Abstract
The principles that govern the organization of genomes, which are needed for an understanding of how chromosomes are packaged and function in eukaryotic cells, could be deciphered if the threedimensional (3D) structures are known. Recently, singlecell imaging techniques have been developed to determine the 3D coordinates of genomic loci in vivo. Here, we introduce a computational method (Distance Matrix to Ensemble of Structures, DIMES), based on the maximum entropy principle, with experimental pairwise distances between loci as constraints, to generate a unique ensemble of 3D chromatin structures. Using the ensemble of structures, we quantitatively account for the distribution of pairwise distances, threebody colocalization, and higherorder interactions. The DIMES method can be applied to both small and chromosomescale imaging data to quantify the extent of heterogeneity and fluctuations in the shapes across various length scales. We develop a perturbation method in conjunction with DIMES to predict the changes in 3D structures from structural variations. Our method also reveals quantitative differences between the 3D structures inferred from HiC and those measured in imaging experiments. Finally, the physical interpretation of the parameters extracted from DIMES provides insights into the origin of phase separation between euchromatin and heterochromatin domains.
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Introduction
In little over a decade, a variety of experimental techniques, combined with computational tools and physical modeling, have greatly contributed to our understanding of chromatin organization in numerous cell types and species^{1,2,3,4,5,6,7,8,9}. These studies have paved the way toward a deeper understanding of the relationship between genome structure and gene expression^{10,11,12,13,14}. The commonly used experimental techniques could be broadly classified as sequencebased or microscopybased. The former include the Chromosome Conformation Capture (3C)^{15} and its variants, including HiC^{16} and MicroC^{17}, which in concert with highthroughput sequencing provide populationaveraged data for the contact matrix or contact maps (CMs)^{18,19}. The elements of the CM are the average probabilities that two loci separated by a given genome length (s, a linear measure) are in spatial proximity. In order to reveal the celltocell chromatin variations in chromatin conformations, singlecell HiC (scHiC) or similar techniques have also been developed^{20,21,22,23,24}. These studies, along with methods that utilize a combination of HiC and imaging techniques^{25}, reveal the statistical and heterogeneous nature of chromatin organization. In addition, methods like SPRITE^{26} and genome architecture mapping (GAM)^{27,28}, which avoid the ligation step in HiC^{9}, have also revealed the organization of chromosomes, including features that are missed in conventional HiC methods, such as the higherorder contacts that go beyond pairwise interactions. How to utilize the data contained in the CM to directly reconstruct an ensemble of the threedimensional structures of the genome is a difficult inverse problem. Datadriven approaches^{29,30,31,32,33,34,35,36,37,38} have been advanced to solve the complicated HiC to 3D structure problem (see the summary in ref. ^{39} for additional related studies and ref. ^{40} for a comprehensive overview of the existing methods).
The imagingbased technique is the most direct route for determining the 3D chromosome structures^{41,42,43}. In combination with the fluorescence in situ hybridization (FISH) technique^{44}, imaging experiments have enabled direct imaging of the position of the genomic loci at the singlecell level. The FISH experiments have revealed global genome organization principles, such as the chromosome territories (CT)^{44}. Recently developed multiscale multiplexed DNA FISHbased imaging methods^{25,45,46,47,48,49,50,51,52} have further advanced the field, which has resulted in measurements of the spatial positions of many loci for a large number of cells, thus providing not only glimpses of the structures over a large length scale but also a quantitative assessment of the fluctuations in the celltocell conformations. For instance, the imaging method was used to obtain the locations of ~65 loci in a 2megabasepair (Mbp) region for chromosome 21 (Chr21)^{47} for a large number of cells. More recently, the method was further improved to image over ~900 targeted loci spread uniformly across the entire chromosome scale (≈242 Mbp for Chr2) and over 1000 genomic loci across all 23 chromosomes^{51}. Compartment features, longrange interactions between loci, and the distribution of the radius of gyration could be directly visualized or calculated using the coordinates of the imaged loci, thus providing direct quantitative information on the nuances of genome organization. Although the resolution in the imaging technique will doubtless increase in the future, currently HiC based methods provide higher resolution at the CM level but not at the 3D structural level. Thus, by combining the experimental data from a variety of sources and computational methods unexpected insights about chromosome organization could be gleaned.
Here, we introduce a method, DIMES (from DIstance Matrix to Ensemble of Structures), that utilizes the mean distance matrix (DM) between loci as input to generate an ensemble of structures using the maximum entropy principle. The data in the two studies^{47,51} are used to quantitatively validate the DIMES method. In order to demonstrate the predictive power of the method, we used DIMES to determine the changes in the organization (expressed as CM) from structural variants (inversion) on Chr1 from the mouse cell line and the effect of single loci deletion on Chr2 from IMR90 cell line. Our approach, when applied to HiC (using the HIPPS method^{53}) and imaging data, reveals important differences between the two methods in the finer details of the structural ensemble of Chr21.
Results
Formulating DIMES using the maximumentropy model
We developed the DIMES method, which utilizes imaging data, to generate an ensemble of 3D chromosome conformations. The input for our theory is the pairwise distances between the genomic loci (Fig. 1). We seek to find a joint distribution of positions of loci, P({x_{i}}), which is consistent with the squared mean pairwise distance \(\langle{{{{{{{{\boldsymbol{x}}}}}}}}}_{i}{{{{{{{{\boldsymbol{x}}}}}}}}}_{j}{}^{2}\rangle=\langle {r}_{ij,\exp }^{2}\rangle\), where \(\langle {r}_{ij,\exp }^{2}\rangle\) is the experimentally measured average squared distance between two loci i and loci j. One could also use the average distance instead of the average squared distance as constraints. However, constraining average squared distances is computationally more efficient because the resulting maximumentropy distribution is a multivariate Gaussian distribution which allows fast sampling.
