出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/04/06 10:30:03」(JST)
Pre-implantation genetic diagnosis (PGD or PIGD) refers to genetic profiling of embryos prior to implantation (as a form of embryo profiling), and sometimes even of oocytes prior to fertilization. PGD is considered in a similar fashion to prenatal diagnosis. When used to screen for a specific genetic disease, its main advantage is that it avoids selective pregnancy termination as the method makes it highly likely that the baby will be free of the disease under consideration. PGD thus is an adjunct to assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation. The term preimplantation genetic screening (PGS) is used to denote procedures that do not look for a specific disease but use PGD techniques to identify embryos at risk.
The procedures may also be called preimplantation genetic profiling to adapt to the fact that they are sometimes used on oocytes or embryos prior to implantation for other reasons than diagnosis or screening.[1]
Procedures performed on sex cells before fertilization may instead be referred to as methods of oocyte selection or sperm selection, although the methods and aims partly overlap with PGD.
In 1967, Robert Edwards and Richard Gardner reported the successful identification of the sex of rabbit blastocysts.[2] It was not until the 1980s that human IVF was fully developed, which coincided with the breakthrough of the highly sensitive polymerase chain reaction (PCR) technology. Handyside and collaborators' first successful tests happened in October 1989, with the first births in 1990[3] though the preliminary experiments had been published some years earlier.[4][5] In these first cases, PCR was used for sex determination of patients carrying X-linked diseases.
PGD became increasingly popular during the 1990s when it was used to determine a handful of severe genetic disorders, such as sickle-cell anemia, Tay Sachs disease, Duchenne’s muscular dystrophy, and Beta-thalassemia.[6]
As with all medical interventions associated with human reproduction, PGD raises strong, often conflicting opinions of social acceptability, particularly due to its eugenic implications. For example, in Germany the use of PGD had been prohibited by the Embryo Protection Act of 1990 [1], though this prohibition was relaxed somewhat in July 2011 [2].
In other countries PGD is permitted in law but its operation is controlled by the state. In the UK, the use of PGD is controlled by the HFEA ([3]) - the UK regulator for fertility treatment and embryo research. The HFEA only permits the use of PGD where the clinic concerned has a licence from the HFEA and sets out the rules for this licensing in its Code of Practice ([4]). Each clinic, and each medical condition, requires a separate application where the HFEA check the suitability of the genetic test proposed and the staff skills and facilities of the clinic. Only then can PGD be used for a patient.
In South Africa, where the right to reproductive freedom is a constitutionally protected right, it has been proposed that the state can only limit PGD to the degree that parental choice can harm the prospective child or to the degree that parental choice will reinforce societal prejudice.[7]
PGD can potentially be used to select embryos to be without a genetic disorder, to have increased chances of successful pregnancy, to match a sibling in HLA type in order to be a donor, to have less cancer predisposition, and for sex selection.
PGD is available for a large number of monogenic disorders —that is, disorders due to a single gene only (autosomal recessive, autosomal dominant or X-linked)— or of chromosomal structural aberrations (such as a balanced translocation). PGD helps these couples identify embryos carrying a genetic disease or a chromosome abnormality, thus avoiding diseased offspring. The most frequently diagnosed autosomal recessive disorders are cystic fibrosis, Beta-thalassemia, sickle cell disease and spinal muscular atrophy type 1. The most common dominant diseases are myotonic dystrophy, Huntington's disease and Charcot-Marie-Tooth disease; and in the case of the X-linked diseases, most of the cycles are performed for fragile X syndrome, haemophilia A and Duchenne muscular dystrophy. Though it is quite infrequent, some centers report PGD for mitochondrial disorders or two indications simultaneously.
PGD is also now being performed in a disease called Hereditary multiple exostoses (MHE/MO/HME).
In addition, there are infertile couples who carry an inherited condition and who opt for PGD as it can be easily combined with their IVF treatment.
