circRNAs as prognostic markers in pediatric acute myeloid leukemia.

Circular RNAs (circRNAs) arise from precursor mRNA processing through back-splicing and have been increasingly recognized for their functions in various cancers including acute myeloid leukemia (AML). However, the prognostic implications of circRNA in AML remain unclear. We conducted a comprehensive genome-wide analysis of circRNAs using RNA-seq data in pediatric AML. We revealed a group of circRNAs associated with inferior outcomes, exerting effects on cancer-related pathways. Several of these circRNAs were transcribed directly from genes with established functions in AML, such as circRUNX1, circWHSC1, and circFLT3. Further investigations indicated the increased number of circRNAs and linear RNAs splicing were significantly correlated with inferior clinical outcomes, highlighting the pivotal role of splicing dysregulation. Subsequent analysis identified a group of upregulated RNA binding proteins in AMLs associated with high number of circRNAs, with TROVE2 being a prominent candidate, suggesting their involvement in circRNA associated prognosis. Through the integration of drug sensitivity data, we pinpointed 25 drugs that could target high-risk AMLs characterized by aberrant circRNA transcription. These findings underscore prognostic significance of circRNAs in pediatric AML and offer an alternative perspective for treating high-risk cases in this malignancy.

Successful regression of newly formed corneal neovascularization by subconjunctival injection of bevacizumab in patients with chemical burns.

Purpose: To investigate the effect and timing of subconjunctival bevacizumab injection on inhibiting corneal neovascularization (CorNV) in patients after chemical burns. Methods: Patients with CorNV secondary to chemical burns were involved. Two subconjunctival injections of bevacizumab (2.5 mg/0.1 mL per involved quadrant) with an interval of 4 weeks were administered, and followed up a year. The area occupied by neovascular vessels (NA), accumulative neovascular length (NL), mean neovascular diameter (ND), best-corrected visual acuity (BCVA) and intraocular pressure (IOP) were evaluated. Complication was also recorded. Results: Eleven patients with CorNV were involved. Eight patients had a history of surgery (four had amniotic grafts, one had keratoplasty, and three had amniotic grafts and keratoplasty). Decreasing in NA, NL, and ND were statistically significant at each time point compared to the baseline (p < 0.01). CorNV that developed within 1 month was considerably regressed, and vessels with fibrovascular membranes were found to be narrower and shorter than pretreatment. BCVA improved in five patients (from one to five lines), remained unchanged in five patients, and decreased in one patient compared to pretreatment. Conclusion: Subconjunctival bevacizumab injection has a particular potential for the regression of CorNV, especially newly formed within 1 month in patients after chemical burns.

Targeting DNA polymerase ß elicits synthetic lethality with mismatch repair deficiency in acute lymphoblastic leukemia.

Mismatch repair (MMR) deficiency has been linked to thiopurine resistance and hypermutation in relapsed acute lymphoblastic leukemia (ALL). However, the repair mechanism of thiopurine-induced DNA damage in the absence of MMR remains unclear. Here, we provide evidence that DNA polymerase ß (POLB) of base excision repair (BER) pathway plays a critical role in the survival and thiopurine resistance of MMR-deficient ALL cells. In these aggressive resistant ALL cells, POLB depletion and its inhibitor oleanolic acid (OA) treatment result in synthetic lethality with MMR deficiency through increased cellular apurinic/apyrimidinic (AP) sites, DNA strand breaks and apoptosis. POLB depletion increases thiopurine sensitivities of resistant cells, and OA synergizes with thiopurine to kill these cells in ALL cell lines, patient-derived xenograft (PDX) cells and xenograft mouse models. Our findings suggest BER and POLB's roles in the process of repairing thiopurine-induced DNA damage in MMR-deficient ALL cells, and implicate their potentials as therapeutic targets against aggressive ALL progression.

PRPS2 mutations drive acute lymphoblastic leukemia relapse through influencing PRPS1/2 hexamer stability.

