Down's syndrome. Current stage of cytogenetic research

Down syndrome in Kuwait

During a 7-year-period (1980-1986) trisomy 21 was confirmed in 635 cases (257 males and 278 females). There were 611 cases of trisomy 21 (96.2%), 12 of different translocations (1.9%), 9 of mosaicism (1.4%), and 3 with nonclassical karyotypes (0.5%). The frequency of chromosome aberrations in our study is compared to that of major world-wide cytogenetic surveys comprising 17,738 Down syndrome cases. These surveys showed that regular trisomy 21 constitutes 92.9%, translocations 4.3%, mosaicism 2.2%, and nonclassical karyotypes 0.5%.

Preferential alternate segregation in the common t(11;22)(q23;q11) reciprocal translocation: sperm FISH analysis in two brothers

Segregation behaviour studies in t(11;22) carriers have reported controversial results. Whereas some authors have detected a preponderance of 3:1 products, no evidence of such prevalence was found by others. This study reports a fluorescence in-situ hybridization (FISH) segregation analysis on decondensed spermatozoa in two brothers, carriers of the same t(11;22)(q23;q11) rearrangement. Data revealed a similar meiotic segregation pattern in both carriers, 2:2 Alternate segregation being the most frequent (42.94 and 45%), while 3:1 genotypes were the least frequent in both patients, with percentages around 10%. The production of three chiasmata, based on the presence of G-light bands along the translocated segments and the presence of recombination sites at 11q and 22q distal regions, are proposed as the cause of a preponderance of the Alternate segregation. Interchromosomal effects involving chromosomes 13, 18, 21, X and Y were also evaluated. An increased frequency of sex chromosome disomies was detected in one patient. Reviewing the literature, a relationship between this phenomenon and the involvement of acrocentric chromosomes in the reorganization is suggested. FISH segregation and interchromosomal effects studies in spermatozoa are encouraged to gather information to establish the best approach for preimplantational genetic diagnosis in reorganization carriers.

The nonrandom participation of human acrocentric chromosomes in Robertsonian translocations

The present study explores the origin of human Robertsonian translocations (RT) and the causes of the nonrandom participation of the different acrocentrics in them. Satellite associations have been analysed in 966 cells from 8 persons, and 1266 RT with ascertainment have been collected from the literature. The observation that the chromosomes preferentially taking part in satellite associations vary between individuals is confirmed. However, since a preferred chromosome appears to associate at random with the others, this phenomenon should not add to the nonrandomness of the RT. Most RT presumably arise through adjacent chromatid exchanges corresponding to mitotic chiasmata, in the pericentric regions of the acrocentrics. Our working hypothesis is that there is a basic exchange rate between any two acrocentrics. The surplus of t(14q21q) is presumed to depend on these two chromosomes having a homologous pericentric region. The 10-20 times higher incidence of t(13q14q) as compared with other RT is best explained by crossing-over between homologous, but relatively inverted, segments in these chromosomes. Of the 246 RT ascertained through repeated abortions or infertility, 56 were found through the latter. Of these, chromosome 14 was involved in 51. The infertility may be caused by a small deletion of 14q, as is often the case in 15q in Prader-Willi syndrome. In all RT ascertained through 21 or 13 trisomy, respectively, the relevant chromosome is one of the participants. Our data thus do not give any support to the idea of interchromosomal effects exerted by RT.

Structural rearrangements in the parents of children with primary trisomy 21

A retrospective cytogenetic study was carried out on the parents of children with regular trisomy 21 Down's syndrome. In a total of 128 parents referred routinely to our laboratory after the birth of their affected child, three structural abnormalities, a reciprocal translocation and two pericentric inversions not involving chromosome 21, were detected. This is about 10 times the frequency expected based on current figures from consecutive newborn studies. In addition, the brother of one of nine older people with trisomy 21 referred for cytogenetic analysis for the first time was found to have a reciprocal translocation. This supports the contention made by others that an interchromosomal effect does exist in man. It is suggested that centres who routinely analyse the parents of their trisomy 21 referrals in an unbiased fashion should review their records. They will almost certainly contain useful information regarding the possible existence of this phenomenon and may even contain clues as to its nature. In addition to its undoubted scientific value, such data should prove useful in the genetic counselling of carriers of structural rearrangements.

