Spontaneous loss of B lineage transcription factors leads to pre-B leukemia in Ebf1+/–Bcl-xLTg mice

Early B-cell factor 1 (EBF1) plays a central role in B-cell lineage specification and commitment. Loss of this critical transcription factor is strongly associated with high-risk, relapsed and therapy-resistant B–cell-acute lymphoblastic leukemia, especially in children. However, Ebf1 haploinsufficient mice exhibit a normal lifespan. To determine whether prolonged survival of B cells would enable tumorigenesis in Ebf1 haploinsufficient animals, we generated Ebf1+/–Bcl-xLTg mice, which express the anti-apoptotic factor Bcl-xL in B cells. Approximately half of Ebf1+/–Bcl-xLTg mice develop aggressive oligoclonal leukemia as they age, which engrafts in congenic wild-type recipients without prior conditioning. The neoplastic cells display a pre-B phenotype and express early developmental- and natural killer cell/myeloid-markers inappropriately. In addition, we found tumor cell-specific loss of several transcription factors critical for maintaining differentiation: EBF1, TCF3 and RUNX1. However, in the majority of tumors, loss of Ebf1 expression was not due to loss of heterozygosity. This is the first spontaneous mouse model of pre-B leukemia to demonstrate inappropriate expression of non-B-cell-specific genes associated with loss of Ebf1, Tcf3 and Runx1 expression.


INTRODUCTION
Early B-cell factor 1 (EBF1) is an important transcriptional regulator of early B-cell development. In synergy with PAX5, EBF1 controls specification of progenitors to the B-cell lineage and maintains lineage commitment throughout development (reviewed in Ramirez et al. 1 ). Ebf1 gene expression begins in common lymphoid progenitors and increases as B-cell maturation proceeds, with the exception of terminally differentiated plasma cells. 2,3 In the absence of EBF1, B-cell development is arrested at the common lymphoid progenitor stage and functional B cells are not generated. 4,5 Loss of lymphoid and B-cell-specific transcription factors including IKZF1 (Ikaros), PAX5 and EBF1 are strongly associated with human B-cell-acute lymphoblastic leukemias (B-ALL, reviewed in ref. 6). Although EBF1 mono-allelic deletions occur in a small fraction (4%) of total B-ALL cases, 25% of relapsed pediatric B-ALL patients carry EBF1 mutations and EBF1 deletion is strongly associated with a low relapse-free survival rate. 7,8 Tumors from high-risk leukemia patients such as those with BCR-ABL1 translocations or TP53/RB1 and JAK mutations are more likely to display EBF1 haploinsufficiency than those from low-risk patients. Deletions in EBF1 genes often interrupt the open reading frame, suggesting that loss of function contributes to disease progression and resistance to chemotherapy. 9 EBF1 is a transcriptional activator, as well as a repressor. 10-13 EBF1 represses several natural killer (NK)/myeloid cell-specific genes, including Cd244 (CD244; 2B4) and Klrb1c (NK1.1). 14 These cell surface markers are expressed promiscuously in pro-B and early pre-B cells of Ebf1 haploinsufficient mice, which also inappropriately expressed the early hematopoietic marker Sca-1 (Ly6a). However, aberrant expression of these factors was confined to specific B-cell subsets in the bone marrow and was not observed in developmentally progressed B cells in the periphery. Compared to wild-type mice, Ebf1 haploinsufficient mice do not exhibit an increased incidence of tumors. 4,15 To investigate whether prolonging survival of pro-B cells would induce tumorigenesis, we crossed Ebf1 haploinsufficient (Ebf1 +/-) mice with transgenic mice (Bcl-x L Tg ) overexpressing the antiapoptotic factor Bcl-x L (Bcl2l1) specifically in B cells. 4, 16 We report that a majority of Ebf1 +/-Bcl-x L Tg mice develop aggressive B-cell leukemia by slightly over one year of age. Development of disease is associated with significantly reduced expression of key transcription factors including EBF1, TCF3 (E2A) and/or RUNX1, which are critical for maintaining B-cell differentiation. 17 Our results are consistent with the hypothesis that promoting survival of Ebf1 haploinsufficient B cells results in tumorigenesis, likely after an accumulation of DNA damage and inactivation of critical transcription factors. The partial loss of cellular identity is manifested in these cells by aberrant expression of cell surface markers of NK, myeloid or early progenitor cells. These mice provide a useful new model for studying roles of EBF1 and the impact of its loss during leukemogenesis.