Tumor relapse is the major cause of treatment failure in childhood acute lymphoblastic leukemia (ALL), yet the underlying mechanisms are still elusive. Here, we demonstrate that phosphoribosyl pyrophosphate synthetase 2 (PRPS2) mutations drive ALL relapse through influencing PRPS1/2 hexamer stability. Ultra-deep sequencing was performed to identify PRPS2 mutations in ALL samples. The effects of PRPS2 mutations on cell survival, cell apoptosis, and drug resistance were evaluated. In vitro PRPS2 enzyme activity and ADP/GDP feedback inhibition of PRPS enzyme activity were assessed. Purine metabolites were analyzed by ultra-performance liquid-chromatography tandem mass spectrometry (UPLC-MS/MS). Integrating sequencing data with clinical information, we identified PRPS2 mutations only in relapsed childhood ALL with thiopurine therapy. Functional PRPS2 mutations mediated purine metabolism specifically on thiopurine treatment by influencing PRPS1/2 hexamer stability, leading to reduced nucleotide feedback inhibition of PRPS activity and enhanced thiopurine resistance. The 3-amino acid V103-G104-E105, the key difference between PRPS1 and PRPS2, insertion in PRPS2 caused severe steric clash to the interface of PRPS hexamer, leading to its low enzyme activity. In addition, we demonstrated that PRPS2 P173R increased thiopurine resistance in xenograft models. Our work describes a novel mechanism by which PRPS2 mutants drive childhood ALL relapse and highlights PRPS2 mutations as biomarkers for relapsed childhood ALL.

Up-regulation of ABCG1 is associated with methotrexate resistance in acute lymphoblastic leukemia cells.

Acute lymphoblastic leukemia (ALL) is a prevalent hematologic malignancy in children, and methotrexate (MTX) is a widely employed curative treatment. Despite its common use, clinical resistance to MTX is frequently encountered. In this study, an MTX-resistant cell line (Reh-MTXR) was established through a stepwise selection process from the ALL cell line Reh. Comparative analysis revealed that Reh-MTXR cells exhibited resistance to MTX in contrast to the parental Reh cells. RNA-seq analysis identified an upregulation of ATP-binding cassette transporter G1 (ABCG1) in Reh-MTXR cells. Knockdown of ABCG1 in Reh-MTXR cells reversed the MTX-resistant phenotype, while overexpression of ABCG1 in Reh cells conferred resistance to MTX. Mechanistically, the heightened expression of ABCG1 accelerated MTX efflux, leading to a reduced accumulation of MTX polyglutamated metabolites. Notably, the ABCG1 inhibitor benzamil effectively sensitized Reh-MTXR cells to MTX treatment. Moreover, the observed upregulation of ABCG1 in Reh-MTXR cells was not induced by alterations in DNA methylation or histone acetylation. This study provides insight into the mechanistic basis of MTX resistance in ALL and also suggests a potential therapeutic approach for MTX-resistant ALL in the future.

Long-Term Safety and Efficacy of CD19 Humanized Selective CAR-T Therapy in B-ALL Patients Who Have Previously Received Murine-Based CD19 CAR-T Therapy.

Murine-based CD19 CAR-T (CD19m CAR-T) therapy can lead to a relatively high CR rate when administered to B-ALL patients for the first time. However, the DOR is sub-optimal and a subset of patients even show primary resistance to CD19m CAR-T. To address these issues, we employed a humanized selective CD19CAR-T (CD19hs CAR-T) and evaluated the long-term safety and efficacy of treating 8 R/R B-ALL patients who had relapsed or failed to achieve CR following CD19m CAR-T infusion (Clinical trials' number: ChiCTR1800014761 and ChiCTR1800017439). Of the 8 patients, 7 achieved CR on Day 30 after the 1st infusion of CD19hs CAR-T. The median CRS grade was 1 without significant neurotoxicity seen in any of the 8 patients. The median DOR was 11 months, significantly longer than the DOR following CD19mCAR-T infusions. Anti-CAR antibodies were induced in patients who had received prior CD19m CAR-T infusions but not in those following a single or repeated CD19hsCAR-T treatment, which probably had contributed to the sub-optimal DOR and/or failure of effective response in these patients. CD19hs CAR-T, in contrast, induced low immunogenicity compared with CD19m CAR-T, suggesting that a repeat dosing strategy might be feasible and efficacious for patients who have relapsed and/or show primary resistance to CD19m CAR-T therapy. In this clinical study, CD19hs CAR-T showed a significant clinical efficacy with mild side effect among patients with R/R B-ALL who had previously received CD19m CAR-T. Clinical Trial Registration: https://www.chictr.org.cn/showprojen.aspx?proj=25199 (ChiCTR1800014761). https://www.chictr.org.cn/showproj.aspx?proj=29174 (ChiCTR1800017439).

