Additional reports and data including postpartum karyotype analysis of the newborns are necessary to provide conclusive decisions.” PGT-A is widely used for a number of indications and with the introduction of Next Generation Sequencing (NGS) and increased identification of mosaicism, clinicians require more informative data to guide them to make safe decisions when considering transfer of mosaic embryos… Patient counseling regarding mosaic embryo transfer is extremely important. 2020) stated: “The transfer of mosaic embryos marks a new era in ART and future studies and reports of cases are needed to help guide clinicians to make safe decisions regarding mosaic embryos. In addition, there is the problem of predicting the potential for the optimal development of a mosaic embryo to a healthy individual, without physical and mental disorders. There is no consistency among results of published studies on the clinical outcomes of segmental mosaicism regarding their capacity to implant and develop to a fetus (Liu et al. Among mosaic embryos, one-third of the mosaic cases were segmental mosaics (Liu et al. ( 2020) were the first to demonstrate that mosaic embryos may have the potential for giving birth to healthy offspring. Until recently, mosaic embryos had not been considered for transfer. Mosaicism is particularly frequent (up to 30% of the cases) in preimplantation embryos (Munné and Wells 2017). The application of molecular technologies resulted in the detection of more carriers of segmental mosaicism. 2010) and in 10% of patients with a ring chromosome (Guilherme et al. For example, a normal cell line was detected in 6–20% of patients with microscopically determined disease-causing deletions (Niebuhr 1978 Cassidy et al. Segmental mosaicism appears to be more frequent than previously thought. Prior to recent publications (Kovaleva and Cotter 2016, 2017a, b), the basic characteristics such as population frequency, cytogenetic profiles, and sex ratio (SR, male-to-female ratio) in various groups of carriers were not identified. Until recently, mosaicism for chromosomal rearrangement (Rea), i.e., segmental SM, was considered extremely rare both in patients referred for cytogenetic testing and in prenatal diagnoses, and therefore its epidemiology had not been studied. Mosaicism is the presence of more than one genetically distinct cell line in a single organism that originates from a genetically homogenous zygote. We believe these data are instructive in the challenging medical genetic counseling of parents faced with no option other than transfer of an embryo with segmental mosaicism. According to data obtained from “post-embryo” studies, clinical manifestations of chromosomal imbalance are associated with a high proportion of abnormal cells, female gender, the type of rearrangement and involved chromosome(s), and maternal age. Female predominance was found among abnormal pregnancy outcomes, among disease-defined carriers of loss and gain/loss rearrangements, and among transmitting carriers of gonadal SM, both affected and asymptomatic. Male predominance was found among normal pregnancy outcomes and among disease-defined carriers of rearrangements resulting in a gain of genomic material. We have identified maternal age and preferential involvement of chromosome 18 in rearrangements associated with clinical manifestations. Paternal age was not found to be associated with SM in postnatally disease-defined individuals. However, there is no concordance regarding excessive involvement of chromosomes 1, 5, and 9 in unbalanced rearrangements and a preferential involvement of larger chromosomes compared to short ones. We observed a concordance with preimplantation diagnoses regarding the clinical significance of the extent of mosaicism as well as a predominance of deletions over other types of rearrangements. We analyzed 580 published cases of SM including (i) postnatally diagnosed affected carriers, (ii) clinically asymptomatic carriers, (iii) prenatally diagnosed carriers, and (iv) miscarriages. Therefore, we suggested comparing factors predisposing to favorable and poor prognoses, identified in postnatal and prenatal cohorts of SM carriers, with those obtained from studies on preimplantation embryos. Moreover, there is the problem of predicting the potential for the optimal development of a mosaic embryo to a healthy individual. However, there is no consistency among results of published studies on the clinical outcomes of embryos with SM, primarily due to the small number of reported cases. In contrast, in preimplantation embryos SM is a frequent finding and, therefore, is even more challenging. Mosaicism for unbalanced chromosomal rearrangements segmental mosaicism (SM) is rare, both in patients referred for cytogenetic testing and in prenatal diagnoses.
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