To date, the combined ADSP/NDSP is the largest population-based study of DS to determine the origin of the chromosomal error and collect demographic information including parental ages and ethnicity. Overall, the proportion of maternal cases (92.0%) was similar to that of other population-based studies (e.g., Gomez et al. 2000
; Mikkelsen et al. 1995
). However, we noted a significant change in the percentage of maternal meiotic errors over time: a greater proportion of errors were maternal in the NDSP (93.2%) than in the ADSP sample (88.5%) (p
= 0.01, ). This pattern can be explained by two important study observations. First, we found that the association between advanced maternal age and the increased occurrence of DS existed only for cases resulting from maternally-derived errors, not from paternally-derived or inferred post-zygotic mitotic errors. This confirms other studies (Antonarakis et al. 1993
; Carothers et al. 2001
; Petersen et al. 1993
). Second, there was a significant increase in maternal age over time between the ADSP and NDSP samples (). Thus, as the maternal age increased, there was a consequent increase in the proportion of maternal age-dependent nondisjunction errors. This implies that the percentage of cases due to specific nondisjunction errors will vary with the maternal age structure of a population, and comparisons from one study to another need to be interpreted carefully.
The documented association between advanced maternal age and both MMI and MMII cases confirms our previous work (Yoon et al. 1996
) and that of others (Antonarakis et al. 1992
; Muller et al. 2000
). Thus, the arrest of meiosis and its resumption after many years may compromise the ability of the oocyte to complete both stages of meiosis properly. Many hypotheses have been suggested to explain the maternal age effect and most imply an age-related degradation of the meiotic machinery. Recent studies have indicated changes in gene expression in younger compared with older oocytes in both mouse (Hamatani et al. 2004
; Pan et al. 2008
) and human studies (Steuerwald et al. 2007
). Gene profiles that were altered by age included those involved in cell cycle regulation, cytoskeletal structure, energy pathways, transcription control, and stress responses. Such changes could play a role in the meiotic spindle abnormalities observed frequently in oocytes of older mothers (Battaglia et al. 1996
; Eichenlaub-Ritter et al. 2004
) and/or in the deterioration of sister chromatid or centromere cohesion complexes as seen in mice by Hodges et al. (2005)
. Further, checkpoint systems that monitor spindle assembly and chromosome movement may not be effective in older oocytes (e.g., Hodges et al. 2002
; LeMaire-Adkins et al. 1997
; Vogt et al. 2008
With this large ADSP/NDSP data set, we dissected the maternal age influence further and found differences in the ratio of MMI to MMII cases across the maternal age continuum. At all ages, MMI errors exceeded MMII errors. However, the ratio of MMI to MMII was less in the youngest and the oldest maternal age groups compared with that in the other maternal age groups. This decreased MMI to MMII ratio was particularly noticeable for women ≥40. Thus, although there are more MMI than MMII errors across all maternal age groups, perhaps additional factors more often present at the beginning and/or the end of reproductive life lead to an increase in meiotic errors in which sister chromatids fail to separate properly. Hodges et al. (2002)
provide strong evidence from the mouse model that oocyte growth in an altered environment leads to an increase in the failure of chromosomes to move toward the equator during MI (congression failure). They further show that congression failure at MI can increase the risk for premature sister chromatid segregation (PSCS) in both MI and MII. There are many factors that may be involved in the control of oocyte growth. These could include the complex orchestration of signaling from the hypothalamic-pituitary-ovarian (HPO) axis as well as others involved in folliculogenesis. Factors common to both early and late reproductive life may involve altered hormone profiles (e.g., increased FSH, cycle variability). Results from Hodges et al. (2002)
implicated both oocyte-somatic cell communication and an altered endocrine environment as factors that increase congression failure.
Studies of human oocytes are also consistent with an increased maternal age being associated with both MMI and MMII errors (for review, see Pellestor et al. 2005
). In a study of 309 karyotypically abnormal human oocytes observed at meiosis II metaphase, Pellestor et al. (2003)
identified both whole chromosome nondisjunction and PSCS. Interestingly, they found that, during meiosis I, single chromatid aneuploidy occurred more frequently than did whole chromosome aneuploidy among the 309 oocytes and had a stronger correlation with maternal age. PSCS at meiosis I could lead to the error being classified as MMI or MMII depending on the action of the chromatids. From our analyses, we cannot determine the underlying mechanism for meiosis II errors. That is, we cannot distinguish MII errors that result from PSCS at meiosis I, whole chromosome nondisjunction at meiosis I followed by a reductional division at meiosis II or a “classical” meiosis II error in which chromatids fail to separate properly after completing a successful meiosis I division. Maternal-age risk factors are most likely associated with one or more of these mechanisms.