In general, there are many, possibly infinite, number of such P({x_{i}}) which satisfy the constraints. Using the maximum entropy principle^{54,55}, we can find a unique distribution P^{MaxEnt}({x_{i}}) whose differential entropy is maximal among all possible distributions. We should point out that the maximum entropy principle has been previously used in the context of genome organization^{56,57,58}, principally to learn the values of the unknown parameters in a chosen energy function deemed to be appropriate for describing chromosome organization. Recently, it was shown that the maximumentropy distribution with the constraints of contact frequency can be mapped to a confined lattice polymer model in ref. ^{59}. The Lagrange multipliers that enforce the constraints are interpreted as the contact energies in the Hamiltonian of the polymer with the position of each monomer occupying the lattice sites. Here, we use the pairwise distances as constraints and derive the corresponding maximumentropy distribution from which the 3D structures may be readily obtained.
The maximumentropy distribution P^{MaxEnt}({x_{i}}) is given by,
where Z is the normalization factor, and k_{ij}’s are the Lagrange multipliers that are determined so that the average value \(\langle{{{{{{{{\boldsymbol{x}}}}}}}}}_{i}{{{{{{{{\boldsymbol{x}}}}}}}}}_{j}{}^{2}\rangle=\langle {r}_{ij,\exp }^{2}\rangle\). It can be shown that P^{MaxEnt}({x_{i}}) is a multivariate normal distribution (see “Methods”). In addition, it can be proven that for any valid \({r}_{ij,\exp }^{2}\), there exists a unique set of k_{ij} (see “Methods”). The values of k_{ij} can be determined using an iterative scaling algorithm^{60} or a gradient descent algorithm (see “Methods” and Supplementary Note 1). For later reference, we define the matrix with elements k_{ij}’s as K. Note that Eq. (1) has the same form as the Boltzmann distribution of the generalized Rouse model (GRM) with k_{ij} ≥ 0, which has been applied to reconstruct chromosome structure by fitting to the HiC contact map^{61,62}. However, it is important to point out that Eq. (1) is derived under the maximum entropy principle, which does not assume the thermal equilibrium condition of the system.
The three steps in the DIMES to generate an ensemble of chromosome structures are: (1) We compute the target mean squared spatial distance matrix from experimental measurements of the coordinates of genomic loci. (2) Using an iterative scaling or gradient descent algorithm, we obtain the values of k_{ij}’s to match the experimental measured \(\langle {r}_{ij,\exp }^{2}\rangle\). (3) Using the values of k_{ij}, the coordinates of the 3D chromosome structures can be sampled from P^{MaxEnt}({x_{i}})—a multivariate normal distribution. The details of the procedures are described in “Methods”.
Validating DIMES
In order to demonstrate the effectiveness of DIMES, we first used the experimental data^{47} in which the authors reported, using a highly multiplexed superresolution imaging approach, coordinates of about 65 individual loci for the 2Mbp segment for Chromosome 21 (Chr21) from four different cell lines (IMR90, K562, HCT116, and A549). Using the calculated mean spatial distance matrices from the reported coordinates as targets, we determined k_{ij} (Eq. (1)), which allowed us to generate an ensemble of structures for this 2Mbp segment. For all cell lines, the mean spatial distance matrices computed from the reconstructed ensemble of structures almost perfectly match the target distance maps (Fig. 2a, b and Supplementary Fig. 9). In addition, we also applied the DIMES to the chromosomescale data (see a later section), and achieved the same level of accuracy.
We then perform crossvalidation of the DIMES method. This is done as follows: a randomly chosen fraction of pairwise distances from the distance map is deemed to be missing data. The resulting distance map containing the missing data is then used as input for DIMES. It is important to note that the missing data is not used to update k_{ij}. Finally, the predicted distances for the missing data are compared with the values obtained from the full distance map. Figure 2c–f compare the input distance maps with missing data and the full predicted distance maps. Remarkably, DIMES quantitatively predicts pairwise distances for the missing data even if only 10% of the distance map is used. Together, these results demonstrate that the model is effective in producing 3D structures that are consistent with the experimental input and is robust with respect to missing data.
Distribution of pairwise distances
Next, we tested if DIMES could recover the properties of the genome organization that are not encoded in the mean spatial distances. We focused on reproducing the distributions of pairwise distances, which can be calculated because pairwise distance data are available for a large number of individual cells. It is worth emphasizing that the input in the DIMES method is the mean spatial distance, which does not contain any information about the distributions.
To quantitatively measure the degree of agreement between the measured and calculated distance distributions using DIMES, we compute the Jensen–Shannon divergence (JSD), defined as, (D(p∣∣m) + D(q∣∣m))/2 where p and q are two probability vectors and D(p∣∣m) is the Kullback–Leibler divergence. The JSD value is bounded between 0 and 1. A zero value means that the two distributions are identical. For each loci pair (I, j), we calculated the JSD, thus generating the JSD matrix. Figure 3a shows the JSD matrix, and Fig. 3b displays the histogram of all the calculated JSDs. The average value of JSDs is merely 0.02, which shows that the overall agreement between the calculated and measured distributions of distances is excellent.
Upon closer inspection of Fig. 3b, we find that the values of JSDs are not randomly distributed. We choose two pairs with relatively large and small JSD values. Comparison between the experimental and calculated P(r_{ij}) for one pair with JSD = 0.08 (Fig. 3c) and with = 0.009 (Fig. 3d) shows that the dispersions are substantial. The pair with JSD = 0.08 (Fig. 3c) samples distances that far exceed the mean value, which implies that there are substantial celltocell variations in the organization of chromosomes^{47,51}. The percentage of the subpopulations, associated with different distances, can in principle be inferred by deconvolution of the full distance distribution^{63}.
