Molecular patterns of response and treatment failure after frontline venetoclax combinations in older patients with AML.

The BCL-2 inhibitor venetoclax combined with hypomethylating agents or low-dose cytarabine represents an important new therapy for older or unfit patients with acute myeloid leukemia (AML). We analyzed 81 patients receiving these venetoclax-based combinations to identify molecular correlates of durable remission, response followed by relapse (adaptive resistance), or refractory disease (primary resistance). High response rates and durable remissions were typically associated with NPM1 or IDH2 mutations, with prolonged molecular remissions prevalent for NPM1 mutations. Primary and adaptive resistance to venetoclax-based combinations was most commonly characterized by acquisition or enrichment of clones activating signaling pathways such as FLT3 or RAS or biallelically perturbing TP53. Single-cell studies highlighted the polyclonal nature of intratumoral resistance mechanisms in some cases. Among cases that were primary refractory, we identified heterogeneous and sometimes divergent interval changes in leukemic clones within a single cycle of therapy, highlighting the dynamic and rapid occurrence of therapeutic selection in AML. In functional studies, FLT3 internal tandem duplication gain or TP53 loss conferred cross-resistance to both venetoclax and cytotoxic-based therapies. Collectively, we highlight molecular determinants of outcome with clinical relevance to patients with AML receiving venetoclax-based combination therapies.

Early T cell receptor beta gene expression is regulated by the pre-T cell receptor-CD3 complex.

We have examined the question of whether there is an additional checkpoint in T cell development that regulates T cell receptor (TCR)-beta expression in CD25+44- thymocytes by mechanisms that are independent of the pre-TCR. Our analysis in various mutant mice indicates that all changes in cytoplasmic TCR-beta expression can be accounted for by pre-TCR-dependent signal mediation, putting into question the function of a putative pro-TCR.

Role of different T cell receptors in the development of pre-T cells.

The development of pre-T cells with productive TCR-beta rearrangements can be mediated by each the pre-T cell receptor (pre-TCR), the TCR-alphabeta as well as the TCR-gammadelta, albeit by distinct mechanisms. Although the TCR-gammadelta affects CD4-8- precursor cells irrespective of their rearrangement status by TCR-beta mechanisms not involving TCR-beta selection, both the pre-TCR and the TCR-alphabeta select only cells with productive TCR-beta genes for expansion and maturation. The TCR-alphabeta appears to be much less effective than the pre-TCR because of the paucity of TCR-alpha proteins in TCR-beta-positive precursors since an early expressed transgenic TCR-alphabeta can largely substitute for the pre-TCR. Thus, the TCR-alphabeta can assume a role not only in the rescue from programmed cell death of CD4+8+ but also of CD4-8- thymocytes. In evolution this double function of the TCR-alphabeta may have been responsible for the maturation of alphabeta T cells before the advent of the pre-TCR-alpha chain.

Allelic exclusion of the T cell receptor beta locus requires the SH2 domain-containing leukocyte protein (SLP)-76 adaptor protein.

Signaling via the pre-T cell receptor (TCR) is required for the proliferative expansion and maturation of CD4(-)CD8(-) double-negative (DN) thymocytes into CD4(+)CD8(+) double-positive (DP) cells and for TCR-beta allelic exclusion. The adaptor protein SH2 domain-containing leukocyte protein (SLP)-76 has been shown to play a crucial role in thymic development, because thymocytes of SLP-76(-/-) mice are arrested at the CD25(+)CD44(-) DN stage. Here we show that SLP-76(-/-) DN thymocytes express the pre-TCR on their surfaces and that introduction of a TCR-alpha/beta transgene into the SLP-76(-/-) background fails to cause expansion of DN thymocytes or developmental progression to the DP stage. Moreover, analysis of TCR-beta rearrangement in SLP-76(-/-) TCR-transgenic mice or in single CD25(+)CD44(-) DN cells from SLP-76(-/-) mice indicates an essential role of SLP-76 in TCR-beta allelic exclusion.