Our recent data that examined recombination profiles along nondisjoined chromosomes 21 by type of nondisjunction error and maternal age provide additional insight into mechanisms underlying nondisjunction. These studies were performed on a subset of cases from the population-based studies presented here (Lamb et al. 2005
; Oliver et al. 2008
). Among MMI cases, we found that the majority of nondisjoined chromosomes 21 were associated with either a lack of an exchange or a telomeric exchange and that these patterns influenced the risk for nondisjunction irrespective of maternal age. In contrast, we found that the nondisjoined chromosomes 21 that were categorized as MMII errors and had a pericentromeric exchange were enriched among older women with this type of error. These data suggested a maternal age-dependent mechanism (Oliver et al. 2008
). In Oliver et al., we offered two alternative explanations for this observation: (1) a pericentromeric exchange initiates or exacerbates the susceptibility to maternal age risk factors, perhaps leading to an increase in PSCS, or (2) a pericentromeric exchange protects the bivalent against age-related risk factors allowing proper segregation of homologues at meiosis I, but not segregation of sisters at meiosis II. The former explanation would represent a two-hit model: the first hit being the pericentromeric recombinant event and the second hit would involve any number of meiotic-related structures or proteins that degrade with oocyte age (Lamb et al. 1996
). The latter explanation implies that true MII errors may occur among older women only if bivalents are protected from age-related factors in some way. This protective factor could be a proximal recombinant event which then allows the sister cohesion complex to remain intact along most of the chromosome arm. Other protective factors could include genetic variants that reduce age-related degradation of meiotic structures or environmental factors that create an optimal environment during the arrested state of the oocyte, to name a few.
Lastly, we tested the hypothesis that the age of the maternal grandmother of the child with trisomy 21 affects the risk for a nondisjunction error. Results from past studies are conflicting, potentially due to differences in design and sampling strategies (Aagesen et al. 1984
; Greenberg 1963
; Malini and Ramachandra 2006
; Papp et al. 1977
; Penrose 1964
; Richards 1970
; Stoller and Collmann 1969
). However our failure to find a relationship between nondisjunction and grand-maternal age is strong evidence against such an effect for the following reasons: we had adequate numbers of cases and controls representing the same populations in the same time frames, we documented standard trisomy 21 by karyotype and included only maternally-derived cases in the comparison.
Although this study has many strengths, it is important to outline its limitations. First, due to limited resources, the NDSP could only recruit mothers who spoke either English or Spanish. Further, case and control families whose infant died or was placed for adoption prior to enrollment were not recruited. These factors probably have limited impact on the results of this study. The most important limitation was that we were not able to include pregnancies with trisomy 21 that were either spontaneously lost or terminated. We included only live births which represent no more than 10–20% of conceptions with trisomy 21(for review, see Hassold and Hunt 2001
), thus, our results must be interpreted with this in mind. For example, we discovered that the mean age of mothers at the time of birth increased over the time period of the study. Martin et al. (2005)
presented a similar increase in mean maternal age based on National Vital Statistics data. We found this increase occurred in mothers of both cases and controls; although the slope of the increase for cases was steeper than that for controls. The steeper increase in maternal age of infants with trisomy 21 from 1989–2004 could be due to increased prenatal screening being offered to younger women (maternal serum screening) beginning in the mid 1990s. It is possible that positive screens among these women led to confirmatory testing and pregnancy termination, thus reducing the proportion of mothers eligible for our study. Although only speculation, there may have been increased acceptance of an infant with trisomy 21, perhaps more often among older women. Potentially, this could also influence this increased slope. Nevertheless, these influences should not affect our interpretation of the comparison of MI to MII errors, as women are blind to the type of nondisjunction error. The participation rates varied by study site; however, there were no significant differences in mean maternal age for enrolled and non-enrolled cases or controls. Thus, the associations with maternal age should be representative of the population of eligible live-born cases.
In summary, in this large population-based study, we have confirmed our previous findings (Yoon et al. 1996
). Specifically, the significant association between advanced maternal age and chromosome 21 nondisjunction was restricted to errors in the egg; the association was not observed in paternal or in post-zygotic mitotic errors. Further, an almost three-fold higher proportion of MMI errors over MMII errors is present at all maternal ages; however, we note that this ratio decreases for mothers <19 years and those ≥40 years at the time of their infant’s birth. The next logical step will be to use both origin of the meiotic error and recombination profiles along the nondisjoined chromosomes 21 to classify types of nondisjunction errors. Although such parameters are still only surrogates for the exact type of error, they will provide more homogeneous groups in which to detect maternal-age associated risk factors.