A Novel Method to Generate MNase Ladders Reveal Rapid Chromatin Remodeling upon Gametogenesis and Mating in Chlamydomonas

Explore & actuate: the future of personalized medicine in oncology through emerging technologies

PURPOSE OF REVIEW

The future of medicine is aimed to equip the physician with tools to assess the individual health of the patient for the uniqueness of the disease that separates it from the rest. The integration of omics technologies into clinical practice, reviewed here, would open new avenues for addressing the spatial and temporal heterogeneity of cancer. The rising cancer burden patiently awaits the advent of such an approach to personalized medicine for routine clinical settings.

RECENT FINDINGS

To weigh the translational potential, multiple technologies were categorized based on the extractable information from the different types of samples used, to the various omic-levels of molecular information that each technology has been able to advance over the last 2 years. This review uses a multifaceted classification that helps to assess translational potential in a meaningful way toward clinical adaptation.

SUMMARY

The importance of distinguishing technologies based on the flow of information from exploration to actuation puts forth a framework that allows the clinicians to better adapt a chosen technology or use them in combination to enhance their goals toward personalized medicine.

Successful reversal of transgene silencing in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii has been successfully engineered to produce compounds of interest following transgene integration and heterologous protein expression. The advantages of this model include the availability of validated tools for bioengineering, its photosynthetic ability, and its potential use as biofuel. Despite this, breakthroughs have been hindered by its ability to silence transgene expression through epigenetic changes. Histone deacetylases (HDAC) are main players in gene expression. We hypothesized that transgene silencing can be reverted with chemical treatments using HDAC inhibitors. To analyze this, we transformed C. reinhardtii, integrating into its genome the mVenus reporter gene under the HSP70-rbcs2 promoter. From 384 transformed clones, 88 (22.9%) displayed mVenus positive (mVenus+ ) cells upon flow-cytometry analysis. Five clones with different fluorescence intensities were selected. The number of integrated copies was measured by qPCR. Transgene expression levels were followed over the growth cycle and upon SAHA treatment, using a microplate reader, flow cytometry, RT-qPCR, and western blot analysis. First, we observed that expression varies with the cell cycle, reaching a maximum level just before the stationary phase in all clones. Second, we uncovered that supplementation with HDAC inhibitors of the hydroxamate family, such as vorinostat (suberoylanilide-hydroxamic-acid, SAHA) at the initiation of culture increases the frequency (% of mVenus+ cells) and the level of transgene expression per cell over the whole growth cycle, through histone deacetylase inhibition. Thus, we propose a new tool to successfully trigger the expression of heterologous proteins in the green algae C. reinhardtii, overcoming its main obstacle as an expression platform.

A potential screening method for epigenetic drugs: uncovering stress-induced gene silencing in Chlamydomonas

Histone modifications and DNA methylation together govern promoter availability, thereby influencing gene expression. This study queries the unicellular chlorophyte, Chlamydomonas reinhardtii using a three step "epigenetic assay" design to phenotypically track the variegation of a randomly integrated Paromomycin resistance transgene(s) (Pm^R). Based on its position of integration, the Pm^R gene expression hinged on two epigenetic hallmarks: the spreading of heterochromatin, and the transmissible memory of epigenetic states across generations. Using a spot-dilution analysis, the loss of antibiotic resistance phenotype was scored from 0 to 4, four being maximally silenced. Appropriate construct designs were used to demonstrate that the cis-spread of heterochromatin could be interfered with a stronger euchromatic barrier (TUB2 promoter). When assayed for metal ion stress, a combination of Mn deficiency with excess Cu or Zn stress was shown to induce gene silencing in Chlamydomonas. Cu stress resulted in the accumulation of intracellular ROS, while Zn stress elevated the sensitivity to ROS. As proof of functional conservation, mammalian epigenetic drugs demonstrably interfered with stress-induced gene silencing. Finally, a selected group of transgenic clones responsive to HDACi sodium butyrate, when tested in a gradient plate format showed similarity in phenotype to the plant-derived compound cinnamic acid. This indicated a possible commonality in their mode of action, unlike curcumin which might have a different mechanism. Thus, using binned libraries, based on a common set of responses to known drugs, a cost-effective high-throughput screening strategy for epigenetically active compounds from plants or other sources is described.

Slaying the last unicorn: discovery of histones in the microalga Nanochlorum eucaryotum

Histones are the principal constituents of eukaryotic chromatin. The four core histones (H2A, H2B, H3 and H4) are conserved across sequenced eukaryotic genomes and therefore thought to be universal to eukaryotes. In the early 1980s, however, a series of biochemical investigations failed to find evidence for histones or nucleosomal structures in the microscopic green alga Nanochlorum eucaryotum. If true, derived histone loss in this lineage would constitute an exceptional case that might help us further understand the principles governing eukaryotic gene regulation. To substantiate these earlier reports of histone loss in N. eucaryotum, we sequenced, assembled and quantified its transcriptome. Following a systematic search for histone-fold domains in the assembled transcriptome, we detect orthologues to all four core histones. We also find histone mRNAs to be highly expressed, comparable to the situation in other eukaryotes. Finally, we obtain characteristic protection patterns when N. eucaryotum chromatin is subjected to micrococcal nuclease digestion, indicating widespread formation of nucleosomal complexes in vivo. We conclude that previous reports of missing histones in N. eucaryotum were mistaken. By all indications, Nanochlorum eucaryotum has histone-based chromatin characteristic of most eukaryotes.