RESULTS
Ebf1 +/-Bcl-x L Tg (EB) mice develop clonal lymphoproliferative disease Mice heterozygous for an Ebf1 knockout allele display a normal lifespan without overt disease development. 4 In order to assess whether increasing the survival of B cells would allow Ebf1 loss of heterozygosity to occur, we generated Ebf1 +/-Bcl-x L Tg (EB) mice that express high levels of the pro-survival factor Bcl-x L under the control of the μ immunoglobulin heavy chain enhancer. 16 These mice display a significantly shortened lifespan compared to control littermates, with a median survival of approximately 64 weeks of age ( Figure 1a). Affected mice display hunched posture, lethargy, pale paws, ruffled hair coat, and enlarged spleen and peripheral lymph nodes including cervical, axillary, subiliac, colic and iliac nodes (Figures 1b and c). The tissues shown are representative of all control mice. PCR analysis of immunoglobulin heavy chain rearrangements ( Figure 1d) and lambda light chain rearrangements (Figure 1e) in DNA isolated from the lymph nodes of seven different affected EB mice revealed monoclonal or oligoclonal cell populations. Most of these cell populations contained both light and heavy chain rearrangements, indicating that the progenitor cells had reached the pre-B, or later stages of B-cell development.
EB mice develop aggressive disease with neoplastic infiltrates in multiple organs We performed histopathologic analyses of clinically affected EB mice as well as aged Ebf1 +/and Bcl-x L Tg littermates. Analysis of multiple organs including the medullary (bone marrow) cavities of the skull (Figures 2a and b), spleen (Figures 2c and d), lungs (Figures 2f and g) and kidney (Figures 2h and i) revealed dense accumulations of neoplastic round cells in EB mice (right panels), which were completely absent in Bcl-x L Tg control mice (left panels). Within the bone marrow, spleen and lymph nodes, neoplastic cells almost completely effaced normal tissue parenchyma. Control mice displayed normal heterogeneous hematopoietic populations in the bone marrow of skull (Figure 2a inset) and long bones, in striking contrast to sheets of large cells with a lymphoblast morphology observed in the bone marrow of affected EB mice (Figure 2b inset). Ki-67 staining revealed many proliferating neoplastic cells within affected tissues, shown in the spleen ( Figure 2e) and mesenteric adipose tissue (Figure 2j). Similar, but less abundant infiltrates of small lymphocytes with a normal morphology were observed in the lung, adipose tissue and kidney in aged Bcl-x L Tg , but not Ebf1 +/control mice. These results are in accordance with published observations that Bcl-x L Tg mice accumulate B cells in the periphery. 18   evidence of lymphoproliferative disease. Immunohistochemical analysis showed EBF1 protein levels are reduced in neoplastic cells in the bone marrow of an affected EB mouse compared to normal bone marrow from an Ebf1 +/control littermate (Figures 2k and l). To verify this finding we performed an immunoblot with whole cell lysates from four different primary EB tumors and utilized a different EBF1 antibody for detection. Lysate from wild-type splenic B cells was used as a positive control for EBF1 expression ( This finding underscores the importance of loss of factors that maintain cellular identity during transformation, even in the context of different oncogenic drivers. Neoplastic cells display a precursor B-cell phenotype with aberrant expression of NK-cell markers Flow cytometric analysis was performed on lymphoid tissues from EB mice as well as aged WT, Ebf1 +/and Bcl-x L Tg littermates. Large populations of B220 + cells were observed in the bone marrow, thymus and peripheral blood leukocytes of affected EB mice ( Figure 3a). Affected mice, but not control or unaffected EB mice, exhibited a uniform population of enlarged lymphoblastic cells in the peripheral blood ( Figure 3b (Table 1). Primary isolates from seven different EB mice differed in BP-1 (Enpep) and CD25 (Il2ra) expression, with some tumors expressing BP-1 (present on late pro and early pre B cells) and others expressing CD25 (displayed on late pre B cells 19 ) ( Figure 3c, Table 1). All tumors examined expressed at least one of these pre-B-cell markers. Surface expression of CD117 (kit), an early developmental marker normally present on common lymphoid progenitors through early pro-B cells, was increased on most tumor cells ( Figure 3c, Table 1). Notably, the NK/myeloid cell receptor CD244 (Cd244) and NK/T-cell protein NK1.1 (Klrb1c) were present on cells expressing the B-cell commitment marker, CD19 ( Figure 3d). All tumor isolates displayed CD244 in large amounts, and most expressed low to moderate levels of NK1.1 (Table 1). However, tumor cells did not express CD335 (NKp46; Ncr1), a marker of active NK cells. Both NK1.1 and CD244 are aberrantly expressed on Ebf1 +/pro-B and CD25pre-B cells. 