KRAS Mutations Are Associated With Solid Growth Pattern and Tumor-Infiltrating Leukocytes in Lung Adenocarcinoma

Natasha Rekhtman; Daphne C Ang; Gregory J Riely; Marc Ladanyi; Andre L Moreira

Disclosures

Mod Pathol. 2013;26(10):1307-1319. 

In This Article

Abstract and Introduction

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.

Introduction

Kristen rat sarcoma viral oncogene (KRAS) mutations are one of the most common oncogenic events in human carcinomas of endodermal origin, occurring at high frequency in adenocarcinomas of lung, pancreatic, and colorectal origin.[1,2]KRAS is an 'old oncogene' in lung cancer, having been first described in these tumors in 1984,[3] but recent years have witnessed a revamped interest in the role of KRAS in lung adenocarcinoma because of the rapid advances in molecularly targeted therapies. Although the efforts to therapeutically target mutant KRAS have thus far proven unsuccessful, KRAS has emerged as a useful negative predictive marker because it occurs in a mutually exclusive fashion with several recently identified targetable mutations, including epidermal growth factor receptor (EGFR)—the molecular target of EGFR tyrosine kinase inhibitors erlotinib and gefitinib. Thus, routine predictive molecular testing of lung adenocarcinomas now commonly combines screening for KRAS together with EGFR mutations.[4,5]

Clinically, KRAS and EGFR mutations define two distinct and contrasting subgroups of lung adenocarcinoma patients. Although KRAS mutations are more common in western than East Asian patients (25–35% vs 5–10%, respectively), EGFR mutations have an inverse prevalence in these ethnic groups (10–20% vs >50%, respectively).[6] In addition, KRAS mutations are more common in smokers, whereas EGFR mutations in never or light smokers.[6] Although the data on prognostic significance of KRAS and EGFR mutations has been conflicting across studies, the adverse prognostic impact of KRAS mutations and the favorable impact of EGFR mutations have been demonstrated in several studies over the years[7–10] and in recent studies from our institution.[11,12] In addition, several studies also suggested that KRAS mutations may be markers of resistance not only to EGFR tyrosine kinase inhibitors[4,5] but also to conventional cisplatin-based chemotherapy.[13–15]

Histologically, it is well established that EGFR mutations occur preferentially in non-mucinous adenocarcinomas with lepidic/bronchioloalveolar and papillary patterns (reviewed in Travis et al[16]). By contrast, KRAS mutations are over-represented in mucinous adenocarcinomas.[17–20] However, mucinous carcinomas account for only a minority of lung adenocarcinomas with KRAS mutations in western populations,[18,20,21] and therefore this association is unlikely to explain the distinct clinical characteristics imparted by KRAS mutations. Several previous studies also suggested that KRAS mutations are associated with poor differentiation,[22–24] but this finding has been inconsistent across publications. Furthermore, because grading of lung carcinomas is not well-established, it is not known which morphological features (growth pattern, cytological features, necrosis, etc) may have imparted this association.

The goal of this study was therefore to re-examine potential histopathological correlates of KRAS mutations, particularly in non-mucinous adenocarcinomas. In addition to recent clarification regarding adverse prognostic significance of KRAS mutations, this re-examination was also prompted by advances in mutation testing methodology, with emergence of methods like mass spectrometry-based genotyping (Sequenom platform), which detect a wide spectrum of KRAS mutations with higher analytical sensitivity than standard Sanger sequencing. The use of a higher-sensitivity method to detect KRAS mutations can be anticipated to yield a more robust molecular baseline for the study of histological and other clinicopathological correlates of mutations.

With these considerations in mind, we performed a detailed histological and clinicopathological analysis of 180 lung adenocarcinomas annotated for KRAS and EGFR mutations by mass spectrometry-based genotyping and sensitive PCR-based assays with the main goal to re-examine potential histopathological characteristics associated with KRAS mutations.

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