Motivation and approach

Most known examples of large inversions have been identified in studies on human disease where inversions have no detectable effect in parents, but increase the risk of a disease-associated rearrangement in the offspring. In the Williams-Beuren syndrome, for example, 25–30% of transmitting parents carry a 1.5 Mbp inversion encompassing a commonly deleted region, whereas the same inversion is present in only 6% of the general population [26]. Similarly, a polymorphic inversion has been reported at 15q11-q13 that gives rise to a de novo deletion resulting with the Angelman syndrome [27]. Two more striking examples are found in the Sotos syndrome [28] and the 17q21.31 microdeletion syndrome [8, 10, 29–31]. In each of these disorders, where a de novo microdeletion arises, every parent studied to date carries an inversion at the same region. All these inversions are enriched in segmental duplications at their breakpoints, leading to an increased susceptibility to non-allelic homologous recombination (NAHR), which in turn elevates risk for disease-causing rearrangements in the offspring. The typical presence of duplicated sequences at the inversion boundaries is also the major challenge for inversion detection.

Creation of a map of inversion polymorphisms will provide valuable information regarding their distribution and frequency in the human genome. Such a map will be important for future studies aimed to unravel how inversions and the segmental duplications architecture associated with inverted haplotypes contribute to genomic susceptibility to disease rearrangements. To fill this gap, the InvFEST [32] database aims to collect a comprehensive set of inversions reported in the literature. Currently InvFEST hosts 86 validated inversions, of which 14 are larger than 1 Mbp.

The common method to discover inversions is to analyze the read pair signature [1, 20], where the mapping strand of the read pairs spanning the inversion breakpoints will be different from what is expected (Fig. 1). For example, the Illumina platform generates read pairs from opposing strands but if the DNA frag ment spans an inversion breakpoint, the read pairs will have a discordant size and they will map to the same strand. When the inversion is large, the real mapping distance between pairs increases, therefore increasing the chance of incorrect mapping due to the common repeats that usually map at the inversion breakpoints and on other chromosomes. Another complication in accurate detection of large inversions arises as other types of SVs might occur within inversions or around inversion breakpoints further confusing the sequence signatures.

Large molecule sequencing for long range contiguity

The HTS platforms generate data at very high rates with minimal cost. However, since both the HTS reads (100–150 bp for Illumina), and the DNA fragments are very short (350–500 bp), the mappability of the HTS data is dramatically reduced in repeat-rich regions that harbor most of the inversion breakpoints. Recently several methods have been developed to address these issues, and effectively to obtain information over a longer range. There are substantial differences between different approaches, yet, all have the same basic principles. First, the DNA is broken into very large molecules (10–100 Kb), then diluted and separated into a number of pools. Each pool is later barcoded and sequenced using the Illumina platform.

The first method that followed this procedure, called pooled clone sequencing (PCS), used fosmid and BACs to clone the input DNA [33] to generate 40-to- 50 Kb molecules. The alternative approaches avoid the cloning step and generate different average molecule sizes. These include the TruSeq Synthetic Long Reads (TSLR¹), 10X Genomics linked- reads [34] (10XG), Dovetail Genomics (Chicago Method²), and CPT-Seq [35]. In the remainder of this paper, we collectively refer to these technologies as long range sequencing for simplicity.

Our approach to discover large genomic inversions up to tens of thousands base pairs using long range sequencing follows from the observation that, DNA molecules (sequenced as linked-reads or pooled clones) that span the inversion breakpoint will be split into two sections when mapped to the reference genome, also separated by a distance approximately the size of the inversion. We call this sequence signature as split clones (Fig. 1), which is similar to the split read sequence signature used by several SV discovery tools such as DELLY [21] and Pindel [36] but has the advantage that can span over repetitive regions. In contrast to split reads, incorrect mappings in large inversion regions using split clone signatures are less pronounced. This is because split clones are identified from many reads where each read pair is mapped concordantly, rather than using shorter alignments of single reads and then encompass larger regions that are longer than the repeats. Combining split-clone signatures with paired-end read signatures, we can distinguish the true paired-end read signatures and here even one pair of paired-end reads would suffice to detect the presence of the inversion and thus this approach allows us to detect very large inversions with low false discovery rate.

