Let-7 microRNAs are developmentally regulated in circulating human erythroid cells

MicroRNA are ~22nt-long small non-coding RNA that negatively regulate protein expression through mRNA degradation or translational repression in eukaryotic cells. Based upon their importance in regulating development and terminal differentiation in model systems, erythrocyte microRNA profiles were examined at birth and in adults to determine if changes in their abundance coincide with the developmental phenomenon of hemoglobin switching. Expression profiling of microRNA was performed using total RNA from 4 adult peripheral blood samples compared to 4 cord blood samples after depletion of plasma, platelets, and nucleated cells. Labeled RNA was hybridized to custom spotted array containing 474 human microRNA species (miRBase release 9.1). Total RNA from Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines provided a hybridization reference for all samples to generate microRNA abundance profile for each sample. Among 206 detected miRNA, 79% of the miRNA were present at equivalent levels in both cord and adult cells. By comparison, 40 microRNA were up-regulated and 4 microRNA were down-regulated in adult erythroid cells (fold change > 2; p < 0.01). Among the up-regulated subset, the let-7 miRNA family consistently demonstrated increased abundance in the adult samples by array-based analyses that were confirmed by quantitative PCR (4.5 to 18.4 fold increases in 6 of 8 let-7 miRNA). Profiling studies of mRNA in these cells additionally demonstrated down-regulation of ten let-7 target genes in the adult cells. These data suggest that a consistent pattern of up-regulation among let-7 miRNA in circulating erythroid cells occurs in association with hemoglobin switching during the fetal-to-adult developmental transition in humans. Expression profiling of microRNA was performed using biological replicates from 4 adult peripheral blood samples compared to 4 cord blood samples after depletion of plasma, platelets, and nucleated cells. Total RNA from Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines provided a hybridization reference for all samples.