The KRAS-G12D mutation induces metabolic vulnerability in B-cell acute lymphoblastic leukemia.

Mutations in RAS pathway genes are highly prevalent in acute lymphoblastic leukemia (ALL). However, the effects of RAS mutations on ALL cell growth have not been experimentally characterized, and effective RAS-targeting therapies are being sought after. Here, we found that Reh ALL cells bearing the KRAS-G12D mutation showed increased proliferation rates in vitro but displayed severely compromised growth in mice. Exploring this divergence, proliferation assays with multiple ALL cell lines revealed that the KRAS-G12D rewired methionine and arginine metabolism. Isotope tracing results showed that KRAS-G12D promotes catabolism of methionine and arginine to support anabolism of polyamines and proline, respectively. Chemical inhibition of polyamine biosynthesis selectively killed KRAS-G12D B-ALL cells. Finally, chemically inhibiting AKT/mTOR signaling abrogated the altered amino acid metabolism and strongly promoted the in vivo growth of KRAS-G12D cells in B-ALL xenograft. Our study thus illustrates how hyperactivated AKT/mTOR signaling exerts distinct impacts on hematological malignancies vs. solid tumors.

A γ-glutamyl hydrolase lacking the signal peptide confers susceptibility to folates/antifolates in acute lymphoblastic leukemia cells.

A key cofactor of several enzymes implicated in DNA synthesis, repair, and methylation, folate has been shown to be required for normal cell growth and replication and is the basis for cancer chemotherapy using antifolates. γ-Glutamyl hydrolase (GGH) catalyzes the removal of γ-polyglutamate tails of folylpoly-/antifolylpoly-γ-glutamates to facilitate their export out of the cell, thereby maintaining metabolic homeostasis of folates or pharmacological efficacy of antifolates. However, the factors that control or modulate GGH function are not well understood. In this study, we show that intact GGH is not indispensable for the chemosensitivity and growth of acute lymphoblastic leukemia (ALL) cells, whereas GGH lacking N-terminal signal peptide (GGH-ΔN ) confers the significant drug resistance of ALL cells to the antifolates MTX and RTX. In addition, ALL cells harboring GGH-ΔN show high susceptibility to the change in folates, and glycosylation is not responsible for these phenotypes elicited by GGH-ΔN . Mechanistically, the loss of signal peptide enhances intracellular retention of GGH and its lysosomal disposition. Our findings clearly define the in vivo role of GGH in ALL cells and indicate a novel modulation of the GGH function, suggesting new avenues for ALL treatment in future.

Chemotherapy and mismatch repair deficiency cooperate to fuel TP53 mutagenesis and ALL relapse.

Chemotherapy is a standard treatment for pediatric acute lymphoblastic leukemia (ALL), which sometimes relapses with chemoresistant features. However, whether acquired drug-resistance mutations in relapsed ALL pre-exist or are induced by treatment remains unknown. Here we provide direct evidence of a specific mechanism by which chemotherapy induces drug-resistance-associated mutations leading to relapse. Using genomic and functional analysis of relapsed ALL we show that thiopurine treatment in mismatch repair (MMR)-deficient leukemias induces hotspot TP53 R248Q mutations through a specific mutational signature (thio-dMMR). Clonal evolution analysis reveals sequential MMR inactivation followed by TP53 mutation in some patients with ALL. Acquired TP53 R248Q mutations are associated with on-treatment relapse, poor treatment response and resistance to multiple chemotherapeutic agents, which could be reversed by pharmacological p53 reactivation. Our findings indicate that TP53 R248Q in relapsed ALL originates through synergistic mutagenesis from thiopurine treatment and MMR deficiency and suggest strategies to prevent or treat TP53-mutant relapse.

Bromodomains and Extra-Terminal (BET) Inhibitor JQ1 Suppresses Proliferation of Acute Lymphocytic Leukemia by Inhibiting c-Myc-Mediated Glycolysis.

