Small cell lung cancer (SCLC) is a neuroendocrine carcinoma that tends to grow rapidly, and accounts for about 15% of all lung cancers. Despite treatment regimens for such aggressive tumors, limited progress has been made in more than two decades; outcomes remain poor with a median overall survival of about 10 months. As such, there is a pressing need for new therapeutic options. Whole genome sequencing studies on patient samples have shown SCLC to be highly mutated; the majority of these genetic alterations are not easily druggable, neither have they been validated for their role in SCLC pathogenesis. Of the most frequently mutated genes in SCLC, mutations in the CREB binding protein (CREBBP) gene can be found in 15-17% of patients. CREBBP is involved in the transcriptional coactivation of many different transcription factors, important for influencing critical processes such as embryonic development, cell growth control and homeostasis by coupling chromatin remodeling to transcription factor binding. Additionally, CREBBP has histone acetylation activity, transferring an acetyl group to histones which impact gene expression. CREBPP deletion, truncating and missense mutations frequently occur in the histone acetyltransferase (HAT) domains to abrogate CREBBP-mediated histone acetylation.
The loss of function of the tumor suppressor p53 is an important contributor to oncogenesis. In mouse models of leukemia and lymphoma, CREBBP has been shown to function as a tumor suppressor, where the loss of CREBBP-mediated acetylation and activation of p53 drives tumorigenesis. There is thus the assumption that in solid tumors such as SCLC that almost invariably harbor TP53 mutations, CREBBP cannot act through this mechanism to exert tumor suppressor function. Therefore, there is a crucial need to establish if CREBBP can act as a tumor suppressor in SCLC. Using several genetic models, Dr. David MacPherson and members of his laboratory (Human Biology Division), along with collaborators from the University of Virginia, showed that CREBBP indeed functions as a tumor suppressor independent of p53 in SCLC. They also showed how this finding could be a promising therapeutic opportunity for SCLC patients, and reported their work in a recent issue of the journal Cancer Discovery.
In addition to p53, the retinoblastoma 1 (Rb1) protein is also a tumor suppressor. Deletions of both of these genes in the lung have been shown to result in SCLC in murine models. Utilizing cell-based and mouse models of p53-deficient, Rb1-deficient SCLC, the authors showed that loss of CREBBP resulted in more aggressive tumor growth and increased death in mice, validating the ability of CREBBP to function as a tumor suppressor in SCLC. Additionally, the tumor suppressor activity of CREBBP was also demonstrated in other neuroendocrine tumors such as pituitary and thyroid tumors using genetic mouse models where Rb1, Tpr53 and Crebbp genes are deleted in neuroendocrine cells. Given these in vivo data, the authors reasoned that Crebbp loss cooperates with Rb1/Trp53 deletion to drive tumorigenesis in neuroendocrine tumors through gene expression, and compared the transcriptomes of tumors with and without Crebbp deletion. CREBBP expression in neuroendocrine tumor cells promotes a cellular adhesion related transcriptional program, where E-CADHERIN (CDH1) was of particular interest. CDH1 protein is important in the epithelial to mesenchymal transitions (EMT) that occur in cancer pathogenesis. CDH1 is also a tumor suppressor, and the authors showed that reduced expression of CDH1 induces a partial EMT program when CREBBP is inactivated. This was further validated through rescue experiments, where restoration of CREBBP expression increased CDH1 expression and impaired proliferation. To dissect the molecular underpinnings of the relationship between CREBBP and CDH1, the authors showed that histone acetylation may be the method by which CREBBP is regulating the expression of CDH1 and other cellular adhesion genes. Histone deacetylases (HDACs) can counteract the histone acetyltransferase activity of CREBBP; the authors employed a HDAC inhibitor, Pracinostat on SCLC cell lines and mouse models and found that loss of CREBBP increased therapeutic responses. Dr. MacPherson explained these findings and how they are relevant in other neuroendocrine tumors: “In our studies, Crebbp loss cooperated with Rb/p53 loss to promote not only SCLC, but neuroendocrine pituitary and thyroid tumors as well. As Crebbp loss regulated similar adhesion-related gene expression programs across all three tumor types, I would predict that other neuroendocrine tumors with Crebbp inactivation would also show increased sensitivity to Pracinostat. This will be important to test in the future.”
This study demonstrates the critical role CREBBP plays in SCLC tumor suppression, and its potential use in targeted therapy. The authors also generated a valuable SCLC mouse model that will be indispensable in examining further mechanisms of CREBBP tumor suppression and potential therapeutic approaches. Dr. MacPherson alluded to what lies ahead: “One step we are taking is trying to accumulate a panel of human patient derived xenograft models of SCLC with or without CREBBP mutation to determine whether inactivation of this gene may predict responsiveness in a model system that is more genetically complex than the mouse models tested thus far. In addition, we need to better understand molecular reasons underlying the heterogeneity in response between CREBBP-null tumors that strongly regressed upon HDAC inhibition compared to those that did not.”
Jia D, Augert A, Kim D-W, Eastwood E, Wu N, Ibrahim AH, Kim K-B, Dunn CT, P S Pillai S, Gazdar AF, Bolouri H, Park K-S, MacPherson D. 2018. Crebbp loss drives small cell lung cancer and increases sensitivity to HDAC inhibition. Cancer Discovery. Sept 4 [Epub ahead of print]
Funding was provided by the National Institutes of Health, American Cancer Society, David R. Jones Fund at University of Virginia, and the International Association for the study of lung cancer.