematopoietic cells. Its inhibition leads to a decrease of the HOXA9 protein level but not of Hoxa9 mRNA and to an over-recruitment of MOZ and MLL onto the HOXA9 promoter. Altogether, our results highlight the role of Symplekin in transcription repression involving a regulatory network between MOZ, MLL and Symplekin.

Valrubicin-loaded immunoliposomes for specific vesicle-mediated cell death in the treatment of hematological cancers.

We created valrubicin-loaded immunoliposomes (Val-ILs) using the antitumor prodrug valrubicin, a hydrophobic analog of daunorubicin. Being lipophilic, valrubicin readily incorporated Val-lLs that were loaded with specific antibodies. Val-ILs injected intravenously rapidly reached the bone marrow and spleen, indicating their potential to effectively target cancer cells in these areas. Following the transplantation of human pediatric B-cell acute lymphoblastic leukemia (B-ALL), T-cell acute lymphoblastic leukemia (T-ALL), or acute myeloid leukemia (AML) in immunodeficient NSG mice, we generated patient-derived xenograft (PDX) models, which were treated with Val-ILs loaded with antibodies to target CD19, CD7 or CD33. Only a small amount of valrubicin incorporated into Val-ILs was needed to induce leukemia cell death in vivo, suggesting that this approach could be used to efficiently treat acute leukemia cells. We also demonstrated that Val-ILs could reduce the risk of contamination of CD34+ hematopoietic stem cells by acute leukemia cells during autologous peripheral blood stem cell transplantation, which is a significant advantage for clinical applications. Using EL4 lymphoma cells on immunocompetent C57BL/6 mice, we also highlighted the potential of Val-ILs to target immunosuppressive cell populations in the spleen, which could be valuable in impairing cancer cell expansion, particularly in lymphoma cases. The most efficient Val-ILs were found to be those loaded with CD11b or CD223 antibodies, which, respectively, target the myeloid-derived suppressor cells (MDSC) or the lymphocyte-activation gene 3 (LAG-3 or CD223) on T4 lymphocytes. This study provides a promising preclinical demonstration of the effectiveness and ease of preparation of Val-ILs as a novel nanoparticle technology. In the context of hematological cancers, Val-ILs have the potential to be used as a precise and effective therapy based on targeted vesicle-mediated cell death.

Proteasome inhibition improves the muscle of laminin α2 chain-deficient mice.

Muscle atrophy, a significant characteristic of congenital muscular dystrophy with laminin α2 chain deficiency (also known as MDC1A), occurs by a change in the normal balance between protein synthesis and protein degradation. The ubiquitin-proteasome system (UPS) plays a key role in protein degradation in skeletal muscle cells. In order to identify new targets for drug therapy against MDC1A, we have investigated whether increased proteasomal degradation is a feature of MDC1A. Using the generated dy(3K)/dy(3K) mutant mouse model of MDC1A, we studied the expression of members of the ubiquitin-proteasome pathway in laminin α2 chain-deficient muscle, and we treated dy(3K)/dy(3K) mice with the proteasome inhibitor MG-132. We show that members of the UPS are upregulated and that the global ubiquitination of proteins is raised in dystrophic limb muscles. Also, phosphorylation of Akt is diminished in diseased muscles. Importantly, proteasome inhibition significantly improves the dystrophic dy(3K)/dy(3K) phenotype. Specifically, treatment with MG-132 increases lifespan, enhances locomotive activity, enlarges muscle fiber diameter, reduces fibrosis, restores Akt phosphorylation and decreases apoptosis. These studies promote better understanding of the disease process in mice and could lead to a drug therapy for MDC1A patients.

Smad4 binds Hoxa9 in the cytoplasm and protects primitive hematopoietic cells against nuclear activation by Hoxa9 and leukemia transformation.