Based on these observations, we developed a novel combinatorial algorithm and statistical heuristics called Valor (variation using long range information) (Fig. 2). Briefly, Valor searches for both read pair and split clone sequence signatures (Fig. 3) using the mapping locations of long range sequencing reads, and requires split clones from different pools to cluster at the same putative inversion breakpoints (Methods). Ambiguity due to multiple possible pairings of split clones are resolved using an approximation algorithm for the maximal quasi clique problem [37]. Other tools such as VariationHunter [38] and CLEVER [39] use the Set-Cover or the equivalent maximum clique formulation [40] to cluster the variants. This approximation fails in clustering large inversions because it aims to detect complete cliques with low cardinality, which results in identifying a single breakpoint within repetitive regions of the genome (Additional file 1: 1.7).

Valor proves its potential when tested on simulated data, and it is able to discover previously characterized large inversions in the genome of a human individual (NA12878) using pooled BAC sequence data. Additionally we tested Valor using 10XG data generated from the same genome [34] and obtained similar results. Kitzman et al. [33] note that large clone (or molecule) size is required to span segmental duplication blocks, and smaller clones such as fosmids may not be sufficient to detect inversions around segmental duplications. In contrast we found on simulated data that fosmids perform as well as BAC clones, if not better, given that fosmids are still statistically larger than most segmental duplications. In conclusion, the theoretical minimum inversion size detectable by Valor is limited by clone length, i.e. 150 Kbp when BACs are used.

Simulations

Using VarSim [41], we implanted 686 simulated inversions in different sizes (500 bp to 10 Mbp) to the human reference genome (GRCh37). VarSim generated a diploid simulated genome, where 252/686 inversions intersected with another simulated inversion. VarSim uses databases of previously validated variation in the human genome and it allows for inserting few novel inversions (set to 1% by default). The simulated inversions are representative of realistic inversions. We then simulated a WGS library at 50X coverage using ART [42] to benchmark the WGS-based tools. The read length was 150 bp and the average fragment size was set to 600 bp with a standard deviation of 60 bp.

In order to test Valor, we randomly generated a set consisting of 300 pools of simulated fosmid (μ = 40 Kbp, σ = 10 Kbp) clones from the same simulated chromosomes at 5X physical coverage. We then simulated paired-end reads at 10X sequence coverage for each pool using ART with a read length of 150 bp, average fragment size of 600 bp and standard deviation of 60 bp. However, by random chance, no fosmid clones spanned breakpoints of 26 of 275 inversions larger than 40 Kbp, and 20 of 167 inversions greater than 80 Kbp.

We mapped both libraries to the reference genome using BWA-MEM [43], sorted and removed duplicates using SAMtools [44] and Picard [45], and realigned around indels using GATK IndelRealigner [46] with default parameters. We used Valor on the simulated clone data set, and three popular SV discovery tools (LUMPY, DELLY, and GASVPro) on the WGS simulation. In our tests, we required at least 50% reciprocal overlap between inversion intervals using the BEDtools suite [47]. Here, due to the presence of heterozygous inversions, a predicted inversion may intersect with two simulated inversions. We classify both such inversions as correctly identified in such cases.

We also performed three more simulation experiments to comprehensively test the Valor performance in the presence of other SVs and segmental duplications, and its ability to differentiate between inversions and inverted duplications (Additional file 1: 1.10).

Characterizing false discovery using a haploid genome

All tools developed for SV discovery suffer from high false discovery rates. We used a haploid genome (CHM1) to characterize the false positive calls. To achieve this, we mapped Illumina WGS reads at 100X coverage to the CHM1.1 assembly [48]. Following the fact that the CHM1 cell line is haploid, and both the assembly and the WGS reads are derived from the same DNA resource, we do not expect to find any real SVs, but assembly errors may present themselves as variation. We applied the same analysis steps described above. Overall, DELLY predicted 5,578, GASVPro found 2,458, and LUMPY called 136 inversions, which shows the high false discovery rate of these tools.

Similarly, we simulated clones (average clone size 10 Kb, standard deviation 1 Kb) from the CHM1 assembly at 5X physical coverage. We then generated Illumina reads using ART at 10X sequence coverage per simulated clone. In this experiment, Valor returned no inversions as expected from the data.

