Uridine-cytidine kinase 2 potentiates the mutagenic influence of the antiviral ß-d-N4-hydroxycytidine.

Molnupiravir (EIDD-2801) is an antiviral that received approval for the treatment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) infection. Treatment of bacteria or cell lines with the active form of molnupiravir, ß-d-N4-hydroxycytidine (NHC, or EIDD-1931), induces mutations in DNA. Yet these results contrast in vivo genotoxicity studies conducted during registration of the drug. Using a CRISPR screen, we found that inactivating the pyrimidine salvage pathway component uridine-cytidine kinase 2 (Uck2) renders cells more tolerant of NHC. Short-term exposure to NHC increased the mutation rate in a mouse myeloid cell line, with most mutations being T:A to C:G transitions. Inactivating Uck2 impaired the mutagenic activity of NHC, whereas over-expression of Uck2 enhanced mutagenesis. UCK2 is upregulated in many cancers and cell lines. Our results suggest differences in ribonucleoside metabolism contribute to the variable mutagenicity of NHC observed in cancer cell lines and primary tissues.

Overexpression of Lmo2 initiates T-lymphoblastic leukemia via impaired thymocyte competition.

Cell competition has recently emerged as an important tumor suppressor mechanism in the thymus that inhibits autonomous thymic maintenance. Here, we show that the oncogenic transcription factor Lmo2 causes autonomous thymic maintenance in transgenic mice by inhibiting early T cell differentiation. This autonomous thymic maintenance results in the development of self-renewing preleukemic stem cells (pre-LSCs) and subsequent leukemogenesis, both of which are profoundly inhibited by restoration of thymic competition or expression of the antiapoptotic factor BCL2. Genomic analyses revealed the presence of Notch1 mutations in pre-LSCs before subsequent loss of tumor suppressors promotes the transition to overt leukemogenesis. These studies demonstrate a critical role for impaired cell competition in the development of pre-LSCs in a transgenic mouse model of T cell acute lymphoblastic leukemia (T-ALL), implying that this process plays a role in the ontogeny of human T-ALL.

Ganciclovir-induced mutations are present in a diverse spectrum of post-transplant malignancies.

BACKGROUND: Ganciclovir (GCV) is widely used in solid organ and haematopoietic stem cell transplant patients for prophylaxis and treatment of cytomegalovirus. It has long been considered a mutagen and carcinogen. However, the contribution of GCV to cancer incidence and other factors that influence its mutagenicity remains unknown. METHODS: This retrospective cohort study analysed genomics data for 121,771 patients who had undergone targeted sequencing compiled by the Genomics Evidence Neoplasia Information Exchange (GENIE) or Foundation Medicine (FM). A statistical approach was developed to identify patients with GCV-associated mutational signature (GCVsig) from targeted sequenced data of tumour samples. Cell line exposure models were further used to quantify mutation burden and DNA damage caused by GCV and other antiviral and immunosuppressive drugs. RESULTS: Mutational profiles from 22 of 121,771 patient samples in the GENIE and FM cohorts showed evidence of GCVsig. A diverse range of cancers was represented. All patients with detailed clinical history available had previously undergone solid organ transplantation and received GCV and mycophenolate treatment. RAS hotspot mutations associated with GCVsig were present in 9 of the 22 samples, with all samples harbouring multiple GCV-associated protein-altering mutations in cancer driver genes. In vitro testing in cell lines showed that elevated DNA damage response and GCVsig are uniquely associated with GCV but not acyclovir, a structurally similar antiviral. Combination treatment of GCV with the immunosuppressant, mycophenolate mofetil (MMF), increased the misincorporation of GCV in genomic DNA and mutations attributed to GCVsig in cell lines and organoids. CONCLUSIONS: In summary, GCV can cause a diverse range of cancers. Its mutagenicity may be potentiated by other therapies, such as mycophenolate, commonly co-prescribed with GCV for post-transplant patients. Further investigation of the optimal use of these drugs could help reduce GCV-associated mutagenesis in post-transplant patients.

Single-cell multiomics reveal the scale of multilayered adaptations enabling CLL relapse during venetoclax therapy.

