Tofacitinib as a Successful Treatment for Hyper-IgE Syndrome and Autoimmunity Associated with Constitutive Activation of JAK3.

Leptin controls body weight by activating the long form of the leptin receptor (LEPRb). Janus kinase 2 (JAK2) is associated with LEPRb and autophosphorylates in response to leptin. JAK2 also phosphorylates LEPRb, STAT3, and multiple other downstream molecules. Surprisingly, here we show that JAK2 is not required for leptin stimulation of STAT3 phosphorylation. Leptin time- and dose-dependently stimulated tyrosine phosphorylation of STAT3 in both human and mouse JAK2-null cells. Leptin also increased the viability of JAK2-null cells. Overexpression of c-Src or Fyn, two Src family members, promoted STAT3 phosphorylation, whereas inhibition of the endogenous Src family members by either pharmacological inhibitors or dominant negative Src(K298M) decreased the ability of leptin to stimulate the phosphorylation of STAT3 and ERK1/2. Leptin also stimulated tyrosine phosphorylation of kinase-inactive JAK2(K882E) in JAK2-null cells. Overexpression of JAK2(K882E) enhanced the ability of leptin to stimulate STAT3 phosphorylation in JAK2-null cells. Tyr1138 in LEPRb was required for leptin-stimulated phosphorylation of STAT3 but not JAK2(K882E). These data suggest that leptin stimulates non-JAK2 tyrosine kinase(s), including the Src family members, which phosphorylate JAK2, STAT3, and other molecules downstream of LEPRb. JAK2 mediates leptin signaling by both phosphorylating its substrates and forming a signaling complex as a scaffolding/adaptor protein. The non-JAK2 kinase(s) and JAK2 may act coordinately and synergistically to mediate leptin response.

Human Tyrosine Kinase 2 Deficiency Reveals Its Requisite Roles in Multiple Cytokine Signals Involved in Innate and Acquired Immunity

JAK2 (Janus kinase 2) is essential for cytokine receptor signaling, and several lines of evidence support a causal role of an activating JAK2 mutation in myeloproliferative disorders. JAK2 activity is autoinhibited by its pseudokinase domain in the basal state, and the inhibition is released by cytokine stimulation; how engagement of the cognate receptor triggers this release is unknown. From a functional screen for gain-of-function JAK2 mutations, we discovered 13 missense mutations, nine in the pseudokinase domain and four in the Src homology 2 (SH2)-pseudokinase domain linker. These mutations identified determinants for autoinhibition and inducible activation in JAK2. Two of the mutants, K539I and N622I, resulted in erythrocytosis in mice. Scanning mutagenesis of the SH2-pseudokinase domain linker indicated that its N-terminal part was essential for interaction of JAK2 with the Epo receptor, whereas certain mutations in the C-terminal region conferred constitutive activation. We further showed that substitutions for Glu(543)-Asp(544) in this linker or Leu(611), Arg(683), or Phe(694) in the hinge proximal region of the pseudokinase domain resulted in activated JAK2 mutants that could not be further stimulated by Epo. These results suggest that the SH2-pseudokinase domain linker acts as a switch that relays cytokine engagement to JAK2 activation by flexing the pseudokinase domain hinge.

Compound Heterozygosity of Two Novel JAK2 Mutations in Hereditary Essential Thrombocythemia Implicates Important Monomer-Monomer Interactions in Thrombopoiesis Signaling

NK Cell Deficiency in Job's Syndrome Patients

JAK2 plays important roles in the regulation of a variety of cellular processes including cell migration, proliferation, and protection from apoptosis. Recently the L611S point mutation in JAK2 has been identified in a child with acute lymphoblastic leukemia. Here we analyzed the mechanism by which JAK2 exhibits its oncogenicity. In BaF3 murine hematopoietic cells, L611S mutant increased the expression of antiapoptotic proteins including X chromosome-linked inhibitor of apoptosis protein, inhibitor of apoptosis protein, and Bcl-XL. We also showed that JAK2 L611S mutant protects BaF3 cells from cytokine withdrawal-induced apoptotic cell death and leads to cytokine-independent cell growth. Furthermore BaF3 cells expressing JAK2 L611S mutant gained the ability to induce tumorigenesis in nude mice. The L611S mutant also exhibited malignancy, including prompt invasion and spreading into various organs, leading to rapid lethality of the mice. Finally we showed that a specific JAK2 inhibitor, AG490, potently inhibited cytokine-independent cell growth induced by JAK2 L611S mutant via the induction of apoptotic cell death. In addition, treatment with AG490 significantly inhibited the JAK2 L611S mutant-induced tumorigenesis in nude mice. Thus, our results both in vitro and in vivo strongly suggest that L611S mutant of JAK2 harbors potent oncogenic activity, and this probably requires the antiapoptotic signaling pathway.

