Morphologic and Other Forms of Heterogeneity in Small Cell Lung Cancer: What Can We Learn from Them?

Abstract

SCLC is a deadly, recalcitrant form of lung cancer that is strongly associated with tobacco exposure. Inactivation of the tumor protein p53 gene (TP53) and retinoblastoma 1 gene (RB1) genes in SCLC is almost universal and is believed to be the initiating molecular event.1George J. Lim J.S. Jang S.J. et al.Comprehensive genomic profiles of small cell lung cancer.Nature. 2015; 524: 47-53Crossref PubMed Scopus (1214) Google Scholar, 2Gazdar A. Bunn P. Minna J. Small-cell lung cancer: what we know, what we need to know and the path forward.Nat Rev Cancer. 2017; 17 ([correction: 765]): 725-737Crossref PubMed Scopus (411) Google Scholar The WHO classification of lung cancers recognizes only one major morphologic form of SCLC, although elements of NSCLC cancers may be present (combined SCLC [CSCLC]).3Travis W.D. Brambilla E. World Health Organization Classification of Lung and Pleural Tumors. Springer-Verlag, Berlin, Germany2015Google Scholar Historically, SCLC has been regarded as a “homogenous” disease (with little documented intertumor or intratumor heterogeneity with respect to histologic characteristics or molecular biology). Consequently, most patients with SCLC are treated similarly with doublet chemotherapy, usually with a platinum agent and etoposide,4Rudin C.M. Ismaila N. Hann C.L. et al.Treatment of small-cell lung cancer: American Society of Clinical Oncology endorsement of the American College of Chest Physicians guideline.J Clin Oncol. 2015; 33: 4106-4111Crossref PubMed Scopus (210) Google Scholar with or without thoracic radiation, after adjustments for tumor extent or stage, and this has remained the standard of care for over 30 years.2Gazdar A. Bunn P. Minna J. Small-cell lung cancer: what we know, what we need to know and the path forward.Nat Rev Cancer. 2017; 17 ([correction: 765]): 725-737Crossref PubMed Scopus (411) Google Scholar CSCLC cases represent examples of intratumor heterogeneity and have also been described in genetically engineered mouse models (GEMMs).5Gazdar A.F. Savage T.K. Johnson J.E. et al.The comparative pathology of genetically engineered mouse models for neuroendocrine carcinomas of the lung.J Thorac Oncol. 2015; 10: 553-564Abstract Full Text Full Text PDF PubMed Scopus (77) Google Scholar In this issue of the Journal of Thoracic Oncology, Zhao et al. report the most comprehensive study of CSCLC described to date.6Zhao X. McCutcheon J.N. Kallakury B. et al.Combined small cell carcinoma of the lung: is it a single entity?.J Thorac Oncol. 2017; 13: 237-245Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar In 170 cases of resected SCLC from the People's Republic of China, the incidence of CSCLC was 6.9%. However, the investigators did not estimate the percentages of the NSCLC components. CSCLC tumors had a shorter survival time than pure SCLC cases; however, the number of CSCLC cases was modest and the p value was borderline. Thus, this finding needs to be confirmed with larger data sets, perhaps pooled from multiple institutions. The differences in survival may be due to the relative resistance of the NSCLC component in CSCLC to cytotoxic therapies. There were no other important clinicopathologic differences between the two tumor types. In three CSCLC cases the investigators were able to microdissect the individual SCLC and NSCLC components and perform targeted exome sequencing. About 75% of the genetic changes were similar in both compartments, including TP53 mutations. Unfortunately, we are not informed of the status of the RB1 gene, perhaps because mutations were not detected. These findings suggest that both morphologic components arose from the same initial clonal event and then underwent subclonal evolution. Although SCLC is often considered to be a poorly differentiated carcinoma on the basis its morphology and presence of small, sparse dense core granules, most SCLC tumors and cell lines express the entire range of neuroendocrine (NE) markers in high