A systematic cytogenetic study of a population of 1170 mentally retarded and/or behaviourly disturbed patients including fragile X-screening. The Hondsberg experience

A cytogenetic study was performed in a population of 1170 mentally retarded and/or behaviourly disturbed patients of the Hondsberg Institute in the south of the Netherlands. The cytogenetic data are presented and discussed. In all patients chromosomal evaluation was performed with Giemsa-banding and Quinacrine fluorescence, and additional banding techniques were performed whenever they were necessary to clarify the chromosomal abnormality. A fragile X screening with M199 cultures was performed in 311 males. In 22.1% of the patients a chromosomal basis was found for their developmental retardation: 14.3% Down syndrome patients, 6.1% other chromosomal abnormalities (mainly partial autosomal trisomies and monosomies and sex-chromosome abnormalities). In 24 males, through 21 index patients, a positive fragile X screening was found, i.e. 6.7% of the screened population and 1.8% of the total population. These results indicate that the diagnostic contribution of the fragile X screening is numerically of equal importance as are advanced chromosome banding techniques, and its contribution to the diagnosis of fragile X syndrome in one index male patient in general leads to the detection of several female relatives at risk to be carrier of this X-linked recessively inherited condition. The causal relationship between the occurrence of mental retardation and chromosomal aberration in genera i.e. autosomal trisomies, partial autosomal trisomies and monosomies, and Xq27-28 fragility is well established and is, to some extent, easy to understand. Whether carriers of other chromosomal rearrangements, mainly of balanced reciprocal and Robertsonian translocations, small extra chromosomes, paracentric inversions and chromosomal variants, have increased risk for mental handicap and/or congenital malformations in their progeny, remains unclear at the present time. Some of these residual problems and questions are discussed in the perspective of their importance for genetic counseling. Detailed data will be presented about the mental development and psychological profile of patients with these different types of chromosomal abnormalities and rearrangements.     

Cluster size polymorphism of active human ribosomal genes and simulation of the conditions of its stability through generations

Based on selective silver nitrate staining of active ribosomal gene (AcRG) clusters in nucleolus organizer regions (NORs) of human metaphase chromosomes, a technique was developed earlier to estimate the AcRG dosage in individual genomes as a sum of arbitrary units (0–3) ascribed to the silver precipitate (AgNOR) on ten NORs. The AcRG dosage was considered to be an additive quantitative trait determined by five polymorphic autosomal loci (with four allelic forms for each locus). A database was created to contain the data on AcRG cluster variants for more than 1000 individual human genomes. In this study, the population frequencies of AcRG cluster variants were determined. The results agreed with the hypothesis that stabilizing selection acts at the zygotic and/or early embryogenetic stage to restrain the AcRG genomic dosage (copy number) within a range from 14.9 to 23.7 arbitrary units (the cell is unviable when the trait is beyond this range). The average zygotic losses due to selection were estimated at 9.1–9.9% for a real population. A computer model where the AcRG dosage of a progeny results from a random combination of the AgNORs of the five acrocentric chromosome pairs of the parents was developed and used to simulate the formation of a certain AcRG genomic dosage through generations in a human panmictic population with nonoverlapping generations. A combination of stabilizing selection by total AcRG copy number and a certain spontaneous mutation rate (the probability of changes in the cluster size of a NOR as a result of unequal crossingover in meiotic prophase) was shown to be a sufficient condition for the restrain of equilibrium population frequencies of AgNOR size variants in a human panmictic population. Using the model, the most probable spontaneous mutation frequency was predicted to be (2.1–2.3) × 10⁻² per NOR per generation for human AgNORs. The predicted frequency was within the 95% confidence interval of the experimental rate, which was determined by studying the inheritance of AgNOR variants in real families.     

Cytogenetic survey in couples with recurrent fetal wastage

Summary Cytogenetic studies have been performed in 1068 couples with antecedent fetal wastage, i.e., at least two spontaneous first trimester abortions or one spontaneous first trimester abortion and one late fetal death, particularly with multiple congenital malformations.Three major types: 33 reciprocal translocations (3.09%), 20 Robertsonian translocations (1.87%) and six other chromosomal abnormalities (0.56%) were found, bringing the total number of chromosomal abnormalities to 59 (5.5%) in 1068 couples under investigation.In contrast to couples with reciprocal translocations, a high excess of female over male carriers was found in the group of Robertsonian translocations.In the evaluation of chromosomal polymorphisms, only variants with particularly large paracentromeric constitutive heterochromatin blocks were taken into account, and their low frequency in the present study is therefore not comparable with that in a general population.The impact of further extensive familial investigation on genetic counseling and the follow-up of prenatal diagnosis are discussed.     

