Preimplantation genetic diagnosis of spinal muscular atrophy

Evolving approaches to prenatal genetic counseling for Spinal Muscular Atrophy in the new treatment era

Spinal muscular atrophy (SMA) is an autosomal recessive genetic disease characterized by muscle weakness and atrophy with usually typical cognition. The first disease-modifying therapy for SMA, nusinersen, was approved by the United States Food and Drug Administration (FDA) in 2016 and leads to improved outcomes, especially when administered presymptomatically. Population-wide carrier screening and newborn screening (NBS) are now recommended by several professional organizations to promote reproductive autonomy, early diagnosis, and treatment. Prenatal genetic counselors (GCs) are important providers of the SMA screening and diagnosis process, but the possible impact of nusinersen on their practice has not been explored. A survey of 182 prenatal GCs in the United States (US) assessed baseline knowledge of nusinersen and likelihood of discussing this option with prospective parents. The majority of GCs (94.5%) were aware of this drug, and almost all (87.3%) felt that this information would affect pregnancy management decisions. However, less than half of GCs (49.2%) felt confident discussing nusinersen, 45.1% were unaware if this treatment was available in their practice setting, and one in five (19.3%) did not know where to find information about SMA treatments. Participants were more confident and knowledgeable about NBS for SMA, and several indicated that NBS would reduce their emphasis on carrier screening and diagnostic testing, not recognizing that an early prenatal diagnosis can enable preparations for complex, time-sensitive treatment. Only 5.0% of participants felt that a prenatal GC should discuss nusinersen with prospective parents. However, encouragingly, nearly all GCs who felt confident discussing this treatment option (86.4%) reported using this information weekly in their real-world practice. These data highlight an opportunity to provide up-to-date education about SMA treatments, as well as the significant impacts of early diagnosis. Additionally, interdisciplinary communication and care may be appropriate to clarify healthcare resources available and support a variety of patient needs. Increasing awareness and confidence about available options can help prenatal GCs empower patient autonomy and shared decision-making in the new era of disease-modifying treatment for SMA.

Preimplantation Genetic Testing for Monogenic Disease of Spinal Muscular Atrophy by Multiple Displacement Amplification: 11 unaffected livebirths

Background: Preimplantation genetic testing for monogenic disease (PGT-M) has become an effective method for providing couples with the opportunity of a pregnancy with a baby free of spinal muscular atrophy (SMA). Multiple displacement amplification (MDA) overcomes the innate dilemma of very limited genetic material available for PGT-M. Objective: To evaluate the use of MDA, combined with haplotype analysis and mutation amplification, in PGT-M for families with SMA. Methods: MDA was used to amplify the whole genome from single blastomeres or trophectoderm (TE) cells. Exon 7 of the survival motor neuron gene 1 (SMN1) and eleven STRs markers flanking the SMN1 gene were incorporated into singleplex polymerase chain reaction (PCR) assays on MDA products. Results: Sixteen cycles (19 ovum pick-up cycles) of PGT-M were initiated in 12 couples. A total of 141 embryos were tested: 90 embryos were biopsied at the cleavage stage and 51 embryos were biopsied at the blastocyst stage. MDA was successful on 94.44% (85/90) of the single blastomeres and on 92.16% (47/51) of the TE cells. And the PCR efficiency were 98.4% (561/570) and 100% (182/182), respectively. In addition, the average allele drop-out (ADO) rates were 13.3% (60/392) and 9.8% (11/112), respectively. The results for SMN1 exon 7 were all matched with haplotype analysis, which allowed an accurate diagnosis of 93.62% (132/141) embryos. Twelve families had unaffected embryos available for transfer and a total of 38 embryos were transferred in 20 embryo transfer cycles. Eight transfers were successful, resulting in a clinical pregnancy rate of 40% (8/20) and an implantation rate of 28.95% (11/38). Finally, 11 healthy babies were born. Among them, 5 SMA carriers were singleton live births and 3 SMA carriers had twin births. Conclusion: Careful handling during the MDA procedure can improve subsequent PCR efficiency and reduce the ADO rate. We suggest that this protocol is reliable for increasing the accuracy of the PGT-M for SMA.

