The hunt for novel biomarkers for the diagnosis and monitoring of kidney diseases has intensified over the last decade, and several important advances are beginning to make their way into clinical medicine. Most notable is anti-PLA₂R antibody testing for the diagnosis and monitoring of membranous nephropathy. In fact, a clinical test for anti-PLA₂R antibodies was recently approved by the FDA for use in humans. However, we are only now beginning to see how this test will shape clinical practice. FSGS is not without exception. SuPAR is a potential biomarker, and efforts are underway to clarify its role. In a recent article in AJKD, Zhang et al explore the utility of measuring microRNAs (miRNAs) in patients with FSGS. miRNAs are beginning to gain traction as a biomarker in multiple fields, most notably oncology. miRNAs are small non-coding RNAs of approximately 20 nucleotides in length. They function to silence or regulate gene expression by base-pairing with complementary sequences within mRNA (or coding RNA). miRNAs are highly conserved and thought to be an evolutionary form of gene regulation. miRNAs are also stable in the circulation, and can enter cells to regulate target gene expression in distant organs. miRNAs are named numerically in the order of their discovery and not for their biological function (e.g., miR-1 was discovered first, and miR-100 came later). Furthermore, capital “miR-” refers to the mature form of the miRNA, while the small case “mir-” refers to the pre-miRNA or pri-miRNA.  Lastly, all capital “MIR” refers to the gene that encodes each of them.

Zhang et al screened and verified two independent Chinese cohorts of patients with FSGS for miRNAs. In the screening and validation phase, they pooled plasma from 9 patients with FSGS and nephrotic syndrome and 9 healthy controls, and ran a panel of known miRNAs. This showed that 161 miRNAs were detectable in both groups. After qPCR verification, they were left with 6 miRs that had 2-fold greater upregulation in the FSGS pool compared to the control pool. Of these 6 miRs, only 3 were at a detectable range: miR-125b, miR-186, and miR-193a-3p. The group focused on these 3 miRs for the duration of the study. The next step was to validate these 3 miRs in a separate cohort of patients (again of Chinese origin) with FSGS and nephrotic syndrome (n=32) and healthy controls (n=30). ROC curves were then performed in order to demonstrate how these miRs discriminate between FSGS and healthy patients, and all showed decent area under the curve (0.882 for miR-125b, 0.789 miR-186, and 0.910 miR-193a-3p, specificity and sensitivity range from 80-93% and 62-81% respectively with miR-186 being most specific and miR-125b being most sensitive). The combination of all 3 miRs produced a specificity of 93% and sensitivity of 91%.

They next looked at how these 3 miRs change according to disease status. They used a separate cohort of patients with FSGS with nephrotic syndrome (n=37) and patients with FSGS in complete remission.  Of the 3 miRs, miR-125b, and miR-186 were able to discriminate between disease and remission, but not miR-193a-3p. In a real tour de force, this group performed a prospective study to see if miR-125b and miR-186 levels changed in individual patients before and after steroid treatment for 8 weeks. These data indicated that miR-186, but not miR-125b, levels returned to baseline after successful treatment with steroids therapy. They concluded that miR-186 is a potential biomarker for FSGS activity as it could reliably distinguish between FSGS and controls, and decreased with remission. The group also demonstrated that neither miR-125b nor miR-186 were increased in other forms of nephrotic syndrome, including membranous and diabetic nephropathy.

This is an intriguing study that will need to be replicated in other FSGS cohorts as this was a fairly homogenous group of younger Chinese patients with FSGS. Furthermore, it is unclear if biopsies were obtained in any of the patient with FSGS. The biologic plausibility of miR-186 or even miR-125b contributing to the pathogenesis is rooted in how these miRs regulated genes such as PTTG1, POXO1, P2X7 receptor, or AKAP12. These are genes involved in cell-cycle control, AKT signaling, insulin signaling, and calcium signaling, and all have been shown to influence podocyte function. Whether or not these studies can be replicated in other cohorts will be the true test of their usefulness. Even then, how these tests will guide patient care will take time to completely elucidate. The Holy Grail is finding a way to manipulate one of these miRs to modify disease progression. As miRs regulate so many genes, off target gene repression could represent a real limitation to this approach. I look forward to seeing where this story goes as the current drug armamentarium to treat FSGS is quite limited.

Dr. Matt Sparks
AJKD Blog Advisory Board

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