Morules with optically clear nuclei in ovarian borderline endometrioid tumor

Optically clear nuclei (OCN) have been observed in morules of some neoplasms and in some conditions unrelated to the development of the morules. We first report a case of ovarian borderline endometrioid tumor (BET) showing the morules associated with OCN. The patient was a 47-year-old premenopausal woman with a left ovarian cystic tumor, atypical endometrial hyperplasia, and elevated serum levels of FSH, LH, estradiol, and CA 125. The resected ovarian tumor measured 6 cm in diameter, and showed a papillary growth. Histologically, the ovarian tumor was consistent with BET, and the morules with OCN were scattered. Immunohistochemically, OCN were proven to be rich in biotin. An aberrant nuclear expression of beta-catenin was observed in both the tumor cells and the morular cells. Our case may suggest the possibility that the appearance of OCN with or without morules in ovarian tumors is related to endometrioid differentiation of the tumor cells, and should be recognized as a diagnostic clue of ovarian endometrioid tumors. Although female sex hormones have been reported to play a role in the occurrence of OCN, the participation of beta-catenin mutation has also been suggested.     

Morules in cribriform-morular variant of papillary thyroid carcinoma: Immunohistochemical characteristics and distinction from squamous metaplasia

Morules are a diagnostic clue to the cribriform-morular variant (C-MV) of papillary thyroid carcinoma, and are superficially similar to squamous metaplasia. In order to clarify the histogenesis of morules and differentiate them from squamous metaplasia, we immunohistochemically compared the morules in five cases of C-MV with squamous metaplasia in six cases of diffuse sclerosing variant (DSV) of papillary thyroid carcinoma. The squamous metaplastic cells were immunopositive for low- and high-molecular-weight cytokeratin, whereas the morular cells were negative or focally positive. Vimentin-positive cells were observed focally in the morules and squamous metaplasia, except for one case of CMV that showed intense positivity. The morular cells showed weak cytoplasmic positivity for beta-catenin, and the cell membrane was not highlighted. Some nuclei of the morular cells were also positive for this antibody. Beta-catenin was intensively positive along the cell membrane of the metaplastic cells, and did not react against the nuclei or cytoplasm. Bcl-2 was positive in the morular cells, but negative in the metaplastic cells. S-100 protein-positive dendritic cells were observed in the metaplastic nests, but not in the morules. We argue that morules appear in connection with nuclear and cytoplasmic aberrant localization of beta-catenin, and are not an early form of squamous metaplasia.     

Aberrant intranuclear localization of biotin, biotin-binding enzymes, and beta-catenin in pregnancy-related endometrium and morule-associated neoplastic lesions.

Biotin-rich intranuclear inclusions, also known as optically clear nuclei, have been observed in various neoplastic and non-neoplastic lesions. They look like nuclei of herpesvirus-infected cells and cause a false-positive immunohistochemical result by avidin-biotin-complex (ABC) method. In the literature, all types of neoplastic lesions with intranuclear inclusions, with one exception, have been characteristically associated with squamoid structures known as morules. By contrast, all reported non-neoplastic lesions with such inclusions lacked morules and were confined to pregnancy-related endometrium. In the present study, adding unreported types of morule-associated neoplastic lesions, we investigated the distribution of biotin, biotin-binding enzymes, and beta-catenin in these lesions by immunohistochemical staining. We detected the intranuclear localization of biotin and of two mitochondrial biotin-binding enzymes (pyruvic acid carboxylase and propionyl CoA carboxylase) in all lesions examined, regardless of whether they were neoplastic or non-neoplastic and irrespective of the presence or absence of morules. The intranuclear localization of beta-catenin was detected in all neoplastic lesions with morules and in ovarian endometrioid adenocarcinoma without morules, but not in non-neoplastic endometrial lesions. These results suggest the following conclusions: (1) lesions with biotin-rich intranuclear inclusions can be classified as non-neoplastic/pregnancy-related endometrial and as neoplastic/pregnancy-unrelated or morular category; (2) the intranuclear biotin in both types of lesion is found in conjunction with biotin-binding enzymes. However, the role of beta-catenin in morule-associated neoplastic lesions, the relationship between beta-catenin and biotin/biotin-binding enzymes, and the mechanism of migration of biotin and biotin-binding enzymes from the cytoplasm to the nucleus remain unclear.     