BACKGROUND Acute lymphocytic leukemia (ALL) is a common blood cancer which induces high mortality in children. Bromodomains and extra-terminal (BET) protein inhibitors, such as JQ1 and ARV-825, are promising cancer therapeutic agents that can be used by targeting c-Myc. A recent work reported that JQ1 effectively attenuates ALL in vitro by suppressing cell proliferation and accelerating apoptosis. The purpose of this research was to probe into the potential mechanism of how JQ1 inhibits ALL cell proliferation in vitro. MATERIAL AND METHODS Cell viability of ALL cells were measured by CTG after treatment by JQ1. Cell cycle analysis was done by EdU and PI staining. Cell apoptosis was assessed by Annexin V/PI staining. Glycolysis was detected using Seahorse and LC-MS kits. The expression of glycolytic rate-limiting enzymes was assessed by RNA-seq, qRT-PCR, and Western blot. RESULTS JQ1 suppressed cell proliferation by arresting the cell cycle and inducing the apoptosis of acute lymphocytic leukemia cells. JQ1 inhibited cell proliferation of B-ALL cells by restraining glycolysis. Conversely, the cell cycle block of B-ALL cells induced by JQ1 was partially abolished after pretreatment with 2-Deoxy-D-glucose (2-DG), an inhibitor of glycolysis. Furthermore, JQ1 restrained the glycolysis of B-ALL cell lines by remarkably downregulating the rate-limiting enzymes of glycolysis, such as hexokinase 2, phosphofructokinase, and lactate dehydrogenase A. Moreover, the cell cycle arrest was reversed in B-ALL cells with overexpressed c-Myc treated by JQ1, which is involved in the enhancement of glycolysis. CONCLUSIONS The BET inhibitor JQ1 suppresses the proliferation of ALL by inhibiting c-Myc-mediated glycolysis, thus providing a new strategy for the treatment of ALL.

Early Application of Bevacizumab After Sclerocorneal Grafting for Patients With Severe Late-Stage Ocular Chemical Burns.

PURPOSE: To investigate whether subconjunctival bevacizumab help prevent corneal graft neovascularization and prolong the graft survival of patients with chemical burns. METHODS: We performed a prospective nonrandomized comparative case series study. Twenty-six eyes received subconjunctival bevacizumab (10 mg/0.4 mL) once and topical immunosuppressive agents after sclerocorneal lamellar keratoplasty as the treatment, and 13 eyes received a topical immunosuppressant alone and served as the control group. The main outcomes were a cumulative probability of graft survival, development of corneal neovascularization, and complications. RESULTS: The postoperative follow-up time was 14.3 months (range, 2-62 mo). The cumulative graft survival time was significantly longer in the treatment group than that in the control group (42.9 ± 5.9 vs. 4.8 ± 0.7 mo; log rank < 0.001). In the treatment group, 19 of the 26 grafts (73.1%) survived as transparent with a mean follow-up of 18.7 ± 3.0 months. At the end of the follow-up, 4 grafts remained free of neovascularization, 2 developed edema without neovascularization, and 15 remained transparent with a stable ocular surface and some neovascular vessels in the peripheral transplant interface. The other 5 grafts became opaque and neovascularized. In the control group, all grafts became opaque and neovascularized within the follow-up period (5.5 ± 0.7 mo). During the follow-up, a corneal epithelial defect developed in 9 eyes in the treatment group and 7 in the control group. CONCLUSIONS: Early application of subconjunctival bevacizumab after sclerocorneal lamellar keratoplasty can significantly prevent corneal neovascularization and promote graft survival for severe late-stage ocular chemical burns.

Therapy-induced mutations drive the genomic landscape of relapsed acute lymphoblastic leukemia.