Polymorphisms in genes involved in folate metabolism as maternal risk factors for Down syndrome

Down syndrome is a complex genetic and metabolic disorder attributed to the presence of three copies of chromosome 21. The extra chromosome derives from the mother in 93% of cases and is due to abnormal chromosome segregation during meiosis (nondisjunction). Except for advanced age at conception, maternal risk factors for meiotic nondisjunction are not well established. A recent preliminary study suggested that abnormal folate metabolism and the 677C-->T polymorphism in the methylenetetrahydrofolate reductase (MTHFR) gene may be maternal risk factors for Down syndrome. The present study was undertaken with a larger sample size to determine whether the MTHFR 677C-->T polymorphism was associated with increased risk of having a child with Down syndrome. Methionine synthase reductase (MTRR) is another enzyme essential for normal folate metabolism. A common polymorphism in this gene was recently associated with increased risk of neural tube defects and might also contribute to increased risk for Down syndrome. The frequencies of the MTHFR 677C-->T and MTRR 66A-->G mutations were evaluated in DNA samples from 157 mothers of children with Down syndrome and 144 control mothers. Odds ratios were calculated for each genotype separately and for potential gene-gene interactions. The results are consistent with the preliminary observation that the MTHFR 677C-->T polymorphism is more prevalent among mothers of children with Down syndrome than among control mothers, with an odds ratio of 1.91 (95% confidence interval [CI] 1.19-3.05). In addition, the homozygous MTRR 66A-->G polymorphism was independently associated with a 2. 57-fold increase in estimated risk (95% CI 1.33-4.99). The combined presence of both polymorphisms was associated with a greater risk of Down syndrome than was the presence of either alone, with an odds ratio of 4.08 (95% CI 1.94-8.56). The two polymorphisms appear to act without a multiplicative interaction.     

Segregation of chromosomes in sperm of Robertsonian translocation carriers

Robertsonian translocations are the most frequent structural chromosomal abnormalities in humans and can affect fertility, with various degrees of sperm alterations in men; or the pregnancy outcome of the carriers. The studies on meiotic segregation of chromosomes in sperm of Robertsonian translocation males find a majority of normal or balanced spermatozoa for the chromosomes related to the translocation (mean 85.42%; range 60-96.60%). Furthermore, recent studies suggest an interchromosomal effect. Studies on spermatozoa from translocation carriers, and in mouse models help the comprehension of the meiotic segregation mechanisms. Results of meiotic segregation analysis in man could be integrated in genetic counselling especially when assisted reproductive technology is required.     

The most frequent chromosomal abnormalities in karyotypes of patients with reproductive problems

Results of cytogenetic and molecular-cytogenetic inspection of 210 matrimonial pairs with the problems of reproduction are presented. Different types of chromosomal aberrations have been detected in the karyotypes of the patients in 46 (10.95%) cases. Such structural chromosomal aberrations as pericentric inversions, Robertsonian translocations, balanced reciprocal translocations, and marker chromosomes as well prevailed the numerical chromosomal aberrations (89.13% and 10.87% cases accordingly). In the general group of the inspected patients there were 19 cases (4.52%) characterized by the low level of X and Y chromosome mosaicism. The authors suppose that the patients with the exposed chromosomal abnormalities need the differentiated approach at their treatment.     

Increased risk of respiratory tract infections in children with Down syndrome: the consequence of an altered immune system

Down syndrome (DS) is the most common chromosomal abnormality among live-born infants. Respiratory tract infections are the most important cause of mortality in individuals with DS at all ages. In recent decades several studies have been performed to elucidate abnormalities of the immune system in DS. However, the influence of the immune system on the occurrence of respiratory tract infections in these children has never been reviewed.     

