I-35: Polar Body Analysis by Array CGH Identifies Women with Varying Susceptibility to Aneuploidy and Suggests that Non-disjunction Is Not The Predominant Mechanism Leading to Aneuploidy in Humans

Background: The maternal age effect for trisomy is well known. However what is less established is whether certain women are more (or less) prone to segregation errors, independent of age. Trisomy arises primarily through maternal meiosis I chromosome segregation errors however the precise mechanism by which these errors occur is unclear. Current dogma attributes the origin of trisomy to malsegregation of a whole chromosome to the same pole as its homologue (non-disjunction). Classical cytogenetic studies however suggest that this model does not fully account for the patterns observed in human oocytes. An alternative model (precocious separation of sister chromatids) has thus been proposed, but recurring criticism of this model purports that technical issues may have led to interpretation errors. The purpose of this study was to determine the relationship between chromosome segregation errors of whole chromosomes and single chromatids in human oocytes to (i) provide a more complete understanding of oogenesis and the origins of aneuploidy, and (ii)help identify the best strategy to avoid aneuploidy in pregnancy. Materials and Methods: Oocytes from 48 patients (aged 29-50 years) were harvested 43-45 hours after administration of human chorionic gonadotrophin and 297 first polar bodies biopsied by micromanipulation. Polar bodies were subjected to whole genome amplification and array comparative genomic hybridization (24 Sure, Bluegnome, UK). Bluefuse software was used to distinguish between whole chromosome (non-disjunction) and chromatid (precocious separation) errors. Results: Of the 297 oocytes biopsied, a total of 285 (96%) first polar bodies were successfully amplified and analysed. Of those analysed, 129 (45.3%) had no detectable chromosome segregation error (and were classified as euploid) and 156 (54.7%) has at least one gain or loss of a chromatid/chromosome. The total number of errors was 390, giving a per polar body error rate of 1.37. There was no difference overall between the frequency of losses (184=55.3%) compared with gains (149=44.7%) but whole chromosome losses were much more frequent than gains. Notably single chromatid errors were nearly 11 times more common than whole chromosome errors. A positive association (p<0.01) between the frequency of segregation errors and maternal age was established, albeit with apparent inter-individual differences. That is, selected women in the younger age group had relatively high levels of aneuploidy and viceversa. Women who went on to have an unaffected live birth were those who had low levels of aneuploidy. The preponderance of single chromatid errors is consistent with the notion that precocious separation, rather than non-disjunction is the primary mechanism leading to human trisomy. These findings were consistent with other recently published findings using similar technology on 1st and 2nd PB plus the embryo. Conclusion: The current study demonstrates clearly that whole chromosome errors are rare in comparison to chromatid errors even in women of advance maternal age. Array CGH is a powerful method of diagnosing aneuploidy in oocytes in women of advanced maternal age enabling them to make informed choices about their future reproductive options. Furthermore, it can also act as a screening tool for women to help them select chromosomally normal oocytes as a strategy to avoid aneuploid pregnancy and live births. It remains to be seen whether interventions or lifestyle changes can improve the rates of chromosome errors in the oocytes of individual women, however, array CGH would accurately monitor this. In addition, SNP genotyping (Karyomapping) on the same samples provides the basis for further elucidating the mechanism of chromosome segregation errors; in particular, the role of recombination patterns in the genesis of trisomy.