To study the mechanisms of relapse in acute lymphoblastic leukemia (ALL), we performed whole-genome sequencing of 103 diagnosis-relapse-germline trios and ultra-deep sequencing of 208 serial samples in 16 patients. Relapse-specific somatic alterations were enriched in 12 genes (NR3C1, NR3C2, TP53, NT5C2, FPGS, CREBBP, MSH2, MSH6, PMS2, WHSC1, PRPS1, and PRPS2) involved in drug response. Their prevalence was 17% in very early relapse (<9 months from diagnosis), 65% in early relapse (9-36 months), and 32% in late relapse (>36 months) groups. Convergent evolution, in which multiple subclones harbor mutations in the same drug resistance gene, was observed in 6 relapses and confirmed by single-cell sequencing in 1 case. Mathematical modeling and mutational signature analysis indicated that early relapse resistance acquisition was frequently a 2-step process in which a persistent clone survived initial therapy and later acquired bona fide resistance mutations during therapy. In contrast, very early relapses arose from preexisting resistant clone(s). Two novel relapse-specific mutational signatures, one of which was caused by thiopurine treatment based on in vitro drug exposure experiments, were identified in early and late relapses but were absent from 2540 pan-cancer diagnosis samples and 129 non-ALL relapses. The novel signatures were detected in 27% of relapsed ALLs and were responsible for 46% of acquired resistance mutations in NT5C2, PRPS1, NR3C1, and TP53. These results suggest that chemotherapy-induced drug resistance mutations facilitate a subset of pediatric ALL relapses.

Blocking ATM-dependent NF-κB pathway overcomes niche protection and improves chemotherapy response in acute lymphoblastic leukemia.

Bone marrow (BM) niche responds to chemotherapy-induced cytokines secreted from acute lymphoblastic leukemia (ALL) cells and protects the residual cells from chemotherapeutics in vivo. However, the underlying molecular mechanisms for the induction of cytokines by chemotherapy remain unknown. Here, we found that chemotherapeutic drugs (e.g., Ara-C, DNR, 6-MP) induced the expression of niche-protecting cytokines (GDF15, CCL3 and CCL4) in both ALL cell lines and primary cells in vitro. The ATM and NF-κB pathways were activated after chemotherapy treatment, and the pharmacological or genetic inhibition of these pathways significantly reversed the cytokine upregulation. Besides, chemotherapy-induced NF-κB activation was dependent on ATM-TRAF6 signaling, and NF-κB transcription factor p65 directly regulated the cytokines expression. Furthermore, we found that both pharmacological and genetic perturbation of ATM and p65 significantly decreased the residual ALL cells after Ara-C treatment in ALL xenograft mouse models. Together, these results demonstrated that ATM-dependent NF-κB activation mediated the cytokines induction by chemotherapy and ALL resistance to chemotherapeutics. Inhibition of ATM-dependent NF-κB pathway can sensitize ALL to chemotherapeutics, providing a new strategy to eradicate residual chemo-resistant ALL cells.

AICAr suppresses cell proliferation by inducing NTP and dNTP pool imbalances in acute lymphoblastic leukemia cells.

It has been shown that 5-amino-4-imidazolecarboxamide riboside (AICAr) can inhibit cell proliferation and induce apoptosis in childhood acute lymphoblastic leukemia (ALL) cells. Although AICAr could regulate cellular energy metabolism by activating AMPK, the cytotoxic mechanisms of AICAr are still unclear. Here, we knocked out TP53 or PRKAA1 gene (encoding AMPKα1) in NALM-6 and Reh cells by using the clustered regularly interspaced short palindromic repeats/Cas9 system and found that AICAr-induced proliferation inhibition was independent of AMPK activation but dependent on p53. Liquid chromatography-mass spectrometry analysis of nucleotide metabolites indicated that AICAr caused an increase in adenosine triphosphate, deoxyadenosine triphosphate, and deoxyguanosine triphosphate levels by up-regulating purine biosynthesis, while AICAr led to a decrease in cytidine triphosphate, uridine triphosphate, deoxycytidine triphosphate, and deoxythymidine triphosphate levels because of reduced phosphoribosyl pyrophosphate production, which consequently impaired the pyrimidine biosynthesis. Ribonucleoside triphosphate (NTP) pool imbalances suppressed the rRNA transcription efficiency. Furthermore, deoxy-ribonucleoside triphosphate (dNTP) pool imbalances induced DNA replication stress and DNA double-strand breaks, followed by cell cycle arrest and apoptosis in ALL cells. Exogenous uridine could rebalance the NTP and dNTP pools by supplementing pyrimidine and then attenuate AICAr-induced cytotoxicity. Our data indicate that RNA transcription inhibition and DNA replication stress induced by NTP and dNTP pool imbalances might play a key role in AICAr-mediated cytotoxic effects on ALL cells, suggesting a potential clinical application of AICAr in future ALL therapy.-Du, L., Yang, F., Fang, H., Sun, H., Chen, Y., Xu, Y., Li, H., Zheng, L., Zhou, B.-B. S. AICAr suppresses cell proliferation by inducing NTP and dNTP pool imbalances in acute lymphoblastic leukemia cells.