Preimplantation genetic profiling (PGP) has been suggested to be applied as a method of assisted reproductive technology to perform embryo selection of an embryo that appears to have the greatest chances for successful pregnancy. However, as the results of PGP rely on the assessment of a single cell, PGP has inherent limitations as the tested cell may not be representative of the embryo because of mosaicism.[8]
A systematic review and meta-analysis of existing randomized controlled trials came to the result that there is no evidence of a beneficial effect of PGP as measured by live birth rate.[8] On the contrary, for women of advanced maternal age, PGP significantly lowers the live birth rate.[8] Technical drawbacks, such as the invasiveness of the biopsy, and chromosomal mosaicism are the major underlying factors for inefficacy of PGP.[8]
When used for women of advanced maternal age and for patients with repetitive IVF failure, PGP is mainly carried out as a screening for detection of chromosomal abnormalities such as aneuploidy, reciprocal and Robertsonian translocations, and few cases for other abnormalities such as chromosomal inversions or deletions. The principle behind it is that, since it is known that numerical chromosomal abnormalities explain most of the cases of pregnancy loss, and a large proportion of the human embryos are aneuploid, the selective replacement of euploid embryos should increase the chances of a successful IVF treatment. Comprehensive chromosome analysis methods include array-comparative genomic hybridization (aCGH), quantitative PCR and SNP arrays.[9] Combined with single blastomere biopsy on day-3 embryos, aCGH is very robust with 2.9% of tested embryos with no results, and associated with low error rates (1.9%).[9]
In addition to screening for specific abnormalities, techniques are in development that can avail for up to full genome sequencing, from which genetic profiling can score the DNA patterns by comparing with ones that have previously been found among embryos in successful or unsuccessful pregnancies.[9]
Human leukocyte antigen (HLA) typing of embryos, so that the child's HLA matches a sick sibling, availing for cord-blood stem cell donation.[10] The child is in this sense a "savior sibling" for the recipient child. HLA typing has meanwhile become an important PGD indication in those countries where the law permits it.[11] The HLA matching can be combined with the diagnosis for monogenic diseases such as Fanconi anaemia or b-thalassemia in those cases where the ailing sibling is affected with this disease, or it may be exceptionally performed on its own for cases such as children with leukaemia. The main ethical argument against is the possible exploitation of the child, although some authors maintain that the Kantian imperative is not breached since the future donor child will not only be a donor but also a loved individual within the family.
A more recent application of PGD is to diagnose late-onset diseases and (cancer) predisposition syndromes. Since affected individuals remain healthy until the onset of the disease, frequently in the fourth decade of life, there is debate on whether or not PGD is appropriate in these cases. Considerations include the high probability of developing the disorders and the potential for cures. For example, in predisposition syndromes, such as BRCA mutations which predispose the individual to breast cancer, the outcomes are unclear. Although PGD is often regarded as an early form of prenatal diagnosis, the nature of the requests for PGD often differs from those of prenatal diagnosis requests made when the mother is already pregnant. Some of the widely accepted indications for PGD would not be acceptable for prenatal diagnosis.
Preimplantation genetic diagnosis provides a method of prenatal sex discernment even before implantation, and may therefore be termed preimplantation sex discernment. Potential applications of preimplantation sex discernment include:
In the case of families at risk of X-linked diseases, patients are provided with a single PGD assay of gender identification. Gender selection offers a solution to individuals with X-linked diseases who are in the process of getting pregnant. The selection of a female embryo offspring is used in order to prevent the transmission of X-linked Mendelian recessive diseases. Such X-linked Mendelian diseases include Duchenne muscular dystrophy (DMD), and hemophilia A and B, which are rarely seen in females because the offspring is unlikely to inherit two copies of the recessive allele. Since two copies of the mutant X allele are required for the disease to be passed on to the female offspring, females will at worst be carriers for the disease but may not necessarily have a dominant gene for the disease. Males on the other hand only require one copy of the mutant X allele for the disease to occur in one's phenotype and therefore, the male offspring of a carrier mother has a 50% chance of having the disease. Reasons may include the rarity of the condition or because affected males are reproductively disadvantaged. Therefore, medical uses of PGD for selection of a female offspring to prevent the transmission of X-linked Mendelian recessive disorders are often applied. Preimplantation genetic diagnosis applied for gender selection can be used for non-Mendelian disorders that are significantly more prevalent in one sex. Three assessments are made prior to the initiation of the PGD process for the prevention of these inherited disorders. In order to validate the use of PGD, gender selection is based on the seriousness of the inherited condition, the risk ratio in either sex, or the options for disease treatment.[12]
A 2006 survey reveals that PGD has occasionally been used to select an embryo for the presence of a particular disease or disability, such as deafness, in order that the child would share that characteristic with the parents.[6]
PGD is a form of genetic diagnosis performed prior to implantation. This implies that the patient’s oocytes should be fertilized in vitro and the embryos kept in culture until the diagnosis is established. It is also necessary to perform a biopsy on these embryos in order to obtain material on which to perform the diagnosis. The diagnosis itself can be carried out using several techniques, depending on the nature of the studied condition. Generally, PCR-based methods are used for monogenic disorders and FISH for chromosomal abnormalities and for sexing those cases in which no PCR protocol is available for an X-linked disease. These techniques need to be adapted to be performed on blastomeres and need to be thoroughly tested on single-cell models prior to clinical use. Finally, after embryo replacement, surplus good quality unaffected embryos can be cryopreserved, to be thawed and transferred back in a next cycle.
