Preferential development of pre-B lymphomas with drastically down-regulated N-myc in the E mu-ret transgenic mice

The Eμ-Ret mouse is a novel model of hyperdiploid B-cell acute lymphoblastic leukemia

The presence of supernumerary chromosomes is the only abnormality shared by all patients diagnosed with high-hyperdiploid B cell acute lymphoblastic leukemia (HD-ALL). Despite being the most frequently diagnosed pediatric leukemia, the lack of clonal molecular lesions and complete absence of appropriate experimental models have impeded the elucidation of HD-ALL leukemogenesis. Here, we report that for 23 leukemia samples isolated from moribund Eμ-Ret mice, all were characterized by non-random chromosomal gains, involving combinations of trisomy 9, 12, 14, 15, and 17. With a median gain of three chromosomes, leukemia emerged after a prolonged latency from a preleukemic B cell precursor cell population displaying more diverse aneuploidy. Transition from preleukemia to overt disease in Eμ-Ret mice is associated with acquisition of heterogeneous genomic abnormalities affecting the expression of genes implicated in pediatric B-ALL. The development of abnormal centrosomes in parallel with aneuploidy renders both preleukemic and leukemic cells sensitive to inhibitors of centrosome clustering, enabling targeted in vivo depletion of leukemia-propagating cells. This study reveals the Eμ-Ret mouse to be a novel tool for investigating HD-ALL leukemogenesis, including supervision and selection of preleukemic aneuploid clones by the immune system and identification of vulnerabilities that could be targeted to prevent relapse.

CD19 CAR immune pressure induces B-precursor acute lymphoblastic leukaemia lineage switch exposing inherent leukaemic plasticity

Adoptive immunotherapy using chimeric antigen receptor (CAR) expressing T cells targeting the CD19 B lineage receptor has demonstrated marked success in relapsed pre-B-cell acute lymphoblastic leukaemia (ALL). Persisting CAR-T cells generate sustained pressure against CD19 that may drive unique mechanisms of resistance. Pre-B ALL originates from a committed pre-B cell or an earlier progenitor, with potential to reprogram into other hematopoietic lineages. Here we report changes in lineage markers including myeloid conversion in patients following CD19 CAR therapy. Using murine ALL models we study the long-term effects of CD19 CAR-T cells and demonstrate partial or complete lineage switch as a consistent mechanism of CAR resistance depending on the underlying genetic oncogenic driver. Deletion of Pax5 or Ebf1 recapitulates lineage reprogramming occurring during CD19 CAR pressure. Our findings establish lineage switch as a mechanism of CAR resistance exposing inherent plasticity in genetic subtypes of pre-B-cell ALL.

CAR-T targeting CD19 have been successfully used in a variety of B-cell malignancies but patients may eventually relapse. Here, the authors show that CD19 CAR-T resistance in pre-B cell ALL can be due to the induction of a myeloid lineage switch through an epigenetic alterations in master regulators of B cell development.

Mammalian target of rapamycin (mTOR) pathway signalling in lymphomas

The mammalian target of rapamycin mTOR is a central element in an evolutionary conserved signalling pathway that regulates cell growth, survival and proliferation, orchestrating signals originating from growth factors, nutrients or particular stress stimuli. Two important modulators of mTOR activity are the AKT and ERK/MAPK signalling pathways. Many studies have shown that mTOR plays an important role in the biology of malignant cells, including deregulation of the cell cycle, inactivation of apoptotic machinery and resistance to chemotherapeutic agents. The development of several mTOR inhibitors, in addition to rapamycin, has facilitated studies of the role of mTOR in cancer, and verified the antitumour effect of mTOR inhibition in many types of neoplasms, including lymphomas. Clinical trials of rapamycin derivatives in lymphoma patients are already in development and there are encouraging preliminary results, such as the substantial response of a subset of mantle cell lymphoma patients to the rapamycin analogue temsirolimus. Based on results obtained from in vitro and in vivo studies of the mTOR pathway in lymphomas, it seems that better understanding of mTOR regulation will reveal aspects of lymphomagenesis and contribute to the development of more powerful, targeted therapies for lymphoma patients.

