The regulated retrotransposon transcriptome of mammalian cells

Enhancing non-coding RNA information content with ADAR editing

The depth and complexity of the non-coding transcriptome in nervous system tissues provides a rich substrate for adenosine de-amination acting on RNA (ADAR). Non-coding RNAs (ncRNAs) serve diverse regulatory and computational functions, coupling signal flow from the environment to evolutionarily coded analog and digital information elements within the transcriptome. We present a perspective of ADARs interaction with the non-coding transcriptome as a computational matrix, enhancing the information processing power of the cell, adding flexibility, rapid response, and fine tuning to critical pathways. Dramatic increases in ADAR activity during stress response and inflammation result in powerful information processing events that change the functional state of the cell. This review examines the pathways and mechanisms of ADAR interaction with the non-coding transcriptome, and their functional consequences for information processing in nervous system tissues.

Retrotransposons and non-protein coding RNAs

Retrotransposons constitute a significant fraction of mammalian genomes. Considering the finding of widespread transcriptional activity across entire genomes, it is not surprising that retrotransposons contribute to the collective RNA pool. However, the transcriptional output from retrotransposons does not merely represent spurious transcription. We review examples of functional RNAs transcribed from retrotransposons, and address the collection of non-protein coding RNAs derived from transposable element sequences, including numerous human microRNAs and the neuronal BC RNAs. Finally, we review the emerging understanding of how retrotransposons themselves are regulated by small RNAs.

The plasticity of the mammalian transcriptome

The dogmatic view of RNA as a mere necessity in the transfer of information between DNA and proteins has during recent years come into question. Novel approaches and new technology has revealed an unprecedented level of inherent complexity in the mammalian transcriptome. Here, the majority of nucleotides are expressed, in sharp contrast to the approximately 1.2% of the human genome harboring protein coding information. Also, >50% of genomic loci contain antisense and interleaved transcription, a conservative estimate since non-coding RNA is highly regulated between tissues and developmental stages, which has only been investigated to a limited extent. Subsequent focus on RNA with no coding potential has revealed numerous species with novel functions, and deep sequencing studies imply that many remain to be discovered. This review gives an overview of the plasticity and dynamics of the mammalian transcriptome and the prevailing interpretation of its effect on the complexity of species.

Exon-trapping mediated by the human retrotransposon SVA

Although most human retrotransposons are inactive, both inactive and active retrotransposons drive genome evolution and may influence transcription through various mechanisms. In humans, three retrotransposon families are still active, but one of these, SVA, remains mysterious. Here we report the identification of a new subfamily of SVA, which apparently formed after an alternative splicing event where the first exon of the MAST2 gene spliced into an intronic SVA and subsequently retrotransposed. Additional examples of SVA retrotransposing upstream exons due to splicing into SVA were also identified in other primate genomes. After molecular and computational experiments, we found a number of functional 3' splice sites within many different transcribed SVAs across the human and chimpanzee genomes. Using a minigene splicing construct containing an SVA, we observed splicing in cell culture, along with SVA exonization events that introduced premature termination codons (PTCs). These data imply that an SVA residing within an intron in the same orientation as the gene may alter normal gene transcription either by gene-trapping or by introducing PTCs through exonization, possibly creating differences within and across species.

Probabilistic resolution of multi-mapping reads in massively parallel sequencing data using MuMRescueLite

Multi-mapping sequence tags are a significant impediment to short-read sequencing platforms. These tags are routinely omitted from further analysis, leading to experimental bias and reduced coverage. Here, we present MuMRescueLite, a low-resource requirement version of the MuMRescue software that has been used by several next generation sequencing projects to probabilistically reincorporate multi-mapping tags into mapped short read data.

AVAILABILITY AND IMPLEMENTATION

MuMRescueLite is written in Python; executables and documentation are available from http://genome.gsc.riken.jp/osc/english/software/.

Endogenous retroviral LTRs as promoters for human genes: a critical assessment

Gene regulatory changes are thought to be major factors driving species evolution, with creation of new regulatory regions likely being instrumental in contributing to diversity among vertebrates. There is growing appreciation for the role of transposable elements (TEs) in gene regulation and, indeed, laboratory investigations have confirmed many specific examples of mammalian genes regulated by promoters donated by endogenous retroviruses (ERVs) or other TEs. Bioinformatics studies have revealed hundreds of additional instances where this is likely to be the case. Since the long terminal repeats (LTRs) of retroviruses naturally contain abundant transcriptional regulatory signals, roles for ERV LTRs in regulating mammalian genes are eminently plausible. Moreover, it seems reasonable that exaptation of an LTR regulatory module provides opportunities for evolution of new gene regulatory patterns. In this Review we summarize known examples of LTRs that function as human gene alternative promoters, as well as the evidence that LTR exaptation has resulted in a pattern of novel gene expression significantly different from the pattern before LTR insertion or from that of gene orthologs lacking the LTR. Available data suggest that, while new expression patterns can arise as a result of LTR usage, this situation is relatively rare and is largely restricted to the placenta. In many cases, the LTR appears to be a minor, alternative promoter with an expression pattern similar to that of the native promoter(s) and hence likely exerts a subtle overall effect on gene expression. We discuss these findings and offer evolutionary models to explain these trends.

From transcription start site to cell biology

The regulation of transcription is a complex process. Recent novel insights concerning the in vivo regulation and expression of protein-coding and non-coding RNAs have added previously unimagined levels of complexity to these processes.