Venetoclax (VEN) inhibits the prosurvival protein BCL2 to induce apoptosis and is a standard therapy for chronic lymphocytic leukemia (CLL), delivering high complete remission rates and prolonged progression-free survival in relapsed CLL but with eventual loss of efficacy. A spectrum of subclonal genetic changes associated with VEN resistance has now been described. To fully understand clinical resistance to VEN, we combined single-cell short- and long-read RNA-sequencing to reveal the previously unappreciated scale of genetic and epigenetic changes underpinning acquired VEN resistance. These appear to be multilayered. One layer comprises changes in the BCL2 family of apoptosis regulators, especially the prosurvival family members. This includes previously described mutations in BCL2 and amplification of the MCL1 gene but is heterogeneous across and within individual patient leukemias. Changes in the proapoptotic genes are notably uncommon, except for single cases with subclonal losses of BAX or NOXA. Much more prominent was universal MCL1 gene upregulation. This was driven by an overlying layer of emergent NF-κB (nuclear factor kappa B) activation, which persisted in circulating cells during VEN therapy. We discovered that MCL1 could be a direct transcriptional target of NF-κB. Both the switch to alternative prosurvival factors and NF-κB activation largely dissipate following VEN discontinuation. Our studies reveal the extent of plasticity of CLL cells in their ability to evade VEN-induced apoptosis. Importantly, these findings pinpoint new approaches to circumvent VEN resistance and provide a specific biological justification for the strategy of VEN discontinuation once a maximal response is achieved rather than maintaining long-term selective pressure with the drug.

MINTIE: identifying novel structural and splice variants in transcriptomes using RNA-seq data.

Calling fusion genes from RNA-seq data is well established, but other transcriptional variants are difficult to detect using existing approaches. To identify all types of variants in transcriptomes we developed MINTIE, an integrated pipeline for RNA-seq data. We take a reference-free approach, combining de novo assembly of transcripts with differential expression analysis to identify up-regulated novel variants in a case sample. We compare MINTIE with eight other approaches, detecting > 85% of variants while no other method is able to achieve this. We posit that MINTIE will be able to identify new disease variants across a range of disease types.

Detecting copy number alterations in RNA-Seq using SuperFreq.

MOTIVATION: Calling copy number alterations (CNAs) from RNA sequencing (RNA-Seq) is challenging, because of the marked variability in coverage across genes and paucity of single nucleotide polymorphisms (SNPs). We have adapted SuperFreq to call absolute and allele sensitive CNAs from RNA-Seq. SuperFreq uses an error-propagation framework to combine and maximize information from read counts and B-allele frequencies. RESULTS: We used datasets from The Cancer Genome Atlas (TCGA) to assess the validity of CNA calls from RNA-Seq. When ploidy estimates were consistent, we found agreement with DNA SNP-arrays for over 98% of the genome for acute myeloid leukaemia (TCGA-AML, n = 116) and 87% for colorectal cancer (TCGA-CRC, n = 377). The sensitivity of CNA calling from RNA-Seq was dependent on gene density. Using RNA-Seq, SuperFreq detected 78% of CNA calls covering 100 or more genes with a precision of 94%. Recall dropped for focal events, but this also depended on signal intensity. For example, in the CRC cohort SuperFreq identified all cases (7/7) with high-level amplification of ERBB2, where the copy number was typically >20, but identified only 6% of cases (1/17) with moderate amplification of IGF2, which occurs over a smaller interval. SuperFreq offers an integrated platform for identification of CNAs and point mutations. As evidence of how SuperFreq can be applied, we used it to reproduce the established relationship between somatic mutation load and CNA profile in CRC using RNA-Seq alone. AVAILABILITY AND IMPLEMENTATION: SuperFreq is implemented in R and the code is available through GitHub: https://github.com/ChristofferFlensburg/SuperFreq/. Data and code to reproduce the figures are available at: https://gitlab.wehi.edu.au/flensburg.c/SuperFreq_RNA_paper. Data from TCGA (phs000178) was accessed from GDC following completion of a data access request through the database of Genotypes and Phenotypes (dbGaP). Data from the Leucegene consortium was downloaded from GEO (AML samples: GSE67040; normal CD34+ cells: GSE48846). SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Intact TP-53 function is essential for sustaining durable responses to BH3-mimetic drugs in leukemias.

