In the general population, CCR is rarely found. The condition is usually associated with congenital anomalies, mental impairment, recurrent spontaneous abortions and infertility [19]. For example, Sinkar and Devi [20] reported a boy who inherited a significantly balanced chromosomal translocation from his deaf–mute mother, showing mental impairment and aphasia. However, Campos et al. [21] showed that the frequency of balanced CCR in the population may be underestimated since it may not cause phenotypic effects and may not be detected by the analysis method used. This is similar to the results of the present study. The CCR patient reported herein had normal phenotyping and intelligence, but the karyotype analysis revealed that the patient was a complex chromosome translocation carrier.
There are four types of CCRs based on the combination of chromosome number and breakpoints. Type I, the simplest and most common, is inherited maternally; it is characterised by an equal number of chromosomes and breakpoints in CCRs, known as triple rearrangements (three chromosomes with one breakpoint on each chromosome), and it is the most common of all CCR cases [22]. Balanced translocations are partly inherited from the parents and partly caused by chromosomes breaking and rejoining during sperm or egg formation or fertilisation of the ovum [23, 24]. During this process, two chromosomes are exchanged after the break, and no increase or decrease of chromosomal fragments is involved. The carrier has normal intelligence and phenotyping, and the clinical manifestations may include infertility, recurrent miscarriages, embryonic arrest and embryonic developmental malformations [25].
Carriers of balanced translocations of two chromosomes form quadriradial chromosomes during meiosis; they produce 18 gametes, of which 1 is normal and 1 is a balanced translocation carrier, with the remaining 16 being unbalanced gametes [26]. The balanced translocations carried by the patient reported in this study involved a total of three chromosomes (1, 9 and 6). They were a carrier of complex translocations, which had an increased chance of chromosomal rearrangement compared with carriers involving only two chromosomes in reciprocal translocations, which are more likely to form unbalanced gametes. These complex balanced translocations were responsible for a history of adverse pregnancy and delivery [27]. In the patient's first pregnancy with foetal arrest, the embryo was examined for chromosomes 1, 6 and 9, with varying degrees of microdeletions or microduplications, and an examination of the patient's chromosomes suggested balanced translocations of chromosomes 1 and 9. In the patient's second pregnancy, the foetus exhibited three or more positive ultrasound soft indicators, which suggested an increased risk of chromosomal foetal abnormalities. The amniotic fluid CNV suggested chromosome 6 microdeletion as a pathogenic variant and chromosome 1 microduplication as a suspected pathogenic variant; both embryonic tissue and amniotic fluid suggested chromosome 6 abnormality, so the patient's chromosomes were re-examined. Finally, the patient was identified as a complex translocation carrier of chromosomes 1, 9 and 6.
The patient's gestational amniotic fluid CNV detected a deletion of approximately 1.60 Mb in the 6q27q27 region, which was a pathogenic variant containing 11 protein-coding genes, including DLL1, THBS2, ERMARD and TBP. The DLL1 gene was evaluated by ClinGen as a single-dose-sensitive gene [28]. DLL1 acts as a ligand for Notch and plays an important role in Notch signalling [29]. Studies have shown that abnormalities in the DLL1/Notch signalling pathway can lead to abnormal embryonic development, dysregulation of biological processes and malignant transformation [30]. In mammalian cells, the DLL1/Notch signalling pathway is associated with the maintenance of homeostasis in stem cells [31]. In addition, when Notch signalling is active, the Notch ligand (DLL1) binds to Notch receptors on the surface of neighbouring cells where it induces the expression of genes that inhibit neural differentiation, thereby maintaining the cell in a proliferative state. The pathogenic variant formed by the microdeletion of the 6q27q27 region detected by the CNV of this patient's pregnancy amniotic fluid may have been related to the loss of function of the protein encoded by this region of the gene.
In the present study, the patient had discrepancies in the results of two karyotypic analyses; the first showed translocations only of chromosomes 1 and 9, while the second showed complex balanced translocations of chromosomes 1, 6 and 9, which were further confirmed by FISH. There are various reasons for the discrepancy in the results of the two examinations, for example, the resolution (high vs low) of the G-Banding karyotype analysis may have led to variations in the results.