Piwi-interacting RNAs (piRNAs) and Colorectal Carcinoma: Emerging Non-invasive diagnostic Biomarkers with Potential Therapeutic Target Based Clinical Implications

PIWI-interacting RNAs (piRNAs) constitute new small non-coding RNA molecules of around 24-31 nucleotides in length, mostly performing regulatory roles for the piwi protein family members. In recent times, developing evidence proposes that piRNAs are expressed in a tissue-specific way in various human tissues and act as moderate vital signalling pathways at the transcriptional or post-transcriptional level in addition to mammalian germline. Recent findings, however, show that the unusual expression of piRNAs is an exclusive and discrete feature in several diseases, including many human cancers. Recently, considerable evidence indicates that piRNAs could be dysregulated thus playing critical roles in tumorigenesis. The function and underlying mechanisms of piRNAs in cancer, particularly in colorectal carcinoma, are not fully understood to date. Abnormal expression of piRNAs is emerging as a critical player in cancer cell proliferation, apoptosis, invasion, and migration in vitro and in vivo. Functionally, piRNAs preserve genomic integrity and regulate the expression of downstream target genes through transcriptional or post-transcriptional mechanisms by repressing transposable elements' mobilization. However, little research has been done to check Piwi and piRNAs' potential role in cancer and preserve genome integrity by epigenetically silencing transposons via DNA methylation, especially in germline cancer stem cells. This review reveals emerging insights into piRNA functions in colorectal carcinoma, revealing novel findings behind various piRNA-mediated gene regulation mechanisms, biogenetic piRNA processes, and possible applications of piRNAs and piwi proteins in cancer diagnosis and their potential clinical significance in the treatment of colorectal carcinoma patients.

The PIWI proteins SMEDWI-2 and SMEDWI-3 are required for stem cell function and piRNA expression in planarians

PIWI proteins are expressed in germ cells in a wide variety of metazoans, where they participate in the synthesis and function of a novel class of small RNAs called PIWI associated RNAs (piRNAs). One function of piRNAs is to preserve the integrity of the germline genome by silencing transposons, though they also participate in epigenetic and post-transcriptional gene regulation. In the planarian Schmidtea mediterranea, the PIWI proteins SMEDWI-1 and SMEDWI-2 are expressed in neoblasts and SMEDWI-2 is required for regeneration and homeostasis. Here, we identify a diverse population of approximately 32-nucleotide small RNAs that strongly resemble vertebrate and insect piRNAs and map to hundreds of thousands of loci in the S. mediterranea genome. The expression of these RNAs occurs predominantly in neoblasts and is not restricted to the germline. RNAi knockdown of either SMEDWI-2 or a newly identified PIWI protein, SMEDWI-3, impairs regeneration and homeostasis and decreases the levels of both piRNAs and neoblasts. Therefore, SMEDWI-2 and SMEDWI-3 are required for piRNA expression, regeneration, and neoblast function in S. mediterranea.