We studied leukemic stem cells (LSCs) in a Smad4(-/-) mouse model of acute myelogenous leukemia (AML) induced either by the HOXA9 gene or by the fusion oncogene NUP98-HOXA9. Although Hoxa9-Smad4 complexes accumulate in the cytoplasm of normal hematopoietic stem cells and progenitor cells (HSPCs) transduced with these oncogenes, there is no cytoplasmic stabilization of HOXA9 in Smad4(-/-) HSPCs, and as a consequence increased levels of Hoxa9 is observed in the nucleus leading to increased immortalization in vitro. Loss of Smad4 accelerates the development of leukemia in vivo because of an increase in transformation of HSPCs. Therefore, the cytoplasmic binding of Hoxa9 by Smad4 is a mechanism to protect Hoxa9-induced transformation of normal HSPCs. Because Smad4 is a potent tumor suppressor involved in growth control, we developed a strategy to modify the subcellular distribution of Smad4. We successfully disrupted the interaction between Hoxa9 and Smad4 to activate the TGF-ß pathway and apoptosis, leading to a loss of LSCs. Together, these findings reveal a major role for Smad4 in the negative regulation of leukemia initiation and maintenance induced by HOXA9/NUP98-HOXA9 and provide strong evidence that antagonizing Smad4 stabilization by these oncoproteins might be a promising novel therapeutic approach in leukemia.

Mice with ribosomal protein S19 deficiency develop bone marrow failure and symptoms like patients with Diamond-Blackfan anemia.

Diamond-Blackfan anemia (DBA) is a congenital erythroid hypoplasia caused by a functional haploinsufficiency of genes encoding for ribosomal proteins. Among these genes, ribosomal protein S19 (RPS19) is mutated most frequently. Generation of animal models for diseases like DBA is challenging because the phenotype is highly dependent on the level of RPS19 down-regulation. We report the generation of mouse models for RPS19-deficient DBA using transgenic RNA interference that allows an inducible and graded down-regulation of Rps19. Rps19-deficient mice develop a macrocytic anemia together with leukocytopenia and variable platelet count that with time leads to the exhaustion of hematopoietic stem cells and bone marrow failure. Both RPS19 gene transfer and the loss of p53 rescue the DBA phenotype implying the potential of the models for testing novel therapies. This study demonstrates the feasibility of transgenic RNA interference to generate mouse models for human diseases caused by haploinsufficient expression of a gene.

Bone marrow-derived extracellular vesicles carry the TGF-ß signal transducer Smad2 to preserve hematopoietic stem cells in mice.

Extracellular vesicles (EVs) released by cells in the bone marrow (BM) are important for regulating proliferation, differentiation, and other processes in hematopoietic stem cells (HSC). TGF-ß signaling is now well known to be involved in HSC's quiescence and maintenance, but the TGF-ß pathway related to EVs is still largely unknown in the hematopoietic system. We found that the EV inhibitor Calpeptin, when injected intravenously into mice, particularly affected the in vivo production of EVs carrying phosphorylated Smad2 (p-Smad2) in mouse BM. This was accompanied with an alteration in the quiescence and maintenance of murine HSC in vivo. EVs produced by murine mesenchymal stromal MS-5 cells also showed presence of p-Smad2 as a cargo. We treated MS-5 cells with the TGF-ß inhibitor SB431542 in order to produce EVs lacking p-Smad2, and discovered that its presence was required for ex vivo maintenance of HSC. In conclusion, we revealed a new mechanism involving EVs produced in the mouse BM that transport bioactive phosphorylated Smad2 as a cargo to enhance the TGF-ß signaling-mediated quiescence and maintenance of HSC.

Heat-Shock Proteins in Leukemia and Lymphoma: Multitargets for Innovative Therapeutic Approaches.

Heat-shock proteins (HSPs) are powerful chaperones that provide support for cellular functions under stress conditions but also for the homeostasis of basic cellular machinery. All cancer cells strongly rely on HSPs, as they must continuously adapt to internal but also microenvironmental stresses to survive. In solid tumors, HSPs have been described as helping to correct the folding of misfolded proteins, sustain oncogenic pathways, and prevent apoptosis. Leukemias and lymphomas also overexpress HSPs, which are frequently associated with resistance to therapy. HSPs have therefore been proposed as new therapeutic targets. Given the specific biology of hematological malignancies, it is essential to revise their role in this field, providing a more adaptable and comprehensive picture that would help design future clinical trials. To that end, this review will describe the different pathways and functions regulated by HSP27, HSP70, HSP90, and, not least, HSP110 in leukemias and lymphomas.

Acute lymphoblastic leukemia-derived extracellular vesicles affect quiescence of hematopoietic stem and progenitor cells.