The common cytokine receptor gamma chain and the pre-T cell receptor provide independent but critically overlapping signals in early alpha/beta T cell development.

Intracellular signals emanating from cytokine and antigen receptors are integrated during the process of intrathymic development. Still, the relative contributions of cytokine receptor signaling to pre-T cell receptor (TCR) and TCR-mediated differentiation remain undefined. Interleukin (IL)-7 interactions with its cognate receptor complex (IL-7Ralpha coupled to the common cytokine receptor gamma chain, gammac) play a dominant role in early thymopoiesis. However, alpha/beta T cell development in IL-7-, IL-7Ralpha-, and gammac-deficient mice is only partially compromised, suggesting that additional pathways can rescue alpha/beta T lineage cells in these mice. We have investigated the potential interdependence of gammac- and pre-TCR-dependent pathways during intrathymic alpha/beta T cell differentiation. We demonstrate that gammac-dependent cytokines do not appear to be required for normal pre-TCR function, and that the rate-limiting step in alpha/beta T cell development in gammac- mice does not involve TCR-beta chain rearrangements, but rather results from poor maintenance of early thymocytes. Moreover, mice double mutant for both gammac and pre-Talpha show vastly reduced thymic cellularity and a complete arrest of thymocyte differentiation at the CD44(+)CD25(+) cell stage. These observations demonstrate that the pre-TCR provides the gammac-independent signal which allows alpha/beta T cell development in gammac- mice. Thus, a series of overlapping signals derived from cytokine and T cell receptors guide the process of alpha/beta thymocyte development.

Pleiotropic changes controlled by the pre-T-cell receptor.

The construction of various gene-deficient mice has facilitated the understanding of the role of various receptors and signaling pathways that control the generation of alphabeta lineage cells. A predominant role is occupied by the pre-TCR, which not only generates large numbers of alphabeta lineage cells but also controls TCRbeta allelic exclusion as well as commitment to the gammadelta lineage versus the alphabeta lineage.

The pre-B cell receptor in B cell development: recent advances, persistent questions and conserved mechanisms.

B cell development is a process tightly regulated by the orchestrated signaling of cytokine receptors, the pre-B cell receptor (BCR) and the B cell receptor (BCR). It commences with common lymphoid progenitors (CLP) up-regulating the expression of B cell-related genes and committing to the B cell lineage. Cytokine signaling (IL-7, stem cell factor, FLT3-L) is essential at this stage of development as it suppresses cell death, sustains proliferation and facilitates heavy chain rearrangements. As a result of heavy chain recombination, the pre-BCR is expressed, which then becomes the primary determiner of survival, cell cycle entry and allelic exclusion. In this review, we discuss the mechanisms of B cell lineage commitment and describe the signaling pathways that are initiated by the pre-BCR. Finally, we compare pre-BCR and pre-TCR structure, signal transduction and function, drawing parallels between early pre-B and pre-T cell development.

Activated Notch counteracts Ikaros tumor suppression in mouse and human T-cell acute lymphoblastic leukemia.

Activating NOTCH1 mutations occur in ~60% of human T-cell acute lymphoblastic leukemias (T-ALLs), and mutations disrupting the transcription factor IKZF1 (IKAROS) occur in ~5% of cases. To investigate the regulatory interplay between these driver genes, we have used a novel transgenic RNA interference mouse model to produce primary T-ALLs driven by reversible Ikaros knockdown. Restoring endogenous Ikaros expression in established T-ALL in vivo acutely represses Notch1 and its oncogenic target genes including Myc, and in multiple primary leukemias causes disease regression. In contrast, leukemias expressing high levels of endogenous or engineered forms of activated intracellular Notch1 (ICN1) resembling those found in human T-ALL rapidly relapse following Ikaros restoration, indicating that ICN1 functionally antagonizes Ikaros in established disease. Furthermore, we find that IKAROS mRNA expression is significantly reduced in a cohort of primary human T-ALL patient samples with activating NOTCH1/FBXW7 mutations, but is upregulated upon acute inhibition of aberrant NOTCH signaling across a panel of human T-ALL cell lines. These results demonstrate for the first time that aberrant NOTCH activity compromises IKAROS function in mouse and human T-ALL, and provide a potential explanation for the relative infrequency of IKAROS gene mutations in human T-ALL.

