Biology subjects

Flensburg, C.

Publications and source records attributed to Flensburg, C..

8 recordsLinked to original sources

bioRxiv2024

Mbd4 and MutSα protect cells from spontaneous deamination of 5-methylcytosine.

5-Methylcytosine (5mC) is a common source of somatic mutations. Deamination of 5mC to thymine generates a G/T mismatch, which occurs spontaneously and must be repaired prior to DNA replication to avoid mutation. We generated genetically engineered mice and cell lines to define DNA repair pathways that protect against methylation damage. We observed a low background mutation rate in mouse bone marrow or colon, typically 0.2-0.5 CG>TG mutations/genome/day. This increased 3-7 fold in cells lacking the glycosylase Methyl-binding domain 4 (Mbd4), one of the few glycosylases capable of excising thymine from G/T mismatches. We found no role for Thymine DNA glycosylase (Tdg) in methylation damage repair. Instead, our results support cooperation between Mbd4 and the mismatch repair (MMR) complex MutS (Msh6:Msh2), evident through elevated rates of methylation damage in Msh6-deficient cells; increasing to 2.6-4.8 CG>TG mutations/genome/day in primary cells and up to 13.9 CG>TG mutations/genome/day in cell lines. Our findings support the view that MutS has DNA repair activity outside of replication. While loss of Mbd4 elevates methylation damage selectively, the broader functionality of MutS explains why mutational signatures linked to Msh6-deficiency are variable and reflect the replicative history of the cell.

molecular biology↗

bioRxiv2021

Germline loss-of-function variants in the base-excision repair gene MBD4 cause a Mendelian recessive syndrome of adenomatous colorectal polyposis and acute myeloid leukaemia

Inherited defects in base-excision repair (BER) predispose to adenomatous polyposis and colorectal cancer (CRC), yet our understanding of this important DNA repair pathway remains incomplete. By combining detailed clinical, histological and molecular profiling, we reveal biallelic germline loss-of-function (LOF) variants in the BER gene MBD4 to predispose to adenomatous polyposis and -uniquely amongst CRC predisposition syndromes- to myeloid neoplasms. Neoplasms from MBD4-deficient patients almost exclusively accumulate somatic CpG>TpG mutations, resembling mutational signature SBS1. MBD4-deficient adenomas harbour mutations in known CRC driver genes, although AMER1 mutations were more common and KRAS mutations less frequent. We did not find an increased risk for colorectal tumours in individuals with a monoallelic MBD4 LOF variant. We suggest that this condition should be termed MBD4-associated neoplasia syndrome (MANS) and that MBD4 is included in testing for the genetic diagnosis of polyposis and/or early-onset AML.

genetics↗

bioRxiv2021

Life without mismatch repair

Mismatch repair (MMR) is a critical defence against mutation, but we lack quantification of its activity on different DNA lesions during human life. We performed whole-genome sequencing of normal and neoplastic tissues from individuals with constitutional MMR deficiency to establish the roles of MMR components, tissue type and disease state in somatic mutation rates. Mutational signatures varied extensively across genotypes, some coupled to leading-strand replication, some to lagging-strand replication and some independent of replication, implying that the various MMR components engage different forms of DNA damage. Loss of MSH2 or MSH6 (MutS), but not MLH1 or PMS2 (MutL), caused 5-methylcytosine-dependent hypermutation, indicating that MutS is the pivotal complex for repairing spontaneous deamination of methylated cytosines in humans. Neoplastic change altered the distribution of mutational signatures, particularly accelerating replication-coupled indel signatures. Each component of MMR repairs 1-10 lesions/day per normal human cell, and many thousands of additional events during neoplastic transformation. HighlightsO_LIMMR repairs 1-10 lesions/day in every normal cell and thousands more in tumor cells C_LIO_LIMMR patterns and rates are shaped by genotype, tissue type and malignant transformation C_LIO_LIMSH2 and MSH6 are pivotal for repairing spontaneous deamination of methylated cytosine C_LIO_LIReplication indels and substitutions vary by leading versus lagging strand and genotype C_LI