The role of p53 in suppression of KSHV cyclin-induced lymphomagenesis

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a cyclin D homolog, K cyclin, that is thought to promote viral oncogenesis. However, expression of K cyclin in cultured cells not only triggers cell cycle progression but also engages the p53 tumor suppressor pathway, which probably restricts the oncogenic potential of K cyclin. Therefore, to assess the tumorigenic properties of K cyclin in vivo, we transgenically targeted expression of K cyclin to the B and T lymphocyte compartments via the E micro promoter/enhancer. Around 17% of E micro -K cyclin animals develop lymphoma by 9 months of age, and all such lymphomas exhibit loss of p53. A critical role of p53 in suppressing K cyclin-induced lymphomagenesis was confirmed by the greatly accelerated onset of B and T lymphomagenesis in all E micro -K cyclin/p53(-/-) mice. However, absence of p53 did not appear to accelerate K cyclin-induced lymphomagenesis by averting apoptosis: E micro -K cyclin/p53(-/-) end-stage lymphomas contained abundant apoptotic cells, and transgenic E micro -K cyclin/p53(-/-) lymphocytes in vitro were not measurably protected from DNA damage-induced apoptosis compared with E micro -K cyclin/p53(wt) cells. Notably, whereas aneuploidy was frequently evident in pre-lymphomatous tissues, end-stage E micro -K cyclin/p53(-/-) tumors showed a near-diploid DNA content with no aberrant centrosome numbers. Nonetheless, such tumor cells did harbor more restricted genomic alterations, such as single-copy chromosome losses or gains or high-level amplifications. Together, our data support a model in which K cyclin-induced genome instability arises early in the pre-tumorigenic lymphocyte population and that loss of p53 licenses subsequent expansion of tumorigenic clones.

Rapamycin is active against B-precursor leukemia in vitro and in vivo, an effect that is modulated by IL-7-mediated signaling

A balance between survival and apoptotic signals regulates B cell development. These signals are tightly regulated by a host of molecules, including IL-7. Abnormal signaling events may lead to neoplastic transformation of progenitor B cells. Signal transduction inhibitors potentially may modulate these abnormal signals. Inhibitors of the mammalian target of rapamycin (mTOR) such as rapamycin have been used as immunosuppressive agents. We hypothesized that rapamycin might demonstrate activity against B-precursor acute lymphoblastic leukemia. We have found that rapamycin inhibited growth of B-precursor acute lymphoblastic leukemia lines in vitro, with evidence of apoptotic cell death. This growth inhibition was reversible by IL-7. One candidate as a signaling intermediate cross-regulated by rapamycin and IL-7 was p70 S6 kinase. Rapamycin also demonstrated in vivo activity in E mu-ret transgenic mice, which develop pre-B leukemia/lymphoma: E mu-ret transgenic mice with advanced disease treated daily with rapamycin as a single agent showed a >2-fold increase in length of survival as compared with symptomatic littermates who received vehicle alone. These results suggest that mammalian target of rapamycin inhibitors may be effective agents against leukemia and that one of the growth signals inhibited by this class of drugs in precursor B leukemic cells may be IL-7-mediated.

