Trisomy and age at menopause: predicted associations given a link with rate of oocyte atresia

Maternal age in the epidemiology of common autosomal trisomies

The birth prevalence rate of each common autosomal trisomy generally increases with advancing maternal age and there is a substantial fetal loss rate between late first trimester and term. The literature is reviewed in order to provide the best estimates of these rates, taking account where possible of biases due to prenatal diagnosis and selective termination of pregnancy. There is an almost exponential increase in Down syndrome birth prevalence between ages 15 and 45 but at older ages the curve flattens. There is no evidence of the claimed relatively high birth prevalence at extremely low ages. Gestation-specific intra-uterine fetal loss rates are estimated by follow-up of women declining termination of pregnancy after prenatal diagnosis, comparison of observed rates with those expected from birth prevalence and comparison of age-specific curves developed for prenatal diagnosis and birth. Down syndrome fetal loss rates reduce with gestation and increase with maternal age. Edwards and Patau syndrome birth prevalence is approximately 1/8 and 1/13 that of Down syndrome overall, although the ratio differs according to maternal age, particularly for Patau syndrome where it reduces steadily from 1/9 to 1/19. Fetal loss rates are higher for Edwards and Patau syndromes than for Down syndrome.

Telomere length is associated with types of chromosome 21 nondisjunction: a new insight into the maternal age effect on Down syndrome birth

Advanced maternal age is a well-documented risk factor of chromosome 21 nondisjunction in humans, but understanding of this association at the genetic level is still limited. In particular, the state of maternal genetic age is unclear. In the present study, we estimated maternal genetic age by measuring telomere length of peripheral blood lymphocytes among age-matched mothers of children with Down syndrome (cases: N = 75) and mothers of euploid children (controls: N = 75) in an age range of 18-42 years. All blood samples were taken within 1 week of the birth of the child in both cases and controls. The telomere length estimation was performed by restriction digestion--Southern blot hybridization method. We stratified the cases on the basis of centromeric STR genotyping into maternal meiosis I (N = 48) and maternal meiosis II (N = 27) nondisjunction groups and used linear regression to compare telomere length as a function of age in the euploid, meiosis I and meiosis II groups. Our results show that all three groups have similar telomere length on average for younger mothers. As age increases, all groups show telomere loss, but that loss is largest in the meiosis II mother group and smallest in the euploid mother group with the meiosis I mother group in the middle. The regression lines for all three were statistically significantly different from each other (p < 0.001). Our results do not support the theory that younger women who have babies with Down syndrome do so because are 'genetically older' than their chronological age, but we provide the first evidence that older mothers who have babies with Down syndrome are 'genetically older' than controls, who have euploid babies at the same age. We also show for the first time that telomere length attrition may be associated in some way with meiosis I and meiosis II nondisjunction of chromosome 21 and subsequent Down syndrome births at advanced maternal age.

Trisomic pregnancy and elevated FSH: implications for the oocyte pool hypothesis

BACKGROUND

Some studies, but not all, support the hypothesis that trisomy frequency is related to the size of the oocyte pool, with the risk increased for women with fewer oocytes (older ovarian age). We tested this hypothesis by comparing hormonal indicators of ovarian age among women who had trisomic pregnancy losses with indicators among women with non-trisomic losses or chromosomally normal births. The three primary indicators of advanced ovarian age were low level of anti-Müllerian hormone (AMH), high level of follicle-stimulating hormone (FSH) and low level of inhibin B.

METHODS

The analysis drew on data from two hospital-based case-control studies. Data were analyzed separately and the evidence from the two sites was combined. We compared 159 women with trisomic pregnancy losses to three comparison groups: 60 women with other chromosomally abnormal losses, 79 women with chromosomally normal losses and 344 women with live births (LBs) age-matched to women with losses. We analyzed the hormone measures as continuous and as categorical variables. All analyses adjust for age in single years, day of blood draw, interval in storage and site.