Breakpoint mapping and array CGH in translocations: comparison of a phenotypically normal and an abnormal cohort

We report the analyses of breakpoints in 31 phenotypically normal and 14 abnormal carriers of balanced translocations. Our study assesses the differences between balanced translocations in normal carriers and those in abnormal carriers, focusing on the presence of genomic imbalances at the breakpoints or elsewhere in the genome, presence of cryptic chromosome rearrangements, and gene disruption. Our hypothesis is that all four features will be associated with phenotypic abnormalities and absent or much less frequent in a normal population. In the normal cohort, we identified neither genomic imbalances at the breakpoints or elsewhere in the genome nor cryptic chromosome rearrangements. In contrast, we identified candidate disease-causing imbalances in 4/14 abnormal patients. These were three breakpoint associated deletions and three deletions unrelated to the breakpoints. All six de novo deletions originated on the paternally inherited chromosome. Additional complexity was also present in one of these cases. Gene disruption by the breakpoints was present in 16/31 phenotypically normal individuals and in 5/14 phenotypically abnormal patients. Our results show that translocations in phenotypically abnormal patients are molecularly distinct from those in normal individuals: the former are more likely to be associated with genomic imbalances at the breakpoints or elsewhere and with chromosomal complexity, whereas the frequency of gene disruption is similar in both normal and abnormal translocation carriers.     

Molecular and cytogenetic analyses of autism in Taiwan

Karyotypic and DNA analyses were both performed on 104 autistic children referred from Taichung Autism Education Academy and Tainan Autism Association in Taiwan. The frequency of fragile sites of the autistic patients did not differ significantly from that of the normal individuals. Of the 12 autistic children with chromosomal abnormalities, 8 had the fragile X, 2 had Down syndrome, and the remaining had other aneuploid constitutions. The results of this study illustrate the contribution of chromosomal abnormalities or variants to the pathogenesis of infantile autism.     

Identification of the translocation breakpoints in the Ts65Dn and Ts1Cje mouse lines: relevance for modeling down syndrome

Down syndrome (DS) is the most frequent genetic disorder leading to intellectual disabilities and is caused by three copies of human chromosome 21. Mouse models are widely used to better understand the physiopathology in DS or to test new therapeutic approaches. The older and the most widely used mouse models are the trisomic Ts65Dn and the Ts1Cje mice. They display deficits similar to those observed in DS people, such as those in behavior and cognition or in neuronal abnormalities. The Ts65Dn model is currently used for further therapeutic assessment of candidate drugs. In both models, the trisomy was induced by reciprocal chromosomal translocations that were not further characterized. Using a comparative genomic approach, we have been able to locate precisely the translocation breakpoint in these two models and we took advantage of this finding to derive a new and more efficient Ts65Dn genotyping strategy. Furthermore, we found that the translocations introduce additional aneuploidy in both models, with a monosomy of seven genes in the most telomeric part of mouse chromosome 12 in the Ts1Cje and a trisomy of 60 centromeric genes on mouse chromosome 17 in the Ts65Dn. Finally, we report here the overexpression of the newly found aneuploid genes in the Ts65Dn heart and we discuss their potential impact on the validity of the DS model.     

Contribution of ultrasonographic examination to the prenatal detection of chromosomal abnormalities in 19 centres across Europe.

The objective of this study was to evaluate the prenatal detection of chromosomal abnormalities by fetal ultrasonographic examination in a large database provided by 19 Registries of Congenital Anomalies from 11 European countries. This study included 1738 cases of chromosomal abnormalities, liveborn, stillborn or termination of pregnancy regardless of maternal age from a population of 664,340 births during the period 1996 - 1998. The most frequent chromosomal anomalies were Down syndrome (n=1050), trisomy 18 (n=191), Turner syndrome (n=125), trisomy 13 (n=86), and triploidy (n=56). Fetal ultrasonographic examination resulted in the prenatal detection of 37.7% of the chromosomal abnormalities, thereby resulting in a reduction of 28.6% in their prevalence at birth due to terminations of pregnancy. The detection rate by ultrasound examination varied according to local policies of prenatal diagnosis : it was lower in countries where routine scan were not performed and higher in countries in which at least one routine anomaly scan during the second trimester of pregnancy was performed. The ultrasound detection varied according to the specific chromosomal anomaly and was lowest for Klinefelter syndrome (5.7%) and highest for triploidy (78.6%). For Down syndrome it was 26.4%. Termination of pregnancy was performed in 75.9% of the cases. Among the 655 cases detected by ultrasound, the most frequent ultrasound signs by category of chromosomal abnormalities were analysed. This study shows that ultrasound screening is an important tool in the prenatal detection of chromosomal abnormalities in Europe, leading to a significant reduction in the prevalence of livebirth children with chromosomal anomalies.     

