Loss-of-phosphorylation of IKZF1 results in gain-of-function associated with immune dysregulation

Background Immune dysregulation often presents as autoimmunity, inflammation and/or lymphoproliferation. Several germline genetic defects have been associated with immune dysregulation including heterozygous gain-of-function (GOF) mutations in IKZF1, an essential transcription factor for hematopoiesis containing zinc finger domains (ZFs). However, in a large part of patients the genetic origin of their immunedysregulation remains undetermined. Objective A family with two members presenting immune dysregulation signs was studied to identify the genetic cause of their disease. Methods Whole exome sequencing, analysis of immunological parameters, functional assays including western blotting, EMSA during cell cycle and T helper differentiation were performed. Results The two patients carried a novel heterozygous mutation in IKZF1 (IKZF1T398M). IKZF1 heterozygous mutations have been previously shown to be responsible of several distinct human immunological diseases by directly impacting the ability of ZFs to bind to DNA or to dimerize. Herein, we showed that the IKZF1T398M which is outside the ZFs caused impaired phosphorylation of IKZF1 resulting in enhanced DNA-binding ability at the S phase of cell cycle, reduction of the G1-S phase transition and decreased proliferation. Confirming these data, similar functional alterations were observed with IKZF1T398A, but not with IKZF1T398D mimicking dephosphorylation and phosphorylation, respectively. In T lymphocytes, expression of IKZF1T398M led to T helper (TH) differentiation skewed towards TH2. Thus, our data indicate that the IKZF1T398M behaves as a GOF variant underlying immune dysregulation. Conclusion Disturbed IKZF1 phosphorylation represents a novel GOF mechanism (gain-of-function by loss-of-phosphorylation; GOF-LOP), associated with immune dysregulation, highlighting the regulatory role of IKZF1 during cell cycle progression through phosphorylation.