Msh2-dependent DNA repair mitigates a unique susceptibility of B cell progenitors to c-Myc-induced lymphomas

Interactions between the DNA Damage Response and the Telomere Complex in Carcinogenesis: A Hypothesis

Contrary to what was once thought, direct cancer originating from normal stem cells seems to be extremely rare. This is consistent with a preneoplastic period of telomere length reduction/damage in committed cells that becomes stabilized in transformation. Multiple observations suggest that telomere damage is an obligatory step preceding its stabilization. During tissue turnover, the telomeres of cells undergoing differentiation can be damaged as a consequence of defective DNA repair caused by endogenous or exogenous agents. This may result in the emergence of new mechanism of telomere maintenance which is the final outcome of DNA damage and the initial signal that triggers malignant transformation. Instead, transformation of stem cells is directly induced by primary derangement of telomere maintenance mechanisms. The newly modified telomere complex may promote survival of cancer stem cells, independently of telomere maintenance. An inherent resistance of stem cells to transformation may be linked to specific, robust mechanisms that help maintain telomere integrity.

Lessons from Using Genetically Engineered Mouse Models of MYC-Induced Lymphoma

Oncogenic overexpression of MYC leads to the fatal deregulation of signaling pathways, cellular metabolism, and cell growth. MYC rearrangements are found frequently among non-Hodgkin B-cell lymphomas enforcing MYC overexpression. Genetically engineered mouse models (GEMMs) were developed to understand MYC-induced B-cell lymphomagenesis. Here, we highlight the advantages of using Eµ-Myc transgenic mice. We thoroughly compiled the available literature to discuss common challenges when using such mouse models. Furthermore, we give an overview of pathways affected by MYC based on knowledge gained from the use of GEMMs. We identified top regulators of MYC-induced lymphomagenesis, including some candidates that are not pharmacologically targeted yet.

Rap1 regulates hematopoietic stem cell survival and affects oncogenesis and response to chemotherapy

Increased levels and non-telomeric roles have been reported for shelterin proteins, including RAP1 in cancers. Herein using Rap1 null mice, we provide the genetic evidence that mammalian Rap1 plays a major role in hematopoietic stem cell survival, oncogenesis and response to chemotherapy. Strikingly, this function of RAP1 is independent of its association with the telomere or with its known partner TRF2. We show that RAP1 interacts with many members of the DNA damage response (DDR) pathway. RAP1 depleted cells show reduced interaction between XRCC4/DNA Ligase IV and DNA-PK, and are impaired in DNA Ligase IV recruitment to damaged chromatin for efficient repair. Consistent with its role in DNA damage repair, RAP1 loss decreases double-strand break repair via NHEJ in vivo, and consequently reduces B cell class switch recombination. Finally, we discover that RAP1 levels are predictive of the success of chemotherapy in breast and colon cancer.

[Proteomic analysis and verification of protein expression after upregulation of human CD99 in Hodgkin lymphoma cell line L428]

Objective: To investigate the proteins expression difference after upregulation of human CD99 in Hodgkin Lymphoma cell line, L428 cell, and verify the function of differential proteins. Methods: The differential proteins were detected by two-dimensional fluorescence difference gel electrophoresis and mass spectrometry analysis, cluster analysis was done by GOfact. Results: There were 38 proteins screened out, of which 21 proteins were positively associated with CD99, while 17 proteins were negative. Among the 38 proteins, 32 proteins participated in biological process, and 35 proteins were involved in the composition and construction. And 28 proteins participated in multifaceted biological activities including antioxidation, protein binding, catalytic activity, regulation of enzyme, signal transduction, molecular structure, regulation of translation and ion transport. Conclusions: The changes of the differential proteins, correlated with cytoskeleton, cell differentiation, signal pathway and regulating gene expression, are closely relevant to the translation between Hodgkin/Reed-Sternberg and B lymphocyte cell.

