Partial trisomy 21

Modeling Down syndrome in animals from the early stage to the 4.0 models and next

The genotype-phenotype relationship and the physiopathology of Down Syndrome (DS) have been explored in the last 20 years with more and more relevant mouse models. From the early age of transgenesis to the new CRISPR/CAS9-derived chromosomal engineering and the transchromosomic technologies, mouse models have been key to identify homologous genes or entire regions homologous to the human chromosome 21 that are necessary or sufficient to induce DS features, to investigate the complexity of the genetic interactions that are involved in DS and to explore therapeutic strategies. In this review we report the new developments made, how genomic data and new genetic tools have deeply changed our way of making models, extended our panel of animal models, and increased our understanding of the neurobiology of the disease. But even if we have made an incredible progress which promises to make DS a curable condition, we are facing new research challenges to nurture our knowledge of DS pathophysiology as a neurodevelopmental disorder with many comorbidities during ageing.

Genotype-phenotype correlation for congenital heart disease in Down syndrome through analysis of partial trisomy 21 cases

Among Down syndrome (DS) children, 40-50% have congenital heart disease (CHD). Although trisomy 21 is not sufficient to cause CHD, three copies of at least part of chromosome 21 (Hsa21) increases the risk for CHD. In order to establish a genotype-phenotype correlation for CHD in DS, we built an integrated Hsa21 map of all described partial trisomy 21 (PT21) cases with sufficient indications regarding presence or absence of CHD (n=107), focusing on DS PT21 cases. We suggest a DS CHD candidate region on 21q22.2 (0.96Mb), being shared by most PT21 cases with CHD and containing three known protein-coding genes (DSCAM, BACE2, PLAC4) and four known non-coding RNAs (DSCAM-AS1, DSCAM-IT1, LINC00323, MIR3197). The characterization of a DS CHD candidate region provides a useful approach to identify specific genes contributing to the pathology and to orient further investigations and possibly more effective therapy in relation to the multifactorial pathogenesis of CHD.

Co-expression network analysis of Down's syndrome based on microarray data

Down's syndrome (DS) is a type of chromosome disease. The present study aimed to explore the underlying molecular mechanisms of DS. GSE5390 microarray data downloaded from the gene expression omnibus database was used to identify differentially expressed genes (DEGs) in DS. Pathway enrichment analysis of the DEGs was performed, followed by co-expression network construction. Significant differential modules were mined by mutual information, followed by functional analysis. The accuracy of sample classification for the significant differential modules of DEGs was evaluated by leave-one-out cross-validation. A total of 997 DEGs, including 638 upregulated and 359 downregulated genes, were identified. Upregulated DEGs were enriched in 15 pathways, such as cell adhesion molecules, whereas downregulated DEGs were enriched in maturity onset diabetes of the young. Three significant differential modules with the highest discriminative scores (mutual information>0.35) were selected from a co-expression network. The classification accuracy of GSE16677 expression profile samples was 54.55% and 72.73% when characterized by 12 DEGs and 3 significant differential modules, respectively. Genes in significant differential modules were significantly enriched in 5 functions, including the endoplasmic reticulum (P=0.018) and regulation of apoptosis (P=0.061). The identified DEGs, in particular the 12 DEGs in the significant differential modules, such as B-cell lymphoma 2-associated transcription factor 1, heat shock protein 90 kDa beta member 1, UBX domain-containing protein 2 and transmembrane protein 50B, may serve important roles in the pathogenesis of DS.

