The humoral immune response of the Camelidae is unique as these animals are the only known mammals that seem to possess functional homodimeric heavy-chain antibodies besides the classical heteromeric antibodies composed of heavy (H) and... more
The humoral immune response of the Camelidae is unique as these animals are the only known mammals that seem to possess functional homodimeric heavy-chain antibodies besides the classical heteromeric antibodies composed of heavy (H) and light (L) chains. By definition, the heavy-chain antibodies lack the L-chain, and it was noticed that their H-chain is devoid of the typical first constant domain (CH1) and contains a dedicated variable domain, referred to as VHH. The VHH exon is assembled from separate V–D–J gene segments. The recombined VHH region is subjected to somatic hypermutations; however, the timing and actual mechanism of the class switch from μ to the dedicated γ-isotype remains elusive. Interestingly, antigen-specific VHHs are easily retrieved after panning of a phage-displayed rearranged V-gene pool cloned from an immunised camelid. These single-domain antigen binding entities possess a number of biophysical properties that offer particular advantages in various medical and biotechnological applications.
Trends Genet . 2021 Nov 2;S0168-9525(21)00289-4. doi: 10.1016/j.tig.2021.10.005.Online ahead of print. RNA-directed DNA repair and antibody somatic hypermutation Andrew Franklin 1, Edward J Steele 2 DOI: 10.1016/j.tig.2021.10.005... more
Trends Genet . 2021 Nov 2;S0168-9525(21)00289-4. doi: 10.1016/j.tig.2021.10.005.Online ahead of print. RNA-directed DNA repair and antibody somatic hypermutation Andrew Franklin 1, Edward J Steele 2 DOI: 10.1016/j.tig.2021.10.005 https://pubmed.ncbi.nlm.nih.gov/34740453/ Abstract Somatic hypermutation at antibody loci affects both deoxyadenosine-deoxythymidine (A/T) and deoxycytidine-deoxyguanosine (C/G) pairs. Deamination of C to deoxyuridine (U) by activation-induced deaminase (AID) explains how mutation at C/G pairs is potentiated. Mutation at A/T pairs is triggered during the initial stages of repair of AID-generated U lesions and occurs through an as yet unknown mechanism in which polymerase η has a major role. Recent evidence confirms that human polymerase η can act as a reverse transcriptase. Here, we compare the popular suggestion of mutation at A/T pairs through nucleotide mispairing (owing to polymerase error) during short-patch repair synthesis with the alternative proposal of mutation at A/T pairs through RNA editing and RNA-directed DNA repair. Keywords antibody; polymerase error; polymerase η; reverse transcriptase; somatic hypermutation.
Soma-to-germline feedback is forbidden under the neo- Darwinian paradigm. Nevertheless, there is a growing realization it occurs frequently in immunoglobulin (Ig) variable (V) region genes. This is a surprising development. It arises from... more
Soma-to-germline feedback is forbidden under the neo- Darwinian paradigm. Nevertheless, there is a growing realization it occurs frequently in immunoglobulin (Ig) variable (V) region genes. This is a surprising development. It arises from a most unlikely source in light of the exposure of co-author EJS to the haplotype data of RL Dawkins and others on the polymorphism of the Major Histocompati- bility Complex, which is generally assumed to be the most polymorphic region in the genome (spanning $4 Mb). The comparison between the magnitude of MHC polymor- phism with estimates for the human heavy chain immunoglobulin V locus (spanning $1 Mb), suggests IGHV could be many orders of magnitude more polymorphic than the MHC. This conclusion needs airing in the literature as it implies generational churn and soma-to-germlinegene feedback. Pedigree-based experimental strategies to resolve the IGHV issue are outlined
The simian virus 40 (SV40) T antigen is a potent oncogene able to transform many cell types and has been implicated in leukemia and lymphoma. In this report, we have achieved sporadic SV40 T-antigen expression in mature B cells in mice,... more
The simian virus 40 (SV40) T antigen is a potent oncogene able to transform many cell types and has been implicated in leukemia and lymphoma. In this report, we have achieved sporadic SV40 T-antigen expression in mature B cells in mice, by insertion of a SV40 T antigen gene in opposite transcriptional orientation in the immunoglobulin (Ig) heavy (H) chain locus between the D and J(H) segments. SV40 T-antigen expression appeared to result from retention of the targeted germline allele and concomitant antisense transcription of SV40 large T in mature B cells, leading to chronic lymphocytic leukemia (CLL). Although B-cell development was unperturbed in young mice, aging mice showed accumulation of a monoclonal B-cell population in which the targeted IgH allele was in germline configuration and the wild-type IgH allele had a productive V(D)J recombination. These leukemic B cells were IgD(low)CD5(+) and manifested nonrandom usage of V, D, and J segments. V(H) regions were either unmutated, with preferential usage of the VH11 family, or manifested extensive somatic hypermutation. Our findings provide an animal model for B-CLL and show that pathways activated by SV40 T antigen play important roles in the pathogenesis of B-CLL.
