David R. Cox

Structure and variability of human chromosome ends.

Mammalian telomeres are thought to be composed of a tandem array of TTAGGG repeats. To further define the type and arrangement of sequences at the ends of human chromosomes, we developed a direct cloning strategy for telomere-associated DNA. The method involves a telomere enrichment procedure based on the relative lack of restriction endonuclease cutting sites near the ends of human chromosomes. Nineteen (TTAGGG)n-bearing plasmids were isolated, two of which contain additional human sequences proximal to the telomeric repeats. These telomere-flanking sequences detect BAL 31-sensitive loci and thus are located close to chromosome ends. One of the flanking regions is part of a subtelomeric repeat that is present at 10 to 25% of the chromosome ends in the human genome. This sequence is not conserved in rodent DNA and therefore should be a helpful tool for physical characterization of human chromosomes in human-rodent hybrid cell lines; some of the chromosomes that may be analyzed in this manner have been identified, i.e., 7, 16, 17, and 21. The minimal size of the subtelomeric repeat is 4 kilobases (kb); it shows a high frequency of restriction fragment length polymorphisms and undergoes extensive de novo methylation in somatic cells. Distal to the subtelomeric repeat, the chromosomes terminate in a long region (up to 14 kb) that may be entirely composed of TTAGGG repeats. This terminal segment is unusually variable. Although sperm telomeres are 10 to 14 kb long, telomeres in somatic cells are several kilobase pairs shorter and very heterogeneous in length. Additional telomere reduction occurs in primary tumors, indicating that somatic telomeres are unstable and may continuously lose sequences from their termini.

Segregation of the Huntington disease region of human chromosome 4 in a somatic cell hybrid

We have developed an X-irradiation:cell fusion procedure that segregates segments of human chromosomes lacking selectable markers and have used this approach to construct somatic cell hybrids retaining fragments of human chromosome 4 as the only human material. To identify hybrids retaining a small chromosomal fragment in the region of the Huntington disease (HD) gene, we used Southern blot analysis to screen 72 hybrid lines for the presence or absence of seven chromosome 4 single-copy loci. These data, combined with in situ hybridization experiments, identified three hybrids of interest. One of these cell lines, C25, stably retains a 10,000- to 20,000-kb fragment of distal 4p in the vicinity of the HD gene, translocated to a hamster chromosome. Field-inversion gel electrophoresis revealed no evidence of rearrangements in the human DNA present in C25. In combination with similar radiation hybrids, C25 is a valuable tool for isolating DNA probes near the HD gene.

Mouse trisomy 16 as an animal model of human trisomy 21 (Down syndrome): Production of viable trisomy 16 ↔ diploid mouse chimeras☆

We have previously proposed that mice trisomic for chromosome 16 will provide an animal model of human trisomy 21 (Down syndrome). However, the value of this model is limited to some extent because trisomy 16 mouse fetuses do not survive as live-born animals. Therefore, in an effort to produce viable mice with cells trisomic for chromosome 16, we have used an aggregation technique to generate trisomy 16 diploid (Ts 16 2n) chimeras. A total of 79 chimeric mice were produced, 11 of which were Ts 16 2n chimeras. Seven of these Ts 16 2n mice were analyzed as fetuses, just prior to birth, and 4 were analyzed as live-born animals. Unlike nonchimeric Ts 16 mouse fetuses which die shortly before birth with edema, congenital heart disease, and thymic and splenic hypoplasia, all but 1 of the Ts 16 2n animals were viable and phenotypically normal. The oldest of the live-born Ts 16 2n chimeras was 12 months old at the time of necropsy. Ts 16 cells, identified by coat color, enzyme marker, and/or karyotype analyses, comprised 50-60% of the brain, heart, lung, liver, and kidney in the 7 Ts 16 2n chimeric fetuses and 30-40% of these organs in the 4 live-born Ts 16 2n animals. Ts 16 cells comprised an average of 40% of the thymus and 80% of the spleen in the Ts 16 2n chimeras analyzed as fetuses, with no evidence of thymic or splenic hypoplasia. However, we observed a marked deficiency to Ts 16 cells in the blood, spleen, thymus, and bone marrow of live-born Ts 16 2n chimeras as compared to 2n 2n controls. These results demonstrate that although the Ts 16 2n chimeras were, with one exception, viable and phenotypically normal, each animal contained a significant proportion of trisomic cells in a variety of tissues, including the brain. Furthermore, our results suggest that although the abnormal development of Ts 16 thymus and spleen cells observed in Ts 16 fetuses is largely corrected in Ts 16 2n fetuses, Ts 16 erythroid and lymphoid cells have a severe proliferative disadvantage as compared to diploid cells in older live-born Ts 16 2n chimeras. Ts 16 2n chimeric mice will provide a valuable tool for studying the functional consequences of aneuploidy and may provide insight into the mechanisms by which trisomy 21 leads to developmental abnormalities in man.

