PERSPECTIVES normal physiology and cancer remains TIMELINE unclear, and is beyond the scope of this article (for a review on these topics, see Thirty years of BCL-2: translating REF. 10). This Timeline article focuses on key advances in our understanding of the function of the BCL-2 protein family in cell death discoveries into novel cell death, in the development of cancer, cancer therapies and as targets in cancer therapy. Early studies on apoptosis Alex R. D. Delbridge, Stephanie Grabow, Andreas Strasser and David L. Vaux In their 1972 paper that adopted the word ‘apoptosis’ to describe a physiological Abstract | The ‘hallmarks of cancer’ are generally accepted as a set of genetic and process of cellular suicide, Kerr and epigenetic alterations that a normal cell must accrue to transform into a fully colleagues11 recognized the presence malignant cancer. It follows that therapies designed to counter these alterations of apoptotic cells in tissue sections of miht e effective as anti-cancer strateies ver the past 3 years, research on certain human cancers. Accordingly, the BCL-2-regulated apoptotic pathway has led to the development of they proposed that increasing the rate of apoptosis of neoplastic cells relative to their small-molecule compounds, nown as BH3-mimetics, that ind to pro-survival rate of production could potentially be BCL-2 proteins to directly activate apoptosis of malignant cells. This Timeline therapeutic. However, interest in cell death article focuses on the discovery and study of BCL-2, the wider BCL-2 protein family and its role in cancer languished until the and, specifically, its roles in cancer development and therapy late 1980s, when genetic abnormalities that prevented cell death were directly linked to malignancy in humans. In the past 30 years, cell death has become and established a new hallmark of Until the early 1980s, most oncogenes a major field of investigation, building cancer2 — evasion of cell death were discovered as genes carried by to a crescendo with the recent award of (apoptosis) (FIG. 1). transforming retroviruses (for example, ‘Breakthrough Therapy Designation’ from Over the past three decades, research v-myc, v-src and v-abl), genes located at the US Food and Drug Administration in hundreds of laboratories has identified recurrent chromosomal translocation (FDA) to ABT-199 (also known as and characterized at least 16 members break points (for example, BCR–ABL venetoclax), a selective inhibitor of of the BCL-2 protein family, and and MYC), or genes that could transfer the anti-apoptotic protein BCL-2, in categorized them into three functional oncogenic properties from malignant recognition of its promise as a treatment groups that each bear one or more cells to non-malignant ones (for example, for patients with chemotherapy- resistant BCL-2 homology (BH) domains (FIG. 2). mutant RAS)12. Whereas the normal chronic lymphocytic leukaemia (CLL)1. These are the pro-survival BCL-2 family counterparts of these oncogenes promoted Although research on cell death members (including BCL-2 itself), cell proliferation in a controlled manner, extends back more than 150 years, the multi-BH-domain pro-apoptotic when they were dysregulated in cancers, until the late 1980s it remained an members (such as BAX and BAK) and they caused uncontrolled cell growth esoteric subject. Today, however, it is the pro-apoptotic ‘BH3-only’ proteins and proliferation. a major research field, with more than (such as BIM (also known as BCL2L11) 20,000 new publications on apoptosis and PUMA (also known as BBC3)). Many BCL2: a novel class of oncogene or programmed cell death appearing of the upstream pathways that control The discovery of BCL-2 started with the each year. The explosion in interest was these proteins have been elucidated, as association of t(14;18) chromosomal sparked by research on one protein, well as the effector processes triggered translocations with human follicular BCL-2, as experiments on BCL-2 showed by their activation that are the ultimate lymphoma by Fukuhara and Rowley13. that mechanisms for cell death are highly cause of cell demolition. Reviews that This enabled others to clone the conserved throughout the evolution provide detailed in-depth discussion of chromosomal break point, and of animals, and because chromosomal BCL-2-regulated apoptosis signalling at a subsequently the cDNA, of a gene on translocations that activate the BCL2 gene molecular level are available3–9. Although chromosome 18, which was termed BCL2 are associated with malignant disease in non-apoptotic roles for BCL-2 family for B cell leukaemia or lymphoma gene humans. Thus, the identification of BCL-2 members have been investigated, including number 2 (REFS 14–18). as an inhibitor of cell death marked roles in autophagy, endoplasmic reticulum The strong association of translocations recognition of the first component of a (ER) calcium dynamics and the unfolded involving BCL2 with follicular lymphoma cell death mechanism