Endometrial Carcinoma: Pathology and Genetics
Moderator: Dr. Michael A. Wells
Section 1 -
PTEN: Gatekeeper of the Endometrium
George L. Mutter, MD
Brigham and Women's Hospital
This presentation will integrate complementary
pathologic, genetic, and hormonal views of endometrial carcinogenesis, emphasizing mutational
inactivation of the PTEN gene as a marker for early lesions, and hormonal effects upon fate of PTEN
mutant clones as mediators of ultimate cancer risk.
Endometrial Intraepithelial Neoplasia (EIN), A Diagnosible Monoclonal Precancer
The histologic presentation of premalignant endometrial lesions prone to malignant
transformation to endometrioid adenocarcinoma is called EIN. In the past, both generalized hormonal
responses and localized premalignant lesions have been lumped under the term "endometrial hyperplasia"
subdivided by architectural complexity and cytologic atypia . Although this practice has
been widespread, it fails to optimally stratify patients according to those pathologic mechanisms and
cancer risks necessary for appropriate therapeutic triaging. Recent molecular studies have provided
evidence that the use of the term hyperplasia is conceptually correct for some but not all of these
lesions. For these reason, we have chosen to present a practically oriented disease classification in
which the hormonal effects of unopposed estrogens (benign hyperplasia) and emergent neoplastic
precancerous lesions (endometrial intraepithelial neoplasia (EIN) are separately diagnosed using
non-overlapping terminology and discrete criteria (Table 1) .
Table I: EIN Diagnostic Criteria. Modified after
|EIN Criterion ||Comments|
|Architecture ||Area of Glands greater than Stroma|
|Cytology ||Cytology differs between architecturally crowded focus and background, or clearly abnormal.|
|Size >1 mm ||Maximum linear dimension exceeds 1mm.|
|Exclude mimics ||Benign conditions with overlapping criteria: Basalis, secretory, polyps, repair, etc.|
|Exclude Cancer ||Carcinoma if mazelike glands, solid areas, polygonal "mosaic-like" glands, myoinvasion, or significant cribriforming|
The risk of developing endometrial cancer, as predicted by an EIN diagnosis are the basis
for therapy. Although there are many previous references citing cancer outcomes of EIN patients
two studies summarize cancer predictive value of EIN diagnosed by subjective
 and objective
histomorphometric  methods. Overall, patients with EIN lesions
have an 89-fold increased cancer risk than those without .
Molecular Biology of EIN.
Insights into how endometrial
precancers behave have been facilitated by application of molecular markers to paraffin embedded human
materials. EIN lesions begin as localized monoclonal outgrowths of mutated endometrial cells with a
changed cytology and architecture that enables their recognition when compared to the background source
polyclonal field . The clonal nature of EIN lesions has been demonstrated by various markers
such as nonrandom X chromosome inactivation and clonal propagation of altered microsatellites
Histomorphometric analysis of premalignant endometrial lesions identified by monoclonal
growth, and lineage continuity with actual carcinomas that developed in the same patients showed that
virtually all precancers have a histologic appearance identical to that seen in histomorphometric studies
to increase cancer risk . These histomorphometric features have been incorporated into
subjective diagnostic criteria.
Each EIN lesion is the end result of multiple mutations that occur in varying permutations
and order of invocation between patients. Within individual patients, those exact genetic alterations
present in an EIN lesion are carried forward to the cancer, establishing them as physical progenitors of
The clone which comprises an EIN lesion may acquire additional mutations
during subsequent clonal expansion, a key element of progression to carcinoma and development of
Comparison of the extent and range of genomic damage
between premalignant and malignant phases indicates a greater cumulative mutational load in cancers, a
feature that must contribute to their differing morphology and behavior. For example, while 55% of EIN
lesions have demonstrable inactivating events (mutation and/or deletion)  of the PTEN tumor
suppressor gene, the proportion rises to 83% in those cancers which follow an EIN lesion .
Similarly, for those lesions with microsatellite instability, the burden of altered microsatellite
alleles increases between EIN and carcinoma
Several other genes known to be structurally altered in endometrial carcinomas are already
abnormal in EIN lesions. Most are somatically acquired rather than inherited defects, as they are intact
in the background endometrial tissues. Activating mutations of the KRAS2 cellular oncogene are clonally
present in the cells of 16% of EIN lesions
Microsatellite instability caused by
defective DNA mismatch repair, is seen in 25-20% of EIN lesions. β-catenin mutations involve 25-30%
 of endometrial cancers and their premalignant counterparts.
