Calibrated super-resolution imaging of metabolically labeled DNA in single human chromosomes (Fang et al., ACS Nano 2018) showed that interphase chromatin is organized into three coexisting levels: dispersed chromatin, discrete nanodomains containing predominantly 1-10 kilobases of DNA within tens-of-nanometer extents, and clusters of such nanodomains, with single-peaked size and density distributions that are independent of nuclear position. The source work left open the mechanistic origin of these nanodomains - protein-driven phase separation (liquid-like demixing), DNA-protein crosslinking or bridging by specific proteins, a mixture of mechanisms, or another process, and which proteins or complexes are causally responsible - as well as their functional relationship to genomic regulatory units such as topologically associated domains (TADs). Later work sharpened but did not resolve either question: reconstituted chromatin undergoes liquid-liquid phase separation in vitro, while living-cell measurements report solid-like mesoscale behavior and a recent single-molecule study implicates linker histone H1 as a liquid-like glue; super-resolution studies in mouse cells show chromatin nanodomains persist without cohesin or CTCF and constitute a layer distinct from TADs, yet no causal mechanism and no causal nanodomain-TAD correspondence has been established or excluded.

Background

Beyond the nucleosome - the repeating unit of chromatin in which about 147 base pairs (bp) of DNA are wrapped around a histone protein octamer - the spatial organization of genomic DNA inside the cell nucleus at the 10-200 nanometer ('mesoscale') range remains incompletely understood. Classical models proposed regular higher-order fibers, most famously the 30-nm fiber, as the dominant organizing principle, while electron-microscopic and live-cell studies increasingly argued that chromatin is largely irregular in situ. Single-molecule localization microscopy - in which densely labeled samples are imaged by stochastically switching on individual fluorophores and localizing each with nanometer precision (the dye-based variant is dSTORM) - transformed this field in two steps. First, STORM imaging of histone proteins revealed 'nucleosome clutches', heterogeneous groups of nucleosomes whose size and internal density vary with cell type (Ricci et al., Cell 2015). Second, live-cell super-resolution imaging showed dynamic chromatin domains roughly 100-200 nm wide that partially mix and split over seconds to minutes (Nozaki et al., Mol Cell 2017). In parallel, chromosome-conformation-capture methods such as in situ Hi-C, which measure DNA-DNA contact frequencies averaged over millions of cells, partitioned chromosomes into topologically associated domains (TADs) - sub-megabase genomic intervals whose DNA preferentially contacts itself (Rao et al., Cell 2014) - and, at coarser scale, into A/B compartments, multi-megabase regions of preferentially self-contacting chromatin that are broadly transcriptionally active (A) or inactive (B). TAD boundaries are formed by loop extrusion, the processive enlargement of a DNA loop by the ring-shaped ATPase motor cohesin until the loop is halted at boundary motifs bound by the insulator protein CTCF; the related condensin complex performs this motor role in mitosis, and super-resolution imaging with sequence-specific Oligopaint DNA probes (pools of fluorescently labeled oligonucleotides that hybridize to designated genomic loci) visualized TAD-like physical units in single cells (Szabo et al., Sci Adv 2018; Bintu et al., Science 2018). Separately, biophysical work proposed that phase separation - the spontaneous demixing of a multivalent protein or protein-nucleosome mixture into dense, liquid-like droplet condensates - could drive formation of condensed chromatin domains, exemplified by the heterochromatin protein HP1 (Strom et al., Nature 2017) and by reconstituted nucleosome arrays (Gibson et al., Cell 2019).

Within this landscape, Fang et al. (ACS Nano 2018) contributed a calibrated, DNA-specific in-situ measurement. DNA was metabolically labeled with the nucleotide analog EdU (5-ethynyl-2'-deoxyuridine), covalently tagged with fluorophores by click chemistry, and imaged by dSTORM, with only a single chromosome labeled per nucleus (achieved by labeling during one S phase and diluting the label through subsequent cell divisions) to suppress out-of-focus background and push the resolution to roughly 20 nm. Localization counts were converted into absolute DNA amounts using an in-system calibration of about 6.8 localizations per kilobase. This revealed that subchromosomal regions of fixed G1-phase human fibroblast nuclei, regardless of nuclear position (interior, periphery, or nucleolar proximity), contain three coexisting levels of DNA organization: dispersed chromatin; discrete nanodomains with spatial extents of tens of nanometers containing predominantly 1-10 kb of DNA; and clusters of such nanodomains. The size and DNA-content distributions were single-peaked, arguing against canonical 30-nm or 120-nm fibers as the dominant structural unit, and nanodomains inside clusters retained the same size and density as isolated ones, suggesting a two-stage hierarchical assembly of roughly invariant modular units. The authors noted that the single-peak, roughly uniform statistics are consistent with, but do not establish, condensation via a phase-separation-like process, and that relating the nanodomains and their clusters to genomic regulatory units such as TADs requires correlative sequence-resolved imaging. The mechanistic question and the nanodomain-TAD relationship are scientifically linked: both concern whether the nanoscale DNA-density building blocks are a causally distinct physical layer of genome organization or a mere by-product of the machinery that builds loop-extrusion-defined domains.

Problem Statement

Two linked questions raised by that study remain open.

(1) Mechanism of nanodomain formation. What physical process condenses DNA into the observed few-kilobase, tens-of-nanometer nanodomains in intact interphase nuclei: protein-driven phase separation (liquid-like demixing of multivalent chromatin-binding proteins with nucleosomes), DNA-protein crosslinking or bridging by specific proteins, a mixture of mechanisms acting on different subsets of nanodomains, or another process? Which proteins or protein complexes (for example, linker histone H1 (the abundant non-nucleosomal histone that binds the DNA entering and exiting each nucleosome), HP1-family proteins, ATP-dependent chromatin remodelers that reposition nucleosomes, the loop-extrusion motors cohesin and condensin, or transcriptional regulators) are causally responsible for forming and maintaining the nanodomains?

(2) Relationship to genomic regulatory structure. How do the nanodomains and their clusters correspond, structurally and functionally, to the domains and compartments defined by DNA-contact mapping (TADs, A/B compartments) and to epigenetically defined domains (for example, heterochromatin domains marked by H3K9 methylation and HP1-family proteins, and Polycomb-repressed domains): is there a causal correspondence between specific nanodomain clusters and specific TADs or other regulatory units, or are nanodomains an independent layer of physical organization with its own regulatory consequences?

In both questions, 'nanodomains' refers to the few-kilobase, tens-of-nanometer DNA-density nanodomains measured by calibrated in-situ super-resolution imaging in human interphase nuclei in the source study, which are distinct in genomic scale from the roughly 100-kilobase 'chromatin nanodomains' (CNDs) later reported in mouse cells (Szabo et al., Nat Genet 2020).

Both questions are mandatory: a solution must address (1) and (2). Each question is resolved by a concrete, experimentally testable answer - a specified physical mechanism together with the proteins or complexes proposed to be causally responsible, or a specified structural and functional relationship between the nanodomains and/or their clusters and the domain classes named in (2) (TADs, A/B compartments, and the specified epigenetic domains) - accompanied by the discriminating measurement or perturbation that would establish or exclude that answer relative to the competing alternatives, and by an explicit statement of which cited proposals the answer adopts, extends, or excludes. Because the questions are open, an already established causal demonstration is not required, but a bare restatement of a mechanism, protein, or relationship already proposed in the cited literature does not by itself resolve a question. An answer that addresses only one of the two questions is incomplete.

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