Experience as knowledge: Disability, distillation and (reprogenetic) decision-making

‘Experiential knowledge’ is increasingly recognised as an important influence on reproductive decision-making. ‘Experiential knowledge of disability’ in particular is a significant resource within prenatal testing/screening contexts, enabling prospective parents to imagine and appraise future lives affected by disability. However, the concept of ‘experiential knowledge’ has been widely critiqued for its idiosyncrasy, its impermanence and consequently its perceived inferiority to (medical) knowledge. This paper explores some of these key critiques of experiential knowledge through an analysis of its constitution and uses in the context of reproductive decision-making. Seventeen UK-resident women with Spinal Muscular Atrophy (SMA), or with SMA in their family, took part in two in-depth interviews: one in 2007–9 and the other in 2013–4. By comparing and contrasting these women’s accounts at two time points, this paper demonstrates the stark contrast between ‘lived experience’ of SMA (the visceral everyday realities of life with the condition) and the various way(s) this experience was transformed into, and presented as, ‘knowledge’ through the processes of making, and accounting, for reproductive decisions. The analysis highlights that multiple, distinct and sometimes competing experiential frameworks are used to conceptualise SMA across time and context. However, rather than evidence of its fallibility, this finding highlights that ‘knowledge’ is an inappropriate vessel with which to capture and transfer ‘experiential knowledge’. Rather, we need to consider how to value such insight in ways that harnesses its inherent strength without leaving it vulnerable to the epistemological critiques attracted by labelling it ‘knowledge’.

Clinical applications of MARSALA for preimplantation genetic diagnosis of spinal muscular atrophy

Conventional PCR methods combined with linkage analysis based on short tandem repeats (STRs) or Karyomapping with single nucleotide polymorphism (SNP) arrays, have been applied to preimplantation genetic diagnosis (PGD) for spinal muscular atrophy (SMA), an autosome recessive disorder. However, it has limitations in SMA diagnosis by Karyomapping, and these methods are unable to distinguish wild-type embryos with carriers effectively. Mutated allele revealed by sequencing with aneuploidy and linkage analyses (MARSALA) is a new method allowing embryo selection by a one-step next-generation sequencing (NGS) procedure, which has been applied in PGD for both autosome dominant and X-linked diseases in our group previously. In this study, we carried out PGD based on MARSALA for two carrier families with SMA affected children. As a result, one of the couples has given birth to a healthy baby free of mutations in SMA-causing gene. It is the first time that MARSALA was applied to PGD for SMA, and we can distinguish the embryos with heterozygous deletion (carriers) from the wild-type (normal) ones accurately through this NGS-based method. In addition, direct mutation detection allows us to identify the affected embryos (homozygous deletion), which can be regarded as probands for linkage analysis, in case that the affected family member is absent. In the future, the NGS-based MARSALA method is expected to be used in PGD for all monogenetic disorders with known pathogenic gene mutation.

Preimplantation genetic diagnosis for monogenic diseases: overview and emerging issues

Preimplantation genetic diagnosis (PGD) is an established reproductive option for couples at risk of conceiving a pregnancy affected with a known genetic disease, who wish to avoid an (additional) affected child, termination of pregnancy or recurrent miscarriages. Early technologies concentrated on different approaches to direct mutation testing for monogenic diseases using single cell PCR protocols, or sex selection by fluorescent in situ hybridization for X-linked monogenic disease. Development of multiplex fluorescent PCR allowed simultaneously testing of linked markers alongside the mutation test, increasing the accuracy by controlling for contamination and identifying allele drop-out. The advent of highly effective whole genome amplification methods has opened the way for new technologies such as preimplantation genetic haplotyping and microarrays, thus increasing the number of genetic defects that can be detected in preimplantation embryos; the number of cases carried out and the new indications tested increases each year. Different countries have taken very different approaches to legislating and regulating PGD, giving rise to the phenomenon of reproductive tourism. PGD is now being performed for scenarios previously not undertaken using prenatal diagnosis, some of which raise significant ethical concerns. While PGD has benefited many couples aiming to have healthy children, ethical concerns remain over inappropriate use of this technology.

