Tilting MYC toward cancer cell death

MYC oncoprotein promotes cell proliferation and serves as the key driver in many human cancers; therefore, considerable effort has been expended to develop reliable pharmacological methods to suppress its expression or function. Despite impressive progress, MYC-targeting drugs have not reached the clinic. Recent advances suggest that within a limited expression range unique to each tumor, MYC oncoprotein can have a paradoxical, proapoptotic function. Here we introduce a counterintuitive idea that modestly and transiently elevating MYC levels could aid chemotherapy-induced apoptosis and thus benefit the patients as much, if not more than MYC inhibition.

Correction: Transient stabilization, rather than inhibition, of MYC amplifies extrinsic apoptosis and therapeutic responses in refractory B-cell lymphoma

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Transient stabilization, rather than inhibition, of MYC amplifies extrinsic apoptosis and therapeutic responses in refractory B-cell lymphoma

Therapeutic targeting of initiating oncogenes is the mainstay of precision medicine. Considerable efforts have been expended toward silencing MYC, which drives many human cancers including Burkitt lymphomas (BL). Yet, the effects of MYC silencing on standard-of-care therapies are poorly understood. Here we found that inhibition of MYC transcription renders B-lymphoblastoid cells refractory to chemotherapeutic agents. This suggested that in the context of chemotherapy, stabilization of Myc protein could be more beneficial than its inactivation. We tested this hypothesis by pharmacologically inhibiting glycogen synthase kinase 3β (GSK-3β), which normally targets Myc for proteasomal degradation. We discovered that chemorefractory BL cell lines responded better to doxorubicin and other anti-cancer drugs when Myc was transiently stabilized. In vivo, GSK3 inhibitors (GSK3i) enhanced doxorubicin-induced apoptosis in BL patient-derived xenografts (BL-PDX), as well as in murine MYC-driven lymphoma allografts. This enhancement was accompanied by and required deregulation of several key genes acting in the extrinsic, death-receptor-mediated apoptotic pathway. Consistent with this mechanism of action, GSK3i also facilitated lymphoma cell killing by a death ligand TRAIL and by a death receptor agonist mapatumumab. Thus, GSK3i synergizes with both standard chemotherapeutics and direct engagers of death receptors and could improve outcomes in patients with refractory lymphomas.

Convergence of Acquired Mutations and Alternative Splicing of CD19 Enables Resistance to CART-19 Immunotherapy

The CD19 antigen, expressed on most B-cell acute lymphoblastic leukemias (B-ALL), can be targeted with chimeric antigen receptor-armed T cells (CART-19), but relapses with epitope loss occur in 10% to 20% of pediatric responders. We detected hemizygous deletions spanning the CD19 locus and de novo frameshift and missense mutations in exon 2 of CD19 in some relapse samples. However, we also discovered alternatively spliced CD19 mRNA species, including one lacking exon 2. Pull-down/siRNA experiments identified SRSF3 as a splicing factor involved in exon 2 retention, and its levels were lower in relapsed B-ALL. Using genome editing, we demonstrated that exon 2 skipping bypasses exon 2 mutations in B-ALL cells and allows expression of the N-terminally truncated CD19 variant, which fails to trigger killing by CART-19 but partly rescues defects associated with CD19 loss. Thus, this mechanism of resistance is based on a combination of deleterious mutations and ensuing selection for alternatively spliced RNA isoforms.

SIGNIFICANCE

CART-19 yield 70% response rates in patients with B-ALL, but also produce escape variants. We discovered that the underlying mechanism is the selection for preexisting alternatively spliced CD19 isoforms with the compromised CART-19 epitope. This mechanism suggests a possibility of targeting alternative CD19 ectodomains, which could improve survival of patients with B-cell neoplasms.

