Elise Millie

Human aneuploidy: Incidence, origin, and etiology

Chromosome abnormalities occur with astonishing frequency in humans, being present in an estimated 1030% of all fertilized eggs. Of the different classes of chromosome abnormality, aneuploidy (trisomy and monosomy) is by far the most common and, clinically, the most important. For example, over 25% of all miscarriages are monosomic or trisomic, making aneuploidy the leading known cause of pregnancy loss. Furthermore, among those conceptions that survive to term, aneuploidy is the leading genetic cause of mental retardation. Over the past 25 years, a considerable body of information has accrued on the incidence of aneuploidy in human gametes, fetuses, and newborns. More recently, the application of molecular biological techniques to the study of aneuploidy has begun to uncover some of the underlying causes of human aneuploidy. In this review, we first summarize the cytogenetic data on the incidence of aneuploidy in humans, and then discuss recent molecular data on the mechanism of origin of different aneuploid conditions, the basis of the maternal age effect on aneuploidy, and the importance of aberrant genetic recombination to the genesis of aneuploidy.

Sperm aneuploidy among Chinese pesticide factory workers: scoring by the FISH method.

BACKGROUND A study of the prevalence of sperm aneuploidy among pesticide factory workers was conducted in Anhui, China. METHODS We recruited 75 men: 32 subjects from a large pesticide-manufacturing plant and 43 subjects from a nearby textile factory free of pesticide exposure. Each subject met the following criteria: age of 20-40 years; continuous work in the plant for 3 months prior to the study, no congenital anomalies or acquired disease of the external genitalia and no history of recent febrile illness or mumps. Within one hour after collection from each subject, semen was evaluated in terms of several parameters and smear slides were prepared. RESULTS Exposure assessment revealed that workers in the pesticide plant were exposed to ethyl parathion or methamidophos, each of which is a potent organophosphate pesticide, at a median level of 0.02 mg/m3 (8-hour time weighted average as measured by personal pump) while workers in the control plant had no such occupational exposure. Twenty-nine semen slides (13 from the exposed group and 16 from the unexposed group) were randomly chosen for aneuploidy scoring by the three-color fluorescence in situ hybridization (FISH) method with scorers being unaware of exposure status. Median semen parameters were as follows for exposed (and unexposed) men: abstinence period, 3 days (4 days); sperm concentration, 52.8x10(6)/ml (53.1x10(6)/ml); proportion of sperm with normal motility, 50.5% (61.3%); and proportion of sperm with normal morphology, 59% (61.5%). The specific chromosome abnormalities of interest were disomy for chromosome 18 and the three different types of sex chromosome disomy (i.e. XX, XY, YY disomy). The crude proportion of all aneuploidy combined was 0.30% and 0.19% for sperm from exposed and unexposed men, respectively. Poisson regression with overdispersion adjustment yielded significantly different crude risks of aneuploidy - 3.03 and 1.94 per 1,000 sperm from exposed and unexposed men, respectively - giving a rate ratio of 1.56 (95% CI, 1.06-2.31). The regression coefficients remained statistically significant after adjustment for inter-technician variability giving a rate ratio of 1.51 (95% CI, 1. 04-2.20). CONCLUSIONS We conclude that occupational exposure to organophosphate pesticides moderately increases the prevalence of sperm aneuploidy.

Akt1/PKB upregulation leads to vascular smooth muscle cell hypertrophy and polyploidization

Vascular smooth muscle cells (VSMCs) at capacitance arteries of hypertensive individuals and animals undergo marked age- and blood pressure-dependent polyploidization and hypertrophy. We show here that VSMCs at capacitance arteries of rat models of hypertension display high levels of Akt1/PKB protein and activity. Gene transfer of Akt1 to VSMCs isolated from a normotensive rat strain was sufficient to abrogate the activity of the mitotic spindle cell-cycle checkpoint, promoting polyploidization and hypertrophy. Furthermore, the hypertrophic agent angiotensin II induced VSMC polyploidization in an Akt1-dependent manner. These results demonstrate that Akt1 regulates ploidy levels in VSMCs and contributes to vascular smooth muscle polyploidization and hypertrophy during hypertension.

Cytogenetic analysis of spontaneous abortions: Comparison of techniques and assessment of the incidence of confined placental mosaicism

Cytogenetic studies on spontaneous abortions traditionally have used one of two methodologies, direct preparations or long-term culture, to determine the chromosome constitution of either the cytotrophoblast or villous stroma, respectively. Few studies have utilized both techniques simultaneously to compare the relative efficiencies of each method and to assess the contribution of confined placental mosaicism (CPM). The present report summarizes cytogenetic studies on 691 consecutive spontaneous abortions using long-term culture, direct preparations, or both. All 691 cases were analyzed by long-term culture and 177 cases were analyzed using both long-term culture and direct preparations. The results indicate that the two methods have similar success rates, 82% for long-term culture and 76% for direct preparation; however, the proportion of normal females was significantly increased in the culture method, presumably attributable to maternal contamination. In 107 cases, results were obtained from both methods with 22 discrepancies identified. However, most of these involved a 46,XX result in culture, consistent with maternal contamination in the cultured preparation. Therefore, to estimate the proportion of CPM we excluded cases with a 46,XX result in culture and found four (6.1%) of the remaining 65 cases to be consistent with CPM. These cases consisted of normal or mosaic aneuploid cytotrophoblast and non-mosaic aneuploid villous stroma. These studies suggest that each method has specific advantages in the analysis of spontaneous abortions. Direct preparations are less prone to maternal contamination, but certain chromosome abnormalities are more likely to be identified using long-term culture.

