Modeling the maternal-age dependency of reproductive failure and genetic fitness

Reproductive Failure and Thrombophilia: Not Enough Evidence for a Tight Bond

OBJECTIVES

The role of hereditary thrombophilia in reproductive failure (RF) is strongly debatable. In this retrospective single-center study, we analyzed pregnancy outcome in 175 women screened for thrombophilia after at least one event of RF.

RESULTS

The prevalence of thrombophilia in our cohort was 33.4%. Pregnancy survival curves were not different according to severity (log-rank, p = 0.302) or type of thrombophilia (log-rank, p = 0.532). In total, 81.7% of 175 subsequent pregnancies were proceeded with LMWH. Concomitant use of ASA was prescribed in 75 pregnancies according to physician choice. The primary endpoint was live birth rate (LBR) that succeeded in 152/175 next pregnancies (86.8%) and late obstetric complications (LOBC) which occurred in 17/175 next pregnancies (9.8%). In logistic regression analysis, neither the severity nor the type of thrombophilia was important for any pregnancy outcome (LBR or LOBC). Considering therapeutic interventions, the use of LMWH ± ASA was not related to LBR or LOBC. The only factor inversely related to LBR was age above the cutoff value of 35.5 years (p = 0.049).

CONCLUSIONS

Incidence of thrombophilia is increased among women with RF, but the severity or type of thrombophilia is not related to pregnancy outcome.

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.

On the origin of trisomy 21 Down syndrome

BACKGROUND

Down syndrome, characterized by an extra chromosome 21 is the most common genetic cause for congenital malformations and learning disability. It is well known that the extra chromosome 21 most often originates from the mother, the incidence increases with maternal age, there may be aberrant maternal chromosome 21 recombination and there is a higher recurrence in young women. In spite of intensive efforts to understand the underlying reason(s) for these characteristics, the origin still remains unknown. We hypothesize that maternal trisomy 21 ovarian mosaicism might provide the major causative factor.

RESULTS

We used fluorescence in situ hybridization (FISH) with two chromosome 21-specific probes to determine the copy number of chromosome 21 in ovarian cells from eight female foetuses at gestational age 14-22 weeks. All eight phenotypically normal female foetuses were found to be mosaics, containing ovarian cells with an extra chromosome 21. Trisomy 21 occurred with about the same frequency in cells that had entered meiosis as in pre-meiotic and ovarian mesenchymal stroma cells.

CONCLUSION

We suggest that most normal female foetuses are trisomy 21 ovarian mosaics and the maternal age effect is caused by differential selection of these cells during foetal and postnatal development until ovulation. The exceptional occurrence of high-grade ovarian mosaicism may explain why some women have a child with Down syndrome already at young age as well as the associated increased incidence at subsequent conceptions. We also propose that our findings may explain the aberrant maternal recombination patterns previously found by family linkage analysis.

Adaptive-filtering of trisomy 21: risk of Down syndrome depends on family size and age of previous child

The neonatal incidence rate of Down syndrome (DS) is well-known to accelerate strongly with maternal age. This non-linearity renders mere accumulation of defects at recombination during prolonged first meiotic prophase implausible as an explanation for DS rate increase with maternal age, but might be anticipated from chromosomal drive (CD) for trisomy 21. Alternatively, as there is selection against genetically disadvantaged embryos, the screening system that eliminates embryos with trisomy 21 might decay with maternal age. In this paper, we provide the first evidence for relaxed filtering stringency (RFS) to represent an adaptive maternal response that could explain accelerating DS rates with maternal age. Using historical data, we show that the proportion of aberrant live births decrease with increased family size in older mothers, that inter-birth intervals are longer before affected neonates than before normal ones, and that primiparae exhibit elevated levels of DS incidence at higher age. These findings are predicted by adaptive RFS but cannot be explained by the currently available alternative non-adaptive hypotheses, including CD. The identification of the relaxation control mechanism and therapeutic restoration of a stringent screen may have considerable medical implications.

Previous miscarriages influence IVF and intracytoplasmatic sperm injection pregnancy outcome

Previous conceptions are one predictor for the outcome of assisted reproductive technology procedures. Approximately 18-34% of clinical pregnancies following assisted reproduction procedures result in spontaneous abortion. The risk of such pregnancy loss is believed to increase with women's age, previous miscarriages and use of frozen-thawed embryos. This study analyses German IVF Registry data to examine the impact of previous miscarriages on the outcome of assisted reproduction procedures. The data set consists of a total of 174,909 assisted reproduction procedures performed between January 1998 and December 2000. Multiple logistic regression is used to assess the correlation between women's age, spousal/partner change, and infertility diagnosis. It is demonstrated that any previous miscarriage will increase the treatment-dependent miscarriage rate in assisted reproduction procedures. A significantly higher impact is shown for one previous miscarriage achieved by assisted reproduction procedures compared with spontaneous conception. Partner change is shown to have no specific impact on the treatment dependent miscarriage rate, whereas a statistically significant increase in miscarriages in all assisted reproduction procedures was found among women older than 34 years of age. Overall, the highest rate of treatment-dependent miscarriages was seen in assisted reproduction procedures with cryopreserved embryo transfer.