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Most “dark matter” transcripts are associated with known genes

ABSTRACT: A series of reports over the last few years have indicated that a much larger portion of the mammalian genome is transcribed than can be accounted for by currently annotated genes, but the quantity and nature of these additional transcripts remains unclear. Here, we have used data from single- and paired-end RNA-Seq and tiling arrays to assess the quantity and composition of transcripts in PolyA+ RNA from human and mouse tissues. Relative to tiling arrays, RNA-Seq identifies many fewer transcribed regions (‘‘seqfrags’’) outside known exons and ncRNAs. Most nonexonic seqfrags are in introns, raising the possibility that they are fragments of pre-mRNAs. The chromosomal locations of the majority of intergenic seqfrags in RNA-Seq data are near known genes, consistent with alternative cleavage and polyadenylation site usage, promoter- and terminator-associated transcripts, or new alternative exons; indeed, reads that bridge splice sites identified 4,544 new exons, affecting 3,554 genes. Most of the remaining seqfrags correspond to either single reads that display characteristics of random sampling from a low-level background or several thousand small transcripts (median length = 111 bp) present at higher levels, which also tend to display sequence conservation and originate from regions with open chromatin. We conclude that, while there are bona fide new intergenic transcripts, their number and abundance is generally low in comparison to known exons, and the genome is not as pervasively transcribed as previously reported.

OTHER RELATED OMICS DATASETS IN: PRJNA120373

ORGANISM(S): Mus musculus Homo sapiens

PROVIDER: GSE19289 | GEO | 2010/05/18

SECONDARY ACCESSION(S): PRJNA120373

REPOSITORIES: GEO

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Project description:Although a large proportion of human transcription occurs outside the boundaries of known genes, the functional significance of this transcription remains unknown. We have compared the expression patterns of known genes as well as intergenic transcripts within the ENCODE regions between humans and chimpanzees in brain, heart, testis and lymphoblastoid cell lines. We find that intergenic transcripts show patterns of tissue-specific conservation of their expression which are comparable to exonic transcripts of known genes. This suggests that intergenic transcripts are subject to functional constraints that restrict their rate of evolutionary change as well as putative positive selection to an extent comparable to that of classical protein-coding genes. In brain and testis, we find that part of this intergenic transcription is caused by wide-spread use of alternative promoters. Further, we find that about half of the expression differences between humans and chimpanzees are due to intergenic transcripts. In order to transfer genetic information encoded in the genomic sequence of an organism into functional features, the genomic sequence must be transcribed. According to the current genome annotation, the human genome produces transcripts from 20,00025,000 protein-coding genes and a smaller number of non-coding genes. Recently, the introduction of tiling arrays that enable the measurement of transcription regardless of previous annotation challenged this view by providing evidence that a large proportion of transcription occurs outside of annotated genes. It has been suggested that these transcripts represent previously unidentified functional RNAs as well as extensions of known genes. However, their lack of conservation between species on the DNA sequence level raises questions about their functionality. In this study we assess the functionality of these novel transcripts by testing the extent to which their expression is conserved between humans and chimpanzees in different tissues. Surprisingly, we find the expression of known and novel transcripts to be conserved to the same relative extent when different tissues are compared. This suggests that both known and novel transcripts are equally affected by adaptive and stabilizing selection in different tissues during human and chimpanzee evolution. Further, in terms of absolute numbers, about half of the expression differences between humans and chimpanzees in any given tissue are due to intergenic transcripts.

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Most “dark matter” transcripts are associated with known genes

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