Deanna M. Church

Database resources of the National Center for Biotechnology Information.

In addition to maintaining the GenBank(R) nucleic acid sequence database, the National Center for Biotechnology Information (NCBI) provides data analysis and retrieval resources for the data in GenBank and other biological data made available through NCBI’s website. NCBI resources include Entrez, PubMed, PubMed Central, LocusLink, the NCBI Taxonomy Browser, BLAST, BLAST Link (BLink), Electronic PCR, OrfFinder, Spidey, RefSeq, UniGene, HomoloGene, ProtEST, dbMHC, dbSNP, Cancer Chromosome Aberration Project (CCAP), Entrez Genomes and related tools, the Map Viewer, Model Maker, Evidence Viewer, Clusters of Orthologous Groups (COGs) database, Retroviral Genotyping Tools, SARS Coronavirus Resource, SAGEmap, Gene Expression Omnibus (GEO), Online Mendelian Inheritance in Man (OMIM), the Molecular Modeling Database (MMDB), the Conserved Domain Database (CDD) and the Conserved Domain Architecture Retrieval Tool (CDART). Augmenting many of the web applications are custom implementations of the BLAST program optimized to search specialized data sets. All of the resources can be accessed through the NCBI home page at: http://www.ncbi.nlm.nih.gov.

Consensus statement: chromosomal microarray is a first-tier clinical diagnostic test for individuals with developmental disabilities or congenital anomalies.

Chromosomal microarray (CMA) is increasingly utilized for genetic testing of individuals with unexplained developmental delay/intellectual disability (DD/ID), autism spectrum disorders (ASD), or multiple congenital anomalies (MCA). Performing CMA and G-banded karyotyping on every patient substantially increases the total cost of genetic testing. The International Standard Cytogenomic Array (ISCA) Consortium held two international workshops and conducted a literature review of 33 studies, including 21,698 patients tested by CMA. We provide an evidence-based summary of clinical cytogenetic testing comparing CMA to G-banded karyotyping with respect to technical advantages and limitations, diagnostic yield for various types of chromosomal aberrations, and issues that affect test interpretation. CMA offers a much higher diagnostic yield (15%-20%) for genetic testing of individuals with unexplained DD/ID, ASD, or MCA than a G-banded karyotype ( approximately 3%, excluding Down syndrome and other recognizable chromosomal syndromes), primarily because of its higher sensitivity for submicroscopic deletions and duplications. Truly balanced rearrangements and low-level mosaicism are generally not detectable by arrays, but these are relatively infrequent causes of abnormal phenotypes in this population (<1%). Available evidence strongly supports the use of CMA in place of G-banded karyotyping as the first-tier cytogenetic diagnostic test for patients with DD/ID, ASD, or MCA. G-banded karyotype analysis should be reserved for patients with obvious chromosomal syndromes (e.g., Down syndrome), a family history of chromosomal rearrangement, or a history of multiple miscarriages.

Database resources of the National Center for Biotechnology

In addition to maintaining the GenBank(R) nucleic acid sequence database, the National Center for Biotechnology Information (NCBI) provides data analysis and retrieval resources for the data in GenBank and other biological data made available through NCBIs Web site. NCBI resources include Entrez, PubMed, PubMed Central (PMC), LocusLink, the NCBITaxonomy Browser, BLAST, BLAST Link (BLink), Electronic PCR (e-PCR), Open Reading Frame (ORF) Finder, References Sequence (RefSeq), UniGene, HomoloGene, ProtEST, Database of Single Nucleotide Polymorphisms (dbSNP), Human/Mouse Homology Map, Cancer Chromosome Aberration Project (CCAP), Entrez Genomes and related tools, the Map Viewer, Model Maker (MM), Evidence Viewer (EV), Clusters of Orthologous Groups (COGs) database, Retroviral Genotyping Tools, SAGEmap, Gene Expression Omnibus (GEO), Online Mendelian Inheritance in Man (OMIM), the Molecular Modeling Database (MMDB), the Conserved Domain Database (CDD), and the Conserved Domain Architecture Retrieval Tool (CDART). Augmenting many of the Web applications are custom implementations of the BLAST program optimized to search specialized data sets. All of the resources can be accessed through the NCBI home page at: http://www.ncbi.nlm.nih.gov.

ClinVar: public archive of relationships among sequence variation and human phenotype

ClinVar (http://www.ncbi.nlm.nih.gov/clinvar/) provides a freely available archive of reports of relationships among medically important variants and phenotypes. ClinVar accessions submissions reporting human variation, interpretations of the relationship of that variation to human health and the evidence supporting each interpretation. The database is tightly coupled with dbSNP and dbVar, which maintain information about the location of variation on human assemblies. ClinVar is also based on the phenotypic descriptions maintained in MedGen (http://www.ncbi.nlm.nih.gov/medgen). Each ClinVar record represents the submitter, the variation and the phenotype, i.e. the unit that is assigned an accession of the format SCV000000000.0. The submitter can update the submission at any time, in which case a new version is assigned. To facilitate evaluation of the medical importance of each variant, ClinVar aggregates submissions with the same variation/phenotype combination, adds value from other NCBI databases, assigns a distinct accession of the format RCV000000000.0 and reports if there are conflicting clinical interpretations. Data in ClinVar are available in multiple formats, including html, download as XML, VCF or tab-delimited subsets. Data from ClinVar are provided as annotation tracks on genomic RefSeqs and are used in tools such as Variation Reporter (http://www.ncbi.nlm.nih.gov/variation/tools/reporter), which reports what is known about variation based on user-supplied locations.

