Background/Purpose: We sought to identify the etiology of early onset autoimmunity and T cell lymphopenia in two siblings through genetic and functional studies.  Newborn screening identified a full term infant (Patient B) with undetectable T-cell receptor excision circles, raising a concern for severe combined immunodeficiency (SCID).  A female sibling (Patient A) had died at 2 years of age secondary to complications from autoimmunity.  At 6 months, Patient A had developed vasculitis leading to digital necrosis, myositis, autoimmune cytopenias, positive autoantibodies (ANA, anti-cardiolipin, thyroid peroxidase, and thryroglobulin), elevated inflammatory markers, and hypocomplementia.  Both children were severely T cell lymphopenic with poor lymphocyte proliferation to mitogens, although natural killer cells, B lymphocytes, and immunoglobulin levels were preserved.  Due to the sibling’s ultimate fatality, Patient B received prophylactic antibiotics and IVIG; the child never developed autoimmunity and underwent a successful bone marrow transplant at 3 months of age.

Methods: Genomic analysis was done on DNA extracted from whole blood.  Recombinase activity was explored using Abelson-immortalized murine Rag1^-/- pro-B cells with an intrachromosomal inverted GFP cassette flanked by recombination signal sequences.  The Abelson pro-B cells were transduced with vectors encoding either wild type or mutant RAG1 alleles.  Recombination-dependent expression of GFP served as an indicator of RAG1 activity.  The Abelson system was also used to assess RAG1 protein expression through immunoblotting.  Expression of TCRVβ families in CD3+ cells was detected using monoclonal antibodies conjugated to fluorochromes.  The B cell repertoire was evaluated by pyrosequencing.   

Results: Testing in Patient A for mutations classically associated with T^–B⁺SCID was unrevealing.  Patient B had compound heterozygous missense mutations in RAG1 (c.2522 G>A, p.R841Q; c.2920 T>C, p.F974L).  Analysis of frozen genomic DNA from Patient A confirmed identical RAG1mutations.  The parents were carriers (paternal allele:  p.R841Q; maternal allele:  p.F974L).  In the Abelson system, the R841 mutant lacked protein expression and recombinase activity while the F974L mutant demonstrated normal protein expression but reduced activity (56.5% of wild type).  In Patient B, the T cell repertoire was oligoclonal with 8/24 TCRVβ families falling in normal range.  The restricted T cell diversity corrected after transplant.  By pyrosequencing, the usage of variable, diversity, and joining (V(D)J) segments was skewed in the B cells.     

Conclusion: Lymphocyte receptor diversity is generated by the activity of RAG1 and 2, which create DNA breaks that allow recombination of V(D)J gene segments.  The clinical presentation of RAG deficiency ranges from T^–B^–SCID to isolated CD4⁺ lymphopenia with the variability partially explained by the residual activity of RAG1.  We describe a novel presentation of hypomorphic RAG deficiency characterized by early onset autoimmunity in the presence of B cells, highlighting the importance of considering immunodeficiencies in children who present with immune dysregulation, as prompt treatment can be lifesaving.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/expanding-the-spectrum-of-recombination-activating-gene-1-deficiency-to-include-early-onset-autoimmunity/