Comparative Gene Mapping of Human Chromosome 21 and Mouse Chromosome 16

Trisomy 21 is the most common chromosomal abnormality found in human newborn infants as well as the single most common cause of human mental retardation and congenital heart disease.’ All individuals with trisomy 21 are mentally retarded, whereas approximately 50% have a congenital heart lesion, most commonly an endocardia1 cushion defect. In addition, trisomy 21 results in abnormal thymic and hematopoietic development as well as an altered immune response, which may be related to the increased incidences of infection and leukemia associated with this Cellular sensitivity to radiation has also been claimed to be increased in persons with trisomy 21.6 Neuropathologic changes consisting of neurofibrillary tangles, senile plaques, and neuronal loss are found in the brains of many patients dying after 35 years of age, and these changes, which are indistinguishable from those seen in Alzheimer’s disease, are often associated with clinical features of presenile dementia.’ Finally, trisomy 21 results in a characteristic pattern of dysmorphic features that allows this disorder to be diagnosed clinically.2 The consistent pattern of structural and functional abnormalities outlined above regularly occurs in persons with trisomy 21 and is known as Down syndrome. Little is known concerning the mechanisms by which trisomy for chromosome 21 interferes with normal development and function to produce the various features of Down syndrome. Since trisomy 21 results in a pattern of developmental abnormalities that is distinct from that of other human trisomies, such as trisomy 13 or trisomy 18, it seems reasonable to assume that the features of Down syndrome are due to imbalance of specific loci on human

Deletion of chromosome 21 and normal intelligence : molecular definition of the lesion

SummaryApplication of a method for the fine structure analysis of unbalanced chromosomal rearrangements using quantitative Southern blot analysis has established that an individual of normal intelligence and largely normal appearance has a significant interstitial deletion of chromosome 21. Using high resolution cytogenetic analysis and molecular analysis with five single copy DNA sequences unique to chromosome 21 and a probe for human SOD1 (CuZn, Superoxide dismutase), we find that the deletion extends from the border of bands 21q11.1–11.2 and extends to the border of bands 21q21.2–q21.3. The latter border is established molecularly by the presence of two copies of SOD1, previously mapped to band 21q22.1, and of four single copy sequences known to be located distal to this region. The presence of SOD1 was confirmed by enzyme dosage analysis. These findings demonstrate that deletion of close to 20,000kb of autosomal material is compatible with normal intelligence. Further, they suggest that chromosome 21 may include a large region of relative developmental neutrality whose molecular basis may now be investigated. Because of the limits of even high resolution cytogenetic analysis, fine structure molecular analyses of this type will be necessary to reliably detect and define similar small chromosomal deletions or insertions. The molecular definition of such aneuploidy provides the basis for increasing the resolution of the human physical genetic map.

The mouse homolog of the human amyloid β protein (AD-AP) gene is located on the distal end of mouse chromosome 16: Further extension of the homology between human chromosome 21 and mouse chromosome 16

The human amyloid beta protein is the major constituent of the brain amyloid plaques found in Alzheimer disease. The gene that encodes this protein is located on chromosome 21, and individuals with Down syndrome (trisomy 21) also exhibit an early onset form of Alzheimer disease. We have used the cloned human amyloid beta protein gene and a panel of somatic cell hybrids to map the location of the mouse homolog of this gene. We report here that the mouse gene is located on chromosome 16 within the region 16C3----ter, in common with three other genes which map within the Down syndrome region of human chromosome 21.