Colocalization of three loci and biological significance
We next asked if the method accounts for higherorder structures, such as threeway contacts, discovered in GAM^{27,28} and SPRITE experiments^{26} and imaging experiments^{47}, and predicted by the theory^{64,65}. First, we computed the probability of colocalization of loci triplets, π_{ijk}(a) = Pr(r_{ij} < a, r_{ik} < a, r_{jk} < a) where a is the distance threshold for contact formation (r_{ij} < a implies a contact). To make a quantitative comparison with experiments, we also computed \({\pi }_{ijk}^{\exp }(a)\) using the experimental data. We then calculated the Pearson correlation coefficient, ρ, between \({\pi }_{ijk}^{\exp }(a)\) and \({\pi }_{ijk}^{{{{{{{{\rm{sim}}}}}}}}}(a)\). Figure 4a shows that the degree of agreement between experiment and theory is best when a is in the range of 200–400 nm. We chose a = 300 nm at which ρ is a maximum. The scatter plot of \({\pi }_{ijk}^{{{{{{{{\rm{DIMES}}}}}}}}}\) versus \({\pi }_{ijk}^{\exp }\) (Fig. 4a) is in excellent agreement with \(\rho ({\pi }_{ijk}^{DIMES},{\pi }_{ijk}^{exp})\approx 0.99\).
The good agreement is also reflected in the Fig. 4b, which compares the heatmaps \({\pi }_{ijk}^{\exp }\) for i = 11 (lower triangle) and the heatmap for \({\pi }_{ijk}^{{{{{{{{\rm{DIMES}}}}}}}}}\) with i = 11 (upper triangle). Due to the polymeric nature of the chromatin, the absolute value of π_{ijk} may not be instructive because π_{ijk} is usually highest when the three loci i, j, k are close along the sequence. To normalize the genomic distance dependence, and capture the significance of the colocalization of triplets, we calculated the Zscore for π_{ijk} defined as Z_{ijk} = (π_{ijk} − μ(π_{ijk}))/σ(π_{ijk}) where μ(π_{ijk}) = ∑_{m,n,q}δ(∣j − i∣∣k − j∣ − ∣m − n∣∣n − q∣)π_{mnq}/∑_{m,n,q}δ(∣j − i∣∣k − j∣ − ∣m − n∣∣n − q∣), and σ(π_{ijk}) is the corresponding standard deviation. Positive Zscore implies that the corresponding triplet has a greater probability for colocalization with respect to the expected value. As an example, a comparison of the Zscores for \({Z}_{ijk}^{\exp }\) and \({Z}_{ijk}^{{{{{{{{\rm{DIMES}}}}}}}}}\) in Fig. 4c for i = 11 shows excellent agreement. The scatter plot in Fig. 4d of \({Z}_{ijk}^{{{{{{{{\rm{DIMES}}}}}}}}}\) versus \({Z}_{ijk}^{\exp }\) shows that the triplet of loci with index (1, 11, 31) has the highest value of Z_{ijk} both in the experimental data and the predictions based on DIMES. Three randomly selected individual conformations with (1, 11, 31), colocalized within distance threshold a = 300 nm (Fig. 4e) adopt diverse structures, attesting to the heterogeneity of chromosome organization^{25}.
To demonstrate the biological significance of the triplet with Z_{ijk} values, we overlay 10 sets of threeway contacts with 10 largest Z_{ijk} values on the mean spatial distance map (Fig. 4f, with orange circles representing the triplet loci). Interestingly, the threeway contacts are localized on the boundaries of the Topologically Associating Domains (TADs)^{66,67}, which are enriched with CTCF motifs^{66,68}. Comparing the location of the triplets and the CTCF peak track (plotted using Chipseq data^{69}) we find that the spatial localization of the triplets is highly correlated with the CTCF peaks. This implies that the CTCF/cohesin complex has a tendency to colocalize in the form of triplets or possibly higherorder multiplets, which is consistent with the recent experimental studies demonstrating the presence of foci and clusters of CTCF and cohesin in cells^{70,71}.
Higherorder structures
Because the DIMES method is quantitatively accurate, we could probe higherorder chromatin structures. To this end, we considered three aspects of chromatin organization, which can be calculated directly from the coordinates of the Chr21 loci.
TADlike patterns in a single cell
Imaging experiments^{47} show that even in a single cell, domainlike or TADlike structures may be discerned, even in inactive X chromosomes whose ensemble HiC map appears to be featureless^{72}. These signatures are manifested as TADlike patterns in the pairwise distance matrix (see the left panels for two cells in Fig. 4B in ref. ^{47}). Similar patterns are observed in the individual conformation generated by DIMES as well (Supplementary Fig. 1). We then compute the boundary probabilities^{47} of individual genomic loci using experimentally measured structures and the structures generated by DIMES. Figure 5a shows that DIMES captures the profile of boundary probabilities nearly quantitatively. Interestingly, such TADlike structures are present even in ideal homopolymer structures (Supplementary Fig. 3). We surmise that the intrinsic features of fluctuating polymer conformations contribute to such TADlike structures. These structures are dynamic in nature because they are a consequence of fluctuations. The specific interactions which distinguish chromosomes from an ideal homopolymer counterpart lead to the statistically preferred distributions of these TADlike structures rather than being random.
Size and shapes
Using the ensemble of Chr21 structures, we wondered if the radius of gyration (R_{g}) and the shape of the genome organization could be accurately calculated. We determined the R_{g} distribution, P(R_{g}), and the shape parameter κ^{2}^{73,74}, for the Chr21 in the 28–30 Mbp region. The results (Fig. 5b, and Supplementary Fig. 10) show that the model achieves excellent agreement with the experiment, both for P(R_{g}) and P(κ^{2}).
