Molecular evidence for true isochromosome 21q

Summary In one family a duplicated 21q was shown to be a true isochromosome, which segregates from mosaic mother to non-mosaic child with full Down syndrome phenotype. Densitometric analysis of Southern blots, using probe pPW228C for the distal long arm of chromosome 21, indicated that the 21q duplication contains two copies of the allele detected by the probe. Maternal mosaic karyotype of 45,XX,-21/46,XX/46, XX,-21,+21i(21q) also suggested transverse mitotic centromere division as the origin of the 21q isochromosomes. Morphologic analysis of chromosome heteromorphisms strengthened this interpretation because the free 21 missing in the cell line with 45 chromosomes was also missing in cells with the isochromosome. In a second family the cytogenetic data also suggested transmission of an i(21q) from mosaic mother to nonmosaic Down syndrome child but molecular evidence did not prove identity of alleles in the duplicated chromosome 21.     

Hypochondroplasia due to FGFR3 gene mutation (N540K) and mosaic form of Down syndrome in the same patient

The simultaneous presence of Down syndrome and achondroplasia has rarely been reported in the literature, and our search revealed only six patients with such an association. We are reporting the first case of a patient with Down syndrome and hypochondroplasia. In this patient, Down syndrome was clinically recognised and confirmed by the cytogenetic finding of mosaic karyotype (47,XX,+21/46,XX) shortly after birth. She was subsequently diagnosed with hypochondroplasia at the age of 6 years when disproportional short stature, stocky habitus and macrocephaly were observed. These phenotypic findings were later confirmed by the presence of fibroblast growth factor receptor 3 (FGFR3) gene mutation N540K. The overlapping common clinical features of Down syndrome and hypochondroplasia resulted in delayed diagnosis of hypochondroplasia in our patient and the associated deleterious effect on her linear growth. Her final height is 126.5 cm, which is −3.76 standard deviations (SD) lower than the median height in patients with Down syndrome, and is under the lower borderline of the adult height range for women with hypochondroplasia.     

Mosaic down syndrome with a marker: molecular cytogenetic characterization of the marker chromosome

Down syndrome is a complex disorder characterized by well defined and distinctive phenotypic features. Approximately 2-3% of all live-born Down individuals are mosaics. Here we report a boy with suspected Down syndrome showing mosaicism for two different cell lines where one cell line is unexpected. The cytogenetic analysis by G-banding revealed a karyotype of 47 XY+21 [20]/46,X+marker [30]. Further, molecular cytogenetic analysis with spectral karyotyping identified the marker as a derivative of Y chromosome. The delineation of Y chromosomal DNA was done by quantitative real-time PCR and aneuploidy detection by quantitative fluorescence PCR. The Y-short tandem repeats typing was performed to estimate the variation in quantity as well as to find out the extent of deletion on Y chromosome using STR markers. Fluorescence in situ hybridization using Y centromeric probe was also performed to confirm the origin of the Y marker. Further fine mapping of the marker was carried out with three bacterial artificial chromosome clones RP11-20H21, RP11-375P13, RP11-71M14, which defined the hypothetical position of the deletion. In our study we defined the extent of deletion of the marker chromosome and also discussed it in relation with mosaicism. This is the first report of mosaic Down syndrome combined with a second de novo mosaic marker derived from the Y chromosome.     

Detection of chromosomal abnormalities using cordocentesis]

Four cases of cytogenetic prenatal diagnosis of fetuses with chromosomal aberrations are presented: (1) the Patau syndrome; (2) and (4) the Down syndrome; (3) the Klinefelter syndrome. Cordocentesis has been shown to be expedient for rapid and accurate determination of fetus karyotype. Indicative for cytogenetic examination were ultrasonic data, maternal age, the values of AFP, HGG and nonconjugated estreol in maternal serum. Comparison of ultrasonic examination of fetuses with the data on abortus autotopsia was undertaken. The results demonstrate importance of ultrasonic, cytogenetic, biochemical and morphological research in prenatal malformation diagnosis.     