Aberrant Cdx2 expression in endometrial lesions with squamous differentiation: important role of Cdx2 in squamous morula formation

Only a few reports have described Cdx2 expression in endometrial lesions of the uterus. Our aim was to determine whether Cdx2 expression is related to squamous differentiation in endometrial lesions. Furthermore, we examined whether there is any correlation between Cdx2 and beta-catenin, a well-known marker of aberrant nuclear accumulation in endometrial squamous foci secondary to mutation. We performed immunohistochemical analysis of 225 cases (29 normal endometrium, 28 nonproliferative conditions, 21 polyps, 46 hyperplasias, and 101 endometrioid carcinomas) that included 72 cases (4 polyps, 16 hyperplasias, and 52 carcinomas) showing morular or keratinizing squamous differentiation (SD(+)). Normal endometrium and nonproliferative conditions showed no staining for Cdx2. Whereas there was a low rate of Cdx2 positivity in SD(-) polyps (5.9%) and hyperplasias (10%), all SD(+) lesions expressed Cdx2 (P < .001). Thirty-eight (73%) of the SD(+) carcinomas were positive for Cdx2, whereas only 6 SD(-) cases (14.0%) were positive (P < .001). Furthermore, the larger the number of squamous foci, the greater the number of Cdx2-positive cells that was found. The labeling indices of Cdx2 were significantly higher in morular components than in keratinizing or glandular ones (P < .001). There was a strong correlation of the labeling indices of Cdx2 and beta-catenin in squamous foci of hyperplasias and carcinomas. Using immunofluorescence, we confirmed the coexpression of the 2 markers. The Cdx2 protein is expressed frequently in endometrial lesions with squamous differentiation, especially morular-type differentiation, and correlates strongly with nuclear beta-catenin expression. These facts suggest that Cdx2 plays an important role in squamous morula formation.     

Shadow cell differentiation from squamoid morule in endometrial adenoacanthoma

Shadow cell differentiation (SCD), commonly found in cutaneous pilomatricoma (PMX), has been said to be extremely rare in extracutaneous tumors and its morphogenesis has not been clarified yet. In the present study, 25 cases of endometrial adenoacanthoma were examined with special reference to SCD and with immunohistochemistry for beta-catenin and CD10. Shadow cell nests (SCNs) were observed in 2 out of 5 cases of adenocarcinoma with squamoid morules and all of 4 cases of adenocarcinoma with squamous differentiation and morules, but not in any cases of adenocarcinoma with squamous differentiation. SCNs were just adjacent to morules with or without a mutual transition. Immunohistochemical examination revealed nuclear accumulation of beta-catenin and expression of CD10 in the squamoid morules around SCNs. These results indicate that SCNs are derived from squamoid morules in endometrial adenoacanthoma, and established a link between matrical basaloid cells in PMX and squamoid morules in endometrial adenoacanthoma, as common original tissues, showing nuclear accumulation of beta-catenin and expression of CD10, of SCNs. It seems that SCD is not so uncommon as previously estimated in endometrial adenoacanthoma.     

Aberrant nuclear localization and gene mutation of beta-catenin in low-grade adenocarcinoma of fetal lung type: up-regulation of the Wnt signaling pathway may be a common denominator for the development of tumors that form morules.

The salient histopathologic features of low-grade adenocarcinoma of the fetal lung type (L-FLAC)/well-differentiated fetal adenocarcinoma (WDFA) include complex glandular structures and morules with biotin-rich optically clear nuclei. Interestingly, these characteristic features are shared by the cribriform-morular variant of papillary thyroid carcinoma, whose morphology is identical to that of familial adenomatous polyposis (FAP)-associated thyroid carcinoma. Furthermore, the single reported case of lung cancer associated with FAP was L-FLAC/WDFA. These observations lead us to hypothesize that up-regulation of the Wnt signaling pathway underlies the development of L-FLAC/WDFA. To verify this hypothesis, 11 cases of L-FLAC/WDFA, including the one FAP-associated case, eight cases of high-grade adenocarcinoma of the fetal lung type (H-FLAC), 24 cases of conventional pulmonary adenocarcinoma (CAC), and 13 fetal lungs were immunostained for beta-catenin. All cases of L-FLAC/WDFA showed predominantly aberrant nuclear/cytoplasmic expression, especially in budding glands and morules, whereas six of eight cases (75%) of H-FLAC and all but one case (96%) of CAC showed predominantly membranous expression. Fetal lungs showed nuclear/cytoplasmic expression restricted to the distal branching airway epithelium. Mutational analysis of exon 3 of the beta-catenin gene in five sporadic cases of L-FLAC/WDFA showed a point mutation at codon 34 and codon 37 in two cases, respectively. The present study indicates that up-regulating disturbances in the Wnt signaling pathway, including mutation of the beta-catenin gene, underlie tumorigenesis of L-FLAC/WDFA. The expression pattern of beta-catenin in L-FLAC/WDFA resembles that of the developing fetal lung airway. With the expression pattern of beta-catenin as a marker, most cases of H-FLAC as well as CAC appear to have different oncogenic pathways from cases of L-FLAC/WDFA. The present study together with other available data also suggests that abnormal up-regulation of the Wnt signaling pathway may be a common denominator for the development of tumors with morular formation from a variety of anatomic sites.     