Available data relevant to the mechanism of somatie hypermutation have been critically evaluated in the context of alternative models: (i) error-generating reverse transcription (RT) followed by homologous recombination: and (ii)... more
Available data relevant to the mechanism of somatie hypermutation have been critically evaluated in the context of alternative models: (i) error-generating reverse transcription (RT) followed by homologous recombination: and (ii) error-prone DNA replication/repair. A set of basic principles conceming somatic hypermutation has also been formulated and a revised and expanded 'RT-Mutatorsome" concept (analogous to tclomerase) is presented which is consistent with these principles and all data on the distribution of somatic mutations in normal and Ig transgenic mice carr\'ing particular \'(D)J and flanking region constructs. It is predicted that in the mouse \ 77 and 1 'K loci, the J-C intronic Enhancer-Nuclear Matrix Attachment Region (Ei/MAR) contains a unique sequence motif or secondary structure which ensures that only I (D)J sequences mutate whilst other regions ofthe genome arc not mutated.
HIV-1 infection or systemic lupus erythematosus (SLE) disrupt B cell homeostasis, reduce memory B cells, and impair function of IgG and IgM antibodies. To determine how disturbances in B cell populations producing polyclonal antibodies... more
HIV-1 infection or systemic lupus erythematosus (SLE) disrupt B cell homeostasis, reduce memory B cells, and impair function of IgG and IgM antibodies. To determine how disturbances in B cell populations producing polyclonal antibodies relate to the IgM repertoire, the IgM transcriptome in health and disease was explored at the complementarity determining region 3 (CDRH3) sequence level. 454-deep pyrosequencing in combination with a novel analysis pipeline was applied to define populations of IGHM CDRH3 sequences based on absence or presence of somatic hypermutations (SHM) in peripheral blood B cells. HIV or SLE subjects have reduced biodiversity within their IGHM transcriptome compared to healthy subjects, mainly due to a significant decrease in the number of unique combinations of alleles, although recombination machinery was intact. While major differences between sequences without or with SHM occurred among all groups, IGHD and IGHJ allele use, CDRH3 length distribution, or gene...
V(D)J recombination is essential to produce an Ig repertoire with a large range of Ag specificities. Although NF-B-binding sites are present in the human and mouse IgH, Ig, and Ig enhancer modules and RAG expression is controlled by NF-B,... more
V(D)J recombination is essential to produce an Ig repertoire with a large range of Ag specificities. Although NF-B-binding sites are present in the human and mouse IgH, Ig, and Ig enhancer modules and RAG expression is controlled by NF-B, it is not known whether NF-B regulates V(D)J recombination mechanisms after RAG-mediated dsDNA breaks. To clarify the involvement of NF-B in human V(D)J recombination, we amplified Ig gene rearrangements from individual peripheral B cells of patients with X-linked anhidrotic ectodermal dysplasia with hyper-IgM syndrome (HED-ID) who have deficient expression of the NF-B essential modulator (NEMO/Ikk). The amplification of nonproductive Ig gene rearrangements from HED-ID B cells reflects the influence of the Ikk-mediated canonical NF-B pathway on specific molecular mechanisms involved in V(D)J recombination. We found that the CDR3H from HED-ID B cells were abnormally long, as a result of a marked reduction in the exonuclease activity on the V, D, and...
Affinity maturation of the antibody response is a fundamental process in adaptive immunity during which B-cells activated by infection or vaccination undergo rapid proliferation accompanied by the acquisition of point mutations in their... more
Affinity maturation of the antibody response is a fundamental process in adaptive immunity during which B-cells activated by infection or vaccination undergo rapid proliferation accompanied by the acquisition of point mutations in their rearranged immunoglobulin (Ig) genes and selection for increased affinity for the eliciting antigen. The rate of somatic hypermutation at any position within an Ig gene is known to depend strongly on the local DNA sequence, and Ig genes have region-specific codon biases that influence the local mutation rate within the gene resulting in increased differential mutability in the regions that encode the antigen-binding domains. We have isolated a set of clonally related natural Ig heavy chain-light chain pairs from an experimentally infected influenza patient, inferred the unmutated ancestral rearrangements and the maturation intermediates, and synthesized all the antibodies using recombinant methods. The lineage exhibits a remarkably uniform rate of im...
Burkitt lymphoma/leukaemia (BL/L) is a heterogeneous disease with respect to epidemiological patterns and cell origin. The occurrence of BL/L with an immature phenotype raises the question whether this phenotype might be a consequence of... more
Burkitt lymphoma/leukaemia (BL/L) is a heterogeneous disease with respect to epidemiological patterns and cell origin. The occurrence of BL/L with an immature phenotype raises the question whether this phenotype might be a consequence of early B-cell transformation or, alternatively, a secondary feature of transformed, mature B cells. It also poses important clinical questions regarding diagnosis and therapeutic procedures. Here we describe the case of a 4-yr-old child with BL/L and FAB L3 morphology, with phenotypic and genotypic characteristics of a CD10+ precursor B-cell acute lymphoid leukaemia (ALL) associated with t(8;14)(q24;q32). Molecular analysis showed expression of RAG1 and RAG2 and an unmutated VDJCμ immunoglobulin rearrangement coinciding with a lack of AICDA expression, indicating an immature B-cell origin. His clinical response suggested that FAB L3 ALL with MYC rearrangement and an aberrant precursor B-cell phenotype is clinically similar to BL/L. Moreover, short, intensive chemotherapeutic protocols seemed to be beneficial. This case also allowed us to refine the description of cellular and molecular variants of BL/L regarding the cell origin and pathogenesis of this biologically heterogeneous disease.