Currently, all PGD embryos are obtained by assisted reproductive technology, although the use of natural cycles and in vivo fertilization followed by uterine lavage was attempted in the past and is now largely abandoned. In order to obtain a large group of oocytes, the patients undergo controlled ovarian stimulation (COH). COH is carried out either in an agonist protocol, using gonadotrophin-releasing hormone (GnRH) analogues for pituitary desensitisation, combined with human menopausal gonadotrophins (hMG) or recombinant follicle stimulating hormone (FSH), or an antagonist protocol using recombinant FSH combined with a GnRH antagonist according to clinical assessment of the patient’s profile (age, body mass index (BMI), endocrine parameters). hCG is administered when at least three follicles of more than 17 mm mean diameter are seen at transvaginal ultrasound scan. Transvaginal ultrasound-guided oocyte retrieval is scheduled 36 hours after hCG administration. Luteal phase supplementation consists of daily intravaginal administration of 600 µg of natural micronized progesterone.
Oocytes are carefully denudated from the cumulus cells, as these cells can be a source of contamination during the PGD if PCR-based technology is used. In the majority of the reported cycles, intracytoplasmic sperm injection (ICSI) is used instead of IVF. The main reasons are to prevent contamination with residual sperm adhered to the zona pellucida and to avoid unexpected fertilization failure. The ICSI procedure is carried out on mature metaphase-II oocytes and fertilization is assessed 16–18 hours after. The embryo development is further evaluated every day prior to biopsy and until transfer to the woman’s uterus. During the cleavage stage, embryo evaluation is performed daily on the basis of the number, size, cell-shape and fragmentation rate of the blastomeres. On day 4, embryos were scored in function of their degree of compaction and blastocysts were evaluated according to the quality of the throphectoderm and inner cell mass, and their degree of expansion.
As PGD can be performed on cells from different developmental stages, the biopsy procedures vary accordingly. Theoretically, the biopsy can be performed at all preimplantation stages, but only three have been suggested: on unfertilised and fertilised oocytes (for polar bodies, PBs), on day three cleavage-stage embryos (for blastomeres) and on blastocysts (for trophectoderm cells).
The biopsy procedure always involves two steps: the opening of the zona pellucida and the removal of the cell(s). There are different approaches to both steps, including mechanical, chemical, and physical (Tyrode’s acidic solution) and laser technology for the breaching of the zona pellucida, extrusion or aspiration for the removal of PBs and blastomeres, and herniation of the trophectoderm cells.
The first and second polar body of the oocyte are extruded at the time of the conclusion of the meiotic division, normally the first polar body is noted after ovulation, and the second polar body after fertilization. PB biopsy is used mainly by two PGD groups in the USA[13][14] and by groups in countries where cleavage-stage embryo selection is banned.[15] They have been used for diagnosing translocations and monogenic disorders of maternal origin, as well as for PGS.
The first PB is removed from the unfertilised oocyte, and the second PB from the zygote, shortly after fertilization. The main advantage of the use of PBs in PGD is that they are not necessary for successful fertilisation or normal embryonic development, thus ensuring no deleterious effect for the embryo. One of the disadvantages of PB biopsy is that it only provides information about the maternal contribution to the embryo, which is why cases of autosomal dominant and X-linked disorders that are maternally transmitted can be diagnosed, and autosomal recessive disorders can only partially be diagnosed. Another drawback is the increased risk of diagnostic error, for instance due to the degradation of the genetic material or events of recombination that lead to heterozygous first PBs. It is generally agreed that it is best to analyse both PBs in order to minimize the risk of misdiagnosis. This can be achieved by sequential biopsy, necessary if monogenic diseases are diagnosed, to be able to differentiate the first from the second PB, or simultaneous biopsy if FISH is to be performed.
The biopsy and analysis of the first and second PBs can be completed before syngamy, which is the moment from which the zygote is considered an embryo and becomes protected by the law.
Cleavage-stage biopsy is generally performed the morning of day three post-fertilization, when normally developing embryos reach the eight-cell stage. The biopsy is usually performed on embryos with less than 50% of anucleated fragments and at an 8-cell or later stage of development. A hole is made in the zona pellucida and one or two blastomeres containing a nucleus are gently aspirated or extruded through the opening. The main advantage of cleavage-stage biopsy over PB analysis is that the genetic input of both parents can be studied. On the other hand, cleavage-stage embryos are found to have a high rate of chromosomal mosaicism, putting into question whether the results obtained on one or two blastomeres will be representative for the rest of the embryo. It is for this reason that some programs utilize a combination of PB biopsy and blastomere biopsy. Furthermore, cleavage-stage biopsy, as in the case of PB biopsy, yields a very limited amount of tissue for diagnosis, necessitating the development of single-cell PCR and FISH techniques. Although theoretically PB biopsy and blastocyst biopsy are less harmful than cleavage-stage biopsy, this is still the prevalent method. It is used in approximately 94% of the PGD cycles reported to the ESHRE PGD Consortium. The main reasons are that it allows for a safer and more complete diagnosis than PB biopsy and still leaves enough time to finish the diagnosis before the embryos must be replaced in the patient’s uterus, unlike blastocyst biopsy. Of all cleavage-stages, it is generally agreed that the optimal moment for biopsy is at the eight-cell stage. It is diagnostically safer than the PB biopsy and, unlike blastocyst biopsy, it allows for the diagnosis of the embryos before day 5. In this stage, the cells are still totipotent and the embryos are not yet compacting. Although it has been shown that up to a quarter of a human embryo can be removed without disrupting its development, it still remains to be studied whether the biopsy of one or two cells correlates with the ability of the embryo to further develop, implant and grow into a full term pregnancy.
