Brice Laffleur

Nuclear Proximity of Mtr4 to RNA Exosome Restricts DNA Mutational Asymmetry

The distribution of sense and antisense strand DNA mutations on transcribed duplex DNA contributes to the development of immune and neural systems along with the progression of cancer. Because developmentally matured B cells undergo biologically programmed strand-specific DNA mutagenesis at focal DNA/RNA hybrid structures, they make a convenient system to investigate strand-specific mutagenesis mechanisms. We demonstrate that the sense and antisense strand DNA mutagenesis at the immunoglobulin heavy chain locus and some other regions of the B cell genome depends upon localized RNA processing protein complex formation in the nucleus. Both the physical proximity and coupled activities of RNA helicase Mtr4 (and senataxin) with the noncoding RNA processing function of RNA exosome determine the strand-specific distribution of DNA mutations. Our study suggests that strand-specific DNA mutagenesis-associated mechanisms will play major roles in other undiscovered aspects of organismic development.

A p53 Defect Sensitizes Various Stages of B Cell Development to Lymphomagenesis in Mice Carrying an IgH 3′ Regulatory Region-Driven c-myc Transgene

Although c-myc is classically described as the driving oncogene in Burkitt’s lymphoma (BL), deregulation and mutations of c-myc have been reported in multiple solid tumors and in other mature B cell malignancies such as mantle cell lymphoma (MCL), myeloma, and plasma cell lymphoma (PCL). After translocation into the IgH locus, c-myc is constitutively expressed under the control of active IgH enhancers. Those located in the IgH 3′ regulatory region (3′RR) are master control elements of class switch recombination and of the transcriptional burst associated with plasma cell differentiation. c-myc-3′RR mice are prone to lymphomas with rather homogeneous, most often BL-like, phenotypes with incomplete penetrance (75% tumor incidence) and long latencies (10–12 mo). To reproduce c-myc–induced mature B cell lymphomagenesis in the context of an additional defect often observed in human lymphomas, we intercrossed c-myc-3′RR with p53+/− mice. Double transgenic c-myc-3′RR/p53+/− mice developed lymphoma with short latency (2–4 mo) and full penetrance (100% tumor incidence). The spectrum of B lymphomas occurring in c-myc-3′RR/p53+/− mice was widened, including nonactivated (CD43−) BL, activated (CD43+) BL, MCL-like lymphoma, and PCL, thus showing that 3′RR-mediated deregulation of c-myc can promote various types of B lymphoproliferation in cells that first acquired a p53 defect. c-myc/p53+/− mice closely reproduce many features of BL, MCL, and PCL and provide a novel and efficient model to dissect the molecular events leading to c-myc–induced lymphomagenesis and an important tool to test potential therapeutic agents on malignant B cells featuring various maturation stages.

Specific impairment of proximal tubular cell proliferation by a monoclonal κ light chain responsible for Fanconi syndrome

BACKGROUND Fanconi syndrome (FS) is a rare renal disorder featuring proximal tubule dysfunction that may occur following tubular reabsorption of a monoclonal light chain (LC), in patients with multiple myeloma. FS may precede the recognition of multiple myeloma by several years. In most cases, crystalline inclusions of monoclonal κ LCs are observed within the lysosomes of proximal tubular cells (PTCs) and probably participate in their functional alteration. METHODS To investigate the mechanism implicated in proximal tubule dysfunction, we compared the effects of κ LC-CHEB obtained from a patient with myeloma-associated FS to those of control κ LC-BON obtained from a patient without evidence of FS, on the viability and proliferation of two different PTC lines. RESULTS Our data suggest that the tubular atrophy in myeloma-associated FS does not result from increased apoptosis of PTCs, but from their impaired capacity to proliferate and renew. Indeed, in vitro incubation of cultured PTCs with physiological amounts of the nephrotoxic κ LC-CHEB was sufficient to cause a depression in DNA synthesis and in cell proliferation. This effect was observed neither with control κ LC-BON nor in the absence of κ LC. CONCLUSIONS The reduced turnover of PTCs may affect tubular repair and regeneration. In addition, the reduced proliferation of myeloma cells producing the same monoclonal κ LC might explain the frequent association of FS with smoldering multiple myeloma.

