The interaction between cytosine methylation and processes of DNA replication and repair shape the mutational landscape of cancer genomes

© The Author(s) 2017. Methylated cytosines (5mCs) are frequently mutated in the genome. However, no studies have yet comprehensively analysed mutation-methylation associations across cancer types. Here we analyse 916 cancer genomes, together with tissue type-specific methylation and replication timing data. We describe a strong mutation-methylation association across colorectal cancer subtypes, most interestingly in samples with microsatellite instability (MSI) or Polymerase epsilon (POLE) exonuclease domain mutations. By analysing genomic regions with differential mismatch repair (MMR) efficiency, we suggest a possible role for MMR in the correction of 5mC deamination events, potentially accounting for the high rate of 5mC mutation accumulation in MSI tumours. Additionally, we propose that mutant POLE asserts a mutator phenotype specifically at 5mCs, and we find coding mutation hotspots in POLE-mutant cancers at highly-methylated CpGs in the tumour-suppressor genes APC and TP53. Finally, using multivariable regression models, we demonstrate that different cancers exhibit distinct mutation-methylation associations, with DNA repair influencing such associations in certain cancer genomes. Taken together, we find differential associations with methylation that are vital for accurately predicting expected mutation loads across cancer types. Our findings reveal links between methylation and common mutation and repair processes, with these mechanisms defining a key part of the mutational landscape of cancer genomes.Link_to_subscribed_fulltex

Proteogenomic analysis prioritises functional single nucleotide variants in cancer samples

© Ma et al. Massively parallel DNA sequencing enables the detection of thousands of germline and somatic single nucleotide variants (SNVs) in cancer samples. The functional analysis of these mutations is often carried out through in silico predictions, with further downstream experimental validation rarely performed. Here, we examine the potential of using mass spectrometry-based proteomics data to further annotate the function of SNVs in cancer samples. RNA-seq and whole genome sequencing (WGS) data from Jurkat cells were used to construct a custom database of single amino acid variant (SAAV) containing peptides and identified over 1,000 such peptides in two Jurkat proteomics datasets. The analysis enabled the detection of a truncated form of splicing regulator YTHDC1 at the protein level. To extend the functional annotation further, a Jurkat phosphoproteomics dataset was analysed, identifying 463 SAAV containing phosphopeptides. Of these phosphopeptides, 24 SAAVs were found to directly impact the phosphorylation event through the creation of either a phosphorylation site or a kinase recognition motif. We identified a novel phosphorylation site created by a SAAV in splicing factor SF3B1, a protein that is frequently mutated in leukaemia. To our knowledge, this is the first study to use phosphoproteomics data to directly identify novel phosphorylation events arising from the creation of phosphorylation sites by SAAVs. Our study reveals multiple functional mutations impacting the splicing pathway in Jurkat cells and demonstrates potential benefits of an integrative proteogenomics analysis for high-throughput functional annotation of SNVs in cancer.Link_to_subscribed_fulltex

The search for cis-regulatory driver mutations in cancer genomes

With the advent of high-throughput and relatively inexpensive whole-genome sequencing technology, the focus of cancer research has begun to shift toward analyses of somatic mutations in non-coding cis-regulatory elements of the cancer genome. Cis-regulatory elements play an important role in gene regulation, with mutations in these elements potentially resulting in changes to the expression of linked genes. The recent discoveries of recurrent TERT promoter mutations in melanoma, and recurrent mutations that create a super-enhancer regulating TAL1 expression in T-cell acute lymphoblastic leukaemia (T-ALL), have sparked significant interest in the search for other somatic cis-regulatory mutations driving cancer development. In this review, we look more closely at the TERT promoter and TAL1 enhancer alterations and use these examples to ask whether other cis-regulatory mutations may play a role in cancer susceptibility. In doing so, we make observations from the data emerging from recent research in this field, and describe the experimental and analytical approaches which could be adopted in the hope of better uncovering the true functional significance of somatic cis-regulatory mutations in cancer.Link_to_subscribed_fulltex