Systematic reanalysis of partial trisomy 21 cases with or without Down syndrome suggests a small region on 21q22.13 as critical to the phenotype

A 'Down Syndrome critical region' (DSCR) sufficient to induce the most constant phenotypes of Down syndrome (DS) had been identified by studying partial (segmental) trisomy 21 (PT21) as an interval of 0.6-8.3 Mb within human chromosome 21 (Hsa21), although its existence was later questioned. We propose an innovative, systematic reanalysis of all described PT21 cases (from 1973 to 2015). In particular, we built an integrated, comparative map from 125 cases with or without DS fulfilling stringent cytogenetic and clinical criteria. The map allowed to define or exclude as candidates for DS fine Hsa21 sequence intervals, also integrating duplication copy number variants (CNVs) data. A highly restricted DSCR (HR-DSCR) of only 34 kb on distal 21q22.13 has been identified as the minimal region whose duplication is shared by all DS subjects and is absent in all non-DS subjects. Also being spared by any duplication CNV in healthy subjects, HR-DSCR is proposed as a candidate for the typical DS features, the intellectual disability and some facial phenotypes. HR-DSCR contains no known gene and has relevant homology only to the chimpanzee genome. Searching for HR-DSCR functional loci might become a priority for understanding the fundamental genotype-phenotype relationships in DS.

Clinical, cytogenetic, and molecular characterization of a girl with some clinical features of Down syndrome resulting from a pure partial trisomy 21q22.11-qter due to a de novo intrachromosomal duplication

We report a girl with a de novo pure partial trisomy 21 with some clinical features of Down syndrome. The girl patient presented a flat broad face, brachycephaly, and a flat nasal bridge. She also had upwardly slanted palpebral fissures, epicanthal folds, blepharitis, brushfield spots, and strabismus. Her mouth was wide with downturned corners, prominent lower lip, narrow and furrowed tongue, and short palate. G-banded chromosomal analysis of metaphases in cells from both skin and blood showed a 46,XX karyotype with additional chromosomal material on the distal short arm of one chromosome 21. Parental chromosomes were normal. Molecular analyses with the short-tandem-repeat (STR) marker D21S2039 (interferon-alpha/beta receptor [IFNAR]) (21q22.1) showed a triallelic pattern. Subtelomeric fluorescent in situ hybridization (FISH) analyses, LSI 13 (retinoblastoma 1 [RB1])/LSI 21(21q22.13-q22.2), and whole chromosome painting probes specific for chromosome 21 showed trisomy for the segment 21q22.13-21q22.2 due to a de novo intrachromosomal duplication. A 500K SNP microarray analysis was then performed and revealed a 13-Mb duplication of 21q22.11-qter. This duplicated material had been translocated onto the end of the "p" arm of one of the chromosome 21s. The karyotype was provisionally defined as 46,XX,add(21)(p12).ish der (21)t(21;21)(p12;q22.11)(WCP21q+,PCP21q++,D215259/D21S341/D21S342++)dn. At the age of 4 years and 10 months, a comprehensive psychological examination was performed and the diagnostic criteria for mental retardation were not fulfilled. In comparison with previously published cases of pure partial trisomy 21, this is a rare finding. Additional studies of such rare patients should aid in the study of the pathogenesis of Down syndrome.

Molecular genetic analysis of Down syndrome

Down syndrome (DS) is caused by trisomy of all or part of human chromosome 21 (HSA21) and is the most common genetic cause of significant intellectual disability. In addition to intellectual disability, many other health problems, such as congenital heart disease, Alzheimer's disease, leukemia, hypotonia, motor disorders, and various physical anomalies occur at an elevated frequency in people with DS. On the other hand, people with DS seem to be at a decreased risk of certain cancers and perhaps of atherosclerosis. There is wide variability in the phenotypes associated with DS. Although ultimately the phenotypes of DS must be due to trisomy of HSA21, the genetic mechanisms by which the phenotypes arise are not understood. The recent recognition that there are many genetically active elements that do not encode proteins makes the situation more complex. Additional complexity may exist due to possible epigenetic changes that may act differently in DS. Numerous mouse models with features reminiscent of those seen in individuals with DS have been produced and studied in some depth, and these have added considerable insight into possible genetic mechanisms behind some of the phenotypes. These mouse models allow experimental approaches, including attempts at therapy, that are not possible in humans. Progress in understanding the genetic mechanisms by which trisomy of HSA21 leads to DS is the subject of this review.