T-cell development in the absence of the pre-T-cell receptor.

The development of pre-T-cells with productive T-cell receptor beta (TCR beta) rearrangements can be furthered by each of the pre-T-cell receptors (pre-TCR), the alpha beta TCR as well as the gamma delta TCR, albeit by distinct mechanisms. While the gamma delta TCR affects CD4-8- precursor cells irrespective of their TCR beta rearrangement status both the pre-TCR and the alpha beta TCR select only cells with productive TCR beta genes for expansion and maturation. The alpha beta TCR is much less effective than the pre-TCR because of the paucity of TCR alpha proteins in TCR beta positive precursors.

Expression of corticotrophin-releasing hormone in the mouse uterus: participation in embryo implantation.

The detection of corticotropin-releasing hormone (CRH) in the pregnant and non-pregnant uterus has driven research to determine the role of this 41 amino acid neuropeptide in the female reproductive system. As concentrations of CRH mRNA and its peptide product are greater in the implantation sites of the early pregnant uterus compared with the regions between implantation sites, CRH has been hypothesised to participate in blastocyst implantation. Using the mouse system as an experimental model, we studied the distribution of CRH in the uterus during the oestrus cycle and early gestational period, and now provide evidence for its involvement in embryo implantation using cell culture techniques. The percentage of CRH-positive uterine cells and the amount of CRH released during anoestrus, pro-oestrus and oestrus were determined by immunofluorescence and ELISA experiments respectively. The highest number of intracellularly CRH-positive cells was obtained during pro-oestrus, whereas the highest CRH concentration in uterine cell culture supernatants was detected during anoestrus. At early stages of gestation, CRH was detected in the endometrium on days 2, 3 and 4 of pregnancy and in the myometrium on days 3 and 4, whereas it was undetectable on day 5. The functional role of CRH during early gestation was evaluated by administering anti-CRH antibody to mice from day 3 to day 8 of pregnancy. This treatment resulted in implantation failure in 60% of the cases, in which implantation sites, although clearly present in the uterus, had failed to host an embryo. These results provide direct evidence about the involvement of CRH in murine embryo implantation and are in agreement with hypotheses postulated in humans.

The controversial role of the Hedgehog pathway in normal and malignant hematopoiesis.

Hedgehog (Hh) is a developmental signaling pathway in which Hh ligands bind Patched (Ptch), which relieves its inhibition of Smoothened (Smo), allowing the Gli family of transcription factors to translocate to the nucleus and activate Hh target genes. The role of Hh signaling in hematopoiesis is controversial and ill defined. Although some groups observed self-renewal defects with decreased replating and reduced efficiency of secondary murine transplants, other groups reported no hematopoietic phenotypes, which may be related to the timing of Hh abrogation. In malignant hematopoiesis, most attention has been focused on the role of Hh signaling in chronic myeloid leukemia (CML), considered by many to be a stem cell disorder that bears the constitutively active BCR-ABL tyrosine kinase. Despite the elimination of most leukemia cells through BCR-ABL inhibition, most patients remain PCR positive, suggesting that the putative CML stem cell may be resistant to kinase antagonism. Groups are now exploring the Hh pathway as an alternate pathway supporting CML stem cell survival. Knockdown or inhibition of Smo abrogates or delays the appearance of CML in several in vitro and in vivo models. These data have lead to clinical trials using BCR-ABL kinase and novel Smo inhibitors in combination.

PHF6 mutations in adult acute myeloid leukemia.

