New Molecular Targets in Lung Adenocarcinoma

Oncology & Hematology Review (US), 2013;9(2):122–8

Abstract:

Lung cancer is designated as either non-small-cell lung cancer (NSCLC) or small-cell lung cancer. There are three subtypes of NSCLC: adenocarcinoma (48%), squamous cell carcinoma (28%), and large-cell carcinoma (24%). Epidermal growth factor receptor (EGFR) mutations, anaplastic lymphoma kinase (ALK) rearrangements, and ROS1 rearrangements are co-associated with lung adenocarcinoma in never-smokers. Histologically, lung adenocarcinoma is sub-divided into papillary, acinar, bronchioalveolar, and solid subtypes. A superseding molecular subclassification is emerging with important therapeutic implications. Secondary resistance to medications targeting these molecular abnormalities does invariably occur. It is anticipated that strategies including drugs with increased receptor binding affinity, altered medication pharmacodynamic profiles, and combinatorial approaches will emerge.
Keywords: Adenocarcinoma, tyrosine kinase, crizotinib, sequencing, clinical correlates
Disclosure: The authors have no conflicts of interest to declare.
Received: August 09, 2013 Accepted October 05, 2013 Citation Oncology & Hematology Review (US), 2013;9(2):122–8
Correspondence: Fergal C Kelleher, MSc, MRCSI, MRCP, Department of Medical Oncology, St Vincent’s University Hospital, Dublin, Ireland. E: fergalkelleher@hotmail.com

Annually, 1.3 million cases of NSCLC occur.1 Despite smoking being the most important environmental causative factor for lung cancer, 10% of lung cancers occur in never-smokers, thus emphasizing the importance of genetic factors.2 Detection methods for genomic alterations include array-based profiling, targeted sequencing, and whole-genome sequencing. The following driver genetic alterations and respective frequencies occur in lung adenocarcinoma: EGFR (5–15%), ALK (5–15%), and KRAS (>15%). Genes with a mutation rate <5% include BRAF, PIK3CA, MAP2K1, MET, and HER2. The ability to detect such driver mutations in a majority of lung cancer patient specimens has been demonstrated in large genomic projects, such as the Lung Cancer Mutation Consortium (LCMC),3 which seeks to not only determine driver mutations but also allow clinicians to use this knowledge to use appropriate targeted therapies or enroll patients into relevant clinical trials. In 2012, next generation sequencing of 183 lung adenocarcinomas was reported.4 New findings that supplemented existing literature included recurrent somatic mutations in the splicing factor gene U2AF1 and truncating mutations in ARID1A and RBM10. Loss of function mutations and deletions in tumor suppressor genes are not easy to therapeutically target but deserve inclusion. These include LKB1, TP53, RB1, NF1, CDKN2A, SMARCA4, and KEAP1.5 Inactivation of p16Ink4 also occurs in lung adenocarcinoma and is associated with cigarette smoking. Finally, somatic focal amplifications of NKX2-1 as well as recurrent inframe fusions of KIF5B and RET can also occur.

Molecular Biology
In an assessment of 188 cases of lung adenocarcinoma in 2008, DNA sequencing was performed on 623 selected genes. Twenty-six genes emerged that were frequently mutated and considered likely involved in carcinogenesis.5 The inferred significantly mutated pathways were the MAPK, Wnt, p53 signaling, cell cycle, and mammalian target of rapamycin (mTOR) pathways. Clinically, KRAS mutations and LKB1 mutations were correlated with smoking status. EGFR mutations were correlated with never-smoker status.6 Pathologically, mutations of LRP1B, TP53, and INHBA were negatively correlated with acinar, papillary, and bronchioalveolar subtypes, but were significantly positively correlated with the solid subtype. By contrast, EGFR mutations were negatively correlated with the solid subtype and positively correlated with the papillary subtype. Molecular alterations of importance in lung adenocarcinoma are also not solely restricted to mutations, but include amplifications or gene rearrangements.