Familial 4.3 Mb duplication of 21q22 sheds new light on the Down syndrome critical region

A 4.3 Mb duplication of chromosome 21 bands q22.13-q22.2 was diagnosed by interphase fluorescent in situ hybridisation (FISH) in a 31 week gestational age baby with cystic hygroma and hydrops; the duplication was later found in the mother and in her 8-year-old daughter. All had the facial gestalt of Down syndrome (DS). This is the smallest accurately defined duplication of chromosome 21 reported with a DS phenotype. The duplication encompasses the gene DYRK1 but not DSCR1 or DSCAM. Previous karyotype analysis and telomere screening of the mother, and karyotype analysis and metaphase FISH of a chorionic villus sample, had all failed to reveal the duplication. The findings in this family add to the identification and delineation of a "critical region" for the DS phenotype on chromosome 21.

Familial 4.3 Mb duplication of 21q22 sheds new light on the Down syndrome critical region

A 4.3 Mb duplication of chromosome 21 bands q22.13-q22.2 was diagnosed by interphase fluorescent in-situ hybridisation (FISH) in a 31-week gestational age baby with cystic hygroma and hydrops; the duplication was later found in the mother and in her 8-year-old daughter by the same method and confirmed by array comparative genomic hybridisation (aCGH). All had the facial gestalt of Down syndrome (DS). This is the smallest accurately defined duplication of chromosome 21 reported with a DS phenotype. The duplication encompasses the gene DYRK1 but not DSCR1 or DSCAM, all of which have previously been implicated in the causation of DS. Previous karyotype analysis and telomere screening of the mother, and karyotype analysis and metaphase FISH of a chorionic villus sample, had all failed to reveal the duplication. The findings in this family add to the identification and delineation of a "critical region" for the DS phenotype on chromosome 21. Cryptic chromosomal abnormalities can be missed on a routine karyotype for investigation of abnormal prenatal ultrasound findings, lending support to the use of aCGH analysis in this setting.

Clinical, cytogenetic, and molecular genetic characterization of two unrelated patients with different duplications of 21q

We present 2 patients with dup(21q). Patient MP01 had mild mental retardation, facial findings characteristic of Down syndrome (DS), and a terminal duplication of chromosome 21. His karyotype was 46,XY,dup(21) (q22.1-qter). Patient MP03 had mild mental retardation, minor anomalies not characteristic of DS, and a duplication of the proximal long arm of chromosome 21, karyotype 46,XX,dup(21) (q11.2-q21.2). The patients were studied with single-copy DNA sequences from 20 loci on chromosome 21 to characterize the extent of the duplicated regions at the DNA level. DNA loci from D21S55 to COL6A1 were triplicated in patient MP01 while loci from D21S13 to D21S8 were triplicated in patient MP03. Our results support the hypothesis of a critical region of chromosome 21, which in triplicate is responsible for many of the facial changes associated with DS. Other genes outside this region may also contribute to other abnormalities observed in DS.

Down syndrome critical region around D21S55 on proximal 21q22.3

We have analysed the DNA of 2 patients with many manifestations of Down syndrome and partial duplication of distinct regions of chromosome 21, respectively, q11.205----q22.300 and q22.300----qter (Rahmani et al.: Proceedings of the National Academy of Sciences of the United States of America 86:5958-5962, 1989). Assessment of the copy number of five chromosome 21 sequences (SOD1, D21S17, D21S55, ETS2, and D21S15) has shown that D21S55 was duplicated in both cases. The size of the common duplicated region can be estimated between 400 and 3,000 Kb, after the results of pulsed-field gel analysis and from the knowledge of regional mapping of the probes D21S17, D21S55, and ETS2. This region, located on the proximal part of 21q22.3, is postulated to contain genes the overexpression of which plays a major role in the pathogenesis of Down syndrome.

