癌症生物学

Telomeres are structures that cap the ends of linear chromosomes and play critical roles in maintaining genome integrity and establishing the replicative lifespan of cells. In stem and cancer cells, telomeres are actively elongated by either telomerase or the alternative lengthening of telomeres (ALT) pathway. This pathway is characterized by several hallmark features, including extrachromosomal C-rich circular DNAs that can be probed to assess ALT activity. These so-called C-circles are the product of ALT-associated DNA damage repair processes and simultaneously serve as potential templates for iterative telomere extension. This bifunctional nature makes C-circles highly sensitive and specific markers of ALT. Here, we describe a C-circle assay, adapted from previous reports, that enables the quantitation of C-circle abundance in mammalian cells subjected to a wide range of experimental perturbations. This protocol combines the Quick C-circle Preparation (QCP) method for DNA isolation with fluorometry-based DNA quantification, rolling circle amplification (RCA), and detection of C-circles using quantitative PCR. Moreover, the inclusion of internal standards with well-characterized telomere maintenance mechanisms (TMMs) allows for the reliable benchmarking of cells with unknown TMM status. Overall, our work builds upon existing protocols to create a generalizable workflow for in vitro C-circle quantitation and ascertainment of TMM identity.

Numerous oncogenes have been identified to cause leukemia. For example, chromosomal translocation generates various fusion genes of the mixed-lineage leukemia (MLL) gene and a partner gene in leukemia, whose gene products transform primary myeloid progenitors into an immortalized state. To characterize the transforming ability of leukemic oncogenes, researchers in the field have developed an ex vivo murine myeloid transformation assay using retroviral gene transduction and its protocol has been improved over the past 10 years. Here, we provide the detailed procedure for this assay.

While telomerase is expressed in ~90% of primary human tumors, most somatic tissue cells except transiently proliferating stem-like cells do not have detectable telomerase activity (Shay and Wright, 1996; Shay and Wright, 2001). Telomeres progressively shorten with each cell division in normal cells, including proliferating stem-like cells, due to the end replication (lagging strand synthesis) problem and other causes such as oxidative damage, therefore all somatic cells have limited cell proliferation capacity (Hayflick limit) (Hayflick and Moorhead, 1961; Olovnikov, 1973). The progressive telomere shortening eventually leads to growth arrest in normal cells, which is known as replicative senescence (Shay et al., 1991). Once telomerase is activated in cancer cells, telomere length is stabilized by the addition of TTAGGG repeats to the end of chromosomes, thus enabling the limitless continuation of cell division (Shay and Wright, 1996; Shay and Wright, 2001). Therefore, the link between aging and cancer can be partially explained by telomere biology. There are many rapid and convenient methods to study telomere biology such as Telomere Restriction Fragment (TRF), Telomere Repeat Amplification Protocol (TRAP) (Mender and Shay, 2015b) and Telomere dysfunction Induced Foci (TIF) analysis (Mender and Shay, 2015a). In this protocol paper we describe Telomere Restriction Fragment (TRF) analysis to determine average telomeric length of cells.

Telomeric length can be indirectly measured by a technique called Telomere Restriction Fragment analysis (TRF). This technique is a modified Southern blot, which measures the heterogeneous range of telomere lengths in a cell population using the length distribution of the terminal restriction fragments (Harley et al., 1990; Ouellette et al., 2000). This method can be used in eukaryotic cells. The description below focuses on the measurement of human cancer cells telomere length. The principle of this method relies on the lack of restriction enzyme recognition sites within TTAGGG tandem telomeric repeats, therefore digestion of genomic DNA, not telomeric DNA, with a combination of 6 base restriction endonucleases reduces genomic DNA size to less than 800 bp.

Telomeres are found at the end of eukaryotic linear chromosomes, and proteins that bind to telomeres protect DNA from being recognized as double-strand breaks thus preventing end-to-end fusions (Griffith et al., 1999). However, due to the end replication problem and other factors such as oxidative damage, the limited life span of cultured cells (Hayflick limit) results in progressive shortening of these protective structures (Hayflick and Moorhead, 1961; Olovnikov, 1973). The ribonucleoprotein enzyme complex telomerase- consisting of a protein catalytic component hTERT and a functional RNA component hTR or hTERC- counteracts telomere shortening by adding telomeric repeats to the end of chromosomes in ~90% of primary human tumors and in some transiently proliferating stem-like cells (Shay and Wright, 1996; Shay and Wright, 2001). This results in continuous proliferation of cells which is a hallmark of cancer. Therefore, telomere biology has a central role in aging, cancer progression/metastasis as well as targeted cancer therapies. There are commonly used methods in telomere biology such as Telomere Restriction Fragment (TRF) (Mender and Shay, 2015b), Telomere Repeat Amplification Protocol (TRAP) and Telomere dysfunction Induced Foci (TIF) analysis (Mender and Shay, 2015a). In this detailed protocol we describe Telomere Repeat Amplification Protocol (TRAP).

The TRAP assay is a popular method to determine telomerase activity in mammalian cells and tissue samples (Kim et al., 1994). The TRAP assay includes three steps: extension, amplification, and detection of telomerase products. In the extension step, telomeric repeats are added to the telomerase substrate (which is actually a non-telomeric oligonucleotide, TS) by telomerase. In the amplification step, the extension products are amplified by the polymerase chain reaction (PCR) using specific primers (TS upstream primer and ACX downstream primer) and in the detection step, the presence or absence of telomerase is analyzed by electrophoresis. TSNT is, an internal standard control, amplified by TS primer. NT is its own reverse primer, which is not a substrate for telomerase. These primers are used to identify false-negative results by if the gel lacks internal control bands.

Telomerase maintains telomeric DNA in eukaryotes during early developments, ~90% of cancer cells and some proliferative stem like cells. Telomeric repeats at the end of chromosomes are associated with the shelterin complex. This complex consists of TRF1, TRF2, Rap1, TIN2, TPP1, POT1 which protect DNA from being recognized as DNA double-stranded breaks. Critically short telomeres or impaired shelterin proteins can cause telomere dysfunction, which eventually induces DNA damage responses at the telomeres. DNA damage responses can be identified by antibodies to 53BP1, gammaH2AX, Rad17, ATM, and Mre11. DNA damage foci at uncapped telomeres are referred to as Telomere dysfunction-Induced Foci (TIFs) (de Lange, 2005; Takai et al., 2003).

The TIF assay is based on the co-localization detection of DNA damage by an antibody against DNA damage markers, such as gamma-H2AX, and telomeres using an antibody against one of the shelterin proteins such as TRF2 (Takai et al., 2003; de Lange, 2002; Karlseder et al., 1999). The method we describe here can be used in normal human and cancer cells.

Other commonly used methods-Telomere Restriction Fragment (TRF) Analysis (Mender and Shay, 2015b) and Telomere Repeat Amplification Protocol (TRAP) (Mender and Shay, 2015a)- in telomere biology can be found by clicking on the indicated links.

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).

Here we describe a method for detection of APBs in human B-lymphocytes. The method can be applied with minor modifications to different cell types including adherent, suspension and primary cells.