The effect of a pattern in palmar interdigital II on a-b ridge count in black and white Down syndrome cases and controls

An unconfirmed study by Fang (Ph.D. thesis, Univ. of London, 1950) in Britain showed that individuals with Down syndrome had lower total a-b ridge counts in palmar Interdigital area II (ID II) than a group of controls. This study compares 603 white Down syndrome cases and 93 black Down syndrome cases with 668 white and 402 black controls. Our results confirm those of Fang in that the Down syndrome cases in both racial groups had lower total a-b ridge counts than their respective controls. In addition, the black controls and Down syndrome cases had lower a-b ridge counts than their white counterparts. The mean a-b ridge count was significantly lower in individuals with a pattern in ID II compared to individuals without a pattern in ID II in both the Down syndrome and control groups. Some of the lower a-b ridge counts in the Down syndrome samples can be accounted for by the fact that there is an increased frequency of a pattern in ID II in Down syndrome cases. Both Down syndrome and normal individuals who had a pattern unilaterally had a lower than expected a-b ridge count on the contralateral hand that did not have a pattern. There was a tendency also for increased asymmetry in Down syndrome cases with a pattern in ID II.     

Imprinted genes: identification by chromosome rearrangements and post-genomic strategies

Imprinted genes are differentially expressed from the maternally and paternally inherited alleles. Accordingly, inheritance of both copies of an imprinted chromosome or region from a single parent leads to the mis-expression of the imprinted genes present in the selected region. Strains of mice with reciprocal and Robertsonian chromosomal translocations or mice with engineered chromosomal rearrangements can be used to produce progeny where both copies of a chromosomal region are inherited from one parent. In combination with systematic differential expression and methylation-based approaches, these mice can be used to identify novel imprinted genes. Advances in genome sequencing and computer-based technologies have facilitated this approach to finding imprinted genes.     

A fragile X female with Down syndrome

Summary Clinical and cytogenetic aspects of a female infant with trisomy 21 and the fragile X [fra (X)] chromosome are reported. Most of the facial characteristics of the patient are those observed in Down syndrome, but some features such as long face with prominent forehead and lower jaw, and large ears are related to the fra (X) syndrome. The origin of an additional chromosome 21 may be ascribed to maternal first meiotic nondisjunction in our case. It has been suspected that female carriers of the fra (X) chromosome may be predisposed to meiotic nondisjunctional events. However, there is probably no relationship between the two chromosomal abnormalities in our case because of the maternal age at the delivery.     

Evaluation of routine prenatal ultrasound examination in detecting fetal chromosomal abnormalities in a low risk population

Prenatal diagnosis performed by fetal ultrasound scan is now a routine part of antenatal care in many countries. We have used our registry of congenital malformations to determine how many fetal anomalies and consequently how many chromosomal abnormalities are detected by this procedure. In our region, evaluation of prenatal diagnosis of chromosomal abnormalities in women of 38 years and younger (chromosomal prenatal diagnosis is offered to women 38 years) with no personal or familial history of chromosomal anomaly was performed in 119 099 consecutive pregnancies of known outcome from 1980 to 1987. At least one ultrasonographic examination seeking congenital malformations was performed in more than 95% of the pregnant women studied. The total number of chromosomal anomalies during the study period was 199, 123 of these being Down syndrome. Only 41 (34.5%) of the 119 fetuses with chromosomal abnormalities and congenital malformation examined had been found to have a malformation at ultrasound examination. This low sensitivity was different for the diverse chromosomal abnormalities. Only 10 out of the 54 fetuses with Down syndrome and malformations (18.5%) were detected and only 3 out of 24 (12.5%) atrioventricular canal defects in those trisomie 21 patients were detected. Only 5 out of 11 (45.4%) fetuses with trisomy 13, 13 out of 26 (50.0%) fetuses with trisomy 18, 7 out of 12 patients with monosomy X (58.3%) and 6 out of 27 (22.2%) fetuses with other chromosomal abnormalities were diagnosed. Moreover, the time of detection of these anomalies was early enough to allow amniocentesis and termination of pregnancy in the case of a chromosomal abnormality in only 15 out of these 41 patients, including 7 cases of cystic hygroma in fetuses with monosomy X. This low sensitivity is not the result of the quality of the ultrasound equipment. It may be explained by the inadequate qualification of some operators and by the insufficient duration of the routine examination. In conclusion, our study has shown that the sensitivity of the detection of chromosomal abnormalities by routine prenatal ultrasound screening is low. Other screening methods are needed.     