Sex-specific chromosome instability in early human development

The predominance of females segregating chromosome aberrations to their offspring has been explained mostly by selection disadvantage of unbalanced products of spermatogenesis. However, analysis of data from the literature supports the idea that somatic cells of early female embryos are similar to female germ cells in that they are prone to malsegregation. The goal of this study was to compare the sex ratio (male to female ratio) of carriers of presumably mitotic-occurring chromosome abnormalities to identify any sex biases. In examining the literature, we found a female prevalence in cases of mosaicism associated with uniparental disomy (UPD) (26 male individuals/conceptions and 45 female individuals/conceptions, sex ratio is 0.58, significantly different from 1.06 in newborn population, P = 0.0292). This predominance was highest at gestational age <16 week (8 male and 22 female conceptuses, sex ratio is 0.36, significantly different from expected figure of 1.28, P = 0.0025), which diminished at later stages of fetal development indicating potential correction of trisomies predominantly in females. There is a threefold prevalence of 46,XX/45,X mosaics over 46,XY/45,X mosaics in prenatally diagnosed cases, which also suggests a gender-specific postzygotic chromosome loss. The male prevalence in Prader-Willi syndrome with maternal UPD of chromosome 15 also can be explained by sex-specific trisomy correction, with predominant loss of a maternal chromosome causing biparental inheritance and therefore, complete correction of trisomy in females (without UPD). Finally, there is a female predominance in carriers of chromosome rearrangement with pericentromere break (mosaicism for Robertsonian translocation/isochromosome, centric fission, nonacrocentric isochromosome, and whole arm rearrangement), in both prenatal (21 males and 36 females, sex ratio is 0.58, P < 0.0184) and postnatal ill-defined cases (14 males and 35 females, sex ratio is 0.40, P = 0.001). Thus, the findings presented in this paper suggest that, in addition to reduction in male fertility, and to probable selection against abnormal cell line(s), there are two mechanisms that contribute to female preponderance among carriers of mosaicism: sex-specific chromosome loss and sex-specific centromere instability. The data obtained suggest that females may have gonadal mosaicism for aneuploidies and structural rearrangements more often than males. This may lead to the maternal origin bias in offspring with trisomies or structural rearrangements.

Discordant prenatal diagnosis of trisomy 21 due to mosaic structural rearrangements of chromosome 21

OBJECTIVES

Trisomy 21 mosaicism associated with a structural rearrangement of chromosome 21 is uncommon. We report on two prenatal diagnoses in which karyotypes showed mosaicism with an aberrant cell line, including a structural rearrangement of chromosome 21.

METHODS

Both these cases were associated with increased nuchal translucency. Conventional and molecular cytogenetic analyses were performed on uncultured and cultured trophoblast and amniotic fluid cells.

RESULTS

In Case 1, analysis of trophoblast cells revealed an abnormal karyotype of 47,XX,+mar.ish der(13/21)(D13Z1/D21Z1+)/46,XX. The amniocentesis showed a free non-mosaic trisomy 21. In Case 2, the trophoblast direct analysis showed a normal male karyotype whereas the long-term culture revealed a mosaicism for a dicentric long-arm isochromosome 21: 46,XY,idic(21)(p11)/45,XY,-21/46,XY. Amniocentesis showed an unbalanced non-mosaic karyotype 46,XY,idic(21)(p11) resulting therefore in trisomy for the long arm of chromosome 21.

CONCLUSION

Our cases underline the importance of combining the direct analysis and long-term culture of trophoblast and emphasise the need for confirmatory studies in other tissues when mosaicism of structural rearrangement is encountered in chorionic villi. The meiotic and mitotic mechanisms of formation of these structural rearrangements of chromosome 21 are discussed.