Not all methods of opening the zona pellucida have the same success rate because the well-being of the embryo and/or blastomere may be impacted by the procedure used for the biopsy. Zona drilling with acid Tyrode’s solution (ZD) was looked at in comparison to partial zona dissection (PZD) to determine which technique would lead to more successful pregnancies and have less of an effect on the embryo and/or blastomere. ZD uses a digestive enzyme like pronase which makes it a chemical drilling method. The chemicals used in ZD may have a damaging effect on the embryo. PZD uses a glass microneedle to cut the zona pellucida which makes it a mechanical dissection method that typically needs skilled hands to perform the procedure. In a study that included 71 couples, ZD was performed in 26 cycles from 19 couples and PZD was performed in 59 cycles from 52 couples. In the single cell analysis, there was a success rate of 87.5% in the PZD group and 85.4% in the ZD group. The maternal age, number of oocytes retrieved, fertilization rate, and other variables did not differ between the ZD and PZD groups. It was found that PZD led to a significantly higher rate of pregnancy (40.7% vs 15.4%), ongoing pregnancy (35.6% vs 11.5%), and implantation (18.1% vs 5.7%) than ZD. This suggests that using the mechanical method of PZD in blastomere biopsies for preimplantation genetic diagnosis may be more proficient than using the chemical method of ZD. The success of PZD over ZD could be attributed to the chemical agent in ZD having a harmful effect on the embryo and/or blastomere. Currently, zona drilling using a laser is the predominant method of opening the zona pellucida. Using a laser is an easier technique than using mechanical or chemical means. However, laser drilling could be harmful to the embryo and it is very expensive for in vitro fertilization laboratories to use especially when PGD is not a prevalent process as of modern times. PZD could be a viable alternative to these issues.[16]
In an attempt to overcome the difficulties related to single-cell techniques, it has been suggested to biopsy embryos at the blastocyst stage, providing a larger amount of starting material for diagnosis. It has been shown that if more than two cells are present in the same sample tube, the main technical problems of single-cell PCR or FISH would virtually disappear. On the other hand, as in the case of cleavage-stage biopsy, the chromosomal differences between the inner cell mass and the trophectoderm (TE) can reduce the accuracy of diagnosis, although this mosaicism has been reported to be lower than in cleavage-stage embryos.
TE biopsy has been shown to be successful in animal models such as rabbits,[17] mice[18] and primates.[19] These studies show that the removal of some TE cells is not detrimental to the further in vivo development of the embryo.
Human blastocyst-stage biopsy for PGD is performed by making a hole in the ZP on day three of in vitro culture. This allows the developing TE to protrude after blastulation, facilitating the biopsy. On day five post-fertilization, approximately five cells are excised from the TE using a glass needle or laser energy, leaving the embryo largely intact and without loss of inner cell mass. After diagnosis, the embryos can be replaced during the same cycle, or cryopreserved and transferred in a subsequent cycle.
There are two drawbacks to this approach, due to the stage at which it is performed. First, only approximately half of the preimplantation embryos reach the blastocyst stage. This can restrict the number of blastocysts available for biopsy, limiting in some cases the success of the PGD. Mc Arthur and coworkers[20] report that 21% of the started PGD cycles had no embryo suitable for TE biopsy. This figure is approximately four times higher than the average presented by the ESHRE PGD consortium data, where PB and cleavage-stage biopsy are the predominant reported methods. On the other hand, delaying the biopsy to this late stage of development limits the time to perform the genetic diagnosis, making it difficult to redo a second round of PCR or to rehybridize FISH probes before the embryos should be transferred back to the patient.
