UCLA

Hematopoietic stem cells (HSCs) are capable of self-renewing to maintain the stem cell pool as well as differentiating into different mature blood cells to replenish the blood system. Genotoxic stress, such as chemotherapy and radiation, could induce DNA damage in the HSCs, increasing the risk of malignant transformation and decrease the normal function of HSCs. Therapies to promote DNA repair in HSCs after exposure to genotoxic stress remains not well developed. This dissertation reports that the epidermal growth factor receptor (EGFR) signaling promotes DNA repair in HSCs through activation of the non-homologous end-joining (NHEJ) pathway and regulates HSCs regeneration. Data in this dissertation demonstrates that epidermal growth factor (EGF) treatment reduces DNA damage in HSCs after radiation and chemotherapy. EGFR signaling preferentially enhances the activity of the NHEJ pathway, as indicated by NHEJ specific molecules such as DNA-dependent protein kinase, catalytic subunit (DNA-PKcs), Artemis, and Ku70. Mechanistically, EGF binds and activates EGFR, which subsequently activates Akt, further leading to the activation of DNA-PKcs. Pharmacological inhibition of Akt and DNA-PKcs confirmed the EGFR/Akt/DNA-PKcs pathway for DNA repair in HSCs in vivo. Systemic administration of EGF accelerated the hematopoietic recovery of irradiated or chemotherapy-treated mice without affecting the relapse of acute myeloid leukemia. Conditional suppression of EGFR in the hematopoietic stem and progenitor cells (HSPCs) impaired DNA repair and functional recovery, underlining the necessity of EGFR signaling in DNA repair in HSCs. Moreover, EGF treatment accelerated the recovery of irradiated human bone marrow HSCs shown by immunophenotyping in vitro and multilineage reconstitution in vivo. EGF treated human HSPCs also presented enhanced DNA repair. Whole-genome sequencing of HSPCs from irradiated EGF-treated mice revealed no significant difference in the coding regions in terms of mutation rate compared to irradiated control mice, despite increased intergenic copy number variant mutations. RNA sequencing of HSPCs from irradiated EGF-treated mice displayed no significant alterations of the transcription of leukemogenesis related genes. This thesis project uncovered the EGFR/Akt/DNA-PKcs pathway for NHEJ DNA repair in HSCs and explored the therapeutic potential of EGF to promote human HSCs regeneration.