Detection of PIWI-piRNAS in the mitochondria of human normal and cancer cells

Abstract

Piwi-interacting RNAs (piRNAs) are a class of small non-coding RNAs that regulate differentiation in primordial germ cells, found mainly in the nucleus and cytoplasm. However, despite the essentiality of mitochondria to intrinsic apoptosis and energy metabolism, the presence of piRNAs (and their associated Piwi proteins) in this organelle remains largely unknown. In this study, a public database comparison of mitochondrial RNA-seq data from human normal (HEK293) and cancer (HeLa and 143B) cells revealed the presence of 29 mature piRNA sequences, with 12 of those piRNA sequences perfectly matching 7 tRNAs (Phe, Val, Leu, His, Ser, Glu, and Thr), suggesting piRNAs and tRNAs derive from a common precursor mitochondrial polycistronic transcript. Consistent with their known interaction with piRNAs, further examination revealed the presence of PIWI proteins in mitochondria. Moreover, the majority of the 29 piRNA sequences overlapped multiple longer sequences (expressed sequence tags, ESTs) that uniquely mapped to the mitochondrial genome. To our knowledge, this is the first report of Piwi and piRNAs in the mitochondria of mammalian normal and cancer cells.