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    A user's guide to the human genome
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doi:10.1038/ng966
volume 32 supplement pp 9 - 17

 
 
Question 1
How does one find a gene of interest and determine that gene's structure? Once the gene has been located on the map, how does one easily examine other genes in that same region?

This question serves as a basic introduction to the three major genome viewers. One gene, ADAM2, will be examined using all three sites so that the reader can gain an appreciation of the subtle differences in information presented at each of these sites.

National Center for Biotechnology Information Map Viewer
The NCBI Human Map Viewer can be accessed from the NCBI's home page, at http://www.ncbi.nlm.nih.gov. Follow the hyperlink in the right-hand column labeled Human map viewer to go to the Map Viewer home page. The notation at the top of the page indicates that this is Build 29, or the NCBI's 29th assembly of the human genome. Build 29 is based on sequence data from 5 April 2002. The previous genome assembly, Build 28, was based on sequence data from 24 December 2001. To search for any mapped element, such as a gene symbol, GenBank accession number, marker name or disease name, enter that term in the Search for box and then press Find. For this example, enter 'ADAM2' and then press Find. The on chromosome(s) box may be left blank for text-based searches such as this one.

The resulting overview page shows a schematic of all of the human chromosomes, pinpointing the position of ADAM2 to the p arm of chromosome 8 (Fig. 1.1). The search results section shows that the gene exists on two NCBI maps, Genes_cyto and Genes_seq. Genes_cyto refers to the cytogenetic map, whereas Genes_seq refers to the sequence map. Clicking on either of those two links opens a view of just that map.

Detailed descriptions of these and other NCBI maps are available at http://www.ncbi.nlm.nih.gov/PMGifs/
Genomes/humansearch.html
. To get the most general overview of the genomic context of ADAM2, including all available maps, click on the item in the Map element column (in this case, ADAM2). This view shows ADAM2 and a bit of flanking sequence on chromosome 8p11.2 (Fig. 1.2). Three maps are displayed in this view, each of which will be discussed below. Additional maps, discussed in other examples in this guide, can be added to this view using the Maps & Options link.


 
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The rightmost map is the master map, the map providing the most detail. The master map in this case is the Genes_seq map, which depicts the intron/exon organization of ADAM2 and is created by aligning the ADAM2 mRNA to the genome. The gene appears to have 14 exons. The vertical arrow next to the ADAM2 gene symbol (within the pink box) shows the direction in which the gene is transcribed. The gene symbol itself is linked to LocusLink, an NCBI resource that provides comprehensive information about the gene, including aliases, nucleotide and protein sequences, and links to other resources10 (see Question 10). The links to the right of the gene symbol point to additional information about the gene.

sv, or sequence view, shows the position of the gene in the context of the genomic contig, including the nucleotide and encoded protein sequences.

ev brings the user to the evidence viewer, a view that displays the biological evidence supporting a particular gene model. This view shows all RefSeq models, GenBank mRNAs, transcripts (whether annotated, known or potential) and expressed sequence tags (ESTs) aligning to this genomic contig. More information on the evidence viewer can be found on the NCBI web site by clicking Evidence Viewer Help on any ev report page.

hm is a link to the NCBI's Human–Mouse Homology Map, showing genome sequences with predicted orthology between mouse and human (Fig. 12.2).

seq allows the user to retrieve the genomic sequence of the region in text format. The region of sequence displayed can easily be changed.

mm is a link to the Model Maker, which shows the exons that result when GenBank mRNAs, ESTs and gene predictions are aligned to the genomic sequence. The user can then select individual exons to create a customized model of the gene. More information on the Model Maker can be found on the NCBI web site by clicking help on any mm report page.

The UniG_Hs map shows human UniGene clusters that have been aligned to the genome. The gray histogram depicts the number of aligning ESTs and the blue lines show the mapping of UniGene clusters to the genome. The thick blue bars are regions of alignment (that is, exons) and the thin blue lines indicate potential introns. In this example, the mapping of UniGene cluster Hs.177959 to the genome follows that of ADAM2, and all the exons align.


 
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The Genes_cyto map shows genes that have been mapped cytogenetically; the orange bar shows the position of the gene. Although ADAM2 has been finely mapped and is represented by a short line, other genes, such as the group below it on a longer line, have been cytogenetically mapped to broader regions of chromosome 8.

Clicking on the zoom control in the blue sidebar allows the user to zoom out to view a larger region of chromosome 8. Zooming out one level shows 1/100th of the chromosome. There are 20 genes in the region, and all 20 are labeled (displayed) in this view (Fig. 1.3). The region of ADAM2 is highlighted in red on all maps. On the basis of the Genes_seq map, ADAM2 is located between ADAM18 and LOC206849.

University of California, Santa Cruz Genome Browser
The home page for the UCSC Genome Browser is http://genome.ucsc.edu/. At present, UCSC provides browsers not only for the most recent version of the mouse and human genome data, but also for several earlier assemblies. To use the Genome Browser, select the appropriate organism from the pull-down menu at the top of the blue sidebar (Human, in this case) and then click the link labeled Browser. On the resulting page, select the version of the human assembly to view. The genome browser from August 2001 is based on an assembly of the human genome done by UCSC using sequence data available on that date. The Dec. 2001 browser displays annotations based on NCBI's build 28 of the human genome, and the Apr. 2002 browser displays annotations on NCBI's build 29. As the annotations presented in this most recent human assembly are not yet as comprehensive as those from the December 2001 assembly, the examples in this text are based on the earlier assembly. Select Dec. 2001 from the pull-down menu to access the assembly from that date (Fig. 1.4).

