Transitional cell carcinoma of the bladder, the second most common tumor of the genitourinary tract, is also the second most common cause of death from these cancers. In 2001, an estimated 54,300 new cases will be diagnosed in the United States, and some 12,400 patients will die of the disease.
Approximately 80% of patients with primary bladder cancer experience low-grade tumors confined to the superficial mucosa, and the majority are amenable to initial transurethral resection and selected administration of intravesical immunotherapy or chemotherapy. The risk of recurrence for patients with superficial bladder tumors can be as high as 70%, however. One-third of recurrent tumors may demonstrate tumor progression to a higher grade and/or stage of disease. Muscle-invasive tumors occur initially in 15% to 30% of all bladder cancer patients; 50% of those treated locally for invasive tumors will relapse with metastatic disease within 2 years of treatment. These data underscore the heterogeneous nature and malignant potential of transitional cell carcinoma of the bladder.
The optimal management of invasive bladder cancer requires the detection and accurate assessment of the tumor’s biological potential. Treatment strategies for patients with bladder cancer are currently dictated by histologic evaluation, including determination of tumor grade and stage as the primary prognostic variables. Although these two conventional histopathologic variables provide a certain degree of stratification of a tumor’s biological characteristics, there remains a significant degree of tumor heterogeneity within the various prognostic subgroups. This makes it difficult to accurately and reliably predict the tumor’s aggressiveness. The ability to precisely predict an individual tumor’s true biological potential would facilitate treatment selection for patients who may benefit from adjuvant therapy and identify patients who may require less aggressive strategies. Intense research efforts are ongoing to best characterize bladder cancer and its varying biological profiles.
Transitional cell carcinoma of the bladder has generally been viewed as two different disease processes. Superficial tumors are most often locally proliferative and recurrent but can become invasive and even metastatic. Superficial tumors that maintain a malignant phenotype may be treated more effectively with early, aggressive intravesical therapy or even cystectomy. The use of molecular markers may guide decision-making in the treatment of superficial bladder cancers. Muscle-invasive cancer, notorious for its potential clinical virulence, is ideally treated aggressively, but there remains a significant incidence of recurrence and disease progression in some patients, who may ultimately benefit from an adjuvant form of therapy.
The need to predict which superficial tumors will recur or progressand which invasive tumors will metastasizehas led to an ongoing search for improved understanding of bladder carcinogenesis and metastasis. With the advent of new molecular techniques, the field of medical molecular biology has exploded in recent years, resulting in detailed analysis of human cells and tissues on the DNA, RNA, and protein levels. The molecular and genetic changes in transitional cell carcinoma of the bladder can be schematically classified into three separate, but intertwined, events: (1) chromosomal alterations, representing the initial event in carcinogenesis; (2) tumor proliferation, caused by loss of cell-cycle regulation; and (3) metastasis, in which the initial tumor breaks from its original confined environment, aided, in part, by processes such as angiogenesis and loss of cell adhesion.
We believe that the accumulation of these successive genetic alterationsrather than a single genetic event in timedetermines a tumor’s phenotype and, subsequently, the patient’s clinical outcome. In this review, we will summarize the recent literature regarding molecular and genetic changes in bladder cancer and comment on potentially improved diagnostic tools and treatment regimens becoming available as a result of our better understanding of these molecular pathways.
Our understanding of tumor biology has evolved rapidly over the past decade. Advances in molecular biology, immunology, and cytogenetics have prompted this progress. An increase in knowledge has provided an opportunity to identify and evaluate tumor characteristics beyond the scope of general histology and gross DNA content and to distinguish the potential behavior of an individual tumor. As the role of tumor markers in the diagnosis and prognosis of bladder cancer grows, it is important to understand the various technologic, methodologic, and analytic issues regarding each diagnostic modality. Several techniques can be applied to evaluate a tumor marker; most are only applied in a research setting, however.
For the clinical urologist, the most widely used evaluation method is immunohistochemistry. Translational research has enhanced the application and evaluation of immunohistochemical techniques with potentially important clinical roles in bladder cancer. Currently, most tumor markers that have been studied and merit a role in the contemporary clinical decision-making process for bladder cancer have evolved from the application of immunohistochemistry.
