The class-specific BCR tonic signal modulates lymphomagenesis in a c-myc deregulation transgenic model

Mouse Models of c-myc Deregulation Driven by IgH Locus Enhancers as Models of B-Cell Lymphomagenesis

Chromosomal translocations linking various oncogenes to transcriptional enhancers of the immunoglobulin heavy chain (IgH) locus are often implicated as the cause of B-cell malignancies. Two major IgH transcriptional enhancers have been reported so far. The E_μ enhancer located upstream of the C_μ gene controls early events in B-cell maturation such as VDJ recombination. The 3' regulatory region (3'RR) located downstream from the C_α gene controls late events in B-cell maturation such as IgH transcription, somatic hypermutation, and class switch recombination. Convincing demonstrations of the essential contributions of both E_μ and 3'RR in B-cell lymphomagenesis have been provided by transgenic and knock-in animal models which bring the oncogene c-myc under E_μ/3'RR transcriptional control. This short review summarizes the different mouse models so far available and their interests/limitations for progress in our understanding of human c-myc-induced B-cell lymphomagenesis.

Eμ and 3'RR transcriptional enhancers of the IgH locus cooperate to promote c-myc-induced mature B-cell lymphomas

Numerous B-cell lymphomas feature translocations linking oncogenes to different locations in the immunoglobulin heavy chain (IgH) locus. During Burkitt lymphoma (BL), IgH breakpoints for c-myc translocation stand either close to JH segments or within switch regions. Transcription, accessibility, and remodeling of the IgH locus are under the control of the 2 potent cis-acting enhancer elements: Eμ and the 3' regulatory region (3'RR). To ensure their respective contributions to oncogene deregulation in the context of the endogenous IgH locus, we studied transgenic mice harboring a knock-in of c-myc in various positions of the IgH locus (3' to JH segments, 5' to Cμ with Eμ deletion and Cα). The observed spectrum of tumors, kinetics of emergence, and transcriptome analysis provide strong evidence that both Eμ and 3'RR deregulate c-myc and cooperate together to promote B-cell lymphomagenesis. Transgenics mimicking endemic BL (with c-myc placed 3' to JH segments) exhibited the highest rate of B-cell lymphoma emergence, the highest Ki67 index of proliferation, and the highest transcriptomic similarities to human BL. The 3'RR enhancer alone deregulated c-myc and initiated the development of BL-like lymphomas, suggesting that its targeting would be of therapeutic interest to reduce c-myc oncogenicity in vivo.

The IgH 3' regulatory region and c-myc-induced B-cell lymphomagenesis

Deregulation and mutations of c-myc have been reported in multiple mature B-cell malignancies such as Burkitt lymphoma, myeloma and plasma cell lymphoma. After translocation into the immunoglobulin heavy chain (IgH) locus, c-myc is constitutively expressed under the control of active IgH cis-regulatory enhancers. Those located in the IgH 3 regulatory region (3RR) are master control elements of transcription. Over the past decade numerous convincing demonstrations of 3RRs contribution to mature c-myc-induced lymphomagenesis have been made using transgenic models with various types of IgH-c-myc translocations and transgenes. This review highlights how IgH 3RR physiological functions play a critical role in c-myc deregulation during lymphomagenesis.

Efficient role of IgH 3' regulatory region deficient B-cells in the development of oil granulomas

Functional B-cells are essential for the formation of oil granulomas. The IgH 3′ regulatory region (3′RR) activates important check-points during B-cell maturation. We investigated if 3′RR-deficient B-cells remain efficient to develop oil granulomas in response to pristine. B-cells expressing an IgH 3′RR-deficient allele were similarly recruited to wild type allele expressing B-cells in the granuloma. No differences were observed between 3′RR-deficient mice and control mice for granuloma numbers, cellular composition and ability to express mRNA transcripts for several pro- and anti-inflammatory cytokines. Altogether these results suggest a normal role for 3′RR-deficient B-cells in the development of an acute B-cell-mediated inflammatory response.

[The IgH 3'RR: Doctor Jekyll and Mister Hyde of B-cell maturation and lymphomagenesis]

The four transcriptional enhancers located in the 3' regulatory region (3'RR) of the IgH locus control the late phases of B-cell maturation, namely IgH locus transcription, somatic hypermutation and class switch recombination. Doctor Jekyll by nature, the 3'RR acts as Mister Hyde in case of oncogenic translocation at the IgH locus taking under its transcriptional control the translocated oncogene. The aim of this review is to show this duality on the basis of the latest scientific advances in the structure and function of the 3'RR and to hIghlight the targeting of the 3'RR as a potential therapeutic approach in mature B-cell lymphomas.