14 In contrast, several EB primary tumor cell isolates strongly displayed both CD25 and CD244, likely indicating they had progressed to the late pre-B stage before transformation occurred and CD244 expression was further upregulated. Aged Bcl-x L Tg littermates did not display a population of cells with surface markers similar to the EB tumor cells (B220 lo CD117 + sIg -, Supplementary Figure S1). Neoplastic cells did not express markers characteristic of T cells or granulocytes (CD3, Ly-6G or CD11b, Table 1). The early developmental phenotype of the neoplastic cells, combined with a high abundance of lymphoblasts in the bone marrow and peripheral blood, suggest a precursor B-cell lymphoblastic lymphoma/leukemia derived from bone marrow precursors as the presumed tissue of origin. 20 To discern whether EB tumor cells are transplantable, 2 × 10 6 isolated lymph node cells from one of six different primary tumors were transferred to congenic immunodeficient recipient mice via intravenous injection ( Table 2). The tumors that developed from these transfers were serially transferred through four to nine different cohorts of recipients, depending on the tumor isolate. All recipients of the tumor cells developed clinical disease within 2-12 weeks. To test whether EB tumors are aggressive enough to cause disease in immunocompetent recipients, 2 × 10 5 cells from two different primary tumor isolates were transferred into healthy CD45.1 recipient mice without prior irradiation or conditioning. All recipients developed clinical disease within 4 weeks of injection. A distinct population of CD45.2 + CD244 + cells was found in the peripheral blood, spleen, lymph nodes and bone marrow (Supplementary Figure S2).
Pre-B lymphoma/leukemia is characterized by loss of lineagecommitment transcription factors A unique population of pre-B cells in the bone marrow of healthy EB mice (Figure 4a, upper left panel) aberrantly expresses CD244. This population is characterized by B220 lo /CD19 + /IgM − staining, includes CD25 + pre-B cells, and has also previously been observed in Ebf1 +/mice. 14 The population is not present in wild-type or Bcl-x L Tg mice, or in peripheral tissues of healthy EB mice ( Figure 4a). All EB tumors were B220 lo /CD244 + (Figure 4a, far right panel, Table 1). Therefore, we hypothesized that this aberrant population comprises the progenitors of EB tumors. To investigate this possibility, total bone marrow from a healthy EB donor was transferred to four sub-lethally irradiated Rag2 − / − IL2rγ − / − recipients. All recipients were collected 16 weeks after transfer, when one bone marrow transfer recipient exhibited signs of clinical disease including abdominal effusion containing mononuclear round cells, splenomegaly and lymphadenopathy. This mouse (K832, Figure 4a far right panel, Table 1) displayed a single uniform population of B220 lo CD19 + IgM -CD25 + CD244 hi cells in the bone marrow which largely effaced all other populations and matched EB tumor cell surface marker phenotype, suggesting that tumor cells developed from the unusual B220 lo CD244 + late pre-B population present in EB and Ebf1 +/bone marrow. In addition, sorted B220 lo CD244 + populations from the bone marrow of these mice carry lambda light chain rearrangements similar to EB tumors (Supplementary Figure S3). Although mouse and human lymphatic tumors are commonly characterized based on cell surface expression of lineage-specific markers as well as distribution and molecular characteristics, 20,21 we cannot exclude the possibility that EB tumors arise from peripheral B cells. To analyze the expression of genes important for B-cell development and lineage maintenance in EB tumor cells, we performed quantitative PCR analysis of cDNA from FACS-sorted wild-type and Bcl-x L Tg BM late pre B cells (B220 + IgM -BP-1 -CD25 + ), healthy EB BM transplant recipients (B220 + CD244 + BM; unaffected EB) and primary EB tumor isolates. Strikingly, significant reductions in Ebf1, Tcf3 (encoding E2A), and Runx1 transcripts were observed in tumor compared to unaffected EB cells (Figure 4b). Reductions of Ebf1, Tcf3 and Runx1 transcripts were also detected in EB tumor cells relative to lymph node cells from Bcl-x L Tg control mice,  Table 1).
