HEMATOPOIETIC STEM CELLS (HSC) COMPRISE LESS THAN 0.1% OF THE BONE MARROW (BM) OF ADULTS YET THEY PRODUCE ALL BLOOD CELLS THAT ARERESPONSIBLE FOR CONSTANT MAINTENANCE AND IMMUNE PROTECTION OF THE BODY. IN ADDITION TO THE REQUIREMENT FOR HIGH CELL OUTPUT THECONCENTRATION OF THE DIFFERENT BLOOD CELL LINEAGES PRODUCED ARE PRECISELY CONTROLLED TO ALLOW NOT ONLY PRESERVATION OF NORMALHOMEOSTASIS BUT ALSO THE PROMPT RESPONSE TO SPECIFIC STIMULI AND DEMANDS. PREVIOUS STUDIES HAVE SUGGESTED THAT EXPOSURE TO GALACTICCOSMIC RAY (GCR) AND SOLAR PARTICLE EVENT (SPE) RADIATION THAT OCCURS DURING SPACE FLIGHT MAY INCREASE THE LIKELIHOOD OFCARCINOGENESIS WITH LEUKEMIAS BEING ONE OF THE MOST FREQUENT RADIOGENIC CANCERS AND THE ONES WITH THE SMALLEST LATENCY PERIOD. WEHYPOTHESIZE THAT GCR AND SPE RADIATION CAN HAVE A DELETERIOUS IMPACT NOT ONLY AT THE LEVEL OF THE MATURE IMMUNE CELLS AND THEIRCOMMITTED PROGENITORS AS IS CURRENTLY THOUGHT BUT AT THE EARLIER DEVELOPMENTAL STAGE OF THE HSC. GENETIC LESIONS INDUCED BY THISEXPOSURE CAN LEAD DIRECTLY TO LEUKEMIC TRANSFORMATION AND MAY ALSO CAUSE THESE CELLS TO GENERATE A FUNCTIONALLY COMPROMISED IMMUNESYSTEM LACKING THE ABILITY TO EFFECTIVELY FIGHT CHALLENGES SUCH AS PRE-LEUKEMIC TRANSFORMATIONS. AS SUCH ASTRONAUTS WOULD BE DOUBLY ATRISK DUE TO BOTH THE POTENTIAL FOR INDUCTION OF LEUKEMIA BY GCR/SPE RADIATION AND THE REDUCED ABILITY OF THE ASTRONAUTS IMMUNESYSTEMS TO IDENTIFY AND ELIMINATE MALIGNANT CELLS IN THE BODY. SINCE IT HAS ALSO BEEN SHOWN THAT BOTH GENDER AND GENETIC FACTORS LIKELYIMPACT UPON SUSCEPTIBILITY TO LEUKEMOGENESIS IT IS ESSENTIAL TO PRECISELY DEFINE THESE RISKS TO THE HUMAN HEMATOPOIETIC SYSTEM AS A RESULTOF EXPOSURE TO GCR/SPE AND TO ACCURATELY MODEL THE IMPACT OF GENDER AND GENETIC/EPIGENETIC BACKGROUND. UNFORTUNATELY EXISTINGANIMAL MODEL SYSTEMS HAVE NOT ALLOWED THE PRECISE ASSESSMENT OF THE RISKS ASSOCIATED WITH ACUTE OR CHRONIC EXPOSURE OF HUMANHEMATOPOIETIC CELLS TO GCR OR SPE NOR THE ELUCIDATION OF THE MECHANISM INVOLVED.IN THE PRESENT PROPOSAL WE HAVE FORMED A HIGHLY SYNERGISTIC TEAM TO DIRECTLY ADDRESS THIS ISSUE. THIS TEAM COMBINES THECOMPLEMENTARY EXPERTISE OF 6 DIFFERENT LABORATORIES: HUMAN HSC AND CELLS OF THE SUPPORTING BM MICROENVIRONMENT (PORADA ANDALMEIDA-PORADA GROUPS) TISSUE ENGINEERING/SCAFFOLDS (SOKER GROUP) BIOINFORMATICS/GENOMICS (WALKER GROUP) AND RADIATION BIOLOGY(WILSON AND ANDERSON GROUPS). WE WILL USE A VARIETY OF BOTH IN VITRO CELL CULTURE AND IN VIVO TRANSPLANTATION SYSTEMS TO RIGOROUSLY TESTTHE NOVEL HYPOTHESIS THAT GCR AND SPE RADIATION CAN DIRECTLY INDUCE DNA DAMAGE AND MUTATIONS WITHIN PRIMITIVE HSC AND CAN ALSOCONTRIBUTE TO DEFECTIVE HEMATOPOIESIS/IMMUNITY THROUGH SO-CALLED BYSTANDER EFFECTS BY DAMAGING THE BM MICROENVIRONMENTAL CELLSTHAT REGULATE HEMATOPOIESIS.WE ASSERT THAT A MODEL SYSTEM WITH SPECIFIC GENDER-RELATED FACTORS AND GENETIC BACKGROUND WILL ENABLE US TO TEST WHETHER THESE DIRECTAND INDIRECT INSULTS CAN CAUSE THE NORMAL HEMATOPOIETIC SYSTEM TO BE POPULATED BY UNSTABLE CLONES THAT ARE FUNCTIONALLY DEFICIENT ANDMAY BE MORE PRONE TO LEUKEMIA. ACCORDINGLY THE OVERALL GOAL OF THIS PROPOSAL IS TO UTILIZE UNIQUE MODEL SYSTEMS DEVELOPED IN OURLABORATORY TO PRECISELY AND REPRODUCIBLY DEFINE THE EFFECTS OF GCR AND SPE RADIATION ON THE HUMAN HEMATOPOIETIC AND IMMUNESYSTEMS AT BOTH THE MOLECULAR AND FUNCTIONAL LEVELS THEREBY DEFINING THE POSSIBLE RISK OF LEUKEMOGENESIS AS A RESULT OF EXPOSURE.