Molecular signatures of cardiac defects in down syndrome lymphoblastoid cell lines suggest altered ciliome and hedgehog pathways

Forty percent of people with Down syndrome exhibit heart defects, most often an atrioventricular septal defect (AVSD) and less frequently a ventricular septal defect (VSD) or atrial septal defect (ASD). Lymphoblastoid cell lines (LCLs) were established from lymphocytes of individuals with trisomy 21, the chromosomal abnormality causing Down syndrome. Gene expression profiles generated from DNA microarrays of LCLs from individuals without heart defects (CHD(-); n?=?22) were compared with those of LCLs from patients with cardiac malformations (CHD(+); n?=?21). After quantile normalization, principal component analysis revealed that AVSD carriers could be distinguished from a combined group of ASD or VSD (ASD+VSD) carriers. From 9,758 expressed genes, we identified 889 and 1,016 genes differentially expressed between CHD(-) and AVSD and CHD(-) and ASD+VSD, respectively, with only 119 genes in common. A specific chromosomal enrichment was found in each group of affected genes. Among the differentially expressed genes, more than 65% are expressed in human or mouse fetal heart tissues (GEO dataset). Additional LCLs from new groups of AVSD and ASD+VSD patients were analyzed by quantitative PCR; observed expression ratios were similar to microarray results. Analysis of GO categories revealed enrichment of genes from pathways regulating clathrin-mediated endocytosis in patients with AVSD and of genes involved in semaphorin-plexin-driven cardiogenesis and the formation of cytoplasmic microtubules in patients with ASD-VSD. A pathway-oriented search revealed enrichment in the ciliome for both groups and a specific enrichment in Hedgehog and Jak-stat pathways among ASD+VSD patients. These genes or related pathways are therefore potentially involved in normal cardiogenesis as well as in cardiac malformations observed in individuals with trisomy 21.     

Cytogenetic study of recurrent miscarriages and their parents

There are substantial evidences that genetic alterations are contributing factors to the risk for recurrent miscarriages. This study was conducted to determine the frequency and contribution of chromosomal abnormalities in miscarriages and in couples with recurrent miscarriages. We studied a total of 41 miscarriages and their parents with a history of 2-11 recurrent miscarriages. Chromosomal analysis from chorionic villus sampling (CVS) and fetal tissues were performed according to standard cytogenetic methods using G-banding technique. Major chromosomal aberrations and polymorphic variants were found in 51 and 4.8%, respectively. The chromosomal abnormalities were structural (34.4%) and numerical (65.1%) of which 26.1, 21.7, 8.7 and 8.7% were fetal sex aneuploid, triploid, mosaics and trisomic, respectively. Unbalanced and balanced rearrangements were found in 17.2% and 8.6% of all abnormalities, respectively. Major chromosomal abnormalities in couples were seen in 4.9%. The chromosomal abnormalities associated with pregnancy losses and recurrent miscarriages are mostly numerical ones. The incidence of balanced translocations found here is 4.9% which is near to the mode (about 3-6%) observed in the previous studies. Those frequencies are greater than in the general population (0.3%). This indicates that balanced translocations, seen in parents, have some importance in causing miscarriage. The major parental chromosomal aberrations are significantly associated with fetal wastage. Mosaicism should be taken into account for cytogenetic analyses of pregnancy losses. Thus, cytogenetic analyses should be recommended in couples with recurrent miscarriages, when clinical data fail to clarify the cause.     