Given the excellent agreement, it is natural to ask whether the DIMES method can capture the size and shape on finer scales. To shed light on this issue, we calculated the distribution of R_{g} and κ^{2} (P(R_{g}; i, j) and P(κ^{2}; i, j)) for every subsegment (i loci to j loci) over the 2Mbp region. In order to assess the accuracy of the predictions, we calculated the JSD between P^{DIMES}(R_{g}; i, j) and \({P}^{\exp }({R}_{g};i,j)\), and between \({P}^{\exp }({\kappa }^{2};i,j)\) and P^{DIMES}(κ^{2}; i, j). On the finer scale, there are deviations between calculations based on DIMES and the experiment. The JSD heatmap (Fig. 5c) for all pairs i and j shows that the deviation is not uniform throughout the 2Mbp region. We picked two regions that show good agreement (red segment) and poor agreement (blue segment) in Fig. 5c. Direct comparison of P(R_{g}) and P(κ^{2}) for these two segments is presented in Fig. 5d. For the blue segment, both the predicted P(R_{g}) and P(κ^{2}) have less dispersion than the experimental data. This suggests that the heterogeneity observed in experiments is even greater than predicted by the DIMES method. For the red segment, the predicted and experimentally measured are in good agreement. Visual inspection of Fig. 5c suggests that the discrepancy is localized mostly in the TAD regions, which might be due to the dynamic nature of these substructures.
Ensemble of structures partition into clusters
We then compared the overall distributions of the ensemble of calculated structures with experiment data. To do this, we first performed tSNE to project the coordinates of each conformation onto a twodimensional manifold using the distance metric, D_{mn},
where \({r}_{ij}^{(m)}\) and \({r}_{ij}^{(n)}\) are the Euclidean distances between the i^{th} and j^{th} loci in conformations m and n, respectively. Based on the density of the tSNE projections (shown as contour lines in Fig. 6), it is easy to identify two peaks, implying that the space of structures partition into two major clusters. The points are then clustered into two major clusters (orange and blue) using Agglomerative Clustering with the Ward linkage^{75}. The percentage of cluster #1 (orange) in the experiment is 36%, which is in excellent agreement with the value (34%) predicted by the DIMES method. We display a representative structure with the lowest average distance to all the other members in the same cluster for the two clusters in Fig. 6. Based on our analyses of the experimental results (conformations on the left in Fig. 6), the representative structure belonging to cluster #1 (in blue) is more compact compared to the one in cluster #2 (in orange). The same trend is found in the DIMES predictions (conformations on the right in Fig. 6).
We also computed the mean distance maps from all the conformations from each cluster (shown on the left and right side in Fig. 6). The distance maps show that the structures in cluster #2 adopt a dumbbell shape whereas those belonging to cluster #1 exhibit no such characteristic. The quantitative agreement between the distance maps from the experiment and the model is excellent.
Structures of the 242Mbplong Chr2
We extend the DIMES method to chromosomescale imaging data in order to compare with the recent superresolution imaging experiments, which reported coordinates of 935 loci genomic segments with each locus being 50kbp long spanning the entire 242Mbp Chr2 of Human IMR90 cell^{51}. Note that there are spaces between the loci that are not imaged. We computed the average squared distance matrix from the measured coordinates and then used DIMES to generate an ensemble of structures. Figure 7a compares the experimental and calculated mean distance matrices using the DIMES method (Fig. 7a). As before, the agreement is excellent (see also Fig. 2). A few randomly chosen conformations from the ensemble are shown in Supplementary Fig. 2a, demonstrating that there are large variations among the structures. Experimental singlecell distance maps (Supplementary Fig. 8a) show that similar variations are also observed in vivo. DIMES reproduces the distributions of pairwise distances at large length scales (Supplementary Fig. 8b).
In addition to recovering the experimental data, our approach produces genomic distance (s) dependent effective interaction strengths between the loci, which gives insights into the organization of Chr2 on genomic length scale. Because the parameters in the DIMES are k_{ij}’s, which could be interpreted as effective “interaction” strengths between the loci, we asked if the K matrix encodes for the A/B compartments (the prominent checkerboard pattern in HiC experiments indicating phase separation between euchromatin and heterochromatin) observed in the distance matrix. (Although k_{ij}s may not represent the actual strength associated with interactions between i and j, we use this terminology for purposes of discussion.) Note that k_{ij} can be either negative or positive, with a negative (positive) value representing the effective repulsion (attraction). The lower triangle in Fig. 7b shows the matrix K. We then computed the correlation matrix ρ from k_{ij} (see Supplementary Note 2 for details), which is shown in the upper triangle in Fig. 7b. The corresponding principal component dimension 1 (PC1) of ρ is shown in the top panel in Fig. 7b. The negative (positive) PC1 corresponds to A (B) compartments. The results in Fig. 7b show that the A/B compartments can be inferred directly from K, indicating that the parameters in DIMES correctly capture the underlying characteristics of the genome organization on all genomic length scales.
Given that the A/B type of each genomic loci are unambiguously identified, we then computed the histogram of k_{ij} and genomicdistance normalized \(\langle {k}_{ij}(s)\rangle=(1/(Ns))\mathop{\sum }\nolimits_{i < j}^{N}\delta (s(ji)){k}_{ij}\) for AA, BB, and AB interactions. The results show that the mean AB interactions are repulsive (negative 〈k_{ij}(s)〉) whereas AA and BB interactions are attractive (positive) (Fig. 7c). This finding explains the compartment features observed in the HiC data and the distance maps. Furthermore, we find that, on an average, AA interactions are more attractive than BB interactions (Fig. 7c), which seems counterintuitive because of the general lore that heterochromatin (formed by B locus) appears to be denser than euchromatin (composed of locus A) in microscopy experiments^{41}. On the other hand, the same analysis on Chromosome 21 shows opposite results, with the BB interactions being stronger on an average than AA interactions (Supplementary Fig. 4b). The genomicdistance normalized 〈k(s)〉 also shows that over the range of 2–10 Mbp, 〈k(s)〉 for BB is consistently stronger than that for AA (Supplementary Fig. 4c, d). The results for Chr2 and Chr21 show that the comparison between the strength of AA and BB interactions is possibly chromosomedependent. As we noted in a previous study^{8}, what is important is that the Flory χ = (ϵ_{AA} + ϵ_{BB} − 2ϵ_{AB})/2 is positive to ensure compartment formation. Here, ϵ_{AA}, ϵ_{BB}, and ϵ_{AB}, are the interaction energy scales involving A and B.
