Unlocking inaccessible historical genomes preserved in formalin

BackgroundMuseum specimens represent an unparalleled record of historical genomic data. However, the wide-spread practice of formalin preservation has thus far impeded genomic analysis of a large proportion of specimens. Limited DNA sequencing from formalin-preserved specimens has yielded low genomic coverage with unpredictable success. We set out to refine sample processing methods and to identify specimen characteristics predictive of sequencing success. With a set of taxonomically diverse specimens collected between 1936 and 2015 and ranging in preservation quality, we compared the efficacy of several end-to-end whole genome sequencing workflows alongside a k-mer-based trimming-free read alignment approach to maximize mapping of endogenous sequence.ResultsWe recovered complete mitochondrial genomes and up to 3X nuclear genome coverage from formalin-fixed tissues. Hot alkaline lysis coupled with phenol-chloroform extraction out- performed proteinase K digestion in recovering DNA, while library preparation method had little impact on sequencing success. The strongest predictor of DNA yield was overall specimen condition, which additively interacts with preservation conditions to accelerate DNA degradation.ConclusionsWe demonstrate a significant advance in capability beyond limited recovery of a small number of loci via PCR or target-capture sequencing. To facilitate strategic selection of suitable specimens for genomic sequencing, we present a decision-making framework that utilizes independent and non-destructive assessment criteria. Sequencing of formalin-fixed specimens will contribute to a greater understanding of temporal trends in genetic adaptation, including those associated with a changing climate. Our work enhances the value of museum collections worldwide by unlocking genomes of specimens that have been disregarded as a valid molecular resource.

Virus-Mediated Targeted DNA Methylation Illuminates the Dynamics of Methylation in an Endogenous Plant Gene

DNA methylation maintains genome stability and regulates gene expression in plants. RNA-directed DNA methylation (RdDM) is critical for appropriate methylation. However, no efficient tools are available for the investigation of the functions of specific DNA methylation. In this study, the cucumber mosaic virus vector was used for targeted DNA methylation. Methylation was rapidly induced but gradually decreased from the 3′ end of the target endogenous sequence in Nicotiana benthamiana, suggesting a mechanism to protect against the ectopic introduction of DNA methylation. Increasing 24-nt siRNAs blocked this reduction in methylation by down-regulating DCL2 and DCL4. RdDM relies on the sequence identity between RNA and genomic DNA; however, this identity does not appear to be the sole determinant for efficient DNA methylation. The current findings provide new insight into the regulation of DNA methylation and promote additional effort to develop efficient targeted DNA methylation in plants.

Role of centromere sites in activation of ParB proteins for partition complex assembly

The ParB-parS partition complexes that bacterial replicons use to ensure their faithful inheritance also find employment in visualization of DNA loci, as less intrusive alternatives to fluorescent repressor-operator systems. The ability of ParB molecules to interact via their N-terminal domains and to bind to non-specific DNA enables expansion of the initial complex to a size both functional in partition and, via fusion to fluorescent peptides, visible by light microscopy. We have investigated whether it is possible to dispense with the need to insert parS in the genomic locus of interest, by determining whether ParB fused to proteins that bind specifically to natural DNA sequences can still assemble visible complexes. In yeast cells, coproduction of fusions of ParB to a fluorescent peptide and to a TALE protein targeting an endogenous sequence did not yield visible foci; nor did any of several variants of these components. In E.coli, coproduction of fusions of SopB (F plasmid ParB) to fluorescent peptide, and to dCas9 together with specific guide RNAs, likewise yielded no foci. The result of coproducing analogous fusions of SopB proteins with distinct binding specificities was also negative. We conclude that in order to assemble higher order partition complexes, ParB proteins need specific activation through binding to their cognate parS sites.

Sequence Elements in cis Influence Heterochromatic Silencing in trans

ABSTRACT The brown Dominant (bw D ) allele contains a large insertion of heterochromatin, which causes the locus to aberrantly associate with heterochromatin in interphase nuclei and silences the wild-type allele in heterozygotes. Transgenes placed near the bw + locus, in trans to bw D , can also be silenced. The strength of silencing (called trans inactivation) varies with the regulatory sequences of the transgene and its distance away from the bw D insertion site in trans. In this study, we examine endogenous sequences in cis that influence susceptibility of a reporter gene to trans inactivation. Flanking deletions were induced in two parental lines containing P-element transgenes showing trans inactivation of the mini-white reporter. These new lines, which have mini-white under the influence of different endogenous sequence elements, now show varied ability to be silenced by bw D . Determination of the deleted regions and the levels of mini-white expression and trans inactivation has allowed us to explore the correlation between cis sequence elements and susceptibility to trans inactivation and to identify a 301-bp sequence that acts as an enhancer of trans inactivation. Intriguingly, this region encompasses the upstream regions of two divergently transcribed genes and contains a sequence motif that may bind BEAF, a protein involved in delimiting chromatin boundaries.

Interference between avian endogenous ev/J 4.1 and exogenous ALV-J retroviral envelopes

A new family of avian retroviral endogenous sequences designated ev/J or EAV-HP has been identified recently. Here an additional avian ev/J 4.1 endogenous sequence, ev/J 4.1 Rb, is reported. ev/J 4.1 Rb has the most extensive amino acid identity ever described for an endogenous envelope protein with the ALV-J avian leukosis virus. Here, we also demonstrate that ev/J 4.1 Rb functionally pseudotypes murine leukaemia virions and leads to a complete reciprocal interference with ALV-J envelopes. This is the first demonstration of such a high level of envelope interference between endogenous and exogenous avian retroviruses. Our results provide additional clues on the co-evolution of retroviral sequences among vertebrates.

Association between the Endogenous Retrovirus HRES-1 and Multiple Sclerosis in the United Kingdom – Evidence of Genetically Different Disease Subsets?

In the present study we determined the frequencies of four haplotypes of the human T-cell lymphotropic virus-related endogenous sequence, HRES-1, in 110 multiple sclerosis (MS) patients and 100 healthy control subjects from the United Kingdom. We found evidence of an association between this endogenous retrovirus and MS (p < 0.01), in particular reflecting an increased frequency of HRES-1 haplotype 1 in the group of patients. There was no significant difference in the distribution of HRES-1 haplotypes between relapsing-remitting MS and the primary progressive form of the disease. The odds ratio for HRES-1 haplotype 1 and MS did not differ significantly between individuals positive for HLA-DR2 and DR2-negative individuals. Comparison of the observations from the present study with previous results implicated HRES-1 as a marker of genetic heterogeneity in MS.