Isolation and field-inversion gel electrophoresis analysis of DNA markers located close to the Huntington disease gene

A radiation-induced hybrid cell line containing 10-20 million base pairs of DNA derived from the terminal part of human 4p16 in a background of hamster chromosomes has been used to construct a genomic library highly enriched for human sequences located close to the Huntington disease (HD) gene. Recombinant phage containing human inserts were isolated from this library and used as hybridization probes against two other radiation hybrids containing human fragments with chromosomal breaks in 4p16 and against a human-hamster somatic cell hybrid that retains only the 4p15-4pter part of chromosome 4. Of 121 human phage tested, 6 were mapped distal to the HD-linked D4S10 locus. Since the HD gene is located between D4S10 and the 4p telomere, all of these sequences are likely to be closer to HD than D4S10, and any one of them may be a distal flanking marker for the disease locus. Long-range restriction map analysis performed with a field-inversion gel system shows that the six new loci are distributed in different places within 4p16. Although it is not possible to establish an order for the six sequences with the FIGE data, the results demonstrate that the region detected by these probes must span at least 2000 kb of DNA.

Recent evolution of DNA sequence homology in the pericentromeric regions of human acrocentric chromosomes

A search for genes located on human chromosome 21 resulted in the isolation of a HeLa cDNA clone, pUNC724, which hybridized to 3.7 and 2.5 kilobase (kb) EcoRI fragments on each of the human acrocentric chromosomes. In situ hybridization further localized pUNC724 to the pericentromeric region of the human acrocentrics. Two other EcoRI fragments that hybridized to pUNC724 were assigned to the long arms of chromosomes 1 and 18. The pUNC724 sequence does not appear to be related to ribosomal or satellite DNA sequences. The juxtaposition of DNA sequences homologous to pUNC724 and ribosomal DNA sequences presumably occurred within the past thirty-five million years, following the divergence of the lines leading to man and the New World owl monkey, Aotus trivirgatus--pUNC724 is not syntenic with the single chromosome containing ribosomal DNA sequences in the owl monkey.

Trisomy 21: Mechanisms and Models*

associated with autosomal trisomy, is to understand how the presence of an extra set of normal genes has an adverse effect on development and function. In our thinking about the mechanisms by which trisomy produces its consequences we have found i t convenient to divide the effects of the trisomic state into those which we consider primary and those regarded as secondary.1.2 By primary effects we refer to the gene dosage effects by which the presence of an extra copy of a gene results in the production of a commensurately increased amount of the primary gene product -in virtually all instances a protein. There are now numerous examples of such gene dosage effects in human and other mammalian aneuploid states, including human trisomy 21. Of the genes now mapped to human chromosome 21, direct (primary) gene dosage effects have been demonstrated for two, superoxide dismutase-1 (S00-7)3,4 and phosphoribosylglycinamide synthetase (PRGS).S,b In each instance, the activity of the enzymes in both erythrocytes and fibroblasts have been T the pathogenesis of Downs syndrome, or of any of the conditions

The murine Hox-2 cluster of homeo box containing genes maps distal on chromosome 11 near the tail-short (Ts) locus

Two probes derived from a mouse recombinant lambda-clone (H24.1), that contains a sequence closely homologous to the Drosophila antennapedia homeo box, were mapped to mouse chromosome (MMU) 11 by filter hybridization of somatic cell hybrid DNA. This sequence is highly homologous to a human homeo box gene (HOX2) and appears to represent one of the two genes in the Hox-2 cluster previously assigned to MMU 11. To regionally map the Hox-2 cluster, we have carried out in situ hybridization of the two H24.1 probes and of an independently isolated Hox-2 probe. The autoradiographic silver grain distributions were similar in all three experiments with a peak over band 11D. This region contains the locus for the tail-short (Ts) mutation which causes skeletal abnormalities in heterozygotes and early embryonic death in homozygotes.