The genomicdistance normalized 〈k_{ij}(s)〉 for Chr2 shows that all the interaction pairs have the highest value at s = 1—a manifestation of the polymeric nature of chromatin fiber. Beyond s = 1, all pairs have negative k(s), indicating repulsive interaction on a small length scale. At s ≈ 5 Mbp, k(s) for all pairs develop positive peaks. At length scale s > 10 Mbp, the BB interactions decay as s increases whereas sdependent AA interactions fluctuate around a positive value. In addition, the histograms of k_{ij} (Fig. 7c) suggest that the average differences among AA, BB, and AB interactions are small (〈k_{AA}〉 = 0.0014,〈k_{BB}〉 = 0.0005, 〈k_{AB}〉 = − 0.0009), which is consistent with the recent liquid HiC experiment^{76}. In summary, the results in Fig. 7 demonstrate that the application of DIMES to largelengthscale imaging data explains the origin of compartments on large length scales. More importantly, the calculated values of k_{ij} provide insights into interactions between the genomic loci on all length scales, which cannot be inferred solely from experiments. Surprisingly, but in accord with experiments^{76}, the differences in the strengths of interaction between the distinct loci are relatively small.
Besides demonstrating the efficacy of DIMES in determining the ensemble of structures accurately, the calculated k_{ij}s explain micro phase segregation of active (A) and inactive (B) loci. It is a surprise that phase separation between A and B emerges from the calculated k_{ij}s, without a polymer model with an assumed energy function. Because k_{ij}s can be calculated using either the HIPPS^{53} or the DIMES method, the differences in compartment formation (segregation between A and B loci) in various chromosomes can be quantitatively inferred.
Applications
Impact of genomic rearrangement on 3D organization
It is known that the 3D organization of chromosomes can change substantially upon genomic rearrangements, such as duplication (increases the length of the genome), deletion (decreases the genome length), or inversion (shuffling of genome sequence while preserving the length). Both deletion and duplication are drastic genomic changes that require recomputing the K using either contact^{53} or distance maps. In contrast, the inversion is a gentler perturbation, and hence the changes in chromosome folding compared to the wild type (WT) can be calculated by treating it as perturbations applied to the K for the WT.
Once the WT K is calculated, a perturbation method could be used to determine the variations in the ensemble of genome structures. In particular, we asked whether the 3D structural changes upon rearrangement in the genomic sequence could be predicted by accounting for the corresponding changes in k_{ij}. For instance, inversion (Fig. 8a) would correspond to an inversion on the K (see Supplementary Note 3). For illustration purposes, we applied our method to the experimental HiC maps for the WT and a variant with an inversion^{77}. To apply DIMES to the two constructs, we first converted the contact probability to the mean spatial distance using the scaling relation \(\langle {r}_{ij}\rangle={{\Lambda }}{p}_{ij}^{1/\alpha }\) where we take α = 4^{53}. Figure 8a shows excellent agreement between the experimentally measured HiC contact map with inversion and the one predicted by the combined HIPPSDIMES approach. The more drastic structural variants (deletion and insertion) require applying the HIPPS or the DIMES method directly to the mutated CMs or DMs.
Structural integrity is determined by loci at the CTCF anchors and A/B boundary
Does the deletion of every single locus have the same effect on the 3D structures? To answer this question, we first investigated the effect of deletion of CTCF/cohesin anchors by applying DIMES to Chr21 28–30 Mbp region^{47}. The method for deletion of a single locus is described in Supplementary Note 3. The 3D structural changes are quantified using the Perturbation Index (PI),
where \(\langle {\widetilde{r}}_{ij}\rangle\)s is the WT average distance between loci i and j and 〈r_{ij}〉s are changed values after locus deletion. PI profile along the 2Mbp region and the Chipseq data for CTCF are shown in Fig. 8b top and middle panels. More importantly, on an average, the PI profile exhibits higher values closer to the CTCF Chipseq peaks (Fig. 8b bottom panel), demonstrating that the genomic loci associated with CTCF anchors have a more significant effect on the 3D structures upon their deletion.
We then probe the prediction of locus deletion for Chr2 imaging data^{51} (Fig. 8c). The PI profile along Chr2 (Fig. 8c top panel) shows that there are large variations among the individual loci, suggesting that deletion of some genomic locus have a larger impact on the 3D structures than others. To ascertain whether the variations in the PI are associated with the known chromosomal structural features such as A/B compartments, we compared the values of PI with the principal component dimension 1 (PC1) which are computed from correlation matrix ρ (Fig. 8d). The loci with PC1 close to zero are interpreted as the A/B boundaries. We find that, statistically, the boundary elements between A/B compartments have higher PI values, indicated by the basin near \(\ln ({{{{{{{\rm{PI}}}}}}}})=1\), whereas elements inside the A/B compartments have lower PIs \(\ln ({{{{{{{\rm{PI}}}}}}}})\, \approx 2\). From this finding, we propose that boundary loci are most important in maintaining chromosome structural integrity.