Cytogenetic study of recurrent miscarriages and their parents

There is substantial evidence that genetic alterations are contributing factors to the risk for recurrent miscarriages. This study was conducted to determine the frequency and contribution of chromosomal abnormalities in miscarriages and in couples with recurrent miscarriages. We studied a total of 41 miscarriages and their parents with a history of 2–11 recurrent miscarriages. Chromosomal analysis from chorionic villus sampling (CVS) and fetal tissues were performed according to standard cytogenetic methods using G-banding technique. Major chromosomal aberrations and polymorphic variants were found in 51 and 4.8%, respectively. The chromosomal abnormalities were structural (34.4%) and numerical (65.1%) of which 26.1, 21.7, 8.7 and 8.7% were fetal sex aneuploid, triploid, mosaics and trisomic, respectively. Unbalanced and balanced rearrangements were found in 17.2 and 8.6% of all abnormalities, respectively. Major chromosomal abnormalities in couples were seen in 4.9%. The chromosomal abnormalities associated with pregnancy losses and recurrent miscarriages are mostly numerical ones. The incidence of balanced translocations found here is 4.9% which is near to the mode (about 3–6%) observed in the previous studies. Those frequencies are greater than in the general population (0.3%). This indicates that balanced translocations, seen in parents, have some importance in causing miscarriage. The major parental chromosomal aberrations are significantly associated with fetal wastage. Mosaicism should be taken into account for cytogenetic analyses of pregnancy losses. Thus, cytogenetic analyses should be recommended in couples with recurrent miscarriages, when clinical data fail to clarify the cause. The text was submitted by the authors in English.     

Interchange trisomy 21 by t(1;21)(p22;q22)mat

Interchange trisomy 21 by t(1:21)(p22:q22)mat: Interchange trisomy 21 by t(1;21)(p22;q22)mat was identified in a sporadic patient with Down syndrome. With a 21q22 specific probe, we observed signals on both normal 21 chromosomes and on the der. We reviewed the 23 published reports of families with reciprocal translocations leading to viable offspring with interchange trisomy 21. The breakpoints in chromosome 21 were mainly located in 21q (19/24 instances, including the present report) and in 19/23 cases the other chromosome involved in the translocation was (pairs 1-12). The underlying 3:1 segregation occurred mainly in carrier mothers; only one patient presented a de novo imbalance and in another case the father was the carrier. In addition, there were 4 instances of concurrence with another unbalanced segregation (adjacent-1 or tertiary trisomy) and 3 families with recurrence of interchange trisomy 21. The mean age of 14 female carriers at birth of interchange trisomy 21 offspring (24.8 yr) was lower that the mean (28.3 yr) found in a larger sample of mothers of unbalanced offspring due to 3:1 segregation (mostly tertiary trisomics) and was not increased with respect to the general population average. Overall, these data agree with previous estimates regarding recurrence risk (9-15%) and abortion rate (about 28%) in female carriers ascertained through an interchange trisomic 21 child.     

Cryptic Subtelomeric Rearrangements and X Chromosome Mosaicism: A Study of 565 Apparently Normal Individuals with Fluorescent In Situ Hybridization

Five percent of patients with unexplained mental retardation have been attributed to cryptic unbalanced subtelomeric rearrangements. Half of these affected individuals have inherited the rearrangement from a parent who is a carrier for a balanced translocation. However, the frequency of carriers for cryptic balanced translocations is unknown. To determine this frequency, 565 phenotypically normal unrelated individuals were examined for balanced subtelomeric rearrangements using Fluorescent In Situ hybridization (FISH) probes for all subtelomere regions. While no balanced subtelomeric rearrangements were identified, three females in this study were determined to be mosaic for the X chromosome. Mosaicism for XXX cell lines were observed in the lymphocyte cultures of 3 in 379 women (0.8%), which is a higher frequency than the 1 in 1000 (0.1%) reported for sex chromosome aneuploidies. Our findings suggest that numerical abnormalities of the X chromosome are more common in females than previously reported. Based on a review of the literature, the incidence of cryptic translocation carriers is estimated to be approximately 1/8,000, more than ten-fold higher than the frequency of visible reciprocal translocations.