Unequal contribution of Akt isoforms in the double-negative to double-positive thymocyte transition.

Pre-TCR signals regulate the transition of the double-negative (DN) 3 thymocytes to the DN4, and subsequently to the double-positive (DP) stage. In this study, we show that pre-TCR signals activate Akt and that pharmacological inhibition of the PI3K/Akt pathway, or combined ablation of Akt1 and Akt2, and to a lesser extent Akt1 and Akt3, interfere with the differentiation of DN3 and the accumulation of DP thymocytes. Combined ablation of Akt1 and Akt2 inhibits the proliferation of DN4 cells, while combined ablation of all Akt isoforms also inhibits the survival of all the DN thymocytes. Finally, the combined ablation of Akt1 and Akt2 inhibits the survival of DP thymocytes. Constitutively active Lck-Akt1 transgenes had the opposite effects. We conclude that, following their activation by pre-TCR signals, Akt1, Akt2, and, to a lesser extent, Akt3 promote the transition of DN thymocytes to the DP stage, in part by enhancing the proliferation and survival of cells undergoing beta-selection. Akt1 and Akt2 also contribute to the differentiation process by promoting the survival of the DP thymocytes.

Haploinsufficiency of the Hmga1 gene causes cardiac hypertrophy and myelo-lymphoproliferative disorders in mice.

The HMGA1 protein is a major factor in chromatin architecture and gene control. It plays a critical role in neoplastic transformation. In fact, blockage of HMGA1 synthesis prevents rat thyroid cell transformation by murine transforming retroviruses, and an adenovirus carrying the HMGA1 gene in the antisense orientation induces apoptotic cell death in anaplastic human thyroid carcinoma cell lines, but not in normal thyroid cells. Moreover, both in vitro and in vivo studies have established the oncogenic role of the HMGA1 gene. In this study, to define HMGA1 function in vivo, we examined the consequences of disrupting the Hmga1 gene in mice. Both heterozygous and homozygous mice for the Hmga1-null allele show cardiac hypertrophy due to the direct role of HMGA1 on cardiomyocytic cell growth regulation. These mice also developed hematologic malignancies, including B cell lymphoma and myeloid granuloerythroblastic leukemia. The B cell expansion and the increased expression of the RAG1/2 endonuclease, observed in HMGA1-knockout spleen tissues, might be responsible for the high rate of abnormal IgH rearrangements observed in these neoplasias. Therefore, the data reported here indicate the critical role of HMGA1 in heart development and growth, and reveal an unsuspected antioncogenic potential for this gene in hematologic malignancies.

DLX genes as targets of ALL-1: DLX 2,3,4 down-regulation in t(4;11) acute lymphoblastic leukemias.

Dlx genes constitute a gene family thought to be essential in morphogenesis and development. We show here that in vertebrate cells, Dlx genes appear to be part of a regulatory cascade initiated by acute lymphoblastic leukemia (ALL)-1, a master regulator gene whose disruption is implicated in several human acute leukemias. The expression of Dlx2, Dlx3, Dlx5, Dlx6, and Dlx7 was absent in All-1 -/- mouse embryonic stem cells and reduced in All-1 +/- cells. In leukemic patients affected by the t(4;11)(q21;q23) chromosomal abnormality, the expression of DLX2, DLX3, and DLX4 was virtually abrogated. Our data indicate that Dlx genes are downstream targets of ALL-1 and could be considered as important tools for the study of the early leukemic cell phenotype.

Loss of Hmga1 gene function affects embryonic stem cell lympho-hematopoietic differentiation.

By interacting with transcription machinery, high-mobility group A 1 (HMGA1) proteins alter the chromatin structure and thereby regulate the transcriptional activity of several genes. To assess their role in development, we studied the in vitro differentiation of embryonic stem (ES) cells that bear one or both disrupted Hmga1 alleles. Here, we report that Hmga1 null ES cells generate fewer T-cell precursors than do wild-type ES cells. Indeed, they preferentially differentiate to B cells, probably consequent to decreased interleukin 2 expression and increased interleukin 6 expression. Moreover, a lack of HMGA1 expression induces changes in hemopoietic differentiation, i.e., a reduced monocyte/macrophage population and an increase in megakaryocyte precursor numbers, erythropoiesis, and globin gene expression. Re-expression of the Hmga1 gene in Hmga1 null ES cells restores the wild-type phenotype. The effect on megakaryocyte/erythrocyte lineages seems, at least in part, mediated by the GATA-1 transcription factor, a key regulator of red blood cell differentiation. In fact, we found that Hmga1-/- ES cells overexpress GATA-1 and that HMGA1 proteins directly control GATA-1 transcription. Taken together, these data indicate that HMGA1 proteins play a prime role in lymphohematopoietic differentiation.

TCL1 participates in early embryonic development and is overexpressed in human seminomas.

Overexpression of the TCL1 oncogene has been shown to play a causative role in T cell leukemias of humans and mice. The characterization of Tcl1-deficient mice in these studies indicates an important developmental role for Tcl1 in early embryogenesis. In wild-type embryos, Tcl1 is abundant in the first three mitotic cycles, during which it shuttles between nuclei and the embryo cortical regions in a cell-cycle-dependent fashion. The absence of this protein in early embryogenesis results in reduced fertility of female mice. The present studies elucidate the mechanism responsible for the reduced female fertility through analysis of the oogenesis stages and early embryo development in Tcl1-deficient mice. Even though Tcl1(-/-) females display normal oogenesis and rates of oocyte maturation/ovulation and fertilization, the lack of maternally derived Tcl1 impairs the embryo's ability to undergo normal cleavage and develop to the morula stage, especially under in vitro culture conditions. Beyond this crisis point, differentiative traits of zygotic genome activation and embryo compaction can take place normally. In contrast with this unanticipated role in early embryogenesis, we observed an overexpression of TCL1 in human seminomas. This finding suggests that TCL1 dysregulation could contribute to the development of this germinal cell cancer as well as lymphoid malignancies.

Overexpression of the HMGA2 gene in transgenic mice leads to the onset of pituitary adenomas.

Overexpression of the HMGA2 gene is a common feature of neoplastic cells both in experimental and human models. Intragenic and extragenic HMGA2 rearrangements responsible for HMGA2 gene overexpression have been frequently detected in human benign tumours of mesenchymal origin. To better understand the role of HMGA2 overexpression in human tumorigenesis, we have generated transgenic mice carrying the HMGA2 gene under the transcriptional control of the cytomegalovirus promoter. High expression of the transgene was demonstrated in all the mouse tissues analysed, whereas no expression of the endogenous HMGA2 gene was detected in the same tissues from wild-type mice. In this study, two independent lines of transgenic mice have been generated. By 6 months of age, 85% of female animals of both transgenic lines developed pituitary adenomas secreting prolactin and growth hormone. The transgenic males developed the same phenotype with a lower penetrance (40%) and a longer latency period (about 18 months). Therefore, these data demonstrate that the overexpression of HMGA2 leads to the onset of mixed growth hormone/prolactin cell pituitary adenomas. These transgenic mice may represent an important tool for the study of this kind of neoplasia.