A genome-wide comparison of recent chimpanzee and human segmental duplications

We present a global comparison of differences in content of segmental duplication between human and chimpanzee, and determine that 33% of human duplications (> 94% sequence identity) are not duplicated in chimpanzee, including some human disease-causing duplications. Combining experimental and computational approaches, we estimate a genomic duplication rate of 4–5 megabases per million years since divergence. These changes have resulted in gene expression differences between the species. In terms of numbers of base pairs affected, we determine that de novo duplication has contributed most significantly to differences between the species, followed by deletion of ancestral duplications. Post-speciation gene conversion accounts for less than 10% of recent segmental duplication. Chimpanzee-specific hyperexpansion (> 100 copies) of particular segments of DNA have resulted in marked quantitative differences and alterations in the genome landscape between chimpanzee and human. Almost all of the most extreme differences relate to changes in chromosome structure, including the emergence of African great ape subterminal heterochromatin. Nevertheless, base per base, large segmental duplication events have had a greater impact (2.7%) in altering the genomic landscape of these two species than single-base-pair substitution (1.2%).

Lineage-Specific Biology Revealed by a Finished Genome Assembly of the Mouse

A finished clone-based assembly of the mouse genome reveals extensive recent sequence duplication during recent evolution and rodent-specific expansion of certain gene families. Newly assembled duplications contain protein-coding genes that are mostly involved in reproductive function.

An evidence-based approach to establish the functional and clinical significance of copy number variants in intellectual and developmental disabilities

Purpose: Copy number variants have emerged as a major cause of human disease such as autism and intellectual disabilities. Because copy number variants are common in normal individuals, determining the functional and clinical significance of rare copy number variants in patients remains challenging. The adoption of whole-genome chromosomal microarray analysis as a first-tier diagnostic test for individuals with unexplained developmental disabilities provides a unique opportunity to obtain large copy number variant datasets generated through routine patient care.Methods: A consortium of diagnostic laboratories was established (the International Standards for Cytogenomic Arrays consortium) to share copy number variant and phenotypic data in a central, public database. We present the largest copy number variant case-control study to date comprising 15,749 International Standards for Cytogenomic Arrays cases and 10,118 published controls, focusing our initial analysis on recurrent deletions and duplications involving 14 copy number variant regions.Results: Compared with controls, 14 deletions and seven duplications were significantly overrepresented in cases, providing a clinical diagnosis as pathogenic.Conclusion: Given the rapid expansion of clinical chromosomal microarray analysis testing, very large datasets will be available to determine the functional significance of increasingly rare copy number variants. This data will provide an evidence-based guide to clinicians across many disciplines involved in the diagnosis, management, and care of these patients and their families.

Shotgun sequence assembly and recent segmental duplications within the human genome

Complex eukaryotic genomes are now being sequenced at an accelerated pace primarily using whole-genome shotgun (WGS) sequence assembly approaches. WGS assembly was initially criticized because of its perceived inability to resolve repeat structures within genomes. Here, we quantify the effect of WGS sequence assembly on large, highly similar repeats by comparison of the segmental duplication content of two different human genome assemblies. Our analysis shows that large (> 15 kilobases) and highly identical (> 97%) duplications are not adequately resolved by WGS assembly. This leads to significant reduction in genome length and the loss of genes embedded within duplications. Comparable analyses of mouse genome assemblies confirm that strict WGS sequence assembly will oversimplify our understanding of mammalian genome structure and evolution; a hybrid strategy using a targeted clone-by-clone approach to resolve duplications is proposed.

Mouse segmental duplication and copy number variation

Detailed analyses of the clone-based genome assembly reveal that the recent duplication content of mouse (4.94%) is now comparable to that of human (5.5%), in contrast to previous estimates from the whole-genome shotgun sequence assembly. However, the architecture of mouse and human genomes differs markedly: most mouse duplications are organized into discrete clusters of tandem duplications that show depletion of genes and transcripts and enrichment of long interspersed nuclear element (LINE) and long terminal repeat (LTR) retroposons. We assessed copy number variation of the C57BL/6J duplicated regions within 15 mouse strains previously used for genetic association studies, sequencing and the Mouse Phenome Project. We determined that over 60% of these base pairs are polymorphic among the strains (on average, there was 20 Mb of copy-number-variable DNA between different mouse strains). Our data suggest that different mouse strains show comparable, if not greater, copy number polymorphism when compared to human; however, such variation is more locally restricted. We show large and complex patterns of interstrain copy number variation restricted to large gene families associated with spermatogenesis, pregnancy, viviparity, pheromone signaling and immune response.

Completing the map of human genetic variation

Large-scale studies of human genetic variation have focused largely on understanding the pattern and nature of single-nucleotide differences within the human genome. Recent studies that have identified larger polymorphisms, such as insertions, deletions and inversions, emphasize the value of investing in more comprehensive and systematic studies of human structural genetic variation. We describe a community resource project recently launched by the National Human Genome Research Institute (NHGRI) to sequence large-insert clones from many individuals, systematically discovering and resolving these complex variants at the DNA sequence level. The project includes the discovery of variants through development of clone resources, sequence resolution of variants, and accurate typing of variants in individuals of African, European or Asian ancestry. Sequence resolution of both single-nucleotide and larger-scale genomic variants will improve our picture of natural variation in human populations and will enhance our ability to link genetics and human health.