Comparing HiC and imaging experiment
Although there are several scHiC experiments, the majority of the studies report HiC data as ensemble averaged contact maps. On the other hand, superresolution imaging experiments directly measure the coordinates of loci for each cell. It is unclear how the chromosome structures inferred from HiC differ from the ones directly measured in imaging experiments. We compare the HiC data^{1} and the imaging data^{51} for Chr2 from the IMR90 cell line. Both the HIPPS and the DIMES methods first convert the contact probability p_{ij} to mean spatial distance 〈r_{ij}〉 using \(\langle {r}_{ij}\rangle={{\Lambda }}{p}_{ij}^{1/\alpha }\). We determined the value of Λ ≈ 0.36μm and α ≈ 5.26 by minimizing the error between the distances inferred from HiC and the experimental measurements from imaging experiments, \(\chi=(2/N(N1))\mathop{\sum }\nolimits_{i < j}^{N}{(\langle {r}_{ij}^{{{{{{{{\rm{HiC}}}}}}}}}\rangle \langle {r}_{ij}^{{{{{{{{\rm{Imaging}}}}}}}}}\rangle )}^{2}\). Figure 9a compares the mean distance matrix inferred from HiC using the HIPPS method and the one computed using the coordinates directly measured in the imaging experiment. Visual inspection suggests that the distance matrix inferred from HiC shows stronger compartmental patterns compared to the imaging result even though on an average the mean pairwise distances r_{ij}’s obtained from the two methods are in good agreement with each other (Fig. 9b). We also find that the locations of A/B compartments obtained from both the experimental methods are in excellent agreement (Fig. 9c).
Although the positions of the A/B compartments obtained from both HiC and imaging data agree with each other, it is unclear whether these two methods could be used to generate 3D structures that are consistent with each other. To ascertain the 3D structures inferred from HiC data and the imaging experiments are consistent with each other, we calculated Q_{k} and F_{k}^{53}; Q_{k} measures the degree of spatial mixing between A and B compartments.
where k is the number of the nearest neighbors of loci i. In Eq. (4), n_{A}(i; k) and n_{B}(i; k) are the number of neighboring loci belonging to A compartment and B compartment for loci i out of k nearest neighbors, respectively [n_{A}(i; k) + n_{B}(i; k) = k]. With N = N_{A} + N_{B}, the expected number of k neighboring loci in the A compartment with random mixing is \({\tilde{n}}_{A}=k{N}_{A}/N\) and \({\tilde{n}}_{B}=k{N}_{B}/N\) where N_{A} and N_{B} are the total number of A and B loci, respectively. With k ≪ N, perfect mixing would result in Q_{k} = 0, and Q_{k} ≠ 0 indicates demixing between the A and B compartments.
The function, F_{k}, not unrelated to contact order, quantifies the multibody longrange interactions of the chromosome structure. We define F_{k} as,
where k again is the number of nearest neighbors and m_{i}(k) is the set of loci that are k nearest neighbors of loci i; F_{0,k} = (1/2)(1 + k/2) is the value of F_{k} for a straight chain. Eq. (5) implies that the presence of longrange interaction increases the value of F_{k}. In both Q_{k} and F_{k}, k is the number of nearest neighbors for a given locus.
Figure 9 d shows that, compared to the results obtained through imaging, the HiC method overestimates the extent of longrange interactions, and underestimates the spatial mixing between A and B compartments, which is reflected in the shift of the distribution of P(Q_{k}) and P(F_{k}) (we chose k = 8 without loss of generality). We also computed the interactions profiles of AA, BB, and AB in the same fashion as shown in Fig. 7c, d from the HiC data. The HiC data suggest that the BB interactions are more attractive than AA, which is the opposite of the results obtained from the imaging data. Furthermore, the extracted 〈k(s)〉 for s between 0 and 10 Mbp differs in the two methods (Figs. 9f and 7d). The HiC data suggest that the interactions within the range of s between 0 and 10 Mbp are attractive (Fig. 9f) whereas the imaging data suggest that interactions for s ≲ 5 Mbp are repulsive, and become attractive for larger s values (Fig. 7d).
Next, we compared the HiC and imaging techniques at a smaller scale. To this end, we applied HIPPS/DIMES to HiC data for Chr21 28–30 Mbp from the IMR90 cell line^{1}. Supplementary Fig. 6a shows the comparison between the mean distance matrix inferred from HiC using HIPPS/DIMES method and that calculated from imaging data. The scatter plot between \({r}_{ij}^{{{{{{{{\rm{Imaging}}}}}}}}}\) and \({r}_{ij}^{{{{{{{{\rm{HiC}}}}}}}}}\) (Supplementary Fig. 6b) shows a higher degree of agreement compared to Chr2 (Fig. 9b). We then compute F_{k} and its distribution P(F_{k}), which shows good agreement between HiC and imaging data (Supplementary Fig. 6c). These results show that structures inferred from HiC and imaging have a higher degree of agreement on the length scale of Mbp compared to the scale of the whole chromosome.
Discussion
We have developed a computational method (DIMES) that solves the following inverse problem: how to generate the threedimensional conformations from the experimentally measured average distance matrix? First, applications to genome data on a length scale of a few TADs and on the scale of the whole chromosome, show that DIMES correctly reproduces the pairwise distances. Second, we demonstrate that DIMES accurately accounts for the higherorder structures beyond pairwise contacts, such as the threebody interactions, radius of gyration, shapes, and the clustering of structures. These results for Chr21 on 2Mbp and the entire 242Mbp Chr2 agree quantitatively with multiplexed superresolution data, thus setting the stage for a wide range of applications. Third, we also demonstrate that the DIMES accurately predicts the changes in the structures due to structural variants. We believe that this is a key prediction because the results for the wild type suffice to predict 3D structures, thus eliminating the need to do additional experiments.
Implications of the DIMES method
Our method is based on the maximum entropy principle, which is used to find the optimal distribution over the coordinates of chromatin loci that are consistent with experimental data. With the choice of the average squared pairwise distances as constraints, the maximumentropy distribution has a special mathematical structure. The distribution in Eq. (1) shows that: (1) It is a multivariate normal distribution whose properties are analytically known. Thus, finding the values of parameters k_{ij} does not require simulations but only an optimization procedure. (2) It has the same mathematical structure as GRM^{78,79} if one sets 1/k_{B}T = 1. Hence, all the properties of the GRM^{63} also hold for Eq. (1). This analogy provides a physical interpretation of k_{ij}, allowing us to explain phase separation between A and B compartments without appealing to polymer models.
