Abstract: The humoral immune response relies on a diverse antibody repertoire, which is expanded through processes such as somatic hypermutation, class-switch recombination and gene conversion. These processes are primarily mediated by activation-induced cytidine deaminase (AID). Gene conversion generates diversity in immunoglobulin heavy and light chains (IGHVs) in species such as chickens and rabbits, though it has not been widely studied. Since 80% of the equine IGHV repertoire originates from only three functional gene segments, we examined gene conversion events in horses to assess their role in antibody diversification. Using a modified version of BrepConvert, which optimized analysis time, we identified gene conversion events in 6.9% of immunoglobulin sequences. The results showed a local preference, with most events occurring at the beginning of framework region 1 (FR1) and within complementarity-determining region 2 (CDR2). Pseudogenes IGHV4-35, IGHV4-53, and IGHV4-38 were utilized most frequently, while functional genes IGHV4-21, IGHV4-22, and IGHV4-29 exhibited the highest event frequencies. Interestingly, while most mismatched regions were only three nucleotides long, 91% of these events are flanked by specific sequences (six nucleotides at the 5' end and one nucleotide at the 3' end). Furthermore, functional pseudogene pairs often share identical leader regions of 5-26 nucleotides, suggesting expanded events. We also identified a potential association between these events and local non-B DNA conformations, as well as with the zinc finger protein ZNF691, which supports the involvement of DNA-binding factors. Together, these findings demonstrate that gene conversion significantly contributes to equine antibody diversity by targeting specific IGHV regions.
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Overview
This study investigates how gene conversion contributes to creating antibody diversity in horses by identifying specific hotspots within immunoglobulin heavy chain variable (IGHV) genes where these events preferentially occur.
The research reveals that gene conversion plays a significant role in equine immune diversity, focusing on particular gene regions and pseudogenes, and highlights potential molecular mechanisms driving this process.
Background
The humoral immune response requires a wide variety of antibodies to recognize diverse pathogens.
Antibody diversity is generated through multiple mechanisms, including:
Somatic hypermutation (introduces point mutations)
Gene conversion (replaces short DNA segments using related sequences as templates)
Activation-induced cytidine deaminase (AID) is a key enzyme mediating these processes.
Gene conversion has been well-studied in species like chickens and rabbits but less so in horses.
In horses, 80% of antibody diversity in IGHV genes derives from just three functional gene segments, raising questions about additional mechanisms like gene conversion to explain diversity.
Study Goals
To detect and characterize gene conversion events within the horse IGHV repertoire.
To identify specific regions in IGHV genes where gene conversion preferentially occurs (“hotspots”).
To analyze which pseudogenes contribute most frequently to these gene conversions.
To explore molecular features associated with gene conversion, such as sequence motifs and DNA conformations.
Methods
Genomic analysis of horse IGHV sequences was performed using a modified tool called BrepConvert optimized for speed and accuracy.
Immunoglobulin sequences were examined to detect gene conversion events, defined as short DNA segments replaced by sequences derived from pseudogenes.
Location of conversion events was mapped to IGHV gene regions including framework regions (FR1) and complementarity-determining regions (CDRs).
Frequency and distribution of events involving different pseudogenes and functional genes were analyzed.
Sequence motifs flanking gene conversion tracts were identified to understand targeting preferences.
Associations were sought between gene conversion sites and structural DNA features (non-B DNA conformations) or DNA-binding proteins like ZNF691.
Key Findings
Gene conversion events were detected in approximately 6.9% of immunoglobulin sequences, indicating it is a significant diversification mechanism in horses.
These events showed a strong local preference:
Predominantly concentrated at the start of the Framework Region 1 (FR1).
Also frequent within Complementarity-Determining Region 2 (CDR2), an important antigen binding site.
Pseudogenes IGHV4-35, IGHV4-53, and IGHV4-38 were most commonly used as templates in gene conversion.
Functional IGHV genes IGHV4-21, IGHV4-22, and IGHV4-29 showed the highest frequencies of conversion events, suggesting active diversification in these segments.
Most mismatched gene conversion tracts were short, typically three nucleotides long.
Approximately 91% of these conversion events were flanked by specific nucleotide sequences:
Six nucleotides at the 5′ junction.
One nucleotide at the 3′ junction.
Functional pseudogene pairs often shared identical leader sequences ranging from 5 to 26 nucleotides, suggesting coordinated or expanded conversion events across genes.
Potential linkages were found between gene conversion hotspots and:
Non-B DNA structures, which are local DNA conformations that differ from the typical right-handed double helix.
The zinc finger protein ZNF691, implicating DNA-binding factors in targeting or regulating gene conversion.