Tertiary trisomy due to a reciprocal translocation of chromosomes 5 and 21 in a four-generation family

Tertiary trisomy, or double trisomy, is a rare occurrence. We present two individuals with a previously unreported tertiary trisomy for chromosomes 5p and 21q in an eight-generation pedigree. Their phenotypes are compared with other partial trisomies of either 5p or 21q from the literature. The propositus was diagnosed with trisomy 21 at 2 years of age after a karyotype study for short stature and developmental delay. His phenotype was described as atypical for Down syndrome. He presented at 9 years of age because of pervasive behavioral problems and obesity. He was brachycephalic with a flattened nasal bridge, but he lacked other characteristics of trisomy 21. Because of lack of phenotypic evidence of Down syndrome, a repeat karyotype was obtained and showed 47,XY, +der(21)t(5;21)(p15.1; q22.1), incorporating partial trisomies of both chromosomes 5 and 21. Mother had a balanced translocation, 46, XX,t(5;21)(p15.1; q22.1); 8 other relatives were examined. The translocation originated from the maternal great-grandmother, but only the propositus and his mentally retarded aunt had a similar phenotye and the derivative chromosome. Fluorescence in situ hybridization showed absence of band 21q22.2 in the derivative chromosome of the propositus and his aunt, indicating that neither had trisomy for the Down syndrome critical region. These cases represent a unique double partial trisomy of chromosome arms 5p and 21q that occurred because of 3:1 malsegregation of a reciprocal translocation. These cases further demonstrate that phenotypic discordance with cytogenetic results dictate further investigation using advanced cytogenetic hybridization.

The Down syndrome critical region

Since the early 1970's numerous attempts have been made to learn whether specific segments of chromosome 21, when triplicated, are responsible for the clinical condition Down syndrome (DS). Studies were reported in which positive or negative clinical diagnoses of DS were made in the presence of partial trisomy of one or another segment of the chromosome. The distal half of the long arm of 21 (21q22) possesses most of the gene transcribing sites of the chromosome. It was this region that was thought to contain loci essential to production of the clinical syndrome. Subsequent studies identified subregions of this band as "minimal" or "critical" sites necessary and sufficient to produce the clinical condition. A major problem with these assignments was that different investigators defined different critical/minimal regions. In 1994 evidence was presented in which regions of most of the long arm of chromosome 21 were said to contribute to the DS phenotype. Soon after, a report described a child with DS and partial tetrasomy of the short arm and proximal long arm of 21, segments clearly distinct from the previously identified critical areas. Thus the clinical diagnosis of DS can be made in the presence of partial aneuploidy of nearly all segments of chromosome 21. It must be concluded that no evidence exists that individual loci on 21 are singularly responsible for specific phenotypic abnormalities in DS. Without exception, each of the clinical findings associated with DS is a multifactorial trait. The analysis of each trait in DS should thus be similar to analyses of the same traits in the general population with a focus on the way aneuploidy affects expression of multifactorial characteristics.

Overexpression of copper-zinc superoxide dismutase in trisomy 21

Down's syndrome (DS), the most frequent of congenital birth defects, results from the trisomy of chromosome 21 in all cells of affected patients. This disease is characterized by developmental anomalies, mental retardation and features of rapid aging, particularly in the brain, where the occurrence of Alzheimer's disease is observed in trisomy 21 patients over the age of 35. Copper-zinc superoxide dismutase (CuZnSOD) is one of the proteins encoded by chromosome 21 (21q22.1). As a consequence of gene dosage excess, CuZnSOD activity is increased by 50% in all DS tissues. This work reports the SOD activity of a population of DS patients with complete trisomy 21, partial trisomy 21, translocations and mosaicism, in order to confirm the gene dosage effect of SOD on the clinical features of DS, and to help to establish which is the critical region of chromosome 21 in DS. CuZnSOD was measured in red blood cells using the Minami and Yoshikawa method. In the population with complete trisomy 21, SOD activity was increased by 42%; in the population with partial trisomy 21, translocations and mosaicism, SOD activity was normal. In the population diagnosed as DS, but not karyotyped, SOD activity was increased by 28%. No differences between sexes or among ages were found. We conclude that the 21q22.1 segment is not the critical region responsible for DS, as we have found normal SOD activity in patients with the clinical features of DS.

