Lung cancer continues to be the leading cause of cancer deaths in the US with more than 200,000 new cases and150,000 deaths each year. About 10-15% of lung cancer cases belong to a type known as small cell lung cancer (SCLC). SCLC is a devastating disease because of its high propensity for metastases. While recent exome sequencing studies have identified common genetic alterations in SCLC, it remains challenging to determine which of these alterations are required for disease progression. One common alteration in SCLC is amplification of MYCL (Mycl in mice). This gene encodes a member of the MYC family of basic helix-loop-helix (bHLH) leucine zipper transcription factors that are well known to be involved in a wide variety of human cancers. However, whether MYCL is required for tumor development is unknown. To address this question, the Fred Hutch Laboratory of Dr. David MacPherson (Public Health Sciences and Human Biology Divisions), along with collaborators from the University of Virginia and other institutions, developed a new cellular SCLC model by transforming pulmonary neuroendocrine cells (PNECs), the believed cells of origin for SCLC. This model, recently published in Genes and Development, allowed the authors to test the role of candidate oncogenes, including MYCL.
First, the investigators developed transgenic mice wherein PNECs were labeled with GFP (Chga-GFP). PNECs purified with fluorescence-activated cell sorting (FACS) only grew in culture if isolated from a SCLC model (Rb/p53/p130 triple mutant). PNECs isolated from the lungs of the SCLC model before macroscopic tumors were evident expressed neuroendocrine markers but did not form subcutaneous tumors in nude mice, so the authors referred to them as preneoplastic precursors (preSCs). Using a retroviral system, the researchers found that overexpression of any member of the Myc family (L-Myc, N-Myc or c-Myc) in preSCs resulted in sphere formation in culture, growth in soft agar, and formation of palpable tumors in nude mice; all of these are hallmarks of human SCLC. A Mycl mouse mutant generated by Dr. Bob Eisenman’s team at Fred Hutch (Basic Sciences Division) allowed the investigators to assess whether Mycl was required for SCLC. Deletion of Mycl in two different in vivo SCLC models (Rb/p53/p130 or Rb/p53/Pten) resulted in a striking suppression of tumor formation and increased survival. Because mice lacking Mycl are developmentally normal, the authors reasoned that inhibition of Mycl or its downstream oncogenic pathways could represent a novel therapeutic strategy. To identify such pathways, the researchers compared gene expression profiles of cells with (L-Myc-preSCs) or without (preSCs) L-Myc overpexression, which uncovered genes involved in ribosome biogenesis and protein synthesis among the top L-Myc regulated genes. Finally, to test whether increased ribosome biogenesis and rRNA expression constituted a vulnerability of SCLC, the investigators treated both in vitro and in vivo SCLC models with a RNA polymerase I inhibitor (CX-5461), that is currently being tested in clinical trials for leukemias and lymphomas. CX-5461 treatment in a different SCLC model (Rb/p53) resulted in only one out of eight mice with progressive disease whereas nine out nine untreated animals succumbed to progressive disease. At the cellular level, CX-5461-induced SCLC suppression was characterized by a reduction in both cell proliferation and pre-rRNA synthesis. In summary, a combination of in vivo SCLC models with pharmacological studies established that Mycl is essential for SCLC development and that a small molecule inhibitor of RNA polymerase I holds promise as a targeted therapy for SCLC. "The results of deleting L-Myc and finding strong suppression of small cell lung cancer in mouse models was exciting, as it suggested that pharmacological inhibition of L-MYC or its effectors could represent a new strategy for treating small cell lung cancer", said Dr. MacPherson.
Kim DW,Wu N,Kim YC,Cheng PF,Basom R,Kim D,Dunn CT,Lee AY,Kim K,Lee CS,Singh A,Gazdar AF,Harris CR,Eisenman RN,Park KS,MacPherson D. 2016. Genetic requirement for Mycl and efficacy of RNA Pol I inhibition in mouse models of small cell lung cancer. Genes Dev, 30(11), 1289-99.
Funding for this work was provided by the National Institutes of Health and the American Cancer Society.