Dysregulation of IL12 Signaling As a Novel Cause of an Autoimmune Lymphoproliferative like Syndrome

Introduction

Autoimmune lymphoproliferative syndrome (ALPS) is characterized by abnormal lymphocyte homeostasis caused by defective apoptosis. Mutations in genes involved in the Fas death receptor pathway (FAS, FASLG or CASP10 genes) are the cause for the pathogenesis of ALPS. However, in 20-30% of all ALPS cases, collectively classified as ALPS-U (undetermined), the genetic defect is still unknown. The objective of this study was to employ whole-exome sequencing to search for novel gene candidates underlying ALPS-U or ALPS-like disease. Resulting candidates should be validated and their impact on the Fas death receptor pathway studied.

Methods

Peripheral blood samples were collected from 26 patients diagnosed with ALPS (based on clinical phenotype and accumulation of DNT cells), relatives and healthy controls. PCR and Sanger sequencing were performed to check for germline and somatically acquired FAS, FASLG and CASP10 mutations. Whole-exome sequencing was carried out for all patients with unknown mutation and their parents. Candidate genes were identified by a KEGG-based protein interaction analysis interface of our in-house developed proprietary MySQL database driven workbench, termed SNuPy (Single Nucleotide Polymorphism Database). STRING 9.1 was used to identify high confidence (≥0.900) interaction partners of the Fas pathway. Single nucleotide variations (SNVs) were verified by PCR and Sanger sequencing. The impact of detected mutations on the candidate genes´ expression and functionality was analyzed by immunoblot. The candidate genes´ impact on Fas death receptor pathway and Fas/FasL expression was examined by flow cytometry, ELISA and qRT-PCR.

Results

Out of 26 analyzed ALPS cases 4 unrelated patients harbored heterozygous germline mutations in the death domain of the Fas receptor (p.Q282K, p.R249G, p.NVQ265-267KQT) or the extracellular domain (p.R191C), respectively. Two siblings with homozygous FASLGtruncating mutation (A69fs*138) and complete loss of FasL surface expression as a consequence were identified. Whole-exome sequencing also identified a homozygous R212* mutation in the IL12/IL23 receptor-component IL12RB1 (Interleukin 12 receptor, beta 1 subunit) in a patient who presented with classical ALPS symptoms (chronic non-malignant, noninfectious lymphadenopathy, splenomegaly, hepatomegaly, elevation of DNT cells, autoimmune cytopenias with polyclonal hypergammaglobulinemia, persistently increased vitamin B12 and IL10 levels). The p.R212* mutation in IL12RB1 leads to premature protein truncation by a stop codon gain, resulting in a complete loss of cell surface expression of IL12RB1 in the patient. IL12 is a factor known to regulate expression of both Fas and FasL. IL12 signaling was abrogated as demonstrated by deficient downstream STAT4 phosphorylation and IFNγ production. Low FasL expression on T-cells and low soluble FasL plasma levels were probably due to lack of IFNγ mediated transcriptional activation of FasL. In contrast to healthy controls, prolonged IL12 stimulation did not trigger upregulation of FasL nor apoptosis in the IL12RB1 deficient and FasL deficient (A69fs*138) patients, indicating that IL12 mediated apoptosis is FasL dependent. Heterozygous carriers of the IL12RB1 or the FASLG mutation showed an intermediate response but were asymptomatic and sub-clinically affected (showing e.g. moderate elevation of DNT cells).

Conclusion

Our data show that IL12 employs Fas signaling to achieve T-cell apoptosis and reveal IL12 signaling deficiency as a new cause of ALPS like disease via its impact on FasL expression.