Neuropathologic effects of extracranial radiation therapy in a mouse model

Abstract

Cancer survivors are increasingly identified with a syndrome of neurocognitive dysfunction often termed cancer-related cognitive impairment (CRCI). Doxorubicin chemotherapy and radiation therapy, mainstay anticancer treatments, have been implicated in contributing to CRCI. Cancer survivors treated with extracranial radiation therapy (ECRT) alone report cognitive impairment for durations up to 5 years after treatment with clinical signs consisting of memory loss and impaired concentration. The exact mechanisms and roles in development of cognitive impairment for brain injury following ECRT to distant sites are unknown. Prior work demonstrated the occurrence of gliosis and astrocytosis following ECRT. The objective of this proposal was to characterize the signaling pathways by which ECRT causes activation of microglia and astrocytes and to evaluate the role of this neuroinflammation in cognitive impairment. We began by establishing a mice model of CRCI. The mice strain selected was SKH1 mice because they are hairless and immunocompetent mice. To establish the model of CRCI, we compared mice treated with systemic doxorubicin, radiation to the hindlimb, concurrent doxorubicin and radiation, or anesthesia alone (control). Mice treated with doxorubicin, hindlimb radiation or both showed comparable hippocampus dependent memory deficits 14 days post-treatment. These mice also had neuroinflammation characterized by increases in activated microglia and astrocytosis and differential gene expression within multiple distinct brain regions compared to control mice 16 days post-treatment. To evaluate signaling pathways possibly inducing memory deficits and neuroinflammation in mice post hindlimb irradiation, we performed a longitudinal timepoint study evaluating two radiation doses. Mice were prescribed 20Gy or 30Gy radiation to the hindlimb while control mice were treated with anesthesia only. The mice were euthanized 6 hours, 24 hours, 5 days, 12 days or 25 days post radiation treatment to allow evaluation of the primary irradiated tissue, spleen, bone marrow, thoracic spinal cord and brain. ECRT caused a dose dependent and time dependent decrease in immune cell constituents in the spleen, and pathological changes in the bone marrow including hypocellularity, vascular congestion and reticular fibrosis. There were both macroscopic and microscopic pathological changes in the irradiated skin from the 30Gy treated mice that included epidermis hyperplasia, intracellular edema, ulceration and dermatitis. Mice treated with 30Gy radiation had significant changes in leukocyte population within their spleen. In the thoracic spinal cord, there was a significant dose and time dependent increase in activated astrocytes post treatment. There was a significant radiation dose dependent change within the brains of mice post treatment that included different glial activation profiles and differential expressed genes associated with neurotransmission and glial cell activation. Collectively, the results of these studies highlight the role of ECRT in CRCI.