Detection of gonadal mosaicism in parents of children with Down syndrome

The paper presents results of a revision of data of both conventional chromosome testing and a study of cytogenetic (QFQ) markers in families with Down syndrome. Retrospective analysis of 151 families found eight families with a carrier of gonadal mosaicism. In all cases, the mother was younger than 35 years old. Therefore a prevalence of parental mosaicism in young couples was estimated to be 6.5% (8/123). Conventional diagnostic testing, not followed by analysis of segregation of QHQ markers, would have resulted in a prevalence of only 1%. A comparison of the results ofcytogenetic analysis with those expected using molecular polymorphisms suggests that cytogenetic testing cannot be entirely replaced by molecular testing. A combination of both methods should be applied when gonadal mosaicism is suspected.     

The ratio of de novo unbalanced translocation to 47, trisomy 21 Down syndrome. A new method for human mutation surveillance and an apparent recent change in mutation rate resulting in human interchange trisomies in one jurisdiction.

The Down syndrome phenotype may be associated with, among other genotypes, an unbalanced Robertsonian translocation producing an "interchange trisomy" with 46 chromosomes, or 47, trisomy 21. Translocations, like specificlocus point mutations, result from a direct change in structural chromosome elements. In contrast 47, trisomy 21 results from meiotic non-disjunction. Mutation rates for interchange trisomies may be followed indirectly by determining the ratio of instances of Down syndrome associated with a new translocation mutation to those produced by 47, trisomy 21, which accounts for the bulk of the Down syndrome phenotype. This genotypic ratio can be analyzed in data from cytogenetic laboratories, clinics, and chromosome registries and does not depend upon intensive chromosome screening of newborn populations. A similar approach can be adopted to follow trends in Patau syndrome. The genotypic ratio, stratified by maternal age, may in addition, provide a sentinel index for changes in human specific-locus mutations and perhaps other adverse health consequences. Analysis of data from the New York State-North-eastern chromosome registry revealed a two- to three-fold increase in the genotypic ratio for both Down syndrome and Patau syndrome for individuals born in 1973, 1974 and 1975 compared to those born in earlier years.     

A patient with Down syndrome with a de novo derivative chromosome 21

Pure partial trisomy of chromosome 21 is a rare event. The patients with this aberration are very important for setting up precise karyotype-phenotype correlations particularly in Down syndrome phenotype. We present here a patient with Down syndrome with a de novo derivative chromosome 21. Karyotype of the patient was designated as 46,XY,der(21)(p13)dup(21)(q11.2q21.3)dup(21)(q22.2q22.3) with regard to cytogenetic, FISH and array-CGH analyses. Non-continuous monosomic, disomic and trisomic chromosomal segments through the derivative chromosome 21 were detected by array-CGH analysis. STR analyses revealed maternal origin of the de novo derivative chromosome 21. The dual-specificity tyrosine (Y)-phosphorylation regulated kinase 1A (DYRK1A) and Down Syndrome Critical Region 1 (DSCR1) genes that are located in Down syndrome critical region, are supposed to be responsible for most of the clinical findings of Down syndrome. However, our patient is the first patient with Down syndrome whose clinical findings were provided in detail, with a de novo derivative chromosome 21 resulting from multiple chromosome breaks excluding DYRK1A and DSCR1 gene regions.     

Immunophenotype of a transient myeloproliferative disorder in a newborn with trisomy 21

Cytologic, immunologic, and cytogenetic studies were performed on the blast cells of a newborn with Down syndrome and transient myeloproliferative disease. This hematologic disorder is uncommon, and occurs primarily in infants with Down syndrome. This boy presented with a high white blood cell count and a high percentage of blast cells, without anemia or thrombocytopenia. Chromosome analysis showed a constitutional trisomy 21 without any other clonal abnormality. A three-color flow cytometric analysis was performed and revealed two different CD45 dim, CD34(+), CD117(+), CD56(+) immature subpopulations: the normal immature myeloid precursor and an immature blast cell population that expressed CD41, CD42, CD61, CD36, CD13, CD1a, and CD2. We postulate that this population could be the leukemic precursor involved in the acute megakaryoblastic leukemia frequently observed in children with Down syndrome.     