abundance.7Zhang W, Girard L, Zhang Y, et al. SCLC tumors and preclinical models display heterogeneity of neuroendocrine phenotypes. Transl Lung Cancer Res, in press.Google Scholar However, about 10% to 16% of tumors and cell lines have greatly reduced or absent expression of NE markers.7Zhang W, Girard L, Zhang Y, et al. SCLC tumors and preclinical models display heterogeneity of neuroendocrine phenotypes. Transl Lung Cancer Res, in press.Google Scholar, 8Rekhtman N. Neuroendocrine tumors of the lung: an update.Arch Pathol Lab Med. 2010; 134: 1628-1638Crossref PubMed Google Scholar These two subtypes represent examples of intertumor heterogeneity. More than 30 years ago my colleagues and I described a “variant” form of SCLC with altered morphologic features, decreased NE markers, and frequent v-myc avian myelocytomatosis viral oncogene homolog (MYC) amplification/overexpression.9Gazdar A.F. Carney D.N. Nau M.M. Minna J.D. Characterization of variant subclasses of cell lines derived from small cell lung cancer having distinctive biochemical, morphological, and growth properties.Cancer Res. 1985; 45: 2924-2930PubMed Google Scholar, 10Carney D.N. Gazdar A.F. Bepler G. et al.Establishment and identification of small cell lung cancer cell lines having classic and variant features.Cancer Res. 1985; 45: 2913-2923PubMed Google Scholar We recently summarized the modest literature about the low NE SCLC tumors and their association with the variant form.7Zhang W, Girard L, Zhang Y, et al. SCLC tumors and preclinical models display heterogeneity of neuroendocrine phenotypes. Transl Lung Cancer Res, in press.Google Scholar Although the variant form is not officially recognized by the WHO, it has been described in SCLC cell lines and tumors and in GEMMs.11Mollaoglu G. Guthrie M.R. Bohm S. et al.MYC drives progression of small cell lung cancer to a variant neuroendocrine subtype with vulnerability to aurora kinase inhibition.Cancer Cell. 2017; 31: 270-285Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar, 12Calbo J. van Montfort E. Proost N. et al.A functional role for tumor cell heterogeneity in a mouse model of small cell lung cancer.Cancer Cell. 2011; 19: 244-256Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar This SCLC subtype has, in addition to reduced NE markers, loss of the transcription factor NK2 homeobox 1 gene (NKX2-1) (also known as thyroid transcription factor 1) and the Notch inhibitor delta like canonical Notch ligand 3 (DLL3), and upregulation of the MYC gene, the Notch, HIPPO, and transforming growth factor β pathways, as well as considerable epithelial-mesenchymal transition. In addition, the low-NE cells express RE1 silencing transcription factor, a transcriptional repressor of NE differentiation (REST) present in most non-NE cells, including in NSCLC. Also, the low-NE subtype is radioresistant and chemoresistant compared with the NE subtype. In a MYC-driven GEMM, conversion of a NE high classic subtype to a NE low-variant subtype has been described,11Mollaoglu G. Guthrie M.R. Bohm S. et al.MYC drives progression of small cell lung cancer to a variant neuroendocrine subtype with vulnerability to aurora kinase inhibition.Cancer Cell. 2017; 31: 270-285Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar indicating that the subtypes are plastic and at least a one-way conversion may occur. Another GEMM has been reported with low NE expression, and activation of Notch pathway and RE1 silencing transcription factor without obvious morphologic changes.13She Y, L’im J, Sage J. Tumor heterogeneity in small cell lungcancer defined and investigated in pre-clinical mouse models. Transl Lung Cancer Res, in press.Google Scholar, 14Lim J.S. Ibaseta A. Fischer M.M. et al.Intratumoural heterogeneity generated by Notch signalling promotes small-cell lung cancer.Nature. 