Abstract
RAG1 or RAG2 mutations are associated with defects in V(D)J recombination activity, causing severe immunodeficiency with a wide spectrum of clinical phenotypes. A R229Q mutation of RAG2 was identified in patients with severe combined immunodeficiency (SCID) or Omenn syndrome (OS). Although some factors determining the clinical features between SCID and OS were not clear, the molecular mechanism of OS was studied in a mouse model in which an EGFP tag is fused to Rag2 with the R229Q mutation. To design the human disease model mimicking severe immunodeficiency, we generated Rag2-R229Q knock-in mice without an epitope tag. Mutant mice showed impaired T and B cell differentiation with reduced V(D)J recombination activity; however, the extent to which the R229Q mutation affects severe immunodeficiency was not severe. While Rag2-R229Q mutation under some conditions may cause severe immunological and clinical phenotypes similar to human SCID or OS, R229Q mutation per se did not cause severe immunodeficiency in mice, suggesting that additional factors other than R229Q mutation are required to induce severe immunodeficiency. Thus, our report implies that the effects of genetic background and/or a tagged protein sequence may alter the mouse immune system, revealing the mechanism of phenotypic heterogeneity arising from an identical mutation.
Highlights
The adaptive immune response to a particular pathogen relies on B and T lymphocytes, which possess genetically rearranged and highly diverse antigen receptors[1]
Based on the structure of the recombination activating gene 1 (RAG1)-RAG2 protein complex, missense mutations leading to severe combined immunodeficiency (SCID) or Omenn syndrome (OS) can be categorized into four groups: (1) mutations destabilizing the tertiary structure of RAG1-RAG2; (2) mutations affecting polar residues involved in DNA binding; (3) mutations surrounding the active sites; and (4) mutations located at the interface of RAG1 and RAG210
Accumulating evidence suggests that identical or similar RAG mutations in relatives can lead to different clinical phenotypes[12,25,26,27], indicating that mutations involved in V(D)J recombinase activity are not solely responsible for the clinical outcome
Summary
The adaptive immune response to a particular pathogen relies on B and T lymphocytes, which possess genetically rearranged and highly diverse antigen receptors[1]. Homozygous Rag2-R229Q/Enhanced green fluorescent protein (EGFP) mutant mice (hereafter Rag[2] KI/EGFP), developed by Marrella and colleagues[13], presented clinical and immunological phenotypes remarkably similar to human OS with severe alopecia, erythroderma, infiltration by T lymphocytes and eosinophils into the skin and gut, and complete absence of B cells. This mouse model expressed the mutant protein, in which the endogenous Rag[2] gene was targeted by a construct containing Rag2-R229Q with an EGFP tag at the N terminus. CRISPR/Cas[9] has been successfully used for generating knockout or knock-in mice that mimic human diseases[15]
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