Knowledge is power? The role of experiential knowledge in genetically 'risky' reproductive decisions

Knowledge of the condition being tested for is increasingly acknowledged as an important factor in prenatal testing and screening decisions. An analysis of the way in which family members living with an inheritable condition use and value this knowledge has much to add to debates about whether and how this type of knowledge could be made available to prospective parents facing screening decisions. This article reports on in-depth interviews (conducted between 2007 and 2009) with 61 people with a genetic condition, spinal muscular atrophy (SMA) in their family. Many participants reported that their intimate familial knowledge of SMA offered them valuable insights with which they could imagine future lives. Other participants, however, found themselves trapped between their experiential knowledge of SMA and their (often) competing responsibility to maintain the wellbeing of their family. Still, others established a hierarchy of knowledge to rank the authenticity of different family member's accounts of SMA in order to discredit or justify their decisions. This article highlights the way in which experiential knowledge of the condition being tested for cannot be unproblematically assumed to be a useful resource in the context of prenatal testing decisions and may actually constrain reproductive decisions.

Experiential knowledge of disability, impairment and illness: The reproductive decisions of families genetically at risk

As the capacities of Reproductive Genetic Technologies expand, would-be parents face an increasing number of reproductive decisions regarding testing and screening for different conditions. Several studies have acknowledged the role that 'experiential knowledge of disability' plays in arriving at decisions around the use of these technologies; however, there is a lack of clarity within this literature as to what constitutes 'experiential knowledge of disability' and an over-reliance on medical diagnoses as a shorthand to describe different types of experience. Drawing on both social model of disability theory and the literature on chronic illness, this article presents an analysis of data from an in-depth qualitative interview study with 64 people with an inheritable condition in their family, Spinal Muscular Atrophy, and reports their views around reproduction and Reproductive Genetic Technologies. An experiential typology is presented which demonstrates the way in which experiences of 'disability', 'embodied experiences of impairment' or 'embodied experiences of illness, death and bereavement' are strategically privileged in accounts of reproductive decisions, in order to validate reproductive decisions taken, and, specifically, justify use (or non-use) of Reproductive Genetic Technologies. By highlighting the experiential categories within which participants embedded their reproductive decisions, this article draws attention to the porous and collapsible nature of diagnostic categories in the context of reproductive decision-making and genetic risk, and suggests new ways of researching 'experiential knowledge of disability' within these contexts which are able to account for the various contours of the embodied lived reality of life with 'disability'.

Embryo Culture Media, Culture Techniques and Embryo Selection: A Tribute to Wesley Kingston Whitten

This review article gives a brief history of the classical experiments that led to the development of the embryo culture medium and in vitro embryo culture. It proposes that, in view of the outstanding and significant pioneering contributions of Wesley Kingston Whitten to the development of embryo culture medium, he be considered the “Father of Embryo Culture Medium”. Furthermore, it describes the nutritional requirements of early embryos and how these requirements with specific references to carbohydrates, amino acids, phosphates, growth factors, etc, have been utilized to formulate increasingly more complex embryo culture media. This has led to the development of progressively more efficacious embryo culture media including the formulation of completely defined and synthetic protein-free embryo culture medium. The review also describes physical factors, growth factors, insemination methods for the fertilization of oocytes and culture methods affecting embryo growth, development, metabolism, oxygen embryotoxicity and survival. In procedural terms, the review also summarizes the evolution of embryo culture techniques from tube culture to, microdrop culture under oil to co-culture to ultra microdrop culture techniques. It includes techniques of in vitro maturation and for the selection of potentially viable embryos of various developmental stages.