Interpretation of k _{ij}’s
We wondered whether k_{ij}’s could be decomposed into two additive terms representing the polymer component and epigenetic component, k_{ij∈αβ} = k_{0}(i, j) + k_{e}(α, β); k_{0}(i, j) is the polymer contribution to k_{ij} which only depends on i and j. k_{e}(α, β) is the epigenetic contribution to k_{ij} that only depends on epigenetic types, e.g., k_{e}(A, A), k_{e}(B, B), and k_{e}(A, B). Assuming that k_{ij∈αβ} = k_{0}(i, j) + k_{e}(α, β) holds, by averaging over s = ∣i − j∣, it follows that 〈k_{AA}(s)〉 − 〈k_{AB}(s)〉, 〈k_{AA}(s)〉 − 〈k_{BB}(s)〉, and 〈k_{BB}(s)〉 − 〈k_{AB}(s)〉 are independent of s. Supplementary Fig. 7 shows that these quantities fluctuate approximately around constant values for 60 Mbp < s < 200 Mbp. This result suggests that, to a first approximation, the parameters k_{ij}’s may be decoupled into contributions from polymer connectivity and that arising from epigenetic states in an additive manner.
Predictive power of DIMES
One could legitimately wonder about the utility of DIMES, especially if future imaging techniques generate the coordinates of individual loci at high resolution. Of course, if this were to occur then it would make all computational approaches as well as HiC experiments for genome organization irrelevant. However, what is worth noting is that by using the DIMES method one can also predict the 3D structures of structural variants accurately. These applications show that the DIMES method accurately accounts for the data generated by highresolution imaging experiments for the WT. With the WT k_{ij}’s at hand, certain mutational effects could be predicted without having to repeat the imaging experiments, which may not become routine for the foreseeable future. Such highthroughput calculations, which can be performed using DIMES, would be particularly useful when analyzing cancer data from different tissues. We believe this is the major advantage of our computational approach.
Methods
Maximumentropy distribution
In this section, we first show that
which is the starting point in the DIMES method (see the main text), is a multivariate normal distribution. In the above equation, x_{i} = [x_{1i}, x_{2i}, x_{3i}] are the 3D coordinates of the i^{th} locus. We write Eq. (6) in a matrix form,
where \({{{{{{{{\boldsymbol{X}}}}}}}}}_{p}={[{x}_{p1},{x}_{p2},\cdots,{x}_{pN}]}^{T}\), subscript p ∈ (1, 2, 3) denotes the three spatial dimensions, and N is the total number of loci. The connectivity matrix, K, is given by,
Eq. (7) is a product of three multivariate normal distributions with covariance matrix Σ = −K^{+} and mean 0, i.e., \({{{{{{{{\boldsymbol{X}}}}}}}}}_{p} \sim {{{{{{{\mathcal{N}}}}}}}}({{{{{{{\boldsymbol{0}}}}}}}},{{{{{{{\boldsymbol{\Sigma }}}}}}}})\) for p = 1, 2, 3 (\({{{{{{{\mathcal{N}}}}}}}}({{{{{{{\boldsymbol{0}}}}}}}},{{{{{{{\boldsymbol{\Sigma }}}}}}}})\) is the multivariate Gaussian distribution). K^{+} is the MoorePenrose inverse (pseudoinverse) of K. Note that MoorePenrose inverse ensures that, even if Σ is not full rank, the distribution is properly defined. In DIMES, Σ has exact one zero eigenvalue, which corresponds to the zero mode (center of the system).
Note that for P^{MaxEnt}({x_{i}}) to be a normalizable probability density distribution, it is known that Σ has to be positive semidefinite (Σ ≥ 0). Σ with zero eigenvalue (not full rank) corresponds to the degenerate case, which is expected for a system that is translationally invariant. Hence, it is necessary that K be negative semidefinite. If k_{ij} ≥ 0 for all i, j, as is the case for the Generalized Rouse Model^{63,78}, K can be proven to be negative semidefinite. Even if k_{ij} < 0 for some i, j, K can still be negative semidefinite. This means that negative k_{ij} values are allowed as long as the matrix K remains semidefinite.
Next, we prove that there exists a unique set of k_{ij}s that satisfy the set of constraints, which in our problem is specified the average squared pairwise distances, \(\langle{{{{{{{{\boldsymbol{x}}}}}}}}}_{i}{{{{{{{{\boldsymbol{x}}}}}}}}}_{j}{}^{2}\rangle=\langle {r}_{ij}^{2}\rangle\). The constraints could be measured or calculated.
The distribution of coordinates of the loci is given by Eq. (6). Following the derivation in our previous work^{53,63}, we have,
where \({\omega }_{ij}^{2}={{{\Sigma }}}_{ii}+{{{\Sigma }}}_{jj}2{{{\Sigma }}}_{ij}\), and Σ = −K^{+}. Note that the whole set of equations \({\omega }_{ij}^{2}={{{\Sigma }}}_{ii}+{{{\Sigma }}}_{jj}2{{{\Sigma }}}_{ij}\) has the same number of unknown variables (Σ_{ij}) as the number of equations, which leads to a unique solution for Σ_{ij} given the values of \(\langle {r}_{ij}^{2}\rangle\). We may obtain K in principle using K = −Σ^{+}. Because Σ is unique, so is K.
Note that this also shows that K can be obtained directly without any optimization procedure. However, there are three issues associated with this method. First, this involves a matrix inversion operation, which is usually numerically unstable for a large matrix. Second, if the target pairwise distance matrix is not a proper distance matrix (for example it does not satisfy triangle inequality) then the resulting Σ is not positive semidefinite, which would result in an invalid distribution (Eq. (7)). Third, the direct calculation does not allow for regularization (see Supplementary Note 1), which is needed in practice.
