Clinical Guide to Biomarkers for Endometrial (Uterine) Cancer

If you or someone you love is navigating endometrial cancer, the alphabet soup of tests can feel overwhelming. Biomarkers are simply measurable clues in the tumor or blood that help answer three practical questions: What am I dealing with, how aggressive is it, and which treatments are most likely to work? This guide walks you through the biomarkers that matter today, what they mean in plain language, and where the science is headed next.

Why biomarkers matter in endometrial cancer

For decades, endometrial cancer was classified mostly by how the cells looked under a microscope. That still matters, but genetics now does the heavy lifting. The landmark Cancer Genome Atlas (TCGA) showed that endometrial cancers fall into four molecular categories with very different behaviors and treatment responses¹. Current international guidelines (NCCN; ESGO/ESTRO/ESP) use these molecular groups to refine risk, guide adjuvant therapy, and identify candidates for immunotherapy².

The modern framework uses testing that is widely available in clinical pathology labs. Think of it like checking both the "software" and the "hardware" of the tumor. The software is the DNA and the way repair systems work. The hardware is the protein patterns that reflect those DNA changes. Together, they provide the clearest picture of risk and options.

The four molecular groups, in practical terms

Most centers now classify endometrial cancers into one of four TCGA-based groups using a pragmatic, stepwise algorithm. You'll often see this referred to as "ProMisE" or a "TCGA surrogate" approach³.

1) POLE-ultramutated (POLEmut)

What it is: POLE is a DNA "spell-checking" enzyme. When its proofreading region (the exonuclease domain) is mutated, the tumor DNA accumulates an enormous number of typos. Paradoxically, these tumors tend to have an excellent prognosis even when they look high grade.

How it is tested: Targeted DNA sequencing of the tumor focuses on the POLE exonuclease domain. Only specific mutations are considered definitively pathogenic; many variants are benign or uncertain. The lab report should flag whether the POLE variant is known pathogenic in the proofreading region.

Why it matters: Across multiple studies and pooled analyses, POLE-mutated tumors show very low recurrence rates⁴. Guidelines now support treatment de-escalation in clearly defined POLEmut cases, even when the tumor appears aggressive. This is one of the rare times in oncology when "more mutation" predicts a gentler course.

Key caveat: Not every POLE variant counts. Variants of uncertain significance should not reclassify a tumor as POLEmut. If the report is unclear, it is reasonable for the care team to request variant-level expert review or orthogonal confirmation. When both POLE mutations and other high-risk features coexist, the POLE designation usually takes precedence for prognosis⁵.

2) Mismatch repair deficient (MMRd) or microsatellite instability-high (MSI-H)

What it is: The mismatch repair (MMR) system corrects small DNA typos. If it is lost, the tumor becomes "MSI-high," riddled with short repetitive sequence errors. This can be sporadic or due to Lynch syndrome, a hereditary condition.

How it is tested: Immunohistochemistry (IHC) for four proteins (MLH1, PMS2, MSH2, MSH6) looks for loss of expression. Molecular MSI testing (PCR or next-generation sequencing) detects the genomic signature of instability. Many centers start with IHC because it hints at which gene is affected.

Why it matters: MMRd/MSI-H status is a powerful predictive biomarker for response to PD-1–based immunotherapy⁶. It also prompts evaluation for Lynch syndrome when the pattern suggests a germline cause (for instance, loss of MSH2/MSH6 without MLH1 promoter methylation). In early-stage disease, MMRd informs risk classification and adjuvant decisions.

Key caveats: MLH1/PMS2 loss is often due to MLH1 promoter methylation in the tumor rather than inherited risk; many labs reflex to methylation testing before recommending germline evaluation. Subclonal or patchy loss can occur and may require repeat IHC, MSI testing, or sequencing. Equivocal results should be interpreted in the context of fixation quality and tumor cellularity.

3) p53 abnormal (p53abn)

What it is: p53 is the genome's quality control officer. When it is broken, tumors often behave more aggressively. This group includes many serous carcinomas and some high-grade endometrioid tumors.