Novel tumorigenic rearrangement, Delta rfp/ret, in a papillary thyroid carcinoma from externally irradiated patient

Molecular analysis of cDNA derived from a papillary thyroid carcinoma (PTC) (follicular variant of papillary thyroid carcinoma on histology) which developed in an externally irradiated patient 4 years after exposure identified a portion of the 5' region, exons 1-3, of the rfp gene juxtaposed upstream of the fragment encoding the tyrosine kinase (TK) domain of the ret gene. The fusion gene, termed Delta rfp/ret, was the result of a balanced chromosomal translocation t(6;10) (p21.3;q11.2) confirmed by interphase FISH painting, with breakpoints occurring in introns 3 and 11 of the rfp and ret genes, respectively. Both Delta rfp/ret and reciprocal ret/rfp chimeric introns had small deletions around breakpoints consistent with presumed misrepair of a radiation-induced double-strand DNA break underlying the rearrangement. No extensive sequence homology was found between the fragments flanking the breakpoints. The fusion protein retained the propensity to form oligomers likely to be mediated by a coiled-coil of the RFP polypeptide as assessed by a yeast two-hybrid system. NIH 3T3 fibroblasts stably transfected with a mammalian expression vector encoding full-length Delta RFP/RET readily gave rise to the tumors in athymic mice suggestive of high transforming potential of the fusion protein. Thus, the Delta rfp/ret rearrangement may be involved in a causative manner in cancerogenesis and provides additional evidence of the role of activated ret oncogene in the development of a subset of papillary thyroid carcinoma.

The Evolution of B Precursor Leukemia in the Eμ-ret Mouse

Eμ-ret mice carrying an RFP/RET fusion gene under the transcriptional control of the immunoglobulin heavy chain enhancer develop B lineage leukemias/lymphomas. We have characterized B-cell development in these mice before the onset of clinical disease to determine the steps involved in leukemogenesis. Flow cytometry reveals that the CD45R+CD43+CD24+BP-1+late pro–B-cell population is markedly expanded in the bone marrow of 3- to 5-week-old Eμ-ret mice. Compared with late pro–B cells from transgene-negative mice, Eμ-ret late pro–B cells have a limited capacity to differentiate in interleukin (IL)-7 and a higher incidence of VDJ rearrangements, but a similar cell cycle profile. In contrast, CD45R+CD43+CD24+BP-1−early pro–B cells from 3- to 5-week-old Eμ-ret mice, which also express the RFP/RET transgene, differentiate in IL-7 similarly to their normal counterparts. Furthermore, early pro–B cells from Eμ-ret and transgene-negative mice have an identical pattern of growth inhibition when exposed to interferons (IFNs)-α/β and -γ, whereas, pro–B-cell leukemia lines derived from Eμ-ret mice are markedly less sensitive to growth inhibition by these IFNs. In 13-week-old well-appearing Eμ-ret mice, late pro–B cells upregulate CYCLIN D1 expression and downregulate CASPASE-1 expression in a pattern that correlates with the emergence of B precursor cells in the peripheral blood and the loss of other B lineage subsets in the bone marrow. Taken together, these results suggest that the expression of the RFP/RET transgene initially prevents the normal elimination of late pro–B cells with nonproductive rearrangements. Secondary events that simultaneously disturb the normal transcriptional regulation of genes involved in the control of the cell cycle and apoptosis may allow for subsequent malignant transformation within the expanded late pro–B-cell population.

RET-deficient mice: an animal model for Hirschsprung's disease and renal agenesis

Receptor tyrosine kinases play a critical role in transducing signals involved in cell growth and differentiation. The c-ret proto-oncogene is a member of the receptor tyrosine kinase gene superfamily originally identified by its transforming ability. Somatic mutations of c-ret are responsible for a large proportion of thyroid papillary carcinomas, while germ-line mutations are responsible for multiple endocrine neoplasia types 2A and 2B, dominantly inherited cancer syndromes characterized by multiple tumours of neuroectodermal origin. In addition to its role in tumour formation. c-ret is thought to have a developmental role since mutations of the gene have been implicated in the aetiology of Hirschsprung's syndrome (congenital megacolon). A targeted mutation in the murine c-ret locus shows that the ret receptor is required for normal development of two lineally unrelated systems, the excretory system and the enteric nervous system.

