The “Editors” Take to RAG: Promise of CRISPR/Cas9/rAAV6-Based Gene Therapy for RAG2 Deficiency

Excerpt

Severe combined immunodeficiencies (SCID) are inborn errors of immunity characterized by an absence of T lymphocytes [1]. The concomitant presence or absence of B and/or natural killer (NK) cells usually points to specific genetic defects. The presence of NK cells coupled to an absence of T and B cells (T⁻B⁻NK⁺ SCID) is the hallmark of T cell receptor (TCR) and B cell receptor (BCR) rearrangement defects, due to mutations affecting the V(D)J recombination machinery. V(D)J recombination allows the generation of highly diverse TCR and BCR repertoires. TCR and BCR rearrangements are initiated by the recombination-activating gene (RAG) complex, a tetramer composed of two RAG1 and RAG2 heterodimers [2]. Upon binding to the recombination signal sequences (RSS) flanking the V, D, and J gene segments, the RAG complex introduces double-strand breaks into the DNA that are processed and joined by the non-homologous end-joining (NHEJ) pathway. These two genes, and their role in V(D)J recombination and T and B cell differentiation, were characterized in a series of landmark studies in mice in the late 1980s and early 1990s [3‐6]. Shortly afterwards, the role of RAG mutations in T and B cell lymphopenia in humans was confirmed [7]. Biallelic RAG mutations abolishing RAG activity cause SCID; biallelic hypomorphic RAG mutations that markedly reduce, but do not completely abolish, recombination activity are the most common cause of Omenn syndrome. Clinically, this syndrome is characterized by early-onset generalized erythroderma, lymphadenopathy, hepatosplenomegaly, and typical laboratory features, including eosinophilia, severe hypogammaglobulinemia with high IgE levels, and autologous, oligoclonal, activated T cells infiltrating multiple organs. Hypomorphic mutations of RAG leading to even milder deficiencies of RAG activity cause a phenotype of combined immunodeficiency (CID), characterized by granulomatous inflammation and various autoimmune manifestations. Since the first report in 1996, more than 400 patients with RAG deficiency have been described [8]. In a recent US report, RAG1/RAG2 deficiency accounted for 20% of SCID cases [9]. …