confirming that levels of these transcripts are reduced even if the tumor cells are peripheral in origin (Supplementary Figure S4). Abundance of Ikzf1 and Pax5 transcription factor mRNA was not significantly different in tumor vs unaffected cells, but was markedly lower than in pre-B cells from wild-type and Bcl-x L Tg mice. RUNX1, E2A, IKZF1 and PAX5 all positively regulate Ebf1 transcription, and the Pax5 gene itself is also positively regulated by EBF1 as part of a positive regulatory feedback loop. Mono-allelic deletions, point mutations and translocations of one or more of these transcription factors often occur in pediatric B-ALL. 9,22 Reduced levels of factors encoded by Ikzf1, Runx1, Tcf3, Pax5 and Ebf1 may each be important individually for predisposing EB mice to tumor development, while concerted loss of these factors, particularly in combination, would potently induce loss of differentiation. Transcript abundance of Rad51 (encoding a common DNA damage response factor) was also significantly reduced in both tumor and unaffected EB cells compared to wild-type and Bcl-x L Tg pre-B cells, in accordance with previous findings that Ebf1 heterozygosity results in decreased expression of this gene and increased DNA damage in pro-B cells. 23 Transcripts of the EBF1 target genes VpreB, Rag1, and Cd79a/mb-1 were also significantly reduced in tumor cells compared to unaffected EB cells, likely as a result of reduced EBF1 activity.
Chromosomal translocations resulting in BCL11A overexpression are often seen in human B-ALL tumors, and kit is a proto-oncogene and early developmental marker upregulated in many hematopoietic neoplasms. Abundance of Kit (CD117) and Bcl11a transcripts were increased in unaffected EB cells compared to wild-type pre-B cells; however, this likely reflects increased Bcl-x L Tg activity because Bcl-x L Tg pre-B cells displayed comparable levels of these transcripts. Additionally, we observed increased Cd244 transcripts in tumor cells of EB mice alone, which correlates with increased cell surface display of CD244 observed in flow cytometric analyses. Putative EBF1 binding sites are present in the Cd244 promoter and upstream regulatory module (JH, unpublished data), and EBF1 negatively regulates Cd244 expression. 14 We also examined Irf4 and Spi1 transcript levels as mice lacking expression of both Spi1 (encodes PU.1) and IRF4 in B cells spontaneously develop pre-B leukemia. 24 However, although levels of Irf4 transcripts are significantly decreased in EB tumor cells, levels of Spi1 transcripts were not significantly altered (Supplementary Figure S5). Irf4 deficiency is not by itself associated with an increased risk of leukemia, but cooperates with oncogenes to promote leukemogenesis. 25 Loss of heterozygosity is not required for loss of EBF1 expression in EB tumors One EB tumor, #4606, lacked surface expression of CD19 in contrast to all other tumors examined, which were CD19 positive ( Figure 5a, Table 1). However, other cell surface markers of this tumor were identical to other EB tumors (B220 lo /CD117 + /CD25 + / CD244 + /sIg -). EBF1 and PAX5 coordinately activate CD19 expression, which is often used as an indicator of commitment to the B-cell lineage. Owing to this finding and the loss of EBF1 expression in EB tumors, we investigated whether loss of heterozygosity occurs in EB tumors similar to EBF1 genetic lesions found in a subset of high-risk pediatric B-ALL. We performed high-throughput sequencing of Ebf1 genes in paired DNA samples comprising EB primary tumor and tail tissue from each host (Figures 5b and c). Sequence analysis software was used to identify regions of change in Ebf1 copy number; however, changes identified in repeat-rich areas with very low sequence coverage were not considered robust. Ebf1 copy number loss occurred in introns 7, 8 and 10 in a single primary tumor, which was verified by qPCR analysis of these regions (data not shown). No other robust changes were observed in seven other primary tumors sequenced. ChIP-seq data from ENCODE/PSU shows that areas of Ebf1 genes are enriched with monomethylated histone H3 lysine 4 in a murine B lymphoma cell line, with the regions of copy number loss in the EB54 tumor indicated (Figure 5c). This histone mark is enriched in genes poised for transcription, and often marks the location of enhancers. The areas of copy number loss identified in the EB tumor isolate fall within regions of increased H3K4me1 occupancy, so it is possible these areas might serve as enhancers. Overall, however, intragenic Ebf1 lesions do not appear responsible for the observed decrease in Ebf1 transcript levels in EB tumors.