Detection and characterization of new genetic mutations in individuals heterozygous for lactate dehydrogenase-B(H) deficiency using DNA conformation polymorphism analysis and silver staining

In northwest European countries maternal age is increasing. This will lead to an increase of the prevalence of Down syndrome conceptuses. Meanwhile, the increased use of prenatal cytogenetic diagnosis (PCD) will lead to a decrease in the prevalence of Down syndrome among livebirths. We were interested to know what the result of these two opposite developments would be in the near future, and we describe here a model to quantify these processes and the resulting livebirth prevalence of Down syndrome. The model is demonstrated for The Netherlands from 1992 to 2001. The predicted livebirth prevalence for The Netherlands in 1992 is 1.36 per 1000. Demographic factors will cause an increase to 1.76 per 1000 in 2001 with present indications for PCD and a utilization ratio of 50%. An increase of the utilization ratio to 90% in 2001 will lead to a prevalence of 1.22 per 1000, a little less than the present prevalence. Alternative screening programs, including maternal serum screening, could lead to a further decrease of the livebirth prevalence. The model described here can be used for evaluation of the consequences of alternative forms of Down syndrome screening.     

Cytogenetic survey of a hospital for the mentally retarded

Summary A cytogenetic survey of all 588 patients in Strathmont Training Centre, an Australian hospital for the mentally retarded, was carried out. Abnormal karyotypes were found in 90 (15.3%) patients, of whom 73 (12.4%) had clinical Down syndrome, 12 (2.04%) other autosomal abnormalities, and 5 (0.85%) sex chromosome abnormalities.     

A blind prometaphase study of Prader-Willi syndrome: frequency and consistency in interpretation of del 15q.

Controversy continues to exist concerning the proportion of individuals with Prader-Willi syndrome who have a chromosome 15 deletion and concerning the reliability with which a cytogenetic service laboratory can accurately perform the appropriate analysis. Blind prometaphase cytogenetic study of 13 individuals from a Prader-Willi syndrome clinic and seven controls has revealed that approximately 70% of accurately diagnosed clinically typical patients with this disorder have an evident deletion of at least 15q12. Blind analysis of panels of chromosome 15 pairs from all cases in this study by the directors of four independent cytogenetic service laboratories demonstrated substantial interobserver consistency in interpretation of results. The possibility of euploid mosaicism for del 15q was investigated, but remains unresolved.     

A cytogenetic study of mentally retarded school children in taiwan with special reference to the fragile X chromosome

Summary A cytogenetic study was made on 341 mentally retarded children in the Provincial Nantou Rehabilitation Center for the Mentally Retarded and the St. Raphael Opportunity Center in Tainan. Of the 89 mentally retarded children with chromosomal abnormalities, 63 had Down syndrome, 13 had the fragile X [fra(X)] syndrome, and the remaining had other aneuploid constitutions. Family studies were possible for 2 of the 13 fra(X) probands. The results of this study illustrate the contribution of chromosomal abnormalities to the pathogenesis of mental retardation in children.     

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

Summary 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.     

Use of short sequence repeat DNA polymorphisms after PCR amplification to detect the parental origin of the additional chromosome 21 in Down syndrome.

The origin of nondisjunction in trisomy 21 has so far been studied using cytogenetic heteromorphisms and DNA polymorphisms using Southern blot analysis. Short sequence repeats have recently been described as an abundant class of DNA polymorphisms in the human genome, which can be typed using the polymerase chain reaction (PCR) amplification. We describe the usage of such markers on chromosome 21 in the study of parental origin of the additional chromosome 21 in 87 cases of Down syndrome. The polymorphisms studied were (a) two (GT)n repeats and a poly(A) tract of an Alu sequence within the HMG14 gene and (b) a (GT)n repeat of locus D21S156. The parental origin was determined in 68 cases by studying the segregation of polymorphic alleles in the nuclear families (either by scoring three different alleles in the proband or by dosage comparison of two different alleles in the proband). Our results demonstrate the usefulness of highly informative PCR markers for the study of nondisjunction in Down syndrome.     

