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Down syndrome caused by chromosome 21 trisomy is the most common genetic cause of mental retardation in humans. Disruption of the phenotype is thought to be the result of gene-dosage imbalance. Variations in chromosome 21 gene expression in Down syndrome were analyzed in lymphoblastoid cells derived from patients and control individuals. Of the 359 genes and predictions displayed on a specifically designed high-content chromosome 21 microarray, one-third were expressed in lymphoblastoid cells. We performed a mixed-model analysis of variance to find genes that are differentially expressed in Down syndrome independent of sex and interindividual variations. In addition, we identified genes with variations between Down syndrome and control samples that were significantly different from the gene-dosage effect (1.5). Microarray data were validated by quantitative polymerase chain reaction. We found that 29% of the expressed chromosome 21 transcripts are overexpressed in Down syndrome and correspond to either genes or open reading frames. Among these, 22% are increased proportional to the gene-dosage effect, and 7% are amplified. The other 71% of expressed sequences are either compensated (56%, with a large proportion of predicted genes and antisense transcripts) or highly variable among individuals (15%). Thus, most of the chromosome 21 transcripts are compensated for the gene-dosage effect. Overexpressed genes are likely to be involved in the Down syndrome phenotype, in contrast to the compensated genes. Highly variable genes could account for phenotypic variations observed in patients. Finally, we show that alternative transcripts belonging to the same gene are similarly regulated in Down syndrome but sense and antisense transcripts are not.

Regions close to human centromeres contain DNA fragments spanning hundreds of kilobases that exhibit a high degree of sequence identity (>95%). Here we report the genomic structure and evolution of a family of four paralogous regions related to a 220-kb genomic fragment present on the long arm of human chromosome 21 (21q22.1). Phylogenetic classification of the paralogous sequences obtained from the draft of the Human Genome Project are in agreement with results from comparative fluorescence in situ hybridization on metaphase chromosomes from human and great apes. The original copy present in 21q22.1 in human was duplicated in great apes after the divergence of the orang-utan and inserted in a pericentromeric region, most likely the ancestor of HSA2q, then disseminated by transposition of a larger fragment to other pericentromeric locations: HSA18p11, HSA13q11 and HSA21q11.1. The degree of dissemination varies among species.

The short arm of human chromosome 21 (21p) contains many different types of repetitive sequences and is highly homologous to the short arms of other acrocentric chromosomes. Owing to its repetitive nature and the lack of chromosome 21p-specific molecular markers, most physical maps of chromosome 21 exclude this region. We constructed a physical map of chromosome 21p using sequence tagged site (STS) content mapping of yeast artificial chromosomes (YACs). To this end, 39 STSs located on the short arm or near the centromere of chromosome 21 were constructed, including four polymorphic simple tandem repeats (STRs) and two expressed sequence tags (ESTs). Thirty YACs were selected from the St. Louis YAC library, the chromosome 21-enriched ICRF YAC library, and the CEPH YAC and megaYAC libraries. These were assembled in a YAC contig map ranging from the centromere to the rDNA gene cluster at 21p12. The total size of the region covered by YACs is estimated between 2.9 and 5 Mb. The integrity of the YAC contig was confirmed by restriction enzyme fingerprinting and fluorescence in situ hybridization (FISH). One gap with an estimated size of 400 kb remained near the telomeric end of the contig. This YAC contig map of the short arm of human chromosome 21 constitutes a basic framework for further structural and functional studies of chromosome 21p.

We present a high-resolution bacterial contig map of 3.4 Mb of genomic DNA in human chromosome 21q11-q21, encompassing the region of elevated disomic homozygosity in Down Syndrome-associated abnormal myelopoiesis and leukemia, as well as the markers, which has shown a strong association with Alzheimer's Disease that has never been explained. The map contains 89 overlapping PACs, BACs, or cosmids in three contigs (850, 850, and 1500 kb) with two gaps (one of 140-210 kb and the second <5 kb). To date, eight transcribed sequences derived by cDNA selection, exon trapping, and/or global EST sequencing have been positioned onto the map, and the only two genes so far mapped to this cytogenetic region, STCH and RIP140 have been precisely localized. This work converts a further 10% of chromosome 21q into a high-resolution bacterial contig map, which will be the physical basis for the long-range sequencing of this region. The map will also enable positional derivation of new transcribed sequences, as well as new polymorphic probes, that will help in elucidation of the role the genes in this region may play in abnormal myelopoiesis and leukemia associated with trisomy 21 and Alzheimer's Disease.

We analyzed the conservation of large paralogous regions (more than 200 kb) on human chromosome regions 21q22.1 and 21q11.2 and on pericentromeric regions of chromosomes 2, 13, and 18 in three nonhuman primate species. Orthologous regions were found by FISH analysis of metaphase chromosomes from Gorilla gorilla, Pan troglodytes, and Pongo pygmaeus. Only one orthologous region was detected in chromosomes of P. pygmaeus, showing that the original locus was at 21q22.1 and that the duplication arose after the separation of Asian orangutans from the other hominoids. Surprisingly, the paralogous regions were more highly conserved in gorilla than in chimpanzee. PCR amplification of STSs derived from sequences of the chromosome 21 loci and low-stringency FISH analysis showed that this duplication occurred recently in the evolution of the genome. Different rates of sequence evolution through substitutions or deletions, after the duplication, may have resulted in diversity between closely related primates.

The polymorphic marker D21S190 was initially isolated from a chromosome 21 phage library and mapped to two loci: one in 21q11.1 (proximal locus) and the other one in 21q22.1 (distal locus). To characterize the region of homology revealed with D21S190, we have screened two different chromosome 21 YAC libraries and one chromosome 21 cosmid library. Fluorescence in situ hybridization on normal human chromosomes of YACs, cosmids, and phages positive with D21S190 confirmed the existence of two homologous regions on the long arm of chromosome 21. Among the positive YACs, four (HY67, 2D7y21, 2D11y21, and 1B1y21) were selected and oriented relative to each other, forming a 2-Mb contig in the distal locus, including D21S54. Hybridization of YAC extremities to a panel of somatic cell hybrids containing various portions of chromosome 21 showed that the proximal locus is located between the breakpoints of 2Fu(r)1 and ACEM and the distal locus between the breakpoints of ACEM and 6918. The proximal and the distal breakpoints of JC6 are both included in the region of homology. We have constructed a restriction map of HY67, 2D7y21, and 2D11y21 spanning 1 Mb and including several markers: D21S294, D21S296, and the new STSs corresponding to YAC extremities. The region of homology encompasses 135-500 kb and has the same orientation in the distal and in the proximal locus, which are at least 12 Mb apart. It is lacking a NotI site but does contain clusters of GC-rich restriction sites, which are candidate regions for as yet unidentified genes.

A library of yeast artificial chromosomes (YACs) was constructed from a human/hamster somatic cell hybrid containing human chromosome 21 (q11-qter). Cells were embedded in agarose, and the DNA was partially digested with EcoRI, released into solution by agarase treatment of the agarose plugs, ligated into pYAC4, and transferred into yeast. Double screening of the yeast transformants with human and hamster genomic DNA allowed the selection of clones hybridizing only with human DNA. The library consists of 321 clones, amounting to 1.5 equivalents (61 Mb) of chromosome 21. The mean YAC size calculated from 178 clones is 190 +/- 100 kb. Screening of the library with eight sequence-tagged sites gave six positives. Among 21 YACs tested by in situ hybridization, 17 mapped to chromosome 21.