RESULTS

AMH and inhibin B did not differ between women with trisomic losses and any of the three comparison groups. Mean ln(FSH) was 0.137 units (95% confidence interval (CI): 0.055, 0.219) higher for trisomy cases compared with LB controls; it was also higher, though not significantly so, for trisomy cases compared with women with other chromosomally abnormal losses or chromosomally normal losses. The adjusted odds ratio in relation to high FSH (≥ 10 mIU/ml) was significantly increased for trisomy cases versus LB controls (adjusted odds ratio (OR): 3.8, 95% CI: 1.6, 8.9).

CONCLUSIONS

The association of trisomy with elevated FSH is compatible with the oocyte pool hypothesis, whereas the absence of an association with AMH is not. Alternative interpretations are considered, including the possibility that elevated FSH may disrupt meiotic processes or allow recruitment of abnormal follicles.

On the origin of the maternal age effect in trisomy 21 Down syndrome: the Oocyte Mosaicism Selection model

We have recently documented that trisomy 21 mosaicism is common in human foetal ovaries. On the basis of this observation we propose that the maternal age effect in Down syndrome (DS) is caused by the differential behaviour of trisomy 21 in relation to disomy 21 oocytes during development from foetal life until ovulation in adulthood. In particular, we suggest that trisomy 21 oocytes, lagging behind those that are disomic, may escape the timed pruning of the seven million in foetal life to the 300–400 finally selected for ovulation. The net effect of this preferential elimination will be an accumulation of trisomy 21 oocytes in the ovarian reserve of older women. We here highlight the implications of this Oocyte Mosaicism Selection (OMS) model with respect to the prevalent view that the maternal age effect is complex, dependent on many different biological and environmental factors. We examine conclusions drawn from recent large-scale studies in families, tracing DNA markers along the length of chromosome 21q between parents and DS children, in comparison to the OMS model. We conclude that these family linkage data are equally compatible with the maternal age effect originating from the accumulation of trisomy 21 oocytes with advancing maternal age. One relatively straightforward way to get to grips with what is actually going on in this regard would be to compare incidence of trisomy 21 oocytes (and their pairing configurations) in foetal ovaries with that in oocytes at the meiosis I stage from adult women.

Skewed X chromosome inactivation and trisomic spontaneous abortion: no association

Several studies suggest that highly skewed X chromosome inactivation (HSXI) is associated with recurrent spontaneous abortion. We hypothesized that this association reflects an increased rate of trisomic conceptions due to anomalies on the X chromosome that lead both to HSXI and to a diminished oocyte pool. We compared the distribution of X chromosome inactivation (XCI) skewing percentages (range: 50%-100%) among women with spontaneous abortions in four karyotype groups-trisomy (n = 154), chromosomally normal male (n = 43), chromosomally normal female (n = 38), nontrisomic chromosomally abnormal (n = 61)-to the distribution for age-matched controls with chromosomally normal births (n = 388). In secondary analyses, we subdivided the nontrisomic chromosomally abnormal group, divided trisomies by chromosome, and classified women by reproductive history. Our data support neither an association of HSXI with all trisomies nor an association of HSXI with chromosomally normal male spontaneous abortions. We also find no association between HSXI and recurrent abortion (n = 45).

Biological aging and the etiology of aneuploidy

A prevalent hypothesis concerning the cause of the rise in aneuploid conceptions with maternal age is that the changes that accompany normal ovarian aging increase the rate of meiotic errors in the oocyte. Biological aging of the ovary is accompanied by a decline in both the total oocyte pool and the number of antral follicles maturing per cycle, as well as changes in the levels of circulating reproductive hormones. The biological aging hypothesis predicts that aneuploidy rates should be higher in women with a prematurely reduced oocyte pool, and that women with trisomic conceptions should show signs of earlier ovarian aging than women of the same chronological age without trisomic conceptions. Comprehensive studies of aneuploidy in groups of women with known causes of premature ovarian failure remain to be done, though anecdotal evidence does suggest increased rates of pregnancy loss and aneuploidy. Smoking, which is a well-documented cause of earlier ovarian aging, is not associated with an increase in aneuploid conceptions. Evidence from women with unilateral ovariectomies is inconsistent. Support for the biological aging hypothesis was provided by one study showing that menopause occurred about a year earlier in women with a trisomic spontaneous abortion compared to women with chromosomally normal conceptions. Associations between high FSH and pregnancies with Down syndrome and chromosomally abnormal spontaneous abortions have also been reported. However, the most direct test of the hypothesis, which compared antral follicle counts and hormonal levels in women with trisomic pregnancies and those with chromosomally normal pregnancies, failed to find a difference in the expected direction. A prospective study of FSH levels in women with subfertility also failed to find an association with the rate of pregnancy loss. The bulk of evidence thus suggests that, if the processes of biological aging are indeed related to aneuploidy, they probably involve factors other than those measured by oocyte or antral follicle pool size and reproductive hormone levels.