In cis “benign” SOCS1 variants linked to enhanced interferon signaling and autoimmunity

Oncogenic Cooperation Between IL7R-JAK-STAT Pathway Mutations.

Studies of JAK2 mutations in myeloproliferative disorders

The 4A11 antigen is a unique cytokine-inducible antigen up-regulated on rheumatoid arthritis synovial endothelium compared with normal endothelium. In soluble form, this antigen, Lewisy-6/H-5-2 (Ley/H), or its glucose analog, 2-fucosyllactose (H-2g), mediates angiogenesis. The Ley/H antigen is structurally related to the soluble E-selectin ligand, sialyl Lewisx, and is selectively expressed in skin, lymphoid organs, thymus, and synovium, suggesting that it may be important in leukocyte homing or adhesion. In the present study, we used H-2g as a functional substitute to demonstrate a novel property for Ley/H antigen in inducing leukocyte-endothelial adhesion. H-2g significantly enhanced the expression of human dermal microvascular endothelial cells (HMVECs) intercellular adhesion molecule-1 (ICAM-1), but not vascular cell adhesion molecule-1, E-selectin, and P-selectin. Immunoprecipitation and Western blotting showed glycolipids Ley-6, H-5-2, or the glucose analog H-2g quickly activated human microvascular endothelial cell line-1 (HMEC-1) Janus kinase 2 (JAK2) and that the JAK2 inhibitor, AG-490, completely inhibited HMVEC ICAM-1 expression and HL-60 adhesion to HMEC-1s. Use of a JAK/signal transducer and activator of transcription (STAT) profiling system confirmed that H-2g selectively activated STAT3 but not STAT1 and STAT2. AG-490 inhibited H-2g-induced Erk1/2 and PI3K-Akt activation, suggesting that JAK2 is upstream of the Erk1/2 and PI3K-Akt pathways. Furthermore, the JAK2 inhibitor AG-490, the Erk1/2 inhibitor PD98059, or the phosphatidylinositol 3-kinase inhibitor LY294002 or antisense oligodeoxynucleotides directed against JAK2, Erk1/2, or phosphatidylinositol 3-kinase blocked H-2g-induced HMVEC ICAM-1 expression and HL-60 adhesion to HMEC-1s. Hence, H-2g signals through JAK2 and its downstream signal transducers STAT3, Erk1/2, and phosphatidylinositol 3-kinase result in ICAM-1 expression and cell adhesion. Potential treatment strategies through the inhibition of JAK-dependent pathways to target H-2g signals may provide a useful approach in inflammation-driven diseases like rheumatoid arthritis.

Plasma metalloproteinase levels are dysregulated in signal transducer and activator of transcription 3 mutated hyper-IgE syndrome

The HDAC inhibitor Givinostat modulates the hematopoietic transcription factors NFE2 and C-MYB in JAK2V617F myeloproliferative neoplasm cells

Heterozygous signal transducer and activator of transcription 3 mutations in hyper-IgE syndrome result in altered B-cell maturation

