Risk Factor For Down Syndrome Research Articles

Acrocentric chromosome double NOR is not a risk factor for Down syndrome

Jackson-Cook et al. (American Journal of Human Genetics 37:1049-1061, 1985) predicted a high risk of Down syndrome (DS) children for parents carrying a double NOR on acrocentric chromosomes. Hassold et al. (Human Genetics 76:381-384, 1987) could not confirm Jackson-Cook et al.'s findings, thus casting doubts on their conclusions. We studied the NORs of 1) 60 parents of 30 unselected DS subjects; 2) 30 unselected healthy subjects without trisomic offspring, who asked for chromosome analysis; and 3) 100 slides randomly chosen among 1,000 prepared by routine standard techniques and belonging to subjects who were chromosomally normal. By applying rigorously established techniques and scoring criteria we found 4 subjects (6.7%) with strictly defined double NORs (dNORs) in the DS parents sample, 2 subjects (6.7%) in the first control sample, and 3 (3%) in the second control sample. No significant difference among the observed frequencies of dNORs in the 3 samples could be demonstrated. Therefore, our data do not support Jackson-Cook et al.'s statements on the association of dNOR carrier status with DS offspring and on a highly increased risk of meiotic nondisjunction of chromosome 21 for a dNOR carrier parent. A tentative interpretation of the apparently contrasting cytogenetic findings would indicate that sampling and assignment biases are the main causes of this discrepancy.

Abnormal folic acid-homocysteine metabolism as maternal risk factors for Down syndrome in Japan.

Japan has been considered as "a folate sufficient area", since traditional Japanese food contains an adequate amount of folic acid. However, the recent westernized food style of young Japanese mothers may affect the intake of folic acid among them. This food style may contribute to the occurrence of Down syndrome, which has proved to be linked to abnormal folate and homocysteine metabolisms. To preliminarily evaluate the levels of folic acid,homocysteine and other relevant factors which are associated with folate metabolism, among Japanese women who had pregnancies affected by Down syndrome. Blood samples from 31 women who had pregnancies affected by Down syndrome (DS) were obtained. 60 age-matched control blood samples were also obtained from mothers who had not experienced miscarriages or abnormal pregnancies (CONT). Plasma homocysteine and serum folic acid, vitamin B12, and B6 were measured and compared between DS and CONT. Furthermore, the frequency of MTHFR polymorphism (C677T) was also investigated. Plasma levels of homocysteine were significantly increased in DS mothers (p = 0.004). In contrast, serum levels of folic acid were significantly decreased in DS mothers (p = 0.0001). There were no significant differences in the vitamin B12 and B6 levels between DS and CONT. Also, the frequency of 5,10-methylenetetrahydrofolate reductase gene (MTHFR) homozygous polymorphism showed no differences between DS and CONT. Different levels of serum folic acid and plasma homocysteine between both groups may contribute to the occurrence of Down syndrome even in Japan. Although there was no significant difference in the frequency of MTHFR polymorphism between the groups, probably because of an inadequate number of samples, further studies may contribute to the understanding of the occurrence of Down syndrome in Japan.

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.

Abnormal folate metabolism and mutation in the methylenetetrahydrofolate reductase gene may be maternal risk factors for Down syndrome

Down syndrome, or trisomy 21, is a complex genetic disease resulting from the presence of 3 copies of chromosome 21. The origin of the extra chromosome is maternal in 95% of cases and is due to the failure of normal chromosomal segregation during meiosis. Although advanced maternal age is a major risk factor for trisomy 21, most children with Down syndrome are born to mothers <30 y of age. On the basis of evidence that abnormal folate and methyl metabolism can lead to DNA hypomethylation and abnormal chromosomal segregation, we hypothesized that the C-to-T substitution at nucleotide 677 (677C-->T) mutation of the methylenetetrahydrofolate reductase (MTHFR) gene may be a risk factor for maternal meiotic nondisjunction and Down syndrome in young mothers. The frequency of the MTHFR 677C-->T mutation was evaluated in 57 mothers of children with Down syndrome and in 50 age-matched control mothers. Ratios of plasma homocysteine to methionine and lymphocyte methotrexate cytotoxicity were measured as indicators of functional folate status. A significant increase in plasma homocysteine concentrations and lymphocyte methotrexate cytotoxicity was observed in the mothers of children with Down syndrome, consistent with abnormal folate and methyl metabolism. Mothers with the 677C-->T mutation had a 2.6-fold higher risk of having a child with Down syndrome than did mothers without the T substitution (odds ratio: 2.6; 95% CI: 1.2, 5.8; P < 0.03). The results of this initial study indicate that folate metabolism is abnormal in mothers of children with Down syndrome and that this may be explained, in part, by a mutation in the MTHFR gene.

Down syndrome and ionizing radiation.

