Molecular basis of maternal age-related increase in oocyte aneuploidy.

Aneuploidy is one of the most common and serious pregnancy complications in humans. Most conceptuses with autosomal aneuploidy die in utero, resulting in early preg- nancy loss. However, some fetuses with aneuploidy survive to term but suffer from disorders associated with congenital anomalies and mental retardation, such as Down syndrome with trisomy 21. Three general characteristics of this condition are well acknowledged: (i) in most cases the extra chromosome is of maternal origin; (ii) most cases are derived from a malseg- regation event in meiosis I; and (iii) the frequency of these errors increases with maternal age. The basis for the age- dependent increase in meiosis I errors has been a long-standing enigma. Many investigators have addressed the nature of this biological phenomenon through genomic analyses of extra chro- mosome 21 using polymorphic markers to determine the fre- quency or location of crossovers that should ensure faithful chromosome segregation. Cytogenetic analyses of in vitro unfer- tilized oocytes have also been performed. However, no definitive conclusions regarding meiosis I errors have yet been reached from such studies. Recent findings in conditional knock-out mice for meiosis-specific cohesin have shed further light on this issue. The present review focuses on the current understanding of age-related aneuploidy and provides an overview of the mechanisms involved. We refer to recent data to illustrate some of the new paradigms that have arisen in this field.