A comparison of the human genome with that of the chimpanzee is an attractive approach to attempts to understand the specificity of a certain phenotype's development. The two karyotypes differ by one chromosome fusion, nine pericentric inversions, and various additions of heterochromatin to chromosomal telomeres. Only the fusion, which gave rise to human chromosome 2, has been characterized at the sequence level. During the present study, we investigated the pericentric inversion by which chimpanzee chromosome 19 differs from human chromosome 17. Fluorescence in situ hybridization was used to identify breakpoint-spanning bacterial artificial chromosomes (BACs) and plasmid artificial chromosomes (PACs). By sequencing the junction fragments, we localized breakpoints in intergenic regions rich in repetitive elements. Our findings suggest that repeat-mediated nonhomologous recombination has facilitated inversion formation. No addition or deletion of any sequence element was detected at the breakpoints or in the surrounding sequences. Next to the break, at a distance of 10.2-39.1 kb, the following genes were found: NGFR and NXPH3 (on human chromosome 17q21.3) and GUC2D and ALOX15B (on human chromosome 17p13). The inversion affects neither the genomic structure nor the gene-activity state with regard to replication timing of these genes.

As part of the effort to sequence the genome of Rattus norvegicus, we constructed a physical map comprised of fingerprinted bacterial artificial chromosome (BAC) clones from the CHORI-230 BAC library. These BAC clones provide ∼13-fold redundant coverage of the genome and have been assembled into 376 fingerprint contigs. A yeast artificial chromosome (YAC) map was also constructed and aligned with the BAC map via fingerprinted BAC and P1 artificial chromosome clones (PACs) sharing interspersed repetitive sequence markers with the YAC-based physical map. We have annotated 95% of the fingerprint map clones in contigs with coordinates on the version 3.1 rat genome sequence assembly, using BAC-end sequences and in silico mapping methods. These coordinates have allowed anchoring 358 of the 376 fingerprint map contigs onto the sequence assembly. Of these, 324 contigs are anchored to rat genome sequences localized to chromosomes, and 34 contigs are anchored to unlocalized portions of the rat sequence assembly. The remaining 18 contigs, containing 54 clones, still require placement. The fingerprint map is a high-resolution integrative data resource that provides genome-ordered associations among BAC, YAC, and PAC clones and the assembled sequence of the rat genome.

The study of breakpoints that occurred during primate evolution promises to yield valuable insights into the mechanisms underlying chromosome rearrangements in both evolution and pathology. Karyotypic differences between humans and chimpanzees include nine pericentric inversions, which may have potentiated the parapatric speciation of hominids and chimpanzees 5-6 million years ago. Detailed analysis of the respective chromosomal breakpoints is a prerequisite for any assessment of the genetic consequences of these inversions. The breakpoints of the inversion that distinguishes human chromosome 4 (HSA4) from its chimpanzee counterpart were identified by fluorescence in situ hybridization (FISH) and comparative sequence analysis. These breakpoints, at HSA4p14 and 4q21.3, do not disrupt the protein coding region of a gene, although they occur in regions with an abundance of LINE and LTR-elements. At 30 kb proximal to the breakpoint in 4q21.3, we identified an as yet unannotated gene, C4orf12, that lacks an homologous counterpart in rodents and is expressed at a 33-fold higher level in human fibroblasts as compared to chimpanzee. Seven out of 11 genes that mapped to the breakpoint regions have been previously analyzed using oligonucleotide-microarrays. One of these genes, WDFY3, exhibits a three-fold difference in expression between human and chimpanzee. To investigate whether the genomic architecture might have facilitated the inversion, comparative sequence analysis was used to identify an approximately 5-kb inverted repeat in the breakpoint regions. This inverted repeat is inexact and comprises six subrepeats with 78 to 98% complementarity. (TA)-rich repeats were also noted at the breakpoints. These findings imply that genomic architecture, and specifically high-copy repetitive elements, may have made a significant contribution to hominoid karyotype evolution, predisposing specific genomic regions to rearrangements.

No abstract