Endometrial Carcinoma: Pathology and Genetics
Moderator: Dr. Michael A. Wells
Section 2 -
Molecular Pathology of Endometrioid Carcinoma
Hospital de la Santa Creu i Sant Pau
Autonomous University of Barcelona Medical School
In the Western World, endometrial cancer is the most common malignant tumor of the female genital
tract. After an increase in the 70s that resulted from the unrestricted use of estrogen replacement
therapy in postmenopausal women, the incidence rates became stable over the last two decades (10-20 per
100,000 person-years). Recently, the progressive use of tamoxifen - a non-steroidal estrogen agonist and
antagonist - for the treatment of breast cancer has been associated with increased risk of endometrial
cancer but there is not complete agreement among different studies. In this presentation, current
knowledge about molecular pathology of endometrioid carcinomas and their precursors will be discussed.
Two Types of Endometrial Carcinoma
Over the past two decades, the tendency has been to classify endometrial carcinoma (EC) 
into two different types: Type I tumors (about 80%) are endometrioid carcinomas, often preceded by
complex and atypical hyperplasia and associated with estrogenic stimulation. They occur predominantly in
pre or perimenopausal women and are associated with obesity, hyperlipidemia, anovulation, infertility,
and late menopause. Typically, most endometrioid carcinomas are limited to the uterus and follow a
favorable course. In contrast, type II tumors (about 10%) are non-endometrioid
(largely papillary serous) carcinomas, arising occasionally in endometrial polyps or from
precancerous lesions that develop in atrophic endometria (endometrial "intraepithelial"
carcinoma) . Type II tumors are not associated with estrogen stimulation or hyperplasia,
readily invade the myometrium and vascular spaces, and carry a high mortality rate. It has also been
found that the molecular alterations involved in the development of endometrioid (type I) carcinomas are
different from those of the non-endometrioid (type II) carcinomas
A dualistic model of endometrial carcinogenesis has been proposed.
According to this
model, normal endometrial cells would transform into endometrioid carcinomas (EEC) through replication
errors, so-called "microsatellite instability" (MI), and subsequent accumulation of mutations in
oncogenes and tumor supressor genes, whereas alterations of p53 
and loss of heterozygosity (LOH) on several chromosomes would drive the process of neoplastic
transformation into the acquisition of a non-endometrioid carcinoma (NEEC) phenotype
There are several evidences in favor of this pathogenetic proposal; the majority of low grade EEC
express estrogen receptors but not p53, and 25 to 30% of them exhibit MI. In contrast, most NEEC are
negative or only weakly immunoreactive for estrogen receptors, strongly positive for p53 immunostaining,
and do not exhibit MI .
Although the dualistic model appears applicable to paradigmatic cases at both clinicopathological and
molecular levels, exceptions occur. After all, there is a great overlapping in the clinical,
pathological, immunohistochemical and molecular characteristics of the tumors. For instance, it has been
shown that occasionally, NEEC may develop from preexisting EEC as a result of tumor progression.
Obviously, these tumors may share the pathologic and molecular features of types I and II EC .
Molecular Alterations of Endometrioid Carcinomas of the Endometrium (EEC)
Four main molecular alterations have been described in EEC: MI (25-30% of the cases)
, PTEN mutations (30-60%)
k-RAS mutations (10-30%)
and beta-catenin (CTNNB1) mutations with nuclear protein accumulation (25-38%) .
Although MI and PTEN or k-RAS mutations may
coexist in many cases, these three molecular alterations are not usually associated with beta-catenin
Microsatellite Instability (MI)
MI was initially noted in colorectal cancers of patients with the hereditary non-polyposis colon
cancer syndrome (HNPCC), but also in some sporadic colon cancers. EC is the second most common tumor
found in HNPCC patients. MI has been demonstrated in 75% of EC associated with HNPCC, but also in 25-30%
of sporadic EC
MI occurs more frequently in EEC than in NEEC. To give further support to the hypothetical dualistic
model of endometrial carcinogenesis, we investigated the presence of MI in 42 sporadic EC .
