P1408: MONOALLELIC AND BIALLELIC GERMLINE MUTATIONS AFFECTING THE TRANSCRIPTION FACTOR HELIOS CAUSE PLEIOTROPIC DEFECTS OF IMMUNITY

Abstract

Background: Helios, encoded by IKZF2, is a member of the Ikaros family of transcription factors with pivotal roles in T-follicular helper, NK- and T-regulatory cell physiology. Somatic IKZF2 mutations are frequently found in lymphoid malignancies. Aims: Although germline mutations in IKZF1 and IKZF3, encoding Ikaros and Aiolos, have recently been identified in patients with phenotypically similar immunodeficiency syndromes, the effect of germline mutations in IKZF2 on human hematopoiesis and immunity remains to be elucidated. Methods: We performed whole exome sequencing in six patients with immune dysregulation and their family members. We then applied Co-immunoprecipitation, electrophoretic mobility shift assay and immunofluorescence microscopy techniques to assess dimerization and DNA binding capabilities of the mutations. Proteomic analysis by proximity-dependent Biotin Identification (BioID) was used to assess affinity of mutant Helios to its interaction partners, while single-cell RNA sequencing was applied to peripheral blood mononuclear cells (PBMCs) of two patients to study the transcriptional effect of the truncating and biallelic IKZF2 mutations. Results: We identified six patients with monoallelic germline IKZF2 mutations (one nonsense (p.R291X)- and 4 distinct missense variants) and one patient with a biallelic germline IKZF2 mutation (p.I325V). While patients with heterozygous variants were affected by immune dysregulation (systemic lupus erythematosus, immune thrombocytopenia or EBV-associated hemophagocytic lymphohistiocytosis), the patient with the homozygous variant suffered from recurrent respiratory tract infections complicated by bronchiectasis. Immunophenotype analysis revealed reduced propensities of NK cells, CD4+ T-cells with an inverted CD4/CD8 ratio, T-follicular helper cells and hypogammaglobulinemia across patients, while the p.I325V patient also had a reduction in relative and absolute B-cells and low relative proportions of Treg, MAIT and iNKT populations. Single-cell RNA sequencing of PBMCs from the patients carrying the monoallelic R291X truncating variant or the biallelic I325V variant revealed upregulation of pro-inflammatory genes associated with T-cell receptor activation and T-cell exhaustion. Our biochemical and proteomic analysis has revealed that only the truncating R291X mutation results in loss of Helios DNA binding and dimerization properties. However, BioID analysis revealed aberrant interaction of 4/6 Helios mutants with core components of the Nucleosome Remodeling histone Deacetylase (NuRD) complex conveying Helios-mediated epigenetic and transcriptional dysregulation. Summary/Conclusion: Thus, germline mutations in IKZF2 cause a novel syndrome associating immunodeficiency and profound immune dysregulation and disrupt interaction with the NuRD complex.