SHIMS 3.0: Highly efficient single-haplotype iterative mapping and sequencing using ultra-long nanopore reads.

The reference sequence of structurally complex regions can only be obtained through a highly accurate clone-based approach that we call Single-Haplotype Iterative Mapping and Sequencing (SHIMS). In recent years, improvements to SHIMS have reduced the cost and time required by two orders of magnitude, but internally repetitive clones still require extensive manual effort to transform draft assemblies into reference-quality finished sequences. Here we describe SHIMS 3.0, using ultra-long nanopore reads to augment the Illumina data from SHIMS 2.0 assemblies and resolve internally repetitive structures. This greatly minimizes the need for manual finishing of Illumina-based draft assemblies, allowing a small team with no prior finishing experience to sequence challenging targets with high accuracy. This protocol proceeds from clone-picking to finished assemblies in 2 weeks for about $80 (USD) per clone. We recently used this protocol to produce reference sequence of structurally complex palindromes on chimpanzee and rhesus macaque X chromosomes. Our protocol provides access to structurally complex regions that would otherwise be inaccessible from whole-genome shotgun data or require an impractical amount of manual effort to generate an accurate assembly.

GC-biased gene conversion in X-chromosome palindromes conserved in human, chimpanzee, and rhesus macaque.

Gene conversion is GC-biased across a wide range of taxa. Large palindromes on mammalian sex chromosomes undergo frequent gene conversion that maintains arm-to-arm sequence identity greater than 99%, which may increase their susceptibility to the effects of GC-biased gene conversion. Here, we demonstrate a striking history of GC-biased gene conversion in 12 palindromes conserved on the X chromosomes of human, chimpanzee, and rhesus macaque. Primate X-chromosome palindrome arms have significantly higher GC content than flanking single-copy sequences. Nucleotide replacements that occurred in human and chimpanzee palindrome arms over the past 7 million years are one-and-a-half times as GC-rich as the ancestral bases they replaced. Using simulations, we show that our observed pattern of nucleotide replacements is consistent with GC-biased gene conversion with a magnitude of 70%, similar to previously reported values based on analyses of human meioses. However, GC-biased gene conversion since the divergence of human and rhesus macaque explains only a fraction of the observed difference in GC content between palindrome arms and flanking sequence, suggesting that palindromes are older than 29 million years and/or had elevated GC content at the time of their formation. This work supports a greater than 2:1 preference for GC bases over AT bases during gene conversion and demonstrates that the evolution and composition of mammalian sex chromosome palindromes is strongly influenced by GC-biased gene conversion.

Large palindromes on the primate X Chromosome are preserved by natural selection.

Mammalian sex chromosomes carry large palindromes that harbor protein-coding gene families with testis-biased expression. However, there are few known examples of sex-chromosome palindromes conserved between species. We identified 26 palindromes on the human X Chromosome, constituting more than 2% of its sequence, and characterized orthologous palindromes in the chimpanzee and the rhesus macaque using a clone-based sequencing approach that incorporates full-length nanopore reads. Many of these palindromes are missing or misassembled in the current reference assemblies of these species' genomes. We find that 12 human X palindromes have been conserved for at least 25 million years, with orthologs in both chimpanzee and rhesus macaque. Insertions and deletions between species are significantly depleted within the X palindromes' protein-coding genes compared to their noncoding sequence, demonstrating that natural selection has preserved these gene families. The spacers that separate the left and right arms of palindromes are a site of localized structural instability, with seven of 12 conserved palindromes showing no spacer orthology between human and rhesus macaque. Analysis of the 1000 Genomes Project data set revealed that human X-palindrome spacers are enriched for deletions relative to arms and flanking sequence, including a common spacer deletion that affects 13% of human X Chromosomes. This work reveals an abundance of conserved palindromes on primate X Chromosomes and suggests that protein-coding gene families in palindromes (most of which remain poorly characterized) promote X-palindrome survival in the face of ongoing structural instability.

DAZL mediates a broad translational program regulating expansion and differentiation of spermatogonial progenitors.

Fertility across metazoa requires the germline-specific DAZ family of RNA-binding proteins. Here we examine whether DAZL directly regulates progenitor spermatogonia using a conditional genetic mouse model and in vivo biochemical approaches combined with chemical synchronization of spermatogenesis. We find that the absence of Dazl impairs both expansion and differentiation of the spermatogonial progenitor population. In undifferentiated spermatogonia, DAZL binds the 3' UTRs of ~2,500 protein-coding genes. Some targets are known regulators of spermatogonial proliferation and differentiation while others are broadly expressed, dosage-sensitive factors that control transcription and RNA metabolism. DAZL binds 3' UTR sites conserved across vertebrates at a UGUU(U/A) motif. By assessing ribosome occupancy in undifferentiated spermatogonia, we find that DAZL increases translation of its targets. In total, DAZL orchestrates a broad translational program that amplifies protein levels of key spermatogonial and gene regulatory factors to promote the expansion and differentiation of progenitor spermatogonia.

Chromothripsis from DNA damage in micronuclei.

Genome sequencing has uncovered a new mutational phenomenon in cancer and congenital disorders called chromothripsis. Chromothripsis is characterized by extensive genomic rearrangements and an oscillating pattern of DNA copy number levels, all curiously restricted to one or a few chromosomes. The mechanism for chromothripsis is unknown, but we previously proposed that it could occur through the physical isolation of chromosomes in aberrant nuclear structures called micronuclei. Here, using a combination of live cell imaging and single-cell genome sequencing, we demonstrate that micronucleus formation can indeed generate a spectrum of genomic rearrangements, some of which recapitulate all known features of chromothripsis. These events are restricted to the mis-segregated chromosome and occur within one cell division. We demonstrate that the mechanism for chromothripsis can involve the fragmentation and subsequent reassembly of a single chromatid from a micronucleus. Collectively, these experiments establish a new mutational process of which chromothripsis is one extreme outcome.

Radiologic techniques and effectiveness of angiography to diagnose and treat acute upper gastrointestinal bleeding.

The use of catheter-based techniques to treat upper gastrointestinal hemorrhage has evolved considerably over the past few decades. At present, the state-of-the-art interventional suites provide optimal imaging. Coupled with advanced catheter technology, the two may be used to manage and treat the patient with acute upper gastrointestinal hemorrhage. This article summarizes these techniques and, when possible, compares them with other methods such as surgery and endoscopy. The specific role of transcatheter embolotherapy is highlighted, alongside an additional discussion on pharmacologic infusion of vasopressin.