Mortality, pathological findings and causes of death in the de Lange syndrome

The mortality of 48 de Lange patients born 1917-82 was found slightly raised compared with the expected number of deaths, and the survival rates appeared to be lowered. No sex differences were observed. The causes of death and the pathological findings were evaluated in 11 patients. Pneumonia was the most frequent cause of death (6 patients). Congenital malformations seemed to be contributory causes of death in 5 patients, the most frequent being congenital heart disease and gastro-intestinal malformations. A wide variety of malformations were found but no consistent macroscopic or microscopic abnormalities could be demonstrated. An incidence figure of 1:50 000 was found for the years 1967-82.

[Etiology of mongolism]

Beside the rare cares of translocation mongolism, Down syndrome is caused by meiotic malsegregation of chromosomes, No. 21. The meiotic error can take place in both sexes and was found twice as frequently in the female as in the male. The same 2:1 ratio was found concerning nondisjunction in the first and in the second meiotic division. The rate of meiotic errors, which occur at random, is largely dependent on age; this tendency is more pronounced in oogenesis than in spermatogenesis. The results of prenatal chromosomal diagnosis indicate that the recurrence risk of trisomy 21 is not above the age-dependent average.

Decreasing mosaicism in Down's syndrome

A girl with trisomy-21/normal mosaicism has been observed for approximately 10 years. Her appearance is reminiscent of, but not typical for Down's syndrome, and her intellectual abilities are impaired but not to the same severe degree expected in Down's syndrome. These observations are consistent with the belief that, in persons ascertained by clinical resemblance to Down's syndrome. The cytogenetic studies performed longitudinally demonstrated a significant decrease in the proportion of blood lymphocytes showing the trisomic line. The maximal decrease occurred in the first year of life. A direct preparation from the bone marrow and a fibroblast culture at 2.5 months showed 3 and 4% trisomic cells, respectively, corresponding to the level of trisomic cells from the blood cultures during the second year of life.

Down's syndrome in Western Australia: cytogenetics and incidence

235 cases of Down's syndrome were ascertained in a 10-year study of Down's syndrome in Western Australia. Although cytogenetic studies performed on 222 subjects confirmed that 95% of cases were trisomic due to nondisjunction, 4% were trisomic due to translocation, and 1% were mosaic, the ratio of inherited/sporadic translocations differed from that usually reported. Comparison of the results with those of an earlier Australian survey of Down's syndrome demonstrated a real fall in the incidence of Down's syndrome in Australia but no significant change in maternal age-specific incidences.

Incidence study of Down's syndrome in Copenhagen, 1960-1971; with chromosome investigation

The aim of the study was to obtain incidence figures for Down's syndrome throughout a period where a considerable change in the age distribution of child-bearing mothers has taken place and to study if the expected fall in incidence has occurred. In parts of the Copenhagen Metropolitan area 235 liveborn patients with Down's syndrome were ascertained in the period 1960 to 1971 in a population of 1-2 million with a total of 204771 births. All patients available were examined cytogenetically (75%). In 160 (90-4%) a regular trisomy 21 was observed. In 6-2% of the cases translocations and in 2-3% of the cases mosaics were found. Two double trisomies and a double trisomy mosaic were observed. Throughout the period 1960-71 the percentage of women over 30 years delivering children decreased from 23-4% in the beginning of the period to 16-2% at the end of the period. In the first part of the period 52-6% of the cases were born to mothers over 30, at the end of the period 40% of Down's syndrome mothers were of that age. However, the incidence was unchanged throughout the whole period, about 1-15 per 1000 births. For some age groups a steady rise in incidence of trisomy 21 cases was found throughout the whole period. These findings may be explained by better ascertainment of patients at the end of the period; however, environmental factors may also play a role.

Segregation analysis of a large t(21q22q) family

A large family in which a t(21q22q) chromosome is segregating is described. Segregation analysis using data from this family and previously published families shows the risk to a female carrier of having a child with translocation Down's syndrome to be 0·0684±0·0270. The risk to a male carrier is probably much smaller with an upper limit of 0·0286. The segregation ratio for phenotypically normal offspring of carrier parents of both sexes is 0·5053±0·0524.