PCS data

We tested Valor using a real PCS data set generated from the genome of an individual of European descent (NA12878). We used genomic DNA from NA12878 to construct the library. High molecular weight DNA was isolated, partially EcoRI digested, and subcloned into pCC1BAC vector (Epicentre) to create a ~140 Kbp insert library using previously described protocols [49]. We then split a portion of this library to 3 sets of 96 pools each, with 230 clones per pool for set 1, 389 clones per pool for set 2 and 153 clones per pool for set 3. Each pool was expanded by direct liquid outgrowth after infection. We next constructed 96 barcoded sequencing libraries per each set, for a total of 288 sequencing libraries [50]. Libraries from each set were indexed with barcodes, combined and sequenced using the Illumina HiSeq platform (101 bp paired-end reads). Upon sequencing a total of 74,112 clones (22,080 in Set 1, 37,344 in Set 2 and 14,688 in Set 3) we obtained 3.38X expected physical depth of coverage. After read mapping and clone reconstruction, 87.58% of the genome was covered by one or more clones.

10X genomics linked reads

Whole genome sequencing of NA12878 and CHM1

The simulation results above showed that the Valor’s performance was comparable to the performance of LUMPY and DELLY, but benchmark on real data is warranted. We tested LUMPY, DELLY, and GASVPro using a PCR-free WGS data set at 50X coverage. We downloaded the BAM file that corresponds to NA12878 from the European Nucleotide Archive, sequenced as part of the “Illumina Platinum Genomes” project (ENA project ID: PRJEB3381), and applied the aforementioned SV discovery tools. DELLY was able to find all of the known large inversions; however, it returned a total of 3,094 inversion calls. We note that the known inversions that DELLY correctly identified had low quality score, thus simple score-based filtration from the large call set would also remove the true positives. LUMPY returned 161 inversion calls, but it failed to discover any of the previously known large inversions. GASVPro found 167 inversions of size >500 Kbp (48 after merging overlapping calls), and it was able to find one of the large inversions. Such results are expected since these tools are designed for smaller inversions.

In addition we used the CHM1 resource to test the sensitivity of WGS-based tools and compared the results from the WGS-based tools against a well-curated and validated database of SVs generated from the same library [23]. We mapped the CHM1 Illumina WGS reads to the human reference genome (GRCh37), and used the same tools to discover SVs. Briefly, out of 33 validated inversions reported in [23]), DELLY identified 13 and LUMPY found 8 inversions. GASVPro failed to characterized any known and validated inversions. Since there are no real PCS data sets generated from CHM1, and the average validated inversion length is ~7 Kb, we could not test Valor with the same approach.

Experimental validation

After the initial split clone clustering and maximal quasi clique approximation (Methods), we filtered those inversion clusters without sufficient read pair signature support (<2). Additionally, when PCS data is used, we also filtered those that were within segmental duplications or intersected with gaps. We found that Valor could correctly identify two inversions that were previously validated within the same genome (Table 2). These inversions include a 5 Mbp inversion in 8p23.1 [51, 52] and a 2 Mbp inversion in 15q13.3 [53]. We then selected 4 additional inversions for experimental validation that were not experimentally tested before (Table 2), but 3 of them could not be tested by FISH due to the amount of segmental duplications at the breakpoints. Our validation efforts resulted in the discovery of a novel, previously undocumented 2 Mb inversion at the 16p12.3 locus (Fig. 4). Moreover we compared our calls with previously validated inversions reported in InvFEST and found a match for one more inversion at the 17q12 locus.

Library preparation

We first generated a single whole-genome BAC library with long inserts (~140 Kbp). This procedure is a modification of the original haplotyping method previously described by Kitzman et al. (2011), that generates fosmid libraries with ~40 Kbp inserts. Here we used BAC clones, since long inserts are required to span the large duplication blocks where inversion breakpoints typically map [5, 33]. We then randomly partitioned the library into pools such that each pool is essentially a haploid mixture of clones derived from either the maternal or paternal DNA at each genomic location. High-throughput sequencing of each pool provides haplotype information for each clone in that pool. We mapped the paired-end reads generated for each pool separately to the human reference genome assembly using BWA-MEM [43]. We did not generate the 10XG data in this study, it was made freely available by the original authors [34] as pre-aligned BAM files.

