Low doses of X-rays decrease the risk of diploidy in mouse oocytes

Lack of association between polycystic ovary syndrome and embryonic aneuploidy

OBJECTIVE

To determine whether women with polycystic ovary syndrome (PCOS) are at increased risk for embryonic aneuploidy.

DESIGN

Retrospective cohort study.

SETTING

Academic teaching department, privately owned preimplantation genetics laboratory, and academically affiliated, private infertility center.

PATIENT(S)

The study included 174 women, ages 27-45 years, who underwent a single cycle of controlled ovarian hyperstimulation with gonadotropins for in vitro fertilization. Amongst those, 74 were proven patients with PCOS, and 100 were control women with proven absence of PCOS.

INTERVENTION(S)

Preimplantation genetic diagnosis for chromosomes X, Y, 13, 15, 16, 17, 18, 21, and 22.

MAIN OUTCOME MEASURE(S)

Embryonic morphology, in vitro fertilization cycle outcome parameters, and euploidy and aneuploidy rates.

RESULT(S)

Women with PCOS demonstrated similar overall percentages of euploid embryos (49.1% +/- 28.1) when compared with control women (51.8% +/- 30.1). However, a statistically significantly higher oocyte yield in patients with PCOS (22.8 +/- 9.8 vs. 16.5 +/- 7.6) resulted in statistically significantly higher absolute numbers of euploid embryos (3.3 +/- 2.1 vs. 2.4 +/- 2.0). When stratified for age (<38 y and >or=38 y) and egg numbers (10-20 and >20), euploidy rates still did not vary between study and control patients. High-responder patients with PCOS showed, however, statistically significantly reduced clinical- (42.9% vs. 69.0%) and ongoing-pregnancy rates (40.5% vs. 65.5%) compared with high-responder control women.

CONCLUSION(S)

Women with PCOS are not at increased risk for embryonic aneuploidy in the course of in vitro fertilization treatment. Indeed, because of their larger oocyte numbers, they produce more euploid embryos but have lower pregnancy rates after high oocyte yields. This lower pregnancy rate is, thus, not genetically caused and requires further investigation.

Effect of maternal age on the incidence of digynic triploidy in LT/Sv strain mice: implications for the ovulation of primary and secondary oocytes in this strain

LT/Sv strain mice ovulate both primary and secondary oocytes, which are fertilisable, giving rise to digynic triploid and normal diploid fertilised conceptuses, respectively. Since the proportion of primary and secondary oocytes ovulated in our earlier studies varied widely among females, we investigated whether the proportion of primary oocytes ovulated in LT/Sv strain mice was influenced by maternal age. Females 6, 12, 18, 24, and 30 weeks old were mated with F1 hybrid males. The females were autopsied on the 10th day of gestation, and the intact conceptuses or extraembryonic membranes analysed cytogenetically. Since no selective loss of the triploids occurs up to the 10th day of gestation, analysis at this time also provides indirect information on the proportion of primary and secondary oocytes ovulated. We observed 1) that the overall incidence of triploidy decreased from 55% in the 6-week-old females to 6% in the 30-week-old group; 2) that the number of females from which both triploid and diploid embryos were recovered decreased with increased maternal age; 3) that a substantial decrease in the proportion of triploid embryos was observed in those females in which both triploid and diploid embryos were recovered, in relation to increased maternal age; 4) that there was no overall decrease in the total number of implants with increasing age; and 5) that there was no increase in the incidence of resorptions with increasing maternal age. We believe that no comparable relationship between the ovulation of primary and secondary oocytes and maternal age has previously been reported.

Coincident maternal meiotic nondisjunction of chromosomes X and 21 without evidence of autosomal asynapsis

A family in which the proband showed phenotypic signs of both the Turner and Down syndromes was studied cytogenetically and with restriction fragment length polymorphisms. The proband's karyotype was 46,X,+21, showing double aneuploidy without any signs of mosaicism. The single X and one chromosome 21 were of paternal origin while two chromosome 21 were of maternal origin. The nondisjunction of chromosome 21 took place in maternal meiosis II. If it is assumed that the absence of mosaicism renders postzygotic mitotic loss of the X chromosome unlikely, then the X chromosome would have been lost in maternal meiosis I or II. Recombination had occurred between the nondisjoined chromosomes 21. We conclude that double nondisjunction took place in one patient and that asynapsis was not a prerequisite for the autosomal nondisjunction.

Effect of exogenous hormones on the ovulation of primary and secondary oocytes in LT/Sv strain mice

LT/Sv strain mice ovulate both primary and secondary oocytes. These are fertilizable and give rise to digynic triploid and normal diploid conceptuses, respectively. A previous study [Kaufman and Speirs, 1987] had indicated that just over 20% of embryos recovered on the 10th day of gestation from spontaneously ovulating females had a triploid chromosome constitution. This value was considerably lower than might have been expected by extrapolation from earlier studies in which LT/Sv mice had been given exogenous gonadotrophins. In the present study, therefore, cytogenetic analysis of fertilized eggs was performed at the first cleavage mitosis in (1) spontaneously ovulating females mated to F1 hybrid males, and (2) superovulated females mated to similar males. Additional females from group (1) were autopsied on the 10th day of gestation, and the ploidy of embryos isolated at this stage of gestation was determined. Exposure to exogenous gonadotrophins significantly increased the proportion of eggs that were ovulated as primary oocytes (34.4%), compared to the situation observed following spontaneous ovulation (24.4%). All the triploids encountered in both series were of the digynic type and characteristically (for LT/Sv mice) had an oocyte-derived set with 40 chromosomes present, and a sperm-derived set containing 20 chromosomes. Similar numbers of eggs were recovered from spontaneously ovulating females on the 1st and 10th days of gestation, and the incidence of triploidy observed on the 10th day was 22.1%. The influence of exogenous hormones in increasing the "spontaneous" level of triploidy in LT/Sv and in other strains of mice is briefly reviewed.

