Protein tyrosine phosphatase non-receptor type 2 (PTPN2) is a key negative regulator of the JAK/STAT pathway and is essential for immune homeostasis. Loss-of-function variants in PTPN2 have been implicated in monogenic autoimmunity.
We identified novel heterozygous PTPN2 variants in three unrelated Argentinean pediatric patients presenting immune dysregulation. A patient with polyautoimmunity and polyinflammation was found to carry compound heterozygous variants c.515T>C (p.Ile172Thr) and c.865T>C (p.Trp289Arg), inherited from healthy heterozygous parents. In a second case, a child with Evans syndrome harbored the c.515T>C (p.Ile172Thr) variant. The third case involved a girl evaluated for recurrent bronchitis and diagnosed with an inborn error of immunity (IEI), carrying the c.865T>C (p.Trp289Arg) variant. Notably, her mother had previously been diagnosed with common variable immunodeficiency and died of gastric cancer.
To assess the functional consequences of these variants, we cloned wild-type (WT) PTPN2 cDNA into a mammalian expression plasmid with an HA-tag and introduced the two mutations via site-directed mutagenesis. HEK293T cells were transfected with WT/mutant PTPN2 constructs. Protein expression was evaluated by Western blot. Functional assays included co-transfection of PTPN2-HA constructs, WT-STAT3, and a STAT3-responsive GFP reporter, followed by IL-6 stimulation. Additionally, STAT1 activity was assessed using a STAT1-luciferase reporter system after IFN-γ stimulation.
Mutant PTPN2 proteins were expressed at levels comparable to WT, suggesting preserved protein stability. However, cells expressing both PTPN2 variants (compound heterozygous state) showed significantly enhanced STAT3 activity upon IL-6 stimulation. STAT1 pathway analysis revealed altered luciferase activity in response to IFN-γ in cells transfected with each or both mutant constructs, suggesting broader dysregulation of JAK/STAT signaling.
These novel PTPN2 variants may contribute to immune dysregulation by enhancing STAT signaling. Functional evidence supports a pathogenic role consistent with the observed clinical phenotypes. Further studies are needed to define their contribution to autoimmunity and IEIs, and to explore therapeutic implications.