Standardization of the technique and its interpretation will ultimately be necessary for successful and consistent application of immunohistochemistry. With proper controls and standardization, immunohistochemistry will remain one of the principle techniques used to evaluate various tumor markers. Although efforts have been made to standardize immunohistochemistry, the limitations of this technique must be realized before we can develop a consensus as to how to perform the procedure. Despite these limitations, immunohistochemistry maintains some translational application in the analysis of various bladder cancer markers. Other more sophisticated and costly techniques, such as single-strand conformational polymorphism, DNA sequencing, or polymerase chain reaction (PCR)-based analyses, are currently used in the research setting but are not yet applicable to clinical decision-making.
Bladder cancer is an excellent model for the study of molecular changes at the DNA level, owing to its distinctly different subtypes (superficial and muscle-invasive) and their different propensities to progress. Such DNA alterations in bladder cancer have been studied in a variety of ways, ranging from cytogenetics to DNA ploidy to loss of heterozygosity. DNA alterations can result from a number of genetic insults such as point and insertional/deletional mutations, translocations, and loss of alleles. Each insult may affect the translated protein product. The large fund of molecular knowledge developed in recent years about carcinogenesis has provided some evidence about the different genetic pathways for bladder cancer.
Earlier work in the field of molecular oncology focused on oncogenes, which are normal cellular genes that contribute to the malignant phenotype of a tumor by overexpressing the normal gene product or, in some cases, by expressing a protein product with altered function. Overexpression of the normal gene product is usually achieved by gene amplification or chromosomal translocation of the gene to an area downstream of a powerful promoter. However, expression of a mutated protein product can also lead to activation of the malignant phenotype. Oncogenes believed to be important in human malignancies include cH-ras, c-myc, MDM2, and HER2/neu (aka c-erbB2).
cH-ras GeneThe cH-ras gene is an active oncogene thought to be involved in the development and progression of human bladder cancer. Mutational studies of the ras gene family have demonstrated that alterations in codons 12 and 61 of the H-ras gene occur in up to 20% of bladder cancers.[6-8] One study, employing PCR amplification followed by oligonucleotide-specific hybridization, reported that 36% of bladder tumors had the same mutation on codon 12 of the H-ras gene. In general, the activation of H-ras occurs by a single-point mutation (G d A) in codon 12, although other mutations have been described.
Clinically, Fontana and colleagues demonstrated a statistically significant relationship between the overexpression of the c-ras oncogene and early recurrence in patients with superficial bladder cancer. These data suggest a potential prognostic role for the c-ras oncogene in patients with superficial bladder cancer, but currently these techniques apply only in a research setting.
c-myc GeneAn important regulator of cellular proliferation, the myc gene family encodes for nuclear phosphoproteins containing DNA-binding activity. The c-myc oncogene has been shown to be overexpressed in several human tumors including bladder cancer.[12,13] Deregulation of the myc gene family occurs with chromosomal translocation and gene amplification, and studies have demonstrated that myc overexpression promotes cellular proliferation. Although the genetic mechanism causing overexpression of the c-myc gene in bladder cancer is unknown, its overexpression has been shown to be associated with high-grade bladder cancer.
Kotake and associates demonstrated that expression of the c-myc gene product correlates with the nuclear grade of bladder cancer. In a study with conflicting results, however, Lipponen found no independent prognostic value for Myc proteins with respect to prognosis for patients with transitional cell carcinoma of the bladder. Currently, the prognostic significance of c-myc gene expression is unknown, and further evaluation will be required to determine its prognostic role.
HER2/neuThe proto-oncogene HER2/neu has been extensively studied and implicated in a number of tumors, including breast, prostate, and bladder cancers. The HER2/neu oncogene encodes a transmembrane glycoprotein similar to the epidermal growth factor (EGF) receptor, having tyrosine kinase activity and the ability to stimulate cellular growth.
Initial studies of HER2/neu were performed in breast carcinoma and demonstrated a significant relationship between gene expression, tumor progression, and overall survival. Subsequently, several studies reported that HER2/neu expression in bladder cancer patients is associated with higher-stage tumors,[20-22] increased tumor progression, greater incidence of metastasis, and reduced overall survival. These investigations suggest a prognostic value of HER2/neu expression in human bladder cancer; other studies have reported conflicting results, concluding that evaluation of the oncogene provides no additional prognostic value over the previously established predictors of grade and stage for transitional cell carcinoma.[23,24]
In light of these discrepant results, further evaluation will be required to accurately determine the prognostic value of HER2/neu in bladder cancer.