Analysis of microsatellite instability and X-inactivation in ovarian borderline tumors lacking numerical abnormalities by comparative genomic hybridization.

The genetic events underlying development of ovarian borderline tumors (tumors of low malignant potential) are not well understood. In our previous studies of microdissected samples from serous borderline tumors, comparative genomic hybridization (CGH) and/or fluorescence in situ hybridization (FISH) analyses showed that 3 of 13 tumors had detectable numerical abnormalities; the remaining 10 had none. In the present study, we examined microsatellite instability (MSI) and clonality in this same set of tumors. Although absence of chromosomal imbalances has been associated with the presence of MSI in some types of solid tumors, the extent of MSI in borderline tumors, and its role in their pathogenesis, is unclear. In our set of 13 tumors, no MSI was detected despite analysis with microsatellite markers recommended by the National Cancer Institute for assessment of MSI. Quantitative X-inactivation studies were informative at the androgen receptor gene AR in 9 of the 13 tumors and revealed that each of the 9 tumors was clonal. In two patients, bilateral tumors showed identical patterns of skewed X-inactivation. These studies confirm the clonality of borderline tumors and suggest that some borderline tumors may develop through mechanisms other than chromosomal imbalances or microsatellite instability.