Acetyltransferases GCN5 and PCAF Are Required for B Lymphocyte Maturation in Mice

B lymphocyte development has two DNA recombination processes: V(D)J recombination of the immunoglobulin (Igh) gene variable region, and class switching of the Igh constant regions from IgM to IgG, IgA, or IgE. V(D)J recombination is required for the successful maturation of B cells from pro-B to pre-B to immature-B and then to mature B cells in the bone marrow. CSR occurs outside of the bone marrow when mature B cells migrate to peripheral lymphoid organs, such as spleen and lymph nodes. Both V(D)J recombination and CSR depend on an open chromatin state that makes DNA accessible to specific enzymes, recombination activating gene (RAG), and activation-induced cytidine deaminase (AID). Acetyltransferases GCN5 and PCAF possess redundant functions acetylating histone H3 lysine 9 (H3K9). Here, we generated a mouse model that lacked both GCN5 and PCAF in B cells. Double-deficient mice possessed low levels of mature B cells in the bone marrow and peripheral organs, an accumulation of pro-B cells in bone marrow, and reduced CSR levels. We concluded that both GCN5 and PCAF are required for B-cell development in vivo.

Acetyltransferases GCN5 and PCAF are Required for B Lymphocyte Maturation

B lymphocyte development has two DNA recombination processes: V(D)J recombination of the immunoglobulin (Igh) gene variable region and class switching of the Igh constant regions from IgM to IgG, IgA, or IgE. V(D)J recombination is required for successful maturation of B cells from pro-B to pre-B to immature-B and then to mature B cells in the bone marrow. CSR occurs outside of the bone marrow when mature B cells migrate to peripheral lymphoid organs, such as spleen and lymph nodes. Both V(D)J recombination and CSR depend on an “open chromatin” state that makes DNA accessible to specific enzymes, recombination activating gene (RAG), and activation-induced cytidine deaminase (AID). Acetyltransferases GCN5 and PCAF possess redundant functions acetylating histone H3 lysine 9 (H3K9). Here, we generated a mouse model that lacks both GCN5 and PCAF in B cells. We found that double-deficient mice possess low levels of mature B cells in the bone marrow and peripheral organs, an accumulation of pro-B cells in bone marrow, and reduced CSR levels. We conclude that both GCN5 and PCAF are required for B cell development in vivo.

Acetyltransferases GCN5 and PCAF are Required for B Lymphocyte Maturation in Mice

B lymphocyte development has two DNA recombination processes: V(D)J recombination of the immunoglobulin (Igh) gene variable region and class switching of the Igh constant regions from IgM to IgG, IgA, or IgE. V(D)J recombination is required for successful maturation of B cells from pro-B to pre-B to immature-B and then to mature B cells in the bone marrow. CSR occurs outside of the bone marrow when mature B cells migrate to peripheral lymphoid organs, such as spleen and lymph nodes. Both V(D)J recombination and CSR depend on an “open chromatin” state that makes DNA accessible to specific enzymes, recombination activating gene (RAG), and activation-induced cytidine deaminase (AID). Acetyltransferases GCN5 and PCAF possess redundant functions acetylating histone H3 lysine 9 (H3K9). Here, we generated a mouse model that lacks both GCN5 and PCAF in B cells. We found that double-deficient mice possess low levels of mature B cells in the bone marrow and peripheral organs, an accumulation of pro-B cells in bone marrow, and reduced CSR levels. We conclude that both GCN5 and PCAF are required for B cell development in vivo.

Guardian of the Genome: An Alternative RAG/Transib Co-Evolution Hypothesis for the Origin of V(D)J Recombination

The appearance of adaptive immunity in jawed vertebrates is termed the immunological ‘Big Bang’ because of the short evolutionary time over which it developed. Underlying it is the recombination activating gene (RAG)-based V(D)J recombination system, which initiates the sequence diversification of the immunoglobulins and lymphocyte antigen receptors. It was convincingly argued that the RAG1 and RAG2 genes originated from a single transposon. The current dogma postulates that the V(D)J recombination system was established by the split of a primordial vertebrate immune receptor gene into V and J segments by a RAG1/2 transposon, in parallel with the domestication of the same transposable element in a separate genomic locus as the RAG recombinase. Here, based on a new interpretation of previously published data, we propose an alternative evolutionary hypothesis suggesting that two different elements, a RAG1/2 transposase and a Transib transposon invader with RSS-like terminal inverted repeats, co-evolved to work together, resulting in a functional recombination process. This hypothesis offers an alternative understanding of the acquisition of recombinase function by RAGs and the origin of the V(D)J system.

Differential expression of the recombination activating gene 1 activating protein 1 in cancers of the breast.

