Prevalence of chromosomal alterations in first-trimester spontaneous pregnancy loss

Introduction

Worldwide, approximately 10-15% of all clinically recognized pregnancies end in pregnancy loss, which accounts for an estimated 23 million pregnancy losses every year. Over 90% of pregnancy losses occur in the first trimester, before 8-9 weeks gestation. Notably, considerable additional loss that may go unnoticed takes place even earlier. Approximately one in ten women experience at least one pregnancy loss, termed “sporadic pregnancy loss” (SPL), while 1 in 50 and 1 in 143 women experience two or three pregnancy losses respectively, known as “recurrent pregnancy loss” (RPL) [].

Chromosomal aneuploidies, which occur during oogenesis and early embryogenesis, are a primary cause of pregnancy loss. In the last decade, several studies with a combined sample size of over 42,500, showed that chromosomal aneuploidies are responsible for an average yield of approximately 53.7% of pregnancy losses [, , , , , , , ]. Chromosomal aneuploidy manifests due to chromosomal instability (CIN), which is the inability to maintain the same chromosome number from one cell generation to the other. CIN is common in early embryogenesis but not present in birth, indicating that only embryos with sufficient genome integrity survive to term. CIN also leads to mosaic embryos, which contain both chromosomally normal and abnormal cells. It has been hypothesized that self-correction of embryos may be controlled by cellular fragmentation, blastomere exclusion of abnormal cells and selective allocation of abnormal cells to non-fetal cells, or by rescue mechanisms. These can lead to confined placental mosaicism, where the placental tissue has an abnormal number of chromosomes whereas the fetal tissue has 46 chromosomes.

Although the incidence of some whole chromosome aneuploidies increases with maternal age, a recent ACOG Practice Bulletin demonstrated that the risk of segmental or rare chromosomal aneuploidies remains the same in all age groups []. The latter has important implications, particularly in the prenatal testing field, emphasizing the need for inclusive screening for women of all ages.

Aim

The present study aimed to determine the prevalence of chromosomal aneuploidies in pregnancy loss, and whether there is a difference in the allocation of aneuploid cells between the embryonic and the placenta tissues.

Methodology

• The chromosomal landscape of embryonic (extra-embryonic mesoderm) and placental (chorionic villi) lineages of first-trimester pregnancy losses were evaluated, to determine whether these tissues carried different aneuploidies or differed in the level of mosaicism.
• 1745 spontaneous pregnancy losses were analyzed via conventional karyotyping over a course of 35 years. The vast majority of samples were karyotyped via conventional GTG banding, and in a small number of cases where this was not possible, via CGH (3.4%) or FISH (3.4%).
• 94 samples in which no chromosomal aneuploidies were found via karyotype analysis, classified as “karyotypically normal”, were selected to undergo additional genome-wide single nucleotide polymorphism (SNP) genotyping utilizing microarrays, followed by genome haplarithmisis. This was done to determine whether more advanced technological methods made a difference in detecting chromosomal aneuploidies, including mosaic aneuploidies which cannot be detected via conventional karyotyping.

Genome haplarithmisis is a genetic conceptual method that enables simultaneous haplotyping and copy-number profiling, and can distinguish the origin of the aneuploidy.

Results

• In the first step, analysis of conventional karyotyping only, results indicated that 50.4% of pregnancy losses carried chromosomal aneuploidies. The results of the additional SNP-analysis revealed that 35.1% of tested samples initially termed as “karyotypically normal”, indeed carried one or more chromosomal aneuploidies. Therefore, by extrapolation, the overall rate of chromosomal aneuploidies in this study reached 67.8%.

Apart from the present study, several studies that performed additional analysis of karyotypically normal samples via chromosomal microarray detected an overall average rate of 19.4% additional aneuploidies [Supplementary table of reference 1]. This demonstrates that when more technologically advanced methods are used, results are more representative.

• Extra-embryonic mesoderm biopsy samples had a higher level of mosaicism relative to chorionic villi samples. This contrasts with viable pregnancies where mosaic aneuploidies are often restricted to the chorionic villi, and raises hypotheses about the origins of mosaic mutations in pregnancy loss.
• Although SPLs and RPLs had similar prevalence of chromosomal aneuploidies, SPLs had significantly more segmental aneuploidies, and RPLs had more whole chromosome aneuploidies.

Significance

• Conventional cytogenetic methods have several disadvantages, leading to low diagnostic yield []. The need to explore sophisticated genome analysis methods, such as next generation sequencing (NGS) and genome haplarithmisis, carries clear advantages for accurately and efficiently diagnosing the causes of pregnancy loss, and has even been emphasized by ESHRE guidelines [].
• The authors argue that when mosaic chromosomal aneuploidies are of meiotic origin, they likely affect both the fetal and placental tissues and are a high risk for spontaneous pregnancy loss. On the other hand, if the mosaic aneuploidy originates from a mitotic event, it is likely that the aneuploidy is specific to the placenta due to CPM and may be compatible with a healthy pregnancy. As a result, the study suggests that the rules for selecting mitotic mosaic embryos for transfer in an IVF setting could be relaxed.

Conclusion

As many as two-thirds of all pregnancy losses are due to fetal chromosomal aneuploidies. Scrutinizing the full genomic architecture of the conceptus and identifying the origin of chromosomal aneuploidy is necessary to improve our understanding of the mechanisms involved and the risk factors, and ultimately improve the clinical management of pregnancy loss.