Progression of diabetic nephropathy (DN) is commonly defined by an increase in albuminuria
from normoalbuminuria to microalbuminuria and from microalbuminuria to macroalbuminuria.
Although early detection of DN can prevent or slow its progression, a major difficulty
in inducing remission in patients with early DN is the identification of biomarkers
that could help identify patients more likely to progress to end-stage renal disease
(ESRD). Traditional risk factors such as albuminuria do not effectively predict DN
progression, and other predictors of DN have yet to be characterized and validated.
The need for discovering sensitive and easily detectable biomarkers to monitor the
decline in renal function and to separate progressors from nonprogressors of DN is
therefore of paramount importance.
Recently, microRNAs (miRNAs) have emerged as one such potential class of biomarkers.
Mature miRNAs are a class of evolutionarily conserved, short (20–22 nucleotides long),
noncoding RNA that are potent regulators of gene expression. After several synthesis
and processing steps, mature miRNAs are loaded into the RNA-induced silencing complex,
which directs the miRNAs to its target messenger RNAs (mRNAs). Once bound to its target
mRNA, the RNA-induced silencing complex can facilitate several forms of transcriptional
repression depending on the strength of the miRNA-mRNA interaction and seed-sequence/target
site complementarity, ultimately resulting in the loss of protein expression (1).
Thus, the recent discovery of miRNAs has clearly introduced an additional layer of
intricacy to our understanding of gene regulation.
Within the past decade, a vast number of studies exploring the significant contribution
of miRNAs to human health and disease have underscored the critical relevance of miRNAs
to basic and translational biology. Importantly, several promising miRNA therapeutics
(either replacement or inhibition therapies) have begun early-stage phase 1/2 clinical
trials based on promising preclinical findings (2–4). Additionally, a rapidly growing
number of studies have highlighted tissue-specific and urinary miRNAs as potential
biomarkers in a number of pathological conditions (5–8).
In this issue of Diabetes, Pezzolesi et al. (9) demonstrate for the first time the
potential role of experimentally validated, cytokine-regulated plasma-circulating
miRNAs as biomarkers for monitoring the progression of DN in subjects with type 1
diabetes (T1D). The authors integrated the existing knowledge of biologically relevant
miRNAs to DN pathogenesis and progression as a screening tool to identify miRNAs that
can be potentially used as clinical biomarkers. The central premise of the work was
to screen levels of miRNAs regulated by transforming growth factor (TGF)-β1, a well-documented
pathogenic cytokine in DN, in a set of rapidly progressing proteinuric T1D patients.
The authors compared miRNA expression levels from the plasma of three groups of T1D
patients: 1) rapid progressors, patients with proteinuria and diabetes but with normal
renal function at the time of enrollment who progressed rapidly with a fast rate of
estimated glomerular filtration rate decline (mean ± SD duration of 5.1 ± 2.8 years);
2) nonprogressors, patients with proteinuria and diabetes with normal renal function
who maintained normal and stable renal function over the course of follow-up (mean
± SD duration of 11.1 ± 5.0 years); and 3) control subjects, patients with diabetes
and normoalbuminuria who maintained normal and stable renal function during follow-up
(mean ± SD duration of 7.4 ± 1.6 years). Using the baseline levels of known TGF-β1–regulated
miRNAs, the investigators narrowed their study to five highly detectable TGF-β1–regulated
miRNAs (i.e., let-7b-5p, let-7c-5p, miR-21-5p, miR-29a-3p, and miR-29c-3p). Of these
five miRNAs, only circulating levels of let-7b-5p (P = 0.01) and miR-21-5p (P = 0.006)
were associated with a significant increase in the risk of ESRD progression. miR-29a-3p
(P = 0.0007) and let-7c-5p (P = 0.0002) were significantly associated with a greater
than 50% reduction in the risk of rapid progression to ESRD. Interestingly, while
levels of miR-29c-3p did not show any association with risk of rapid progression to
ESRD (P = 0.68), levels of circulating miR-29c-3p were significantly increased in
patients with proteinuria and diabetes from both rapid progressors (P = 0.0009) and
nonprogressors (P = 0.0003) compared with normoalbuminuric control subjects (Fig.
1).
Figure 1
Schematic depicting TGF-β1–regulated circulating miRNAs as predictors of progression
of DN.
The work by Pezzolesi et al. (9) parallels previous studies done to investigate circulating
miRNA levels as biomarkers for disease progression in other kidney diseases (10,11).
A major strength of the study is that the investigators took advantage of patient
samples from the extensively characterized Joslin cohort (12). Furthermore, the findings
of the study validated conclusions from previously published preclinical studies related
to the candidate miRNAs investigated (13). Additionally, while the authors acknowledge
that there are indeed other potential miRNAs that may predict progression of DN, either
regulated by TGF-β1 or other factors, the identification of a select group of well-characterized
miRNAs should allow for future studies to pinpoint the potential role of miRNAs in
the progression of DN in a much more focused fashion.
One caveat that should be taken into consideration is that there is a general tendency
to extrapolate information regarding the levels of circulating miRNAs to the levels
of miRNAs within the tissues, which presumably contribute to specific phenotypes.
Indeed, although the five miRNAs selected for analysis by Pezzolesi et al. (9) are
regulated by TGF-β1, how the kidney levels of these five miRNAs contribute to the
risk of rapid progression of DN remains relatively unexplored. Another conundrum regarding
the interpretation of miRNAs as biomarkers for assessing the risk of progression to
ESRD in patients with DN is the classic chicken-and-egg causality dilemma as it remains
unclear whether perturbation of miRNA levels in the kidney contribute to DN progression
or the progression of DN alters levels of circulating miRNAs. Finally, while the current
study investigated circulating plasma levels of miRNAs, a growing number of studies
demonstrate the ease and robust detection of urinary miRNAs in patients with DN and
other kidney pathologies (14–16). Future research would ultimately determine the most
reliable method for detecting miRNA levels related to DN progression, taking ease
and quantity of sample acquisition into account.
In summary, the findings presented by Pezzolesi et al. (9) merit further investigation
in order to test the applicability and reproducibility in additional cohorts with
T1D and type 2 diabetes. Ultimately, a prospective randomized trial is needed to establish
the role of miRNAs as predictors of progression in patients with DN.