Mir-17∼92 Confers Motor Neuron Subtype Differential Resistance to ALS-Associated Degeneration

Progressive degeneration of motor neurons (MNs) is the hallmark of amyotrophic lateral sclerosis (ALS). Limb-innervating lateral motor column MNs (LMC-MNs) seem to be particularly vulnerable and are among the first MNs affected in ALS. Here, we report association of this differential susceptibility with reduced expression of the mir-17∼92 cluster in LMC-MNs prior to disease onset. Reduced mir-17∼92 is accompanied by elevated nuclear PTEN in spinal MNs of presymptomatic SOD1^G93A mice. Selective dysregulation of the mir-17∼92/nuclear PTEN axis in degenerating SOD1^G93A LMC-MNs was confirmed in a double-transgenic embryonic stem cell system and recapitulated in human SOD1^+/L144F-induced pluripotent stem cell (iPSC)-derived MNs. We further show that overexpression of mir-17∼92 significantly rescues human SOD1^+/L144F MNs, and intrathecal delivery of adeno-associated virus (AAV)9-mir-17∼92 improves motor deficits and survival in SOD1^G93A mice. Thus, mir-17∼92 may have value as a prognostic marker of MN degeneration and is a candidate therapeutic target in SOD1-linked ALS. VIDEO ABSTRACT.

Deficiency in the DNA glycosylases UNG1 and OGG1 does not potentiate c-Myc-induced B-cell lymphomagenesis

C-Myc overexpression mediates lymphomagenesis; however, secondary genetic lesions are required for its full oncogenic potential. The origin and the mechanism of formation of these mutations are unclear. Using the lacI mutation detection system, we show that secondary mutations occur early in B-cell development and are repaired by Msh2. The mutations at the lacI gene were predominantly at C:G base pairs and CpG motifs, suggesting that they were formed due to cytosine deamination or oxidative damage of G. Therefore, we investigated the role of Ogg1 and UNG glycosylases in c-Myc-driven lymphomagenesis but found that their deficiencies did not influence disease outcome in the Eµ c-Myc mouse model. We also show that Rag proteins do not contribute to secondary lesions in this model. Our work suggests that mutations at C:G base pairs that are repaired primarily by the mismatch repair system arise early in B-cell ontogeny to promote c-Myc-driven lymphomagenesis.

Challenging perspectives on the cellular origins of lymphoma

Both B and T lymphocytes have signature traits that set them apart from other cell types. They actively and repeatedly rearrange their DNA in order to produce a unique and functional antigen receptor, they have potential for massive clonal expansion upon encountering antigen via this receptor or its precursor, and they have the capacity to be extremely long lived as ‘memory’ cells. All three of these traits are fundamental to their ability to function as the adaptive immune response to infectious agents, but concurrently render these cells vulnerable to transformation. Thus, it is classically considered that lymphomas arise at a relatively late stage in a lymphocyte's development during the process of modifying diversity within antigen receptors, and when the cell is capable of responding to stimulus via its receptor. Attempts to understand the aetiology of lymphoma have reinforced this notion, as the most notable advances to date have shown chronic stimulation of the antigen receptor by infectious agents or self-antigens to be key drivers of these diseases. Despite this, there is still uncertainty about the cell of origin in some lymphomas, and increasing evidence that a subset arises in a more immature cell. Specifically, a recent study indicates that T-cell lymphoma, in particular nucleophosmin-anaplastic lymphoma kinase-driven anaplastic large cell lymphoma, may originate in T-cell progenitors in the thymus.

Msh2 deficiency leads to dysmyelination of the corpus callosum, impaired locomotion, and altered sensory function in mice

A feature in patients with constitutional DNA-mismatch repair deficiency is agenesis of the corpus callosum, the cause of which has not been established. Here we report a previously unrecognized consequence of deficiency in MSH2, a protein known primarily for its function in correcting nucleotide mismatches or insertions and deletions in duplex DNA caused by errors in DNA replication or recombination. We documented that Msh2 deficiency causes dysmyelination of the axonal projections in the corpus callosum. Evoked action potentials in the myelinated corpus callosum projections of Msh2-null mice were smaller than wild-type mice, whereas unmyelinated axons showed no difference. Msh2-null mice were also impaired in locomotive activity and had an abnormal response to heat. These findings reveal a novel pathogenic consequence of MSH2 deficiency, providing a new mechanistic hint to previously recognized neurological disorders in patients with inherited DNA-mismatch repair deficiency.

