**4. Precancerous lesions in endometriosis**

Endometriosis itself is generally considered a benign disease; however, endometriosis shares certain features with cancer, including the ability of cells from different lineages (i.e., epithelial cells, stromal cells, and the vasculature) to proliferate in ectopic sites. Thus, earlier studies have focused on the clonal or malignant potential of endometriosis by analyzing the loss of heterozygosity (LOH) at several candidate tumor suppressor gene loci. Positive results, such as the detection of LOH at the p53, p16 or PTEN gene, were observed in the majority of the endometriosis samples ((Jiang, 1996; Jiang, 1998; Sato, 2000), for review of other studies with similar results, see (Prowse, 2005)). Another approach, which assesses the clonality of endometriosis samples by analyzing methylation-related marker genes, also demonstrated the clonal nature of endometriosis (Jimbo, 1997). The findings, together with the LOH analysis, led to the conclusion that endometriosis was a neoplasm that may even have malignant potential. However, recent studies deny the malignant or neoplastic potential of endometriosis, demonstrating that most endometriosis tissues are not monoclonal (Mayr, 2003). Furthermore, neither LOH of

Endometriosis-Associated Ovarian Cancer: The Role of Oxidative Stress 315

extraovarian and ovarian tumors, respectively. Adenosarcoma and endometrial stromal sarcoma are the major histological types of sarcomas (Baiocchi, 1990; Heaps, 1990; Slavin, 2000). At least partially, differences in the incidences of tumor types (carcinoma versus sarcoma) depend on the tumor site, and further studies are needed to elucidate this mechanism. Other rare malignant tumors, such as squamous cell carcinoma, malignant mesodermal mixed tumor, and yolk sac tumor, are also reported to develop from endometriosis (Irving, 2011). Although its incidence is very low compared with endometrioid adenocarcinoma or clear cell carcinoma, serous adenocarcinoma has also been associated with endometriosis (Fukunaga, 1997; Modesitt, 2002; Yoshikawa, 2000). Much more rarely, mucinous carcinomas with unusual morphology resembling Mullerian mucinous borderline tumors have also been reported in association with endometriosis

Fig. 2. A. Clear cell carcinoma (left) arising in a endometriotic cyst. B. Hemosiderin

Genetic mutations specifically associated with ovarian cancer subtypes have been reported (reviewed by (Kurman&Shih, 2011)). Focusing on endometrioid adenocarcinomas, genetic mutations of K-ras, p53, PTEN, beta-catenin, and ATR have been reported ( Mizuuchi, 1992; Milner, 1993; Palacios&Gamallo, 1998; Tashiro, 1997; Zighelboim, 2009). Mouse models of endometrioid adenocarcinoma have been reported, either with oncogenic K-ras and conditional PTEN deletion (Dinulescu, 2005) or dysfunction of both the Wnt/beta-catenin and PI3CA/PTEN pathways (Wu, 2007). However, specific genetic alterations of clear cell carcinoma were mostly unknown. Recently, a frequently activated mutation of the PI3CA gene was observed in clear cell carcinoma samples (Kuo, 2009). Most recently, several studies based on novel sequencing technology have elucidated that a significant proportion of clear cell carcinomas harbor a mutation of the ARID1A gene, which encodes the chromatin-remodeling complex protein BAF250A (Jones, 2010; Wiegand, 2010). ARID1A mutation and the consequent loss of BAF250A expression were found not only in clear cell carcinoma samples, but also in endometrioid adenocarcinomas, especially high-grade types

**6. Genetic abnormalities and phenotypes of endometriosis-associated** 

deposition (arrows) is observed in the stroma of clear cell carcinoma.

(Lee&Nucci, 2003).

**ovarian cancer** 

tumor suppressor genes, promoter methylation of oncogenes, nor oncogenic mutations of known tumor-related genes was frequently observed in the majority of the cases, further denying the neoplastic theory (Prowse, 2005; Vestergaard AL, 2011). In contrast with these results, a third approach (fluorescent in situ hybridization [FISH]) used to investigate chromosomal aberrations in endometriosis samples revealed a significantly elevated proportion of aneusomic (monosomic > trisomic) cells in endometriosis in multiple groups (Koerner, 2006) (Bischoff, 2002). However, both endometriosis tissue and normal endometrium also contain a certain proportion of aneusomic cells (Koerner, 2006), and telomerase expression, telomere elongation, higher expression of DNA replication markers and lower expression of DNA damage response markers are all observed in endometriosis tissue, but not in normal endometrium (Hapangama, 2008; Hapangama, 2009). Thus, it may be reasonable to conclude that although endometriosis is generally considered non-neoplastic, the relative rates of abnormal cells are higher in endometriosis than in normal endometrium.

In this case, then, which cells are premalignant? Is there a focal area representing the precancerous state of endometriosis that is morphologically distinguishable from other, presumably benign, areas? "Atypical endometriosis" is the term used to describe this state, which has been found in cases of extraovarian and ovarian cancer as atypical epithelium showing hyperchromatism and stratification continuous with the malignant tumor (Brooks&Wheeler, 1977; Lagrenade&Silverberg, 1988). Fukunaga et al. found atypical endometriosis in 61% of endometriosis-associated ovarian cancers, in contrast with 1.7% of benign endometriosis samples (Fukunaga, 1997). Immunohistochemical markers distinguishing atypical endometriosis from benign endometriosis have not been fully established, but staining patterns of Ki67, Bcl-2, and p53 have been reported as useful markers (Nezhat, 2002; Ogawa, 2000). Extraovarian endometriosis may also show atypical changes. Hyperplastic changes, including atypical hyperplasia and malignant changes, were observed in more than half of the adenomyosis cases associated with endometrioid adenocarcinoma arising from the endometrium (Jacques&Lawrence, 1990; Kucera, 2011), and histologically atypical hyperplasia has been reported in some cases of gastrointestinal endometriosis (Yantiss, 2000).