Patient-derived xenografted (PDX) models were generated through the transplantation of primary acute lymphoblastic leukemia (ALL) cells into immunodeficient NSG mice. We observed that ALL cells from mouse bone marrow (BM) produced extracellular vesicles (EVs) with specific expression of inducible heat shock protein HSP70, which is commonly activated in cancer cells. Taking advantage of this specific expression, we designed a strategy to generate fluorescent HSP70-labeled ALL EVs and monitor the impact of these EVs on endogenous murine BM cells ex vivo and in vivo. We discovered that hematopoietic stem and progenitor cells (HSPC) were mainly targeted by ALL EVs, affecting their quiescence and maintenance in the murine BM environment. Investigations revealed that ALL EVs were enriched in cholesterol and other metabolites that contribute to promote the mitochondrial function in targeted HSPC. Furthermore, using CD34+ cells isolated from cord blood, we confirmed that ALL EVs can modify quiescence of human HSPC. In conclusion, we have discovered a new oncogenic mechanism illustrating how EVs produced by proliferative ALL cells can target and compromise a healthy hematopoiesis system during leukemia development.

HSP90 inhibitor NVP-BEP800 affects stability of SRC kinases and growth of T-cell and B-cell acute lymphoblastic leukemias.

T-cell and B-cell acute lymphoblastic leukemias (T-ALL, B-ALL) are aggressive hematological malignancies characterized by an accumulation of immature T- or B-cells. Although patient outcomes have improved, novel targeted therapies are needed to reduce the intensity of chemotherapy and improve the prognosis of high-risk patients. Using cell lines, primary cells and patient-derived xenograft (PDX) models, we demonstrate that ALL cells viability is sensitive to NVP-BEP800, an ATP-competitive inhibitor of Heat shock protein 90 (HSP90). Furthermore, we reveal that lymphocyte-specific SRC family kinases (SFK) are important clients of the HSP90 chaperone in ALL. When PDX mice are treated with NVP-BEP800, we found that there is a decrease in ALL progression. Together, these results demonstrate that the chaperoning of SFK by HSP90 is involved in the growth of ALL. These novel findings provide an alternative approach to target SRC kinases and could be used for the development of new treatment strategies for ALL.

High-fat diet intensifies MLL-AF9-induced acute myeloid leukemia through activation of the FLT3 signaling in mouse primitive hematopoietic cells.

Using a MLL-AF9 knock-in mouse model, we discovered that consumption of a high-fat diet (HFD) accelerates the risk of developing acute myeloid leukemia (AML). This regimen increases the clusterization of FLT3 within lipid rafts on the cell surface of primitive hematopoietic cells, which overactivates this receptor as well as the downstream JAK/STAT signaling known to enhance the transformation of MLL-AF9 knock-in cells. Treatment of mice on a HFD with Quizartinib, a potent inhibitor of FLT3 phosphorylation, inhibits the JAK3/STAT3, signaling and finally antagonizes the accelerated development of AML that occurred following the HFD regimen. We can therefore conclude that, on a mouse model of AML, a HFD enforces the FLT3 signaling pathway on primitive hematopoietic cells and, in turn, improves the oncogenic transformation of MLL-AF9 knock-in cells and the leukemia initiation.

High-fat diet disturbs lipid raft/TGF-ß signaling-mediated maintenance of hematopoietic stem cells in mouse bone marrow.

Despite recent in vivo data demonstrating that high-fat diet (HFD)-induced obesity leads to major perturbations in murine hematopoietic stem cells (HSC), the direct role of a HFD is not yet completely understood. Here, we investigate the direct impact of a short-term HFD on HSC and hematopoiesis in C57BL/6J mice compared with standard diet-fed mice. We detect a loss of half of the most primitive HSC in the bone marrow (BM) cells of HFD-fed mice, which exhibit lower hematopoietic reconstitution potential after transplantation. Impaired maintenance of HSC is due to reduced dormancy after HFD feeding. We discover that a HFD disrupts the TGF-ß receptor within lipid rafts, associated to impaired Smad2/3-dependent TGF-ß signaling, as the main molecular mechanism of action. Finally, injecting HFD-fed mice with recombinant TGF-ß1 avoids the loss of HSC and alteration of the BM's ability to recover, underscoring the fact that a HFD affects TGF-ß signaling on HSC.

