A recurrent translocation is mediated by homologous recombination between HERV-H elements

<p>Abstract</p> <p>Background</p> <p>Chromosome rearrangements are caused by many mutational mechanisms; of these, recurrent rearrangements can be particularly informative for teasing apart DNA sequence-specific factors. Some recurrent translocations are mediated by homologous recombination between large blocks of segmental duplications on different chromosomes. Here we describe a recurrent unbalanced translocation casued by recombination between shorter homologous regions on chromosomes 4 and 18 in two unrelated children with intellectual disability.</p> <p>Results</p> <p>Array CGH resolved the breakpoints of the 6.97-Megabase (Mb) loss of 18q and the 7.30-Mb gain of 4q. Sequencing across the translocation breakpoints revealed that both translocations occurred between 92%-identical human endogenous retrovirus (HERV) elements in the same orientation on chromosomes 4 and 18. In addition, we find sequence variation in the chromosome 4 HERV that makes one allele more like the chromosome 18 HERV.</p> <p>Conclusions</p> <p>Homologous recombination between HERVs on the same chromosome is known to cause chromosome deletions, but this is the first report of interchromosomal HERV-HERV recombination leading to a translocation. It is possible that normal sequence variation in substrates of non-allelic homologous recombination (NAHR) affects the alignment of recombining segments and influences the propensity to chromosome rearrangement.</p

Differential expression analysis with global network adjustment

&lt;p&gt;Background: Large-scale chromosomal deletions or other non-specific perturbations of the transcriptome can alter the expression of hundreds or thousands of genes, and it is of biological interest to understand which genes are most profoundly affected. We present a method for predicting a gene’s expression as a function of other genes thereby accounting for the effect of transcriptional regulation that confounds the identification of genes differentially expressed relative to a regulatory network. The challenge in constructing such models is that the number of possible regulator transcripts within a global network is on the order of thousands, and the number of biological samples is typically on the order of 10. Nevertheless, there are large gene expression databases that can be used to construct networks that could be helpful in modeling transcriptional regulation in smaller experiments.&lt;/p&gt; &lt;p&gt;Results: We demonstrate a type of penalized regression model that can be estimated from large gene expression databases, and then applied to smaller experiments. The ridge parameter is selected by minimizing the cross-validation error of the predictions in the independent out-sample. This tends to increase the model stability and leads to a much greater degree of parameter shrinkage, but the resulting biased estimation is mitigated by a second round of regression. Nevertheless, the proposed computationally efficient “over-shrinkage” method outperforms previously used LASSO-based techniques. In two independent datasets, we find that the median proportion of explained variability in expression is approximately 25%, and this results in a substantial increase in the signal-to-noise ratio allowing more powerful inferences on differential gene expression leading to biologically intuitive findings. We also show that a large proportion of gene dependencies are conditional on the biological state, which would be impossible with standard differential expression methods.&lt;/p&gt; &lt;p&gt;Conclusions: By adjusting for the effects of the global network on individual genes, both the sensitivity and reliability of differential expression measures are greatly improved.&lt;/p&gt

Large Inverted Duplications in the Human Genome Form via a Fold-Back Mechanism

<div><p>Inverted duplications are a common type of copy number variation (CNV) in germline and somatic genomes. Large duplications that include many genes can lead to both neurodevelopmental phenotypes in children and gene amplifications in tumors. There are several models for inverted duplication formation, most of which include a dicentric chromosome intermediate followed by breakage-fusion-bridge (BFB) cycles, but the mechanisms that give rise to the inverted dicentric chromosome in most inverted duplications remain unknown. Here we have combined high-resolution array CGH, custom sequence capture, next-generation sequencing, and long-range PCR to analyze the breakpoints of 50 nonrecurrent inverted duplications in patients with intellectual disability, autism, and congenital anomalies. For half of the rearrangements in our study, we sequenced at least one breakpoint junction. Sequence analysis of breakpoint junctions reveals a normal-copy disomic spacer between inverted and non-inverted copies of the duplication. Further, short inverted sequences are present at the boundary of the disomic spacer and the inverted duplication. These data support a mechanism of inverted duplication formation whereby a chromosome with a double-strand break intrastrand pairs with itself to form a “fold-back” intermediate that, after DNA replication, produces a dicentric inverted chromosome with a disomic spacer corresponding to the site of the fold-back loop. This process can lead to inverted duplications adjacent to terminal deletions, inverted duplications juxtaposed to translocations, and inverted duplication ring chromosomes.</p></div

Sequenced breakpoint junctions.

<p>Sizes of deletions, duplications, and spacers in bp are shown. The numbers of sequenced disomy-inversion (Dis-inv), inversion-telomere (Inv-tel), and inversion-translocation (Inv-tra) junctions are listed. Spacers without a sequenced Dis-inv junction were measured from most distal duplicated probe to the most proximal deleted probe on the array. The full list of inverted duplication CNVs is provided in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004139#pgen.1004139.s003" target="_blank">Table S1</a>.</p

Complex junctions from EGL106 and 18q-65c.

<p>Insertion orientation (+/−) is indicated relative to the reference genome. (A) Alignment of telomere (black), inverted duplication (orange), inserted sequence (blue), and junction sequence (EGL106) from the telomere-inversion junction is shown above. The inverted duplication, disomic sequence (grey), and inversion-disomy junction sequence (EGL106) alignment is shown below. Microhomology at the junction is boxed. (B) Above, disomic, inserted, and inverted duplication sequences are aligned to the disomy-inversion junction sequence (18q-65c). Below, inverted duplication and telomere sequences are aligned to the inversion-telomere junction sequence (18q-65c). Inserted sequences and their neighboring homologous sequences are underlined.</p

Fold-back model of inverted duplication formation.

<p>(A) 5′ and 3′ strands of the chromosome with telomeres (triangles) and centromere (circle) are shown. Short inverted sequences (grey rectangles with arrows) lie adjacent to the terminal deletion breakpoint. The inverted duplication mechanism occurs as described in the <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004139#s3" target="_blank">Discussion</a>. The resulting inverted duplication is indicated by orange arrows. (B) After a breakage-fusion-bridge cycle, the inverted duplication chromosome may be repaired as a terminal deletion, translocation, or ring chromosome.</p

Inverted duplication junctions.

<p>(A) Location of disomy-inversion and inversion-telomere junctions in an inverted duplication terminal deletion chromosome. (B) 18q-233c's disomy-inversion junction spans a hybrid LINE made up of L1PA2 and L1Hs elements. On a normal chromosome 18, these elements are positioned in opposite orientation. (C) Local genomic context of 18q-233c's spacer and breakpoints relative to the reference genome assembly. The distal end of the disomic spacer (grey box) includes the L1PA2, and the proximal region corresponding to the beginning of the inverted duplication (orange box) includes the L1Hs. The disomy-inversion junction sequence (black rectangles with white arrows) aligns to the distal end of the spacer (positions 1–465 of the junction) and the start of the inverted duplication (positions 140–834 of the junction). Interspersed repeats are shown as black rectangles. No segmental duplications are present in the breakpoint regions.</p