Altered topoisomerase I activity and recombination activating gene expression in a human leukemia cell line resistant to doxorubicin

Nonamer dependent RAG cleavage at CpGs can explain mechanism of chromosomal translocations associated to lymphoid cancers

Chromosomal translocations are considered as one of the major causes of lymphoid cancers. RAG complex, which is responsible for V(D)J recombination, can also cleave non-B DNA structures and cryptic RSSs in the genome leading to chromosomal translocations. The mechanism and factors regulating the illegitimate function of RAGs resulting in oncogenesis are largely unknown. Upon in silico analysis of 3760 chromosomal translocations from lymphoid cancer patients, we find that 93% of the translocation breakpoints possess adjacent cryptic nonamers (RAG binding sequences), of which 77% had CpGs in proximity. As a proof of principle, we show that RAGs can efficiently bind to cryptic nonamers present at multiple fragile regions and cleave at adjacent mismatches generated to mimic the deamination of CpGs. ChIP studies reveal that RAGs can indeed recognize these fragile sites on a chromatin context inside the cell. Finally, we show that AID, the cytidine deaminase, plays a significant role during the generation of mismatches at CpGs and reconstitute the process of RAG-dependent generation of DNA breaks both in vitro and inside the cells. Thus, we propose a novel mechanism for generation of chromosomal translocation, where RAGs bind to the cryptic nonamer sequences and direct cleavage at adjacent mismatch generated due to deamination of meCpGs or cytosines.

Therapeutic potential and molecular mechanism of a novel sulfonamide anticancer drug, indisulam (E7070) in combination with CPT-11 for cancer treatment

PURPOSE

Indisulam (N-(-3-chloro-7-indolyl)-1,4-benzenedisulfonamide; E7070) is an experimental anticancer agent. Microarray analysis indicates that indisulam downregulates several genes involved in drug resistance, and this finding led us to test the effect of combining indisulam with other anticancer drugs. We investigated the antitumor effect and mechanism of synergism when indisulam was administered in combination with CPT-11.

METHODS

In vitro cytotoxic activity was examined using a cell counter kit, and the combination effect was determined by isobologram analysis. The level of topoisomerase IIα was measured by Western blotting. The in vivo antitumor effect was assessed in mice inoculated with human colorectal cancer SW620 cells.

RESULTS

Isobologram analysis indicated that a 24-h exposure to indisulam and SN-38, an active metabolite of CPT-11, had a synergistic effect in HCT116 and SW620 cells and an additive effect in HCT15 and WiDr cells. Prolongation of exposure to 48 h resulted in a synergistic effect in HCT15 and WiDr cells. Treatment with SN-38 alone increased the amount of intracellular topoisomerase IIα in all cell lines tested. Co-treatment with indisulam suppressed the SN-38-induced upregulation of topoisomerase IIα after 24 h of exposure in HCT116 and SW620 cells and after 48 h of exposure in HCT15 and WiDr cells. This apparent association between a synergistic effect and suppression of SN-38-mediated upregulation of topoisomerase IIα suggests that indisulam enhances SN-38 cytotoxicity by suppressing topoisomerase IIα upregulation to compensate for topoisomerase I inhibition by SN-38. Synergy was also observed in xenografted tumors and was accompanied by complete suppression of topoisomerase IIα upregulation induced by CPT-11 treatment.

CONCLUSION

These observations prompted the clinical evaluation of indisulam and CPT-11 combination therapy.

Sequential topoisomerase targeting and analysis of mechanisms of resistance to topotecan in patients with acute myelogenous leukemia