Increase of PRPP enhances chemosensitivity of PRPS1 mutant acute lymphoblastic leukemia cells to 5-Fluorouracil.

Relapse-specific mutations in phosphoribosyl pyrophosphate synthetase 1 (PRPS1), a rate-limiting purine biosynthesis enzyme, confer significant drug resistances to combination chemotherapy in acute lymphoblastic leukemia (ALL). It is of particular interest to identify drugs to overcome these resistances. In this study, we found that PRPS1 mutant ALL cells specifically showed more chemosensitivity to 5-Fluorouracil (5-FU) than control cells, attributed to increased apoptosis of PRPS1 mutant cells by 5-FU. Mechanistically, PRPS1 mutants increase the level of intracellular phosphoribosyl pyrophosphate (PRPP), which causes the apt conversion of 5-FU to FUMP and FUTP in Reh cells, to promote 5-FU-induced DNA damage and apoptosis. Our study not only provides mechanistic rationale for re-targeting drug resistant cells in ALL, but also implicates that ALL patients who harbor relapse-specific mutations of PRPS1 might benefit from 5-FU-based chemotherapy in clinical settings.

Chemotherapy-induced niche perturbs hematopoietic reconstitution in B-cell acute lymphoblastic leukemia.

BACKGROUND: Considerable efforts have been devoted toward the uncovering of the molecular mechanisms underlying the maintenance of hematopoietic stem cells (HSCs) by the normal bone marrow (BM) niche. Previously, we demonstrated that a chemotherapy-induced niche, which is mainly composed of mesenchymal stem cells (MSCs), protects the residual B-cell acute lymphoblastic leukemia (B-ALL) cells from the insult of chemotherapeutic drugs. However, the roles of chemotherapy-induced niche on HSCs functions in B-ALL remain unclear. METHODS: We established an oncogenic N-MYC-driven B-ALL mouse model, which were subsequently treated with common chemotherapy drug cytarabine (Ara-C) and daunorubicin (DNR). After treatment, the structures of the BM niche were imaged by immunofluorescence staining. Then, the self-renewal and differentiation capability of the MSCs in the BM after Ara-C and DNR treatment were studied by ex vivo culture and gene expression analysis with RNA-seq and qRT-PCR. The effects of chemotherapy-induced niche on the hematopoietic reconstitution of HSCs were determined with series transplantation assay. Furthermore, the cell cycle, ROS level, mitochondrial membrane potential and cell apoptosis of HSCs were detected by flow cytometry. RESULTS: The MSCs, which is the main component of chemotherapy-induced BM niche, have decreased self-renewal capability and are prone to differentiate into adipocytes and chondrocytes. The results of gene expression analysis with RNA-seq showed that the MSCs have reduced levels of cytokines, including SCF, CXCL12, ANGPT1, VCAM1, and IL7. Furthermore, the chemotherapy-induced niche perturbed the hematopoietic reconstitution of HSCs in our N-MYC-driven B-ALL mouse model by promoting HSCs to enter cell cycle and increasing intracellular ROS levels and mitochondrial membrane potential of HSCs, which lead to the cell apoptosis of HSCs. CONCLUSIONS: Chemotherapy-induced BM niche perturbs the hematopoietic reconstitution of HSCs by increasing intracellular ROS level and inducing cell apoptosis.

HPRT1 activity loss is associated with resistance to thiopurine in ALL.

