Pathogenesis of Endometrioid Endometrial Carcinoma

(Robbins & Cotran Pathologic Basis of Disease + Robbins & Kumar Basic Pathology)

FLOWCHART

┌─────────────────────────────────────────────────────────────────────┐
│               PREDISPOSING / RISK FACTORS                           │
│                                                                     │
│  Obesity ──► ↑ Peripheral conversion of androgens to ESTROGENS     │
│  Anovulatory cycles / Estrogen-secreting ovarian tumors             │
│  Exogenous unopposed estrogen therapy                               │
│  Hypertension · Type 2 Diabetes (co-occur with obesity)            │
│  Germ-line mutations: PTEN (Cowden syndrome), MMR (Lynch syndrome)  │
└──────────────────────────┬──────────────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────────────┐
│            UNOPPOSED ESTROGENIC STIMULATION                         │
│                                                                     │
│  Estrogen (E2) binds estrogen receptor (ER) in endometrial cells   │
│         ▼                                                           │
│  PI3K/AKT pathway activated by estrogen receptor signaling         │
│         ▼                                                           │
│  ↑ Cell proliferation · ↑ Expression of ER-dependent target genes  │
└──────────────────────────┬──────────────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────────────┐
│            ENDOMETRIAL HYPERPLASIA (Precursor Lesion)               │
│                                                                     │
│  Hyperplasia WITHOUT atypia                                         │
│     - Architectural crowding, cystic gland dilation                │
│     - Low risk of progression                                       │
│          │                                                          │
│          ▼  (additional mutations)                                  │
│  Hyperplasia WITH ATYPIA (Atypical Hyperplasia / EIN)               │
│     - Glandular crowding + cellular atypia                          │
│     - Rounded, vesicular nuclei with prominent nucleoli             │
│     - High risk of progression → CARCINOMA                         │
│     - Shares identical mutations with co-existing carcinoma        │
│       (PTEN, ARID1A, PIK3CA, KRAS) → confirms precursor role       │
└──────────────────────────┬──────────────────────────────────────────┘
                           │
          ┌────────────────┼────────────────────────────┐
          │ EARLY MUTATIONS│                            │
          ▼                ▼                            ▼
┌──────────────┐  ┌──────────────────┐      ┌───────────────────────┐
│ PTEN mutation│  │ MMR gene defects │      │ ARID1A mutation        │
│ (30%–80%)    │  │ (~20% sporadic;  │      │ (~1/3 of tumors)       │
│ Tumor supp.  │  │ MLH1 silenced by │      │ Chromatin regulator;  │
│ gene → loss  │  │ promoter hyper-  │      │ loss enhances PI3K/   │
│ of PTEN →   │  │ methylation;     │      │ AKT pro-oncogenic     │
│ ↑↑ PI3K/AKT │  │ Lynch → germ-line│      │ gene expression       │
│ signaling    │  │ MMR mutations)   │      └───────────────────────┘
└──────┬───────┘  └────────┬─────────┘
       │                   │
       └─────────┬─────────┘
                 ▼
┌─────────────────────────────────────────────────────────────────────┐
│           CENTRAL HALLMARK: ↑↑ PI3K / AKT SIGNALING                │
│                                                                     │
│  Multiple mutations act in concert to amplify PI3K/AKT pathway:   │
│                                                                     │
│  ① PTEN loss-of-function (30%–80%)                                 │
│      PTEN is the phosphatase that degrades PIP3                    │
│      → loss → unopposed PIP3 accumulation → ↑ AKT activation      │
│                                                                     │
│  ② PIK3CA activating mutations (~40%)                              │
│      Encodes catalytic subunit of PI3K                             │
│      → gain-of-function → ↑ PI3K activity → ↑ AKT signaling       │
│                                                                     │
│  ③ KRAS activating mutations (~25%)                                │
│      KRAS stimulates PI3K/AKT signaling                           │
│      → further amplification of pathway                            │
│                                                                     │
│  ④ ARID1A loss (~33%)                                              │
│      Enhances PI3K/AKT-mediated pro-oncogenic gene expression      │
│                                                                     │
│  Endometrial carcinomas are UNIQUE in that single tumors may        │
│  harbor MULTIPLE mutations all converging on PI3K/AKT pathway     │
│  → successive increases in signal strength drive progression       │
└──────────────────────────┬──────────────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────────────┐
│        GENOMIC INSTABILITY (creates "Mutator Phenotype")            │
│                                                                     │
│  A) MMR gene defects (MLH1, MSH2, MSH6, PMS2)                     │
│     → Microsatellite instability (MSI)                             │
│     → Accelerated accumulation of mutations in oncogenes/TSGs      │
│     → Particularly common in Lynch syndrome-associated tumors      │
│                                                                     │
│  B) DNA polymerase ε (POLE) proofreading mutations (<10%)          │
│     → Ultra-mutated phenotype (highest somatic mutation burden     │
│       of any cancer type)                                          │
│     → Paradoxically, POLE-mutated tumors have BETTER prognosis    │
│       (high neoantigen load → immune recognition)                  │
│                                                                     │
│  TP53 mutations: rare in endometrioid type (contrast to serous);   │
│  when present, they are late events                                │
└──────────────────────────┬──────────────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────────────┐
│              ENDOMETRIOID ENDOMETRIAL CARCINOMA                     │
│                                                                     │
│  - Well differentiated; mimics proliferative endometrial glands    │
│  - Exophytic or infiltrative growth                                │
│  - Graded 1 to 3 by degree of differentiation                     │
│  - Spreads by: myometrial invasion → direct extension →            │
│    regional lymph node metastasis                                  │
│  - ~80%–85% of all endometrial carcinomas                          │
│  - Generally better prognosis than serous type                     │
└─────────────────────────────────────────────────────────────────────┘

Summary Table: Key Molecular Alterations

Histology of Precursor Lesions (Robbins & Kumar)

Key Concepts to Remember

1. Estrogen-driven pathway - This is the hallmark distinguishing endometrioid (Type I) from serous (Type II) carcinoma. Type I arises with estrogen excess; Type II arises in atrophic endometrium with TP53 mutations.
2. PI3K/AKT is the central node - Multiple mutations converge on this single pathway, which is unique to endometrial carcinomas. Estrogen receptor signaling itself augments this pathway.
3. Stepwise progression confirmed - Identical mutations (PTEN, ARID1A, KRAS, PIK3CA) found in both atypical hyperplasia and co-existing carcinoma in hysterectomy specimens confirm the hyperplasia → carcinoma sequence.
4. Genomic instability as accelerator - MMR defects (Lynch syndrome or somatic MLH1 methylation) and POLE mutations do not initiate cancer but accelerate mutation accumulation.
5. POLE paradox - Despite creating the highest mutation burden of any cancer, POLE-mutant endometrial carcinomas have better prognosis due to heightened immune recognition.