Primary prevention of Down's syndrome

BACKGROUND

Antenatal screening has the capacity to detect more than 90% of Down's syndrome pregnancies leading to therapeutic abortion. Successes in recent years with such so-called 'secondary' prevention have not been matched with progress in primary prevention. Despite considerable research over many decades the principle cause of the disorder is unknown.

METHODS

This paper considers three potential primary prevention strategies, (1) avoiding reproduction at advanced maternal age, (2) pre-implantation genetic diagnosis for couples who are at high risk of Down's syndrome, and (3) folic acid supplementation. The principle aetiological hypotheses are also reviewed.

INTERPRETATION

A strategy of completing the family before a maternal age of 30 could more than halve the birth prevalence of this disorder. Women with a high a priori risk should have access to pre-implantation genetic diagnosis, which can lead to a reasonably high pregnancy rate with an extremely low risk of a Down's syndrome. The evidence suggesting an aetiological role for defective folate and methyl metabolism is not sufficient to justify an active preventative strategy of folic acid supplementation without performing a large clinical trial. Current supplementation policies designed to prevent neural tube defects may incidentally prevent Down's syndrome, provided a sufficiently high dose of folic acid is used. Further progress in primary prevention is hampered by limited aetiological knowledge and there is an urgent need to refocus research in that direction.

Poor response after hormonal stimulation for in vitro fertilization is not related to ovarian aging

OBJECTIVE

To investigate whether the diminished efficacy of ART in young poor responders compared to young normal responders is due to a quantitative or a qualitative oocyte factor.

DESIGN

Retrospective comparative analysis.

SETTING

University-based infertility center.

PATIENT(S)

Nine thousand six hundred forty-four patients who underwent ART procedures at our hospital from 1993 until 2001.

INTERVENTION(S)

Controlled ovarian hyperstimulation, ultrasonographic monitoring of the ovarian response, oocyte retrieval, ART procedure, embryo transfer, and follow-up of pregnant patients until 12 weeks of amenorrhea.

MAIN OUTCOME MEASURES

Clinical rates of pregnancy and miscarriage.

RESULT(S)

Nine thousand six hundred forty-four ART cycles were analyzed. The pregnancy rate for poor responders was significantly lower than for normal responders (17% vs. 35%). In cycles in which two good-quality embryos were transferred, the pregnancy rate was similar in poor responders and normal responders (33% vs. 42%). The rate of miscarriage was no higher in poor responders than normal responders.

CONCLUSION(S)

Young poor responders have a lower pregnancy rate than young normal responders because they have fewer oocytes, which leads to fewer good-quality embryos to choose from for transfer. The quality of their oocytes and embryos is not inferior to that of normal responders.

Trisomic pregnancy and earlier age at menopause

We tested the hypothesis that the connection between advanced maternal age and autosomal trisomy reflects the diminution of the oocyte pool with age. Because menopause occurs when the number of oocytes falls below some threshold, our hypothesis is that menopause occurs at an earlier age among women with trisomic pregnancies than it does among women with chromosomally normal pregnancies. To determine their menstrual status, we interviewed women from our previous study of karyotyped spontaneous abortions who, in 1993, were age >/=44 years. Premenopausal women completed interviews every 4-5 mo, until menopause or until the study ended in 1997. The primary analyses compare 111 women whose index pregnancy was a trisomic spontaneous abortion with two groups: women whose index pregnancy was a chromosomally normal loss (n=157) and women whose index pregnancy was a chromosomally normal birth (n=226). We used a parametric logistic survival analysis to compare median ages at menopause. The estimated median age at menopause was 0.96 years earlier (95% confidence interval -0.18 to 2.10) among women with trisomic losses than it was among women with chromosomally normal losses and chromosomally normal births combined. Results were unaltered by adjustment for education, ethnicity, and cigarette smoking. Our results support the hypothesis that trisomy risk is increased with decreased numbers of oocytes. Decreased numbers may indicate accelerated oocyte atresia or fewer oocytes formed during fetal development.