beta-Catenin expression pattern, beta-catenin gene mutations, and microsatellite instability in endometrioid ovarian carcinomas and synchronous endometrial carcinomas.

beta-Catenin gene mutations and microsatellite instability (MI) have been reported in endometrioid ovarian carcinomas. In colon but not endometrial cancer, beta-catenin gene mutations are associated with a replication error phenotype and MI. In this study the authors investigate whether beta-catenin mutations and MI are two independent oncogenic pathways in endometrioid ovarian carcinomas. They also evaluate the usefulness of these molecular markers in determining the primary origin of simultaneous tumors in the ovary and endometrium. This study was performed on 26 patients diagnosed with primary endometrioid ovarian carcinoma, five of whom also had pathologically diagnosed primary synchronous endometrioid endometrial carcinoma. Immunohistochemical and molecular analyses indicated that there were 25 primary ovarian tumors with four primary synchronous endometrial cancers and one ovarian metastasis of a primary endometrial carcinoma. All studies were performed on formalin-fixed, paraffin-embedded tissue samples. The beta-catenin expression pattern (nuclear vs. membranous) was analyzed immunohistochemically. Mutations in exon 3 of the beta-catenin gene were studied by polymerase chain reaction, single-strand conformational polymorphism, and direct sequencing. MI status was established by studying BAT-26 and BAT-25 mononucleotide repeats. In the group with 21 single ovarian tumors, 18 (85%) had beta-catenin nuclear expression, eight (38%) had beta-catenin gene mutations (always associated with beta-catenin nuclear expression), and four (19%) had MI. Only one case (5%) had both beta-catenin gene mutations and MI. The mutations affected one of the serine/threonine residues targeted for phosphorylation by glycogen synthase kinase-3beta or adjacent residues. At codon 32, a GAC-to-TAC (D32Y) change was found; at codon 33, two TCT-to-TGT (S33C) changes were found; at codon 37, three TCT-to-TTT (S37F) changes and one TCT-to-TGT (S37C) change were found; and, lastly, one ACC-to-GCC change at codon 41 (T41A) was detected. Four of the 25 endometrioid ovarian carcinomas (16%) had an associated synchronous endometrial carcinoma. There was a higher percentage of beta-catenin mutations (n = 3, 75%) in synchronous ovarian carcinomas than in single ones, although with a similar percentage of MI (n = 1, 25%). beta-catenin mutations were S37C in two cases and D32G in one. One of the four endometrial carcinomas showed an S33C beta-catenin mutation, and two carcinomas had MI. None of the four tumors had both beta-catenin gene mutation and MI. beta-catenin gene mutations were always associated with a nuclear beta-catenin expression pattern, whereas MI was associated with a membranous pattern. In one patient both the ovarian and the endometrial carcinomas had beta-catenin gene mutations, in another patient both tumors showed MI, whereas in the remaining two patients the ovarian carcinomas showed beta-catenin gene mutations and the endometrial carcinomas showed MI. To summarize, the results of this study suggest that beta-catenin mutations and MI could represent two independent pathways in endometrioid ovarian carcinomas because they occur simultaneously very infrequently (in 5% of these cases). beta-catenin mutations are always associated with a nuclear beta-catenin expression pattern, whereas cases with a replication error -plus phenotype showed no abnormal beta-catenin subcellular localization. The study of the beta-catenin expression pattern, beta-catenin mutations, and MI, together with conventional clinicopathologic findings, could aid in distinguishing between the metastatic or independent origin of simultaneous endometrioid ovarian and endometrial carcinomas. Tumors with identical immunohistochemical and molecular features should therefore be considered to have a common origin.     

Beta-catenin and E-cadherin expression patterns in high-grade endometrial carcinoma are associated with histological subtype.