Sampling of cumulus cells can be performed in addition to a sampling of polar bodies or cells from the embryo. Because of the molecular interactions between cumulus cells and the oocyte, gene expression profiling of cumulus cells can be performed to estimate oocyte quality and the efficiency of an ovarian hyperstimulation protocol, and may indirectly predict aneuploidy, embryo development and pregnancy outcomes.[9][9]
Fluorescent in situ hybridization (FISH) and Polymerase chain reaction (PCR) are the two commonly used, first-generation technologies in PGD. PCR is generally used to diagnose monogenic disorders and FISH is used for the detection of chromosomal abnormalities (for instance, aneuploidy screening or chromosomal translocations). Over the past few years, various advancements in PGD testing have allowed for an improvement in the comprehensiveness and accuracy of results available depending on the technology used.[21] Recently a method was developed allowing to fix metaphase plates from single blastomeres. This technique in conjunction with FISH, m-FISH can produce more reliable results, since analysis is done on whole metaphase plates[22]
In addition to FISH and PCR, single cell genome sequencing is being tested as a method of preimplantation genetic diagnosis.[23] This characterizes the complete DNA sequence of the genome of the embryo.
FISH is the most commonly applied method to determine the chromosomal constitution of an embryo. In contrast to karyotyping, it can be used on interphase chromosomes, so that it can be used on PBs, blastomeres and TE samples. The cells are fixated on glass microscope slides and hybridised with DNA probes. Each of these probes are specific for part of a chromosome, and are labelled with a fluorochrome. Currently, a large panel of probes are available for different segments of all chromosomes, but the limited number of different fluorochromes confines the number of signals that can be analysed simultaneously.
The type and number of probes that are used on a sample depends on the indication. For sex determination (used for instance when a PCR protocol for a given X-linked disorder is not available), probes for the X and Y chromosomes are applied along with probes for one or more of the autosomes as an internal FISH control. More probes can be added to check for aneuploidies, particularly those that could give rise to a viable pregnancy (such as a trisomy 21). The use of probes for chromosomes X, Y, 13, 14, 15, 16, 18, 21 and 22 has the potential of detecting 70% of the aneuploidies found in spontaneous abortions.
In order to be able to analyse more chromosomes on the same sample, up to three consecutive rounds of FISH can be carried out. In the case of chromosome rearrangements, specific combinations of probes have to be chosen that flank the region of interest. The FISH technique is considered to have an error rate between 5 and 10%.
The main problem of the use of FISH to study the chromosomal constitution of embryos is the elevated mosaicism rate observed at the human preimplantation stage. A meta-analysis of more than 800 embryos came to the result that approximately 75% of preimplantation embryos are mosaic, of which approximately 60% are diploid–aneuploid mosaic and approximately 15% aneuploid mosaic.[24] Li and co-workers[25] found that 40% of the embryos diagnosed as aneuploid on day 3 turned out to have a euploid inner cell mass at day 6. Staessen and collaborators found that 17.5% of the embryos diagnosed as abnormal during PGS, and subjected to post-PGD reanalysis, were found to also contain normal cells, and 8.4% were found grossly normal.[26] As a consequence, it has been questioned whether the one or two cells studied from an embryo are actually representative of the complete embryo, and whether viable embryos are not being discarded due to the limitations of the technique.
Kary Mullis conceived PCR in 1985 as an in vitro simplified reproduction of the in vivo process of DNA replication. Taking advantage of the chemical properties of DNA and the availability of thermostable DNA polymerases, PCR allows for the enrichment of a DNA sample for a certain sequence. PCR provides the possibility to obtain a large quantity of copies of a particular stretch of the genome, making further analysis possible. It is a highly sensitive and specific technology, which makes it suitable for all kinds of genetic diagnosis, including PGD. Currently, many different variations exist on the PCR itself, as well as on the different methods for the posterior analysis of the PCR products.
When using PCR in PGD, one is faced with a problem that is inexistent in routine genetic analysis: the minute amounts of available genomic DNA. As PGD is performed on single cells, PCR has to be adapted and pushed to its physical limits, and use the minimum amount of template possible: one strand. This implies a long process of fine-tuning of the PCR conditions and a susceptibility to all the problems of conventional PCR, but several degrees intensified. The high number of needed PCR cycles and the limited amount of template makes single-cell PCR very sensitive to contamination. Another problem specific to single-cell PCR is the allele drop out (ADO) phenomenon. It consists of the random non-amplification of one of the alleles present in a heterozygous sample. ADO seriously compromises the reliability of PGD as a heterozygous embryo could be diagnosed as affected or unaffected depending on which allele would fail to amplify. This is particularly concerning in PGD for autosomal dominant disorders, where ADO of the affected allele could lead to the transfer of an affected embryo.
The establishment of a diagnosis in PGD is not always straightforward. The criteria used for choosing the embryos to be replaced after FISH or PCR results are not equal in all centres. In the case of FISH, in some centres only embryos are replaced that are found to be chromosomally normal (that is, showing two signals for the gonosomes and the analysed autosomes) after the analysis of one or two blastomeres, and when two blastomeres are analysed, the results should be concordant. Other centres argue that embryos diagnosed as monosomic could be transferred, because the false monosomy (i.e. loss of one FISH signal in a normal dipoloid cell) is the most frequently occurring misdiagnosis. In these cases, there is no risk for an aneuploid pregnancy, and normal diploid embryos are not lost for transfer because of a FISH error. Moreover, it has been shown that embryos diagnosed as monosomic on day 3 (except for chromosomes X and 21), never develop to blastocyst, which correlates with the fact that these monosomies are never observed in ongoing pregnancies.