Simple and rapid determination of homozygous transgenic mice via in vivo fluorescence imaging

Setting up breeding programs for transgenic mouse strains require to distinguish homozygous from the heterozygous transgenic animals. The combinational use of the fluorescence reporter transgene and small animal in-vivo imaging system might allow us to rapidly and visually determine the transgenic mice homozygous for transgene(s) by the in vivo fluorescence imaging. RLG, RCLG or Rm17LG transgenic mice ubiquitously express red fluorescent protein (RFP). To identify homozygous RLG transgenic mice, whole-body fluorescence imaging for all of newborn F2-generation littermates produced by mating of RFP-positive heterozygous transgenic mice (F1-generation) derived from the same transgenic founder was performed. Subsequently, the immediate data analysis of the in vivo fluorescence imaging was carried out, which greatly facilitated us to rapidly and readily distinguish RLG transgenic individual(s) with strong fluorescence from the rest of F2-generation littermates, followed by further determining this/these RLG individual(s) showing strong fluorescence to be homozygous, as strongly confirmed by mouse mating. Additionally, homozygous RCLG or Rm17LG transgenic mice were also rapidly and precisely distinguished by the above-mentioned optical approach. This approach allowed us within the shortest time period to obtain 10, 8 and 2 transgenic mice homozygous for RLG, RCLG and Rm17LG transgene, respectively, as verified by mouse mating, indicating the practicality and reliability of this optical method. Taken together, our findings fully demonstrate that the in vivo fluorescence imaging offers a visual, rapid and reliable alternative method to the traditional approaches (i.e., mouse mating and real-time quantitative PCR) in identifying homozygous transgenic mice harboring fluorescence reporter transgene under the control of a ubiquitous promoter in the situation mentioned in this study.

The IgH 3' regulatory region influences lymphomagenesis in Igλ-Myc mice

The IgH 3′regulatory region (3′RR), encompassing the four transcriptional enhancers hs3a-hs1,2-hs3b-hs4, has a key role on class switch recombination, somatic hypermutation, IgH transcription and B-cell fate. In plasma cells, transcribed IgH and IgL loci often colocalized in transcription factories and an IgL transcription defect might translate into lowered IgH transcription. We explored whether the 3′RR would affect lymphomagenesis in Igλ-Myc transgenic mice prone to lymphoproliferations. Breeding Igλ-Myc transgenics in a background deficient for the 3′RR influences lymphomagenesis toward less mature lymphomas (16% vs 54%, p = 0.01, Z test for two population proportions). In a 3′RR-deficient background mature tumors less often expressed the CD43 antigen (54% vs 0%, p = 0.02), a membrane glycoprotein expressed on activated mature B-cells. In contrast, in a 3′RR-deficient background tumors more often expressed the CD5 antigen (32% vs 12%, p = 0.05) that may serve to control autoimmunity and that is suspected to play a role in leukemic transformation. Lymphoma myc transcript levels, the Ki67 index of proliferation, the clonality, the usage of V(D)J segments, and their somatic hypermutation status were not affected in the 3′RR-deficient background. In conclusion, most probably through its action during the maturation process, the 3′RR can influence lymphomagenesis even when not linked with an oncogene.

Targeting the oncogene B lymphoma deregulator IgH 3' regulatory region does not impede the in vivo inflammatory response in mice

The IgH 3′ regulatory region (3′RR), encompassing the four transcriptional enhancers hs3a-hs1,2-hs3b-hs4, is a potent lymphoma oncogene deregulator but its role in B cell-mediated inflammatory responses is unknown. We investigated the 3′RR involvement in the in vivo pristane-induced inflammatory response in BALB/c mice. The lack of the 3′RR in BALB/c mice had no wide effect on the incidence, the kinetic of development and the cellular composition of peritoneal ascites. Ascite pro-inflammatory cytokines levels (IL-6, IL-21, IL-12/23, TNF-α) were unchanged while anti-inflammatory cytokines levels (IL-10, interferon-γ) were slightly increased in 3′RR-deficient BALB/c mice as compared to wt BALB/c mice. In conclusion, the 3′RR is dispensable for the efficient recruitment of immune cells and the normal development of an inflammatory response in the in vivo pristane-induced inflammatory model. The 3′RR might be considered as a potential suitable target for anti-lymphoma pharmacological therapy without potent adverse effect on normal immune and inflammatory responses.