Individuals with severe atopic dermatitis (AD) may present with very high serum IgE levels (> 10 000 IU mL−1). Signal transducer and activator of transcription 3 hyper-IgE syndrome (STAT3-HIES), a rare immunosuppressive disorder, shares pathogenic similarities with AD, involving dysregulation in the STAT3 signalling pathway and clinical, genetic and serological features similar to AD. Despite these parallels, patients with AD with very high IgE levels are not routinely tested for genetic overlap with STAT3-HIES. This might be relevant in therapeutic intervention. We reviewed the literature to identify individuals with possible overlap of both conditions to help identify a novel mixed phenotype (AD/STAT3-HIES). Intervention with Janus kinase (JAK) inhibitors in this subgroup might be beneficial given their shared dysregulation in STAT3 signalling pathways. This review investigates the identification of the AD/STAT3-HIES phenotype and its response to Janus kinase (JAK) inhibitors. A literature review was undertaken on PubMed, Scopus, Ovid and Google Scholar. Search terms included ‘atopic dermatitis’, ‘hyper-IgE syndrome’, ‘HIES’, ‘Job syndrome’, ‘STAT3’, ‘JAK/STAT pathway’ and ‘Janus kinase inhibitors’. Inclusion criteria were studies published in English between January 2005 and December 2023 relevant to the clinical features, treatment, pathogenesis and genetics of AD or STAT3-HIES. Out of 152 studies identified during the search, 29 met the inclusion criteria. The Table outlines the features of AD, STAT3-HIES and areas of overlap (AD/STAT3-HIES). Clinically, AD and STAT3-HIES share characteristics including eczematous lesions and staphylococcal skin infections. STAT3-HIES is distinguished from AD by the presence of immunoparesis and extracutaneous manifestations. Both diseases commonly exhibit elevated IgE levels. In severe AD, studies reported IgE levels ranging from 356 to 2379 IU mL−1, while patients with STAT3-HIES demonstrated levels between 1000 and 46 850 IU mL−1. Genetically, both diseases are associated with the STAT3 and IL6 genes. JAK inhibitors proved effective in treating AD in multiple studies. To date, only one ex vivo study suggested a potential treatment utility of JAK inhibitors in dominant negative STAT3 mutation. We postulate that patients with AD with elevated IgE levels beyond the typical range may benefit from further genotyping (STAT3 and IL6 gene mutations) to identify the AD/STAT3-HIES phenotype. Future clinical trials could assess the potential benefits of targeted intervention with JAK inhibitors in this subgroup and/or in patients with AD with very high serum IgE.ADAD/STAT3-HIESSTAT3-HIESClinicalPruritic eczematous lesionsPruritic eczematous lesionsPruritic eczematous lesionsStaphylococcal skin infectionsStaphylococcal skin infectionsStaphylococcal skin infectionsCold staphylococcal abscessRecurrent pneumoniaSkeletal and dental abnormalitiesConnective tissue abnormalitiesSerologyElevated IgEElevated IgEElevated IgEGenetic associationsSTAT3STAT3STAT3IL6RIL6RIL6RFLGIL6STIL2/IL21ZNF341IL7RTYK2IL15RA/IL2RAImmunologicalElevated interleukin-4Elevated interleukin-4Elevated interleukin-4Elevated interleukin-13Elevated interleukin-13Elevated interleukin-13

X-linked hyper-immunoglobulin M (X-HIGM) syndrome and autosomal recessive hyper-immunoglobulin E syndrome (HIES) are rare inborn errors of immunity characterized by recurrent infections due to immune system impairment. In this study, we identified a novel hemizygous CD40 ligand (CD40L) mutation and compound heterozygous dedicator of cytokinesis-8 (DOCK8) mutations in two Han Chinese families with X-HIGM and HIES, respectively. We aimed to investigate the association between their genotypes and phenotypes. Genomic DNA was extracted from peripheral blood samples obtained from the families. Whole exome sequencing and Sanger sequencing were performed to identify and verify pathogenic variants in the two families. Clinical analyses of the probands were also performed. A novel hemizygous mutation of CD40L in exon 2 (c.257delA) was identified in the first proband, resulting in the substitution of glycine with glutamic acid at codon 86 of the protein. This leads to premature termination of translation at downstream codon 9 (p.E86Gfs*9). Sanger sequencing confirmed that the variant was inherited from the mother. The second proband carried two novel compound heterozygous mutations in DOCK8: one at exon 14 (c.1546C > G) inherited from the father, and the other at intron 41 (c.5355 + 6C > T; splicing) inherited from the mother. This study enhances our understanding of the pathogenetic mutation spectrum of CD40L and DOCK8 genes, facilitating the prenatal diagnosis of X-HIGM and HIES and enabling timely treatment of patients.