Detection of ALT Associated Promyelocytic Leukemia Nuclear Bodies (APBs) by Immunofluorescence-FISH (IF-FISH)

The activation of functions that counteract the physiological shortening of telomeres in rapidly proliferating cell is prerequisite for the progression of cancer cells to full malignancy (Collado et al., 2007). In most human cancers, the length of telomere is maintained through up-regulation of telomerase whereas a telomerase-independent pathway, termed Alternative Lengthening of Telomeres (ALT) is active in about 10-15% of cancers (Johnson and Broccoli, 2007; Heaphy et al., 2011). One characteristic feature of ALT is the formation of ALT-associated Promyelocytic Leukemia nuclear bodies (APBs) (Lang et al., 2010; Yeager et al., 1999). APBs contain Promyelocytic Leukemia nuclear bodies (PML-NB) components such as PML, SP100 and SUMO, telomeric DNA and telomere associated proteins including the shelterin components TRF1, TRF2, POT1, TIN2, TPP1 and Rap1 (Yeager et al., 1999). In addition, APBs contain proteins involved in DNA repair. In particular, the presence of components of the homologous recombination machinery suggests that APBs may promote telomere elongation by facilitating the homologous recombination of telomeric templates (Nabetani et al., 2004; Stavropoulos et al., 2002). This is also supported by the requirement of the homologous recombination-associated MRN complex for APB formation (Wu et al., 2000). Furthermore, APBs are suggested to be active sites of ATM and ATR dependent DNA repair (Nabetani et al., 2004). Finally, the number of APBs increases in G2 phase of the cell cycle when recombination is mainly active (Grobelny et al., 2000). We have shown that infection of normal and malignant B lymphocytes with the human oncogenic herpesvirus Epstein-Barr virus (EBV) is associated with the induction of APBs and with numerous signs of chromosomal and genomic instability (Kamranvar et al., 2007; Kamranvar and Masucci, 2011; Kamranvar et al., 2013).