Mapping of the Down syndrome phenotype on chromosome 21 at the molecular level

Phenotypic and molecular analysis of individuals with partial trisomy 21 can be used to determine which regions of chromosome 21 are involved in the pathogenesis of specific features of Down's Syndrome. Using dosage analysis of 27 sequences we defined, at the molecular level, the extent of the chromosome 21 duplication in ten individuals with partial trisomy 21. Phenotype-genotype correlations led to the definition of minimal regions, the duplications of which are linked to the expression of 23 clinical features of Down's Syndrome. The D21S55 region or Down's Syndrome Chromosome Region 1 (DCR1) (1/20 of the long arm), on 21q22.2-21q22.3 proximal, is involved in four cardinal features of the disease: mental retardation, growth retardation, muscular hypotonia and joint hyperlaxity, and in eight of the 18 more common morphological anomalies of the face, hands and feet. Overlapping the DCR1, the D21S55-MX1 region or DCR2 (1/10 of the long arm), spanning 21q21.2 down to the 1/4th proximal part of 21q22.3, is involved in the features defined by the DCR1 plus congenital heart defect and five additional morphological anomalies. Thus, our results indicate that duplication of a relatively small region of chromosome 21 plays a critical role in the pathogenesis of the Down's phenotype.

Critical role of the D21S55 region on chromosome 21 in the pathogenesis of Down syndrome

The duplication of a specific region of chromosome 21 could be responsible for the main features of Down syndrome. To define and localize this region, we analyzed at the molecular level the DNA of two patients with partial duplication of chromosome 21. These patients belong to two groups of Down syndrome patients characterized by different partial trisomies 21: (i) duplication of the long arm, proximal to 21q22.2, and (ii) duplication of the end of the chromosome, distal to 21q22.2 We assessed the copy number of five chromosome 21 sequences (SOD1, D21S17, D21S55, ETS2, and D21S15) and found that D21S55 was duplicated in both cases. By means of pulsed-field gel analysis and with the knowledge of regional mapping of the probes D21S17, D21S55 and ETS2, we estimated the size of the common duplicated region to be between 400 and 3000 kilobases. This region, localized on the proximal part of 21q22.3, is suspected to contain genes the overexpression of which is crucial in the pathogenesis of Down syndrome.

Trisomy 21q223 and Down's phenotype correlation evidenced by in situ hybridization

Two cases of trisomy 21q223 with the Down's phenotype were analysed by in situ hybridization with specific probes previously located in the sub-bands 21q221 (SOD-A) and 21q223 (BCEI and COL6A). These studies give evidence that the clinical picture of Down's syndrome is at least to a great extent correlated with trisomy for the 21q223 band.

Unusual segregation in a family with a 11/21 translocation

A familial 11/21 translocation is described where the proband has an unbalanced translocation and the oldest translocation carrier shows mosaicism with a partial trisomy no. 21.

Free proximal trisomy 21 without the Down syndrome

Analysis of partial duplication of chromosome 21 suggests that band 21q22 contains determinants for the Down syndrome. We report two cases of free proximal trisomy 21 without manifestations of the Down syndrome. Phenotypic anomalies included marked microcephaly, short stature, hypoplastic nails, and mental retardation/developmental delay. Our cases are consistent with the assignment of band 21q22 as the causal duplicated segment in the Down syndrome.

