Human DNA Helicase B as a Candidate for Unwinding Secondary CGG Repeat Structures at the Fragile X Mental Retardation Gene

The fragile X syndrome (FXS) is caused by a CGG repeat expansion at the fragile X mental retardation (FMR1) gene. FMR1 alleles with more than 200 CGG repeats bear chromosomal fragility when cells experience folate deficiency. CGG repeats were reported to be able to form secondary structures, such as hairpins, in vitro. When such secondary structures are formed, repeats can lead to replication fork stalling even in the absence of any additional perturbation. Indeed, it was recently shown that the replication forks stall at the endogenous FMR1 locus in unaffected and FXS cells, suggesting the formation of secondary repeat structures at the FMR1 gene in vivo. If not dealt with properly replication fork stalling can lead to polymerase slippage and repeat expansion as well as fragile site expression. Despite the presence of repeat structures at the FMR1 locus, chromosomal fragility is only expressed under replicative stress suggesting the existence of potential molecular mechanisms that help the replication fork progress through these repeat regions. DNA helicases are known to aid replication forks progress through repetitive DNA sequences. Yet, the identity of the DNA helicase(s) responsible for unwinding the CGG repeats at FMR1 locus is not known. We found that the human DNA helicase B (HDHB) may provide an answer for this question. We used chromatin-immunoprecipitation assay to study the FMR1 region and common fragile sites (CFS), and asked whether HDHB localizes at replication forks stalled at repetitive regions even in unperturbed cells. HDHB was strongly enriched in S-phase at the repetitive DNA at CFS and FMR1 gene but not in the flanking regions. Taken together, these results suggest that HDHB functions in preventing or repairing stalled replication forks that arise in repeat-rich regions even in unperturbed cells. Furthermore, we discuss the importance and potential role of HDHB and other helicases in the resolution of secondary CGG repeat structures

Detection of early stage pancreatic cancer using 5-hydroxymethylcytosine signatures in circulating cell free DNA

Circulating DNA detected in plasma can be used for diagnostic purposes. Here, the authors show that the 5-hydroxymethyl cytosine biomarker from plasma-derived cell free DNA can be used to detect early stage pancreatic cancer

Detection of early stage pancreatic cancer using 5-hydroxymethylcytosine signatures in circulating cell free DNA

AbstractPancreatic cancer is often detected late, when curative therapies are no longer possible. Here, we present non-invasive detection of pancreatic ductal adenocarcinoma (PDAC) by 5-hydroxymethylcytosine (5hmC) changes in circulating cell free DNA from a PDAC cohort (n = 64) in comparison with a non-cancer cohort (n = 243). Differential hydroxymethylation is found in thousands of genes, most significantly in genes related to pancreas development or function (GATA4, GATA6, PROX1, ONECUT1, MEIS2), and cancer pathogenesis (YAP1, TEAD1, PROX1, IGF1). cfDNA hydroxymethylome in PDAC cohort is differentially enriched for genes that are commonly de-regulated in PDAC tumors upon activation of KRAS and inactivation of TP53. Regularized regression models built using 5hmC densities in genes perform with AUC of 0.92 (discovery dataset, n = 79) and 0.92–0.94 (two independent test sets, n = 228). Furthermore, tissue-derived 5hmC features can be used to classify PDAC cfDNA (AUC = 0.88). These findings suggest that 5hmC changes enable classification of PDAC even during early stage disease.</jats:p

Proteomic Analyses Identify a Novel Role for EZH2 in the Initiation of Cancer Cell Drug Tolerance

Acquisition of drug resistance remains a chief impediment to successful cancer therapy, and we previously described a transient drug-tolerant cancer cell population (DTPs) whose survival is in part dependent on the activities of the histone methyltransferases G9a/EHMT2 and EZH2, the latter being the catalytic component of the polycomb repressive complex 2 (PRC2). Here, we apply multiple proteomic techniques to better understand the role of these histone methyltransferases (HMTs) in the establishment of the DTP state. Proteome-wide comparisons of lysine methylation patterns reveal that DTPs display an increase in methylation on K116 of PRC member Jarid2, an event that helps stabilize and recruit PRC2 to chromatin. We also find that EZH2, in addition to methylating histone H3K27, also can methylate G9a at K185, and that methylated G9a better recruits repressive complexes to chromatin. These complexes are similar to complexes recruited by histone H3 methylated at K9. Finally, a detailed histone post-translational modification (PTM) analysis shows that EZH2, either directly or through its ability to methylate G9a, alters H3K9 methylation in the context of H3 serine 10 phosphorylation, primarily in a cancer cell subpopulation that serves as DTP precursors. We also show that combinations of histone PTMs recruit a different set of complexes to chromatin, shedding light on the temporal mechanisms that contribute to drug tolerance

Validation of a Pancreatic Cancer Detection Test in New-Onset Diabetes Using Cell-Free DNA 5-Hydroxymethylation Signatures

AbstractBACKGROUNDPancreatic cancer (PaC) has poor (10%) 5-year overall survival, largely due to predominant late-stage diagnosis. Patients with new-onset diabetes (NOD) are at a six-to eightfold increased risk for PaC. We developed a pancreatic cancer detection test for the use in a clinical setting that employs a logistic regression model based on 5-hydroxymethylcytosine (5hmC) profiling of cell-free DNA (cfDNA).METHODScfDNA was isolated from plasma from 89 subjects with PaC and 596 case-control non-cancer subjects, and 5hmC libraries were generated and sequenced. These data coupled with machine-learning, were used to generate a predictive model for PaC detection, which was independently validated on 79 subjects with PaC, 163 non-cancer subjects, and 506 patients with non-PaC cancers.RESULTSThe area under the receiver operating characteristic curve for PaC classification was 0.93 across the training data. Training sensitivity was 58.4% (95% confidence interval [CI]: 47.5– 68.6) after setting a classification probability threshold that resulted in 98% (95% CI: 96.5–99) specificity. The independent validation dataset sensitivity and specificity were 51.9% (95% CI: 40.4–63.3) and 100.0% (95% CI: 97.8–100.0), respectively. Early-stage (stage 1 and 2) PaC detection was 47.6% (95% CI: 23%–58%) and 39.4% (95% CI: 32%–64%) in the training and independent validation datasets, respectively. Sensitivity and specificity in NOD patients were 55.2% [95% CI: 35.7–73.6] and 98.4% [95% CI: 91.3–100.0], respectively. The PaC signal was identified in intraductal papillary mucinous neoplasm (64%), pancreatitis (56%), and non-PaC cancers (17%).CONCLUSIONSThe pancreatic cancer detection assay showed robust performance in the tested cohorts and carries the promise of becoming an essential clinical tool to enable early detection in high-risk NOD patients.</jats:sec