Detection of circulating extracellular mRNAs by modified small-RNA-sequencing analysis

Extracellular mRNAs (ex-mRNAs) potentially supersede extracellular miRNAs (ex-miRNAs) and other RNA classes as biomarkers. We performed conventional small-RNA-sequencing (sRNA-seq) and sRNA-seq with T4 polynucleotide kinase (PNK) end treatment of total extracellular RNAs (exRNAs) isolated from serum and platelet-poor EDTA, acid citrate dextrose (ACD), and heparin plasma to study the effect on ex-mRNA capture. Compared with conventional sRNA-seq, PNK treatment increased the detection of informative ex-mRNAs reads up to 50-fold. The exRNA pool was dominated by RNA originating from hematopoietic cells and platelets, with additional contribution from the liver. About 60% of the 15- to 42-nt reads originated from the coding sequences, in a pattern reminiscent of ribosome profiling. Blood sample type had a considerable influence on the exRNA profile. On average approximately 350-1100 distinct ex-mRNA transcripts were detected depending on plasma type. In serum, additional transcripts from neutrophils and hematopoietic cells increased this number to near 2300. EDTA and ACD plasma showed a destabilizing effect on ex-mRNA and noncoding RNA ribonucleoprotein complexes compared with other plasma types. In a proof-of-concept study, we investigated differences between the exRNA profiles of patients with acute coronary syndrome and healthy controls. The improved tissue resolution of ex-mRNAs after PNK treatment enabled us to detect a neutrophil signature in ACS that escaped detection by ex-miRNA analysis.