Selective targeting of BCL-2 with the BH3-mimetic venetoclax has been a transformative treatment for patients with various leukemias. TP-53 controls apoptosis upstream of where BCL-2 and its prosurvival relatives, such as MCL-1, act. Therefore, targeting these prosurvival proteins could trigger apoptosis across diverse blood cancers, irrespective of TP53 mutation status. Indeed, targeting BCL-2 has produced clinically relevant responses in blood cancers with aberrant TP-53. However, in our study, TP53-mutated or -deficient myeloid and lymphoid leukemias outcompeted isogenic controls with intact TP-53, unless sufficient concentrations of BH3-mimetics targeting BCL-2 or MCL-1 were applied. Strikingly, tumor cells with TP-53 dysfunction escaped and thrived over time if inhibition of BCL-2 or MCL-1 was sublethal, in part because of an increased threshold for BAX/BAK activation in these cells. Our study revealed the key role of TP-53 in shaping long-term responses to BH3-mimetic drugs and reconciled the disparate pattern of initial clinical response to venetoclax, followed by subsequent treatment failure among patients with TP53-mutant chronic lymphocytic leukemia or acute myeloid leukemia. In contrast to BH3-mimetics targeting just BCL-2 or MCL-1 at doses that are individually sublethal, a combined BH3-mimetic approach targeting both prosurvival proteins enhanced lethality and durably suppressed the leukemia burden, regardless of TP53 mutation status. Our findings highlight the importance of using sufficiently lethal treatment strategies to maximize outcomes of patients with TP53-mutant disease. In addition, our findings caution against use of sublethal BH3-mimetic drug regimens that may enhance the risk of disease progression driven by emergent TP53-mutant clones.

CHK2 Inhibition Provides a Strategy to Suppress Hematologic Toxicity from PARP Inhibitors.

Patients with cancer treated with PARP inhibitors (PARPi) experience various side effects, with hematologic toxicity being most common. Short-term treatment of mice with olaparib resulted in depletion of reticulocytes, B-cell progenitors, and immature thymocytes, whereas longer treatment induced broader myelosuppression. We performed a CRISPR/Cas9 screen that targeted DNA repair genes in Eµ-Myc pre-B lymphoma cell lines as a way to identify strategies to suppress hematologic toxicity from PARPi. The screen revealed that single-guide RNAs targeting the serine/threonine kinase checkpoint kinase 2 (CHK2) were enriched following olaparib treatment. Genetic or pharmacologic inhibition of CHK2-blunted PARPi response in lymphoid and myeloid cell lines, and in primary murine pre-B/pro-B cells. Using a Cas9 base editor, we found that blocking CHK2-mediated phosphorylation of p53 also impaired olaparib response. Our results identify the p53 pathway as a major determinant of the acute response to PARPi in normal blood cells and demonstrate that targeting CHK2 can short circuit this response. Cotreatment with a CHK2 inhibitor did not antagonize olaparib response in ovarian cancer cell lines. Selective inhibition of CHK2 may spare blood cells from the toxic influence of PARPi and broaden the utility of these drugs. IMPLICATIONS: We reveal that genetic or pharmacologic inhibition of CHK2 may offer a way to alleviate the toxic influence of PARPi in the hematologic system.

Finding a suitable library size to call variants in RNA-Seq.