2017; 545: 360-364Crossref PubMed Scopus (225) Google Scholar These profound differences in biological and other characteristics have important implications for the conventional and targeted therapies of these two subtypes. Before we can put these examples of intratumor and intertumor heterogeneity into context, one other form of morphologic conversion needs to be discussed. Transdifferentiation is the switch, in malignant or nonmalignant stem cells, from one form of differentiation to another.2Gazdar A. Bunn P. Minna J. Small-cell lung cancer: what we know, what we need to know and the path forward.Nat Rev Cancer. 2017; 17 ([correction: 765]): 725-737Crossref PubMed Scopus (411) Google Scholar It occurs during embryonic development and is well described in prostate carcinomas, in which a switch from adenocarcinoma to NE morphologic type and differentiation is a mechanism of androgen resistance.15Li Y. Donmez N. Sahinalp C. et al.SRRM4 drives neuroendocrine transdifferentiation of prostate adenocarcinoma under androgen receptor pathway inhibition.Eur Urol. 2017; 71: 68-78Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar It should be understood that transdifferentiation does not represent combined tumors, nor is it a form of heterogeneity. Several cases of transdifferentiation in the lung have been reported, describing the genotypic and histologic evolution of adenocarcinoma into SCLC as a mechanism of acquired resistance to EGFR tyrosine kinase inhibitor therapy.16Sequist L. Waltman B.A. Dias-Santagata D. et al.Genotypic and histolgical evolution of lung cancers acquiring resistance to EGFR inhibitors.Science Transl Med. 2011; 3: 1-11Crossref Scopus (2691) Google Scholar, 17Niederst M.J. Sequist L.V. Poirier J.T. et al.RB loss in resistant EGFR mutant lung adenocarcinomas that transform to small-cell lung cancer.Nat. Commun. 2015; 6: 6377Crossref PubMed Scopus (402) Google Scholar These resistant transdifferentiated tumors with SCLC features retained the original EGFR mutation and developed loss of RB1, demonstrating that the original adenocarcinoma had transdifferentiated into SCLC,17Niederst M.J. Sequist L.V. Poirier J.T. et al.RB loss in resistant EGFR mutant lung adenocarcinomas that transform to small-cell lung cancer.Nat. Commun. 2015; 6: 6377Crossref PubMed Scopus (402) Google Scholar as opposed to the SCLC arising independently. The examples of tumors containing both SCLC and NSCLC elements (CSCLC tumors) or of switching from one form to the are important for understanding the origin and interrelationships of lung cancers. For about 30 years, Pearse forcefully presented his amine precursor uptake and decarboxylase (APUD) cell concept.18Pearse A.G. 5-hydroxytryptophan uptake by dog thyroid 'C' cells, and its possible significance in polypeptide hormone production.Nature. 1966; 211: 598-600Crossref PubMed Scopus (116) Google Scholar, 19Pearse A.G. The diffuse neuroendocrine system: peptides, amines, placodes and the APUD theory.Prog Brain Res. 1986; 68: 25-31Crossref PubMed Scopus (75) Google Scholar APUD cells share the common function of secreting low-molecular-weight biogenic amines and polypeptide hormones. Although this concept was basically correct, we now know that the APUD properties are only a small part of the much larger repertoire of NE cell properties, and these cells are now referred to as NE cells. However, Pearse also noted that some NE cells (such as the C cells of the thyroid and adrenal medullary cells) were of neural crest origin, and he postulated that all APUD cells arose from the neural crest, indicating that SCLC and NSCLC tumors arose from very different precursor cells. The respiratory epithelium is a complex structure with differing histologic characteristics and functions, changing from a proximal air transfer system to a distal gas exchange system. Although we now recognize that only some NE cells are of neural crest origin, others, such as the pulmonary NE cells, arise from a local primordial multipotent stem cell or its progeny.20Baylin S.B. “APUD” cells: fact and fiction.Trends Endocrinol Metab. 1990; 1: 198-204Abstract Full Text PDF PubMed Scopus (13) Google Scholar, 21Rosai J. The origin of neuroendocrine tumors and the neural crest saga.Mod Pathol. 