POTENTIAL ROLE OF THE INACTIVATED X-CHROMOSOME IN OVARIAN EPITHELIAL TUMOR-DEVELOPMENT

Background: Ovarian epithelial tumors can be divided into subcategorie s often regarded as different stages of neoplastic transformation. Cys tadenomas belong to the least aggressive subgroup and are noninvasive and nonmetastatic. Ovarian tumors of low malignant potential (LMP) are intermediate between cystadenomas and carcinomas and show markedly re duced invasive and metastatic abilities. Invasion and metastasis are t he hallmarks of carcinomas, which constitute the most aggressive subgr oup and can be further subdivided into different grades. Purpose: We p erformed comparative allelotype analyses of ovarian cystadenomas, LMP tumors, and carcinomas, reasoning that such analyses could provide clu es about the molecular determinants of their phenotypic differences. B ecause we realized that allelic losses involving the X chromosome migh t be associated with LMP tumor development, we determined whether such losses were interstitial and whether they involved the active or the inactive X chromosome. Methods: Frequencies of loss of heterozygosity (LOH) at specific loci in every chromosomal arm were determined in 16 ovarian cystadenomas, 23 ovarian LMP tumors, 15 low-grade ovarian carc inomas, and 35 high-grade ovarian carcinomas by use of either the poly merase chain reaction (PCR) or Southern blot analyses. We took advanta ge of the fact that DNA methylation is an important mechanism of X-chr omosome inactivation to determine whether losses involving the X chrom osome were in the active or the inactive copy. We analyzed the methyla tion status of retained alleles on the X chromosome by determining whe ther they could be amplified by PCR after digestion with the methylati on-sensitive restriction endonuclease Hpa II. Results: High-grade carc inomas contained frequent (>50%) LOH in four autosomal chromosome arms , i.e., 6q, 13q, 17p, and 17q. Except for 13q, these same chromosomal arms showed frequent LOH in low-grade carcinomas. LOH in autosomal chr omosomes was comparatively rare in LMP tumors and was absent in cystad enomas. In contrast, half (eight of 16) of LMP tumors informative for a locus in the proximal portion of chromosome Xq showed LOH at that lo cus. These losses were the result of interstitial deletions in six of the eight cases and involved the inactive copy of the X chromosome exc lusively. Similar losses in the X chromosome were not seen in either c ystadenomas or low-grade carcinomas. Conclusions and Implications: LOH at multiple loci is associated with the development of ovarian carcin omas but not with the development of cystadenomas and LMP tumors. Howe ver, the integrity of a locus in chromosome Xq that possibly escapes X -chromosome inactivation is important for the control of LMP tumor dev elopment. The fact that this locus does not appear to be involved in t he genesis of low-grade carcinomas suggests that LMP tumors are not pr ecursors of such carcinomas.