Loss of function mutations and deletions encompassing the plant homeodomain finger 6 (PHF6) gene are present in about 20% of T-cell acute lymphoblastic leukemias (ALLs). Here, we report the identification of recurrent mutations in PHF6 in 10/353 adult acute myeloid leukemias (AMLs). Genetic lesions in PHF6 found in AMLs are frameshift and nonsense mutations distributed through the gene or point mutations involving the second plant homeodomain (PHD)-like domain of the protein. As in the case of T-ALL, where PHF6 alterations are found almost exclusively in males, mutations in PHF6 were seven times more prevalent in males than in females with AML. Overall, these results identify PHF6 as a tumor suppressor gene mutated in AML and extend the role of this X-linked tumor suppressor gene in the pathogenesis of hematologic tumors.

The ubiquitous nature of cancer: the role of the SCF(Fbw7) complex in development and transformation.

The ubiquitin-proteasome system (UPS) is a multi-subunit pathway that allows for ubiquitin modification of proteins and leads to either degradation or other non-proteolytic processes such as trafficking or transcriptional activation. Given its role as a regulator of cellular homeostasis it is not surprising that members of the UPS are frequently aberrantly expressed in a number of disease states including cancer. This review will focus on one member of the UPS, the F-box protein, Fbw7 (also known as Sel-10, Ago, hCDC4) and mechanisms by which Fbw7 interacts with its substrates in the context of development and tumorigenesis will be discussed. In addition, antagonists of this pathway as well as current and future therapeutics for the UPS will be examined.

Essential role of the pre-T cell receptor in allelic exclusion of the T cell receptor beta locus.

Following the recent realization that TCR beta transgenes can severely inhibit the rearrangement of endogenous Vbeta gene segments in the absence of pre-TCR alpha (pT alpha) chains, we tested whether the pre-TCR has an essential role in TCR beta allelic exclusion under more physiological conditions by analyzing TCR rearrangement in immature thymocytes by single-cell PCR. Our results in pT alpha+ mice are consistent with an ordered model of TCR beta rearrangement beginning on one allele and continuing on the other only when the first attempt is unsuccessful. By contrast, a higher proportion of thymocytes from pT alpha-/- mice exhibited two productive TCR beta alleles. Thus, the pre-TCR-independent suppression of rearrangement by TCR beta transgenes represents a transgene artifact, whereas under physiological conditions the pre-TCR is essential for allelic exclusion.

Nitric oxide production by pre-implantation embryos in response to embryotoxic factors.

In this report, we examined whether nitric oxide (NO) is involved in early embryo death. We have chosen various experimentally defined embryotoxic stimuli in mice and determined their ability to induce NO production by 2-cell stage embryos. The embryotoxic factors used were interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), lipopolysaccharide (LPS), 5-Azacytidine (5-AzaC) and the murine embryotoxic antibody DF4. We showed that in all cases the embryotoxic stimuli induced NO production by early stage embryos that correlated with the induction of the inducible and/or endothelial isoforms of NO synthase. This study was also extended to the human system where sera from women who aborted were tested for their ability to act embryotoxically by inducing NO in early mouse embryos and mature murine placenta. The results obtained confirmed the embryotoxic character of NO found in these particular sera leading to the hypothesis that NO plays a potential role in early embryo death.

Inhibition of nitric oxide production rescues LPS-induced fetal abortion in mice.

In this report, we examined the involvement of the cytokines tumor necrosis factor (TNF)-alpha, interferon (IFN)-gamma, interleukin (IL)-4, and IL-10 as well as nitric oxide (NO) in the lipopolysaccharide (LPS)-induced experimental abortion model in BALB/c mice. Although in vivo administration of LPS in pregnant mice showed a 72% decrease of serum IL-10, no significant difference in serum TNF-alpha, IFN-gamma, and IL-4 levels, compared to controls, could be detected. At the same time, a correlation of fetal abortion and maternal splenomegaly with an important increase of NO synthesis in the serum was obtained. Simultaneous administration of LPS and aminoguanidine (AG; an inhibitor to NO synthase) rescued the LPS-induced fetal abortion, reduced maternal spleen weight to physiological levels, and decreased serum NO concentration to control levels. In vitro experiments showed that LPS directly induced NO production in primary placental cells and the TPOPHO-1 trophoblast cell line by stimulating the inducible isoform of NO synthase, which ultimately could be blocked by the NO synthase inhibitors AG and L-NAME. The results indicate that LPS, despite its beneficial involvement in intracellular infections, participates in inflammatory/autoimmune damage during pregnancy, leading to embryotoxicity, which is closely linked to the NO pathway.

