p53 mutant protein—interpretation of abnormal expression patterns, heterogeneity, and clinical significance

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This article focuses on the abnormal expression patterns and clinical significance of p53 mutant protein in immunohistochemical detection, systematically elaborating the morphological characteristics of three abnormal patterns—overexpression, complete absence, and cytoplasmic—and their corresponding TP53 mutation types, analyzing the interpretive challenges of heterogeneous expression caused by subclonal mutations and the multi-slide staining strategy, and exploring the application value of abnormal staining patterns in molecular classification and prognostic assessment of endometrial carcinoma.

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p53 Mutant Protein——Interpretation of Abnormal Expression Patterns, Heterogeneity, and Clinical Significance

Summary: This article focuses on the abnormal expression patterns and clinical significance of p53 mutant protein in immunohistochemical detection, systematically elaborating the morphological features of three abnormal patterns—overexpression, complete absence, and cytoplasmic—and their corresponding TP53 mutation types, analyzing the interpretive challenges of heterogeneous expression caused by subclonal mutations and multi-section staining strategies, and discussing the application value of abnormal staining patterns in molecular classification and prognostic assessment of endometrial cancer.

I. Classification of p53 abnormal expression patterns and their correspondence with mutation types.

Regarding the interpretation of p53 immunohistochemistry results, a three-tier classification system is currently recommended. Among these, overexpression and complete absence are both interpreted as abnormal/mutant patterns. Subsequent studies on ovarian cancer confirmed that optimized immunohistochemical detection has excellent consistency with the underlying TP53 gene mutation status, with specificity reaching 100%. In other words, if p53 staining shows an abnormal pattern, an underlying TP53 gene mutation is almost certainly present. It is noteworthy that some splice-site mutations or truncating mutations can still produce detectable but non-functional p53 protein, thereby presenting a normal wild-type staining pattern; this occurs in approximately 5% of tubo-ovarian high-grade serous carcinomas. Through optimized immunohistochemical methods, a fourth rare cytoplasmic pattern has also been observed. In summary, p53 staining is currently categorized into four patterns based on their correspondence with TP53 mutation status.

II. Morphological features and interpretive points of the overexpression pattern.

The overexpression pattern is the most common type of abnormal expression, accounting for approximately 66% of tubo-ovarian high-grade serous carcinomas, and corresponds to non-synonymous missense mutations. Its morphological feature is strong staining in almost all tumor cell nuclei, with staining intensity significantly exceeding that of internal controls such as central fibroblasts. When interpreting, attention should be paid to cytoplasmic background staining: strong and diffuse nuclear overexpression is often accompanied by low-intensity cytoplasmic background, which indicates a strong chromogenic reaction but should not be interpreted as an abnormal cytoplasmic expression pattern. Identification of the overexpression pattern is relatively straightforward; the main pitfall lies in confusion with the "high expression" wild-type pattern in highly proliferative tumors, and the key to differentiation lies in the uniformity of staining intensity and comparison with internal controls.

III. Prerequisites for interpreting the complete absence pattern and artifact recognition.

The complete absence pattern accounts for approximately 25% of tubo-ovarian high-grade serous carcinomas and corresponds to loss-of-function mutations. The prerequisite for its interpretation is that non-neoplastic cells such as lymphocytes, fibroblasts, or endothelial cells must show positive staining as an effective internal control. Cases without positive internal controls should be considered uninterpretable, a problem particularly prominent in biopsy specimens with scant tissue. At the same time, vigilance is needed regarding occasional non-specific faint nuclear background caused by highly sensitive staining methods; this artifact is easily misjudged as a wild-type pattern in true complete absence cases. Therefore, interpretation of the complete absence pattern must be established on the basis of adequate internal control staining; without this prerequisite, the interpretation is invalid.

IV. Recognition of the cytoplasmic pattern and its indication of nuclear localization signal mutations.

The cytoplasmic pattern is a type only recently confirmed as a distinct abnormal expression pattern, accounting for approximately 4% of tubo-ovarian high-grade serous carcinomas, and is associated with mutations that disrupt the nuclear localization signal of p53 protein. It is characterized by definite cytoplasmic staining, which may be accompanied by varying degrees of nuclear staining. It should be emphasized that strong and diffuse nuclear overexpression with low-intensity cytoplasmic background should be interpreted as the overexpression pattern, not the cytoplasmic pattern. Recognition of this pattern is highly dependent on the immunohistochemical protocol, as it may not be visualized under weaker staining conditions. Truly abnormal cytoplasmic staining should be definitively positive, rather than ambiguous faint background—the latter can be disregarded.

V. Interpretive challenges of heterogeneous expression caused by subclonal mutations.

Unlike in endometrial serous carcinoma, where TP53 mutations serve as early founder mutations and are therefore present in any subclone, some endometrioid carcinomas with a mutant phenotype may acquire TP53 mutations later in the disease course. Such subclonal TP53 mutations can lead to heterogeneous p53 expression, characterized by the coexistence of normal wild-type areas and abnormal mutant-type staining areas. The interpretive challenge lies in determining whether these subclonal foci represent a true abnormal pattern, or wild-type staining intensity variation that has not yet reached the threshold for abnormal overexpression, or intensity variation within the overexpression pattern. It is worth noting that the phenomenon in which the abnormal component of heterogeneous staining is present in only some tumor sections may lead to misdiagnosis as serous carcinoma when it is actually POLE-mutant endometrioid carcinoma. Therefore, when morphological features suggest POLE mutation but p53 staining is abnormal, staining of multiple tumor sections is recommended.

VI. Clinical value of abnormal staining patterns in molecular classification and prognostic assessment.

In endometrial cancer, p53 immunohistochemistry may aid in diagnosing high-grade carcinomas associated with poor prognosis and can serve as part of a panel of tissue typing indicators. A small number of low-grade endometrioid adenocarcinomas harbor TP53 mutations and exhibit mutant-type immunoreactivity. Mutant p53 staining may help avoid misdiagnosing serous carcinomas with intermediate-grade nuclear features as grade 1 or grade 2 endometrioid carcinomas. Abnormal p53 staining alone is insufficient to distinguish endometrioid carcinoma from serous carcinoma, because a considerable proportion of grade 3 endometrioid carcinomas also show mutant p53 immunoreactivity. Grade 3 endometrioid carcinomas with mutant p53 expression have a worse prognosis than those with wild-type expression. Mutant p53 immunostaining can serve as an indicator of the poorest prognostic type in the Cancer Genome Atlas-based molecular classification of endometrial cancer. Given that the sensitivity of p53 immunohistochemistry for detecting TP53 mutations is not 100%, a small subset of morphologically typical endometrial serous carcinomas will show a p53 wild-type immunoreactive pattern while still harboring TP53 mutations; therefore, tumors with wild-type p53 staining can still be diagnosed as serous carcinoma.

VII. Conclusion.

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This article is reviewed and published by the technical expert team of UA

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