J Med Genet: first published as 10.1136/jmg.24.5.257 on 1 May 1987. Downloaded from Gene mapping and medical genetics Journal of Medical Genetics 1987, 24, 257-270 Molecular genetics of human chromosome 21 PAUL C WATKINS*, RUDOLPH E TANZIt, SHIRLEY V CHENGt, AND JAMES F GUSELLAt From *Integrated Genetics Inc, Framingham, Massachusetts 01701; and tNeurogenetics Laboratory, Massachusetts General Hospital, and Department of Genetics, Harvard Medical School, Boston, Massachusetts 02114, USA. SUMMARY Chromosome 21 is the smallest autosome, comprising only about 1-9% of human DNA, but represents one of the most intensively studied regions of the genome. Much of the interest in chromosome 21 can be attributed to its association with Down's syndrome, a genetic disorder that afflicts one in every 700 to 1000 newborns. Although only 17 genes have been assigned to chromosome 21, a very large number of cloned DNA segments of unknown function have been isolated and regionally mapped. The majority of these segments detect restriction fragment length polymorphisms (RFLPs) and therefore represent useful genetic markers. Continued molecular genetic investigation of chromosome 21 will be central to elucidating molecular events leading to meiotic non-disjunction and consequent trisomy, the contribution of specific genes to the pathology of Down's syndrome, and the possible role of chromosome 21 in Alzheimer's disease and other as yet unmapped genetic defects. Until recently, our knowledge of the organisation of expansioA of the number of characterised DNA http://jmg.bmj.com/ chromosome 21 DNA was limited to the level of segments for this chromosome. Similarly, the resolution afforded by cytogenetic approaches. construction of somatic cell hybrid lines containing Using a variety of staining techniques, the long arm translocated derivatives of chromosome 21 has has been subdivided into three bands, recognisable facilitated the regional'mapping of both genes and by their width and intensity of staining. With high DNA markers. These advances have led directly to resolution techniques applied to prometaphase the construction of preliminary physical and genetic chromosomes, some five to nine bands are dis- maps of the chromosome which promise to become tinguishable. By contrast, three distinct regions can increasingly detailed. As they evolve, collections of on October 1, 2021 by guest. Protected copyright. be identified on the short arm of chromosome 21. genetically and physically localised DNA segments The first is a highly fluorescent area near the will probably form the basis for attempts to develop telomere that exhibits extensive variation with a complete restriction map for chromosome 21, as a quinacrine staining. Proximal to this fluorescent prelude to determining the entire DNA sequence of staining region is a secondary constriction that stains the chromosome. In parallel with these efforts, the with silver and contains ribosomal RNA genes.' 2 knowledge gained from mapping approaches will Finally, there is a small area next to the centromere facilitate investigations of the molecular mechan- that is variable by C banding techniques and isms underlying the aetiology and pathogenesis of believed to contain highly repetitive DNA Down's syndrome and permit an evaluation of sequences.3 the potential role of chromosome 21 in numerous in- In the past few years, the necessary foundation for herited disorders, particularly familial Alzheimer's achieving a complete understanding of the structure disease. of chromosome 21 at the molecular level has begun to emerge. The availability of chromosome specific Genes assigned to chromosome 21 recombinant DNA libraries has resulted in a rapid Received for publication 19 January 1987. As with the other human chromosomes,4 genes have Accepted for publication 2(0 January 1987. been assigned to chromosome 21 by a number of 257 J Med Genet: first published as 10.1136/jmg.24.5.257 on 1 May 1987. Downloaded from 258 Paul C Watkins, Rudolph E Tanzi, Shirley V Cheng, and James F Gusella different approaches, particularly somatic cell properties of the NOR also appear to be associated genetics. In addition to conventional strategies, with an active functional state of the rDNA.9 A type however, lymphocytes and fibroblasts from Down's of NOR variant, designated double NOR (dNOR), syndrome patients have been used to show increased may play a role in promoting meiotic non- gene expression or enzyme activity for genes on disjunction, thus predisposing subjects to producing chromosome 21 whose products can be quantified. offspring with Down's syndrome. 