genomics↗

bioRxiv2020

Detecting copy number alterations in RNA-Seq using SuperFreq

Calling copy number alterations (CNAs) from RNA-Seq is challenging, because differences in gene expression mean that read depth across genes varies by several orders of magnitude and there is a paucity of informative single nucleotide polymorphisms (SNPs). We previously developed SuperFreq to analyse exome data of tumours by combining variant calling and copy number estimation in an integrated pipeline. Here we have used the SuperFreq framework for the analysis of RNA sequencing (RNA-Seq) data, which allows for the detection of absolute and allele sensitive CNAs. SuperFreq uses an error-propagation framework to combine and maximise the information available in the read depth and B-allele frequencies of SNPs (BAFs) to make CNA calls on RNA-seq data. We used data from The Cancer Genome Atlas (TCGA) to evaluate the CNA called from RNA-Seq with those generated from SNP-arrays. When ploidy estimates were consistent, we found excellent agreement with CNAs called from DNA of over 98% of the genome for acute myeloid leukaemia (TCGA-AML, n=116) and 87% for colorectal cancer (TCGA-CRC, n=377), which has a much higher CNA burden. As expected, the sensitivity of CNA calling from RNA-Seq was dependent on gene density. Nonetheless, using RNA-Seq SuperFreq detected 78% of CNA calls covering 100 or more genes with a precision of 94%. Recall dropped markedly for focal events, but this also depended on the signal intensity. For example, in the CRC cohort SuperFreq identified 100% (7/7) of cases with high-level amplification of ERBB2, where the copy number was typically >20, but identified only 6% (1/17) of cases with moderate amplification of IGF2, typically 4 or 5 copies over a smaller region (median 5 flanking genes for IGF2, compared to 20 for ERBB2). We were able to reproduce the relationship between mutational load and CNA profile in CRC using RNA-Seq alone. SuperFreq offers an integrated platform for identification of CNAs and point mutations from RNA-seq in cancer transcriptomes. The software is implemented in R and is available through GitHub: https://github.com/ChristofferFlensburg/SuperFreq.

bioinformatics↗

bioRxiv2019

Finding a suitable library size to call variants in RNA-seq

BackgroundRNA-Seq 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. While several studies analyse the effect that library size has on differential expression analyses, sensitivity analysis for variant detection has received far less attention. ResultsWe simulated shallower sequencing depths by downsampling 45 AML samples 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%). The sensitivity in recovering indels 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, assessing 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 3% average loss in sensitivity using 40M fragments. ConclusionsBetween 30M and 40M fragments 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.

bioinformatics↗

bioRxiv2018

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. SuperFreq is implemented in R and available on github at https://github.com/ChristofferFlensburg/SuperFreq.

bioinformatics↗

bioRxiv2017

Germline loss of MBD4 predisposes to leukaemia due to a mutagenic cascade driven by 5mC

Cytosine methylation is essential for normal mammalian development, yet also provides a major mutagenic stimulus. Methylcytosine (5mC) is prone to spontaneous deamination, which introduces cytosine to thymine transition mutations (C>T) upon replication1. Cells endure hundreds of 5mC deamination events each day and an intricate repair network is engaged to restrict this damage. Central to this network are the DNA glycosylases MBD42 and TDG3,4, which recognise T:G mispairing and initiate base excision repair (BER). Here we describe a novel cancer predisposition syndrome resulting from germline biallelic inactivation of MBD4 that leads to the development of acute myeloid leukaemia (AML). These leukaemias have an extremely high burden of C>T mutations, specifically in the context of methylated CG dinucleotides (CG>TG). This dependence on 5mC as a source of mutations may explain the remarkable observation that MBD4-deficient AMLs share a common set of driver mutations, including biallelic mutations in DNMT3A and hotspot mutations in IDH1/IDH2. By assessing serial samples taken over the course of treatment, we highlight a critical interaction with somatic mutations in DNMT3A that accelerates leukaemogenesis and accounts for the conserved path to AML. MBD4-deficiency was also detected, rarely, in sporadic cancers, which display the same mutational signature. Collectively these cancers provide a model of 5mC-dependent hypermutation and reveal factors that shape its mutagenic influence.

cancer biology↗

bioRxiv2017

Dynamic changes in clonal architecture during disease progression in follicular lymphoma

Follicular lymphoma (FL) is typically a slow growing cancer that can be effectively treated. Some patients undergo transformation to diffuse large B cell lymphoma (DLBCL), which is frequently resistant to chemotherapy and is generally fatal. Targeted sequencing of DNA and RNA was applied to identify mutations and transcriptional changes that accompanied transformation in a cohort of 16 patients, including 14 with paired samples. In most cases we found mutations that were specific to the FL clone dominant at diagnosis, supporting the view that DLBCL does not develop directly from FL, but from an ancestral progenitor. We identified frequent mutations in TP53, cell cycle regulators (cyclins and cyclin-dependent kinases) and the PI3K pathway, as well as recurrent somatic copy number variants (SCNVs) on chromosome 3, 7 and 17p associated with transformation. An integrated analysis of RNA and DNA identified allele specific expression changes in oncogenes, including MYC, that could be attributed to structural rearrangements. By focusing on serial samples taken from two patients, we identified evidence of convergent tumour evolution, where clonal expansion was repeatedly associated with mutations targeting the same genes or pathways. Analysis of serial samples is a powerful way to identify core dependencies that support lymphoma growth.

cancer biology↗