Breast cancer affects women at relatively high frequency (1). We mined published microarray datasets (2, 3) to determine in an unbiased fashion and at the systems level genes most differentially expressed in the primary tumors of patients with breast cancer. We report here significant differential expression of the gene encoding the recombination activating gene 1 activating protein 1, RAG1AP1, when comparing primary tumors of the breast to the tissue of origin, the normal breast. RAG1AP1 mRNA was present at significantly higher quantities in tumors of the breast as compared to normal breast tissue. Analysis of human survival data revealed that expression of RAG1AP1 in primary tumors of the breast was correlated with recurrence-free survival in patients with basal type cancer. RAG1AP1 may be of relevance to initiation, maintenance or progression of cancers of the female breast.

Identification of a Novel RAG1 Hypomorphic Mutation in a Child Presenting with Disseminated Vaccine-Strain Varicella.

Background: Recombination-activating gene 1 (RAG1) and recombination-activating gene 2 (RAG2) encode unique lymphocyte endonuclease proteins that are crucial in T and B cell development through V(D)J recombination. RAG1 gene defects lead to variable phenotypes, ranging from immunocompetent to severe combined immunodeficiency (SCID). Curative therapy for severe manifestations can be achieved through hematopoietic stem cell transplantation (HSCT). Advances in genomic sequencing have led to the discovery of new variants and it is recognized that the level of recombinase activity correlates with disease severity. Aim: To report the clinical presentation, immunological work-up, decision process to undergo HSCT, and confirmatory genetic diagnosis in a patient who was well until her initial presentation with disseminated vaccine-strain varicella. Methods: Clinical data was gathered through retrospective chart review. Immunological investigations, targeted gene sequencing, and thymic biopsy results were reviewed. Further genetic analysis, including whole exome and whole genome sequencing was performed. Results: Whole exome sequencing identified a single missense mutation in RAG1, R474C (c.1420C>T), which would not account for the clinical presentation. Healthy individuals with only one mutation have been reported. Subsequently, whole genome sequencing revealed a novel second heterozygous missense variant, H945D (c.2833 G>T) in the RAG1 gene. Conclusion: Hypomorphic RAG1 mutations with residual activity have a diverse phenotypic expression. Identifying and understanding the implications of these mutations is crucial for disease prognostication and tailoring management. Statement of Novelty: We present a novel RAG1 missense variant, with likely complete or partial loss of function, in a patient with significant impairment in cellular immunity.

The recombination activating gene 2, RAG2, is differentially expressed in the tumors of breast cancer patients treated with trastuzumab.

Trastuzumab (Herceptin) is a monoclonal antibody targeting the extracellular domain of the human epidermal growth factor receptor 2 (HER2) (1) utilized for the treatment of adjuvant and metastatic breast cancer (2) in the United States and worldwide. We mined published microarray data (3, 4) to discover in an unbiased manner the most significant transcriptional changes associated with trastuzumab treatment. We identified the recombination activating gene, RAG2, as among those most differentially expressed in the primary tumors of patients with breast cancer treated with trastuzumab. A single administration of trastuzumab was sufficient to transcriptionally activate RAG2 in primary tumors of the breast, demonstrating increased expression of a molecule thought to possess cell type-specific expression in B-lymphocytes as a direct transcriptional consequence of treatment with trastuzumab.

Immunodeficient Rabbit Models: History, Current Status and Future Perspectives

Production of immunodeficient (ID) models in non-murine animal species had been extremely challenging until the advent of gene-editing tools: first zinc finger nuclease (ZFN), then transcription activator-like effector nuclease (TALEN), and most recently clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR)/Cas9. We and others used those gene-editing tools to develop ID rabbits carrying a loss of function mutation in essential immune genes, such as forkhead box protein N1 (FOXN1), recombination activating gene 1/2 (RAG1/2), and interleukin 2 receptor subunit gamma (IL2RG). Like their mouse counterparts, ID rabbits have profound defects in their immune system and are prone to bacterial and pneumocystis infections without prophylactic antibiotics. In addition to their use as preclinical models for primary immunodeficient diseases, ID rabbits are expected to contribute significantly to regenerative medicine and cancer research, where they serve as recipients for allo- and xeno-grafts, with notable advantages over mouse models, including a longer lifespan and a much larger body size. Here we provide a concise review of the history and current status of the development of ID rabbits, as well as future perspectives of this new member in the animal model family.

Altered 3D chromatin structure permits inversional recombination at the IgH locus

Immunoglobulin heavy chain (IgH) genes are assembled by two sequential DNA rearrangement events that are initiated by recombination activating gene products (RAG) 1 and 2. Diversity (DH) gene segments rearrange first, followed by variable (VH) gene rearrangements. Here, we provide evidence that each rearrangement step is guided by different rules of engagement between rearranging gene segments. DH gene segments, which recombine by deletion of intervening DNA, must be located within a RAG1/2 scanning domain for efficient recombination. In the absence of intergenic control region 1, a regulatory sequence that delineates the RAG scanning domain on wild-type IgH alleles, VH and DH gene segments can recombine with each other by both deletion and inversion of intervening DNA. We propose that VH gene segments find their targets by distinct mechanisms from those that apply to DH gene segments. These distinctions may underlie differential allelic choice associated with each step of IgH gene assembly.