Frequent loss of heterozygosity at 7q31.1 in primary prostate cancer is associated with tumor aggressiveness and progression.

Cytogenetic analyses have demonstrated that chromosome region 7q22-32 is commonly altered in prostate adenocarcinomas. In addition, in recent fluorescence in situ hybridization studies, we have observed that aneusomy of chromosome 7 is frequent in prostate cancer and is associated with higher tumor grade, advanced pathological stage, and early prostate cancer death. These findings suggest that genetic alterations of chromosome 7 play a significant role in the development of prostate cancer. To better define the chromosome 7 alterations, PCR analysis of 21 microsatellite loci was performed on 54 paired prostate cancer and control DNAs. Overall, chromosome 7 allelic imbalance was identified in 16 of 54 cases (30%). Allelic imbalances of loci mapped to 7q were observed in 15 of the 16 cases. The allelic imbalances were classified as losses in 15 tumors (28%) and as gains in 1 (2%) by comparative multiplex PCR analysis. The most common site of allelic loss included loci D7S523 and D7S486 at 7q31.1. A comparison with clinicopathological features of the tested tumors revealed that the allelic loss of 7q31.1 correlated with higher tumor grade (P = 0.012) and lymph node metastasis (P = 0.017). These results indicate that 7q31 may be the site of a putative suppressor gene(s) important for the pathogenesis of prostate carcinoma, and that the genetic alterations at 7q31.1 may participate in tumor progression and metastasis.

Evidence of functional RB protein in epithelial ovarian carcinomas despite loss of heterozygosity at the RB locus.

The presence of retinoblastoma (RB) protein was evaluated by immunohistochemical staining and correlated with loss of heterozygosity (LOH) at the RB locus in 52 primary epithelial ovarian carcinomas. Forty-eight tumors were informative at the RB locus by molecular genetic analysis. Twenty-five tumors (52%) showed loss of heterozygosity at the RB locus. RB protein expression was found in 23 of these tumors. The remaining two tumors were negative for RB protein product by immunohistochemical staining. All 23 tumors showing no LOH at the RB locus had a normal RB protein pattern. All but three tumors revealed either no LOH with any marker or, if LOH was found for one chromosome 13 marker, all other informative markers also showed LOH. The three recombinant tumors included two which retained alleles at one or more loci distal and one which retained alleles proximal to the RB locus. LOH at the RB locus was significantly more common in invasive high-grade (grades 3 and 4) tumors as compared to invasive low-grade (grades 1 and 2) tumors (P < 0.001). Our data suggest that while molecular genetic studies reveal frequent LOH at the RB locus, particularly in high-grade tumors, normal RB protein expression is present in the majority (96%) of these tumors. This implies that another, unidentified, gene or genes located on chromosome 13 may be important in the progression of most epithelial ovarian carcinomas. Additionally, it is likely that the specific chromosome 13 alteration(s) associated with sporadic ovarian neoplasms will be extremely difficult to identify using allelic loss and deletion mapping studies.

Comparison of loss of heterozygosity patterns in invasive low-grade and high-grade epithelial ovarian carcinomas.

Loss of heterozygosity (LOH) studies were performed to investigate the genetic differences which separate low-grade (LG), high-grade (HG), and borderline epithelial ovarian carcinomas. Fresh tumor samples and blood were obtained from 58 patients (20 LG, 34 HG, and 4 borderline tumor specimens) undergoing surgery for ovarian carcinoma at Mayo Clinic. Tumors were graded using a modified Broder's classification with invasive grades 1 and 2 considered LG, invasive grades 3 and 4 considered HG, and tumors with no evidence of stromal invasion classified as borderline. Polymorphism analysis was performed using 76 restriction fragment length polymorphisms and variable number of tandem repeats and 59 microsatellite markers representing all chromosome arms. Chromosome arms 6p, 17p, 17q, and 22q were found to be frequently lost in LG as well as HG tumors. Chromosome arms 13q and 15q were lost to a significantly greater extent in HG tumors compared to LG neoplasms (P = 0.003 and P = 0.08, respectively). Conversely, 3p loss was seen more frequently with LG tumors (P = 0.02). The majority of LG tumors (65%) did not show frequent LOH in the allelotype analysis. In fact, a subset of 7 (7 of 20) LG tumors accounted for 76% of the total allelic loss in the LG category. These tumors showed LOH almost identical to that of the HG neoplasms. Borderline tumors showed a low rate of allelic loss. There were no common events found between borderline and invasive tumors. Our data suggest that most HG tumors and a subset of LG tumors share genetic alterations at putative tumor suppressor genes detected by LOH studies. Chromosome 6 and 17 losses appear to be early events while 13q and 15q losses appear to be critical late events. However, a majority of LG tumors appear to develop as a consequence of an alternative mechanism(s) which is not detected by LOH studies. Possibilities include: (a) inactivation of tumor suppressor genes without LOH; (b) dominant negative gene(s) in which only one allele requires mutation; and (c) changes in dominant acting oncogenes. This unidentified phenomenon may be operative in borderline tumors as well.