Both beta-catenin and E-cadherin are epithelial cell adhesion molecules. In addition, beta-catenin is an important element of the Wnt signal transduction pathway, which has been implicated in embryogenesis and carcinogenesis, including the development of endometrial and ovarian endometrioid carcinomas. We hypothesized that the expression pattern of these two adhesion molecules may depend upon the histological subtype of endometrial carcinomas. Therefore, we compared the immunohistochemical expression of beta-catenin and E-cadherin in a set of uterine adenocarcinomas matched for high histologic grade, that is, poorly differentiated (International Federation of Gynecology and Obstetrics [FIGO] Grade III) uterine endometrioid carcinomas and uterine serous carcinomas. Seventeen FIGO Grade III endometrioid adenocarcinomas and 17 serous carcinomas were evaluated histologically and immunohistochemically with commercially available monoclonal antibodies against beta-catenin and E-cadherin. Nuclear expression of beta-catenin was observed in 8 of 17 (47%) endometrioid adenocarcinomas but in none of the serous carcinomas (P = .003). Moderate or strong E-cadherin expression was identified in 7 of 17 (41%) serous carcinomas as opposed to in only 1 of 17 (6%) endometrioid adenocarcinomas (P = .02). The majority of endometrioid adenocarcinomas showed strong beta-catenin expression coupled with weak E-cadherin expression; serous carcinomas did not exhibit a comparable trend. Our results indicate that the expression of beta-catenin and E-cadherin in high-grade endometrial cancers is strongly associated with histological subtype. These data provide further support for the distinct molecular profiles of endometrioid adenocarcinoma and serous carcinoma. Notably, differences in cell adhesion molecule expression could account for variations in patterns of tumor dissemination. The immunohistochemical staining pattern may also be useful for diagnostic purposes.     

Oestrogen receptor alpha and beta mRNA expression in human endometrium throughout the menstrual cycle.

We examined the localization of oestrogen receptor (ER) beta mRNA in the human endometrium throughout the menstrual cycle using non-radioactive in-situ hybridization with Brigati-tailed oligonucleotides. The findings were compared with those of ERalpha in order to examine the possible biological significance of ERbeta in the human endometrium. Both ERalpha and ERbeta mRNA expression were detected in all major human uterine cell types, including glandular epithelial cells, stromal cells and smooth muscle cells of the uterine wall, at every menstrual cycle stage. However, ERalpha mRNA expression was more prominent than that of ERbeta in all cell types throughout the menstrual cycle. In proliferative phase endometrium, ERalpha mRNA was expressed in both glandular epithelial and stromal cells, while ERbeta mRNA was expressed predominantly in glandular epithelial cells. Although the same pattern was observed in the secretory phase, both the ERalpha and ERbeta mRNA expression was relatively weaker. These results suggest that oestrogenic effects occur predominantly through ERalpha, but that ERbeta may also play a role in the modulation of oestrogenic action, especially on glandular epithelial cells in the human endometrium throughout the menstrual cycle.     

Markers of proliferative activity are predictors of patient outcome for low-grade endometrioid adenocarcinoma but not papillary serous carcinoma of endometrium.

On the basis of pathogenesis, two types of endometrial cancer can be recognized. Type 1 endometrial carcinomas are relatively indolent tumors that develop after prolonged estrogen stimulation, on a background of endometrial hyperplasia. Type 2 endometrial carcinomas are aggressive tumors that are not associated with hyperplasia or estrogen excess. The aim of this study is to evaluate the prognostic significance of tumor proliferative activity in early-stage endometrial cancer by using mitotic index and immunostaining, comparing Type 1 (endometrioid) and Type 2 (papillary serous carcinoma) tumors. The mitotic index, MIB-1, and p53 immunostaining in 39 tumors from patients with low-grade Stage Ia or Ib endometrioid adenocarcinoma; as well as 23 tumors from patients with Stage I papillary serous carcinoma. In low-grade endometrioid adenocarcinoma, mitotic and MIB-1 indices were statistically significant independent prognostic indicators (P =.004 and P =.018, respectively), and both were strongly correlated with p53 expression (P =.01 and P =.006, respectively). The mean mitotic index was 5 mitoses/10 high-power fields, and mean MIB-1 index was 27.5%. There was no significant correlation between mitotic or MIB-1 indices and patient outcome or p53 expression in papillary serous carcinoma. The mean mitotic index was 31 mitoses/10 high-power fields, and mean MIB-1 index was 30.5% in these tumors. p53 expression and proliferative indices are strongly correlated in low-grade endometrioid adenocarcinoma. MIB-1 and mitotic indices are independent prognostic indicators in these tumors. Papillary serous carcinoma of endometrium is rapidly proliferative in tumors even at an early stage, and quantification of proliferative activity in these tumors does not allow prediction of patient outcome.     