1' To date, 17 genes have been assigned to chromo- Studies of species specific rDNA polymorphisms some 21 although 10 of these assignments remain of man and ape" have led to the suggestion that provisional. genetic exchanges occur between ribosomal genes on non-homologous acrocentric chromosomes. This THE RIBOSOMAL RNA GENE CLUSTER view is indirectly supported by evidence from The ribosomal RNA gene cluster on chromosome 21 cytological studies showing an association during (RNR4),l 2 like those on the other four acrocentric mitotic metaphase of the satellite region of different chromosome pairs, contains approximately 30 acrocentric chromosomes, 12-14 and by electron tandemly repeated rRNA gene copies5 per chromo- microscopic analysis of human meiotic oocytes some and is located in the secondary constriction, or showing that rRNA genes from different acrocen- stalk region, of the short arm, 21p126 (fig 1). The trics are structurally confined to the same nucleolar secondary constrictions are associated with cell fibrillar centre.15 The sites of unequal homologous nucleoli (nucleolar organising regions or NORs) and recombination leading to the 'concerted evolution' exhibit heritable,7 chromosome specific differences of the ribosomal gene family may be small DNA in affinity for ammoniacal silver stain.8 The staining repeat elements found near the 28S rRNA gene. 16 A Ip UN 724 pi220 13 pi2F4 pPW526-4R 12 ] RNR4 (rRNA) I24 S ] ~~pUN724 13 11.2 16 26 11.1 pPW265D CW21pc 46 http://jmg.bmj.com/ 11.1 I 48 11.2 61 4 4 2 9 6 21 27 10 7 52 59 20 54 62 22 58 63 30 4 65 31 36 67 15 on October 1, 2021 by guest. Protected copyright. _PRGS GARTF 37 SOD '7 38 iFNRA 22.1 25 50 18 19 IFNRB 22.2 ji 4445 55,55760 Ru--iTs --T _ BCE 49 22.3 23 53,56,71 _ 15 64. PFKL FIG 1 Location ofgenes and cloned DNA segments that are regionally assigned to chromosome 21. Genes and cloned DNA segments are designated by gene symbols assigned by the Human Gene Mapping Workshop Nomenclature Committee. / Cloned DNA segments are shown as numbers that make up the last part of the 'D alphanumeric' symbol (D21S..) used to identify the chromosome 21 origin ofthe DNA segments. Laboratory clone designations are usedfor DNA segments that have not yet been assigned 'D numbers'. Genes or DNA segments that detect polymorphisms are underlined. Brackets to the left ofgenes and DNA segments indicate the most likely location for the genes and DNA segments based on in situ hybridisation or somatic cell hybrid studies. In some cases, the assignment represents the shortest region ofoverlap that includes data from more than one study. See references 23, 35, 38, 65, 88, 109, 130, 137, 174, 175 for regional mapping genes or DNA segments by Southern analysis ofsomatic cell hybrids, or references 64, 83, 93 for localisation bv in situ hybridisation studies. J Med Genet: first published as 10.1136/jmg.24.5.257 on 1 May 1987. Downloaded from Gene mapping and medical genetics 259 tendency of the ribosomal genes to engage in non- homologous chromosome recombination has been proposed as a possible mechanism, via the for- mation of dicentric chromosomes, for promoting D21S13 meiotic non-disjunction. 7 SUPEROXIDE DISMUTASE 1 Two distinct forms of superoxide dismutase, the 15 enzyme responsible for catalysing the removal of D21S4 superoxide radicals, exist in eukaryotic cells: a mitochondrial manganese containing enzyme and a cytoplasmic copper/zinc containing enzyme. The :7J locus (SODJ) for the cytoplasmic Cu/Zn form D21SI/D21SIlI (SOD1) was one of the first gene assignments to chromosome 21.18 A SOD] cDNA clone has since been isolated,'9 characterised,2" and sequenced.2' Somatic cell hybrids were initially used to regionally D21 S8/D21Sl2 ]_ 16 map SOD] to band 21q22.22 The same approach has subsequently refined the localisation to 21q22.123 SOD (fig 1). Because SODI maps to the q22 band of the long arm, a region of the chromosome considered to be essential for the full expression of the Down's D21S58 syndrome phenotype, it is one of the candidate 18 genes considered for possible involvement in the pathogenesis of Down's syndrome. Subjects with D21S17 Down's syndrome show an increase of about 50% in D21S3/D21S23 SODI activity owing to higher levels of SOD1 protein,2"27 while similar studies have shown that monosomy 21 patients have the expected half normal level of SOD1 activity.28 An RFLP at the FIG 2 A genetic linkage map ofthe long arm of SOD] locus has been used to achieve the first chromosome 21. The marker order is the most likely order http://jmg.bmj.com/ localisation of a functional gene on the genetic based on an analysis offamilies that were jointly informative linkage map of chromosome 212131 (fig 2). for at least three ofthe markers. The genetic distance (indicated as percent recombination) between the markers is ENZYMES FOR PURINE BIOSYNTHESIS based on the results oftwo point analysis using the computer Phosphoribosylglycinamide synthetase (PRGS or programme LIPED. GARS), phosphoribosylaminoimidazole synthetase (PAIS or AIRS), and glycinamide ribonucleotide transformylase (GARTF) are responsible for the biosynthesis, GARTF, is also encoded by a gene on October 1, 2021 by guest. Protected copyright. catalysis of the third, fourth, and sixth steps, mapped to 21qll2-*q22-2 (fig 1) by studies that respectively, of the de novo purine biosynthetic raise the possibility that the three enzymes PRGS, pathway.