Because DNA methylation frequently silences gene expression during both cellular differentiation and tumorigenesis, we investigated whether the Ebf1α (distal) and Ebf1β (proximal) promoters are methylated in EB tumors. 26,27 Transcripts from both promoters were significantly decreased in unaffected EB lo, low surface expression, but higher than negative controls; ND, not determined. The table lists cell surface markers present or absent on six different primary tumor isolates from Ebf1 +/-Bcl-x L Tg mice and one (K832) which spontaneously developed in an Ebf1 +/-Bcl-x L Tg bone marrow transfer, as determined by flow cytometric analysis. Six different Ebf1 +/-Bcl-x L Tg primary tumor cell isolates were intravenously injected into sublethally irradiated Rag1-deficient recipient mice. Tumors were serially transferred through 4-9 hosts depending on the isolate. Numbers represent animals killed due to disease development with respect to the total number of recipients in the cohort (for example, 4/4 = 4 mice developed clinical disease out of 4 mice injected).     Figure S6). However, only transcripts from the Ebf1β promoter, the major source of Ebf1 transcripts in B cells, 27 were significantly decreased compared to wild-type pre-B cells. We next used bisulfite conversion of genomic DNA followed by PCR amplification and pyrosequencing to examine the methylation of CpGs 1-3 (distal-proximal) in the Ebf1β promoter and CpGs 15-22 (distal-proximal) in the Ebf1α promoter. However, these particular CpGs were largely demethylated in control non-lymphatic tissue (tail and ear clips) from EB mice, and methylation levels were significantly decreased in lymph node samples from all mice, both EB tumor and Ebf1 +/or Bcl-x L Tg controls (data not shown). Other Ebf1 promoter CpGs could be differentially methylated, or increased DNA methylation may not be responsible for the loss of EBF1 expression observed in EB tumors. Serial or concerted loss of Runx1 and E2A transcription factors, possibly through genetic lesions induced by DNA damage, might account for EBF1 loss. Since the distal Ebf1β promoter is much more active than Ebf1α and is regulated by Pax5, reduced expression of this factor may account for concurrent reductions in Ebf1 transcripts. Accessibility and modification of chromatin in the Ebf1 promoter and/or enhancers may also contribute, possibly through binding of transcriptional repressors and/or inhibitory chromatin remodelers such as Mi2-nucleosome remodeling and deacetylase complexes.
To investigate whether EB mice recapitulate selected gene expression signatures found in human B-ALL, we correlated the expression of three pro-survival Bcl2 family members (MCL1, BCL2 and BCL2L1 [BCL-XL]) and three key transcription factors (EBF1, TCF3 and RUNX1) in samples from a study of 177 pediatric patients with high-risk B-ALL (Figure 5d). 8 B-ALL tumors often exhibit expression of multiple pro-survival Bcl2 family members. We found that low expression of each of the transcription factors is correlated with higher BCL2 expression; however, only low expression of TCF3 and EBF1 correlated with levels of BCL2L1 and MCL1 expression. Interestingly, BCL2L1 expression most strongly correlated with low expression of TCF3, which of the three transcription factors examined is also the most profoundly deficient in EB tumors. We also compared PAX5, IKAROS and IRF4 expression with expression of the pro-survival proteins, and found no significant correlation except between IKAROS and BCL2L1 (results not shown). Therefore, EB mice may serve as an appropriate model for precursor B-ALL since they exhibit multiple deficiencies in lineage determinant transcription factors and concurrent upregulation of one or more Bcl2 family members.