How it is tested: p53 IHC serves as a reliable surrogate for TP53 mutation when interpreted with established patterns. "Abnormal" encompasses strong diffuse overexpression, complete absence ("null"), or aberrant cytoplasmic staining. A "wild-type" pattern is variable, patchy nuclear staining.

Why it matters: p53abn tumors carry higher risks of recurrence and typically benefit from chemotherapy-based adjuvant strategies⁷. They are also the subset where HER2 amplification is more common, especially in serous histology, which opens the door to HER2-directed therapy.

4) No specific molecular profile (NSMP)

What it is: Tumors that are not POLEmut, not MMRd/MSI-H, and not p53abn fall into this "default" category. Many are classic, lower-grade endometrioid cancers with hormone receptor expression.

How it is tested: NSMP is a diagnosis of exclusion after performing the steps above. Some centers add targeted testing for CTNNB1 (beta-catenin), which can identify a subset with higher recurrence risk despite low grade.

Why it matters: Prognosis is intermediate overall. Estrogen and progesterone receptor status is often positive, which supports endocrine therapy options in advanced or recurrent settings.

Key caveats: NSMP is a heterogeneous bucket. Emerging markers such as L1CAM and CTNNB1 may refine risk within this group, but practices vary and thresholds are not fully standardized⁸.

Putting the pieces together: a practical testing pathway

Most pathology workflows for a new endometrial cancer proceed in a simple sequence on the biopsy or surgical specimen:

• Confirm histology and grade on the tissue
• Perform MMR IHC (MLH1, PMS2, MSH2, MSH6), with reflex MLH1 methylation testing if MLH1/PMS2 is lost
• Perform p53 IHC
• Sequence the POLE exonuclease domain when morphology or age suggests potential benefit, or as part of an upfront panel when available
• Add HER2 testing in serous and serous-like tumors, and often in high-grade endometrioid tumors
• Assess ER and PR in endometrioid tumors, especially in advanced or recurrent disease

This process assigns most tumors to a TCGA group and flags the major predictive biomarkers that change management. Many centers now order a single next-generation sequencing panel that can report MSI, POLE, TP53, and common pathway mutations alongside TMB, which streamlines testing⁹.

Key predictive biomarkers that change therapy

MMRd/MSI-H for immunotherapy

Cancers that are MMRd or MSI-H are more visible to the immune system. Clinical trials demonstrate meaningful responses to PD-1 inhibitors in this group, including durable control in many patients¹⁰. This is one of the most robust biomarker-treatment relationships in endometrial cancer today. PD-L1 IHC is not required if the tumor is MMRd/MSI-H. Although PD-L1 is widely used in other cancers, it is less reliable as a stand-alone predictor here.

HER2 amplification for HER2-directed therapy

HER2 testing by IHC with in situ hybridization confirmation is recommended in uterine serous carcinoma and often in high-grade endometrioid tumors with serous-like features. Positive, amplified tumors have improved outcomes with the addition of HER2-directed therapy to standard chemotherapy in clinical trials, and guidelines endorse testing in these histologies¹¹.

Key caveats: Endometrial cancers can show heterogeneous HER2 expression. Scoring criteria differ from breast and gastric cancers, and specialized pathology experience helps. If results are borderline, repeat testing on a larger surgical specimen can clarify status.

Hormone receptors (ER and PR) for endocrine therapy

ER and PR positivity supports the use of hormonal strategies in select advanced or recurrent endometrioid cancers, particularly in patients seeking less intensive approaches. Response is more likely when both receptors are strongly expressed¹².

Tumor mutational burden (TMB)

High TMB can associate with better immunotherapy response in several cancers. In endometrial cancer, TMB is often high when tumors are POLEmut or MMRd. Outside those groups, the predictive power of TMB alone is less certain, and thresholds vary by testing platform¹³. If TMB is reported, it should be interpreted alongside MSI/MMR and POLE results.