Significance and Implications
This research demonstrates that gene conversion is a key mechanism contributing to antibody diversity in horses.
It identifies specific IGHV sub-regions and pseudogenes as hotspots, which informs how the immune system maintains diversity despite limited initial gene segments.
Findings about conserved sequence motifs and DNA structures provide insight into possible molecular control mechanisms of gene conversion.
The association with the ZNF691 protein suggests gene conversion might be influenced or directed by specific DNA-binding proteins, opening avenues for further mechanistic studies.
The improved computational tool used (modified BrepConvert) provides a methodology that can be applied to study gene conversion in other species as well.
Conclusion
Gene conversion plays a substantial role in diversifying the horse antibody repertoire by favoring certain IGHV regions and using specific pseudogenes as donors.
The study enhances our understanding of how equine immune systems generate variability, which can be crucial for disease resistance and vaccine design.
These discoveries pave the way for more detailed functional and mechanistic investigations into antibody gene diversification in horses and potentially other species.
Cite This Article
APA
Pinto JEL, Gervásio JHB, Ng JC, Gomes-Silva A, de Matos Guedes HL, Cunha L, Castilho LR, da Silva JL, Franco GR, Navas C, Felicori LF.
(2026).
Identification of gene conversion events in horse IGHV suggests preferential hotspots for diversification.
Immunogenetics, 78(1), 8.
https://doi.org/10.1007/s00251-026-01400-7
Laboratory of Synthetic Biology and Biomimetics, Department of Biochemistry and Immunology, Institute of Biological Sciences (ICB), Federal University of Minas Gerais, Belo Horizonte, MG, Brazil.
Gervásio, João Henrique Brandão
Laboratory of Synthetic Biology and Biomimetics, Department of Biochemistry and Immunology, Institute of Biological Sciences (ICB), Federal University of Minas Gerais, Belo Horizonte, MG, Brazil.
Model-Based Evolutionary Genomic Unit, Okinawa Institute of Science and Technology (OIST), Tancha, 904-0412, Okinawa, Japan.
Ng, Joseph Chi-Fung
Research Department of Structural and Molecular Biology, University College London, London, WC1E 6BT, UK.
Gomes-Silva, Adriano
Interdisciplinary Laboratory of Medical Research, Oswaldo Cruz Institute, Oswaldo Cruz Foundation, Rio de Janeiro, RJ, Brazil.
Mycobacteriosis Clinical Research Laboratory, Evandro Chagas National Institute of Infectious Diseases, Oswaldo Cruz Foundation, Rio de Janeiro, RJ, Brazil.
de Matos Guedes, Herbert L
Laboratory of Immunobiotechnology, Department of Immunology, Paulo de Góes Institute of Microbiology, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil.
Laboratory of Clinical Immunology, Oswaldo Cruz Institute, Oswaldo Cruz Foundation, Rio de Janeiro, RJ, Brazil.
Cunha, Luiz
Vital Brazil Institute, Niterói, RJ, Brazil.
Castilho, Leda R
Cell Culture Engineering Laboratory, COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil.
da Silva, Jerson Lima
Institute of Medical Biochemistry Leopoldo de Meis, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil.
Franco, Glória Regina
Laboratory of Biochemical Genetics, Department of Biochemistry and Immunology, Institute of Biological Sciences (ICB), Federal University of Minas Gerais, Belo Horizonte, MG, 30161-970, Brazil.
Navas, Carlena
Laboratory of Synthetic Biology and Biomimetics, Department of Biochemistry and Immunology, Institute of Biological Sciences (ICB), Federal University of Minas Gerais, Belo Horizonte, MG, Brazil.
Felicori, Liza Figueiredo
Laboratory of Synthetic Biology and Biomimetics, Department of Biochemistry and Immunology, Institute of Biological Sciences (ICB), Federal University of Minas Gerais, Belo Horizonte, MG, Brazil. lizaffelicori@gmail.com.
MeSH Terms
Animals
Gene Conversion
Horses / genetics
Horses / immunology
Immunoglobulin Heavy Chains / genetics
Immunoglobulin Variable Region / genetics
Complementarity Determining Regions / genetics
Somatic Hypermutation, Immunoglobulin / genetics
Antibody Diversity / genetics
Cytidine Deaminase / genetics
Conflict of Interest Statement
Declarations. Competing interests: The authors declare no competing interests. Conflict of interest: The authors declare that the research documented in this manuscript was conducted in the absence of any commercial or financial relationships that could be construed as conflicts of interest. Ethics approval: The experimental project on the Use of Animals was approved by the Ethics Committee of the Federal University of Minas Gerais (CEUA-UFMG) under the protocol number 190/2018.
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