Acute lymphoblastic leukemia (ALL) is an aggressive hematological tumor resulting from the malignant transformation of lymphoid progenitors. Thiopurine is a widely used drug in the maintaining treatment of ALL. After a period of chemotherapy, 20% of pediatric patients and over 50% of adult patients will relapse. To investigate the mechanisms of drug resistance in vitro, we established the thiopurine resistant cell lines Reh-6MPR (6-MP Resistant cell) and Reh-6TGR (6-TG Resistant cell) by stepwise selection of the ALL cell line Reh. Cell viability assay revealed that 6MPR and 6TGR cells were almost 1000-fold more resistant to thiopurine comparing with the control Reh cells, and thiopurine conversion was significantly impaired in the resistant cells. Mechanistically, a same novel hypoxanthine phosphoribosyl transferase 1 (HPRT1) mutation c.495_496insA (p.V165fs) was found by whole exome sequencing in both resistant cells. The HPRT1 mutation dramaticly decreased the production of [13C5,15N4]-IMP from [13C5,15N4]-hypoxanthine (HX), showed a loss-of-funciton mechanism. Notably, re-expression the wildtype HPRT1 in Reh-6MPR cell can reverse the drug resistance and thiopurine conversion in Reh-6MPR cells. These results highlight the importance of HPRT1's activity in thiopurine resistance.

Negative feedback-defective PRPS1 mutants drive thiopurine resistance in relapsed childhood ALL.

Relapse is the leading cause of mortality in children with acute lymphoblastic leukemia (ALL). Among chemotherapeutics, thiopurines are key drugs in ALL combination therapy. Using whole-exome sequencing, we identified relapse-specific mutations in the phosphoribosyl pyrophosphate synthetase 1 gene (PRPS1), which encodes a rate-limiting purine biosynthesis enzyme, in 24/358 (6.7%) relapsed childhood B cell ALL (B-ALL) cases. All individuals who harbored PRPS1 mutations relapsed early during treatment, and mutated ALL clones expanded exponentially before clinical relapse. Our functional analyses of PRPS1 mutants uncovered a new chemotherapy-resistance mechanism involving reduced feedback inhibition of de novo purine biosynthesis and competitive inhibition of thiopurine activation. Notably, the de novo purine synthesis inhibitor lometrexol effectively abrogated PRPS1 mutant-driven drug resistance. These results highlight the importance of constitutive activation of the de novo purine synthesis pathway in thiopurine resistance, and they offer therapeutic strategies for the treatment of relapsed and thiopurine-resistant ALL.

Multimodal biomarker investigation on efficacy and mechanism of action for the mammalian target of rapamycin inhibitor, temsirolimus, in a preclinical mammary carcinoma OncoMouse model: a translational medicine study in support for early clinical development.

The mammalian target of rapamycin (mTOR) has proven to be a valid therapeutic target in a number of human cancers, and it is a candidate for clinical trials in human breast cancer. We report on a biomarker-based translational medicine approach to assess the efficacy and mechanism of action for the mTOR inhibitor temsirolimus (CCI-779) in a mammary carcinoma OncoMouse model [polyomavirus middle T antigen (PyMT)]. The mTOR signaling pathway biomarkers were assessed using a reverse-phase protein array. Pharmacokinetics studies were conducted in both the tumor and plasma compartments. Pharmacodynamic biomarkers for compound-target engagement of tumor phospho-S6 proteins were assayed by Western blot. Temsirolimus (intravenously once a week for 2 weeks) was administered in both early and advanced stages of tumors. Biomarkers for temsirolimus effects on tumor progression were assessed by three-dimensional ultrasound imaging in combination with immunohistochemistry to assess vascular density (Texas red-dextran and CD31 immunostaining) and macrophage burden (F4/80 antigen). Tumor growth was significantly arrested in temsirolimus (25 ± 14% from 8 to 10 weeks, p < 0.05, and 26 ± 17% from 11 to 13 weeks, p < 0.01), compared with 493 ± 160 and 376 ± 50% increases, respectively, in vehicle-treated groups. Temsirolimus reduced tumor vascular density, 36 to 48 and 58 to 60%, p < 0.05, by the Texas red-dextran method or CD31-positive vessel count, respectively. Temsirolimus reduced tumor macrophage burden by 46% at 13 weeks (p < 0.05). Temsirolimus inhibited (p < 0.05) the phosphoproteins S6 pS235/236 and S6 pS240/244 up to 81 and 87%, respectively. We conclude that the multimodal biomarkers of temsirolimus efficacy and mechanism of action (phosphoproteins) strongly suggest that it might translate to therapeutic efficacy in human tumors that bear congruency to features present in the mammary carcinoma of PyMT tumors.