Is follicular atresia biphasic?

OBJECTIVE

To examine the rate of human follicular depletion and the interpretation of curved scatters on log-linear plots.

DESIGN

Four mathematical models were tested with use of data drawn from published autopsy studies and histologic analyses of ovaries.

SETTING

None.

PATIENT(S)

None.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

None.

RESULT(S)

Human oocyte depletion data do not support the inference of a biphasic follicular atresia. On original measurement scales there is no perturbation in the data between ages 37 and 40, and the instantaneous rate of follicle loss is lower after age 40 than ever before.

CONCLUSION(S)

There is no abrupt increase in the "rate" of follicular atresia that corresponds with a drop in fecundability or an increase in risk of chromosomal abnormalities at approximately age 38. The apparent abrupt increase in rate of follicular depletion is an artifact of log-linear transformation.

Age and the ovarian follicle pool assessed with transvaginal ultrasonography

OBJECTIVE

We tested whether transvaginal ultrasonography could detect the age-related decrease in follicle counts that has been observed in autopsy studies.

STUDY DESIGN

Thirty-one healthy volunteers in three age groups (22 to 25, 30 to 33, and 39 to 42 years) underwent ultrasonography in the follicular and luteal phases of the menstrual cycle. At the conclusion of the study the 124 ovarian scans were randomly ordered and antral follicles > or = 2 mm were counted by an evaluator unaware of age. Ordinary least-squares linear regression was used to estimate the associations of age with the total antral follicle count and with ln (1 + follicle count).

RESULTS

The numbers of antral follicles > or = 2 mm decreased by about 60% between 22 and 42 years. Age-related decreases were similar for both phases of the cycle and held for both smaller (2 to 3.5 mm) and larger (>3.5 mm) follicles.

CONCLUSION

We hypothesize that ultrasonographically derived counts of follicles provide a measure of reproductive age that may help to predict age-related phenomena.

Parental age-related aneuploidy in human germ cells and offspring: a story of past and present

Parental age is the most important aetiological factor in trisomy formation in humans. Cytogenetic studies on germ cells reviewed here imply that (i) 2-4% sperm are aneuploid and 8.6% oocytes from IVF are hyperploid (ii) a paternal age effect may exist, and (iii) oocytes of aged women contain precociously separated chromatids in metaphase II. Trisomy data suggest that most aneuploidy is generated during meiosis I of oogenesis and is maternal age-dependent. Trisomy 18 is unique, originating mostly from maternal meiosis II errors. The extra gonosome in 47, XXY derives mostly from a paternal meiosis I error. Trisomy of individual chromosomes may remain low, linearly rise, or exponentially increase with advanced maternal age. Maternal age related trisomies involve achiasmatic and normochiasmate chromosomes, and chromosomes with disturbed recombination and distally located chiasmata. Hypotheses on the origin of the maternal age effect are critically reviewed. One model is presented that relates to altered cell cycle and protein phosphorylation in oocytes of aged mammals and accounts for most of the observed data in humans and in experimental studies. Aneuploidy may thus involve a predetermined component but is possibly also influenced by extrinsic factors reducing oocyte quality or depleting the oocyte pool precociously. Areas of future research are proposed to elucidate (i) the significance of early disturbances in the prenatal ovary, (ii) parameters diminishing the quality of oocytes in dictyate stage, and (iii) mechanisms enabling oocytes to process all chromosomal configurations successfully during later stages of oogenesis. Studies with newly developed and existing animal models appear indispensable to identify exposures affecting chromosome disjunction during meiosis, especially in the aging female.