Actin cytoskeletal defects in Primary Immunodeficiencies: characterization of radiation-induced DNA-damage repair and actin cytoskeleton dynamics in patients affected by ARPC1B deficiency

The increasing identification of Primary Immunodeficiencies (PIDs) caused by new inborn errors of immunity in genes encoding for actin regulatory proteins highlights the essential role of actin dynamics in immune function. Indeed alterations of filamentous (F)-actin assembly and disassembly affects the cytoskeleton remodelling impairing numerous cell functions including mitosis and cytokinesis, mobility, phagocytosis, intracellular signalling and intercellular interactions as well as particularly DNA-damage repair processes and chromatin remodeling. ARPC1B belonging to the Arp2/3 complex, is a key molecule that promotes actin polymerization and cytoskeleton dynamics facilitating also multiple DSBs clustering and DNA end-resection mostly during homology repair (HDR). Homozygous LOF variants in ARPC1B gene cause the ARPC1B deficiency, a recently described combined immunodeficiency with immunedysregulation and impaired cell shape and/or function of platelets, lymphocytes and neutrophils due to actin cytoskeleton disruption. The present PhD project started from the characterization of a CID patient diagnosed with ARPC1B deficiency showing a higher cell sensitivity to radiation-induced DNA damage. The principal aims was to confirm and investigate the radiosensitivity in patients’ ARPC1B-deficient cells in order to establish if this is a new hallmark of the disease. The second aim was to deeper explore, through the analysis of cell cycle progression, the ability of patients’ cells to resolve the DNA damage repair and the viability of damaged cells over time. Different approaches were used to assessed the project proposals including immunological, molecular and genetics investigations, radiosensitive tests. The results revealed the radiation-induced DNA damage sensitivity as a common trait of patients affected by ARPC1B deficiency, even if the radiosensitive feature occurred at different levels of severity and the increased amount of damaged ARPC1B patients’ cells arrested in G2/M cell cycle phase. These findings highlight the importance to evaluate radiosensitivity and defective repair mechanisms in the increasing number of PIDs with cytoskeleton defect and/or immune-dysregulation.