Reciprocal balanced translocations typically involved two chromosomal breakpoints, whereas CCR had three or more breakpoints.Theoretically, a balanced translocation carrier had only 1/9 chance of delivering offspring with a normal phenotype.Due to the nature of CCR and the number of chromosomes involved, it was particularly difficult to analyze the meiotic behavior of CCR and its consequences. Therefore, there were relatively few data on meiotic separation of human CCR[13].Previous studies had suggested that in the population with recurrent spontaneous abortion the frequency of CCR was 0.1%[14].The risk of offspring with an unbalanced karyotype was different in carriers of CCR.Natural pregnancy was discouraged in CCR carriers. Prenatal diagnosis was recommended even if a couple with CCR became pregnant naturally[15].
Preimplantation genetic diagnosis (PGD) ,provided to carriers of CCR, might reduce the risk of spontaneous abortion and the opportunity for offspring to carry chromosomal imbalances, and also could increased the chance of pregnancy by selecting against cleavage-stage embryos carrying unbalanced CCR[16].The identification of CCR and the accurate description of breakpoints depended on the quality of chromosome analysis. Preliminary identification of CCR was performed using traditional cytogenetic methods based on GTG-banding and high-resolution karyotype.By introducing molecular cytogenetics technology (fluorescence in situ hybridization,FISH), the properties of CCR had been greatly improved.Various FISH methods for studying CCR had been tested, and a growing number of reports suggested that CCR may be more complex and common than initially thought[9, 10, 16–19]. However, FISH is limited to detecting specific chromosomes and requires a large number of probes to obtain reliable results. With the development of molecular technology,the application of PGD based on NGS provided an accurate method for detecting the unbalanced segmental rearrangement of embryos[4, 10, 20, 21]. Unlike the traditional low-throughput PCR technique, the next generation sequencing technology can comprehensively detect 23 pairs of chromosomes at high resolution. In addition, NGS can detect de novo copy number variants. High automation, high throughput, and high repeatability make NGS the most widely used technology in PGD.In this case we also employed CMA as control for comparison.
To our knowledge,this is the first study to summarize clinical outcome of PGD patients with CCR reported by previous literatures(Table 2).Combining previous literature and our results (0 balanced embryo out of a total 7 diagnosed in one PGD cycle), we assessed the odds of a balanced embryo in a CCR carrier to be about 9.3%(28/302), while the chance was 20–30% in the case of two chromosomal translocations[19].The transferable embryo rate was approximately 71.4%(20/28) and healthy live born delivery rate was 55%(11/20) according to the data demonstrated in Table 2.The couple was informed that the rate of balanced embryo acquired via PGD in CCR carrier is extremely low,and this should be taken into consideration in planning the next controlled ovarian hyperstimulation cycle.
In this case,the couple was re-karyotyped by reason of discrepancy in cytogenetic result and CMA.The cryptic translocation was finally found in chr10 with high resolution G-banding technique(550-bands level).This indicated that a high level of banding is necessary in cytogenetic examination.It could avoid misdiagnoses in initial consultation and reduce patient’s dissatisfaction and unnecessary medical expenses.We did not conduct FISH verification on the breakpoint of chr10 for the reason as follow.First,customed FISH probes were expensive, which could add extra cost.Second, 550-bands level karyotype was sufficient to identify translocation on chr10 as demonstrated by Fig. 1.Third,we found the deletion of chr10 in the CMA results of the fetus,meanwhile there were chr10 gain or loss in 5 of the 7 biospied embryos according the NGS results(Table 1).
In conclusion,NGS featured high automation, relatively low cost, high throughput, and high repeatability, which made them commonly used during prenatal diagnosis and PGD.The combination of molecular and cytogenetic technology can provide more accurate diagnosis and better fertility services for CCR patients.