The IgH locus 3’ cis -regulatory super-enhancer co-opts AID for allelic transvection

Immunoglobulin heavy chain (IgH) alleles have ambivalent relationships: they feature both allelic exclusion, ensuring monoallelic expression of a single immunoglobulin (Ig) allele, and frequent inter-allelic class-switch recombination (CSR) reassembling genes from both alleles. The IgH locus 3′ regulatory region (3′RR) includes several transcriptional cis-enhancers promoting activation-induced cytidine deaminase (AID)-dependent somatic hypermutation (SHM) and CSR, and altogether behaves as a strong super-enhancer. It can also promote deregulated expression of translocated oncogenes during lymphomagenesis. Besides these rare, illegitimate and pathogenic interactions, we now show that under physiological conditions, the 3′RR super-enhancer supports not only legitimate cis-, but also trans-recruitment of AID, contributing to IgH inter-allelic proximity and enabling the super-enhancer on one allele to stimulate biallelic SHM and CSR. Such inter-allelic activating interactions define transvection, a phenomenon well-known in drosophila but rarely observed in mammalian cells, now appearing as a unique feature of the IgH 3′RR super-enhancer.

RNA Exosome and Non-coding RNA-Coupled Mechanisms in AID-Mediated Genomic Alterations

The eukaryotic RNA exosome is a well-conserved protein complex with ribonuclease activity implicated in RNA metabolism. Various families of non-coding RNAs have been identified as substrates of the complex, underscoring its role as a non-coding RNA processing/degradation unit. However, the role of RNA exosome and its RNA processing activity on DNA mutagenesis/alteration events have not been investigated until recently. B lymphocytes use two DNA alteration mechanisms, class switch recombination (CSR) and somatic hypermutation (SHM), to re-engineer their antibody gene expressing loci until a tailored antibody gene for a specific antigen is satisfactorily generated. CSR and SHM require the essential activity of the DNA activation-induced cytidine deaminase (AID). Causing collateral damage to the B-cell genome during CSR and SHM, AID induces unwanted (and sometimes oncogenic) mutations at numerous non-immunoglobulin gene sequences. Recent studies have revealed that AIDs DNA mutator activity is regulated by the RNA exosome complex, thus providing an example of a mechanism that relates DNA mutagenesis to RNA processing. Here, we review the emergent functions of RNA exosome during CSR, SHM, and other chromosomal alterations in B cells, and discuss implications relevant to mechanisms that maintain B-cell genomic integrity.

B Cell Intrinsic Mechanisms Constraining IgE Memory

Memory B cells and long-lived plasma cells are key elements of adaptive humoral immunity. Regardless of the immunoglobulin class produced, these cells can ensure long-lasting protection but also long-lasting immunopathology, thus requiring tight regulation of their generation and survival. Among all antibody classes, this is especially true for IgE, which stands as the most potent, and can trigger dramatic inflammatory reactions even when present in minute amounts. IgE responses and memory crucially protect against parasites and toxic components of venoms, conferring selective advantages and explaining their conservation in all mammalian species despite a parallel broad spectrum of IgE-mediated immunopathology. Long-term memory of sensitization and anaphylactic responses to allergens constitute the dark side of IgE responses, which can trigger multiple acute or chronic pathologic manifestations, some punctuated with life-threatening events. This Janus face of the IgE response and memory, both necessary and potentially dangerous, thus obviously deserves the most elaborated self-control schemes.

G-quadruplex DNA targeting alters class-switch recombination in B cells and attenuates allergic inflammation

Targeting immunoglobulin switch regions with a G4-DNA binding drug offers a novel immunomodulatory strategy reducing class-switched antibody secretion.