A familial reciprocal translocation t(1;15) in three generations identified in a regular trisomy 21 patient

The concurrence of a reciprocal translocation and an aneuploidy represent a rare coincidence and an interchromosome effect between these two events has been suggested. We report the case of a family with a t(1;15) in three generations which was identified through the evaluation ofa patient with classical trisomy 21 or Down syndrome. The cytogenetic analysis with GTG banding showed that the proband had a regular trisomy 21 and a balanced translocation t(1;15). FISH and microsatellite analysis were carried out in the family in order to discard an interchromosomal effect. The implications for genetic assessment are discussed.     

Chromosomale Mosaike in der klinischen Zytogenetik

In contrast to molecular genetics, in cytogenetic analyses single cells are analyzed individually. This affords an opportunity to detect cells with chromosomal mosaicism. This article provides an overview on problems arising in the detection and interpretation of chromosomal mosaicism in cytogenetic diagnostics. A particular challenge in the diagnostics is to distinguish between clinically relevant genuine mosaicism on the one hand and cultured artifacts, pseudomosaics, age effects, maternal contamination and chimerism on the other. The probability of overlooking mosaicism in cytogenetic routine diagnostics is very high, as on average only 15 of the 10¹² cells in the body are examined and usually only a single tissue is analyzed. However, some cytogenetic mosaics are typical for certain syndromes, such as Pallister-Killian syndrome, cat eye syndrome or Ullrich-Turner syndrome and others are characteristic for certain disorders including some hematological malignancies.     

Parental trisomy 21 mosaicism.

A family with three children with trisomy 21 in which the mother is a phenotypically normal, trisomy 21/normal mosaic was studied. Chromosome 21 fluorescent heteromorphisms were used to document that two of the three number 21's in two of the Down syndrome offspring were of maternal origin. Five cytogenetic surveys in which both parents of a child with trisomy 21 were studied have been reviewed. From these data, it is estimated that 3% of couples producing a child with trisomy 21 can be explained by parental mosaicism. From 17 informative sibships, with one parent mosaic, the segregation ratio was estimated to be 0.43 +/- 0.11.     

Down syndrome rates and relaxed selection at older maternal ages.

Preferential survival in older mothers of fetuses with Down syndrome has been proposed as contributing to the maternal-age effect of this condition. If correct, this provocative hypothesis, which may be termed "relaxed selection," has major implications for approaches to prevention of Down syndrome live births in older women. Several predictions of this hypothesis are examined here by comparisons of parental ages among various populations. These revealed that: (1) mean maternal age of Down syndrome live births is slightly lower than that of Down syndrome spontaneous fetal deaths; (2) mean maternal age of those with mutant D/21 translocation Down syndrome is about the same as that of controls; (3) the ages of Down syndrome mothers who have Down syndrome live births is slightly lower than ages of Down syndrome mothers who have unaffected live births; and (4) in recent data on 47, +21 cases in which the extra chromosome 21 is of paternal origin, the mean maternal ages are 4-5 years lower than the maternal ages of cases of maternal origin (in contrast to earlier reports). All of these observations are contrary to the hypothesis that relaxed selection contributes significantly to the maternal-age association of Down syndrome. If there is any effect of relaxed selection, it is likely to be very weak and/or act primarily upon abortions that occur before recognition of pregnancy.     

Molecular cytogenetic study of Robertsonian translocation 13;14 and Down syndrome in a 3-year-old infant

We describe here a rare case of Robertsonian translocation 13;14 of maternal origin combined with regular trisomy 21 (46,XX,der(13;14)(q10;q10)mat,+21) with Down syndrome phenotype. Molecular cytogenetic studies allowed us to determine the maternal origin of additional chromosome 21 and the non-disjunction of chromosome 21 to occur in meiosis I. On the basis of data obtained we discuss the possible involvement of structural alterations of chromosomes 13 and 14 in the chromosome 21 non-disjunction.