Is p16-Positive Good or Bad in Squamous Cell Carcinoma?

Squamous cell carcinoma (SCC) arises from squamous cells found in the skin and mucosal surfaces. One key biomarker in SCC is p16, a protein that regulates cell growth. Its expression provides important insights into tumor behavior and prognosis.

Whether p16 positivity is beneficial or detrimental in SCC depends on factors such as its association with human papillomavirus (HPV) and its impact on treatment outcomes.

Role Of p16 In Cell Cycle

p16, also known as cyclin-dependent kinase inhibitor 2A (CDKN2A), regulates the cell cycle by inhibiting cyclin-dependent kinases (CDKs) 4 and 6. These kinases phosphorylate the retinoblastoma protein (pRb), which controls the transition from the G1 to S phase and prevents uncontrolled proliferation. When p16 binds to CDK4/6, it prevents pRb phosphorylation, keeping it in its active, growth-suppressive state and halting cell cycle progression.

Loss of p16 disrupts this control, leading to unchecked CDK4/6 activity, hyperphosphorylation of pRb, and activation of E2F transcription factors that drive DNA synthesis and tumor progression. Many cancers silence p16 through genetic mutations, promoter methylation, or deletions, contributing to malignancy. However, in HPV-associated cancers, p16 overexpression occurs as a response to oncogenic stress.

HPV-driven malignancies manipulate p16 differently. The viral oncoprotein E7 inactivates pRb, leading to unchecked E2F activity and continuous proliferation. In response, p16 is upregulated, but this overexpression does not halt tumor growth because E7 continuously degrades pRb. This paradox makes p16 a surrogate marker for HPV-associated cancers, distinguishing them from tumors where p16 loss drives malignancy. The significance of p16 expression varies based on tumor type and molecular context.

p16 Expression In Squamous Cell Carcinoma

p16 expression in SCC depends on the tumor’s origin and molecular characteristics. In HPV-associated SCCs, particularly in the oropharynx, p16 is frequently overexpressed due to E7-mediated pRb inactivation. While this upregulation does not suppress tumor growth, it serves as a marker for HPV-driven cancers. Studies indicate that p16-positive oropharyngeal SCCs respond better to radiation and chemotherapy, leading to improved prognosis. This correlation is strong enough that clinical guidelines use p16 immunohistochemistry to stratify patients and guide treatment decisions.

In contrast, SCCs in non-oropharyngeal sites, such as the lung, esophagus, or skin, often exhibit p16 loss due to CDKN2A gene alterations. In lung SCC, for example, CDKN2A deletions or promoter methylation correlate with aggressive behavior and resistance to therapy. Unlike HPV-related SCCs, where p16 overexpression results from viral oncogenesis, p16 deficiency in these tumors contributes directly to malignancy.

p16 status also influences treatment strategies. In HPV-positive SCCs, the enhanced radiosensitivity of p16-positive tumors has led to discussions about treatment de-escalation to reduce long-term toxicity while maintaining high cure rates. Clinical trials, such as RTOG 1016 and De-ESCALaTE HPV, have explored reduced-intensity regimens for these cases. Conversely, in p16-negative SCCs, particularly those with CDKN2A deletions, CDK4/6 inhibitors like palbociclib are being investigated as potential therapies.

Common Tests For p16

Assessing p16 expression in SCC relies on immunohistochemistry (IHC), which detects p16 protein levels in tumor tissue. A strong, diffuse nuclear and cytoplasmic staining pattern in more than 70% of tumor cells is typically considered positive. IHC is particularly useful in identifying HPV-associated SCCs, especially in the oropharynx. However, discrepancies between p16 positivity and actual HPV infection can occur, requiring further validation.

Polymerase chain reaction (PCR) detects HPV DNA, identifying high-risk subtypes like HPV16 and HPV18. While PCR confirms viral involvement, it does not assess p16 protein expression, making it a complementary test rather than a replacement for IHC. In situ hybridization (ISH) detects HPV RNA or DNA within tumor cells, helping differentiate active infections from incidental viral presence. High-risk HPV E6/E7 mRNA ISH is particularly useful for confirming oncogenic viral activity.

Fluorescence in situ hybridization (FISH) and next-generation sequencing (NGS) provide additional insights. FISH identifies CDKN2A gene deletions, which are common in non-HPV SCCs where p16 loss drives tumor progression. NGS offers a detailed genomic profile, detecting mutations, copy number variations, and epigenetic modifications affecting p16 expression. While these advanced techniques provide deeper molecular characterization, they are typically reserved for research or complex cases.

Observations In p16-Positive Cases

p16-positive SCCs exhibit distinct clinical and pathological characteristics. In HPV-associated SCCs, these tumors respond better to radiation and chemotherapy, showing higher locoregional control rates and improved survival. This sensitivity has led to discussions about modifying treatment to reduce toxicity while maintaining effectiveness, particularly in younger patients.

Histopathologically, p16-positive SCCs often display a basaloid morphology with increased mitotic activity but lower keratinization and necrosis compared to p16-negative tumors. Despite their active proliferation, they generally have fewer genetic alterations linked to traditional carcinogenesis pathways, indicating a different oncogenic trajectory.

Metastatic patterns also differ, with p16-positive tumors showing a higher likelihood of nodal involvement but a lower risk of distant metastasis. This contributes to their more favorable prognosis compared to p16-negative SCCs.