Calculating the mean squared pairwise distance matrix from experiment data
If the input data is direct measurements of the 3D coordinates of certain loci in the chromatin, then the mean squared pairwise distances between all pairs are computed as,
where \({{{{{{{{\boldsymbol{x}}}}}}}}}_{i}^{(i)}\) and \({{{{{{{{\boldsymbol{x}}}}}}}}}_{j}^{(i)}\) are the 3D coordinates of i and j loci in the m^{th} single cell and M is the total number of cells for which measurements are made.
If the input data is the HiC contact frequencies, then the mean squared pairwise distances between all the pairs are computed using^{53},
where Λ sets the length scale and α determines the powerlaw relation between spatial distances and contact probability. In practice, we choose α = 4 if it is not determined experimentally. The value of Λ can only be determined when the length scale of the system is known. For instance, if the average radius of the gyration of the system is known, Λ could be determined by matching the average radius of the gyration of the chromosome from the model with the known value. In practice, we simply set Λ = 1 if not explicitly specified. Note that the structures obtained with Λ = 1 can be simply rescaled.
Optimization
Let us denote the target mean squared pairwise distances as \(\langle {r}_{ij,\exp }^{2}\rangle \equiv {a}_{ij}\). The updating scheme for the values of k_{ij} in the iterative scaling method^{80} is,
t is the step number, and \(\langle {r}_{ij}^{2}\rangle (t)\) is the value of mean squared distance between loci i and j with parameters k_{ij}(t) at step t. The value of the constant learning rate, γ, is chosen to be γ = 10 because it gives good convergence speed while ensuring that the result converges.
For gradient descent (GD), the updating scheme for k_{ij} is,
where γ is the constant learning rate.
For both of these two methods, the value of \(\langle {r}_{ij}^{2}\rangle (t)\) needs to be evaluated. In DIMES, the fact that the maximumentropy distribution is a multivariate normal distribution allows us to compute \(\langle {r}_{ij}^{2}\rangle (t)\) directly using the Eq. (9). With values of k_{ij}(t), the matrix K(t) is constructed according to Eq. (8). Then, the matrix Σ(t) is calculated using Σ(t) = −K^{+}(t) where K^{+}(t) is the MoorePenrose inverse of K(t). Finally, \(\langle {r}_{ij}^{2}\rangle (t)\) is computed using \(\langle {r}_{ij}^{2}\rangle (t)=3\left[{{{\Sigma }}}_{ii}(t)+{{{\Sigma }}}_{jj}(t)2{{{\Sigma }}}_{ij}(t)\right]\).
Once the values of \(\langle {r}_{ij}^{2}\rangle (t)\) are obtained, the values of k_{ij} are updated using Eq. (12) for iterative scaling or Eq. (13) for GD.
Generation of structures
After T number of iteration steps, a reasonably converged K(T) is obtained. Denote K ≡ K(T). To generate an ensemble of structures, we sample the distribution given by Eq. (7). Following the derivation in our previous work^{53,63}, the coordinates of loci, X_{p} (p denotes the three spatial dimensions), can be computed as a linear combination of normal modes, X_{p} = V^{T}R_{p} where R_{p} are the normal modes. V is obtained by eigendecomposition of K, i.e., VKV^{T} = Ω = diag(ω_{1}, ω_{2}, ⋯ , ω_{N}). Each component of the normal modes, R_{i,p}, is a Gaussian random variable with distribution \({{{{{{{\mathcal{N}}}}}}}}(0,{\omega }_{i}^{1})\). Note that different R_{i,p} are independent of each other.
The procedures used to generate an ensemble of structures involve the following steps:

(a)
Perform eigendecomposition of K. Obtain V and the eigenvalues ω_{1}, ω_{2}, ⋯ , ω_{N}.

(b)
Draw a total of N random numbers from distributions \({{{{{{{\mathcal{N}}}}}}}}(0,{\omega }_{i}^{1})\) for i = 1, 2, ⋯ , N. Denote these random numbers as R_{1}, R_{2}, ⋯ , R_{N}.

(c)
Define \({{{{{{{\boldsymbol{R}}}}}}}}={[{R}_{1},{R}_{2},\cdots,{R}_{N}]}^{T}\). Compute X using X = V^{T}R.

(d)
Repeat (b) and (c) two more times, each for a spatial dimension. Finally, we obtain X_{1}, X_{2}, X_{3}, representing the x, y, and z coordinates of N number of loci.

(e)
Repeat (b)  (d) M times, resulting in a total number of M randomly sampled 3D structures.
Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
Data availability
The data that support this study are available from the corresponding authors upon reasonable request. The Multiplexed FISH imaging data used in this study are publicly available from the GitHub repository at https://github.com/BogdanBintu/ChromatinImagingand Zenodo repository^{81} at https://zenodo.org/record/3928890#.Yizd1xDMKFF. The HiC data used in this study are publicly available from the GEO database under accession numbers [GSE92294] and [GSE63525].
Code availability
The code for the DIMES method presented in this work and its detailed user instruction can be accessed at the GitHub repository https://github.com/anyuzx/HIPPSDIMES^{82}. The data analysis is performed using Python 3.9 in Jupyter Lab. The Python packages used in data analysis are Scipy, Numpy, and Pandas.
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Acknowledgements
We thank Davin Jeong and Sucheol Shin for several pertinent comments on the work. This work was supported by a grant from the National Science Foundation (CHE 1900033, D. T.) and the Welch Foundation through the CollieWelch Chair (F0019, D. T.).
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G.S. and D.T. designed research; G.S. and D.T. performed research; G.S. and D.T. analyzed data; G.S. and D.T. wrote the paper.
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Shi, G., Thirumalai, D. A maximumentropy model to predict 3D structural ensembles of chromatin from pairwise distances with applications to interphase chromosomes and structural variants. Nat Commun 14, 1150 (2023). https://doi.org/10.1038/s41467023364124
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DOI: https://doi.org/10.1038/s41467023364124
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