Overview

Somatic Mutations in Muscle and Bone Exposed to Simulated Space Radiation and Microgravity

Principal Investigator:
Henry J. Donahue, Ph.D.

Organization:
Virginia Commonwealth University

The accumulation of somatic (non-heritable) DNA mutations over time is a hallmark and potential mechanism of aging and other diseases. Our long-term goal is to understand how simulated space radiation, including that from protons and high atomic number and energy (HZE) ions, and microgravity, all of which are experienced during long duration space flight, interact to alter the burden of somatic mutations in bone and muscle, whether these changes contribute to disease phenotypes in these tissues, and to develop therapeutic countermeasures. Our central hypothesis is that space radiation induces somatic mutations in bone and muscle and alters known and novel signal transduction pathways that underlie development of cancers, immune system dysfunction and metabolic disturbances resulting from mitochondrial impairment. Furthermore, exposure to simulated microgravity increases the mutagenic potential of exposure to simulated space radiation potentiating the occurrence of mutations. The rationale for this proposed research is that there are no systematic and comprehensive studies of somatic mutations in muscle or bone exposed to space-like radiation and simulated microgravity simultaneously, and therefore the landscape and importance of such changes remains unknown. This gap in knowledge represents a significant barrier to ongoing and future studies of phenotypic variation and susceptibility to musculoskeletal disorders as well as the identification of therapeutic targets and development of innovative countermeasures to preserve astronaut health during extended missions and upon return to Earth.

Technical Summary

SPECIFIC AIMS

The accumulation of somatic (non-heritable) DNA mutations over time is a hallmark and potential mechanism of aging and other degenerative diseases. Our long-term goal is to uncover the mechanisms by which space radiation from solar particle event (SPE) radiation and galactic cosmic radiation (GCR) interact with simulated microgravity to alter the burden of somatic mutations in bone and muscle, whether these changes contribute to disease phenotypes in these tissues, and to develop therapeutic countermeasures.

Specific Aim. Characterize somatic genetic variation in nuclear and mitochondrial DNA, and resulting changes in RNA, from skeletal muscle and bone in male mice undergoing simulated microgravity (hind limb suspension, HLS) in the presence and absence of radiation from high charge and energy (HZE) particles, specifically those from ⁵⁶Fe²⁵ and protons.

The experiments and principal endpoints implicit in this aim and outlined below will specifically address the following biologically important hypotheses:

Mutational signatures (patterns and frequencies of base substitutions, insertions, deletions microsatellite mutations, structural variants, etc.) in bone and muscle will be altered in mice exposed to HZE and proton radiation during a period of HLS, compared to either radiation or HLS alone.
HZE and proton radiation will induce somatic mutations in both nuclear and mitochondrial DNA, resulting in altered gene expression and/or function, in both muscle and bone.
Accumulation of somatic mutations induced by the interaction of radiation and HLS in mitochondrial DNA will be greater than in nuclear DNA because of the relatively higher rate of mitochondrial mutations and turnover, thereby impairing mitochondrial function.
Somatic variation produced by radiation and HLS is maladaptive, enhances the catabolic state, and accelerates the development of functional decline in both bone (osteopenia) and muscle (sarcopenia).
Future development of therapeutic countermeasures will be accelerated as a result of the comprehensive dataset generated. Additionally, novel pathways will be identified, which provide insight into mechanisms underlying bone and muscle loss during the mission period and predict individual susceptibility to future bone and muscle disorders.

Our central hypothesis is that space radiation induces somatic mutations in bone and muscle and alters known and novel signal transduction pathways that underlie development of cancers, immune system dysfunction and metabolic disturbances resulting from mitochondrial impairment. Furthermore, exposure to simulated microgravity increases the mutagenic potential of exposure to simulated space radiation potentiating the occurrence of mutations. The rationale for this proposed research is that there are no systematic and comprehensive studies of somatic mutations in muscle or bone exposed to space-like radiation and simulated microgravity simultaneously, and therefore the landscape and importance of such changes remains unknown. This gap in knowledge represents a significant barrier to ongoing and future studies of phenotypic variation and susceptibility to musculoskeletal disorders as well as to the identification of therapeutic targets and development of innovative countermeasures to preserve astronaut health during extended missions and upon return to Earth. As we are currently funded by NSBRI to examine the interdependence of muscle and bone loss in response to HLS, the central hypothesis, specific aim and principal endpoints are considered a natural extension of our ongoing multidisciplinary research effort.

Earth Applications

On Earth, bone and muscle loss are frequently experienced by individuals confined to bed for long periods of time – whether as a result of neurological injury, trauma, or neuromuscular disease. These patients, as well as healthy patients subjected to experimental extended bed rest, display a significant loss of muscle and bone. In the case of recoverable disease or when ambulation is still possible, this leaves patients at risk for falls and catastrophic fracture. There is also evidence for muscle and bone loss following radiation therapy for cancer, although this radiation is acknowledged to be a larger dose than encountered in space. The proposed study will identify somatic mutations that occur not only as a result of space travel but may also occur under conditions experienced by patients on Earth. These mutations may predict the development of disease in certain patient populations. Development of somatic mutations following space travel may increase susceptibility to certain diseases upon return to Earth. Quantifying the number and type of somatic mutations that occur in bone and muscle following exposure to HZE+proton radiation and unloading will lead to both innovative countermeasures for space travel as well as a better understanding of the genetic bases of disease on Earth leading to identification of novel therapeutics.