Submicroscopic duplication of chromosome 21 and trisomy 21 phenotype (Down syndrome)

A patient with the phenotype of trisomy 21 (Down syndrome) was found to have a normal karyotype in blood lymphocytes and fibroblasts. Assessment of the chromosome 21 markers SOD1, CBS, ETS2, D21S11, and BCEI showed partial trisomy by duplication of a chromosome segment carrying the SOD1, CBS, and ETS2 loci and flanked by the BCEI and D21S11 loci, which are not duplicated. This submicroscopic duplication at the interface of 21q21 and 21q22.1 reduces to about 2000-3000 kb the critical segment the trisomy of which is responsible for the phenotype of trisomy 21.

Molecular genetics of human chromosome 21

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.

Down syndrome with duplication of a region of chromosome 21 containing the CuZn superoxide dismutase gene without detectable karyotypic abnormality

We report the case of an 18-month-old boy with many typical Down syndrome features but a normal cytogenetic analysis. High-resolution banding techniques on lymphocytes and fibroblasts of the propositus and his parents did not show any detectable abnormality including that of trisomy 21 mosaicism. However, CuZn superoxide dismutase (CuZn SOD) in the patient's red cells was increased as in trisomy 21. DNA analysis (Southern blots) using a human CuZn SOD probe showed that the genotype of the propositus contained three CuZn SOD genes. In situ hybridization on metaphase chromosomes with the same probe confirmed the gene location in a segment enclosing the distal part of 21q21 and 21q22.1. There was no significant labeling on other chromosomes of the patient. These results indicate that the Down syndrome phenotype of this patient is due to microduplication of a chromosome 21 fragment containing the CuZn SOD gene.

Mosaic Down's syndrome with de novo 45,XX,-21,-22,+t(21q;22q)/46,XX,-21,+t(21q;21q) rearrangement

The occurrence of mosaic Down's syndrome with two independent Robertsonian translocation cell lines is very rare. Such a patient is reported here, in whom an unbalanced Robertsonian translocation between two chromosomes 21 was detected in the majority of cells. The patient also revealed a minor cell line with a second Robertsonian translocation involving a chromosome 21 and a 22. The chromosome translocations detected in this patient were de novo in origin.

Three new cases of partial monosomy 21 resulting from one ring 21 chromosome and two unbalanced reciprocal translocations

Three patients with partial monosomy of the long arm of chromosome 21 are reported. Each one presents several features of a 21q--syndrome but in cases 2 and 3, other chromosomes are involved, contributing to the variability of the clinical picture. Synthesis of clinical, enzymatic and cytogenetic findings confirms that the superoxide dismutase A (SOD-A) locus is in sub-band 21q22-1. However, it is not possible to localize precisely the segments responsible for the different clinical features of 21q--syndrome.

Prenatal diagnosis of mosaicism 46,XX/46,XX,-21,+t(21q21q)

Mosaicism for a structural chromosome abnormality in amniotic cell cultures indicative of true fetal mosaicism is a rare event. In addition to the laboratory findings the clinical interpretation for counseling in such cases is based on observation of the same abnormality in liveborns as well as previous experience with prenatal diagnosis of the same or similar abnormalities. We report here the prenatal diagnosis of 46,XX/46,XX,-21,+t( 21q21q ) which was confirmed in fetal skin cell and amnion cell cultures.

Down syndrome--a disruption of homeostasis

A major question in human genetics concerns the relationship between the extra chromosome material in the Down syndrome (DS) and its effects. It is suggested here that a generalized disruption of evolved genetic balance in cells of affected individuals leads to decreased developmental and physiological buffering against genetic and environmental forces. Examples of consequences in DS of this model of disruption of homeostasis are presented: i) increased variance for metric traits, ii) amplified instability of developmental pathways, iii) reduced precision of physiological homeostatic controls, and iv) generalized increased morbidity. Evolution has selected for interacting systems. When this evolved balance is disrupted, as in autosomal aneuploidy, the organism is generally disrupted. The model emphasizes the role of environment in producing much of the DS phenotype. Traits less buffered than others in the general population are the ones most disturbed in DS and account for much of the DS phenotype.

