Molecular cytogenetic investigations of chromosomal abnormalities in prostate and urinary bladder cancers

Abstract

Tumorigenesis is suggested to be caused by base pair mutations and chromosomal abnormalities leading to inactivation of tumor suppressor genes an activation of oncogenes. However, still it is not clear for the vast majority of these changes how they contribute to the tumorigenesis of the prostate and bladder cancers. This study focused on the molecular cytogenetic investigations of chromosomal alterations in prostate and bladder cancers and the estimation of the significance of the chromosomal alterations in relation to the clinical practice. Interphase cytogenetics with fluorescence in situ hybridization (FISH) is demonstrated as a powerful technique suitable for studies of the specimens from the prostate and bladder cancers. Deletion of the short arm of chromosome 8 has been reported to be a frequent phenomenon in prostate cancer. Distal deletion of 8p23 was found in 50% of the tumors. Interstitial deletion of 8p22 was found in 53% of the tumors. In total 71% of the specimens demonstrated deletions of any type in chromosome 8p. The degree of 8p deletion was well correlated with tumor grade and tumor metastases. These data support the hypothesis that tumor suppressor gene(s) may be located in the chromosomal region 8p. Deletion of 8p may be of crucial importance for the pathogenesis of prostate cancer.

Numerical aberrations of chromosome 16 and loss of 16q24 were studied by FISH in 31 primary and 22 metastatic tumors. Numerical aberrations of chromosome 16, 16q24 deletion and alteration of E-cadherin expression were respectively found in 29%, 35% and 29% of the primary tumors, and 73%, 73% and 73% of the metastases. High tumor grade and DNA aneuploidy were also found to have significant correlation with metastases. Deletion of chromosome 16q24 and/or decreased E-cadherin function appeared at a high frequency in metastases of prostate cancer. The results suggest that they may be important risk factors, contributing to a metastatic potential of the tumor.

When summarizing the clinical significance of chromosomal alterations, we found that multiple chromosomal changes were related to tumor metastasis and high malignant grade. Deletion of 8p22 was significantly increased in diploid metastases, while deletion of 16q24 was commonly associated with gross DNA changes. Alterations of chromosomes 8 and 16 together with other tumor properties such as high-grade, DNA aneuploidy and high S-phase fraction were not only significantly related to the poor prognosis and the short-term cancer specific survival, but deletions of 8p and 16q may indicate specific steps in the pathway of prostate cancer progression.

Using interphase FISH technique, we found mutations of the p53 gene well correlated with the deletions and overexpression in transitional cell carcinoma of the urinary tract. Deletions of p53 were significantly associated with tumor grade, stage, S-phase fraction, and DNA ploidy, while p53 overexpression correlated only with grade. The close correlation between p53 deletion and the malignant potential of human urothelial tumors indicates the clinical importance. We also studied the alterations of chromosomes in both tumor and histologically normal appearing mucosa from the same urinary bladder. Chromosomal alterations in the tumor were also frequently found in the "normal" mucosa. Normal mucosa with certain genetic abnormalities could indicate the risk of recurrence of bladder cancer, particularly in superficial tumors.

In one paper we reported on the chromosomal composition of 24 uroepithelial carcinomas and five cell lines from bladder cancer and eight established prostate cancer cell lines by spectral karyotyping (SKY). In general, more chromosomal alterations were detected in high-grade invasive bladder carcinomas than in low-grade papillary tumors indicating a progressive accumulation of acquired genetic alterations in bladder carcinogenesis. A clear correlation was observed between tumor stage and grade and specific net chromosomal imbalances involved. The SKY analysis showed complex karyotypes for all cell lines. Several complex chromosome translocations and pinpointed rearrangement breakpoints were also revealed. Identification of chromosomal alterations in these cancers by SKY may prove to be helpful in attempts to clone the genes involved in cancer tumorigenesis of prostate and bladder cancers. Most recurrent breakpoints were observed to participate in deletion and translocation events in these tumors suggesting the importance of tumor suppressor and oncogenes in these regions.

Molecular cytogenetic techniques that we apply in this thesis are useful for the observation of genetic alteration in urinary bladder and prostate tumors. Our results show that some specific chromosomal alterations are closely related to tumor progression and the biological activity of the tumor. They may play an important role in tumorigenesis and tumor development. In the future the assessment of these genetic changes in clinical samples may become important diagnostic and predictive tools.