Ethnic differences exist in different studied populations with differing molecular tumor characteristics and reported frequencies. The clinician must use interpretive caution when applying data between different groups—Asian and Western populations differences are a notable example. The incidence in lung adenocarcinoma of commonly mutated genes and rearrangements in ethnically different populations is detailed in Figure 1. A study comprising an ethnically heterogeneous series of nonsmall- cell lung cancer (NSCLC) found LKB1 mutations in 17% of NSCLC of US origin compared with 5% of Korean cases (p=0.001).6 EGFR mutations are more frequent in NSCLC arising in patients of Asian ethnicity. Patterns of co-occurrence and mutual exclusivity of genomic alterations are also important. In another case series of NSCLC (n=1,683) ALK rearrangements were mutually exclusive of EGFR or KRAS mutations.7

Technical Considerations
The American College of Pathologists devised evidence-based guidelines in 2013 to select the appropriate patients and samples for EGFR- and ALK-directed therapeutics.8 The principle recommendations are to test for EGFR mutations and ALK fusions to guide patient treatment selection, in patients with lung adenocarcinoma. This directive is irrespective of gender, race, smoking history, or other clinical risk factors.

Detection methods for genetic alterations in lung adenocarcinoma include real-time polymerase chain reaction (RT-PCR), immunohistochemistry (IHC), fluorescent in situ hybridization (FISH), and next generation sequencing. The US Food and Drug Administration (FDA) has approved a FISH assay for ALK rearrangements using dual-labeled break-apart probes. IHC for ALK rearrangements has a sensitivity of 90% and specificity of 97.8 %; however, IHC for ALK rearrangements has been subject to challenge because low expression levels encoded by ALK fusion transcripts may occur.9–13 More reassuringly, 100% correlation with FISH has been more recently established using an ultrasensitive IHC technique.14 In one series IHC was concordant with FISH in 229 of 231 dual-informative samples. Difficulties with RT-PCR for ALK rearranged disease include variability in the partner gene breakpoint and partner gene identity within the gene fusion. In ROS1-rearranged NSCLC, FISH is an appropriate though expensive and technically demanding diagnostic method. RT-PCR does not detect all FISH-positive cases. The inference is that other ROS1 partners or different breakpoints to CD74-ROS1 and SLC34A2-ROS1 may emerge. Similarly, in RET fusion positive lung adenocarcinoma RT-PCR is insufficient to detect RET fusion partners or isoforms. IHC is also currently insufficiently reliable for diagnostic purposes.15–17 In terms of EGFR, the FDA have approved the therascreen EGFR RGQ (Rotor-Gene Q 5plex HRM instrument) PCR Kit to detect EGFR exon 19 deletions or exon 21 (L858R) substitutions.

Epidermal Growth Factor Receptor
Therapeutic responsiveness of NSCLC to the anilinoquinazoline inhibitors gefitinib and erlotinib is correlated with EGFR mutations. These are small deletions that affect amino acids 747–750 or point mutations, the most frequent of which is replacement of leucine by arginine at codon 858 (L858R).18,19 Other less-common mutations include G719S in exon 18 and L861Q in exon 21. EGFR mutations occur in approximately 9% of cases of NSCLC. Treatment responsiveness is clinically correlated with females, nonsmokers, adenocarcinoma histology, and East Asian ethnicity. Patient selection for treatment with EGFR tyrosine kinase inhibitors (TKIs) should be determined by genotyping rather than by clinical features. This was found in a subset analysis of the First Line IRESSA™ Versus Carboplatin/ Paclitaxel in Asia (IPASS) trial of advanced lung adenocarcinoma.20 In that study, gefitinib was found to be superior to chemotherapy with 1-year PFS rate of 24.9% versus 6.7%; hazard ratio (HR) for death or disease progression 0.74 favoring gefitinib (p<0.001). Within the EGFR mutated cohort, gefinib was more efficacious compared with chemotherapy (HR for death or disease progression 0.48), but gefitinib was ineffective in the EGFR wild type cohort (HR 2.85).



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Keywords: Adenocarcinoma, tyrosine kinase, crizotinib, sequencing, clinical correlates