Small but Full of Potential: piRNA and Colorectal Cancer Diagnosis

Improving the chances of effectively treating colorectal cancer depends largely on how early it is detected and diagnosed, and many novel biomarkers are being studied for this very purpose. At present, genetic material, proteins, volatile organic compounds (VOCs) and gut microbiota populations are among some of the biomarkers that are being applied to cancer detection and diagnosis.

In the case of genetic biomarkers, many of the well-known examples are genetic mutations that impact cellular activity and lead to cancerous cell growth. These mutations typically occur in the coding section of the DNA, which, based on the central dogma, is transcribed into RNA and then translated into a protein that performs a specific cellular function.

However, our DNA is also made up of many non-coding sections. These sections were originally thought to be unimportant, but many types of RNA transcribed from these non-coding sections — referred to as non-coding small RNAs — have proven to play important roles in gene expression and regulation.

Of the small non-coding RNAs (transcribed from non-coding DNA), we know significantly less about piwi-interacting RNAs (piRNAs) compared to the well-studied microRNA (miRNA), which has been found to play important roles in physiology and pathology, and even as a biomarker for cancer. Nonetheless, piRNAs are shown to have untapped potential in cancer diagnosis, but what exactly is a piRNA?

Piwi-interacting RNA, or piRNA for short, are short single stranded RNAs between 27 to 30 nucleotides long. piRNAs make up a large class of small non-coding RNA (sncRNAs) that bind to PIWI proteins and form RNA-protein complexes. While the process remains unclear, the biogenesis pathway of piRNA serves dual purposes: generating piRNAs and mediating gene expression.

Both piRNAs and their RNA-protein complexes have roles as upstream mediators of epigenetic control as well as transcriptional gene silencing, depending on the recruitment of specific proteins. As such, piRNAs, along with miRNAs and small interfering RNAs (siRNAs) are classified as regulatory RNAs. In particular, piRNAs regulate gene expression either by degrading the target messenger RNA (mRNA) or by inhibiting protein translation through hetero-silencing, where the targets are different from the genes from which they originate.

piRNA is usually found in germline cells and reproduction organs such as the testes and ovaries. However, they are also found in somatic cells in lower quantities. piRNAs are found in highly conserved clusters consisting of evolutionary ancient fragmented transposon copies. Transposons are elements that can get transferred and interspersed between different locations on the genome, creating mutations such as insertions, deletions, and other changes to the chromosome’s structure.

The role of piRNA and its related proteins in silencing transposal activities is well studied. That transposon coding genes have been observed to lose epigenetic control due to the lack of methylation in the absence of PIWI proteins suggests that piRNA may play an important role in cancer detection.

While piRNAs are found in somatic cells in lesser quantities and varieties compared to germline cells, studies have found their association with tumorigenesis to be significant. By binding to the promoter region of transposons, the piRNA-protein complexes facilitate DNA methylation and silencing of transposons. In doing so, the complexes help to maintain genomic integrity and prevent chromosomal aberrations that are critical for tumor development.

Studies have shown that up- or downregulation of piRNAs may cause impairment to regular cell function, leading to a disruption of apoptosis and upregulation of cell proliferation — the hallmark of cancer development. Particularly, piRNAs have been found to be dysregulated in cancer tissues compared to normal tissue.

While the association between piRNAs and cancer development is established, it is also the ease with which it can be detected that has garnered it attention as a potential diagnostic and prognostic biomarker for various cancers, including colorectal cancer.

piRNAs remain stable in biological samples including body fluids such as plasma and blood, suggesting that piRNAs can be used as a biomarker for liquid biopsy. Furthermore, the dysregulation of piRNAs in colorectal cancer tissue is also reflected in these samples. Detection of piRNA-protein complexes can first be detected through co-immunoprecipitation followed by sequencing or qRT-PCR.

So far, piRNA piR-823 has been most closely associated with colorectal cancer. When overexpressed, colorectal cancer cell proliferation is suppressed. Clinical studies have also provided indication that piRNAs are also presented in altered expressions in metastatic conditions.

While further studies and research are required to firmly establish piRNAs as a cancer biomarker, its potential cannot be discounted. Having multiple ways to improve the diagnosis and prognosis of colorectal cancer could pave the way to more effective cancer treatments.