The glands of squamous morule-bearing endometrial lesions demonstrate a variety of appearances, most commonly (50%) of normal density and cytology, but less frequently with mild gland crowding (18%) or EIN (32%). These histologic types are associated with differing clinical outcomes (), with most of the cases that progressed to cancer (5/7) being preceded by morules located within EIN glandular lesions. Progression interval was quite short for EIN lesions (average 0.8 years) compared to that for isolated squamous morules (2 cases developed carcinoma after 4 and 14.6 years). The likelihood of progression of morule-bearing EIN lesions to carcinoma (19% in this series), although significantly less than the 41% seen previously for all types of EIN(9
), justifies therapeutic intervention. For those patients with isolated squamous morules but no EIN, re-evaluation or close surveillance will eventually uncover carcinoma in a fraction (6.5%) of patients.
Conservation of identical mutations in admixed squamous and glandular elements of premalignant (EIN, atypical hyperplasia) and malignant (cancer) endometrial neoplasia of individual patients confirms the hypothesis that both squamous and glandular elements are integral components of a common progenitor cell. Squamous elements, therefore, are not a reactive process involving nonneoplastic tissues. Somatic PTEN
mutations are sufficiently unique as to be good lineage-specific markers. Because each occurrence of a new PTEN mutation is widely scattered across all 9 exons of the approximately 3kb gene(8
), they generally differ between patients and amongst independent multicentric events occurring within the same patient. Additional evidence for a shared lineage is supplied by other published studies showing conservation of the same inactive X chromosome copy(13
), and particular beta-catenin mutations(14
), between squamous and glandular components of neoplastic endometrial lesions.
Acquired hormonal incompetence and amitotic state in the squamous but not glandular component may be a manifestation of terminal cellular commitment, making the functionally inert squamous subpopulation an unlikely participant in estrogen-driven lesion progression from premalignant to malignant disease. This is the basis for the recommendation that gland crowding assessments used to diagnose EIN (area of glands must exceed that of stroma) must be performed to the exclusion of squamous components(2
). Malignancies which follow morule-containing EIN lesions are usually pure adenocarcinomas rather than squamous carcinomas(15
). Other investigators have previously shown coordinate loss of estrogen and progesterone receptors, and reduction of Ki-67 staining specifically in the squamous but not glandular elements of endometrial adenocarcinomas(16
). Our work extends this observation to premalignant EIN lesions. Similarly, BCL2 expression in endometrial epithelial tissues requires an intact progesterone response pathway(18
), so loss of PR in morule components is a likely contributor to the lower levels of BCL2 expression seen in squamous morules(19
The combination of high somatic activation rates in sporadic squamous-containing lesions, known transforming activities, and experimental association with induction of a squamous phenotype(20
), have now established mutation of the gene encoding beta-catenin(20
), rather than PTEN
, as the most frequent event seen in squamous-containing endometrial glandular premalignant lesions. The frequency of PTEN inactivation in squamous morule-containing EIN lesions (13%, 1/8) is significantly lower compared to the 63% (22/35) inactivation reported previously for all EIN lesions(7
). The gene encoding beta-catenin is mutated in up to 86% of endometrial carcinomas with squamous differentiation, in contrast to 13% lacking a squamous component(14
). Similarly, beta-catenin gene mutations are frequent in premalignant endometrial lesions with squamous morules (54%) but rare (9%) in those without(21
Progestin therapy has been long known to be effective in the treatment of precancerous, and even malignant, glandular endometrial lesions(22
). For those EIN lesions with squamous morules, we would predict morules to be relatively resistant to the apoptotic effects of progestins(26
), disappearing only when shedding of the adjacent endometrium carries them away with a withdrawal bleed. Indication of differentiation-state specific response to progestins is provided by comparison of squamous and glandular EIN histopathology before and after high dose progestin therapy. shows a morule-bearing EIN lesion which after several months of progestin therapy responds by developing degenerative nuclear pyknosis and cytoplasmic shrinkage of the glandular, but not squamous, components.
Selective effects of progestin therapy on glandular and squamous elements of EIN
We can now summarize a model of endometrial morule-containing precancerous lesions consistent with the evidence presented above. Intraglandular morules arise as a transdifferentiation event from a pre-existing clone in which the glandular and squamous elements contain conserved beta-catenin mutations. The frequency and distribution of squamous morules within pre-existing glandular lesions is more consistent with a tendency for squamous differentiation within the neoplastic field, rather than independent new mutational events within each morule. Only the glandular elements retain competence to respond to the cancer-promoting effects of estrogens, or involuting effects of progestins. Cancers which result are thus derived from the glandular elements, and primarily have a glandular phenotype. If the lesion is treated with progestins, or exposed to progestins during natural menstrual cycles, the glandular components are prone to involution, thereby proportionately enriching for the squamous elements. A diagnostic problem presents itself when only isolated squamous morules are found in an endometrial biopsy, as it is not always possible to tell whether it represents a successfully treated glandular lesion with residua of hormonally inert morules, or an incompletely sampled neoplastic process. Both are possible mechanisms for presentation of isolated squamous morules in an otherwise unremarkable endometrial biopsy.
The diagnostic classification of squamous morule-bearing endometrial biopsies is primarily accomplished based upon the histologic appearance of the glandular component. In cancer-free patients with endometrial squamous morules, prospective endometrial cancer risk is primarily determined by presence or absence of an EIN glandular lesion, in keeping with the notion that it is the glandular rather than squamous components which are susceptible to the cancer risk modulating effects of estrogens and progestins. The finding of isolated squamous morules without any evident glandular abnormalities, however, justifies resampling and careful followup.