Validation of preimplantation genetic diagnosis by PCR analysis: genotype comparison of the blastomere and corresponding embryo, implications for clinical practice

The aim of this study was to validate the overall preimplantation genetic diagnosis (PGD)-PCR procedure and to determine the diagnostic value. Genotyped embryos not selected for embryo transfer (ET) and unsuitable for cryopreservation after PGD were used for confirmatory analysis. The PGD genotyped blastomeres and corresponding embryos were compared, and morphology was scored on Day 4 post fertilization. To establish the validity of the PGD-PCR procedure and the diagnostic value, misdiagnosis rate, false-negative rate and negative predictive value were calculated. Moreover, comparison on the validity was made for the biopsy of one or two blastomeres. For the total embryo group (n = 422), a misdiagnosis rate of 7.1% and a false-negative rate of 3.1% were found. The negative predictive value was 96.1%. Poor morphology Day 4 embryos (Class 1) were over-represented in the embryo group in which the blastomere genotype was not confirmed by the whole embryo genotype. The misdiagnosis rate of Class 1 embryos was 12.5% and the false-negative rate 17.1%. Exclusion of these embryos resulted in a misdiagnosis rate of 6.1%, a false-negative rate of 0.5% and a negative predictive value of 99.3%. The two blastomere biopsies revealed a significant higher positive predictive value, lowering the misdiagnosis rate, whereas the negative predictive value remained the same. In conclusion, the PGD-PCR procedure is a valid diagnostic method to select unaffected embryos for ET. The misdiagnosis and false-negative rates decrease by rejecting Class 1 embryos for ET. The biopsy of a second blastomere improves the positive predictive value, lowering the misdiagnosis rate.

Efficient strategies for preimplantation genetic diagnosis of spinal muscular atrophy

OBJECTIVE

To develop and apply efficient multiplex preimplantation genetic diagnosis (PGD) protocols for spinal muscular atrophy (SMA).

DESIGN

Two multiplex PGD protocols were developed allowing the detection of the common homozygous deletion of the telomeric spinal muscular atrophy gene (SMN1), together with two microsatellites located on each side of SMN1.

SETTING

The molecular genetics laboratory of the university hospital in Montpellier.

PATIENT(S)

A couple who had already given birth to a child affected with SMA.

INTERVENTION(S)

In vitro fertilization using intracytoplasmic sperm injection (ICSI) and blastomere biopsy.

MAIN OUTCOME MEASURE(S)

Improvement of PGD for SMA.

RESULT(S)

Two different multiplex protocols were set up on 81 (multiplex A) and 64 single cells (multiplex B) from normal controls, affected patients, and individuals with homozygous SMN2 deletion. In one PGD cycle that used one of these protocols, two embryos were transferred, which resulted in the birth of a healthy baby.

CONCLUSION(S)

Analysis of microsatellite markers in addition to the SMN1 deletion allows the detection of contamination, the study of ploidy of the biopsied blastomeres, and the performance of an indirect genetic diagnosis, thereby increasing the reliability of the results. This PGD assay may be applied to all families with the common deletion of SMN1 and also to couples in whom one of the partners carries a small intragenic mutation in SMN1, identified in about 6% of affected individuals who do not lack both copies of SMN1.

Case report: birth after preimplantation genetic diagnosis of a subtle mutation in SMN1 gene

Spinal muscular atrophy (SMA) preimplantation genetic diagnosis (PGD) has been available since 1998. Protocols are based on the detection of the homozygous deletion of exon 7, which are present in 90-98% of SMA patients. A couple where the woman was a heterozygous carrier of the usual SMN1 Del7 mutation and the man was a heterozygous carrier of pMet263Arg substitution in exon 6 of SMN1 gene was referred for PGD. The usual PGD test being unsuitable for this couple, we developed a novel duplex polymerase chain reaction (PCR)-based PGD test for the detection of the mutation pMet263Arg by allele specific amplification, combined with the amplification of D5S641 extragenic polymorphic marker. PCR conditions were established using single control lymphoblasts and lymphocytes from the pMet263Arg substitution carrier. Amplification was obtained in 100% of the 86 single cells tested, amplification refractory mutation system (ARMS) PCR was specific in 100% of single cells tested and a complete genotype (mutation plus D5S641) was achieved in 88% of them. A PGD cycle was performed successfully and a pregnancy was obtained. An unaffected girl was born and postnatal diagnosis confirmed PGD results. This is the first PGD described for SMA because of another mutation than the major homozygous exon 7 deletion of SMN1. In the future, a similar strategy could be adopted for other subtle mutations of this gene.