BACKGROUND: RNA sequencing allows the study of both gene expression changes and transcribed mutations, providing a highly effective way to gain insight into cancer biology. When planning the sequencing of a large cohort of samples, library size is a fundamental factor affecting both the overall cost and the quality of the results. Here we specifically address how overall library size influences the detection of somatic mutations in RNA-seq data in two acute myeloid leukaemia datasets. RESULTS : We simulated shallower sequencing depths by downsampling 45 acute myeloid leukaemia samples (100 bp PE) that are part of the Leucegene project, which were originally sequenced at high depth. We compared the sensitivity of six methods of recovering validated mutations on the same samples. The methods compared are a combination of three popular callers (MuTect, VarScan, and VarDict) and two filtering strategies. We observed an incremental loss in sensitivity when simulating libraries of 80M, 50M, 40M, 30M and 20M fragments, with the largest loss detected with less than 30M fragments (below 90%, average loss of 7%). The sensitivity in recovering insertions and deletions varied markedly between callers, with VarDict showing the highest sensitivity (60%). Single nucleotide variant sensitivity is relatively consistent across methods, apart from MuTect, whose default filters need adjustment when using RNA-Seq. We also analysed 136 RNA-Seq samples from the TCGA-LAML cohort (50 bp PE) and assessed the change in sensitivity between the initial libraries (average 59M fragments) and after downsampling to 40M fragments. When considering single nucleotide variants in recurrently mutated myeloid genes we found a comparable performance, with a 6% average loss in sensitivity using 40M fragments. CONCLUSIONS: Between 30M and 40M 100 bp PE reads are needed to recover 90-95% of the initial variants on recurrently mutated myeloid genes. To extend this result to another cancer type, an exploration of the characteristics of its mutations and gene expression patterns is suggested.

Chemotherapy and Venetoclax in Elderly Acute Myeloid Leukemia Trial (CAVEAT): A Phase Ib Dose-Escalation Study of Venetoclax Combined With Modified Intensive Chemotherapy.

PURPOSE: The B-cell lymphoma 2 (BCL-2) inhibitor venetoclax has an emerging role in acute myeloid leukemia (AML), with promising response rates in combination with hypomethylating agents or low-dose cytarabine in older patients. The tolerability and efficacy of venetoclax in combination with intensive chemotherapy in AML is unknown. PATIENTS AND METHODS: Patients with AML who were ≥ 65 years (≥ 60 years if monosomal karyotype) and fit for intensive chemotherapy were allocated to venetoclax dose-escalation cohorts (range, 50-600 mg). Venetoclax was administered orally for 14 days each cycle. During induction, a 7-day prephase/dose ramp-up (days -6 to 0) was followed by an additional 7 days of venetoclax combined with infusional cytarabine 100 mg/m2 on days 1-5 and idarubicin 12 mg/m2 intravenously on days 2-3 (ie, 5 + 2). Consolidation (4 cycles) included 14 days of venetoclax (days -6 to 7) combined with cytarabine (days 1-2) and idarubicin (day 1). Maintenance venetoclax was permitted (7 cycles). The primary objective was to assess the optimal dose schedule of venetoclax with 5 + 2. RESULTS: Fifty-one patients with a median age of 72 years (range, 63-80 years) were included. The maximum tolerated dose was not reached with venetoclax 600 mg/day. The main grade ≥ 3 nonhematologic toxicities during induction were febrile neutropenia (55%) and sepsis (35%). In contrast to induction, platelet recovery was notably delayed during consolidation cycles. The overall response rate (complete remission [CR]/CR with incomplete count recovery) was 72%; it was 97% in de novo AML and was 43% in secondary AML. During the venetoclax prephase, marrow blast reductions (≥ 50%) were noted in NPM1-, IDH2-, and SRSF2-mutant AML. CONCLUSION: Venetoclax combined with 5 + 2 induction chemotherapy was safe and tolerable in fit older patients with AML. Although the optimal postremission therapy remains to be determined, the high remission rate in de novo AML warrants additional investigation (ANZ Clinical Trial Registry No. ACTRN12616000445471).

A missense mutation in the MLKL brace region promotes lethal neonatal inflammation and hematopoietic dysfunction.

MLKL is the essential effector of necroptosis, a form of programmed lytic cell death. We have isolated a mouse strain with a single missense mutation, MlklD139V, that alters the two-helix 'brace' that connects the killer four-helix bundle and regulatory pseudokinase domains. This confers constitutive, RIPK3 independent killing activity to MLKL. Homozygous mutant mice develop lethal postnatal inflammation of the salivary glands and mediastinum. The normal embryonic development of MlklD139V homozygotes until birth, and the absence of any overt phenotype in heterozygotes provides important in vivo precedent for the capacity of cells to clear activated MLKL. These observations offer an important insight into the potential disease-modulating roles of three common human MLKL polymorphisms that encode amino acid substitutions within or adjacent to the brace region. Compound heterozygosity of these variants is found at up to 12-fold the expected frequency in patients that suffer from a pediatric autoinflammatory disease, chronic recurrent multifocal osteomyelitis (CRMO).

