KRAS mutations are associated with solid growth pattern and tumor-infiltrating leukocytes in lung adenocarcinoma

Abstract

KRAS mutations define a clinically distinct subgroup of lung adenocarcinoma patients, characterized by smoking history, resistance to EGFR-targeted therapies, and adverse prognosis. Whether KRAS-mutated lung adenocarcinomas also have distinct histopathological features is not well established. We tested 180 resected lung adenocarcinomas for KRAS and EGFR mutations by high-sensitivity mass spectrometry-based genotyping (Sequenom) and PCR-based sizing assays. All tumors were assessed for the proportion of standard histological patterns (lepidic, acinar, papillary, micropapillary, solid, and mucinous), several other histological and clinical parameters, and TTF-1 expression by immunohistochemistry. Among 180 carcinomas, 63 (35%) had KRAS mutations (KRAS+), 35 (19%) had EGFR mutations (EGFR+), and 82 (46%) had neither mutation (KRAS-/EGFR-). Solid growth pattern was significantly over-represented in KRAS+ carcinomas: the mean±s.d. for the amount of solid pattern in KRAS+ carcinomas was 27±34% compared with 3±10% in EGFR+ (P<0.001) and 15±27% in KRAS-/EGFR- (P=0.033) tumors. Furthermore, at least focal (≥20%) solid component was more common in KRAS+ (28/63; 44%) compared with EGFR+ (2/35; 6%; P<0.001) and KRAS-/EGFR- (21/82; 26%; P=0.022) carcinomas. KRAS mutations were also over-represented in mucinous carcinomas and were significantly associated with the presence of tumor-infiltrating leukocytes and heavier smoking history. EGFR mutations were associated with non-mucinous non-solid patterns, particularly lepidic and papillary, lack of necrosis, lack of cytological atypia, hobnail cytology, TTF-1 expression, and never/light smoking history. In conclusion, extended molecular and clinicopathological analysis of lung adenocarcinomas reveals a novel association of KRAS mutations with solid histology and tumor-infiltrating inflammatory cells and expands on several previously recognized morphological and clinical associations of KRAS and EGFR mutations. Solid growth pattern was recently shown to be a strong predictor of aggressive behavior in lung adenocarcinomas, which may underlie the unfavorable prognosis associated with KRAS mutations in these tumors.

FIGURE 1

Distribution of patterns according to mutation: the box plots. Y axis indicates the amount of pattern per tumor (0-100%). Upper and lower box borders = 25th and 75th percentiles, whisker = 10th and 90th percentiles, horizontal line = median, plus sign = mean, and dots = outliers. If box borders or a median are not visible – their value is 0. Bolded P values (Mann-Whitney test) are statistically significant. Mucinous pattern was excluded from this analysis due to the overall low number of cases with this pattern. SD standard deviation

FIGURE 2

An inverse graduated effects of the amount of solid and lepidic patterns on the likelihood of KRAS and EGFR mutations. The denominator for shown percentages is the number of cases with indicated amount of pattern.

FIGURE 3

Examples of solid component in adenocarcinomas with KRAS mutations. At least focal (>20%) solid component was present in 55% of KRAS+ carcinomas (A-E) compared to 4% of EGFR+ carcinomas (F). These solid areas, some of which have “squamoid” appearance, were distinguished from a true squamous component by immunohistochemistry for TTF-1 (inset in A and B), and/or negative p40/ΔNp63 (not shown). Images also illustrate a spectrum of cytologic atypia from minimal (A, B) to moderate (C, D) to marked (E), and spectrum of tumor-associated leukocytes from minimal (A,B) to moderate (D, E) to marked (C) in KRAS+ carcinomas.