Omenn Syndrome in Two Infants with Different Hypomorphic Variants in Janus Kinase 3

Abstract

Biallelic null or hypomorphic variants in JAK3 cause SCID and less frequently Omenn syndrome. We investigated homozygous hypomorphic JAK3 mutations in two patients, and expression and function of a novel JAK3^R431P variant in Omenn syndrome. Immunophenotyping of PBMC from the patient with the novel JAK3^R431P variant was undertaken, by flow cytometry and Phosflow after stimulation with IL-2, IL-7, and IL-15. JAK3 expression was investigated by Western blotting. We report two patients with homozygous hypomorphic JAK3 variants and clinical features of Omenn syndrome. One patient had a previously described JAK3^R775H variant, and the second had a novel JAK3^R431P variant. One patient with a novel JAK3^R431P variant had normal expression of JAK3 in immortalised EBV-LCL cells but reduced phosphorylation of STAT5 after stimulation with IL-2, IL-7, and IL-15 consistent with impaired kinase activity. These results suggest the JAK3^R431P variant to be hypomorphic. Both patients are alive and well after allogeneic haematopoietic stem cell transplantation. They have full donor chimerism, restitution of thymopoiesis and development of appropriate antibody responses following vaccination. We expand the phenotype of hypomorphic JAK3 deficiency and demonstrate the importance of functional testing of novel variants in disease-causing genes.

Keywords: Haematopoietic stem cell transplant; Janus kinase 3; Omenn syndrome; Severe combined immunodeficiency.

Fig. 1

Immunophenotyping of healthy control and patient P2’s peripheral blood cells by flow cytometry. A Contour flow plots showing relative abundance of B-cells and reduced T-cells, absence of CD8 + cells and naïve CD4 + cells, hyper activation of CD4 + cells, and skewing of NK-cells towards CD56-CD16 + phenotype in patient P2 with the JAK3^R431P variant. B Quantification of flow cytometry data in controls and patient P2. Data presented as mean ± SD

Fig. 2

Impaired pSTAT5 response to interleukin IL-2, IL-7, and IL-15 stimulation in patient P2’s PBMC subsets. A Unaffected expression of JAK3^R431P protein in EBV-LCL cell lines by immunoblotting. B Histograms of pSTAT5 in patient P2’s and healthy controls’ (C1, C2) peripheral CD4 cells in response to IL-2, IL-7, and IL-15 stimulation. Dashed area: IL-stimulation, empty line: unstimulated. C Impaired pSTAT5 response to interleukin IL-2, IL-7, and IL-15 stimulation in patient P2’s cell subsets. C: Controls C1 and C2, P: patient P2, pSTAT5: phosphorylated STAT5. Data presented as mean ± SD