The frequency of heterozygous carriers of reciprocal translocations is about 0.16–0.2% , which means that the probability of two carriers being a couple is less than 4 × 10− 6. The case with recurrent miscarriage in the present report represents rare instance in which both parents had an independent balanced reciprocal translocation affecting four chromosomes: 46,XX,t(10;16)(q25.2;q12.2) and 46,XY,t(9;14)(p21.1;q12). Since there are extremely rare PGT-SR data on a couple with two independent reciprocal translocations in reviewing the literature, the reproductive risk assessment and genetic counseling for this case would be unique and complex. However, a considerable number of PGT-SR data with regard to one reciprocal translocation might provide bases for predicting PGT-SR outcomes in couples with two reciprocal translocations. Generally, genetic counseling for a couple with one reciprocal translocation focuses on the risk of unbalanced gametes from one parent. Zhang et al. published a PGT-SR data indicated that the proportion of alternate segregation pattern, which can produce normal/balanced gametes, was 40.7% (749/1842) on average by testing 1842 embryos from 356 carriers of single reciprocal translocations . Given that each partner’s translocation is thought to segregate independently, the risks for generating abnormal gametes might be additive for couples with two reciprocal translocations. Thus, the probability of normal/balanced zygotes for such couples was estimated to be 16.6% (40.7% × 40.7%) without considering non-translocation chromosomes abnormalities. Another large practical PGT-SR data of the European Society of Human Reproduction and Embryology (ESHRE) PGT Consortium showed that 19.5% (4681/23960) day-3 embryos were transferable after genetic testing when one of the partners is a carrier for a reciprocal translocation , and this rate increased to 30.0% (142/473) when biopsy at the blastocyst stage . Therefore, theoretically, the transferable embryos rate might be as low as 3.8% (19.5% × 19.5%) in cleavage-stage embryos or 9% (30.0% ×30.0%) in blastocysts for both spouses having a reciprocal translocation. In fact, in 2010 Beyazyurek et al.  reported only one PGT-SR study performed for a consanguineous couple carrying the same familial reciprocal translocation between chromosomes 1 and 16 (Table 1 and Fig. 3A), and the result showed that only one out of 15 (6.7%) day-3 embryos was detected as balanced and transferable which is close to the empirical rate that we extrapolated (3.8%). By contrast, surprisingly, we found that 2/3 (66.7%) day-5 blastocysts were balanced for the current couple. The transferable embryo rate was significantly higher than that which would be expected theoretically could be partly explained by chromosome self-correction at the blastocyst stage, as has been suggested by a few authors [14, 15]. In fact, it has been reported that the euploidy rate was found to be significantly higher for blastocyst stage embryos as compared to that of cleavage stage embryos (60.3 and 33.4%, respectively) . In addition, another possible explanation is that these two translocations may tend to produce a lower proportion of unbalanced gametes, thus forming a higher proportion of euploidy embryos. Besides, this may due to the small number biopsied blastocysts for PGT-SR, thus more such cases reported and sperm fluorescence in situ hybridization (FISH) analysis  would be helpful to predict PGT-SR outcomes. However, other factors including the location of translocation breakpoints, the age of the carriers and chromosome type also made the difference between empirical and practical rates.
In reviewing the literature, there are 15 couples in which both wife and husband had a balanced reciprocal translocation without clinical expression (Table 1), and Fig. 3 illustrates nuclear pedigrees for them [13, 18−31]: 8 in whom both spouses had an identical balanced reciprocal translocation because of consanguineous marriage (Families A, B, C, D, E, F, K and O), one in whom consanguineous partners were inherited with two similar balanced reciprocal translocations (Family G) and 6 in whom unrelated members of couples carried two different reciprocal translocations (Families H, I, J, L, M and N). Figure 3 shows that there were 52 recognized pregnancies among the 15 couples including 50 natural and 2 PGT-SR pregnancies (Family A), resulting in 19 phenotypically normal live births (2 with a normal karyotype, 11 with a single parental balanced reciprocal translocation and 6 with double parental balanced reciprocal translocations); 14 phenotypically abnormal live births (3 unbalanced offspring, 5 balanced offspring with two identical reciprocal translocations and 6 neonatal deaths without karyotype examination); and 19 abortions (1 termination of pregnancy for an abnormal fetus with two balanced reciprocal translocations and 18 spontaneous abortions). The probability of a clinically recognized pregnancy through natural conception ending with healthy live birth was only 17/50 (34.0%) when both partners carried a balanced reciprocal translocation. However, the overall risk for abnormal live births and abortions/stillbirths in these families was 14/50 (28.0%) and 19/50 (38.0%), respectively. It is worth noting that, 6 offspring were homozygous carriers of translocations in 5 consanguineous couples (marked in bold in Families B, C, D, E, and K). Even though they had apparently balanced karyotypes, multiple abnormal phenotypes were observed. Several studies have suggested that the disease causing genes were disrupted by the breaks and that the affected offspring were homozygous for a recessive gene defect, masked by the unaffected heterozygous parents with a same balanced reciprocal translocation [18−21, 27]. Thus, in this context, the genetic risk of the reciprocal translocations should be specially investigated. It is of great importance to identify whether the translocation breakpoints give rise to gene interruption in PGT-SR treatment and prenatal diagnosis for couples with two balanced reciprocal translocation. The review of the literature indicates that the fetus could indeed inherit unbalanced gametes from mother, father, or both; thus the risk of having abnormal live offspring and of spontaneous abortion might be cumulative in couples with two reciprocal translocations. We believe that PGT-SR would be a useful and practical tool in the aspect of increasing healthy birth rates and decreasing recurrent abortions for such couples.
In recent years, precise translocation breakpoint identification has been increasingly used for estimating the phenotypic outcomes of balanced reciprocal translocations and distinguishing normal and translocation-carrying embryos in PGT-SR cycles. To date, several approaches have been developed to identify transferable translocation-free embryos in PGT-SR treatments, such as mate−pair sequencing , MicroSeq-PGD  and MaReCs . In this study, we applied WGLCS, an accurate approach which can limit the breakpoints to ± 1 Kb region, to initially map the four breakpoint regions of the reciprocal translocations . Then, junction-spanning PCR combined with Sanger sequencing were used to characterize the precise breakpoints. Subsequently, the carrier status of the two balanced/euploid embryos was determined using breakpoint-specific PCR. The sequencing results showed that there were several nucleotides insertions and/or deletions occurred at the breakpoint junctions during the translocation formation. This junction is common in human chromosomal translocations and may arise from a non-homologous end-joining (NHEJ) mechanism [35−37]. Hence, balanced translocations may have imbalances in the molecular level. However, these four breakpoints on chromosome 10, 16, 9 and 14 were mapped in the intergenic regions, which did not cause gene interruption. Therefore, we speculated that no matter their offspring carrying heterozygous balanced reciprocal translocation or double heterozygous balanced reciprocal translocations would have normal phenotype.