Loss of H3K27me3 imprinting in the Sfmbt2 miRNA cluster causes enlargement of cloned mouse placentas

Abnormal placentation in cloned animals remains an unsolved problem. We demonstrated the involvement of micro RNAs (miRNAs) in the abnormal enlargement (hyperplasia) of placentas in cloned mice. Using a comparative transcriptome analysis of cloned placentas, we noted the consistent upregulation of clustered miRNAs within Sfmbt2, a paternally expressed imprinted gene. This region was biallelically activated by loss of imprinting (LOI) in cloned placentas. Deletion of the maternal allele of the whole miRNA cluster resulted in the correction of their expression levels and upregulation of their putative target genes with antitumor or apoptotic functions. Consequently, the placental size was reduced to the normal level and histology was ameliorated. In contrast, correcting the expression of the LOI genes (Sfmbt2, Gab1, and Scl38a4) in cloned placentas had no impact on placental size. Thus, we identified that LOI of clustered miRNAs within Sfmbt2 in cloned placentas was the major cause of abnormal placental enlargement. Mouse total RNAs were collected from in vitro fertilized and somatic cell nuclear transferred placentas with three genotypes and two embryonic days and sequenced.