With a prevalence of about 10% of all RCC cases, pRCC is the second most common histological subtype of RCC [
47] and can be differentiated into two different subsets by distinctive histological and molecular findings [
48]. Both type 1 and type 2 are characterized by mostly papillary and tubular structures. However, in type 1, these respective structures are predominantly covered by small cells with pale cytoplasm and small oval nuclei, whereas in type 2, they are covered by larger eosinophilic cells with large spherical nuclei [
49]. A large proportion of type 1 tumors have multiple chromosomal alterations, the most frequent of which are gains of chromosomes 7 and 17. In addition, MET mutations are common in type 1 tumors. In type 2, CDKN2A alterations, either by mutations or by hypermethylation, occur frequently [
50]. Overall, patients with pRCC have a better survival outcome than those with ccRCC; however, type 2 tumors are usually more aggressive and have a greater metastatic potential [
9]. Therefore, type 2 pRCC has a poorer prognosis than type 1 papillary and even ccRCC [
51,
52].
Diagnostic Potential of miRNAs in pRCC
The number of miRNAs with potential for diagnostic purposes is described in this paragraph. miR-21 is upregulated in malignant renal tumors compared to healthy renal tissue [
53,
54] and is linked to tumor growth, cancer progression and metastases [
55]. Interestingly, different levels of miR-21 expression have been found among different renal tumor subtypes. There is a significantly higher expression of miR-21 in clear-cell and papillary subtypes, in contrast with chromophobe RCC and oncocytoma [
53,
56]. In addition, in pRCC, increased miR-21 expression (gene locus 17q23.1) is linked to copy number changes of the genome, since pRCC cells feature a high frequency of trisomy 17 and, therefore, an increase of the related gene products [
56]. As mentioned earlier, numerical chromosomal alterations are more frequently associated with type 1 pRCC [
50]. Thus, miR-21 could be used to distinguish between these respective RCC subtypes on a molecular basis with relatively high sensitivity (83%) and specificity (90%); however, a differentiation of ccRCC and pRCC cannot be achieved by using miR-21 only, meaning that molecular diagnostics are not a substitute for an experienced pathologist [
53].
On the other hand, Powers and colleagues [
56] identified 3 miRNAs with distinctive levels of expression in ccRCC and pRCC. miR-126, miR-126* and miR-143 were significantly upregulated in ccRCC compared to pRCC, which made it possible to distinguish between the respective two RCC subtypes [
56].
The downregulation of miR-126 in pRCC relative to ccRCC was confirmed by another study that also aimed to correctly discriminate between different RCC subtypes [
57]. A two-step model for differentiating ccRCC and pRCC from chRCC and oncocytoma was proposed by Di Meo et al. [
57] based on the expression rates of miR-221, miR-222 and miR-126. The first step takes into consideration the differential expression rates of either miR-221 or miR-222. Both, miR-221 and miR-222 display decreased expression levels in carcinomas with clear-cell or papillary morphology. The second step involves discriminating between ccRCC and pRCC based on the expression rate of miR-126, which is increased in ccRCC compared to pRCC [
57]. The location of miR-126 on chromosome 9 (gene locus 9p34.3) and the frequent loss of chromosome 9p characteristic for type 2 pRCC might be a feasible explanation for the lower expression compared to ccRCC [
50,
57]. Regarding the role of miR-126 in carcinogenesis, miR-126 targets the 3′-UTR of vascular endothelial growth factor A (VEGF-A) [
58] and additionally, expression levels correlate inversely with the expression of epidermal growth factor-like domain 7 (EGFL7) [
59]. Both, VEGF-A and EGFL7 are involved in tumor angiogenesis [
58,
59]. Furthermore, miR-126 may be involved in the regulation of the PI3K/Akt pathway [
60].
Wach and colleagues [
61] conceived a study in which, besides the discrimination of both healthy and cancer tissue, as well as ccRCC and pRCC subtypes, they were also able to differentiate type 1 and 2 of pRCC by using a multistep combination of miRNAs. In the first step, the miRNAs used to distinguish between healthy and tumor tissue were miR-145, miR-200c, miR-210 and mi-R502-3p. In the second step, miR-145 and miR-503-3p were used to classify ccRCC versus pRCC, whereas in the third step, type 1 and 2 were distinguished by utilizing miR-210 and miRNA let-7c. Both, miR-210 and let-7c were upregulated in type 1 pRCC as compared with type 2. The subtypes were classified correctly in 86.5%, 77.6% and 86.4% for the first, second and third discrimination, respectively [
61].
miR-210 is linked to hypoxia in cancer tissue and is directly involved in the hypoxia pathway. Hypoxia-induced factor 1 alpha (HIF1α) binds to its promotor region, thus inducing the transcription of miR-210 in a proposed positive feedback loop [
62,
63]. Nonetheless, higher HIF1α levels and, consequently, elevated miR-210 expression may also occur in non-hypoxic states due to mutations of the Von Hippel Lindau (VHL) gene, which leads to insufficient HIF1α-degradation [
64]. To explain the relatively low miR-210 expression in type 2 pRCC, Wach et al. suggest less dependence on hypoxia in type 2 compared to type 1 pRCC [
61]. Furthermore, miR-210 is upregulated in various other cancer entities, including ccRCC [
62,
65], and has been introduced as a possible diagnostic and prognostic biomarker in RCC, as well as for other malignancies, such as colorectal cancer [
66,
67].
