Escenario 1

Although initially CLL was considered a static disease with uncontrolled accumulation of lymphocytes due to failed apoptosis, the current understanding is that proliferation occurs in secondary lymphoid organs in areas known as proliferation centres of tissue microenvironment leading to expansion and prolong survival of CLL cells in vivo (Chiorazzi, 2007; Reed, 2008; Wu et al., 2018)

The major cause of inefficacy of most targeted therapies is acquired resistance towards apoptotic mechanisms, as evading apoptosis is one of the hallmarks of CLL cells. Owing to the fact that apoptosis causes minimum inflammation and damage to the tissue, targeting apoptotic pathways has been a centre of attraction for the development of new improved anti-cancer drugs.

Apoptosis is a cell suicide mechanism that leads to characteristic morphological alterations such as membrane blebbing, reduced cytoplasm, chromatin condensation and fragmentation of genomic DNA eventually causing breakdown of a cell (Parrish et

al., 2013; Kalkavan and Green, 2017). It is an irreversible process regulated by a family of cysteine dependent aspartate specific proteases known as caspases. The caspases are divided into initiator caspases and executor caspases. The initiator caspases (8-9- 2-10) are responsible for interaction with adaptor proteins in response to apoptotic stimuli. Once activated initiator caspases then cleave effector caspases (3-7-6) thus triggering the activation of caspase cascade. Following initiation, effector caspases cleave a wide range of substrates including poly-ADP-ribose protein (PARP) that results in structural breakdown of cells and finally in apoptosis (Strasser, 2011; Koff et al., 2015).

Apoptosis can be induced by two known pathways (Figure 1.9), either by intrinsic pathway following intracellular activation involving mitochondria (mitochondrial pathway) or by activation of cell surface receptors known as “death receptors”, the extrinsic pathway (Billard, 2014). Activation of the intrinsic pathway is triggered by a wide range of intracellular signals such as radiation, cytotoxic drugs, cellular stress and growth factor withdrawals, and is closely regulated by the Bcl-2 family proteins. Upon stimulation of apoptosis, the balance between pro-apoptotic proteins (Bax, Bak, Bcl-xS

and Bcl-GL)and anti-apoptotic proteins or prosurvival proteins (Bcl-2, Bcl-XL, BCl-W,

Mcl-1 and Af/BF1-1) is disturbed resulting in the release of Bax and Bak sequestered by prosurvival member of Bvel-2 family. The anti-apoptotic proteins contain BH1-4 domain and a transmembrane domain, and the proapoptotic proteins have two multi protein domains (Bax, Bak, Bcl-xs and Bcl-GL) and only one BH3 (Bim/Bod, Bad, Bid, Bmf, Bcl-Gs, Blk, Hrk/DP5, Noxa and PUMA/Bbc3). On receiving apoptotic signals BH3 domain proteins bind with prosurvival proteins and antagonise them to release activated Bax and Bak which increases permeabilisation of mitochondrial membrane. This allows the release of cytochrome C which forms an apoptosome containing adaptor protein Apaf-1, inactive initiator procaspase 9 and cytochrome C. Procaspase 9 becomes activated in the apoptosome complex and cleaves caspases -8 and -9 thus triggering the activation of effector caspases. Once the effector caspase cascade is activated, the cell undergoes morphological and biochemical changes regulated by effector caspase 3 and becomes committed to apoptosis (Strasser, 2011; Billard, 2014; Czabotar et al., 2014).

Figure 1.9. Schematic overview of extrinsic (Mitochondria independent) and intrinsic (Mitochondria dependent) apoptosis pathways. The extrinsic pathway activation involves the binding of ligands to the death receptor that results in the formation of death inducing signaling complex (DISC). DISC enables activation of caspace-8, which then cleaves and activates executer caspases 3 and 7 which then lead to demise of the cell. The intrinsic pathway is initiated following cellular stress causing activation of BH3 that leads to the recruitment of pro-apoptotic proteins Bax and Bak, and the release of cyctochrome C into the cytosol which forms an apoptosome with procaspase-9 and apoptotic protease activating factor-1 (Apaf-1). Activated caspase- 9 then cleaves caspase-3 and 7 allowing apoptosis process to continue to the cell death (Yuan et al., 2012).

The extrinsic pathway also known as mitochondria independent pathway involves the ligation of death receptors which are members of the tumour necrosis factor receptor family such as (TNF-R, FAS, DR4) to their respecting ligands (TNF-L, FAS-L, TRAIL) thus exposing a death domain (DD) on the cytoplasmic site of the receptor. The binding of death receptor to its ligand recruits an adaptor protein and forms a death inducing signal complex (DISC). Within this DISC initiator caspase 8 and effector caspase 3 are cleaved and activated which leads to the demise of the cell.

Apoptosis resistance in CLL is influenced by many factors. CLL cells demonstrate overexpression of Bcl-2 family proteins and the increased Bcl-2 /Bax ratio that induces cell survival instead of apoptosis has led to a significantly poor prognosis in CLL

patients (Packham et al., 2005, 2014). Another anti-apoptotic Bcl-2 family protein Mcl- 1 has been observed to promote cell survival in CLL as high expressions of MCL-1 correlated with increased resistance to apoptosis and inability to attain complete remission following cytotoxic therapy (Pepper et al., 2008; Palma et al., 2015).

Another factor that can cause dysregulation of apoptosis is alterations or loss of function of the P53 gene. Tp53 protein is a transcription factor also known as tumour suppressor protein 53 (TP53). It regulates cellular responses to DNA damages by induction of apoptosis, DNA recombination and cellular senescence. The ataxia telangiectasia-mutated gene (ATM) is the activated gene in the P53 signaling pathway which can block cell cycle progression and stimulate DNA repair. Any defects in P53/ATM gene can contribute to a reduced response to apoptosis signals and defective DNA repair mechanism and as a result an increased survival of CLL cells. A variety of factors can damage P53 gene such as radiation, various chemicals, and viruses such as the Human papillomavirus (HPV) and therefore over 50% of all human cancers have been detected with the mutation of P53/ATM genes in cytogenetic analysis (Zenz et al., 2010). In addition to above mentioned factors, changes in the expression of other apoptotic regulators such asthe nuclear factor-kappa B (NF-κB) transcription factor and phosphatidylinositol-3 kinase (P1-3K) signalling pathways can induce cell survival. Many cytokines, including B-cell activation factor (BAFF), a proliferation inducing ligand (APRIL), CD40 ligand and interleukin-4 (IL-4) are activated in CLL cells and promote the survival of CLL cells, and CLL cells appear to be resistant to apoptosis if abnormalities are identified in signalling via cell-surface death receptors. Intracellular signaling following antigenic stimulation via BCR is also been recognised to influence apoptotic pathways in CLL due to presence of overexpression of apoptotic kinases in activated CLL cells (Chen et al.,2010; Hanahan, 2010).

Factors affecting various apoptosis mechanisms are schematically summarised in Figure 1.10.

Figure 1.10. Factors involved in the apoptosis resistance. The mechanisms by which the evasion of apoptosis occurs in cancer, including defective genes, reduced caspase function, damaged signaling pathways and overexpression of Bcl-2 proteins (Wong, 2011).

Since dysregulation of apoptosis is a critical factor in the pathogenesis of CLL, strategies that induce apoptosis of malignant cells and bypass Bcl-2 will be therapeutically useful. This would require an improved understanding of a correlation between defective apoptosis in CLL and responses to therapy.