A novel IL2RG mutation presenting with atypical T − B + NK + phenotype: Rapid elucidation of NK cell origin

To the Editor: IL-2R g chain is the signaling component of IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21, cytokine receptors that are essential in the ontogeny and function of NK and T cells. Mutations in the gene encoding for the g chain of the interleukin-2 receptor (IL-2RG), mapped to X chromosome, accounts for 50% of diagnosed SCID. X-Linked SCID is characterized by early-onset of severe infections, facilitated by the absence of T and NK lymphocytes (T BþNK phenotype) [1,2]. However, different mutations in the IL-2RG gene have been reported associated with a phenotype variant T BþNKþ [3]. Here we describe a novel mutation of the IL-2RG gene consisting of a triple insertion (ACC) at exon 5, in which NK cells were found in peripheral blood, representing a burden for initial patient diagnosis. The patient was an 8-month-old male born full term to a non-consanguineous couple. Patient’s grand-grand mother was of gypsy origin. There was no family history consistent with immunodeficiency. Since 5months of age the patient suffered several episodes of diarrhea and failure to thrive. At 8 months of age, he was admitted to the hospital due to a severe diarrhea and dehydration. Staphylococcus auricularis, Staphylococcus epidermidis, and Candida parasilopsis were identified in blood cultures. Candida albicans and Pseudomona aeruginosa were detected in stool’s cultures. Immunological findings showed hypogammaglobulinemia IgG: 41mg/dl (reference range 165–590mg/dl); IgA: 5mg/dl (15–50mg/dl); IgM: 25mg/dl (30–135mg/dl). Lymphocyte counting showed CD3þ: 8 cells/ml (reference range 2,000–5,900 cells/ml); CD19þ: 1,664 cells/ml (610–2600 cells/ml); CD16þCD56þ: 353 cells/ml (160–950 cells/ml). Despite the observed T BþNKþ phenotype, IL-2Rgc chain deficiency was investigated, as it is the most frequent cause of SCID. DNA sequence analysis of the IL-2RG gene revealed a triple insertion (ACC) at exon 5, resulting in the inclusion of a histidine in the gc chain sequence. This pathological variant p. His242_Pro243 insHis has not been previously described. Results from the female-carrier studies demonstrated that this mutation appeared de novo in the patient’s mother, as shewas the only carrier female within the family. All other family females including five sisters, the grand mother as well as the grand-grand mother of the patient, were found free of the mutation. In spite of the presence of NK cells in the peripheral blood of our patient, no signs of maternal lymphoid engraftment were evident. Maternal T cell engraftment has been reported in up to 50% of diagnosed SCID. However, as it is occurs in our patient, some of them never develop graft versus host disease [4]. Nevertheless, the assessment of the origin of the circulating NK cells was approached bymeans of a flow cytometric strategy.We reasoned that if NK cells detected were from maternal origin, they should constitutively express at the cell surface, as NK cells from healthy donors, IL2Rgc chain (CD132) and IL-21R chain (CD360). Conversely, both markers should be absent from patient’s resting B cells. Multicolor flow cytometry assay confirmed co-expression of CD132 and CD360 on the NK cell population, showing the maternal origin of NK cells. The patient received hematopoietic stem cell transplantation from an HLA-identical, non-related donor, which outcome was the immunological reconstitution and successful clinical recovery. In conclusion, we presented here a novel IL-2Rgc mutation, appearing de novo in the patient’s mother and presenting NK cells in the peripheral blood. An original and informative approach allowed us to rapidly elucidate the maternal origin of NK avoiding other laborious molecular analysis and facilitating the achievement of an accurate diagnose and appropriate therapy for this patient.