Dinucleotide repeat (GT)n markers on chromosome 21

Genetic and physical mapping of the GLUR5 glutamate receptor gene on human chromosome 21

Glutamate receptors (GluRs) mediate excitatory neurotransmission and may have important roles in central nervous system disorders. To characterize the human GLUR5 gene, which is located on human chromosome 21q22.1, we isolated cDNAs, genomic phage lambda clones, and yeast artificial chromosomes (YACs) and developed sequence tagged sites (STSs) and simple sequence length polymorphisms (SSLPs) for GLUR5. Genetic mapping with a tetranucleotide AGAT repeat named GLUR5/AGAT (six alleles observed, 70% heterozygosity) placed GLUR5 5 cM telomeric to APP (D21S210) and 3 cM centromeric to SOD1 (D21S223). The human GLUR5 gene is located near the familial amyotrophic lateral sclerosis (FALS) locus; linkage analysis of GLUR5 SSLPs in FALS pedigrees yielded negative lod scores, consistent with the recent association of the FALS locus with the SOD1 gene. Physical mapping of GLUR5 using a YAC contig suggested that the GLUR5 gene spans approximately 400-500kb, and is within 280kb of D21S213. The large size of the GLUR5 gene raises questions regarding its functional significance. Our GLUR5 YAC contig includes clones found in the Genethon chromosome 21 YAC contig, and reference to the larger contig indicates the orientation centromere--D21S213-GLUR5 5' end-GLUR5/AGAT--GLUR5 3' end--SOD1. The development of GLUR5/AGAT should permit rapid determination of the status of the GLUR5 gene in individuals with partial trisomy or monosomy of chromosome 21. Such studies may provide insights concerning the possible role of GLUR5 in Down syndrome.

Identification of flanking markers for the familial amyotrophic lateral sclerosis gene ALS1 on chromosome 21

Amyotrophic lateral sclerosis (ALS) is a progressive, adult-onset, neurodegenerative disorder characterized by the death of large motor neurons from the cerebral cortex, brainstem, and spinal cord. The etiology of ALS remains unknown; however, approximately 10% of the cases are familial in nature. In the majority of these families, the mode of transmission is autosomal dominant. Recently, linkage of an autosomal dominant familial ALS (FALS) gene to the locus ALS1 on chromosome 21q was established. In addition, evidence was provided for genetic heterogeneity, with approximately 55% of families most likely linked to chromosome 21. The development of a number of highly informative simple sequence repeat polymorphisms in the region of linkage-21q21 through 21q22.1-has permitted us to confirm both the assignment of ALS1 to 21q and the genetic heterogeneity of FALS. In addition, we have been able to refine the mapping of ALS1, based on recombination events in two of the linked families. Flanking markers for the FALS gene are D21S213 on the centromeric side and D21S219 on the telomeric side. The candidate region is approximately 4 Mb and contains the genes copper/zinc superoxide dismutase (CuZnSOD); the fourth member of the class II cytokine receptor family (CRF2-4); and the interferon-alpha receptor (IFNAR).

Genetic linkage analysis of familial amyotrophic lateral sclerosis using human chromosome 21 microsatellite DNA markers

Amyotrophic lateral sclerosis (ALS: Lou Gehrig's Disease) is a lethal neurodegenerative disease of upper and lower motorneurons in the brain and spinal cord. We previously reported linkage of a gene for familial ALS (FALS) to human chromosome 21 using 4 restriction fragment length polymorphism DNA markers [Siddique et al.: N Engl J Med 324:1381-1384, 1991] and identified disease-associated mutations in the superoxide dismutase (SOD)-1 gene in some ALS families [Rosen et al.: Nature 362:59-62, 1993]. We report here the genetic linkage data that led us to examine the SOD-1 gene for mutations. We also report a new microsatellite DNA marker for D21S63, derived from the cosmid PW517 [VanKeuren et al.: Am J Hum Genet 38:793-804, 1986]. Ten microsatellite DNA markers, including the new marker D21S63, were used to reinvestigate linkage of FALS to chromosome 21. Genetic linkage analysis performed with 13 ALS families for these 10 DNA markers confirmed the presence of a FALS gene on chromosome 21. The highest total 2-point LOD score for all families was 4.33, obtained at a distance of 10 cM from the marker D21S223. For 5 ALS families linked to chromosome 21, a peak 2-point LOD score of 5.94 was obtained at the DNA marker D21S223. A multipoint score of 6.50 was obtained with the markers D21S213, D21S223, D21S167, and FALS for 5 chromosome 21-linked ALS families. The haplotypes of these families for the 10 DNA markers revealed recombination events that further refined the location of the FALS gene to a segment of approximately 5 megabases (Mb) between D21S213 and D21S219.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular analysis to assign parental origin and distinguish de novo i(21q) from t(21q21q) in two Down syndrome fetuses

OBJECTIVE

We sought to determine the origin of two prenatal cases of chromosome 21 rearrangements not amenable to clarification by conventional cytogenetic methodology.

METHODS

Hypervariable repeat polymorphisms (chromosome 21) were used to determine the type of structural rearrangement and the parental origin of the rearranged chromosome. The repeats used were highly polymorphic and located very close to the centromere; thus, the likelihood of differences among the parental alleles and overall informativeness were increased.

RESULTS

The rea(21q21q) chromosomes were identified as a Robertsonian translocation in one fetus and an isochromosome in the other. The extra chromosome material was found to be maternal in origin in both cases.

CONCLUSION

The ability to clarify the origin of abnormal chromosomal rearrangements provides valuable information concerning possible mechanisms of aneuploidy, as well as clinical data that may have an impact in assessing a patient's risk for abnormal offspring.