Expression of wild-type estrogen receptor beta protein in human breast cancer: specific correlation with HER2/neu overexpression

Expression of estrogen receptor beta (ERbeta) protein in human breast cancer and correlation with clinicopathological factors have been reported by many investigators, but many of them used ERbeta antibodies that react with both wild-type ERbeta (ERbetawt) and splicing variant isoform. Therefore, the frequency and correlation with clinicopathological factors of ERbetawt expression remain to be established. In the present study a monoclonal antibody EMR02, specific for ERbetawt, was used in formalin-fixed paraffin-embedded sections from 225 female primary breast cancer patients diagnosed as having invasive ductal carcinoma. Expression of ERalpha, progesterone receptor (PgR) and HER2/neu were also investigated by immunohistochemistry. For ERbetawt, ERalpha and PgR, positivity was defined as nuclear staining in >10% of the cancer cells. HER2/neu overexpression was defined as a Hercep test score 3+. Positivity for ERbetawt, ERalpha, PgR and HER2/neu overexpression was 55%, 74%, 61% and 25%, respectively. The expression of ERbetawt had a positive correlation with ERalpha (P=0.018) and PgR (P=0.02). There was significant positive correlation between ERbetawt expression and HER2/neu overexpression (P<0.0001). According to multivariate logistic regression analysis the most significant association was between ERbetawt expression and HER2/neu overexpression (P<0.0001). These results suggest that clinical significances of ERbetawt expression in human breast cancer patients may be more complex.     

Expression of metastasis-associated protein 1 (MTA1) in benign endometrium and endometrial adenocarcinomas

Endometrial carcinoma is one of the most common malignancies of the female genital tract. Metastasis-associated protein 1 (MTA1) is a component of the Mi-2/nucleosome remodeling and deacetylating complex and acts as a potent corepressor of estrogen receptor in breast cancer cells. MTA1 expression has been demonstrated in various cancers but has never been explored in endometrial carcinoma. We investigated the expression profile of MTA1 in different stages of benign endometrium as well as in endometrial endometrioid adenocarcinoma using immunohistochemistry and Western blotting. In the proliferative and secretory phases, MTA1 was expressed in both the glandular and the stromal compartments and was localized in nucleus and cytoplasm of these cells. MTA1 expression in secretory phase was less prominent when compared with the proliferative phase. In postmenopausal sections, MTA1 staining was observed in both glandular and stromal compartments and was localized in both nucleus and cytoplasm. Western blot analysis of 6 tumor specimens showed increased expression of MTA1 in all the tumors analyzed. Immunohistochemical staining performed on tumor microarray containing 70 endometrial endometrioid adenocarcinomas of various grades showed increased expression of MTA1 in 53 (75.7%) tumors. In grade 1 and grade 2 tumors, MTA1 was present in both nucleus and cytoplasm. Interestingly, in grade 3 tumors, MTA1 was localized in the cytoplasm only. Our results suggest a potential role of MTA1 in endometrial carcinomas.     

β-Catenin Expression Pattern in Stage I and II Ovarian Carcinomas : Relationship with β-Catenin Gene Mutations, Clinicopathological Features, and Clinical Outcome

