The development of B cells is dependent on the sequential DNA rearrangement of immunoglobulin loci that encode subunits of the B cell receptor. The pathway navigates a crucial checkpoint that ensures expression of a signalling-competent immunoglobulin heavy chain before commitment to rearrangement and expression of an immunoglobulin light chain. The checkpoint segregates proliferation of pre-B cells from immunoglobulin light chain recombination and their differentiation into B cells. Recent advances have revealed the molecular circuitry that controls two rival signalling systems, namely the interleukin-7 (IL-7) receptor and the pre-B cell receptor, to ensure that proliferation and immunoglobulin recombination are mutually exclusive, thereby maintaining genomic integrity during B cell development.

Matrix metalloproteinases (MMPs) are a major group of enzymes that regulate cell-matrix composition. MMP genes show a highly conserved modular structure. Ample evidence exists on the role of MMPs in normal and pathological processes, including embryogenesis, wound healing, inflammation, arthritis, cardiovascular diseases, pulmonary diseases and cancer. The expression patterns of MMPs have interesting implications for the use of MMP inhibitors as therapeutic agents. Insights might be gained as to the preference for a general MMP inhibitor as opposed to an inhibitor designed to be specific for certain MMP family members as it relates to a defined disease state, and may give clues to potential side effects. The signalling pathways that lead to induction of expression of MMPs are still incompletely understood, but certain patterns are beginning to emerge. Regarding inhibition of MMP expression at the level of kinase pathways, it is possible that selective chemical inhibitors for distinct signalling pathways (e.g. MAPK, PKC) will hopefully, soon be available for initial clinical trials. Overexpression of selective dual specificity MAPK phosphatases have been shown to prevent MMP promoter activation which could also be used as a novel strategy to prevent activation of AP-1 and ETS transcription factors and MMP promoters in vivo. Interactions between members of different transcription factors provide fine-tuning of the transcriptional regulation of MMP promoter activity. MMPs play a crucial role in tumor invasion. Although the expression of MMPs in malignancies has been studied widely, the specific role of distinct MMPs in the progression of cancer may be more complex than has been assumed. For example, it has recently been shown that MMP-3, MMP-7, MMP-9 and MMP-12 can generate angiostatin from plasminogen, indicating that their expression in peritumoral area may in fact serve to limit angiogenesis and thereby inhibit tumor growth and invasion. The recent view about the role of stromal cells in the progression of cancer cell growth and metastasis is particularly interesting, and additional studies about the regulation of MMP gene expression and activity in malignancies are needed to understand the role and regulation of MMPs in tumor cell invasion.

T-cell acute lymphoblastic leukemia (T-ALL), unlike other ALL types, is only infrequently associated with chromosomal aberrations, but it was recently shown that most individuals with T-ALL carry activating mutations in the NOTCH1 gene. However, the signaling pathways and target genes responsible for Notch1-induced neoplastic transformation remain undefined. We report here that constitutively active Notch1 activates the NF-kappaB pathway transcriptionally and via the IkappaB kinase (IKK) complex, thereby causing increased expression of several well characterized target genes of NF-kappaB in bone marrow hematopoietic stem cells and progenitors. Our observations demonstrate that the NF-kappaB pathway is highly active in established human T-ALL and that inhibition of the pathway can efficiently restrict tumor growth both in vitro and in vivo. These findings identify NF-kappaB as one of the major mediators of Notch1-induced transformation and suggest that the NF-kappaB pathway is a potential target of future therapies of T-ALL.

The molecular crosstalk between the interkeukin-7 receptor (IL-7R) and pre-BCR in B lymphopoiesis has been enigmatic. We demonstrate that in pre-B cells, the IL-7R, but not the pre-BCR, was coupled to the phosphatidylinositol-3-OH kinase (PI(3)K)–Akt module, signaling by which prevents Rag expression. Attenuation of IL-7 signaling resulted in up-regulation of Foxo1 and Pax5, which co-activated many pre-B cell genes, including Rag1,2 and Blnk. Induction of the latter gene enabled pre-BCR signaling via the Syk-BLNK module and promoted immunoglobulin light chain rearrangement. BLNK signaling also antagonized Akt activation, thereby augmenting Foxo1 and Pax5 accumulation. This self-reinforcing molecular circuit appears to sense limiting concentrations of IL-7 and functions to control the expansion and differentiation of pre-B cells.

