Clarifying the spectrum of driver oncogene mutations in biomarker-verified squamous carcinoma of lung: lack of EGFR/KRAS and presence of PIK3CA/AKT1 mutations

Abstract

Purpose: There is persistent controversy as to whether EGFR and KRAS mutations occur in pulmonary squamous cell carcinoma (SQCC). We hypothesized that the reported variability may reflect difficulties in the pathologic distinction of true SQCC from adenosquamous carcinoma (AD-SQC) and poorly differentiated adenocarcinoma due to incomplete sampling or morphologic overlap. The recent development of a robust immunohistochemical approach for distinguishing squamous versus glandular differentiation provides an opportunity to reassess EGFR/KRAS and other targetable kinase mutation frequencies in a pathologically homogeneous series of SQCC.

Experimental design: Ninety-five resected SQCCs, verified by immunohistochemistry as ΔNp63(+)/TTF-1(-), were tested for activating mutations in EGFR, KRAS, BRAF, PIK3CA, NRAS, AKT1, ERBB2/HER2, and MAP2K1/MEK1. In addition, all tissue samples from rare patients with the diagnosis of EGFR/KRAS-mutant "SQCC" encountered during 5 years of routine clinical genotyping were reassessed pathologically.

Results: The screen of 95 biomarker-verified SQCCs revealed no EGFR/KRAS [0%; 95% confidence interval (CI), 0%-3.8%], four PIK3CA (4%; 95% CI, 1%-10%), and one AKT1 (1%; 95% CI, 0%-5.7%) mutations. Detailed morphologic and immunohistochemical reevaluation of EGFR/KRAS-mutant "SQCC" identified during clinical genotyping (n = 16) resulted in reclassification of 10 (63%) cases as AD-SQC and five (31%) cases as poorly differentiated adenocarcinoma morphologically mimicking SQCC (i.e., adenocarcinoma with "squamoid" morphology). One (6%) case had no follow-up.

Conclusions: Our findings suggest that EGFR/KRAS mutations do not occur in pure pulmonary SQCC, and occasional detection of these mutations in samples diagnosed as "SQCC" is due to challenges with the diagnosis of AD-SQC and adenocarcinoma, which can be largely resolved by comprehensive pathologic assessment incorporating immunohistochemical biomarkers.

Figure 1. Microscopic features of EGFR/KRAS-mutant “squamous cell carcinomas” with a revised diagnosis of adenosquamous carcinoma or solid adenocarcinoma

Patient 4 (A–F) and patient 9(D–I) illustrate incomplete sampling of AD-SQC leading to the diagnosis of “SQCC” with EGFR or KRAS mutations. While only squamous component was present in the index sample (core biopsies; A–C and G–I), glandular component was manifest in a cytology sample from another site (pleural fluid; D–F) or subsequent same-site resection (J–L). Glandular and squamous components have distinct morphology and ΔNp63/TTF-1 immunoprofiles as seen indifferent small samples (A–C vs D–F) or different areas of a resected tumor (J–L). Patient12 (M–O) illustrates a solid growth pattern in ADC which resembles SQCC morphologically (“squamoid” or “pseudo-squamous” appearance), but it is readily identified as being of a glandular rather than squamous lineage by IHC. ‡ ID corresponds to patient and sample numbers in Table 4. a adenocarcinoma component, s squamous component, H&E hematoxylin and eosin stain, PAP Papanicolaou stain; arrowheads in G–I – benign pneumocytes (TTF-1+), arrowheads in M–O – benign bronchial basal cells (ΔNp63+)