Diagnosis and misdiagnosis in PGD using PCR have been mathematically modelled in the work of Navidi and Arnheim and of Lewis and collaborators.[27][28] The most important conclusion of these publications is that for the efficient and accurate diagnosis of an embryo, two genotypes are required. This can be based on a linked marker and disease genotypes from a single cell or on marker/disease genotypes of two cells. An interesting aspect explored in these papers is the detailed study of all possible combinations of alleles that may appear in the PCR results for a particular embryo. The authors indicate that some of the genotypes that can be obtained during diagnosis may not be concordant with the expected pattern of linked marker genotypes, but are still providing sufficient confidence about the unaffected genotype of the embryo. Although these models are reassuring, they are based on a theoretical model, and generally the diagnosis is established on a more conservative basis, aiming to avoid the possibility of misdiagnosis. When unexpected alleles appear during the analysis of a cell, depending on the genotype observed, it is considered that either an abnormal cell has been analysed or that contamination has occurred, and that no diagnosis can be established. A case in which the abnormality of the analysed cell can be clearly identified is when, using a multiplex PCR for linked markers, only the alleles of one of the parents are found in the sample. In this case, the cell can be considered as carrying a monosomy for the chromosome on which the markers are located, or, possibly, as haploid. The appearance of a single allele that indicates an affected genotype is considered sufficient to diagnose the embryo as affected, and embryos that have been diagnosed with a complete unaffected genotype are preferred for replacement. Although this policy may lead to a lower number of unaffected embryos suitable for transfer, it is considered preferable to the possibility of a misdiagnosis.
Preimplantation genetic haplotyping (PGH) is a PGD technique wherein a haplotype of genetic markers that have statistical associations to a target disease are identified rather than the mutation causing the disease.[29]
Once a panel of associated genetic markers have been established for a particular disease it can be used for all carriers of that disease.[29] In contrast, since even a monogenic disease can be caused by many different mutations within the affected gene, conventional PGD methods based on finding a specific mutation would require mutation-specific tests. Thus, PGH widens the availability of PGD to cases where mutation-specific tests are unavailable.
PGH also has an advantage over FISH in that FISH is not usually able to make the differentiation between embryos that possess the balanced form of a chromosomal translocation and those carrying the homologous normal chromosomes. This inability can be seriously harmful to the diagnosis made. PGH can make the distinction that FISH often cannot. PGH does this by using polymorphic markers that are better suited at recognizing translocations. These polymorphic markers are able to distinguish between embryos that carried normal, balanced, and unbalanced translocations. FISH also requires more cell fixation for analysis whereas PGH requires only transfer of cells into polymerase chain reaction tubes. The cell transfer is a simpler method and leaves less room for analysis failure.[30]
Embryo transfer is usually performed on day three or day five post-fertilization, the timing depending on the techniques used for PGD and the standard procedures of the IVF centre where it is performed.
With the introduction in Europe of the single-embryo transfer policy, which aims at the reduction of the incidence of multiple pregnancies after ART, usually one embryo or early blastocyst is replaced in the uterus. Serum hCG is determined at day 12. If a pregnancy is established, an ultrasound examination at 7 weeks is performed to confirm the presence of a fetal heartbeat. Couples are generally advised to undergo PND because of the, albeit low, risk of misdiagnosis.
It is not unusual that after the PGD, there are more embryos suitable for transferring back to the woman than necessary. For the couples undergoing PGD, those embryos are very valuable, as the couple's current cycle may not lead to an ongoing pregnancy. Embryo cryopreservation and later thawing and replacement can give them a second chance to pregnancy without having to redo the cumbersome and expensive ART and PGD procedures.