During B lymphopoiesis, Igk recombination requires pre-B cell receptor (pre-BCR) expression and escape from interleukin 7 receptor (IL-7R) signaling. By activating the transcription factor STAT5, IL-7R signaling maintains proliferation and represses Igk germline transcription by unknown mechanisms. We demonstrate that STAT5 tetramer bound the Igk intronic enhancer (Eκi), leading to recruitment of the histone methyltransferase Ezh2. Ezh2 marked H3K27me3 throughout Jκ to Cκ. In the absence of Ezh2, IL-7 failed to repress Igk germline transcription. H3K27me3 modifications were lost after termination of IL-7R–STAT5 signaling and E2A bound Eκi, resulting in acquisition of H3K4me1 and H4Ac. Genome-wide analyses revealed a STAT5 tetrameric binding motif associated with transcriptional repression. These data demonstrate how IL-7R signaling represses Igk germline transcription and provide a general model for STAT5-mediated epigenetic transcriptional repression.

Signals through the pre-B cell antigen receptor (pre-BCR) and IL-7 receptor (IL-7R) coordinate pre-B cell expansion with subsequent Igκ recombination. While many downstream effectors of each receptor are known, how they integrate to mediate development has remained unclear. Herein, we report that pre-BCR mediated activation of the Ras/MEK/ERK signaling pathway silences Ccnd3 transcription and coordinates cell cycle exit with the induction of E2A and the initiation of Igκ recombination. These activities are opposed by IL-7R mediated STAT 5 activation which promotes Ccnd3 expression and concomitantly inhibits Igκ transcription by binding to E κi and inhibiting E2A recruitment. Our data reveal how pre-BCR signaling poises pre-B cells to undergo differentiation upon escape from IL-7R signaling.

The pre–T cell receptor (TCR) is expressed early during T cell development and imposes a tight selection for differentiating T cell progenitors. Pre-TCR–expressing cells are selected to survive and differentiate further, whereas pre-TCR− cells are “negatively” selected to die. The mechanisms of pre-TCR–mediated survival are poorly understood. Here, we describe the induction of the antiapoptotic gene BCL2A1 (A1) as a potential mechanism regulating inhibition of pre–T cell death. We characterize in detail the signaling pathway involved in A1 induction and show that A1 expression can induce pre–T cell survival by inhibiting activation of caspase-3. Moreover, we show that in vitro “knockdown” of A1 expression can compromise survival even in the presence of a functional pre-TCR. Finally, we suggest that pre-TCR–induced A1 overexpression can contribute to T cell leukemia in both mice and humans.

SUMMARY The RAG1 endonuclease, together with its cofactor RAG2, is essential for V(D)J recombination but is a potent threat to genome stability. The sources of RAG1 mis-targeting and the mechanisms that have evolved to suppress it are poorly understood. Here, we report that RAG1 associates with chromatin at thousands of active promoters and enhancers in the genome of developing lymphocytes. The mouse and human genomes appear to have responded by reducing the abundance of “cryptic” recombination signals near RAG1 binding sites. This depletion operates specifically on the RSS heptamer, whereas nonamers are enriched at RAG1 binding sites. Reversing this RAG-driven depletion of cleavage sites by insertion of strong recombination signals creates an ectopic hub of RAG-mediated V(D)J recombination and chromosomal translocations. Our findings delineate rules governing RAG binding in the genome, identify areas at risk of RAG-mediated damage, and highlight the evolutionary struggle to accommodate programmed DNA damage in developing lymphocytes.