Atlas of Genetics and Cytogenetics in Oncology and Haematology

Background on apoptosis Cell death can be achieved by two fundamentally different mechanisms, apoptosis and necrosis. Apoptosis is characterized by several morphological features that include condensation of nuclei and internucleosomal degradation of DNA, cell membrane blebbing, and formation of apoptotic bodies. By contrast, necrosis is recognized by swelling of the cell and organelles, followed by disintegration of the cell and the release of cytoplasmic material. Although the necrotic signalling pathway remains largely uncharacterized at the molecular level, apoptosis is known to be dependent on a family of intracellular cysteine proteases, called CASPASES (Cysteine Aspartate Specific ProteASEs). Caspases (Nicholson DW, 1999) are initially synthesized as inactive cytosolic proteases (proenzymes) which have a p20 and a p10 domain that, in response to apoptotic stimulation, are cleaved to form the active enzyme. Active caspases then cleave other caspases, Bcl-2 family members, or act on various nuclear substrates, such as lamins and caspase-activated DNase (CAD), finally leading to apoptosis. Caspases contain a cysteine residue within the active site, and which cleave their substrate exclusively after an aspartic acid residue, a sequence of 3 amino acids before aspartate determines substrate specificity. Caspases can be divided into inflammatory caspases (-1, -4, -5, -11, -12, -13 and 14), which cleave and activate proinflammatory cytokines, and proapoptotic caspases, which cleave and activate proapoptotic substrates. Proapoptotic caspases comprise initiator caspases (-2, -8, -9 and -10), which, in turn, cleave and activate effector or executioner caspases (-3, -6 and -7). Apoptosis can be induced either from the cell surface, by ligand-dependent triggering of death receptors (e.g: CD95/Fas; TNF-R1; TNF related apoptosis-inducing ligand receptor 1, TRAIL-R1/DR4 and TRAIL-R2/DR5), or by the stimulation of intracellular receptor proteins, such as apoptotic protease-activating factor 1 (Apaf-1), which is activated by its ligand cytochrome c once it is released from damaged mitochondria (Ranger AM et al., 2001). Both systems transmit apoptotic signals through proteinprotein interactions that are mediated by motifs called the death domain (DD), the death effector domain (DED) and the caspase recruitment domain (CARD). Death domains (DD) are 80-100 residue long motifs found both in cytoplasmic proteins (FADD: Fas-associated protein with Death Domain; TRADD: TNF Receptorassociated protein with Death Domain; and RIP: Receptor Interacting Protein) and in transmembrane proteins including members of the TNF-receptor superfamily, like Fas, TNF-R1, TRAIL-R1/DR4 and TRAIL-R2/DR5. DD serve as recruiting modules