Cytopathology in focus—POU2F3: A new IHC marker for small cell lung cancer with low neuroendocrine marker expression

Randa H. Obid, MD, MPH
Jordan P. Reynolds, MD
Derek B. Allison, MD

May 2024—Classically, immunohistochemical expression of small cell lung cancer (SCLC) is characterized by strong expression of neuroendocrine markers, notably synaptophysin, chromogranin A, insulinoma-associated protein 1 (INSM1), and CD-56. However, up to 20 percent of SCLCs demonstrate low or no expression of neuroendocrine (NE) markers by IHC and are termed “NE-low/negative.” While SCLC was previously thought to be a genetically homogeneous type of aggressive carcinoma, new evidence based on genomic profiling suggests that SCLC can be grouped into four molecular subtypes according to key transcription regulators: achaete-scute complex homolog 1 (ASCL1), neurogenic differentiation 1 (NEUROD1), POU class 2 homeobox 3 (POU2F3), and yes-associated protein 1 (YAP1), referred to as SCLC-A, SCLC-N, SCLC-P, and SCLC-Y, respectively.¹ Unlike the other molecular subtypes, SCLC-P, which represents seven to 14 percent of SCLCs, has been known to consistently show low or absent expression of all traditional neuroendocrine markers, which can make a definitive diagnosis challenging, particularly on a small biopsy specimen. Correspondingly, a recent study demonstrated the importance of a new diagnostic nuclear IHC marker, POU2F3, for these NE-low/negative SCLCs (also known as SCLC-P subtype). POU2F3 is a transcription regulator of chemosensory and immunomodulatory tuft cells and the brush cells in the lung airway.

Recently, an article by Wang, et al.,² found POU2F3 to be a highly sensitive and specific IHC marker for NE-low/negative SCLC. The cohort study found that POU2F3 demonstrated strong nuclear staining in 13.4 percent of SCLC cases with negative/minimal staining for the traditional neuroendocrine markers and TTF-1. It found the sensitivity and specificity of POU2F3 in NE-low/negative SCLC to be 82.1 percent and 99.4 percent, respectively.

Another study, by Baine, et al.,³ found POU2F3 expression in 12 percent of SCLC, and it was strongly associated with low expression of typical neuroendocrine markers. Moreover, POU2F3 was expressed in 75 percent of SCLC, with entirely negative or minimal neuroendocrine marker expression, and helped support the diagnosis of SCLC in such cases. In this study, however, basaloid squamous cell carcinomas showed positivity in 22 percent while large cell neuroendocrine carcinomas showed positivity in 12 percent. As a result, additional IHC markers, such as p40, and a reliance on traditional morphologic assessment is crucial when evaluating and choosing to use POU2F3 as a diagnostic marker. Furthermore, only a small subset of tumor types has been evaluated for POU2F3 staining, and, as a result, caution should be exercised when diagnosing a metastasis from an unknown primary site of origin. For instance, Koh, et al.,⁴ found that 12.6 percent of extrapulmonary neuroendocrine carcinomas were positive for POU2F3, a percentage that is similar to primary SCLC. In addition, a study by Yamada, et al.,⁵ found a tuft cell-like signature in thymic squamous cell carcinomas that were positive for POU2F3 in 72 percent of cases. However, preliminary results from other tumor types that can mimic SCLC, such as Merkel cell carcinoma, melanoma, lymphoma, and round cell carcinomas, showed no expression in one study.²

Although the molecular subtypes of SCLC will undoubtedly have future clinical prognostic and therapeutic relevance, the current clinical standard of care for SCLC remains unchanged. As a result, the importance of POU2F3 IHC is in making a specific and accurate diagnosis for NE-low/negative SCLC so the patient can be counseled and treated effectively. As new developments in our understanding of these molecular pathways come to light, having accurately defined cohorts that are representative of all subtypes of SCLC, not just conventional NE-positive cases, is paramount.

In conclusion, recent evidence suggests the use of POU2F3 as a helpful IHC marker in cases that are suspected to be SCLC with low/negative NE expression. In the proper clinical setting and morphology, this marker can be extremely helpful in making a definitive diagnosis of SCLC. However, caution should be exercised until more is known about POU2F3 staining in other tumor types, particularly noting that it does not seem to be specific for small cell carcinoma of pulmonary origin.

1. Baine MK, Febres-Aldana CA, Chang JC, et al. POU2F3 in SCLC: clinicopathologic and genomic analysis with a focus on its diagnostic utility in neuroendocrine-low SCLC. J Thorac Oncol. 2022;17(9):1109–1121.
2. Wang Y, Jin Y, Shen X, et al. POU2F3: a sensitive and specific diagnostic marker for neuroendocrine-low/negative small cell lung cancer. Am J Surg Pathol. 2023;47(9):1059–1066.
3. Baine MK, Hsieh MS, Lai WV, et al. SCLC subtypes defined by ASCL1, NEUROD1, POU2F3, and YAP1: a comprehensive immunohistochemical and histopathologic characterization. J Thorac Oncol. 2020;15(12):1823–1835.
4. Koh J, Kim H, Moon KC, et al. Molecular classification of extrapulmonary neuroendocrine carcinomas with emphasis on POU2F3-positive tuft cell carcinoma. Am J Surg Pathol. 2023;47(2):183–193.
5. Yamada Y, Simon-Keller K, Belharazem-Vitacolonna D, et al. A tuft cell-like signature is highly prevalent in thymic squamous cell carcinoma and delineates new molecular subsets among the major lung cancer histotypes. J Thorac Oncol. 2021;16(6):1003–1016.

Dr. Obid is a (PGY-2) pathology resident, University of Louisville. Dr. Reynolds is professor of laboratory medicine and pathology and director of cytology, Mayo Clinic Florida. Dr. Allison is associate professor of pathology and urology and vice chair for research, Department of Pathology and Laboratory Medicine, University of Kentucky College of Medicine. All are members of the CAP Cytopathology Committee.