2011; 24: S53-S57Crossref PubMed Scopus (65) Google Scholar We know from GEMMs that SCLC tumors usually arise from the pulmonary NE cells predominantly located in the basal layers of the larger airways, although on occasion they may arise from more distally located non-NE cells.22Semenova E.A. Nagel R. Berns A. Origins, genetic landscape, and emerging therapies of small cell lung cancer.Genes Dev. 2015; 29: 1447-1462Crossref PubMed Scopus (156) Google Scholar, 23Sutherland K.D. Proost N. Brouns I. Adriaensen D. Song J.Y. Berns A. Cell of origin of small cell lung cancer: inactivation of Trp53 and Rb1 in distinct cell types of adult mouse lung.Cancer Cell. 2011; 19: 754-764Abstract Full Text Full Text PDF PubMed Scopus (329) Google Scholar These findings provide evidence that SCLC shares a common origin with other lung carcinomas and explain the origins of combined tumors such as CSCLCs and adenosquamous carcinomas. They also explain transdifferentiation. In summary, we can come to several conclusions regarding heterogeneity and transdifferentiation of SCLC that have an impact on our understanding of the biology of lung cancers with important therapeutic implications. Combined lung cancers, including CSCLC, occur and almost certainly share a common origin. CSCLC may have a worse prognosis than pure SCLC tumors, indicating that therapies must target both cellular components. Unfortunately, CSCLC presents diagnostic difficulties, as most SCLC tumors are diagnosed from small biopsy specimens or cytologic materials and minor NSCLC components may not be present in the diagnostic sample. Another example of heterogeneity is the occurrence of high and low NE subtypes of SCLC. As the differences between these subtypes are extensive, encompassing many pathways in addition to NE properties, they are likely to respond very differently to conventional and targeted therapies. However, both subtypes offer many potential, although different, therapeutic targets. Finally, transdifferentiation of adenocarcinoma to SCLC should be recognized as an occasional but important mechanism of resistance after tyrosine kinase inhibitor therapy of EGFR-mutant NSCLC. Although current data are sparse, we presume that transdifferentiated tumors will respond to SCLC-targeted therapies. These observations also demonstrate the plasticity of lung cancers, occurring either spontaneously or in response to therapy, and indicate the necessity of sequential monitoring the phenotype and genotype of lung cancers for selection of optimal therapy. Although sequential sampling of tumor tissue may prove difficult, the high number of circulating tumor cells in pretherapy and posttherapy SCLC cases permits diagnostic and molecular characterization from liquid biopsies or generation of xenograft tumors from circulating tumor cells.24Killock D. Lung cancer: liquid biopsy of SCLC chemosensitivity.Nat Rev Clin Oncol. 2017; 14: 2Crossref PubMed Scopus (4) Google Scholar, 25Wang X. Ma K. Wang Y. He H. Hu J.F. Li W. Evaluation of circulating tumor cells in predicting therapeutic response in small cell lung cancer patients.Arch Med Res. 2016; 47: 454-459Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar, 26Foy V. Fernandez-Gutierrez F. Faivre-Finn C. Dive C. Blackhall F. The clinical utility of circulating tumour cells in patients with small cell lung cancer.Transl Lung Cancer Res. 2017; 6: 409-417Crossref PubMed Scopus (20) Google Scholar Clearly, there are many important lessons to be learned from the morphologic and biological heterogeneity of SCLC. This work was supported by grants from the National Cancer Institute: Specialized Program in Research Excellence in Lung Cancer” (P50 CA70907) and the Small Cell Lung Cancer Consortium Coordinating Center (U24CA213274) Combined Small Cell Carcinoma of the Lung: Is It a Single Entity?Journal of Thoracic OncologyVol. 13Issue 2PreviewSCLC accounts for 15% and 20% of all lung cancers, with combined SCLC (CSCLC) comprising 2% to 5%. Little is known about the clinical characteristics and molecular changes associated with the various histologic components. Full-Text PDF Open Archive