Beyond mRNAs and Mirnas: Unraveling the Full-Spectrum of the Normal Human Platelet Transcriptome Through Next-Generation Sequencing

Abstract 3298

The anucleate platelets play a critical role in the formation of thrombi and prevention of bleeding. While the repertoire of platelet transcripts is a reflection of the megakaryocyte at the time of platelet differentiation, post-transcriptional events are known to occur. Furthermore, a strong correlation between the expressed mRNAs and proteome has been identified. Having a complete understanding of the platelet transcriptome is important for generating insights into the genetic basis of platelet disease traits.

To capture the complexity of the platelet transcriptome, we performed RNA sequencing (RNA-seq) in leukocyte-depleted platelets from 10 males, with median age of 24.5 yrs and unremarkable medical history. Their short and long RNA platelet transcriptomes were analyzed on the SOLiD 5500xl sequencing platform. We generated ∼3.5 billion sequence reads ∼40% of which could be mapped uniquely to the human genome.

Our analysis revealed that ∼9,000 distinct protein-coding mRNAs and ∼800 microRNAs (miRNAs) were present in the transcriptome of each of the 10 sequenced individuals. Comparison of the levels of mRNA expression across the 10 individuals showed an exceptional level of consistency with pair-wise Pearson correlation values ≥0.98. The miRNA expression profiles across the 10 individuals showed a similar consistency with pair-wise Pearson correlation values ≥0.98.

Surprisingly, we found that these mRNAs and miRNAs accounted for a little over 1/2 of all of the uniquely mapped sequence reads suggesting the abundant presence of additional non-protein coding RNA (ncRNA) transcripts. Using the annotated entries of the latest release of the ENSEMBL database, we investigated the genetic make-up of these other transcripts.

We found that ∼25% of each individual's uniquely mapped reads corresponded to non-protein coding transcripts from mRNA-coding loci. These reads accounted for more than 10,000 distinct such transcripts. In addition, each of the individuals in our cohort expressed an average of ∼1,500 pseudogenes and ∼200 long intergenic non-coding RNAs (lincRNAs).

The short RNA profiles of the ten individuals revealed an abundance of diverse categories of ncRNAs including the signal recognition particle RNA (srpRNA), small nuclear RNA (snRNA) and small cytoplasmic RNAs (scRNA). These ncRNAs are involved in the processing of pre-mRNAs and their presence and prevalence in the anucleate platetet suggests the existence of a complex network of mRNA processing that persists after the megakaryocyte fragmentation.

We also investigated the RNA-omes of the ten individuals for evidence of transcription of the pyknon category of ncRNAs. Pyknons are of particular interest because each has numerous intergenic and intronic copies whereas nearly all known human protein-coding genes contain one or more pyknons in their mRNA. Recent experimental work has shown that intergenic instances of the pyknons are transcribed in a tissue- and cell-state specific manner. An average of ∼100,000 pyknons are transcribed in each of the 10 sequenced individuals suggesting the possibility of a far-reaching network of interactions that link exonic space to distant non-exonic regions and are active in platelets.

Lastly, we found that a large variety of distinct repeat element categories are expressed in the RNA-omes (both short and long) of these individuals. Among the most abundantly represented categories of repeat elements were DNA transposons, long terminal repeat (LTR) retrotransposons, and non-LTR retrotransposons such as long interspersed elements (LINEs) and short interspersed elements (SINEs).

In summary, our RNA-seq analyses have revealed a spectrum of platelet transcripts that transcends protein-coding genes and miRNAs. Indeed, the transcripts that have their source in genomic features not previously discussed or analyzed in the platelet context represent a very significant portion of all platelet transcripts. This in turn suggests an unanticipated richness, and presumably commensurate complexity, for the platelet transcriptome. While the role of these novel non-protein coding RNAs is currently unknown it is expected that at least some of them may be of functional significance which will in turn permit a better understanding of the molecular mechanisms that regulate platelet physiology and may contribute to processes beyond thrombosis and hemostasis.