Differentiating Total- or Partial-Body Irradiation in Baboons Using mRNA Expression Patterns: A Proof of Concept.

To better predict clinical outcome after radiation exposure, it is very important to know the absorbed dose and body areas exposed. Previously we found that 22 miRNAs appeared to predict total- and partial-body irradiation (TBI and PBI, respectively) patterns and were suggestive of the percentage of the body exposed in a baboon model. Motivated by these results, we performed a similar analysis on the transcriptional level (mRNAs) using whole genome microarrays. From 17 irradiated baboons, blood samples were taken before, and at 1, 2, 7, 28 and 75-106 days postirradiation to an equivalent TBI dose of 2.5 or 5 Gy applied either to the total body or to different parts of the body such as the upper body (UBE) or left hemibody (LHB). We compared quantile normalized log2-transformed gene expression values with three exposure pattern comparisons, namely TBI vs. PBI, TBI vs. LHB and UBE vs. LHB using Kruskal-Wallis and logistic regression analysis for receiver-operator characteristic (ROC) calculation. We found several hundred significantly (P < 0.05) and ≥2-fold deregulated mRNAs per exposure pattern comparison with a peak of 163-860 mRNAs at day 28. Lower numbers on day 2 (60 mRNAs) and day 7 (91-162 mRNAs) were observed, with the lowest number of deregulated mRNAs at day 75-106 (22-58 mRNAs). The 14 most promising mRNAs (e.g., LTF, DEFA3, OLFM4) appeared 10.1-46.2-fold upregulated and the exposure groups were completely or almost completely discriminated (ROC between 0.8-1.0). Several of the mRNA gene expression changes were significantly associated with the percentage of the body exposed. The numbers of overlapping genes used for diagnosis on consecutive days postirradiation were mostly 0 or less than 10. Bioinformatic analysis confirmed that at each time point different biological processes predominated. Our results suggest mRNA changes over time may be used to retrospectively determine radiation exposure patterns as partial or total body. mRNA gene expression changes likely could be applied over a longer time frame (2-75 days postirradiation) than miRNA, but due to the transient gene expression changes a different set of candidate mRNAs appears to be required at each day after irradiation.