Comprehensive in Vitro Characterization of CXCR4WHIM variants to Decipher Genotype-Phenotype Correlations in WHIM Syndrome

▪ Background: WHIM (Wart, Hypogammaglobulinemia, Infections, Myelokathexis) syndrome is a rare, autosomal-dominant primary immunodeficiency with neutropenia and lymphopenia. The clinical presentation may include recurrent infections, and increased susceptibility to human papillomavirus. In >80% of cases, WHIM syndrome is caused by heterozygous gain-of-function (GOF) mutations in C-X-C chemokine receptor 4 (CXCR4), with >12 variants reported in WHIM syndrome to date (nonsense [NS], frameshift [FS], and missense[MS]) spanning 27 C-terminal amino acids . These mutations cause hyperactivation of downstream signaling and retention of WBC in the bone marrow (McDermott D, et al. Immunol Rev. 2019;287(1):91-102; Beaussant S, et al. Orphanet J Rare Dis. 2012;7(71):1-14). To date, a comprehensive study characterizing the functional abnormalities caused by pathogenic CXCR4 mutations and correlating these measures with clinical presentation in patients has not been conducted. Here, we aimed to establish genotype-phenotype correlations for all known pathogenic variants using in vitro functional assays. These assays characterize CXCR4 receptor trafficking and downstream signaling, which will enable the long-term goal of assessing pathogenicity of novel CXCR4 variants of uncertain significance (VUS). We further aimed to assess the in vitro response of each variant to mavorixafor, an investigational CXCR4 antagonist. Methods: We used the CXCR4-negative K562 cell line as a model system to express all 14 known CXCR4 variants identified in patients diagnosed with WHIM syndrome (previous reports, ClinVar, and genetic screening initiatives [Invitae PATH4WARD]). The effects of the mutations on CXCR4 receptor trafficking (internalization and degradation), downstream signaling (Ca 2+ mobilization, cAMP inhibition, ERK and AKT activation), and chemotaxis were studied in parallel in a series of assays in cells stimulated with the natural ligand CXCL12. All in vitro functional parameters characterized were investigated for potential correlations with the clinical phenotypes reported for each variant, including disease manifestations and biomarkers. Results: The most conserved feature of the 14 CXCR4 variants was an impaired receptor internalization in response to CXCL12, evidenced by higher percentage of CXCR4 receptors remaining on the cell surface compared to untreated control, with truncated variants showing maximum impairment and the MS variant E343K being least affected. The de