Kidney CancerIntegrated Phenotypic/Genotypic Analysis of Papillary Renal Cell Carcinoma Subtypes: Identification of Prognostic Markers, Cancer-related Pathways, and Implications for Therapy
Introduction
Papillary renal cell carcinoma (PRCC) is the second most common adult renal malignancy [1], [2]. Although PRCC morphology has been described since 1976, it was recognized in 1997 as having two histologic subtypes. Type 1 is characterized by small, pale cells with low-grade nuclei. Conversely, type 2 has abundant eosinophilic cytoplasm and higher-grade nuclei with prominent nucleoli, in a pseudo-stratified arrangement [2], [3].
It has been shown that PRCC2 is associated with a higher grade and stage than PRCC1 [2], [4]. Studies found that type 2 is associated with poor survival [2], [4]. However, most studies have repeatedly shown that histologic grade is associated with survival in PRCC [5], [6]. By morphologic definition, PRCC2 is a higher-grade tumor and seems to be associated with worse prognosis.
Studies have opted to distinguish between the two subtypes at the molecular level. Chromosomal changes commonly accompanying PRCC1 as gains of chromosomes 7, 17, and loss of Y are not recapitulated in PRCC2. Type 2 chromosomal changes are less consistent, including losses of 3p [7], [8], 1p [1], [8], or 9p [9]. Gene expression profiling for the two types revealed two distinct tumors [7]. The Cancer Genome Atlas (TCGA) PRCC study also revealed unique molecular signatures, with type 2 being further divided into three clusters [9]. Altered MET gene status is present in 80% of PRCC1, while PRCC2 is reported to have overexpression of the NRF2-ARE pathway [9], [10].
Some PRCCs are difficult to classify as they do not fulfill all criteria for either subtype or they have mixed morphology [1], [7], [8], [11]. The percentage of these PRCCs not otherwise specified (NOS) can be as high as 47% [1]. This highlights the need for a more accurate molecular classification that relies on biology to complement the morphology for PRCC classification. Another subtype of PRCC was described in which the cells are oncocytic and have a non-overlapping histology. The limited number of cases described for this category seemed to have excellent clinical outcome [11], [12].
Current targeted therapies for metastatic RCCs include mTOR and VEGF inhibitors. The response rate of PRCC to these modalities is lower than that of clear-cell renal cell carcinoma (CCRCC) [13], [14], [15], [16]. However, studies analyzing the PRCC response to targeted therapies did not stratify patients into types 1 and 2 [14], [15], [16].
We analyzed the molecular attributes responsible for the pathogenesis of the two subtypes of PRCC using two independent cohorts. We identified clinically useful diagnostic and prognostic biomarkers to stratify the subtypes. Considering the dilemma of NOS tumors, our aim was to identify new molecular markers of diagnostic and prognostic validity.
Section snippets
Specimen collection and morphologic subtyping
As detailed in the Supplementary material, we analyzed 281 PRCC cases from the TCGA KIRP cohort, in addition to 35 cases of PRCC from our institution. Tumors were classified as PRCC1 or PRCC2 according to the original PRCC criteria [3]. Cases that did not meet all the criteria or showed mixed morphology were classified as PRCC NOS. Cases that had an oncocytic non-overlapping papillary morphology 11,12] were designated oncocytic PRCC NOS (Fig. 1A).
RNA extraction and miRNA expression analysis
Pure tumor areas were selected from
Histologic subtyping of PRCC
A total of 290 of the 317 histologic slides available from both cohorts combined had sufficient tissue for accurate subtyping. Using the classic morphologic criteria [3], [11], 72 cases were subtyped as PRCC1, 70 as PRCC2, 131 as NOS, and 17 as oncocytic non-overlapping NOS (Fig. 1A–C).
Molecular subtyping of PRCC
To determine if PRCC1 and PRCC2 are biologically distinct, we performed unsupervised clustering of the gene expression profiles for 80 cases of PRCC (40 cases with classic PRCC1 morphology and 40 cases with
Discussion
We provide strong evidence of what other studies have implied: PRCC types 1 and 2 represent two distinct biological cancers, and should be clinically managed as such [7], [9]. In our study, cases that lacked any of the original histologic criteria were labeled as PRCC NOS (45% of cases). Our ratio of NOS is similar to what Chevarie-Davis et al [1] found in their PRCC study. In all likelihood, our NOS category had cases belonging to PRCC1 or PRCC2, and perhaps cases belonging to a third,
Conclusions
PRCC1 and PRCC2 are two distinct tumors. We provide the first evidence of molecular pathways that could have significant implications for PRCC management. Our study demonstrates that ABCC2 has great potential as a prognostic and predictive biomarker in the PRCC patient population. Our findings highlight the need to augment morphologic criteria with molecular markers to guide clinical management.
Author contributions: George M. Yousef had full access to all the data in the study and takes
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