On the role of the pre-T cell receptor in alphabeta versus gammadelta T lineage commitment.

The role of the pre-T cell receptor (TCR) in lineage commitment to the gammadelta versus alphabeta lineage of T cells was addressed by analyzing TCRbeta chain rearrangements in gammadelta T cells from wild-type and pre-TCR-deficient mice by single cell polymerase chain reaction. Results show that the pre-TCR selects against gammadelta T cells containing rearranged Vbeta genes and that gammadelta T cell precursors but not gammadelta T cells express the pre-TCRalpha protein. Furthermore, pre-TCR-induced proliferation could not be detected in gammadelta T cells. We propose that the pre-TCR commits developing T cells to the alphabeta lineage by an instructive mechanism that has largely replaced an evolutionary more ancient stochastic mechanism of lineage commitment.

Constitutive pre-TCR signaling promotes differentiation through Ca2+ mobilization and activation of NF-kappaB and NFAT.

Pre-T cell antigen receptor (pre-TCR) signaling plays a crucial role in the development of immature T cells. Although certain aspects of proximal pre-TCR signaling have been studied, the intermediate signal transducers and the distal transcription modulators have been poorly characterized. We report here a correlation between pre-TCR signaling and a biphasic rise in the cytosolic Ca2+ concentration. In addition, we show that constitutive pre-TCR signaling is associated with an increased rate of Ca2+ influx through store-operated plasma membrane Ca2+ channels. We show also that the biphasic nature of the observed pre-TCR-induced rise in cytosolic Ca2+ differentially modulates the activities of the transcription factors NF-kappaB and NFAT in developing T cells.

Crucial function of the pre-T-cell receptor (TCR) in TCR beta selection, TCR beta allelic exclusion and alpha beta versus gamma delta lineage commitment.

The analysis of T-cell receptor (TCR) beta selection, TCR beta allelic exclusion and TCR beta rearrangement in gamma delta T cells from normal and pre-TCR-deficient mice has shown that the pre-TCR has a crucial role in T-lymphocyte development: The pre-TCR is by far the most effective receptor that generates large numbers of CD4+8+ T cells with productive TCR beta rearrangements. In the absence of the pre-TCR, TCR beta rearrangement proceeds in developing cells irrespective of whether they already contain a productive TCR beta gene. The pre-TCR directs developing T cells to the alpha beta lineage because gamma delta T cells from pT alpha-/- mice proceed much further in TCR beta rearrangement than gamma delta T cells from wild-type mice. It is argued that the pre-TCR commits developing T cells to the alpha beta lineage by an instructive mechanism, which has largely replaced an evolutionarily more ancient mechanism that involves stochastic alpha beta lineage commitment.

Somatic activation of beta-catenin bypasses pre-TCR signaling and TCR selection in thymocyte development.

Mutation or ablation of T cell factor 1 and lymphocyte enhancer factor 1 indicated involvement of the Wnt pathway in thymocyte development. The central effector of the Wnt pathway is beta-catenin, which undergoes stabilization upon binding of Wnt ligands to frizzled receptors. We report here that conditional stabilization of beta-catenin in immature thymocytes resulted in the generation of single positive T cells that lacked the alpha beta TCR and developed in the absence of pre-TCR signaling and TCR selection. Although active beta-catenin induced differentiation in the absence of TCRs, its action was associated with reduced proliferation and survival when compared to developmental changes induced by the pre-TCR or the alpha beta TCR.