ATM-dependent spontaneous regression of early Eμ-myc-induced murine B-cell leukemia depends on natural killer and T cells

Mechanisms of spontaneous tumor regression have been difficult to characterize in a systematic manner due to their rare occurrence and the lack of model systems. Here, we provide evidence that early-stage B cells in Eμ-myc mice are tumorigenic and sharply regress in the periphery between 41 and 65 days of age. Regression depended on CD4(+), CD8(+), NK1.1(+) cells and the activation of the DNA damage response, which has been shown to provide an early barrier against cancer. The DNA damage response can induce ligands that enhance immune recognition. Blockade of DNAM-1, a receptor for one such ligand, impaired tumor regression. Hence, Eμ-myc mice provide a model to study spontaneous regression and possible mechanisms of immune evasion or suppression by cancer cells.

Reduction of Prep1 levels affects differentiation of normal and malignant B cells and accelerates Myc driven lymphomagenesis

The Prep1 homeodomain transcription factor has recently been recognized as a tumor suppressor. Among other features, haploinsufficiency of Prep1 is able to strongly accelerate the B-lymphomagenesis in EμMyc mice. Now we report that this occurs concomitantly with a change in the type of B-cell lymphomas generated by the Myc oncogene. Indeed, the tumors generated in the EμMyc-Prep1^+/− mice are much more immature, being mostly made up of Pro-B or Pre-B cells, while those in the EμMyc-Prep1^+/+ mice are more differentiated being invariably IgM⁺. Moreover, we show that Prep1 is in fact required for the differentiation of Pro-B and Pre-B cells into IgM⁺ lymphocytes and/or their proliferation, thus showing also how a normal function of Prep1 affects EμMyc lymphomagenesis. Finally, we show that the haploinsufficiency of Prep1 is accompanied with a major decrease of Myc-induced apoptosis and that the haploinsufficieny is sufficient for all these effects because the second allele of Prep1 is not lost even at late stages. Therefore, the tumor-suppressive activity of Prep1 is intertwined with both the interference with Myc-induced apoptosis as well as with natural developmental functions of the protein.

Conditional inactivation of MLH1 in thymic and naive T-cells in mice leads to a limited incidence of lymphoblastic T-cell lymphomas

Defects in the mismatch repair system (MMR) underlie hereditary non-polyposis colorectal cancer (HNPCC)/Lynch syndrome and also a significant number of sporadic colorectal cancers. Mice carrying a null allele for the MMR gene Mlh1 are preferentially prone to the development of lymphomas of B- and T-cell origin and to a lesser extent gastrointestinal tumors. Consistent with these findings in mice, MMR defects have also been observed in sporadic and hereditary hematological malignancies. To study the role of MLH1 for lymphomagenesis in more detail, we generated a new mouse model carrying a conditional Mlh1 allele (Mlh1(flox/flox)). Mating of these mice with EIIa-Cre recombinase transgenic mice allowed the constitutive inactivation of MLH1, and the resulting Mlh1(Δex4/Δex4) mouse line displays complete MMR deficiency and a cancer predisposition phenotype similar to Mlh1−/− mice. For T-cell specific MMR inactivation we combined the Mlh1(flox/flox) allele with the Lck-Cre transgene. In the resulting Mlh1(TΔex4/TΔex4) mice, MLH1 inactivation is limited to DP/SP thymocytes and naive peripheral T-cells. The development of T-cell lymphomas in Mlh1(TΔex4/TΔex4) mice is significantly reduced compared to Mlh1−/− mice, implying that MMR functions either at very early stages during T-cell development or even earlier in lymphoid precursor cells to suppress lymphomagenesis.

Msh6 protects mature B cells from lymphoma by preserving genomic stability

Most human B-cell non-Hodgkin's lymphomas arise from germinal centers. Within these sites, the mismatch repair factor MSH6 participates in antibody diversification. Reminiscent of the neoplasms arising in patients with Lynch syndrome III, mice deficient in MSH6 die prematurely of lymphoma. In this study, we characterized the B-cell tumors in MSH6-deficient mice and describe their histological, immunohistochemical, and molecular features, which include moderate microsatellite instability. Based on histological markers and gene expression, the tumor cells seem to be at or beyond the germinal center stage. The simultaneous loss of MSH6 and of activation-induced cytidine deaminase did not appreciably affect the survival of these animals, suggesting that these germinal center-like tumors arose by an activation-induced cytidine deaminase-independent pathway. We conclude that MSH6 protects B cells from neoplastic transformation by preserving genomic stability.