The immunohistochemical expression pattern of beta-catenin has been correlated with beta-catenin gene mutations, clinicopathological features, and disease outcome in 69 stage I and II ovarian carcinomas. beta-Catenin expression was localized in the nuclei, in addition to the cytoplasm and membrane, in 11 tumors (16%): nine endometrioid carcinomas with widespread nuclear expression and two serous carcinomas with focal nuclear expression. The remaining 58 carcinomas (84%) only had membranous beta-catenin expression. All but one of the endometrioid carcinomas with nuclear beta-catenin expression had considerable squamous metaplasia, and five of these cases had large areas of endometrioid tumor of low malignant potential. In addition, beta-catenin nuclear expression was observed in atypical epithelial cells in endometriotic glands adjacent to an endometrioid carcinoma. Sequencing was performed on 25 tumors and corresponding normal tissue: all 13 endometrioid tumors as well as 12 carcinomas of other histological types (four serous, two clear cell, two mucinous, and two mixed). There were oncogenic mutations in the phosphorylation sequence for GSK-3beta in exon 3 of the beta-catenin gene in seven endometrioid carcinomas with beta-catenin nuclear expression. Three mutations affected codon 32 (D32G, D32Y, and D32Y), one affected codon 33 (S33C), two affected codon 37 (S37C and S37F), and one affected codon 41 (T41A). No mutations were observed in the other 18 carcinomas analyzed, comprising two endometrioid and two serous carcinomas with beta-catenin nuclear expression, and 14 carcinomas of different histological types with only membranous expression. In the univariate and multivariate survival analyses, beta-catenin nuclear expression was selected as an indicator of good prognosis, because no patient whose tumor expressed beta-catenin in the nuclei showed relapses or died, in contrast to the 19 relapses and deaths among patients with tumors that only had beta-catenin membranous expression, including three of the four patients with endometrioid carcinomas. Oncogenic beta-catenin mutation is characteristic of a group of endometrioid carcinomas with a good prognosis, most of which originate from previous benign or borderline lesions. Endometrioid carcinomas with exclusively membranous expression of beta-catenin seem to represent a different subgroup of carcinomas that probably have a worse prognosis. In early-stage ovarian cancer, determination of the beta-catenin expression pattern could prove to be a useful marker for selecting low-risk patients.     

β- And γ-catenin expression in endometrial carcinoma. Relationship with clinicopathological features and microsatellite instability

The activation of the adenomatous polyposis coli (APC)/beta-catenin/T-cell factor (Tcf) pathway due to beta-catenin gene mutation has been recently implicated in the development of some endometrial carcinomas. beta- and gamma-catenin are structurally and functionally related molecules that participate in cell adhesion and signal transduction. Nuclear accumulation of beta- and gamma-catenin have been related to the activation of the APC/beta-catenin/Tcf pathway. In this study, we investigate the immunohistochemical expression pattern (nuclear vs membranous) of beta- and gamma-catenin in 40 endometrial carcinomas and their correlation with clinicopathological features and microsatellite instability (MI) status. MI was detected at three or more loci in 12 tumors: 11 were endometrioid and one was non-endometrioid. Nuclear catenin expression was found in 13 carcinomas: ten carcinomas had nuclear beta-catenin expression and three carcinomas had nuclear gamma-catenin expression. The nuclear catenin expression pattern significantly correlated with the histological type, International Federation of Gynecology and Obstetrics (FIGO) grade, and the presence of a second neoplasm. Nuclear catenin expression was always observed in low-grade endometrioid carcinomas; it was also more frequently associated with a second carcinoma. No correlation was observed between the catenin expression pattern and the level of myometrial infiltration, stage, associated endometrial hyperplasia, the existence of a source of estrogenic stimulation, and MI. However, four of 13 endometrioid carcinomas in this series had both catenin nuclear expression and MI. These data suggest that at least two different neoplastic pathways can lead to endometrial carcinomas with an endometrioid phenotype. In one, MI would be a key event, while in the other, the APC/beta-catenin/Tcf signaling pathways could be activated. Probably, in some cases, both pathways could simultaneously occur.     

Molecular pathology of endometrial hyperplasia and carcinoma.

Four different genetic abnormalities may occur in endometrioid adenocarcinomas of the endometrium (mircosatellite instability and mutations in the PTEN, k-RAS and beta-catenin genes), whereas nonendometrioid carcinomas of the endometrium often have p53 mutations and loss of heterozygosity on several chromosomes. Occasionally, a nonendometrioid carcinoma may develop as a result of dedifferentiation of a preexisting endometrioid carcinoma; in such a case, the tumor exhibits overlapping clinical, morphologic, immunohistochemical, and molecular features of the 2 types. The insaturation of microsatellite instability in endometrial carcinogenesis seems to occur late in the transition from complex hyperplasia to carcinoma, and it is preceded by progressive inactivation of MLH-1 by promoter hypermethylation. Moreover, the endometrioid adenocarcinomas that exhibit microsatellite instability show a stepwise progressive accumulation of secondary mutations in oncogenes and tumor suppressor genes that contain short-tandem repeats in their coding sequences. Mutations in the PTEN and k-RAS genes are also frequent in endometrioid adenocarcinomas of the endometrium, particularly in the tumors that exhibit microsatellite instability, whereas beta-catenin mutations do not seem to be associated with such a phenomenon.