Comparative Genomics Approaches to Study Organism Similarities and Differences
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Journal of Biomedical Informatics 35 (2002) 142–150 www.academicpress.com Methodological Review Comparative genomics approaches to study organism similarities and differences Liping Wei,a,b,* Yueyi Liu,c Inna Dubchak,d John Shon,a and John Parka a Nexus Genomics, Inc., 229 Polaris Ave., Suite 6, Mountain View, CA 94043, USA b Hang Zhou Center, Beijing Genomics Institute, Hangzhou, China c Stanford Medical Informatics, 251 Campus Dr. X215, Stanford University, Stanford CA 94305, USA d Genome Sciences Department, MS 84-171, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received 7 June 2002 Abstract Comparative genomics is a large-scale, holistic approach that compares two or more genomes to discover the similarities and differences between the genomes and to study the biology of the individual genomes. Comparative studies can be performed at different levels of the genomes to obtain multiple perspectives about the organisms. We discuss in detail the type of analyses that offer significant biological insights in the comparisons of (1) genome structure including overall genome statistics, repeats, genome rearrangement at both DNA and gene level, synteny, and breakpoints; (2) coding regions including gene content, protein content, orthologs, and paralogs; and (3) noncoding regions including the prediction of regulatory elements. We also briefly review the currently available computational tools in comparative genomics such as algorithms for genome-scale sequence alignment, gene identification, and nonhomology-based function prediction. Ó 2002 Elsevier Science (USA). All rights reserved. 1. Introduction genomes and the vast range of genome sizes. A full list of genomes, completed or in progress, and their Biomedicine today has a powerful new resource for sequences can be retrieved from the NCBI genome discovery—a rapidly growing number of sequenced ge- resources at http://www.ncbi.nlm.nih.gov/entrez/query. nomes. It was only as recent as 1995 when the first fcgi?db ¼ Genome. complete genome sequence of a free-living organism— There is undoubtedly much excitement over the se- the bacterium Haemophilus influenzae Rd—was pub- quencing of a complete genome. It is argued, however, lished [1]. Since then, through the large-scale DNA se- that a genome taken in isolation from a single organism quencing efforts of many public and private does not reveal much by itself. Genomes, and genes, organizations, the complete genomes of 15 archaea, 67 need to be studied in comparison with other species (or bacteria, and 8 eukaryota have been revealed (as of July subspecies, or strains), in the phylogenetic context of the 2002) [2]. In addition, the draft genome sequences of evolutionary process [7]. Comparative genomics is a several major organisms have become available, in large-scale, holistic approach that compares two or particular, the draft genome sequences of human and more genomes to discover the similarities and differences rice [3–6]. The sequencing of the genomes of about 800 between the genomes and to study the biology of the other organisms is currently in progress. Table 1 com- individual genomes. Comparative genomics applies to pares the sizes of several complete and draft sequences whole genomes or syntenic regions of different species, of archaea, bacteria, and eukaryotae genomes, and different subspecies, or different strains of the same showcases both the large size of some of the eukaryotae species. Comparative genomics research includes both developing computational tools and using the tools to analyze genomes for biological discoveries. * Corresponding author. The practical applications of comparative genomics E-mail address: [email protected] (L. Wei). are many and its scientific impact profound [8]. For 1532-0464/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S1532-0464(02)00506-3 L. Wei et al. / Journal of Biomedical Informatics 35 (2002) 142–150 143 Table 1 Comparison of the sizes of eight complete eukaryotae genome sequences and examples of complete bacteria and archaea genomes and eukaryotae draft genomes Domain Organism Genome size (kbp) Archaea Thermoplasma acidophilum 1565 Archaeoglobus fulgidus 2178 Sulfolobus solfataricus 2992 Methanosarcina acetivorans str. C2A 5751 Bacteria Salmonella typhi 180 Helicobacter pylori 26695 1668 Haemophilus influenzae Rd 1830 Escherichia coli K12 4639 Eucaryota Guillardia theta nucleomorph 551 Encephalitozoon cuniculi 2500 Saccharomyces cerevisiae S288C 12,069 Caenorhabditis elegans 97,000 Arabidopsis thaliana 115,400 Drosophila melanogaster 137,000 Oryza sativa L. ssp. indica (draft) 420,000 Homo sapiens (draft) 3,000,000 