PubMed Health⌕ Search

Biomedical subjects

Almudena R Ramiro

Publications and source records attributed to Almudena R Ramiro.

12 recordsLinked to original sources
PubMed2006

Switching on chromosomal translocations.

Activation-induced deaminase initiates three different antibody diversification reactions: class switch recombination, somatic hypermutation (SHM), and gene conversion. We have shown that, in addition to antibody diversification, activation-induced deaminase can also initiate Burkitt's lymphoma-like c-myc/IgH translocations. However, distinct DNA damage- and oncogene-induced checkpoints operate in B cells to produce a normal intrachromosomal class switch recombination event or an aberrant interchromosomal fusion. These findings open the way to study the molecular pathways taking place at the early stages of malignant transformation.

Cell Transformation, Neoplastic↗
PubMed2006

Activation-induced deaminase: light and dark sides.

Activation-induced deaminase (AID) is required for class switch recombination (CSR) and somatic hypermutation (SHM), which are responsible for secondary diversification of antibodies in germinal centers. AID initiates these processes by deamination of cytosines on the immunoglobulin (Ig) locus, a potentially mutagenic activity. AID expression is restricted to germinal-center B cells, but the mechanisms that regulate its target specificity are not completely understood. Here, we review the most recent findings on the regulation of AID targeting and discuss how AID activity on non-Ig genes is relevant to the generation of chromosome translocations and to lymphomagenesis.

Animals↗
PubMed2006

Role of genomic instability and p53 in AID-induced c-myc-Igh translocations.

Chromosomal translocations involving the immunoglobulin switch region are a hallmark feature of B-cell malignancies. However, little is known about the molecular mechanism by which primary B cells acquire or guard against these lesions. Here we find that translocations between c-myc and the IgH locus (Igh) are induced in primary B cells within hours of expression of the catalytically active form of activation-induced cytidine deaminase (AID), an enzyme that deaminates cytosine to produce uracil in DNA. Translocation also requires uracil DNA glycosylase (UNG), which removes uracil from DNA to create abasic sites that are then processed to double-strand breaks. The pathway that mediates aberrant joining of c-myc and Igh differs from intrachromosomal repair during immunoglobulin class switch recombination in that it does not require histone H2AX, p53 binding protein 1 (53BP1) or the non-homologous end-joining protein Ku80. In addition, translocations are inhibited by the tumour suppressors ATM, Nbs1, p19 (Arf) and p53, which is consistent with activation of DNA damage- and oncogenic stress-induced checkpoints during physiological class switching. Finally, we demonstrate that accumulation of AID-dependent, IgH-associated chromosomal lesions is not sufficient to enhance c-myc-Igh translocations. Our findings reveal a pathway for surveillance and protection against AID-dependent DNA damage, leading to chromosomal translocations.

Animals↗
PubMed2004

AID is required for c-myc/IgH chromosome translocations in vivo.

Chromosome translocations between c-myc and immunoglobulin (Ig) are associated with Burkitt's lymphoma in humans and with pristane- and IL6-induced plasmacytomas in mice. These translocations frequently involve Ig switch regions, suggesting that they might be the result of aberrant Ig class switch recombination (CSR). However, a direct link between CSR and chromosome translocations has not been established. We have examined c-myc/IgH translocations in IL6 transgenic mice that are mutant for activation induced cytidine deaminase (AID), the enzyme that initiates CSR. Here we report that AID is essential for the c-myc/IgH chromosome translocations induced by IL6.

Animals↗
PubMed2004

Somatic hypermutation is limited by CRM1-dependent nuclear export of activation-induced deaminase.

Somatic hypermutation (SHM) and class switch recombination (CSR) are initiated in activated B lymphocytes by activation-induced deaminase (AID). AID is thought to make lesions in DNA by deaminating cytidine residues in single-stranded DNA exposed by RNA polymerase during transcription. Although this must occur in the nucleus, AID is found primarily in the cytoplasm. Here we show that AID is actively excluded from the nucleus by an exportin CRM1-dependent pathway. The AID nuclear export signal (NES) is found at the carboxyl terminus of AID in a region that overlaps a sequence required for CSR but not SHM. We find that AID lacking a functional NES causes more hypermutation of a nonphysiologic target gene in transfected fibroblasts. However, the NES does not impact on the rate of mutation of immunoglobulin genes in B lymphocytes, suggesting that the AID NES does not limit AID activity in these cells.