PTEN, a Marker for Endometrioid Carcinogenesis
PTEN, a tumor suppressor located at 10q23 inactivated early in endometrial carcinogenesis,
is an informative marker for exploring the premalignant phases of disease. 63% of EIN lesions lack
immunohistochemically detectable PTEN protein in a clonal distribution . Despite this very
strong association, and the fact that experimental PTEN inactivation in mice leads to a high incidence of
endometrial cancer , changes in addition to PTEN inactivation must occur before affected
cells acquire histopathologic features diagnostic of EIN.
In vitro cell line data has suggested that the tumor
suppressor functions of PTEN, including G1 arrest and enabling of apoptosis, are mediated by a cascade
which maintains the putative downstream factor Akt in a dephosphorylated state
Although PTEN presents itself as a major determinant of Akt-mediated apoptosis and G1 arrest these are,
however, basic cellular functions controlled by a complex web of regulatory pathways that probably
include elements outside the PTEN-Akt axis. The finding that p27 and cyclin D1 do not necessarily behave
according to a simple linear model aligned with PTEN and Akt  suggests that these downstream
events are indirect or subject to modification.
Latent Precancers: a Preclinical Phase of Disease Identified by the PTEN Biomarker
Initially, somatically acquired endometrial gland mutations in the PTEN tumor suppressor
gene are not accompanied by any cytologic or architectural modifications evident at the light microscopic
level. This "latent precancer" phase is subclinical in every respect, falling below the threshold of
detection using routine diagnostic methods, and without a greatly increased prospective cancer risk. 43%
of normal premenopausal naturally cycling women have small numbers of these immunohistochemically
detected PTEN deficient endometrial glands, which when microdissected bear acquired mutations and
deletions of the PTEN gene itself . Progression from this stage to carcinoma must be
extremely inefficient, as the lifetime risk of endometrial cancer is only 2.6% . These
first events of endometrial carcinogenesis occur with such sufficient frequency that they can be
considered a feature of "normal" endometrial biology rather than part of a pathologic state. In the
latent phase, mutated cells may participate in successive endometrial regeneration over the course of
many menstrual cycles, persisting for years as discernible clones, and demonstrate normal morphogenesis
in conjunction with associated stroma .
Cancer Risk Modulation is Codetermined by Latent Precancer Fate
Hormonal and genetic mechanisms are linked in the very earliest stages of endometrial
carcinogenesis through the selective effects of hormones upon genetically defective compared to intact
Hormonal environment is one systemic factor which may modulate physiologic demand for PTEN
protein, thereby defining a shifting normal baseline against which the functional implications of PTEN
loss must be measured. Normal PTEN expression increases in endometrial glands during the estrogenic
follicular phase of the menstrual cycle , and declines dramatically upon introduction of the
antiestrogenic hormone progesterone. A rapidly dividing estrogen stimulated endometrial gland has a
greater PTEN requirement than a quiescent progesterone exposed non mitotic gland, and it is reasonable to
conclude that these settings would respond differently to loss of PTEN function. Consistent with this
notion is the fact that the primary epidemiologic risk factor for PTEN-deficient endometrial carcinomas
(endometrioid histologic subtype) is protracted estrogen exposure .
Under conditions of a normal monthly menstrual cycle progesterone exposures are
insufficient to ablate latent precancers, only 17% of which disappear a year later . If the
dose and duration of progestins are increased to therapeutic levels, PTEN mutant latent precancers
undergo a 90% rate of involution, thereby resetting the carcinogenesis "clock" . These
events are all inapparent at the level of routine histology.
Comments on Use of PTEN Immunohistochemistry.
PTEN tumor suppressor inactivation permits visual delineation of affected EIN lesions by
routine immunohistochemistry. One third of EIN lesions express normal levels of the PTEN protein, making
it relatively insensitive to render decisions regarding individual patients. Of equal concern, frequent
inactivation of this marker at a preclinical stage means that demonstration of isolated PTEN-null glands
in the absence of otherwise diagnostic EIN cannot be considered a high cancer risk state .
It may, however, have some value when localizing lesions are found to be PTEN null, and thus likely EIN,
in otherwise difficult or equivocal diagnostic situations such as within endometrial polyps 
or secretory endometrium. Lesions that express PTEN, the normal state for endometrial tissues, are
non-informative. The assay requires use of appropriate reagents that have been validated in paraffin
sections against mutational data (antibody 6h2.1), applied to freshly cut sections following antigen
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