T-cell acute lymphoblastic leukemia displays autocrine production of Interleukin-7.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy characterized by an accumulation of immature T cells. Although patient outcomes have improved, novel targeted therapies are needed to reduce the intensity of chemotherapy and improve the prognosis of high-risk patients. Interleukin-7 (IL-7) modulates the survival and proliferation of normal and malignant T cells. Targeting the IL-7 signaling pathway is thus a potentially effective therapeutic strategy. To achieve such aim, it is essential to first understand how the IL-7 signaling pathway is activated. Although IL-7 production has been observed from multiple stromal tissues, T-ALL autocrine IL-7 secretion has not yet been described. Interestingly, using T-ALL cell lines, primary and patient-derived xenotransplanted (PDX) T-ALL cells, we demonstrate that T-ALL cells produce IL-7 whereas normal T cells do not. Finally, using knock down of IL7 gene in T-ALL cells, we describe to what extent IL-7 autocrine secretion is involved in the T-ALL cells propagation in bone marrow and how it affects the number of leukemia-initiating cells in PDX mice. Together, these results demonstrate how the autocrine production of the IL-7 cytokine mediated by T-ALL cells can be involved in the oncogenic development of T-ALL and offer novel insights into T-ALL spreading.

LPCAT3 deficiency in hematopoietic cells alters cholesterol and phospholipid homeostasis and promotes atherosclerosis.

BACKGROUND AND AIMS: LPCAT3 plays a major role in phospholipid metabolism in the liver and intestine. However, the impact of LPCAT3 on hematopoietic cell and macrophage functions has yet to be described. Our aim was to understand the functions of LPCAT3 in macrophages and to investigate whether LPCAT3 deficiency in hematopoietic cells may affect atherosclerosis development. METHODS: Mice with constitutive Lpcat3 deficiency (Lpcat3-/-) were generated. We used fetal hematopoietic liver cells to generate WT and Lpcat3-/- macrophages in vitro and to perform hematopoietic cell transplantation in recipient Ldlr-/- mice. RESULTS: Lpcat3-deficient macrophages displayed major reductions in the arachidonate content of phosphatidylcholines, phosphatidylethanolamines and, unexpectedly, plasmalogens. These changes were associated with altered cholesterol homeostasis, including an increase in the ratio of free to esterified cholesterol and a reduction in cholesterol efflux in Lpcat3-/- macrophages. This correlated with the inhibition of some LXR-regulated pathways, related to altered cellular availability of the arachidonic acid. Indeed, LPCAT3 deficiency was associated with decreased Abca1, Abcg1 and ApoE mRNA levels in fetal liver cells derived macrophages. In vivo, these changes translated into a significant increase in atherosclerotic lesions in Ldlr-/- mice with hematopoietic LPCAT3 deficiency. CONCLUSIONS: This study identifies LPCAT3 as a key factor in the control of phospholipid homeostasis and arachidonate availability in myeloid cells and underlines a new role for LPCAT3 in plasmalogen metabolism. Moreover, our work strengthens the link between phospholipid and sterol metabolism in hematopoietic cells, with significant consequences on nuclear receptor-regulated pathways and atherosclerosis development.

Mining SAGE data allows large-scale, sensitive screening of antisense transcript expression.

As a growing number of complementary transcripts, susceptible to exert various regulatory functions, are being found in eukaryotes, high throughput analytical methods are needed to investigate their expression in multiple biological samples. Serial Analysis of Gene Expression (SAGE), based on the enumeration of directionally reliable short cDNA sequences (tags), is capable of revealing antisense transcripts. We initially detected them by observing tags that mapped on to the reverse complement of known mRNAs. The presence of such tags in individual SAGE libraries suggested that SAGE datasets contain latent information on antisense transcripts. We raised a collection of virtual tags for mining these data. Tag pairs were assembled by searching for complementarities between 24-nt long sequences centered on the potential SAGE-anchoring sites of well-annotated human expressed sequences. An analysis of their presence in a large collection of published SAGE libraries revealed transcripts expressed at high levels from both strands of two adjacent, oppositely oriented, transcription units. In other cases, the respective transcripts of such cis-oriented genes displayed a mutually exclusive expression pattern or were co-expressed in a small number of libraries. Other tag pairs revealed overlapping transcripts of trans-encoded unique genes. Finally, we isolated a group of tags shared by multiple transcripts. Most of them mapped on to retroelements, essentially represented in humans by Alu sequences inserted in opposite orientations in the 3'UTR of otherwise different mRNAs. Registering these tags in separate files makes possible computational searches focused on unique sense-antisense pairs. The method developed in the present work shows that SAGE datasets constitute a major resource of rapidly investigating with high sensitivity the expression of antisense transcripts, so that a single tag may be detected in one library when screening a large number of biological samples.

