Perspectives on miRNAs as Epigenetic Markers in Osteoporosis and Bone Fracture Risk: A Step Forward in Personalized Diagnosis

Abstract

Aging is associated with an increased incidence of age-related bone diseases. Current diagnostics (e.g., conventional radiology, biochemical markers), because limited in specificity and sensitivity, can distinguish between healthy or osteoporotic subjects but they are unable to discriminate among different underlying causes that lead to the same bone pathological condition (e.g., bone fracture risk). Among recent, more sensitive biomarkers, miRNAs — the non-coding RNAs involved in the epigenetic regulation of gene expression, have emerged as fundamental post-transcriptional modulators of bone development and homeostasis. Each identified miRNA carries out a specific role in osteoblast and osteoclast differentiation and functional pathways (osteomiRs). miRNAs bound to proteins or encapsulated in exosomes and/or microvesicles are released into the bloodstream and biological fluids where they can be detected and measured by highly sensitive and specific methods (e.g., quantitative PCR, next-generation sequencing). As such, miRNAs provide a prompt and easily accessible tool to determine the subject-specific epigenetic environment of a specific condition. Their use as biomarkers opens new frontiers in personalized medicine. While miRNAs circulating levels are lower than those found in the tissue/cell source, their quantification in biological fluids may be strategic in the diagnosis of diseases that affect tissues, such as bone, in which biopsy may be especially challenging. For a biomarker to be valuable in clinical practice and support medical decisions, it must be (easily) measurable, validated by independent studies, and strongly and significantly associated with a disease outcome. Currently, miRNAs analysis does not completely satisfy these criteria, however. Starting from in vitro and in vivo observations describing their biological role in bone cell development and metabolism, this review describes the potential use of bone-associated circulating miRNAs as biomarkers for determining predisposition, onset, and development of osteoporosis and bone fracture risk. Moreover, the review focuses on their clinical relevance and discusses the pre-analytical, analytical, and post-analytical issues in their measurement, which still limits their routine application. Taken together, research and clinical findings may be helpful for creating miRNA-based diagnostic tools in the diagnosis and treatment of bone diseases.

The Clinical Potential of Circulating miRNAs as Biomarkers: Present and Future Applications for Diagnosis and Prognosis of Age-Associated Bone Diseases

“…Currently, according to the most recent update (October 2018) of miRBase (www.mirbase.org),~2600 human mature miRNAs have been identified [11]. This database also contains information about universal miRNA nomenclature, which is well schematized in [12]. miRNA coding genes have been found in each human chromosome, including the Y and X one, and can be situated in protein-coding gene sequences (introns or untranslated regions) as well as in non-coding gene sequences [13][14][15][16][17].…”

“…Finally, the post-analytical phase is related to all the procedure that bring to the association of the analytical output to the subject from whom the sample has been taken. In the case of miRNA analysis, the analytical output is represented by a raw value (e.g., the threshold cycle [C T ] from real-time PCR) that needs to be normalized throughout the application of specific, although not universally accepted, algorithms [12,28,29]. Further information about the pre-analytical, analytical and post-analytical variables affecting miRNA determination are reported in Section 1.2.2.…”

“…In particular, miRNAs play a fundamental role in bone remodeling especially by regulating of osteoclast and osteoblasts differentiation and function. Thus, altered expression levels of these miRNAs are known to affect osteoblastogenesis and osteoclastogenesis as well as function, apoptosis, and proliferation of bone cells, acting on key regulators of these processes (i.e., Runx2, Wnt signaling pathway, PDCD4, RANKL, c-Fos, Osterix, ALP) [12,65,66]. These alterations lead to an altered bone cell and matrix homeostasis and, as a consequence, to osteoporosis development and progression.…”

microRNAs in the Antitumor Immune Response and in Bone Metastasis of Breast Cancer: From Biological Mechanisms to Therapeutics

“…Hence, miRNAs can be found in the blood, where they are relatively abundant (as well as virtually in all body fluids), and they can act on cellular targets in other districts. In such a condition, they can be used as markers to monitor a biological process and, since their expression is strikingly dependent upon multiple stimuli they can likely be used for personalized monitoring [12].…”

“…An abnormal miRNA expression is demonstrated in several disorders including tumors, and inflammatory and autoimmune diseases [3,[12][13][14]. In general terms, miRNA expression is downregulated in tumors (since they are inhibitors of gene expression) compared to their non-tumor counterparts and a few miRNAs can mark a specific neoplasia [15].…”