Application of the Next-Generation Sequencing Technology to Reveal Mechanism of Small Cell Lung Cancer Transformation from Adenocarcinoma.

To the Editor: Small cell lung cancer (SCLC) transformation is one of the most common mechanisms of resistance to epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) in non-SCLC (NSCLC). However, whether such transformation is one from NSCLC to SCLC or the coexistence of both remains controversial.[1] In this study, we performed next-generation sequencing (NGS) with a panel of 168 tumor genes for consecutive repeat biopsy tissue samples of a lung adenocarcinoma patient harboring EGFR exon 19 deletion (ex19del).

In a nonsmoking 26-year-old female patient with chest pain, computed tomography (CT) scan revealed a right lung mass and pleural effusion. Positron emission tomography CT demonstrated right pleura metastases. Cell mass from pleural effusion (Sample A) revealed metastatic lung adenocarcinoma, positive for EGFR ex19del. The patient received gefitinib, with partial response as best response. Four months later, CT scan demonstrated disease progression with increasing pleural effusion and lung nodules. A CT-guided percutaneous transthoracic needle biopsy (PTNB) was performed (Sample B), showing a combined tumor presenting adenocarcinoma and SCLC features with the original EGFR ex19del. The patient was treated with gefitinib combined with five cycles of chemotherapy of cisplatin and etoposide. After partial response for 3 months, the mass was found to progress again. Repeat PTNB (Sample C) revealed pure SCLC (EGFR ex19del). To further explore the cause of rapid progress, we examined Sample C with fluorescence in situ hybridization, finding no anaplastic lymphoma kinase fusion or c-MET amplification. Ten milliliter of blood was collected for the cell-free DNA (cfDNA) analysis. Meanwhile, Sample A, Sample B, Sample C, and blood sample were tested by targeted NGS with a panel of 168 tumor genes. The result showed that Sample A, B, C, and blood sample all held TP53 C176S and EGFR ex19del; the allele frequency of TP53 C176S and EGFR ex19del was 9.1% and 12.7% in Sample A, 87.3% and 86.6% in Sample B, 97.2% and 98.4% in Sample C, and 78.0% and 74.7% in blood sample, respectively; the mutations in chromosomes were always present at the same sites. Unfortunately, the patient died soon. The patient's timeline of therapies and diagnostic procedures is shown in Figure 1.

Figure 1

The patient's timeline of therapies and diagnostic procedures. (a) Chest positron emission tomography CT; (b-d) chest CT; (e and f) timeline of treatment; (g-i) pathological pictures of Sample A, B, C; Sample A and C: HE staining, Sample B: HE and immunohistochemistry staining. green, blue arrow: small cell lung cancer and adenocarcinoma in Sample B. AF: Allele frequency; EP: Etoposide cisplatin; EGFR: Epidermal growth factor receptor; EGFR-TKI: EGFR-tyrosine kinase inhibitor; TTF-1: Thyroid transcription factor-1; CT: Computed tomography; HE: Hematoxylin-eosin.

Our study confirmed repeat PTNB was the effective way to fully understand the mechanisms of resistance and to allow discrimination of tumors with SCLC transformation.[2] The NGS results of consecutive repeat biopsy samples, which showed the presence of TP53 C176S and EGFR ex19del and the same change trend shared by them, confirmed that SCLC transformation is one caused by a phenotypic switch that originates from the common cell type rather than intratumor heterogeneity. Such result is consistent with that of Mainardi et al.[3] that alveolar Type II cells have the capacity to develop into adenocarcinoma. Besides, loss of TP53 could efficiently transform alveolar Type II cells, leading to SCLC.[4] In conclusion, NGS of repeat PTNB biopsy can be applied to monitor and overcome resistance to EGFR-TKI therapy.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given her consent for her images and other clinical information to be reported in the journal. The patient understands that names and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

This work was supported by a grant from the Sci-Tech Support Program of Science and Technology Department of Sichuan Province (No. 2016SZ0073).

Conflicts of interest

There are no conflicts of interest.