Oligoclonal Expansion of Revertant T Cells Leads to Omenn Syndrome in a Patient with RAG1-Deficient Severe Combined Immunodeficiency.

Omenn syndrome (OS) is an autosomal recessive primary immunodeficiency characterized by early-onset generalized erythroderma, lymphadenopathy, hepatosplenomegaly, hypogammaglobulinemia, and elevated serum IgE levels. OS patients carry mutations of the recombination activating genes (RAG1 and RAG2) that are essential for V(D)J recombination. OS mutations maintain a residual recombination activity that allows limited T-cell receptor (TCR) gene rearrangements in the thymus, whereas null mutations cause a complete block of T- and B-cell development and lead to severe combined immune deficiency with absence of mature T and B lymphocytes (T-B- SCID). Revertant mosaicism due to back mutations or second-site mutations has been described in an increasing number of primary immunodeficiencies. The occurrence of reversion is considered rare and revertant cells have been shown to carry a single revertant sequence. Here we describe somatic mosaicism due to multiple events of second-site mutations in a patient affected with Omenn syndrome. We found that the patient is homozygous for a single base C deletion after nucleotide 2113 of the RAG1 gene in DNA obtained from his granulocytes. This novel mutation leads to a frameshift at codon 668 and a premature termination, and likely abrogates RAG function. The patient, however, showed typical features of OS including expansion of activated and oligoclonal T cells in peripheral blood. Molecular analysis using various sorted samples revealed that several different second-site mutations, all of which restored the RAG1 reading frame and resulted in missense mutations located in the RAG2 interacting domain, were simultaneously demonstrated in DNA from CD3+ T cells. On the contrary, such sequences were not detectable in DNA obtained from granulocytes, CD14+ monocytes and CD56+ NK cells. We also showed that TCRVβ8+ and TCRVβ1+ T cells, which were predominantly expanded within CD4+ and CD8+ T cells respectively, had different second-site mutations. The presence of multiple second-site mutations in a single patient is an unprecedented finding that supports the possibility that genetic reversions may be more common than previously thought. In addition, these observations indicate that T-B- SCID with reverse mutations that impair but do not abolish the V(D)J recombination process is a novel mechanism for the pathogenesis of OS.