Oncogene-linked in situ immunotherapy of pre-B lymphoma arising in E mu/ret transgenic mice

We attempted to induce anti-tumour immunity for rejecting pre-B lymphoma derived from E mu/ret transgenic mice (TGM). We established pre-B-lymphoma cell lines of C57BL/6 x Balb/c background (H-2b/d) into which H-2k alloantigen and C3H background were introduced (retL1-6 and retL6-6), and we inoculated BCF1 mice with these immunising tumour cells. After these tumours were rejected by alloantigen (H-2k/C3H background)-specific effector cells, the mice were challenged with the pre-B-lymphoma cell line derived from the original E mu/ret TGM (ret0-2). All non-immunised control mice died within 80 days, whereas half the immunised mice survived for over 300 days. The immunity was also effective against primary pre-B-lymphoma cells from E mu/ret TGM and the ret-driven melanoma cell line (MEL-ret), but not against the pre-B-lymphoma cell line from E mu/myc TGM. This immunity was at least in part mediated by cell-mediated cytotoxicity that was specific to the ret oncogene product or ret-regulated antigen. Next we immunised E mu/ret TGM by inoculating them with retL6-6 cells once every 2 weeks beginning at the age of 1 month. Interestingly, this immunisation enabled the TGM to survive longer than the non-immunised control group (P < 0.05). Moreover, 2 of 11 transgenic mice receiving such immunisation were free from both macroscopic and microscopic tumours at the time when all of the 12 non-immunised control TGM had died from their tumour. This provides a new model for oncogene-linked immunotherapy research.

Neuroblastoma in a transgenic mouse carrying a metallothionein/ret fusion gene

We have recently succeeded in producing transgenic mice carrying a hybrid gene consisting of mouse metallothionein promoter-enhancer and the ret oncogene (MT/ret). (Iwamoto et al., 1991b). A retroperitoneal tumour developed in one of 17 MT/ret transgenic founder mice. Histological analysis revealed that the tumour consisted of undifferentiated neuroblasts and differentiated ganglion cells, the latter of which were strongly positive for neuron specific enolase. Expression of the ret transgene was observed at high levels in RNA from the tumour, but not in those of other normal tissues. In addition, a 100kDa ret protein was detected in the cell lysate of the tumour. Taken together with our previous data, these results suggest a possible role for the ret oncogene in the proliferation of neural crest cells.

Redox-linked ligand-independent cell surface triggering for extensive protein tyrosine phosphorylation

Exposure of lymphocytes to 0.2-2 mM HgCl2, a thiol-reactive heavy metal, induced extensive tyrosine phosphorylation of multiple cellular proteins. The phosphorylation started as quickly as 5 s after exposure to HgCl2, and was irreversible. Another 3 thiol-reactive chemicals also displayed similar, though less marked, actions, whereas dithiothreitol, a reducing agent, antagonized the HgCl2 action. The demonstrated new action of HgCl2 indispensably required membrane-intact cells as a target. Whereas exposure of lymphocytes to > 0.2 mM HgCl2 caused rapid cell death, 0.01-0.1 mM HgCl2 affected the cells so as to accelerate their c-fos transcription. These results suggest a novel redox-linked mechanism of cell surface triggering of intracellular protein kinase activity, which is independent of receptor-ligand interactions.

The ret oncogene products are membrane-bound glycoproteins phosphorylated on tyrosine residues in vivo

We identified ret oncogene products in NIH 3T3 cells transformed by the ret oncogene (NIH(ret)) and a cell line (Lym-ret) established from pre-B cell lymphoma which developed in E mu-ret transgenic mice. Using the polyclonal antibody against the kinase domain of ret, two glycoproteins with apparent molecular weights of 100 kd and 96 kd were found in both cell lines, although the expression level of the 100 kd protein was much higher than that of the 96 kd protein. Cell fractionation experiments indicated that the 100 kd protein was present predominantly in the membrane fraction while the 96 kd protein was found in both membrane and cytosol fractions. Western blot analysis indicated that the 100 kd ret protein was phosphorylated on tyrosine residues in vivo.