Inferring clones

We use only the concordantly mapped read pairs (i.e. fragment size within 3 standard deviations of the average) to infer the clone locations. We first merge spanning intervals of such pairs using BEDtools [47] merge command, while allowing up to a distance of 2 × μ_fragment between spanning intervals. Here we denote the spanning interval of a read pair as the interval between the starting map location of the proximal read and the end map location of the distal read. Depending on the data properties, the merge distance can be adapted to reconstruct the clones more precisely. For example when using 10XG data, due to very low clone coverage (~0.1X) the merge distance is set to larger values, up to 10 Kb. After the merge, we filter out those candidate clone intervals that are covered by less than 40% (i.e. breadth of coverage).

Inversion discovery

After inferring clone locations, we also collect read pairs with inversion signature [1] (i.e. both reads mapping to the same strand). We then search for potential split clones within each pool by pairing inferred clone intervals where the summation of their lengths is within the expected size range for full clones. This is formulated as: μ_clone ± 3σ_clone, where μ_clone is the mean clone size and σ_clone is the standard deviation.

Assuming that the inferred clone locations are sorted by mapping locations, our algorithm can detect split clones in O(n) amortized run time, where n is the number of inferred clones. The constant coefficient increases with the sequence coverage.

Here we set the max gap = −1 × max overlap = μ_clone. Note that adding more split clones to the same cluster will narrow down the gap size in breakpoint estimate. Not all of the split clones we identify signal an inversion event. In an ideal case, where there are no mapping errors, other forms of SV, or areas with low mappability may also show themselves as split clone signature for inversions. To ensure only split clones that signal a true inversion are detected, we also require read pair support for inversions [1, 20], and we discard any split clones that are not supported by read pairs. This step of the algorithm runs in O(m + n), where m is the number of read pairs with inversion signature and n is the number of split clones.

Each pair of split clones gives a signature about the existence of an inverted haplotype. There may be many incorrectly identified split clone inversion signatures, or a short clone may have multiple potential “mate”s with similar properties. Therefore, clustering multiple split clone pairs that share inversion breakpoint locations and inversion lengths can help resolve the inversion breakpoints more accurately (Fig. 3c). To both resolve ambiguities from multiple possible split clone pairings, and unambiguously identify inversions, we construct an undirected graph, where each PSC is a node, and an edge between two nodes indicates that share predicted breakpoints.

We initially formulated the inversion detection using split clones as a Set-Cover problem similar to VariationHunter. We then observed in both simulation and real data sets that due to segmental duplications, deletions and nested inversions around the breakpoints, Set-Cover approximation selected only one of the inversion breakpoints correctly (Additional file 1: 1.7). We therefore formulate the problem as finding maximal quasi cliques in the inversion cluster graph.

A subgraph G’ = (V’, E’) of an undirected graph G = (V, E) is a (α, β)-Quasi Clique (0 < α, β < 1) if each node in V’ is connected to at least α.|V’| other nodes and |E’| > = β.|V’ |(|V’| − 1) edges where n = |G’| [37]. In other words, the ratios α and β represent how complete the subgraph G’ is. In contrast to the maximal clique problem or the set cover problem, this formulation allows for the existence of incomplete clusters, and tolerates some split clones to be included in a true cluster, and as a result, increases flexibility and avoids getting stuck in a local optimum.

To find an approximate solution for the maximal quasi clique problem, we use an approximation algorithm previously suggested by [37], and we set the tabu, γ, and λ parameters to |graph|/10 rounds, 50%, and 60%, respectively. We obtained the values for these parameters by another grid optimization on experimental graphs depicting worst case scenarios (Additional file 1: 1.6).

When a quasi clique is found, the nodes within the clique denote a set of PSCs that are clustered together to mark an inversion. The breakpoint of this cluster is obtained by intersecting its split clones using a heuristic based on read pair support and the gap size. Next, the read pair support for the breakpoints within a distance is recalculated using the discordant read pairs. We report the final clusters after removing those that intersect with duplications and assembly gaps (>40%). A flowchart summarizing the Valor algorithm is available in Fig. 2.

Experimental validation

We tested the presence of an inversion in the cell line of the NA12878 individual predicted to carry an inverted haplotype. For this purpose, we used interphase triple-color FISH using two probes inside and one outside the inversion.