DISCUSSION
Leukemia is often thought to arise due to multiple lesions, or 'hits', including (1) loss of genes that mediate hematopoietic differentiation and (2) mutations that confer proliferative and/or survival advantages. 28 Our observations suggest that loss of Ebf1 genes is unlikely to be a primary driver of leukemogenesis by itself, but can synergize with mutations in other genes that increase the survival and relative fitness of leukemic cells. Heltemes-Harris et al. 15 described two murine models of B-ALL characterized by Ebf1 or Pax5 haploinsufficiency and constitutive STAT5 activation (CaStat5). They also investigated whether extending survival of Ebf1 haploinsufficient cells affected leukemogenesis by generating Ebf1 +/-Bcl-x L Tg (EB) mice, but did not report tumor development within~250 days (36 weeks) of age. Our findings are compatible with these results, as by 36 weeks of age only 2/30 (6.7%) of our EB mice had developed tumors. The substantial increase in tumor incidence in CaStat5 mice as a result of Ebf1-or Pax5-haploinsufficiency underscores the importance of these transcription factors in maintaining cellular differentiation despite strong oncogenic signaling. In accordance, the Sigvardsson laboratory described similar development of pre-B leukemia in a subset of Ebf1 +/-Pax5 +/mice with high molecular heterogeneity between tumors. 23 Based on cell surface marker expression and analysis of transcripts of various B-cell and stage-specific factors, it appears pre-B tumors derived from EB mice display similar molecular heterogeneity. These observations lend support to the hypothesis that tumor development in the context of lineage-determinant transcription factor haploinsufficiency is a multistep process.
Translocations involving the key B lineage transcription factors RUNX1 (ETV6-RUNX1), TCF3 (TCF3-PBX1) and EBF1 (EBF1-PDGFRβ) have all been described in human B progenitor leukemias, and deletions affecting these loci frequently occur in high-risk pediatric B-ALL. 8 Our finding of greatly reduced levels of these transcription factors in EB tumors compared to late pre-B cells and cells in peripheral lymph nodes indicates that further loss of lineage-determinant transcription factors in a haploinsufficient context potentiates tumorigenesis and/or transformation. Tumor development in EB mice likely begins with an accumulation of DNA damage due to reduced Ebf1 dosage, combined with increased cell survival due to high levels of the antiapoptotic BCL-2 family member BCL-X L . Similar to BCL2, BCL-X L synergizes with c-Myc to drive B progenitor leukemia. 29 However, BCL-2 family members do not always serve redundant roles in tumorigenesis. Delbridge et al. 30 demonstrated that BIM(Bcl2l11)dependent apoptosis acts as an important tumor suppressor to prevent cancer development as a result of Rag1/2-induced DNA lesions in p53 deficient mice. Interestingly, loss of Bcl2-modifying factor did not potentiate Rag-mediated tumor development, as did loss of BIM, even though the proteins are both pro-apoptotic members of the BCL-2 family. Similarly, Ebf1 +/-Bcl-2 Tg mice do not develop tumors (M. Sigvardsson, unpublished data). This observation may be due to dual inhibition of both BAX and BAK, which are direct effectors of mitochondrial apoptosis, by Bcl-x L , but not Bcl-2. Moreover, Bcl-x L is more stable than its sister pro-survival proteins BCL-2 and Bcl2 family apoptosis regulator MCL-1. 31 In addition, Yagi and colleagues 32 determined that expression of dominant-negative Ikaros, found in a subset of acute myeloid leukemia patients, upregulates BCL-X L but not BCL-2. Expression of BCL-2 and BCL-X L is also differentially regulated during development. In mice, BCL-X L expression is high in pro-and pre-B cells, as well as antigen stimulated B cells, but low in immature and resting mature B cells, while BCL-2 is highly expressed in pro-B cells and resting mature B cells. 16,18,32 Mice deficient in BCL-2 exhibit normal B-cell development but lose mature cells to apoptosis. [33][34][35] In contrast, BCL-X L-deficiency results in embryonic lethality and BCL-X L-deficient lymphocytes display severe defects in B-cell development, with a pronounced reduction in small (late) pre-B and immature B cells. 36 These findings highlight the importance of apoptosis in preventing survival of neoplastic cells with DNA damage, and the lack of complete functional redundancy between anti-apoptotic factors. The collaboration of anti-apoptotic signaling pathways such as JAK/STAT or BCL-ABL activating mutations with loss of key transcription factors such as EBF1 that regulate DNA damage repair and maintain differentiation likely leads to development of aggressive, drug-resistant B-ALL in human patients.