The results of our study supported the concept that MI is a common genetic abnormality in EC (28%), and
appears to be more frequent in EEC (33%) than in NEEC (11%). However, the occasional detection of MI in
NEEC, the lack of an inverse correlation between p53 and MI, and the frequent existence of tumors
exhibiting mixed pathologic, immunohistochemical, and molecular features of EEC and NEEC, indicate that
individual tumors do not invariably follow the so-called dualistic model of endometrial
Molecular Consequences of MI in Endometrioid Carcinomas
It has been demonstrated that cancers showing MI, the so-called mutator phenotype, have mismatch
repair deficiencies that result in the accumulations of mutations in repeated sequences in the coding
mononucleotide repeats of some particular oncogenes and tumor suppressor genes. Transforming growth
factor beta receptor type II (TGF- b RII),
BAX, insulin-like growth factor II receptor (IGF
RII), hMSH3, and hMSH6 are all putative
targets of such phenomenon. These mutations are interpreted as secondary events in the mutator phenotype
pathway in cancers with MI
Our results indicate that frameshift mutations occurring at coding mononucleotide repeats in these
putative target genes are quite frequent in MI positive EC; mutations in one or more of these
microsatellites were detected in 16 of the 24 tumors (66.6%) . An interesting result of our
study was the fact that the mutations were heterogeneously distributed in the tumors; they were found in
some tumor areas but not in others. The heterogeneous distribution of the mutations suggests that they
may be involved in tumor progression. The advantage of growth provided by each specific combination of
mutations in a particular area of the neoplasia could lead to its overgrowth in comparison with other
In a previous report, we suggested that BAX frameshift mutations could
play an important role in the progression of EC with MI . This speculation was based on the
hypothesis that the presence of inactivating BAX mutations in tumors would
explain the low frequency of p53 mutations in the neoplasias associated with MI, by relieving the
selective pressure for p53 mutations during tumor progression. In the presence of BAX mutations, p53 mutations would not be necessary to inhibit BAX transactivation. To give further support to such hypothesis, we compared the
pattern of mutations in the primary EC and their lymph node metastases . Interestingly, in
two cases BAX mutations were found in the primary EC but not in their lymph
node metastases, suggesting that the tumor subclones that exhibited BAX
mutations were not responsible for the dissemination of the neoplasm. In contrast, IGFIIR frameshift mutations were detected in three metastatic tumors, but only one
of them also had the mutation in the corresponding primary neoplasm . The frequent finding
of these mutations in the metastatic tumors gives support to the hypothesis that IGFIIR mutations are related to tumor progression in EC with MI .
MI is Secondary to DNA Altered Methylation
As mentioned above, MI was initially found in colorectal carcinomas from patients with the hereditary
non-polyposis colon cancer, but also in some sporadic colon cancers. In these patients, germline and
somatic mutations in the MSH2 and MLH1 genes
have been detected in chromosomes 2p and 3p . However, the frequency of mismatch repair
genes mutations in sporadic colonic, gastric or endometrial carcinomas with MI is very low, which
suggests that other mechanisms of gene inactivation must be involved .
It has recently been described that MLH1 promoter hypermethylation may
lead to loss of MLH1 expression and subsequent development of MI in
EC . We have detected MLH-1 promoter hypermethylation in 11 of
12 of EC with MI (91%), but in none of the MI negative tumors. On the other hand, MLH-1 promoter hypermethylation was detected in 8 of 116 (7%) cases of endometrial
hyperplasia, and it was almost exclusively restricted to atypical hyperplasias with coexisting
carcinomas  (Fig.3). These data suggest that hypermethylation of MLH-1 may be an early event in the pathogenesis of EEC, that precedes the
development of MI .
The identification of CpG island methylation in several genes (p16, TSP-1, IGF-2, HIC-1 and
MLH-1) in tumors with MI suggests that altered methylation may be a
preliminary alteration in the development of the microsatellite mutator phenotype.
From endometrial hyperplasia to endometrioid carcinoma several genetic alterations occur. In sporadic
tumors, MI results from promoter hypermethylation ofhMLH1 andleads to mutations in several critical
target genes, containing microsatellites,which are involved in apoptosis,
cell proliferation and cell differentiation (Fig.4). These wide range of mutations would be responsible
for tumor heterogeneity. Also, PTEN plays a role in endometrial
tumorigenesis from hyperplasia to carcinoma. PTEN mutations could result
from MI (45%), promoter hypermethylation (16%), or LOH (24%) (Fig.5). PTEN
haplo-insufficiency and coexistent PIK3CA/PTEN mutations (26%) may account
for PTEN inactivation in some of the cases with only monoallelic PTEN alteration. PTEN inactivation releases the
PI3K-AKT pathway, inhibiting apoptosis and resulting in tumor growth advantage. Non-endometrioid
carcinomas are characterized by p53 mutations, which probably result in
chromosome instability and subsequent LOH or amplifications of large chromosomal regions.
Non-endometrioid carcinomas may also derive from endometrioid carcinoma with MI through tumor progression
and subsequent p53 mutations (Fig.6).
Fig. 1. Dualistic model for endometrial carcinogenesis
Fig. 2. Most common genetic alterations in endometrioid carcinoma.
Fig. 3. hMLH1 promoter hypermethylation is an early event in endometrial tumorigenesis.
Fig. 4. From hyperplasia to endometrioid carcinoma. Proposed pathogenesis.
Fig. 5. PTEN alterations in endometrioid carcinomas.
Fig. 6. Endometrial carcinoma. Proposed pathogenesis as an alternative to the dualistic model.
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