Supported types of queries are listed below the text input boxes. Enter 'ADAM2' in the box labeled position and then click Submit. The results of this search are presented in two categories, Known Genes and mRNA Associated Search Results (Fig. 1.5). The section marked Known Genes shows the mapping of the NCBI Reference mRNA sequences to the genome. The mRNA Associated Search Results represent the mapping of other GenBank mRNA sequences to the genome. Click on the Known Genes link for ADAM2 (arrow, Fig. 1.5) to see the genomic context of the ADAM2 mRNA Reference Sequence (NM_001464).


 
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The resulting zoomed-in view shows a region of chromosome 8 from base pair 36234934 to 36280132, located within 8p12 (Fig. 1.6). The blue track entitled Known Genes (from RefSeq) shows the intron–exon structure of known genes. The vertical boxes indicate exons and the horizontal lines introns. The ADAM2 gene seems to have 14 exons. The direction of transcription is indicated by the arrowheads on the introns. The tracks labeled Acembly Gene Predictions, Ensembl Gene Predictions and Fgenesh++ Gene Predictions are the results of gene predictions (see Question 7). Alignments of other database nucleotide sequences are shown in the Human mRNAs from GenBank, spliced EST, UniGene and Nonhuman mRNAs from GenBank tracks. Translated alignments of mouse and Tetraodon genomic sequence are in the mouse and fish BLAT tracks. Tracks displaying single-nucleotide polymorphisms (SNPs), repetitive elements and microarray data are shown at the bottom. Additional details about each track are available by selecting the track name in the Track Controls at the bottom.

To view the genomic context of ADAM2, zoom out 10times by clicking on the zoom out 10times box in the upper right corner. ADAM2 is located between TEM5 and ADAM18 (Fig. 1.7).

Ensembl
The Ensembl7 project, http://www.ensembl.org/, provides genome browsers for four species: human, mouse, zebrafish and mosquito. Click on Human to view the main entry point for the human genome. The current version of human Ensembl is version 6.28.1, based on the NCBI's 28th build of the genome. To perform a text search, enter 'ADAM2' in the text box, and limit the search by selecting Gene from the pull-down search. Click on the upper button labeled Lookup. A single result is returned with a link to the ADAM2 gene (Fig. 1.8).

Click on either of the ADAM2 links to retrieve the GeneView window. The returned page contains four sections of data. The first section (Fig. 1.9) is an overview of ADAM2, including links to accession numbers and protein domains and families. Links to the Ensembl view of highly similar mouse sequences are presented in the Homology Matches section. Some of these fields will be described in more detail in later examples. The second section of the GeneView window provides information on the gene transcript (Fig. 1.10). The sequence of the cDNA is shown, as is a graphic of its intron–exon structure. A limited amount of the genomic context around the gene is shown schematically as well. Exon sequences are shown in the third section of the GeneView (Fig. 1.11) and splice sites in the fourth (Fig. 1.12). If more than one transcript is predicted for the gene, each is allocated its own transcript, exon and splice-site sections.


 
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The complete genomic context of ADAM2 is viewed by returning to the first section of the GeneView (Fig. 1.9) and clicking on one of the two links within the Genomic Location box. The top portion of the resulting ContigView (Fig. 1.13) depicts the chromosome, with the region of interest outlined in red. The Overview shows the genomic context of the gene, including the chromosome bands, contigs, markers and genes that map to near 8p12. Clicking on any of these items recenters the display around that item. The section of interest is boxed in red on the DNA(contigs) map. The genes annotated by Ensembl as being around ADAM2 are Q96KB2 and ADAM18.

The bottom panel of the ContigView, the Detailed View (Fig. 1.14), shows a zoomed-in view of the boxed region, highlighting all features that have been mapped to this region of the human genome. The navigator buttons between the Overview and the Detailed View move the display to the left and right and zoom in and out. The features to be displayed can be changed by selecting the Features pull-down menu and then checking which features to view.

The Features shown in Fig. 1.14 are the defaults. The DNA (contigs) map separates items on the forward strand (above) from those on the reverse (below). The only feature on the reverse strand in this view is a single Genscan transcript, predicted by the GENSCAN gene prediction program11 (see Question 7). The forward strand shows five types of features. Starting at the bottom, the ADAM2 transcript is shown in red, indicating that it is a known transcript corresponding to a near-full-length cDNA sequence, protein sequence or both already available in the public sequence database. Black transcripts are predicted based on EST or protein sequence similarity. EST Transcr. links to individual aligning ESTs, whereas the UniGene track near the top displays UniGene clusters. The Genscan model on the forward strand contains many exons found in the known transcript. The Proteins and Human proteins boxes indicate protein sequences that align to this version of the genome, whereas NCBI Transcr. links to the NCBI Map Viewer. Positioning the computer mouse over any feature brings up the feature's name and links to more detailed information.

The NCBI, UCSC and Ensembl sometimes use different symbols for the same genes, so it can be difficult to compare the views obtained by the different browsers. Furthermore, the three sites maintain independent annotation pipelines and do not all attempt to align the same mRNA sequences to the genome. The NCBI is currently displaying build 29, Ensembl shows build 28, and UCSC offers both builds 28 (December 2001) and 29 (April 2002), although all examples from UCSC in this guide will be illustrated using the better-annotated build 28. Because of the differences between the two assemblies, there are subtle discrepancies between what is shown at the NCBI and what is available at UCSC and Ensembl. However, it is fairly easy to navigate among the three sites. The NCBI, for example, links to Ensembl and UCSC through the black boxes at the top of LocusLink entries for human genes, and Ensembl directs users to NCBI and UCSC through the "Jump to" link in its ContigView. Some versions of UCSC's Genome Browser have links to Ensembl and NCBI's Map Viewer in the blue bar at the top of each browser page.


 
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