This review examines the epidemiologic and experimental studies into the possible role ionizing radiation might play in Down Syndrome (trisomy 21). It is prompted by a report of a temporal cluster of cases of this chromosomal disorder observed in West Berlin exactly 9 mo after the radioactive cloud from Chernobyl passed. In approximately 90% of cases, Down Syndrome is due to the nondisjunction of chromosome 21, most often in the oocyte, which may be exposed to ionizing radiation during two separate periods: before the completion of the first meiosis or around the time of ovulation. Most epidemiologic studies into trisomies and exposure to ionizing radiation examine only the first period; the Chernobyl cluster is related to the second. Analysis of these epidemiologic results indicates that the possibility that ionizing radiation might be a risk factor in Down Syndrome cannot be excluded. The experimental results, although sometimes contradictory, demonstrate that irradiation may induce nondisjunction in oogenesis and spermatogenesis; they cannot, however, be easily extrapolated to humans. The weaknesses of epidemiologic studies into the risk factors for Down Syndrome at birth (especially the failure to take into account the trisomy cases leading to spontaneous abortion) are discussed. We envisage the utility and feasibility of new studies, in particular among women exposed to prolonged or repeated artificially-produced ionizing radiation.

Parity and Down syndrome

Previous studies have suggested a positive correlation between grand multiparity and the incidence of Down syndrome (DS). In order to study different parities as risk factors for DS, the Swedish health registries were used, and 2,615 infants with Down syndrome were selected from 2,184,590 infants born in 1973-1993. A statistically significant risk decrease for primiparas, and a significant risk increase for grand multiparas (5+), was found (age-adjusted odds ratios: 0.87 (95% CI: 0.80-0.96) and 1.40 (95% CI: 1.18-1.65), respectively. Potential confounders, such as the effect of truncated maternal 1-year age classes, citizenship, socioeconomic level, etc., were evaluated but were found to have only marginal effects. Evidence suggesting that the extension of prenatal diagnosis during the study period has decreased the incidence of DS among women of parity 1-4, but not among women of parity 5+, was found. The hypothesis that Swedish grand multiparas may have another attitude toward prenatal diagnosis than women of lower parities was confirmed when, in a data set containing information on 872 amniocenteses, a significantly lower rate of grand multiparity than expected was found. For the negative association between primiparity and DS, no obvious confounder was found.

Paternal age as a risk factor for Down syndrome

Although the effect of maternal age as a risk factor for Down syndrome (DS) is well known, the role of paternal age in the cause of DS has not been clearly established. To investigate this phenomenon we conducted a case-control study between July 1989 and February 1990. The cases were 318 children and teenagers with DS studied at the Specialized Educational Institutions of Lima City, Perú. They were paired with 1,196 control individuals that were selected from the birth records of 2 general hospitals of the city. For each case we tried to obtain 4 controls, paired by their date of birth, sex, and maternal age. The means of paternal age in the 2 groups were compared, first globally and then by groups of maternal age (< 21 years, 21-29 years, 30-34 years, 35-39 years and > 39 years). None of the comparisons gave a statistically significant difference between the 2 groups, using either the Student t-test or the Mann-Whitney U-test. The results obtained in this study give no evidence that paternal age can be considered a risk factor for the conception of a child with DS.

Comparison of Assay Kits for Unconjugated Estriol Shows that Expressing Results as Multiples of the Median Causes Unacceptable Variation in Calculated Risk Factors for Down Syndrome

We compared the performance of two methods for assaying unconjugated estriol in serum: the modified Amerlex third-trimester RIA kit, as used in seminal papers on unconjugated estriol in Down syndrome screening, and the new optimized Amerlex-M second-trimester kit. The significant difference between the results of each assay could cause unacceptable changes in the detection rate and false-positive rate of Down syndrome screening programs, especially if previously published values for estriol are used in the risk calculation. It is not possible to define new calculation parameters for every assay kit because new parameters will need to be defined every time kit changes occur, which would require a large collection of samples from Down syndrome pregnancies for standardization. Possible solutions to this problem are discussed.

Vaginal bleeding in pregnancies associated with fetal Down syndrome.

It has been suggested that vaginal bleeding in pregnancy is associated with an increased risk of fetal Down syndrome. To investigate this, information on vaginal bleeding, collected at the first antenatal visit, was abstracted from the medical notes of 70 pregnancies associated with Down syndrome and 140 unaffected controls matched for maternal age. Fourteen cases (20 per cent) and 23 controls (16 per cent) had some evidence of bleeding (relative risk estimated as the odds ratio = 1.3; P = 0.26, one-sided; 95 per cent confidence interval 0.6-2.8). When these results were pooled with those from the two other controlled studies already published the combined relative risk estimate was 1.8 (P = 0.02, two-sided; 95 per cent confidence interval 1.1-3.0). On the basis of present evidence, there is some reason to regard vaginal bleeding prior to the first antenatal visit as a risk factor for Down syndrome but the association and its biological significance remains uncertain.