Initial sequencing and analysis of the human genome - Vitagenes

articlesSegmental history of the human genomeIn bacteria, genomic segments often convey important informationabout function: genes located close to one another often encodeproteins in a common pathway and are regulated in a commonoperon. In mammals, genes found close to each other only rarelyhave common functions, but they are still interesting because theyhave a common history. In fact, the study of genomic segments canshed light on biological events as long as 500 Myr ago and as recentlyas 20,000 years ago.Conserved segments between human and mouseHumans and mice shared a common ancestor about 100 Myr ago.Despite the 200 Myr of evolutionary distance between the species, asigni®cant fraction of genes show synteny between the two, beingpreserved within conserved segments. Genes tightly linked in onemammalian species tend to be linked in others. In fact, conservedsegments have been observed in even more distant species: humansshow conserved segments with ®sh 350,351 and even with invertebratessuch as ¯y and worm 352 . In general, the likelihood that a syntenicrelationship will be disrupted correlates with the physical distancebetween the loci and the evolutionary distance between the species.Studying conserved segments between human and mouse hasseveral uses. First, conservation of gene order has been used toidentify likely orthologues between the species, particularly wheninvestigating disease phenotypes. Second, the study of conservedsegments among genomes helps us to deduce evolutionary ancestry.Fly (number of genes)4003503002502820058 71501032117 15 12 9100 36437 23 2129 20 1334 2211351933 18 145027 263130242516600 100 200 300 400 500 600 700 800 900Human (number of genes)Figure 44 Relative expansions of protein families between human and ¯y. These datahave not been normalized for proteomic size differences. Blue line, equality betweennormalized family sizes in the two organisms. Green line, equality between unnormalizedfamily sizes. Numbered InterPro entries: (1) immunoglobulin domain [IPR003006]; (2) zinc®nger, C2H2 type [IPR000822]; (3) eukaryotic protein kinase [IPR000719]; (4) rhodopsinlikeGPCR superfamily [IPR000276]; (5) ATP/GTP-binding site motif A (P-loop)[IPR001687]; (6) reverse transcriptase (RNA-dependent DNA polymerase) [IPR000477];(7) RNA-binding region RNP-1 (RNA recognition motif) [IPR000504]; (8) G-proteinb WD-40 repeats [IPR001680]; (9) ankyrin repeat [IPR002110]; (10) homeobox domain[IPR001356]; (11) PH domain [IPR001849]; (12) EF-hand family [IPR002048]; (13) EGFlikedomain [IPR000561]; (14) Src homology 3 (SH3) domain [IPR001452]; (15) RING®nger [IPR001841]; (16) KRAB box [IPR001909]; (17) leucine-rich repeat [IPR001611];(18) ®bronectin type III domain [IPR001777]; (19) PDZ domain (also known as DHR orGLGF) [IPR001478]; (20) TPR repeat [IPR001440]; (21) helicase C-terminal domain[IPR001650]; (22) ion transport protein [IPR002216]; (23) helix±loop±helix DNA-bindingdomain [IPR001092]; (24) cadherin domain [IPR002126]; (25) intermediate ®lamentproteins [IPR001664]; (26) C2 domain [IPR000008]; (27) Src homology 2 (SH2) domain[IPR000980]; (28) serine proteases, trypsin family [IPR001254]; (29) BTB/POZ domain[IPR000210]; (30) tyrosine-speci®c protein phosphatase and dual speci®city proteinphosphatase family [IPR000387]; (31) collagen triple helix repeat [IPR000087]; (32)esterase/lipase/thioesterase [IPR000379]; (33) neutral zinc metallopeptidases, zincbindingregion [IPR000130]; (34) ATP-binding transport protein, 2nd P-loop motif[IPR001051]; (35) ABC transporters family [IPR001617]; (36) cytochrome P450 enzyme[IPR001128]; (37) insect cuticle protein [IPR000618].32And third, detailed comparative maps may assist in the assembly ofthe mouse sequence, using the human sequence as a scaffold.Two types of linkage conservation are commonly described 353 .`Conserved synteny' indicates that at least two genes that reside on acommon chromosome in one species are also located on a commonchromosome in the other species. Syntenic loci are said to lie in a`conserved segment' when not only the chromosomal position butthe linear order of the loci has been preserved, without interruptionby other chromosomal rearrangements.An initial survey of homologous loci in human and mouse 354suggested that the total number of conserved segments would beabout 180. Subsequent estimates based on increasingly detailedcomparative maps have remained close to this projection 353,355,356(http://www.informatics.jax.org). The distribution of segmentlengths has corresponded reasonably well to the truncated negativeexponential curve predicted by the random breakage model 357 .The availability of a draft human genome sequence allows the ®rstglobal human±mouse comparison in which human physical distancescan be measured in Mb, rather than cM or orthologous genecounts. We identi®ed likely orthologues by reciprocal comparisonof the human and mouse mRNAs in the LocusLink database, usingmegaBLAST. For each orthologous pair, we mapped the location ofthe human gene in the draft genome sequence and then checked thelocation of the mouse gene in the Mouse Genome Informaticsdatabase (http://www.informatics.jax.org). Using a conservativethreshold, we identi®ed 3,920 orthologous pairs in which thehuman gene could be mapped on the draft genome sequence withhigh con®dence. Of these, 2,998 corresponding mouse genes had aknown position in the mouse genome. We then searched forde®nitive conserved segments, de®ned as human regions containingorthologues of at least two genes from the same mouse chromosomeregion (, 15 cM) without interruption by segments from otherchromosomes.We identi®ed 183 de®nitive conserved segments (Fig. 46). Theaverage segment length was 15.4 Mb, with the largest segment being90.5 Mb and the smallest 24 kb. There were also 141 `singletons',segments that contained only a single locus; these are not counted inthe statistics. Although some of these could be short conservedsegments, they could also re¯ect incorrect choices of orthologues orproblems with the human or mouse maps. Because of this conservativeapproach, the observed number of de®nitive segments islikely be lower than the correct total. One piece of evidence for thisconclusion comes from a more detailed analysis on human chromosome7 (ref. 358), which identi®ed 20 conserved segments, ofwhich three were singletons. Our analysis revealed only 13 de®nitivesegments on this chromosome, with nine singletons.The frequency of observing a particular gene count in a conservedsegment is plotted on a logarithmic scale in Fig. 47. If chromosomalbreaks occur in a random fashion (as has been proposed) anddifferences in gene density are ignored, a roughly straight lineshould result. There is a clear excess for n = 1, suggesting that 50%or more of the singletons are indeed artefactual. Thus, we estimatethat true number of conserved segments is around 190±230, in goodagreement with the original Nadeau±Taylor prediction 354 .Figure 48 shows a plot of the frequency of lengths of conservedsegments, where the x-axis scale is shown in Mb. As before, there is afair amount of scatter in the data for the larger segments (where thenumbers are small), but the trend appears to be consistent with arandom breakage model.We attempted to ascertain whether the breakpoint regions haveany special characteristics. This analysis was complicated by imprecisionin the positioning of these breaks, which will tend to blur anyrelationships. With 2,998 orthologues, the average interval withinwhich a break is known to have occurred is about 1.1 Mb. Wecompared the aggregate features of these breakpoint intervals withthe genome as a whole. The mean gene density was lower inbreakpoint regions than in the conserved segments (13.8 versus908 © 2001 Macmillan Magazines Ltd NATURE | VOL 409 | 15 FEBRUARY 2001 | www.nature.com