Significantly reduced levels of Ebf1, Tcf3, Pax5 and Ikzf1 transcripts in unaffected EB pre-B cells allow increased transcription of targets normally repressed such as markers of other lineages (for example, NK-cell-specific genes) and early progenitors (for example, kit). This progenitor-like cellular milieu poises the cells for acquisition of additional defects that fully extinguish expression of lineage-determinant transcription factors and induce and/or potentiate oncogenic transformation. A similar process likely occurs in B-ALL patients, as many cancers including B-ALL express anti-apoptotic Bcl2 family members. The EB murine model of B-ALL recapitulates spontaneous generation of pre-B-cell tumors with molecular defects similar to those seen in leukemia patients with common transcription factor haploinsufficiencies, deletions or translocations, and does not depend on expression of oncogenic drivers such as c-myc and constitutively active Stat5 which are not found in most B-ALL patients. Therefore, this model may prove valuable for further study of tumorigenesis and therapy response in aggressive and/or relapsed B-ALL.

MATERIALS AND METHODS Mice
Ebf1 +/mice 4 were obtained from Y. Zhuang (Duke University, Durham, NC, USA) and previously backcrossed onto a C57BL/6J background. Bcl-x L Tg mice (Tg(Emu-Bcl2L1)87Nnz) 16,18 were obtained from T. Behrens (University of Minnesota, Minneapolis, MN, USA) and crossed to Ebf1 +/mice to generate Ebf1 +/-Bcl-x L Tg mice and littermate controls. Genotyping of Ebf1 was performed as previously described. 37 Mice were bred and maintained in specific pathogen-free facilities at National Jewish Health and the University of Washington. All experiments were reviewed and approved by the National Jewish Health and/or University of Washington Institutional Animal Care and Use Committees.
Adoptive transfer experiments were conducted by intravenous tail vein injection of 1 × 10 5 or 2 × 10 6 cells isolated from spleens or lymph nodes of mice bearing primary tumors, with or without prior sub-lethal irradiation (400 rads). All tumor-bearing mice were killed upon development of clinical signs of disease. Thirty EB, 21 Bcl-x L Tg and 15 Ebf1 +/animals including approximately equal representation of both sexes were generated as part of this study. Animals used for analysis ranged in age from 16 to 71 weeks old, and age-matched controls were killed with each affected EB mouse. Animals were censored, according to pre-established criteria, if euthanasia was performed for humane reasons unrelated to neoplasia, such as chronic ulcerative dermatitis. No randomization was used, and no blinding was performed. Sample size estimates were based on previous experience with s.d.'s in the assays used.

Histopathology
All histopathology was performed at The University of Washington Histology and Imaging Core. Tissues were collected, fixed in 10% neutral buffered formalin, routinely embedded and sectioned at 4 μm. Sections were stained with hematoxylin and eosin. Ki-67 (M7249, Dako, Santa Clara, CA, USA) and EBF1 (PA541632, Thermo Fisher Scientific, Waltham, MA, USA) antibodies were used for immunohistochemistry of select formalin-fixed sections from experimental mice. Antigen retrieval was performed by incubation in HIER 2 (EDTA) for 20 min at 100°C (Ki-67) or incubation in Proteinase K for 15 min at 37°C (EBF1), and dilutions of primary antibodies used were 1:50 (Ki-67) or 1:150 (EBF1). Three EB, three Ebf1 +/-, and two Bcl-x L Tg mice were analyzed.
