Human miRNAs on chromosome 21 are differentially expressed in Down syndrome fetal hearts.

We previously demonstrated dosage-dependent upregulation of chro- mosome 21 (Hsa21) genes and dysregulation of mitochondrial and ECM genes in heart tissues of Down syndrome (DS) fetuses. Some of these dysregulated genes might be responsible for the DS cardiac phenotype, but it is evident that also other functional non-coding sequences, such as miRNAs, might play an important role. MiRNAs are highly expressed in the heart and regulate cardiac development and function. Five miRNAs, according to Sanger miRBase, are on Hsa21: miR-99a, miR-125b, let-7c, miR-155 and miR-802 . Nothing is known about their expression in trisomic tissues. We evaluated by qRT-PCR the expression of Hsa21 miRNAs in heart tissues from DS fetuses and controls. We found that miR-99a, miR- 125b, let-7c and miR-155 were expressed in 20-22 weeks fetal heart . MiR-802 was not expressed. MiR-99a, miR-155 and let-7c were over- expressed in trisomic hearts, whereas miR-125b was normoregulated. Target genes of upregulated miRNAs were obtained by merging Pic- Tar, TargetScan and MiRanda prediction lists. As miRNAs could affect protein expression by either interfering with RNA translation or promoting mRNA degradation, we evaluated the mRNA expression of target genes of overexpressed miRNAs by using the data set of our previous study. Seventeen targets of miR-99a, 12 of miR-155 and 15 of let-7c were expressed in fetal heart and downregulated in trisomic samples. Target genes possibly involved in DS phenotype were found such as SLC25A4, let-7c target, downregulated in DS hearts and involved in mitochondrial function, and CYP26B1, miR-99a target, showing a dos- age-dependent effect on ventricular septal defects.