Meiotic nondisjunction in oocytes from aged Djungarian hamsters correlates with an alteration in meiosis rate but not in univalent formation

The effect of maternal ageing on the meiotic rate, on chiasma and univalent frequency as well as on heteroploidy in secondary oocytes from Djungarian hamsters was examined. The frequency of hyperhaploid oocytes increased from 0.6% in young (8-14 weeks) to 2.8% in middle-aged (26-46 weeks) and reached 3.6% in the oldest females (49-75 weeks). On the basis of malsegregated bivalents per oocyte, nondisjunction occurred most often in the middle-aged group (5.42 X 10(-2) bivalents per oocyte). Hereby, the large meta- and submetacentric A-D chromosomes were preferentially involved. Furthermore, the pattern of nondisjunction was not different from that expected on the basis of chromosome length or induced by colchicine. The large A-D chromosomes did not show any alteration in chiasma or univalent frequency. Terminalized chiasmata were only detected in the E group and univalents increased slightly, but not significantly in the small chromosomes (G group). At higher ages, both chromosome groups were not preferentially involved in nondisjunction. Presegregation slightly increased with age and affected more or less all bivalents, whereas the incidence of diploidy significantly decreased. With respect to the rate of meiosis in oocytes from aged females, the resumption was delayed at metaphase I. Our data suggest that failures in the control of oocyte proliferation are involved in nondisjunction rather than the "production-line." Furthermore, a model is proposed to explain nondisjunction of specific bivalents at certain maternal ages.

Inheritance of a meiosis I error expressed in mouse oocytes and modulated by a maternal factor

NMRI/Han mice ovulate significant numbers of diploid oocytes after gonadotrophin stimulation, most of them arrested at metaphase I. In contrast, females from other mouse strains ovulate only oocytes having completed meiosis I. We investigated the heritability of the trait Dipl I by analysing F1hybrid females from crosses between the sensitive NMRI/Han stock and two mouse strains (C3H/HeHan and BALB/c) considered as non-sensitive, and females from backcrosses to NMRI/Han males (only crosses with C3H/HeHan). The results show (1) that the trait Dipl I is inheritable; (2) that an X-chromosomal (location proximally from the X–Y pairing region) or autosomal recessive mode of inheritance is excluded; and (3) that the expression of the trait, measured as frequency of diploidy, is modulated by a maternally transmitted factor.

Mechanisms of nondisjunction: facts and perspectives

An ordered segregation requires distinct processes of differentiation within the germ cell for recognition and segregation of homologous chromosomes/chromatids. These include synchronous maturation of the nucleus and cytoplasm, chromosome pairing and assembly at the metaphase plate, and movement within the spindle; all of them may be under direct/indirect regulatory control by the surrounding somatic compartments. Interference, e.g., by hormonal alterations at any of these steps, may alter the normal program of differentiation, thus increasing the risk of chromosomal malsegregation. Hence, many different causative mechanisms may exist which basically, nevertheless, act via (a) nonsegregation, (b) chance segregation, both during meiosis 1 and 2, or (c) presegregation during meiosis 1. In our animal models of the Djungarian hamster and the NMRI/Han mouse strain, we are analyzing the mechanisms of nondisjunction and presegregation during meiosis 1 in oocytes, as well as during aging of the females, by the application of hormones (gonadotrophins and steroids) and specific microtubular inhibitors (colchicine and methylbenzimidazolcarbamate). We suggest that chromosomes play a relatively passive role, although chromosomal properties, e.g., length, chiasma number, NORs, and position within the spindle, may provide an individual risk for each bivalent to be affected by nondisjunction. Failures in the endocrine control of follicular and germ cell maturation are considered to be primary causes for nondisjunction in young and aging oocytes. The understanding of the differentiation processes resulting in the maturation of follicles "at risk" may provide us with the tool to prevent the generation of aneuploidy in man.

The genetic basis of non-disjunction: increased incidence of hyperploidy in oocytes from F1 hybrid mice

Oocytes from parental mice strains NMRI/Han, C57/bl and Balb/c and from F1 hybrid lines were analysed for aneuploidy due to non-disjunction after gonadotropin-stimulated ovulation. No hyperploid oocytes were present in five of the strains studied. F1 hybrids from crosses of NMRI/HanxC57/bl did ovulate, however, a significantly increased number of hyperploid oocytes, although females from their parental strains show a rather low incidence of non-disjunction. The evidence for a genetic basis for non-disjunction is assessed and possible causative factors are discussed.