Antibodies and flow cytometry Cell staining was performed using the following antibodies with specificities for mouse antigens: CD45R (B220) (

RNA isolation and qPCR
Bone marrow late pre-B cells (B220 + IgM -BP-1 -CD25 + ) were sorted from five wild-type and three Bcl-x L Tg mice using a FACSAria III (BD Biosciences, San Jose, CA, USA). Tumor cell progenitors (B220 + CD244 + ) were isolated by FAC-sorting bone marrow of three Rag2 -/-Il2rγ -/recipients of Ebf1 +/-Bcl-x L Tg bone marrow transfer. Tumor cells (B220 + CD244 + ) were collected from lymph nodes of four clinically affected Ebf1 +/-Bcl-x L Tg mice. Cells were lysed in TRI Reagent (Molecular Research Center, Cincinnati, OH, USA) and RNA purified by phase separation and precipitation according to the manufacturer's instructions. cDNA was prepared using SuperScript II reverse transcriptase (Thermo Fisher Scientific, Waltham, MA, USA) according to the manufacturer's instructions. Quantitative PCR was performed using SYBR Green Master Mix (Thermo Fisher Scientific) and 50 nM of each primer on a Mx3005 instrument (Stratagene, Santa Clara, CA, USA). HPRT was used as a reference to calculate ΔCT for each cDNA sample in every qPCR run, and all samples were run in triplicate for each primer set. For all graphs, each point plotted represents the relative expression of the tested gene (2^− ΔΔCT ) for a cDNA sample from one mouse. Sequences of primers used for qPCR are found in Supplementary Tables 1 and 2.
Analysis of Ig rearrangements Immunoglobulin heavy and light chain rearrangements were performed as previously described. 37 Briefly, DNA was purified from lymph nodes or spleens, PCR was performed, and amplification products were resolved by agarose gel electrophoresis for detection of heavy chain rearrangements. Ig lambda rearrangements were detected using PCR with AmpliTaq DNA polymerase (Thermo Fisher Scientific) and a cocktail of primers specific for Vλ1/Vλ2 (5′-AATCTGCACTCACCACATCACCTGGTG-3′) and antisense primers corresponding to introns of J-Cλ1 (5′-ATCTCTTTTAGCCCCCTGTG-3′), J-Cλ2 (5′-CAGAGAGAAAAAAGACCCTTGC-3′) and J-Cλ3 (5′-CAGAGAGATAAAT GACCCTTGC-3′). Amplification was followed by phenol:chloroform extraction of PCR products, desalting on G50 spin columns, and sequential restriction enzyme digestion with BsrD1 and HindIII (New England Biolabs, Ipswich, MA, USA). Each lane of the gels shown contains DNA from a different mouse.
Sequencing of Ebf1 genes DNA was purified by proteinase K digestion, phenol/chloroform extraction and ethanol precipitation of paired tumor (lymph node or spleen) and normal tissue (tail or ear clip) samples from seven clinically affected Ebf1 +/-Bcl-x L Tg mice, one unaffected Ebf1 +/-Bcl-x L Tg mouse, and B220 + CD244 + FACS-sorted bone marrow from three healthy and one affected Ebf1 +/-Bcl-x L Tg bone marrow transfer recipient mice. A custom SureSelect-XT DNA kit (Agilent, Santa Clara, CA, USA) was used to sequence Ebf1 genes using a MiSeq instrument (Illumina, San Diego, CA, USA). UCSC mm9, NCBI Build 37 was used as the basis for probe design in the kit and was also used as the alignment reference. The control-FREEC application was used to estimate copy number gain and loss across the Ebf1 gene in tumor vs normal samples, and the Integrative Genomics Viewer (Broad Institute, Boston, MA, USA) was used to visualize sequence coverage across the gene.

Statistical analyses
All statistical analyses were performed using GraphPad Prism version 6.01 for Windows, GraphPad Software, La Jolla CA, USA. A log-rank (Mantel-Cox) test was performed to compare tumor-free survival curves in Figure 1a. For all qPCR results (Figure 4b, Supplementary Figures S4-S6), Student's unpaired two-tailed t-tests with equal variance were performed. Center values shown on graphs represent means and error bars represent s.d.'s, and individual values are plotted for all groups with N (biological replicates) less than or equal to five. An F-test to compare variances was performed on each data set. For some transcripts examined (for example, CD244 and kit) EB progenitor and/or tumor cells exhibited higher variance between samples than controls. Correlations between gene expression microarray data (Figure 5d) were performed using Pearson r with two-tailed p-values.