Less common but actionable fusions

NTRK and RET fusions are rare in endometrial cancer but targetable. They are typically detected on broad RNA or DNA panels. Routine screening is not mandated across all cases; panels that include fusion detection will report them when present.

Prognostic and pathway biomarkers you may see on reports

Several genes frequently mutated in endometrioid tumors map to the PI3K/AKT and RAS pathways, including PTEN, PIK3CA, PIK3R1, and KRAS¹⁴. These alterations inform biology but do not yet have a standard, guideline-backed targeted therapy in frontline endometrial cancer outside of clinical trials. CTNNB1 mutations within NSMP correlate with higher local recurrence risk, particularly in low-grade, early-stage disease¹⁵. L1CAM overexpression, when tested, can flag a higher-risk phenotype in otherwise lower-risk tumors¹⁶. These markers refine conversations about surveillance intensity and adjuvant therapy within guideline frameworks.

Serum and liquid biomarkers: what blood tests can and cannot do

CA-125

CA-125 is a protein often elevated in ovarian cancer. In endometrial cancer, it can be elevated when disease has spread beyond the uterus or involves the peritoneum. It can help with baseline staging and trend monitoring in select cases, especially when initially high¹⁷.

Limitations: CA-125 is not specific. It can rise with menstruation, fibroids, endometriosis, pregnancy, liver disease, and even heart failure. A normal CA-125 does not exclude disease, and a high value does not diagnose it.

HE4 and the ROMA score

HE4 is another serum marker more commonly used in evaluating ovarian masses. In endometrial cancer, HE4 may correlate with stage and recurrence risk in research cohorts, but its day-to-day role is limited. The ROMA algorithm was developed for ovarian cancer triage and is not validated for diagnosing endometrial cancer.

Limitations: HE4 rises with age and chronic kidney disease. Smokers can have higher values. These interferences reduce specificity.

Circulating tumor DNA (ctDNA)

ctDNA assays detect tumor-specific DNA fragments in the blood. Early studies in endometrial cancer suggest ctDNA may detect minimal residual disease and forecast recurrence months before imaging¹⁸. This is promising, especially for tailoring surveillance, though routine use is still evolving and not yet standard in guidelines.

Good biomarker results start with good specimens. A few nuts and bolts influence accuracy:

• Specimen type: Endometrial biopsy or curettage provides the initial tissue for diagnosis and most molecular testing. Hysterectomy specimens can confirm and refine results, particularly for HER2 and p53 in heterogeneous tumors.
• Fixation: Formalin fixation that is too short or too long can alter IHC patterns for p53, MMR, ER, and PR¹⁹. Most labs follow validated windows to minimize artifacts.
• Tumor cellularity: Sequencing and MSI assays need enough tumor cells. If cellularity is low, pathologists may macrodissect enriched tumor areas.
• Heterogeneity: Mixed histologies can produce mixed biomarker profiles. Sampling more than one area can resolve discrepancies.
• Assay variability: PD-L1 assays use different antibodies and scoring systems across cancers; in endometrial cancer, MMR/MSI status is the more robust immunotherapy biomarker. TMB thresholds vary by platform.
• Variant interpretation: For POLE, only well-characterized exonuclease domain mutations confer the ultramutated phenotype. VUS should not drive management.

Hereditary risk: Lynch syndrome and beyond

Universal screening for MMR deficiency in endometrial cancer is now standard to identify patients who may have Lynch syndrome²⁰. When IHC shows loss of MSH2/MSH6 or isolated MSH6 loss, or when MLH1/PMS2 loss occurs without MLH1 promoter methylation, germline testing is considered to evaluate inherited risk. This matters for personal surveillance and for family members. Germline POLE mutations are rare and associated with polyposis syndromes; routine germline POLE testing is not performed unless clinical features suggest it.