Studies of non-disjunction in trisomies 2, 7, 15, and 22: does the parental origin of trisomy influence placental morphology?

Recently, there have been several molecular studies of trisomic fetuses and liveborns which have examined the parent and meiotic stage of origin of nondisjunction. However, little is known about the possible phenotypic effects of the origin of trisomy. For trisomic spontaneous abortions, no distinct phenotype has been described, although some have been reported to have features, such as trophoblastic hyperplasia, similar to hydatidiform moles. In the present report, we describe molecular and histological studies of spontaneous abortions with trisomies 2, 7, 15, or 22, conditions occasionally linked to trophoblastic hyperplasia. Our results provide strong evidence for chromosome specific mechanisms of nondisjunction, with trisomy 2 having a high frequency of paternally derived cases and trisomy 7 typically originating postzygotically. In studies correlating parental origin of trisomy with phenotype, we found no difference in the proportion of cases with trophoblastic hyperplasia, fetal tissue, nucleated red blood cells, or hydropic villi among paternally or maternally derived trisomies 2, 7, 15, or 22. However, paternally derived trisomies tended to abort earlier than maternally derived trisomies. This suggests that parental origin might affect the developmental stage at which abortion occurs but not other features of placental phenotype.

The effect of Y-chromosome alpha-satellite array length on the rate of sex chromosome disomy in human sperm.

Trisomy is the leading known cause of mental retardation and pregnancy loss in humans, yet virtually nothing is known of the underlying nondisjunctional mechanisms. Since studies of other organisms suggest an association between centromere size or sequence and meiotic nondisjunction, we recently initiated studies to examine the effect of centromere size variation on human nondisjunction. In the present report, we summarize studies correlating variation in the size of the Y-chromosome centromere with sex chromosome nondisjunction. In one set of studies, we used pulsed-field gel electrophoresis to estimate Y-chromosome alpha-satellite array lengths in normal males, and correlated these values with Y-chromosome sperm disomy levels as determined by fluorescence in situ hybridization. In a second set of studies, we determined the Y-chromosome alpha-satellite array length of 47,XYY males, since the karyotypes of these individuals are a consequence of Y chromosome nondisjunction. Neither set of studies provided evidence for an effect of Y-chromosome alpha-satellite array length on Y-chromosome nondisjunction. Thus, if there is an association between Y-chromosome centromere size and nondisjunction, the effect is subtle and below the detection levels of the present study or involves extreme size variants that were not represented in the present study population.

FISH studies of the sperm of fathers of paternally derived cases of trisomy 21: no evidence for an increase in aneuploidy.

Abstract Paternal nondisjunction accounts for approximately 5% of cases of trisomy 21. To test the hypothesis that, in some such cases, the fathers might be predisposed to meiotic nondisjunction, we utilized fluorescence in situ hybridization (FISH) to screen for aneuploidy in sperm. We analyzed sperm samples from ten males with a trisomy 21 offspring of paternal origin. Among these individuals, the overall frequency of disomy 21 was 0.15%, comparable to estimates of disomy 21 in the general male population. Furthermore, none of the ten fathers of trisomy 21 individuals had significantly elevated levels of disomic sperm. Thus, our results provide no evidence that the occurrence of a trisomy 21 conceptus of paternal origin imparts an increased risk of trisomy in subsequent pregnancies.

Meiotic Exchange and Segregation in Female Mice Heterozygous for Paracentric Inversions

Inversion heterozygosity has long been noted for its ability to suppress the transmission of recombinant chromosomes, as well as for altering the frequency and location of recombination events. In our search for meiotic situations with enrichment for nonexchange and/or single distal-exchange chromosome pairs, exchange configurations that are at higher risk for nondisjunction in humans and other organisms, we examined both exchange and segregation patterns in 2728 oocytes from mice heterozygous for paracentric inversions, as well as controls. We found dramatic alterations in exchange position in the heterozygotes, including an increased frequency of distal exchanges for two of the inversions studied. However, nondisjunction was not significantly increased in oocytes heterozygous for any inversion. When data from all inversion heterozygotes were pooled, meiotic nondisjunction was slightly but significantly higher in inversion heterozygotes (1.2%) than in controls (0%), although the frequency was still too low to justify the use of inversion heterozygotes as a model of human nondisjunction.