Functional Mutations Form at CTCF-Cohesin Binding Sites in Melanoma Due to Uneven Nucleotide Excision Repair across the Motif

© 2016 The Author(s) CTCF binding sites are frequently mutated in cancer, but how these mutations accumulate and whether they broadly perturb CTCF binding are not well understood. Here, we report that skin cancers exhibit a highly specific asymmetric mutation pattern within CTCF motifs attributable to ultraviolet irradiation and differential nucleotide excision repair (NER). CTCF binding site mutations form independently of replication timing and are enriched at sites of CTCF/cohesin complex binding, suggesting a role for cohesin in stabilizing CTCF-DNA binding and impairing NER. Performing CTCF ChIP-seq in a melanoma cell line, we show CTCF binding site mutations to be functional by demonstrating allele-specific reduction of CTCF binding to mutant alleles. While topologically associating domains with mutated CTCF anchors in melanoma contain differentially expressed cancer-associated genes, CTCF motif mutations appear generally under neutral selection. However, the frequency and potential functional impact of such mutations in melanoma highlights the need to consider their impact on cellular phenotype in individual genomes.Link_to_subscribed_fulltex

Lynch Syndrome Associated with Two MLH1 Promoter Variants and Allelic Imbalance of MLH1 Expression

© 2015 The Authors. **Human Mutation published by Wiley Periodicals, Inc. Lynch syndrome is a hereditary cancer syndrome caused by a constitutional mutation in one of the mismatch repair genes. The implementation of predictive testing and targeted preventative surveillance is hindered by the frequent finding of sequence variants of uncertain significance in these genes. We aimed to determine the pathogenicity of previously reported variants (c.-28A > G and c.-7C > T) within the MLH1 5â²untranslated region (UTR) in two individuals from unrelated suspected Lynch syndrome families. We investigated whether these variants were associated with other pathogenic alterations using targeted high-throughput sequencing of the MLH1 locus. We also determined their relationship to gene expr ession and epigenetic alterations at the promoter. Sequencing revealed that the c.-28A > G and c.-7C > T variants were the only potentially pathogenic alterations within the MLH1 gene. In both individuals, the levels of transcription from the variant allele were reduced to 50% compared with the wild-type allele. Partial loss of expression occurred in the absence of constitutional epigenetic alterations within the MLH1 promoter. We propose that these variants may be pathogenic due to constitutional partial loss of MLH1 expression, and that this may be associated with intermediate penetrance of a Lynch syndrome phenotype. Our findings provide further evidence of the potential importance of noncoding variants in the MLH1 5â²UTR in the pathogenesis of Lynch syndrome.Link_to_subscribed_fulltex

Brevi spunti sul d.lgs. n. 150/2009 e sulla sua applicabilità agli Enti locali

© 2014 Taylor & Francis Group, LLC. Gene silencing in cancer frequently involves hypermethylation and dense nucleosome occupancy across promoter regions. How a promoter transitions to this silent state is unclear. Using colorectal adenomas, we investigated nucleosome positioning, DNA methylation, and gene expression in the early stages of gene silencing. Genome-wide gene expression correlated with highly positioned nucleosomes upstream and downstream of a nucleosome-depleted transcription start site (TSS). Hypermethylated promoters displayed increased nucleosome occupancy, specifically at the TSS. We investigated 2 genes, CDH1 and CDKN2B, which were silenced in adenomas but lacked promoter hypermethylation. Instead, silencing correlated with loss of nucleosomes from the -2 position upstream of the TSS relative to normal mucosa. In contrast, permanent CDH1 silencing in carcinoma cells was characterized by promoter hypermethylation and dense nucleosome occupancy. Our findings suggest that silenced genes transition through an intermediary stage involving altered promoter nucleosome positioning, before permanent silencing by hypermethylation and dense nucleosome occupancy.Link_to_subscribed_fulltex