Can cytokeratin 7 (CK7) and thyroid transcription factor‑1 (TTF‑1) be positive in melanoma?

CK7 and TTF-1 Expression in Melanoma

CK7 and TTF-1 are typically negative in melanoma, but rare exceptions exist—CK7 can be positive in approximately 10-31% of melanomas at the mRNA level (though protein expression is much lower), while TTF-1 positivity in melanoma is exceedingly rare and should prompt reconsideration of the diagnosis.

Standard Immunohistochemical Profile

According to established guidelines, melanoma follows a predictable immunohistochemical pattern 1:

• Cytokeratins (including CK7): Negative
• TTF-1: Negative
• S-100: Positive (though not entirely specific)
• Vimentin: Variable (±)

This negative cytokeratin profile is what fundamentally distinguishes melanoma from carcinomas during the workup of cancers of unknown primary 2, 1.

CK7 Expression: The Nuanced Reality

While guidelines consistently list melanoma as CK7-negative, research reveals more complexity:

At the mRNA level, CK expression occurs more frequently than protein detection. A 2009 study found CK18 mRNA positivity in 31.3% of melanoma tissue samples (10% of primary cutaneous, 40% of primary mucosal, and 48% of metastatic melanomas) 3. However, protein expression by immunohistochemistry was only focal in 10% of cases, and pan-cytokeratin (AE1/AE3) staining was completely negative 3.

Clinical significance: When CK18 mRNA was detected, it correlated with significantly poorer prognosis (relative risk 5.430, P<0.001), suggesting this represents a more aggressive melanoma phenotype rather than misdiagnosis 3.

TTF-1 Expression: Extremely Rare

TTF-1 positivity in melanoma is extraordinarily uncommon and should raise immediate diagnostic concerns:

Primary cutaneous neuroendocrine carcinoma (Merkel cell carcinoma) is the key differential diagnosis. Standard MCC is CK20-positive and TTF-1-negative, which helps distinguish it from small cell lung cancer (SCLC) that is typically TTF-1-positive and CK20-negative 4, 5, 6.

However, rare case reports describe TTF-1-positive, CK20-negative skin lesions that behave clinically like primary cutaneous neuroendocrine carcinoma without evidence of systemic disease 6. These represent diagnostic dilemmas rather than true melanomas.

One exceptional case involved melanoma arising from melanin-producing medullary thyroid carcinoma, where the melanoma component was negative for TTF-1 (while the MTC component would be TTF-1-positive) 7.

Diagnostic Algorithm When Encountering Unexpected Staining

When you encounter a suspected melanoma with positive CK7 or TTF-1:

1. First, verify melanoma markers: Confirm S-100, SOX10, Melan-A, and HMB-45 positivity 1, 8

2. If CK7 is positive:

   • Consider in-situ hybridization for CK mRNA if available (more sensitive than immunohistochemistry) 3
   • Recognize this may represent aggressive melanoma phenotype
   • Rule out carcinoma mimics, particularly if other cytokeratins are also positive

3. If TTF-1 is positive:

   • Strongly reconsider the diagnosis—this is not characteristic of melanoma
   • Exclude metastatic lung adenocarcinoma (check Napsin A, which is positive in ~60% of lung adenocarcinomas) 8, 9
   • Exclude small cell carcinoma (check chromogranin, synaptophysin, CD56) 4, 5
   • Consider atypical Merkel cell carcinoma if CK20 is negative 6
   • Evaluate for thyroid primary if thyroglobulin is also positive 2

4. Use comprehensive panels: The 2023 ESMO guidelines recommend initial screening with broad-spectrum keratin (AE1/AE3), CD45, and SOX10/S100 to establish lineage before pursuing additional markers 8

Critical Pitfalls

• Don't rely on single markers: Melanoma diagnosis requires a panel approach, not individual stain interpretation
• Recognize that exceptions exist: Approximately 10-30% of melanomas may show focal or mRNA-level cytokeratin expression without being carcinomas 3
• TTF-1 positivity should trigger diagnostic reconsideration: This finding is so atypical for melanoma that alternative diagnoses (lung primary, thyroid primary, atypical neuroendocrine tumor) must be thoroughly excluded 9, 10, 6
• Clinical context matters: Mucosal and metastatic melanomas show higher rates of aberrant marker expression than primary cutaneous melanomas 3