in any organism, protein response, their importance in suggested that it was an oncogene, but NATURE REVIEWS | CANCER VOLUME 16 | FEBRUARY 2016 | 99 © 2016 Macmillan Publishers Limited. All rights reserved PERSPECTIVES • Cloning of human MCL1 (REF. 57) BAX loss of Bim-knockout mice show that • Cloning of mouse A1 (REF. 59) function mutations Discovery that platelets • ABT-199 granted BH3-only proteins are essential require BCL-X for survival176 • Cloning of chicken, human and reported in L Genetic analysis to breakthrough 58 for initiation of apoptosis in at mouse BCLX • Two distinct pathways, one haematopoietic least some circumstances97 determine which therapy designa- Identification of cell death • Cloning of human BAX62 regulated by BCL-2 and the cancers and in Inhibitors of oncogenic kinases pro-survival BCL-2 family tion by the US FDA1 t(14;18) inhibitory function of BCL-2, • BCL-2 can block death in cells other by ‘death receptors’, colon cancer with kill tumour cells by activating member is needed for • Phase I clinical trial translocation its oncogenic activity and its Subcellular localization lacking mitochondrial DNA34 can lead to apoptosis47 a hypermutation Roles of BH3-only proteins BIM, and this killing is enhanced sustained growth of results for ABT-199 identified13 ability to synergize with MYC21 of BCL-2 resolved30,33–35 • Bcl2-knockout mice created83,85 • Cloning of mouse Bad67 phenotype142,143 determined97,145,146,193,194 by BH3-mimetic compounds150,151 cancer186–190 reported180 1978 1984 1986 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2003 2004 2005 2006 2007 2010 2013 2014 2015 Cloning of the t(14;18) break • BCL2 transgenic mice BCL-2 inhibits • BCL-2 and CED-9 • Cloning of mouse Bid • Discovery of BIM76 • Loss of BIM rescues developmental • Different BH3-only Development and point involving BCL2 (REF. 14) developed22 cell killing are sequence and development of • Two CD95 (also defects in BCL-2-deficient mice86 proteins have different first clinical trials with • BCL-2 protects against a induced by homologues53 the BH3-only protein known as APO-1 • Discovery of human PUMA79,80 abilities to bind to ABT-199 reported179 wide range of cytotoxic anti-cancer • Requirement for a concept; evidence or FAS) signalling different pro-survival Cloning of human 25 28,29 88 16,17 stress agents drugs mitochondrial for direct activation pathways, only • Mcl1 knockout mice created BCL-2 family BCL2 cDNA 72,74 component shown in of BAX by a BH3-only one of which can • Discovery of mouse NOXA78 members Clinical trial results a cell-free system of protein70 be inhibited by • Bax and Bak double knockout mice developed; • Development of with ABT-263 first Synergy of defects in apoptosis apoptosis113 BCL-2, identified48 BH3-mimetics168 reported171 • Structure of BCL-XL demonstration that BAX and BAK have and cell proliferation control in reported167 107 lymphomagenesis demonstrated essential overlapping roles in apoptosis in vivo36 • Not all apoptosis is blocked by BCL-2 (REF. 46) • Discovery that BCL-2 inhibits mitochondrial release • BCL-2 is a functional of cytochrome c and of the role of cytochrome c in homologue of Caenorhabditis activation of the caspase cascade115,116 elegans CED-9 (REF. 52) • Translocation of BAX to mitochondria117 • Inhibition of BAX channels by BCL-2 (REF. 118) Figure 1 | Timeline of key discoveries. This timeline descries ey discoveries related to the identification of BCL-2, determination of its function, its role Nature Reviews | Cancer and that of other BCL-2 family memers in cancer, and efforts to taret BCL-2 family memers for the treatment of cancer DA, ood and Dru Administration CL, myeloid cell leuaemia did not constitute proof, and the amino Although BCL-2 was originally death alone was not sufficient to render a acid sequence did not provide clues reported to reside on the cytosolic face of cell fully transformed. In 1983, Land et al.39 to its function. Because expression of intracellular membranes30,31, subsequent had shown that transformation of fibroblasts several known oncogenes, including articles suggested that it is localized to required expression not only of a RAS SV40 large T antigen, v-abl and v-fms, the inner mitochondrial membrane26 or the oncogene, but also of MYC. Furthermore, enabled interleukin-3 (IL-3)-dependent plasma membrane32. This was resolved by in a patient, Croce et al.40 had observed the mouse myeloid FDC-P1 cells to grow Monaghan et al.33, Jacobson et al.34 and transformation of a follicular lymphoma in the absence of that cytokine, and to Lithgow et al.35 who showed that the first bearing a BCL2 translocation into acute form tumours in mice (see, for example, reports were correct, and that BCL-2 resides pre-B cell leukaemia after acquisition REFS 19,20), a BCL2 expression construct on the outer mitochondrial membrane, the of a second chromosomal translocation was transduced into these cells and they ER membrane and the nuclear envelope, involving MYC.