Although the similarities between humans and mice are typically highlighted, morphologically and genetically, there are many differences. To better understand these two species on a molecular level, we performed a comparison of the expression profiles of 15 tissues by deep RNA sequencing and examined the similarities and differences in the transcriptome for both protein-coding and -noncoding transcripts. Although commonalities are evident in the expression of tissue-specific genes between the two species, the expression for many sets of genes was found to be more similar in different tissues within the same species than between species. These findings were further corroborated by associated epigenetic histone mark analyses. We also find that many noncoding transcripts are expressed at a low level and are not detectable at appreciable levels across individuals. Moreover, the majority lack obvious sequence homologs between species, even when we restrict our attention to those which are most highly reproducible across biological replicates. Overall, our results indicate that there is considerable RNA expression diversity between humans and mice, well beyond what was described previously, likely reflecting the fundamental physiological differences between these two organisms.transcriptome | epigenome | species comparison | noncoding transcripts T he mouse has served as a valuable model organism for human biology and disease. It is widely assumed that biochemical, cellular, and developmental pathways in the mouse are highly conserved with humans and that many processes are clearly preserved at a molecular and genetic level. Moreover, recent detailed studies have examined gene expression in a limited number of tissues in humans and mice. These studies have indicated that gene expression is often conserved and is more similar between the comparable tissues of different organisms rather than within tissues of the same organism. In contrast, the transcript isoform repertoire was found to be markedly different between species (1, 2).
Gene Expression Is More Similar Among Tissues Within a Species Than Between Corresponding Tissues of the Two SpeciesTo examine the similarities between humans and mice in much greater detail, we produced RNA-seq data from 13 human tissues [as part of the Encyclopedia Of DNA Elements (ENCODE)], another 11 human tissues [as part of the Roadmap Epigenomics Mapping Consortium (REMC) (3)], and 13 mouse tissues (for mouse ENCODE). We also included in our analysis other data from mouse ENCODE and the Illumina Human BodyMap 2.0 (HBM) (SI Materials and Methods). Sequencing was performed to a depth of 11,313,824-166,188,101 mappable reads (median of 68,399,538 with and an interquartile range of 31,557,836,199). In total, our analysis used 93 datasets encompassing the most tissue-diverse RNA-seq dataset to date spanning several major projects. Thirteen of the mouse and human orthologous datasets were produced by the same laboratory. For our analysis regarding noncoding transcripts, we incorporated an ad...