instance, animal models can be chosen as monitors for Table 2 pathogenesis and therapy for human genetic diseases Examples of genome-scale alignment and visualization tools based on explicit gene orthology between the animal and Algorithms/tools URL human; effective treatment may result from analysis of BLASTN and http://www.ncbi.nlm.nih.gov/BLAST/ the genetic differences between taxonomically related MEGABLAST pathogen strains that have varying host responses. Sci- GLASS http://crossspecies.lcs.mit.edu/ entific insights can be gained about species evolution in MUMmer http://www.tigr.org/software/mummer/ PatternHunter http://www.bioinformaticssolutions.com/ terms of gene birth and death, phylogeny of mammal products/ph.php orders, species origins, survival, and adaptations, just to PipMaker http://bio.cse.psu.edu/pipmaker/ name a few. VISTA http://www-gsd.lbl.gov/vista/ In the past five years there has been an explosion of WABA http://www.cse.ucsc.edu/kent/xenoAli/ computational and biological advances in the young field of comparative genomics. In this paper, we first technically challenging problem because of the large size briefly summarize the currently available computational of whole genomes (see Table 1), long insertions and tools for genome-scale sequence alignment. We then go deletions, large-scale rearrangement of genomic seg- into detail on how researchers have used comparative ments, and so on. Most traditional sequence alignment genomics to study the similarities and differences of or- algorithms such as Smith-Waterman and BLASTN are ganisms and strains. We focus on three important areas no longer usable. In recent years a growing number of of comparative analysis: genome structure, coding re- new algorithms have been developed for genome-scale gions, and noncoding regions. Finally, we discuss other alignment and visualization [9–17]. Table 2 lists some of related technologies and remaining technical challenges. the algorithms. Almost all of the alignment algorithms Before we start, it is important to define the differences first identify large conserved sequence elements between between the terms homology and similarity. Homology the two genome sequences, and then generate the overall implies common ancestry of two genes or gene products. alignment. We refer readers interested in detailed dis- Similarity is what we can measure from alignment of cussions and comparisons of these alignment algorithms sequences or structures. Similarity may be used as evi- to several recent reviews [18–20]. dence for homology, but does not necessarily imply ho- mology. Most of this paper describes the analysis of similarity, which is a means towards studying homology. 3. Comparative analysis of genome structure Analysis of the global structure of genomes, such as 2. Computational tools for genome-scale sequence align- nucleotide composition, syntenic relationships, and gene ment ordering offer insight into the similarities and differences between genomes. Such comparisons provide informa- The first step in comparative genomics analysis is tion on the organization and evolution of the genomes, often the alignment of two genome sequences. It is a and highlight the unique features of individual genomes. 144 L. Wei et al. / Journal of Biomedical Informatics 35 (2002) 142–150 This analysis is now an indispensable part of any ‘‘shared synteny,’’ ‘‘syntenic,’’ are sometimes used ex- study on comparative sequence analysis. It has been changeably with ‘‘synteny.’’ Identification and analysis applied to the analysis of the human genome [3,4], sev- of syntenic regions provides information on the orga- eral yeast genomes [21], two nematode genomes [16], as nization and evolution of genomes. well as the comparison of individual chromosomes from Synteny is detected either by identification of long, human and mouse [22–25]. conserved sequence elements, or by comparison of The structure of different genomes can be compared conserved proteins using BLASTP, or both [9,25,28]. at three levels: (1) overall nucleotide statistics, (2) ge- Important statistics about the syntenic regions include nome structure at DNA level, and (3) genome structure (1) the length of the regions and percentage of DNA at gene level. sequence identity between conserved syntenic regions, (2) the percentage of genomic sequences that are within 3.1. Comparison of overall nucleotide statistics syntenic regions, (3) the distribution of these regions along the genomes, (4) the gene content, gene density, Overall nucleotide statistics, such as genome size, and gene order of conserved syntenic regions, and (5) overall (G+C) content, regions of different (G+C) con- content of DNA repeats. Description of interesting tent, and genome signature such as codon usage biases, syntenic