Structural variation of chromosome 21 and symptoms of Down's syndrome

The analysis of the fine structure of the chromatids permits the identification of different regions on the long arm of chromosome 21. The preponderant role of the distal third of the long arm in the syndrome of trisomy 21 is now well established. Thus, trisomy of only band 21q22 results in a state identical to that caused by complete trisomy 21. If the trisomy involves only a part of band 21q22, the intensity of the symptoms is diminished, but the appearance of the patient is still reminiscent of Down's syndrome. Monosomy for band 21q22 results in a pathologic condition in which the morphological anomalies are the inverse of those observed in trisomic patients. This syndrome, as a "contre-type" to trisomy 21, is lethal. Trisomy of the proximal long arm region of chromosome 21 (21q21 leads to 21q22) is not associated with malformations but is accompanied by mental retardation. Monosomy of the same region results in a pathologic condition, which does not have features of the contre-type of trisomy 21.

Partial trisomy 21

This report presents and 18 1/2-year-old patient with clinical features of Down syndrome and severe mental retardation due to partial trisomy 21. Cytogenetic studies using Giemsa banding chromosomes revealed translocation of the 21q21 qter segment onto the short arms of chromosome #8 (46,XY,--8, + t(8qter 8p23::21q21 21 qter)). It is recommended that patients with features of Down syndrome whose chromosome analysis was done prior to introduction of banding studies undergo repeat karyotyping.

A rare case of mosaic Down syndrome 46,XY/46,XY, -21, +i(21q)

Detailed studies were carried out on a patient with a rare type of mosaicism which gave rise to an effective 21 trisomy. The clinical signs of Down syndrome were minimal. The cytogenetic interpretation is that the abnormal clone had an isochromosome derived from a maternal No. 21. The normal cell line appears to be replacing the abnormal clone.

A chromosome survey of a hospital for the mentally subnormal

A cytogenetic survey of 756 resident, but otherwise unselected, mentally retarded patients in a Scottish hospital is reported. The karyotypes of all patients were examined using orcein-stained cells, and those found to be abnormal, other than those with standard trisomy 21, were further investigated using a banding technique. A total of 103 patients were found to have an abnormal chromosome complement, of whom 91 had Down's syndrome (including six with translocations), six had some other autosomal abnormality, and six had an abnormality of the sex chromosome complement, including two with an XXYY complement. Details of the clinical and cytogenetic features are presented.

Partial trisomy 21. Further evidence that trisomy of band 21q22 is essential for Down's phenotype

Cytogenetic analysis of a 6-year-old girl with moderate mental retardation revealed 46 chromosomes with a tandem translocation (21;21) resulting in a partial trisomy 21. Only the terminal band 21q22 was not in triplicate. G-, Q-, R-, and C-banding techniques and silver nitrate staining of the nucleolus organizer regions (NORs) were used to identify this chromosome fully. The phenotype of the patient was not typical for Down's syndrome, providing additional evidence that trisomy of band 21q22 is pathogenetic for the phenotype of Down's syndrome. This is also a new example in human pathology of a stable 'dicentric' chromosome in which one of the centromeric constrictions appears to be nonfunctional.

Partial trisomies of chromosome 21 in man. Two new observations due to translocations 19;21 and 4;21

Partial trisomy of the distal portion of the long arm of chromosome no. 21, resulting from a (familial) translocation between the chromosomes no. 19 and 21 in a female twin with Down's syndrome, supports the hypothesis that triplication of 21q22 is the cause of the physical signs of mongolism. Partial trisomy of the remaining segments of chromosome no. 21 due to a (familial) translocation between the chromosomes no. 4 and 21, however, may only cause mental deficiency.

Down syndrome due to partial trisomy 21q

A patient is reported with a typical Down syndrome phenotype, caused by patrial trisomy of chromosome 21. Based on the present case and data from the literature, it is suggested that the Down phenotype is due to the trisomy of the distal portion of the band (q22) of chromosome 21.