embryos in 88 cycles, 53 clinical pregnancies, and 55 children born free from genes predisposing to neurofibromatosis.
Other cancers representing frequent indications for PGT were different types of Fanconi anemia (83 cycles), colorectal cancer (52 cycles), tuberous sclerosis types 1 and 2 (44 cycles) (see Figure 2.2), familial adenomatous polyposis (42 cycles), multiple endocrine neoplasia (34 cycles), and retinoblastoma (31 cycles); under 30 PGT cycles were performed for various other cancers. Overall, 966 genetic predisposition‐free embryos were transferred in 634 cycles, resulting in 387 (61.0%) clinical pregnancies and birth of 407 children free from the risk of developing these cancers.25, 48
The other emerging PGT‐M indication has been inherited cardiac disease, for which 109 cycles were performed relating to 23 different diseases. The most frequent indications were familial hypertrophic cardiomyopathy, CMH4 (22 cycles), dilated cardiomyopathy, CMD1A (17 cycles), Holt–Oram syndrome, HOS (8 cycles), acyl‐CoA dehydrogenase very‐long‐chain deficiency, ACADVLD (6 cycles), familial hypertrophic cardiomyopathy 1, CMH1 (6 cycles), long QT syndrome 1, LQT1 (6 cycles) and Noonan syndrome 1, NS1 (6 cycles); PGT for another 16 cardiac conditions were performed in five or less number of cycles. Overall, 123 embryos free of genes predisposing to cardiac disease were transferred in 89 cycles (1.38 embryos per transfer on the average), resulting in 55 clinical pregnancies (61.7 percent) and birth of 54 children free from inherited predisposition to these cardiac diseases. If not prevented, many of these conditions may manifest despite presymptomatic diagnosis and follow‐up, with their first and only clinical occurrence being a premature or sudden death.78 The couples at risk for producing progeny with inherited cardiac disease usually request PGT prospectively, with no previous pregnancies attempted, given one of the partners being a carrier of the specific mutation. Many couples already going through IVF for fertility treatment may have questions about the implications of genetic susceptibility factors for offspring, and the appropriateness of using PGT in testing for susceptibility to inherited cardiac disease.25, 48
Of special interest are PGT indications for late‐onset disorders with inherited predisposition to neurological disorders, including neurodegenerative conditions. A total of 960 PGT cycles were performed for these conditions, including 610 for intellectual disability, 210 for Huntington disease, 42 for different movement disorders, such as torsion or myoclonic dystonia, nine for Alzheimer disease, and nine for Prion disease. As many as 1,110 unaffected or genetic predisposition‐free embryos were transferred in 738 cycles (1.5 embryos per transfer on average), yielding 412 clinical pregnancies and birth of 406 infants unaffected or free of the genes predisposing to the above conditions.48 Thus, PGT provides a nontraditional option for patients who may wish to avoid the transmission of a mutant gene predisposing to late‐onset disorders in their future children. Because such diseases that present beyond early childhood and even later may not be expressed in 100 percent of cases, the application of PGT for this group of disorders is still controversial. However, for diseases with no current prospect for treatment, PGT may still be offered as the only relief for at‐risk couples.
HLA typing
Preimplantation HLA typing is an attractive PGT indication. The first case of preimplantation HLA typing was performed in combination with PGT for Fanconi anemia complementation group C (FA‐C), which resulted in a successful hematopoietic reconstitution in the affected sibling by transplantation of stem cells obtained from the HLA‐matched offspring resulting from PGT.5 To improve access to the HLA‐identical bone marrow transplantation in sporadic bone marrow failures, this approach was then applied with the sole purpose of ensuring the birth of an HLA‐identical offspring, not involving PGT, which also resulted in radical treatment of a sibling with a sporadic Diamond–Blackfan anemia (DBA) by stem cell transplantation from an HLA‐identical child born following preimplantation HLA typing.79 Preimplantation HLA typing has become one of the most useful indications for PGT, performed currently with or without testing for the causative gene.79–88
Despite the ethical issues involved,80 preimplantation HLA typing procedures have so far been performed in hundreds of cases with affected children requiring HLA‐compatible stem cell transplantation, including thalassemia, Fanconi anemia, Wiskott–Aldrich syndrome, X‐linked adrenoleukodystrophy, X‐linked hyper‐IgM syndrome, X‐linked hypohidrotic ectodermal dysplasia with immune deficiency, X‐linked chronic granulomatous disease, cancer syndromes, incontinentia pigmenti, leukemias, and inherited and sporadic forms of DBA.81–91
We applied PGT‐HLA in 485 cycles, including HLA typing alone, or combined with PGT‐M for 35 different conditions. Overall, 424 HLA‐matched unaffected embryos were detected and transferred in 291 cycles, resulting in 125 clinical pregnancies and birth of 117 HLA‐matched children, as potential donors for their siblings.25, 48, 92 Although the majority of cases were performed for thalassemia, this approach has a great life‐saving potential for affected siblings with congenital immunodeficiency. We performed 135 PGT cycles for 18 different immunodeficiency conditions, resulting in the birth of 54 children free of immunodeficiency, stem cell donors for transplantation treatment of affected siblings, with a total cure, such as Fanconi anemia and hyper‐IgM syndrome.48
Similar experience has been reported from other large series, such as from Istanbul: 626 PGT‐HLA cycles for 312 couples were performed (122 HLA only and 504 with PGT‐M), resulting in the birth of 128 thalassemia‐free children. Stem cells of 66 of these children were used for cord blood or bone marrow transplantation, which resulted in successful bone marrow reconstitution in all but two of them (transplantation treatment of the remaining 57 sibling pending).93, 94
Chromosomal disorders
The theoretical rate of chromosomally abnormal embryos at fertilization is approximately 40 percent, taking into account both the rate of aneuploidies in oocytes and sperm and fertilization‐related abnormalities.95, 96 Mouse data show that most aneuploidies, although compatible with cleavage, are lost during implantation.97, 98 Additional losses of chromosomally abnormal embryos occur after implantation, clinically recognized as spontaneous abortions, more than half of which are caused by chromosomal abnormalities. As a result of this selection against chromosomal abnormalities before and after implantation, only 0.65 percent of newborns have chromosomal disorders, many of which lead to serious disability and early death (see also Chapter 1).
Prevalence and origin of chromosomal errors
A wide range in the frequency of chromosomal aneuploidy in human oocytes has been reported (17–70 percent), but most of these studies have been performed on poor‐quality oocytes left over after the failure of IVF attempts. A high rate of hypohaploidy observed in oocytes was considered to be artificially induced by spreading techniques, so aneuploidy rate was calculated by doubling the number of hyperhaploid oocytes. This ignores chromatid malsegregation and/or chromosome lagging events, contradicting the results of the observation that the rate of hypohaploidy is higher than the rate of hyperhaploidy.99 Cytogenetic analysis of unfertilized oocytes was also improved by parthenogenetic activation of human oocytes.100
As mentioned, an attempt at noninvasive cytogenetic analysis of oocytes was undertaken in the early 1980s through visualization of the second polar body chromosomes by transplanting the polar body into a fertilized egg.33 The success rate of visualization of polar body chromosomes was then improved by different methods, demonstrating the practical implication of polar body analysis for chromosomal errors originating from maternal