Amino Acid Sequence↗
PubMed2003

C-terminal deletion of AID uncouples class switch recombination from somatic hypermutation and gene conversion.

Class-switch recombination (CSR), somatic hypermutation (SHM), and antibody gene conversion are distinct DNA modification reactions, but all are initiated by activation-induced cytidine deaminase (AID), an enzyme that deaminates cytidine residues in single-stranded DNA. Here we describe a mutant form of AID that catalyzes SHM and gene conversion but not CSR. When expressed in E. coli, AID(delta189-198) is more active in catalyzing cytidine deamination than wild-type AID. AID(delta189-198) also promotes high levels of gene conversion and SHM when expressed in eukaryotic cells, but fails to induce CSR. These results underscore an essential role for the C-terminal domain of AID in CSR that is independent of its cytidine deaminase activity and that is not required for either gene conversion or SHM.

Animals↗
PubMed2003

Transcription enhances AID-mediated cytidine deamination by exposing single-stranded DNA on the nontemplate strand.

Somatic hypermutation and class switch recombination are DNA modification reactions that alter the genes encoding antibodies in B lymphocytes. Both of these distinct reactions require activation-induced deaminase (AID) and transcription. Here we show that in Escherichia coli, as in eukaryotic cells, the mutation frequency is directly proportional to the transcription of target genes. Transcription enhances mutation of the nontemplate DNA strand, which is exposed as single-stranded DNA during the elongation reaction, but not mutation of the template DNA strand, which is protected by E. coli RNA polymerase. Our results establish a direct link between AID and transcription and suggest that the role of transcription in facilitating mutation is to provide AID with access to single-stranded DNA.

Animals↗
PubMed2002

Identification of a myeloid intrathymic pathway of dendritic cell development marked by expression of the granulocyte macrophage-colony-stimulating factor receptor.

In this study, the finding that a significant proportion of all dendritic cells (DCs) resident in vivo in the human postnatal thymus displayed a myeloid-related phenotype prompted us to re-examine the developmental origin of thymic DCs, a cell type hitherto considered to represent a homogeneous lymphoid-derived population. We show here that these novel intrathymic DCs are truly myeloid, as they arise from CD34(+) early thymic progenitors through CD34(lo) intermediates which have lost the capacity to generate T cells, but display myelomonocytic differentiation potential. We also demonstrate that phenotypically and functionally equivalent myeloid precursors devoid of T-cell potential do exist in vivo in the postnatal thymus. Moreover, although interleukin 7 (IL-7) supports the generation of such myeloid intermediates, we show that their developmental branching from the main intrathymic T-cell pathway is linked to the up-regulation of the myelomonocytic granulocyte macrophage-colony-stimulating factor (GM-CSF) receptor, to the down-regulation of the IL-7 receptor and to the lack of pre-T-cell receptor alpha (pTalpha) gene transcriptional activation. Taken together, these data challenge the current view that the thymus is colonized by a lymphoid-restricted progenitor and provide evidence that a more immature precursor population with lymphoid and myelomonocytic potential is actually seeding the human postnatal thymus.

Antigens, CD↗
PubMed2002

Regulation of surface expression of the human pre-T cell receptor complex.

Considerable progress has recently been made in defining the role that pre-antigen receptor complexes, namely the pre-T and pre-B cell receptors, play in lymphocyte development. It is now established that these receptors direct, in a similar way, the survival, expansion, clonality and further differentiation of pre-T and pre-B lymphocytes, respectively. However, less is known about the mechanisms which ensure that only minute amounts of pre-TCR and pre-BCR reach the plasma membrane of developing lymphocytes. In this review, we discuss the implications of recent experimental approaches which address the developmental regulation of human pre-TCR expression and the molecular mechanisms that control surface pre-TCR expression levels.

Animals↗