Special clinical contexts and life-stage considerations

Age, hormones, and comorbidities shape the testing picture:

• Premenopausal patients: Abnormal uterine bleeding has many benign causes, but tissue biopsy is the definitive diagnostic step when cancer is suspected. If cancer is found, molecular testing applies similarly and can inform fertility-sparing discussions in carefully selected, lower-risk, hormone-receptor–positive cases.
• Postmenopausal patients: Postmenopausal bleeding warrants prompt evaluation. Molecular profiling clarifies risk and adjuvant therapy needs beyond stage and grade.
• Tamoxifen exposure: Tamoxifen can promote endometrial polyps and rare cancers. Biomarker testing on any diagnosed tumor follows the same pathway; the exposure itself does not alter interpretation.
• Obesity and metabolic health: Adipose tissue increases estrogen exposure, which fuels many endometrioid tumors via ER/PR pathways. This is one reason endocrine biomarkers matter. The mechanism is similar to how muscles improve glucose handling after a workout by changing hormone signaling at the tissue level.

How biomarkers guide conversations with your care team

Biomarkers are decision tools, not verdicts. They help match the intensity of therapy to the biology and identify precise options when the match is strong. Useful prompts for a clinic visit include:

• Which TCGA group does my tumor fall into, and how confident is the assignment?
• Is there evidence of MMR deficiency or MSI-H, and was MLH1 methylation assessed if MLH1/PMS2 was lost?
• Was POLE sequencing performed and, if so, is the variant known pathogenic in the exonuclease domain?
• What is the p53 IHC result, and does it align with the histology?
• For serous or high-grade tumors, was HER2 testing done, and how was heterogeneity accounted for?
• Are ER and PR positive, and could endocrine therapy be part of the plan in advanced or recurrent settings?
• Are there any rare fusions or high TMB results that might expand options?
• If a serum marker like CA-125 was high at baseline, can we use it for trend monitoring?

What the future likely holds

Two areas are moving fast. First, integrated risk models that blend stage, histology, and the four molecular groups are now the backbone of adjuvant therapy decisions, with ongoing trials testing smarter de-escalation for POLEmut and escalation for p53abn. Second, minimal residual disease assessment using ctDNA may sharpen surveillance and catch recurrence earlier. Both directions aim to treat neither too much nor too little, and to use targeted or immune therapies where biology signals a clear advantage. Early results are encouraging, though more research is needed before these become routine everywhere.

Glossary in plain language

• Immunohistochemistry (IHC): A lab technique that stains for specific proteins in tumor tissue.
• MSI-H (microsatellite instability-high): A genomic pattern showing many small DNA errors, often from broken mismatch repair.
• MMRd (mismatch repair deficient): Loss of one or more mismatch repair proteins by IHC.
• POLE: A DNA polymerase with a proofreading function; certain mutations create an ultramutated but paradoxically favorable tumor.
• p53: A tumor suppressor protein; abnormal patterns signal more aggressive biology.
• HER2: A growth signal receptor; when amplified, it can be targeted with specific therapies.
• TMB (tumor mutational burden): The number of mutations per megabase of tumor DNA; very high values often track with MMRd or POLEmut.

Limitations to keep in mind

No single biomarker tells the whole story. Results can be influenced by how the tissue was handled, which assay was used, and which part of the tumor was sampled. Some tests, like PD-L1 IHC or TMB, do not carry the same weight in endometrial cancer as they do in other cancers. POLE variant interpretation requires care to avoid over-calling significance. Serum markers like CA-125 and HE4 are helpful in specific contexts but lack the accuracy to diagnose or rule out disease on their own. When results are borderline or unexpected, repeating or expanding testing on a larger specimen can resolve discrepancies.

Bottom line

Modern endometrial cancer care is driven by a short list of high-impact biomarkers that are testable on routine tissue: MMR/MSI, POLE, p53, and in the right histologies, HER2 and hormone receptors. These results help forecast risk and steer therapy toward options with the highest likelihood of benefit. The science is advancing quickly, with integrated profiles improving precision and liquid biopsy tools on the horizon. Interpreting biomarker results in the full clinical context is essential, and that is where a thoughtful discussion with your care team makes all the difference.