The application of RNA sequencing for the diagnosis and genomic classification of pediatric acute lymphoblastic leukemia.

Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy, and implementation of risk-adapted therapy has been instrumental in the dramatic improvements in clinical outcomes. A key to risk-adapted therapies includes the identification of genomic features of individual tumors, including chromosome number (for hyper- and hypodiploidy) and gene fusions, notably ETV6-RUNX1, TCF3-PBX1, and BCR-ABL1 in B-cell ALL (B-ALL). RNA-sequencing (RNA-seq) of large ALL cohorts has expanded the number of recurrent gene fusions recognized as drivers in ALL, and identification of these new entities will contribute to refining ALL risk stratification. We used RNA-seq on 126 ALL patients from our clinical service to test the utility of including RNA-seq in standard-of-care diagnostic pipelines to detect gene rearrangements and IKZF1 deletions. RNA-seq identified 86% of rearrangements detected by standard-of-care diagnostics. KMT2A (MLL) rearrangements, although usually identified, were the most commonly missed by RNA-seq as a result of low expression. RNA-seq identified rearrangements that were not detected by standard-of-care testing in 9 patients. These were found in patients who were not classifiable using standard molecular assessment. We developed an approach to detect the most common IKZF1 deletion from RNA-seq data and validated this using an RQ-PCR assay. We applied an expression classifier to identify Philadelphia chromosome-like B-ALL patients. T-ALL proved a rich source of novel gene fusions, which have clinical implications or provide insights into disease biology. Our experience shows that RNA-seq can be implemented within an individual clinical service to enhance the current molecular diagnostic risk classification of ALL.

SuperFreq: Integrated mutation detection and clonal tracking in cancer.

Analysing multiple cancer samples from an individual patient can provide insight into the way the disease evolves. Monitoring the expansion and contraction of distinct clones helps to reveal the mutations that initiate the disease and those that drive progression. Existing approaches for clonal tracking from sequencing data typically require the user to combine multiple tools that are not purpose-built for this task. Furthermore, most methods require a matched normal (non-tumour) sample, which limits the scope of application. We developed SuperFreq, a cancer exome sequencing analysis pipeline that integrates identification of somatic single nucleotide variants (SNVs) and copy number alterations (CNAs) and clonal tracking for both. SuperFreq does not require a matched normal and instead relies on unrelated controls. When analysing multiple samples from a single patient, SuperFreq cross checks variant calls to improve clonal tracking, which helps to separate somatic from germline variants, and to resolve overlapping CNA calls. To demonstrate our software we analysed 304 cancer-normal exome samples across 33 cancer types in The Cancer Genome Atlas (TCGA) and evaluated the quality of the SNV and CNA calls. We simulated clonal evolution through in silico mixing of cancer and normal samples in known proportion. We found that SuperFreq identified 93% of clones with a cellular fraction of at least 50% and mutations were assigned to the correct clone with high recall and precision. In addition, SuperFreq maintained a similar level of performance for most aspects of the analysis when run without a matched normal. SuperFreq is highly versatile and can be applied in many different experimental settings for the analysis of exomes and other capture libraries. We demonstrate an application of SuperFreq to leukaemia patients with diagnosis and relapse samples.

Structures of BCL-2 in complex with venetoclax reveal the molecular basis of resistance mutations.

Venetoclax is a first-in-class cancer therapy that interacts with the cellular apoptotic machinery promoting apoptosis. Treatment of patients suffering chronic lymphocytic leukaemia with this BCL-2 antagonist has revealed emergence of a drug-selected BCL-2 mutation (G101V) in some patients failing therapy. To understand the molecular basis of this acquired resistance we describe the crystal structures of venetoclax bound to both BCL-2 and the G101V mutant. The pose of venetoclax in its binding site on BCL-2 reveals small but unexpected differences as compared to published structures of complexes with venetoclax analogues. The G101V mutant complex structure and mutant binding assays reveal that resistance is acquired by a knock-on effect of V101 on an adjacent residue, E152, with venetoclax binding restored by a E152A mutation. This provides a framework for considering analogues of venetoclax that might be effective in combating this mutation.

