DELETION OF THE P16 AND P15 GENES IN HUMAN BLADDER-TUMORS

Background: Two genes, p16 (also known as CDKN2, INK4A, or MTS1) and p 15 (also described as INK4B or MTS2), are found in tandem at chromosom e 9p21. These genes are designated as candidate tumor suppressor genes because they encode proteins that function as negative cell cycle reg ulators. (The encoded polypeptides inactivate specific cyclin-protein kinase complexes that are required for progression through the cell cy cle.) Molecular genetic studies have revealed that deletion of the p16 and p15 genes occurs frequently in cancer cell lines and in certain m alignant neoplasms. Purpose: We evaluated the frequency of p16 and p15 gene alterations in a well-characterized cohort of human transitional cell bladder cancers, and we explored potential associations between alterations in these genes and tumor stage and/or grade. Methods: Tumo r tissue and normal tissue from 110 patients with transitional cell ca rcinoma of the urinary bladder were examined. The status of the p16 an d p15 genes in these tissues was determined by Southern blotting and h ybridization with gene-specific probes, by coupled polymerase chain re action and single-strand conformation polymorphism analysis (PCR-SSCP) , and by sequencing DNA fragments produced during PCR. Associations be tween alterations in the genes and tumor stage and/or grade were evalu ated using the two-tailed Fisher's exact test. Results: Homozygous del etion (both alleles lost) of the p16 and the p15 genes was observed in 11 and nine bladder tumors, respectively. Eight of the 11 tumors exhi biting complete loss of the p16 gene also displayed homozygous deletio n of the p15 gene. Exclusive loss of either gene was detected in only three tumors. Hemizygous deletion (one allele lost, also referred to a s loss of heterozygosity [LOH]) of the p16 and/or p15 genes was observ ed in eight tumors. Rearrangement of the two genes was indicated in th ree additional tumors. No point mutations were identified in either ge ne. The overall frequency of alteration in this cohort of bladder tumo rs was approximately 18% for each gene (in 20 [18.3%, 95% confidence i nterval {CI} = 11.1%-25.6%] of 109 informative tumors for the p16 gene and in 18 [18%, 95% CI = 10.5%-25.5%] of 100 informative tumors for t he p15 gene). A statistically significant association between p16 gene alteration and bladder tumors of low stage (P<.01) and grade (P<.01) was observed; a significant association between p15 gene alteration an d tumors of low stage (P<.01) was also detected. Conclusions: Alterati on of the p16 and p15 genes, especially coincident homozygous deletion , appears to be a common event in bladder cancer.