Resistance to topoisomerase I (TOP1)-targeting drugs such as topotecan often involves upregulation of topoisomerase II (TOP2), with accompanying increased sensitivity to TOP2-targeting drugs such as etoposide. This trial was designed to investigate sequential topoisomerase targeting in the treatment of patients with high-risk acute myelogenous leukemia. An initial cohort of patients received topotecan and cytosine arabinoside daily for 5 days. Serial samples of circulating mononuclear cells were examined to evaluate peak elevations of TOP2-alpha protein expression. In subsequent cohorts, etoposide was administered daily for 3 days, beginning 6 h after initiation of the topotecan infusion. The etoposide dose was escalated to determine a maximum-tolerated dose. Circulating mononuclear cells were analyzed for TOP1 mutations and ABCG2 protein expression. In addition, systemic and intracellular topotecan concentrations were measured. Thirty-one patients were enrolled. On the basis of TOP1-alpha protein levels in three patients with peripheral blast counts greater than 50%, etoposide administration began 6 h after initiation of the topotecan/cytosine arabinoside infusion. Using this schedule of administration, the maximum-tolerated dose of etoposide was 90 mg/m. No TOP1 mutations were identified, but increases in ABCG2 expression during the infusion were observed in mononuclear cells from two of four evaluable patients. Administration of etoposide 6 h after initiation of a topotecan/cytosine arabinoside infusion is feasible and is associated with clinical activity. Analysis of TOP2-alpha protein levels in this small number of patients indicated that peak increases occurred earlier than expected based on earlier publications. Upregulation of ABCG2 was detected in circulating cells and may represent an inducible form of drug resistance that should be investigated further.

Irinotecan: mechanisms of tumor resistance and novel strategies for modulating its activity

Camptothecins are broad-spectrum anticancer drugs that specifically target DNA topoisomerase I (Topo I). The formation of a cleavable drug-Topo I-DNA complex results in lethal double-strand DNA breakage and cell death. However, de novo or acquired clinical resistance to camptothecins is common. Studies of the camptothecin analog irinotecan suggest the following general mechanisms of resistance: (i) variable levels of the enzymes involved in the conversion of irinotecan; (ii) reduced cellular accumulation from active drug efflux; (iii) reduced levels of Topo I expression; (iv) alterations in the structure of Topo I from different mutations; (v) alterations in the cellular response to camptothecin-Topo I-DNA complex formation, which involves proteasome degradation of Topo I and/or enhanced DNA repair; and (vi) activation of the transcription factor nuclear factor kappa B by DNA damage and subsequent suppression of apoptosis. Multiple approaches using pharmacological and biological modulation to circumvent the above mechanisms of resistance have been incorporated into ongoing clinical trials and are expected to enhance the antitumor activity of irinotecan and reduce its systemic toxicity.

A human lymphoid leukemia cell line with a V(D)J recombinase-mediated deletion of hprt

Large deletions of exons 2 and 3 of the hprt gene are the most common type of hprt mutation in lymphocytes of newborn infants, and their frequency increases in cultured human T-lymphoid cells as a result of exposure to etoposide. Sequenced PCR products for these deletions are consistent with a V(D)J recombinase-mediated mechanism underlying their genesis. Herein, we describe the isolation and characterization of an etoposide-induced mutant CEM cell line that is clonal for a V(D)J recombinase-mediated exon 2 + 3 deletion. Human CCRF-CEM cells were exposed to 5 muM etoposide for 4 h, selected in 6-thioguanine, and an exon 2 + 3 deletion mutant was isolated through serial limiting dilution, using a PCR-based assay for detection of the exon 2 + 3 deletion. Untreated CEM cells and cells treated with 6-thioguanine alone were similarly subcultured. The exon 2 + 3 deletion-containing line was termed SJCEM808 and had a slightly longer doubling time than the control lines, tended to clump in suspension, and was characterized by cell membrane blebbing. Compared to the parent line, SJCEM808 had similar cytogenetic abnormalities, lower CD2, CD1, and CD10 expression, and negligible RAG-1 expression. However, RAG-1 expression was down-regulated in some untreated parental subclones following similar subculturing. The sequence of the exon 2 + 3 deletion mutation exhibited nucleotide insertions, and the breakpoints were adjacent to heptamer signal recognition sequences in intact hprt, consistent with a V(D)J recombinase-mediated mechanism underlying its genesis. There were no MLL gene or interlocus T-cell receptor (TCR) rearrangements. These results indicate that non-homologous recombination following etoposide treatment is neither necessarily accompanied by other large DNA rearrangements nor simply a pre-lethal event, and this cell line may serve as a useful tool for studying illegitimate V(D)J recombinase-mediated deletions.