Original article
Overexpression of BUBR1 is associated with chromosomal instability in bladder cancer

https://doi.org/10.1016/j.cancergencyto.2006.11.012Get rights and content

Abstract

BUBR1, a mitotic checkpoint protein, is a key component of the mitotic spindle checkpoint machinery. Defective BUBR1 has been proposed to contribute to chromosomal instability (CIN). To elucidate the relationship of BUBR1 expression with CIN, expression of BUBR1, numbers of centrosomes, numerical aberrations of chromosomes, and DNA ploidy were examined, and BUBR1 expression status was compared with clinicopathological parameters in 104 human urothelial bladder carcinomas. Expression of BUBR1 and numbers of centrosomes were assessed by immunohistochemistry. Numerical aberrations of chromosomes 7, 9, and 17 were evaluated by fluorescence in situ hybridization. Cancers with a large intercellular variation in centromere copy number were designated as CIN cancers. Tumors with BUBR1 overexpression were associated with CIN, DNA aneuploidy, and centrosome amplification. Array CGH revealed that BUB1B amplification and loss rarely occurred, indicating that the overexpression of BUBR1 in these bladder cancers was independent of BUB1B copy number. Overexpression of BUBR1 significantly correlated with higher histological grade, advanced pathological stage, and high cell proliferation. Overexpression of BUBR1 predicted tumor recurrence and disease progression. These data suggest that overexpression of BUBR1 is potentially a new tumor marker for estimating biological characteristics of bladder cancer.

Introduction

Approximately three-quarters of urothelial bladder carcinomas are low grade, and noninvasive superficial tumors at the time of initial diagnosis; however, 50–70% of tumors recur, and 10–20% will progress to a higher grade or stage [1]. Although these tumor types follow an unfavorable disease course, there are few reliable markers to distinguish tumors with aggressive characteristics from others with low malignant potential.

BUBR1 is a key component of the mitotic spindle checkpoint machinery [2], preventing chromosomal segregation errors by blocking onset of anaphase until proper attachments of all chromosomes to spindles are achieved [3]. It has been reported that a deficiency in this mitotic checkpoint causes chromosomal instability (CIN) and aneuploidy in several human cancers [4], [5], [6]. Defective BUBR1 is also associated with aneuploidy [7] and plays a role in the regulation of apoptosis and CIN [8].

The centrosome is a major microtubule organizing center for the formation of bipolar mitotic spindles, and it plays an important role in accurately distributing chromosomes into daughter cells [9]. An abnormal centrosome leading to the formation of multipolar spindles and unequal segregation of chromosomes is reported to be closely associated with CIN, and is one of the major characteristics of many solid malignant tumors [10], [11].

The relationship between BUBR1 protein expression and specific human malignancies remains to be clarified. To elucidate the relationship between BUBR1 expression and CIN in bladder cancer, we examined the status of BUBR1 protein expression, numerical aberrations of chromosomes, DNA ploidy, and numbers of centrosomes. Furthermore, we compared expression of BUBR1 with clinicopathological parameters in 104 cases of bladder cancer.

Section snippets

Patients and tissue specimens

Tissue specimens were obtained from 104 urothelial bladder carcinomas (80 men and 24 women; average age, 69.9 years; age range, 33–95 years) by transurethral resection (94 tumors), radical cystectomy (4 tumors), and transurethral bladder biopsy (6 tumors), between 1996 and 2005. Tissue specimens were frozen and stored in a freezer at −80°C until use. The study was approved by the Institutional Ethical Committee at Yamaguchi University School of Medicine, Japan (Approval No. 69), and written

BUBR1 expression in human urothelial tissues

BUBR1 was weakly to moderately expressed in three of six cases of normal urothelia (Fig. 1A), and was undetectable in the remaining three cases. BUBR1 expression was weak to moderate in three of five cases of bladder dysplasia (Fig. 1B), and was undetectable in the remaining two cases. In normal and dysplastic cells, strong to moderate expression of BUBR1 was occasionally found in <1% of cells that were in the mitotic phase. On the basis of findings from normal and dysplastic urothelia, BUBR1

Discussion

The chromosomal instability that is commonly associated with human cancers results from failure to accurately segregate chromosomes during mitosis [4], [17]. Defects in BUBR1 contribute to CIN [8]. In the present study, overexpression of BUBR1 protein was detected in tumors with abnormal mitotic cells and CIN. It is known that colorectal cancers and gastrointestinal cancers show high expression of BUBR1 mRNA, compared with matched normal controls [24], [25]. This agrees with our present data:

Acknowledgments

The authors would like to thank Mr. Kenzo Ikemoto and Mrs. Takae Okada for their assistance in preparing materials for immunostaining.

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