PGD/PGS is an invasive procedure that requires a serious consideration, according to Michael Tucker, Ph.D., Scientific Director and Chief Embryologist at Georgia Reproductive Specialists in Atlanta.[31] One of the risks of PGD includes damage to the embryo during the biopsy procedure (which in turn destroys the embryo as a whole), according to Serena H. Chen, M.D., a New Jersey reproductive endocrinologist with IRMS Reproductive Medicine at Saint Barnabas.[31] Another risk is cryopreservation where the embryo is stored in a frozen state and thawed later for the procedure. About 20% of the thawed embryos do not survive.[32][33] There has been a study indicating a biopsied embryo has a less rate of surviving cryopreservation.[34] Another study suggests that PGS results in a significantly lower live birth rate for women of advanced maternal age.[35] Also, another study recommends the caution and a long term follow-up as PGD/PGS increases the perinatal death rate in multiple pregnancies.[36]
In a mouse model study, PGD has been attributed to various long term risks including a weight gain and memory decline; a proteomic analysis of adult mouse brains showed significant differences between the biopsied and the control groups, of which many are closely associated with neurodegenerative disorders like Alzheimers and Down Syndrome.[37]
PGD has raised ethical issues, although this approach could reduce reliance on fetal deselection during pregnancy. The technique can be used for prenatal sex discernment of the embryo, and thus potentially can be used to select embryos of one sex in preference of the other in the context of "family balancing". It may be possible to make other "social selection" choices in the future that introduce socio-economic concerns. PGD allows discrimination against those with disabilities, and also, according to Georgiann Davis, those with intersex traits.[38] Only unaffected embryos are implanted in a woman’s uterus; those that are affected are either discarded or donated to science.[39]
PGD has the potential to screen for genetic issues unrelated to medical necessity, such as intelligence and beauty, and against negative traits such as disabilities. The medical community has regarded this as a counterintuitive and controversial suggestion.[40] The prospect of a "designer baby" is closely related to the PGD technique, creating a fear that increasing frequency of genetic screening will move toward a modern eugenics movement.[41] On the other hand, a principle of procreative beneficence is proposed, which is a putative moral obligation of parents in a position to select their children to favor those expected to have the best life.[42] An argument in favor of this principle is that traits (such as empathy, memory, etc.) are "all-purpose means" in the sense of being of instrumental value in realizing whatever life plans the child may come to have.[43]
In 2006 three percent of PGD clinics in the US reported having selected an embryo for the presence of a disability.[44] Couples involved were accused of purposely harming a child. This practice is notable in dwarfism, where parents intentionally create a child who is a dwarf.[44] In the selection of a saviour sibling to provide a matching bone marrow transplant for an already existing affected child, there are issues including the commodification and welfare of the donor child.[45]
By relying on the result of one cell from the multi-cell embryo, PGD operates under the assumption that this cell is representative of the remainder of the embryo. This may not be the case as the incidence of mosaicism is often relatively high.[46] On occasion, PGD may result in a false negative result leading to the acceptance of an abnormal embryo, or in a false positive result leading to the deselection of a normal embryo.
Another problematic case is the cases of desired non-disclosure of PGD results for some genetic disorders that may not yet be apparent in a parent, such as Huntington disease. It is applied when patients do not wish to know their carrier status but want to ensure that they have offspring free of the disease. This procedure can place practitioners in questionable ethical situations, e.g. when no healthy, unaffected embryos are available for transfer and a mock transfer has to be carried out so that the patient does not suspect that he/she is a carrier. The ESHRE ethics task force currently recommends using exclusion testing instead. Exclusion testing is based on a linkage analysis with polymorphic markers, in which the parental and grandparental origin of the chromosomes can be established. This way, only embryos are replaced that do not contain the chromosome derived from the affected grandparent, avoiding the need to detect the mutation itself.[citation needed]
Some religious organizations disapprove of this procedure. The Roman Catholic Church, for example, takes the position that it involves the destruction of human life.[47] and besides that, opposes the necessary in vitro fertilization of eggs as contrary to Aristotelian principles of nature.[citation needed] The Jewish Orthodox religion believes the repair of genetics is okay, but they do not support making a child that is genetically fashioned[39]
A meta-analysis that was performed indicates research studies conducted in PGD underscore future research. This is due to positive attitudinal survey results, postpartum follow-up studies demonstrating no significant differences between those who had used PGD and those who conceived naturally, and ethnographic studies which confirmed that those with a previous history of negative experiences found PGD as a relief. Firstly, in the attitudinal survey, women with a past history of infertility, pregnancy termination, and repeated miscarriages reported having a more positive attitude towards preimplantation genetic diagnosis. They were more accepting towards pursuing PGD. Secondly, likewise to the first attitudinal study, an ethnographic study conducted in 2004 found similar results. Couples with a past history of multiple miscarriages, infertility, and an ill child, felt that preimplantation genetic diagnosis was a viable option. They also felt more relief; "those using the technology were actually motivated to not repeat pregnancy loss".[48] In summary, although some of these studies are limited due to their retrospective nature and limited samples, the study's results indicate an overall satisfaction of participants for the use of PGD. However, the authors of the studies do indicate that these studies emphasize the need for future research such as creating a prospective design with a valid psychological scale necessary to assess the levels of stress and mood during embryonic transfer and implantation.[48]