Trim33/Tif1γ is involved in late stages of granulomonopoiesis in mice.

Trim33/Tif1γ (Trim33) is a member of the tripartite motif family. Using a conditional hematopoietic-specific Trim33 knock-out (Trim33(Δ/Δ)) mouse, we showed previously that Trim33 deficiency in hematopoietic stem cells leads to severe defects in hematopoiesis, resembling the main features of human chronic myelomonocytic leukemia. We also demonstrated that Trim33 is involved in hematopoietic aging through TGFß signaling. Nevertheless, how Trim33 contributes to the terminal stages of myeloid differentiation remains to be clarified. We reveal here the crucial role of Trim33 expression in the control of mature granulomonocytic differentiation. An important component of Trim33-deficient mice is the alteration of myeloid differentiation, as characterized by dysplastic features, abnormal granulocyte and monocyte maturation, and the expansion of CD11b(+)Ly6G(high)Ly6C(low) myeloid cells, which share some features with polymorphonuclear-myeloid-derived suppressor cells. Moreover, in Trim33(Δ/Δ) mice, we observed the alteration of CSF-1-mediated macrophage differentiation in association with the lack of Csf-1 receptor. Altogether, these results indicate that Trim33 deficiency leads to the expansion of a subset of myeloid cells characterizing the myelodysplastic/myeloproliferative neoplasm.

CD45 phosphatase is crucial for human and murine acute myeloid leukemia maintenance through its localization in lipid rafts.

CD45 is a pan-leukocyte protein with tyrosine phosphatase activity involved in the regulation of signal transduction in hematopoiesis. Exploiting CD45 KO mice and lentiviral shRNA, we prove the crucial role that CD45 plays in acute myeloid leukemia (AML) development and maintenance. We discovered that CD45 does not colocalize with lipid rafts on murine and human non-transformed hematopoietic cells. Using a mouse model, we proved that CD45 positioning within lipid rafts is modified during their oncogenic transformation to AML. CD45 colocalized with lipid rafts on AML cells, which contributes to elevated GM-CSF signal intensity involved in proliferation of leukemic cells. We furthermore proved that the GM-CSF/Lyn/Stat3 pathway that contributes to growth of leukemic cells could be profoundly affected, by using a new plasma membrane disrupting agent, which rapidly delocalized CD45 away from lipid rafts. We provide evidence that this mechanism is also effective on human primary AML samples and xenograft transplantation. In conclusion, this study highlights the emerging evidence of the involvement of lipid rafts in oncogenic development of AML and the targeting of CD45 positioning among lipid rafts as a new strategy in the treatment of AML.

Analysis of remnant reticulocyte mRNA reveals new genes and antisense transcripts expressed in the human erythroid lineage.

BACKGROUND AND OBJECTIVES: We studied the gene expression profile of human purified reticulocytes to provide a transcriptional basis for the study of erythroid biology, differentiation and hematologic disorders. DESIGN AND METHODS: We screened highly purified blood reticulocytes from ten healthy adult volunteers. We chose a modified protocol of serial analysis of gene expression (SAGE), the serial analysis of downsized extracts (SADE). RESULTS: Data analysis revealed that 64% of gene signatures (tags) matched with known genes; mainly hemoglobin. In addition to the abundant globin mRNA, SAGE analysis identified previously described genes and new transcripts. In reticulocytes, which are poor in mRNA, we also identified 9% of EST and 27% of tags that did not match with any known genes. Mining our data, 70% of the unknown tags and 39% of tags identifying EST were found to be specific to the reticulocyte. We demonstrated the presence of a mRNA that matched with the reverse sequence of the hemoglobin b (HBB) transcript. INTERPRETATION AND CONCLUSIONS: This is the first description of an antisense transcript of the human HBB gene suggesting regulation by way of sense-antisense pairing. The well-characterized genes found in the SAGE library were genes specific to the blood cell lineage, housekeeping genes and, interestingly, genes not previously described in the reticulocyte. Furthermore the study provides markers of the erythroid lineage regulated during the differentiation process as observed in in vitro experiments.