Kidney CancerExpression of Aquaporin 1 in Primary Renal Tumors: A Prognostic Indicator for Clear-Cell Renal Cell Carcinoma
Introduction
Renal cell carcinoma (RCC), the most common malignancy of the adult kidney, accounts for approximately 3% of all malignancies [1], [2]. The incidence of RCC is increasing worldwide [1], [2]. More than 40% of patients with RCC die from their cancer [3]. Until now, complete surgical resection has been considered the only effective treatment for patients with clinically localized RCC. However, the disease will recur postoperatively in 20–40% of patients who undergo potentially curative nephrectomy [3]. The principal prognostic factors are tumor stage and histological grade. New prognostic parameters, including clinical, laboratory, and biomolecular factors, have long been desired to provide additional predictive value in clinical practice [4].
Recent studies demonstrate that RCC is morphologically and genetically a mixture of heterogeneous tumors [3], [5], [6], [7]. The clear-cell subtype is the most common, constituting approximately 80% of all RCC and >90% of metastatic cases [3], [5], [6], [7]. Genetically, the Von Hippel-Lindau (VHL) tumor suppressor gene is exclusively and frequently mutated in this subtype [3], [6], [7]. Aberrations in mesenchymal–epithelial transition factor (MET), fumarate hydratase (FH), or transcription factor E (TFE) genes are found in the papillary subtype or its variants. In contrast, the Birt-Hogg-Dubé (BHD) gene is closely associated with the occurrence of distal nephron tumors, such as chromophobe RCC and oncocytoma [7]. Therefore, subtype-specific biomarkers or new molecules should be identified for the accurate diagnosis and evaluation of patients’ clinical courses [8].
Aquaporins are water-transporting proteins that are physiologically expressed in many epithelial tissues and endothelium [9], [10]. In the normal kidney, AQP1, the first member to be identified, is present in the glomerular capillary endothelium and proximal tubular segments [9], [10], [11], [12]. AQP1 expression was also detected in some malignancies, such as lung cancer and glioma [13], [14]. In RCC, Northern blot analysis associated high AQP1 expression with a good prognosis in patients with clear-cell RCC [11]. An immunohistochemistry study suggested that AQP1 is a possible expression marker for low-grade clear-cell RCC [12]. We previously examined microarray gene expression profiles for various renal epithelial tumors and found hundreds of genes differentially expressed in each subtype [15]. Among them, AQP1 is a potent indicator of outcome in patients with clear-cell RCC. However, these observations, including our own, were based on limited numbers of samples and/or relatively short follow-up periods. Neither the detailed relationship between AQP1 and renal tumor biology nor the clinical impact of AQP1 on RCC is well elucidated.
In this experiment, we used real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR), a rapid and sensitive gene-expression detection assay, to measure AQP1 expression levels in a larger RCC patient cohort. We then investigated the relationships between AQP1 expression and various clinicopathologic parameters, VHL gene mutational status, and AQP1’s clinical utilities.
Section snippets
Patients and tumor samples
Primary sporadic tumor samples were collected from 559 patients with sporadic RCC who had consecutively undergone nephrectomy at our university hospital or its affiliated hospitals between March 1986 and November 2004. Fresh tumor specimens without apparent necrosis and corresponding normal kidney tissue specimens were grossly cut out immediately after nephrectomy, snap-frozen with liquid nitrogen, and stored at −80 °C until nucleic acid extraction. The tumors were classified according to the
Upregulation of aquaporin 1 in clear-cell and papillary renal cell carcinomas
AQP1 expression in a total of 559 primary renal tumors was quantified by qRT-PCR. We initially tested the reproducibility of gene expression levels between the former microarray and the qRT-PCR assay. Consequently, we found that the reproducibility was quite high (Spearman rank correlation coefficient: ρ = 0.774, n = 33).
We next evaluated AQP1 expression according to histopathologic subtype-specific patterns. qRT-PCR demonstrated that all normal kidney samples presented substantial AQP1 expression.
Discussion
In the present study, we measured AQP1 expression in a total of 559 renal tumors of various subtypes by qRT-PCR assay and subsequently examined the detailed relationships between the expression levels and clinicopathologic factors as well as patient outcomes. We detected high AQP1 expression in clear-cell and papillary RCCs, both of which are closely connected with the proximal nephron system [5], [6], [7], whereas we found low levels in other distal nephron-related tumors, including
Conclusions
We demonstrated that AQP1 is expressed exclusively in clear-cell and papillary RCCs. The AQP1 expression level is inversely correlated with tumor differentiation, aggressiveness, and the acquisition of metastatic activity in clear-cell RCC and probably also in papillary RCC. The determination of the AQP1 expression level as a biomolecular marker can provide useful prognostic information for patients with clear-cell RCC, specifically for predicting cancer survival times for all tumor stages and
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