Targets of the let-7 family include the oncogenes RAS and MYC, making let-7 family members veritable tumor-suppressing miRNAs. The relative downregulation of let-7c in type 2 pRCC corroborates the recent discovery of MYC overexpression in the respective subtype [
61,
68]. In addition, immunohistochemical MYC straining patterns could discriminate prognostic groups in type 1 pRCC [
69].
Regarding the distinction of RCC subtypes, miR-155 showed higher expression levels in ccRCC compared to pRCC and, therefore, could be useful in the distinction of the two subtypes [
54]. Moreover, miR-155 also carries prognostic information, as its overexpression is related to decreased disease-specific survival (DSS) in RCC, although this only prevailed in the univariate analysis [
54]. miR-155 is located on chromosome 21 (gene locus 21q21.2–21.3) and is linked to tumor proliferation. It directly targets nedd4-family interacting protein 1 (NDFIP1), which is a part of the regulation of PTEN (Phosphatase and tensin homolog) [
70], a commonly known apoptosis-promoting tumor suppressor gene in various solid malignancies that is also associated with poorer survival in kidney cancer [
71]. miR-155 may target the 3′-UTR of PTEN mRNA directly as well, leading to an activation of the PI3K/Akt pathway and thus, promoting tumor progression [
72]. Other functions of miR-155 that are related to carcinogenesis are targeting the tumor suppressor DMTF1 (Cyclin D Binding Myb-Like Transcription Factor 1) and enhancing the Wnt/beta-catenin pathway [
31,
73].
Prognostic Potential of miRNAs in pRCC
miRNAs are not only useful as potential diagnostic biomarkers but have also proven to be of prognostic significance in various cancer entities, such as breast, gastric, colon and prostate cancer [
74,
75,
76,
77]. Other than the previously discussed studies, which mainly addressed the potential of microRNAs in diagnosis and the classification of RCC, some studies also focused on their possible prognostic value in pRCC (
Table 1).
Decreased expression levels of miR-200c and miR-127, as well as high levels of miR-34a, were associated with better overall survival (OS) in patients with pRCC [
78]. However, only miR-34a proved to be an independent prognostic marker in the multivariate analysis in the validation stage [
78]. miR-200c was shown to be dysregulated in many solid tumor entities, such as, but not limited to, bladder, breast, colorectal, gastric and lung cancer [
82]. Moreover, it is involved in the proliferation and differentiation of normal and cancer stem cells and, by modifying the cellular sensitivity to death receptor CD95, in the regulation of apoptosis. miR-200c may also suppress endothelial-to-mesenchymal transmission (EMT) and, therefore, inhibit tumor progression [
82].
miR-34a may function as a tumor suppressor, which could illuminate why higher expression levels favor a better prognosis. E2F3, MET and Fra-1 are associated targets of miR-34a [
83,
84,
85].
Other miRNAs analyzed in respect of their prognostic potential are hsa-miR-1293 and hsa-miR-3199 2 [
79]. In the study, the cut-off was chosen as the median expression level. A significant difference in progression-free survival (PFS) in a 5-year follow-up for high-risk (39.4%) and low-risk (70.3%) groups was reported [
79]. However, the inclusion of both metastasized and non-metastasized patients in the analysis must be noted as a limitation of the study regarding the calculation of PFS. Except for hsa-miR-1193, which was also shown to relate to lung cancer [
86], to date, there are no studies to validate their involvement in carcinogenesis and tumor progression.
In a competing endogenous RNA (ceRNA) network analysis, seven miRNAs have been found that may be promising candidates as prognostic biomarkers in pRCC [
80]. Higher expression levels of hsa-miR-133a, hsa-miR-133b, hsa-miR-145, hsa-miR-216a, hsa-miR-217 and hsa-miR-1297 were associated with detrimental effects on OS. As opposed to this, an increased expression of hsa-miR-211 was connected to a better OS [
80]. However, since no uni- and multivariate models were used in the survival analysis, there is not enough evidence to support the prognostic impact of the respective miRNAs, yet. As for miR-145, at least in the previously mentioned study of Wach et al. [
61], it did not independently predict prognosis, as it failed to reach statistical significance in multivariate models.