Prior to implementing the Assisted Human Reproduction Act (AHR) in 2004, PGD was unregulated in Canada. The Act states, "No person shall knowingly for the purpose of creating a human being, perform any procedure or provide, prescribe or administer any thing that would ensure or increase the probability that an embryo will be on a particular sex, or that would identify the sex of an in vitro embryo, except to prevent, diagnose or treat a sex-linked disorder or disease."[49]
Therefore, sex selection for non-medical purposes was banned. Due to 2012’s national budget cuts, the AHR was removed. The regulation of assisted reproduction was then delegated to each province.[50] This delegation provides provinces with a lot of leeway to do as they please. As a result, provinces like Quebec, Alberta and Manitoba have put almost the full costs of IVF on the public healthcare bill.[51] Dr. Santiago Munne, developer of the first PGD test for Down’s Syndrome and founder of Reprogenetics, saw these provincial decisions as an opportunity for his company to grow and open more Reprogenetics labs around Canada. He dismissed all controversies regarding catalogue babies and states that he had no problem with perfect babies.[52]
Ontario, however, has no concrete regulations regarding PGD. Since 2011, the Ministry of Children and Youth Services in Ontario advocates for the development government-funded ‘safe fertility’ education, embryo monitoring and assisted reproduction services for all Ontarians. This government report shows that Ontario not only has indefinite regulations regarding assisted reproduction services like IVF and PGD, but also does not fund any of these services. The reproductive clinics that exist are all private and located only in Brampton, Markham, Mississauga, Scarborough, Toronto, London and Ottawa.[53] In contrast, provinces such as Alberta and Quebec not only have more clinics, but have also detailed laws regarding assisted reproduction and government funding for these practices.
Before 2010, the usage of PGD was in a legal grey area.[54] In 2010, the Federal Court of Justice of Germany ruled that PGD can be used in exceptional cases.[54] On 7 July 2011, the Bundestag passed a law that allows PGD in certain cases. The procedure may only be used when there is a strong likelihood that parents will pass on a genetic disease, or when there is a high genetic chance of a stillbirth or miscarriage.[55] On 1 February 2013, the Bundesrat approved a rule regulating how PGD can be used in practice.[54]
In India, Ministry of Family Health and Welfare, regulates the concept under - "The Pre-Conception and Prenatal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994". The Act was further been revised after 1994 and necessary amendment were made are updated timely on the official website of the Indian Government dedicated for the cause.[56]
In South Africa, where the right to reproductive freedom is a constitutionally protected right, it has been proposed that the state can only limit PGD to the degree that parental choice can harm the prospective child or to the degree that parental choice will reinforce societal prejudice.[7]
The preimplantation genetic diagnosis is not allowed in Switzerland.[citation needed]
No uniform system for regulation of assisted reproductive technologies, including genetic testing, exists in the United States. The practice and regulation of PGD most often falls under state laws or professional guidelines as the federal government does not have direct jurisdiction over the practice of medicine. To date, no state has implemented laws directly pertaining to PGD, therefore leaving researchers and clinicians to abide to guidelines set by the professional associations. The Center for Disease Control and Prevention (CDC) states that all clinics providing IVF must report pregnancy success rates annually to the federal government, but reporting of PGD use and outcomes is not required. The American Society for Reproductive Medicine (ASRM) states that, "PGD should be regarded as an established technique with specific and expanding applications for standard clinical practice." They also state, "While the use of PGD for the purpose of preventing sex-linked diseases is ethical, the use of PGD solely for sex selection is discouraged."[57]
In the UK, assisted reproductive technologies are regulated under the Human Fertilization and Embryology Act (HFE) of 2008. However, the HFE Act does not address issues surrounding PGD. Thus, the HFE Authority (HFEA) was created in 2003 to act as a national regulatory agency which issues licenses and monitors clinics providing PGD. The HFEA strictly prohibits sex selection for social or cultural reasons, but allows it to avoid sex-linked disorders. They state that PGD is not acceptable for, "social or psychological characteristics, normal physical variations, or any other conditions which are not associated with disability or a serious medical condition." It is however accessible to couples or individuals with a known family history of serious genetic diseases.[58]
In a study of 135 IVF clinics, 88% had websites, 70% mentioned PGD and 27% of the latter were university- or hospital-based and 63% were private clinics. Sites mentioning PGD also mentioned uses and benefits of PGD far more than the associated risks. Of the sites mentioning PGD, 76% described testing for single-gene diseases, but only 35% mentioned risks of missing target diagnoses, and only 18% mentioned risks for loss of the embryo. 14% described PGD as new or controversial. Private clinics were more likely than other programs to list certain PGD risks like for example diagnostic error, or note that PGD was new or controversial, reference sources of PGD information, provide accuracy rates of genetic testing of embryos, and offer gender selection for social reasons.[59]
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| リンク元 | 「preimplantation genetic diagnosis」「着床前診断」 |
| 関連記事 | 「diagnosis」 |