Targeted therapy and disease monitoring in CNTRL-FGFR1-driven leukaemia.

We report two patients with leukaemia driven by the rare CNTRL-FGFR1 fusion oncogene. This fusion arises from a t(8;9)(p12;q33) translocation, and is a rare driver of biphenotypic leukaemia in children. We used RNA sequencing to report novel features of expressed CNTRL-FGFR1, including CNTRL-FGFR1 fusion alternative splicing. From this knowledge, we designed and tested a Droplet Digital PCR assay that detects CNTRL-FGFR1 expression to approximately one cell in 100 000 using fusion breakpoint-specific primers and probes. We also utilised cell-line models to show that effective tyrosine kinase inhibitors, which may be included in treatment regimens for this disease, are only those that block FGFR1 phosphorylation.

On fitness: how do mutations shape the biology of cancer?

The theory of evolution by natural selection shapes our understanding of the living world. While natural selection has given rise to all the intricacies of life on the planet, those responsible for treating cancer have a darker view of adaptation and selection. Revolutionary changes in DNA sequencing technology have allowed us to survey the complexities that constitute the cancer genome, while advances in genetic engineering are allowing us to functionally interrogate these alterations. These approaches are providing new insights into how mutations influence cancer biology. It is possible that with time, this new knowledge will allow us to take control of the evolutionary processes that shape the disease, to develop more effective treatments.

Acquisition of the Recurrent Gly101Val Mutation in BCL2 Confers Resistance to Venetoclax in Patients with Progressive Chronic Lymphocytic Leukemia.

The BCL2 inhibitor venetoclax induces high rates of durable remission in patients with previously treated chronic lymphocytic leukemia (CLL). However, despite continuous daily treatment, leukemia recurs in most patients. To investigate the mechanisms of secondary resistance, we analyzed paired pre-venetoclax and progression samples from 15 patients with CLL progression enrolled on venetoclax clinical trials. The novel Gly101Val mutation in BCL2 was identified at progression in 7 patients, but not at study entry. It was first detectable after 19 to 42 months of therapy, and its emergence anticipated clinical disease progression by many months. Gly101Val reduces the affinity of BCL2 for venetoclax by ∼180-fold in surface plasmon resonance assays, thereby preventing the drug from displacing proapoptotic mediators from BCL2 in cells and conferring acquired resistance in cell lines and primary patient cells. This mutation provides new insights into the pathobiology of venetoclax resistance and provides a potential biomarker of impending clinical relapse. SIGNIFICANCE: Why CLL recurs in patients who achieve remission with the BCL2 inhibitor venetoclax has been unknown. We provide the first description of an acquired point mutation in BCL2 arising recurrently and exclusively in venetoclax-treated patients. The mutation reduces venetoclax binding and is sufficient to confer resistance.See related commentary by Thangavadivel and Byrd, p. 320.This article is highlighted in the In This Issue feature, p. 305.

Clinker: visualizing fusion genes detected in RNA-seq data.

Background: Genomic profiling efforts have revealed a rich diversity of oncogenic fusion genes. While there are many methods for identifying fusion genes from RNA-sequencing (RNA-seq) data, visualizing these transcripts and their supporting reads remains challenging. Findings: Clinker is a bioinformatics tool written in Python, R, and Bpipe that leverages the superTranscript method to visualize fusion genes. We demonstrate the use of Clinker to obtain interpretable visualizations of the RNA-seq data that lead to fusion calls. In addition, we use Clinker to explore multiple fusion transcripts with novel breakpoints within the P2RY8-CRLF2 fusion gene in B-cell acute lymphoblastic leukemia. Conclusions: Clinker is freely available software that allows visualization of fusion genes and the RNA-seq data used in their discovery.