Abstract B33: Transient upregulation of Myc with GSK3-β inhibitors in B-cell lymphomas enhances p53-independent apoptotic responses to chemotherapy

The Myc proto-oncogene is known to regulate p53-dependent apoptosis through the Myc -> ARF -| MDM2 -| p53 pathway. We had previously shown that this pathway could be exploited to increase sensitivity to anti-cancer drugs. Specifically, small increases in Myc levels obtained by inhibiting the Myc-targeting microRNA-34a resulted in enhanced apoptotic response of B-cells to bortezomib (Sotillo et al, Oncogene 2010). However, many Myc-driven tumors are deficient in p53 activity. Thus, we sought to determine whether transient up-regulation of Myc also increases cell death in response to chemotherapeutic drugs in p53-deficient backgrounds. We chose to study this in human Burkitt's lymphoma cell lines and murine B-lymphoid cells isolated from bone marrows of p53ER™ knock-in mice and subsequently transduced with a Myc-expressing retrovirus (Amaravadi et al, J Clin Inv 2007, Yu et al, Blood 2007). This latter system allowed us to distinguish between p53-dependent (cells treated with tamoxifen) and independent (no tamoxifen added) cell death. We also considered that Myc expression is regulated not only transcriptionally, most notably by the bromodomain containing protein 4 (BRD4), but also post-transcriptionally by the glycogen synthase kinase 3β (GSK3-β). GSK3-β phosphorylates Myc on its Thr58 residue, targeting it for proteasomal degradation, and inhibition of GSK3-β activity results in stabilization of Myc protein (Chung et al, J Clin Inv 2012). Using the GSK3-β inhibitor CHIR 99021, we transiently increased Myc protein levels prior to treating B-lymphoma cells with chemotherapeutic drugs. Remarkably, pharmacological stabilization of Myc strongly enhanced p53-independent doxorubicin-induced apoptosis in the p53ER™/Myc system as well as in Burkitt's lymphoma cell lines with inactive p53. This enhancement was accompanied by cleavage of caspase 8 with concomitant de-regulation of the death receptor pathway genes, indicating engagement of the extrinsic apoptotic pathway. In contrast, we did not observe any significant differences in the intrinsic pro-apoptotic genes (BAX, BAK, BIM). To determine whether chemosensitization was mediated by Myc or other GSK3-β targets, we used a BRD4 inhibitor (iBET-151) at concentrations that were sufficient to prevent the CHIR 99021-mediated increase in Myc levels but didn't grossly affect cell viability. Inhibition of Myc with iBET-151 abrogated the increase in apoptosis brought about by CHIR 99021 and doxorubicin, suggesting that Myc is key to chemosensitization by CHIR 99021. Our results suggest that Myc stabilization could be a viable adjuvant therapy, even for tumors with p53 loss or MDM2 amplification.

Citation Format: Elena Sotillo-Piñeiro, Colleen T. Harrington, Daniel Soto De Jesus, Andrei Thomas-Tikhonenko. Transient upregulation of Myc with GSK3-β inhibitors in B-cell lymphomas enhances p53-independent apoptotic responses to chemotherapy. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B33.

Fundamentals of pyrosequencing

CONTEXT

DNA sequencing is critical to identifying many human genetic disorders caused by DNA mutations, including cancer. Pyrosequencing is less complex, involves fewer steps, and has a superior limit of detection compared with Sanger sequencing. The fundamental basis of pyrosequencing is that pyrophosphate is released when a deoxyribonucleotide triphosphate is added to the end of a nascent strand of DNA. Because deoxyribonucleotide triphosphates are sequentially added to the reaction and because the pyrophosphate concentration is continuously monitored, the DNA sequence can be determined.

OBJECTIVE

To demonstrate the fundamental principles of pyrosequencing.

DATA SOURCES

Salient features of pyrosequencing are demonstrated using the free software program Pyromaker ( http://pyromaker.pathology.jhmi.edu ), through which users can input DNA sequences and other pyrosequencing parameters to generate the expected pyrosequencing results.

CONCLUSIONS

We demonstrate how mutant and wild-type DNA sequences result in different pyrograms. Using pyrograms of established mutations in tumors, we explain how to analyze the pyrogram peaks generated by different dispensation sequences. Further, we demonstrate some limitations of pyrosequencing, including how some complex mutations can be indistinguishable from single base mutations. Pyrosequencing is the basis of the Roche 454 next-generation sequencer and many of the same principles also apply to the Ion Torrent hydrogen ion-based next-generation sequencers.

Pyoderma gangrenosum after cholecystectomy

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