Spinal muscular atrophy: from gene to therapy

The molecular basis of spinal muscular atrophy (SMA), an autosomal recessive neuromuscular disorder, is the homozygous loss of the survival motor neuron gene 1 (SMN1). A nearly identical copy of the SMN1 gene, called SMN2, modulates the disease severity. The functional difference between both genes is a translationally silent mutation that, however, disrupts an exonic splicing enhancer causing exon 7 skipping in most SMN2 transcripts. Only 10% of SMN2 transcripts encode functional full-length protein identical to SMN1. Transcriptional activation, facilitation of correct SMN2 splicing, or stabilization of the protein are considered as strategies for SMA therapy. Among various drugs, histone deacetylase inhibitors such as valproic acid (VPA) or 4-phenylbutyrate (PBA) have been shown to increase SMN2-derived RNA and protein levels. Recently, in vivo activation of the SMN gene was shown in VPA-treated SMA patients and carriers. Clinical trials are underway to investigate the effect of VPA and PBA on motor function in SMA patients.

Improved single-cell protocol for preimplantation genetic diagnosis of spinal muscular atrophy

OBJECTIVE

To develop and validate a simple and reliable single-cell analysis protocol for the preimplantation genetic diagnosis (PGD) of spinal muscular atrophy (SMA).

DESIGN

Molecular tests based on specific enzymatic digestion have already been described for SMA diagnosis. We modified the amplified DNA fragments so as to introduce a novel restriction site that provides an internal control for the completeness of the digestion.

SETTING

The genetics and reproduction departments of two teaching hospitals.

PATIENT(S)

Six informed couples at risk of transmitting SMA.

INTERVENTION(S)

All patients underwent standard procedures associated with intracytoplasmic sperm injection.

MAIN OUTCOME MEASURE(S)

Improvement of SMA diagnostic efficiency and accuracy on single cell.

RESULT(S)

One hundred fifty lymphocytes were analyzed with our protocol. One hundred percent diagnostic accuracy was achieved from both homozygous normal and SMN1-deleted leukocytes. Successful molecular analysis was achieved for 36 of 42 biopsied embryos (86%). Twenty-five normal embryos were transferred, but no pregnancy was achieved.

CONCLUSION(S)

We developed an improved protocol for PGD of SMA that is simple, robust, and accurate; unfortunately, no pregnancies were achieved for any of the six patients who have undergone PGD in the program thus far.

Comparison of PCR-RFLP with allele-specific PCR in genetic testing for spinal muscular atrophy

PCR-based methods for the detection of homozygous deletion of exon 7 of the SMN1 gene have been widely used in genetic testing for spinal muscular atrophy (SMA). We compared the most commonly used PCRrestriction fragment length polymorphism (PCR-RFLP) assay with an allele-specific PCR method, evaluating their potential application in direct testing, prenatal prediction, and preimplantation diagnosis, in terms of a range of DNA amounts used in such testing. We showed that PCR-RFLP could identify the SMN1 exon 7 by amplifying 10 pg of genomic DNA, and could differentiate SMN1 from SMN2 at the 100-pg DNA level (DraIdigested SMN2 fragments served as an internal control for PCR efficiency). In contrast, allele-specific PCR for SMN1, despite some advantages in a rapid preimplantation diagnosis, quickly lost its specificity when 100 pg of genomic DNA was used. In addition, the absence of a SMN1 fragment at the 10-pg DNA level may be due to a PCR amplification failure, and, thus, it is difficult to interpret without a proper internal control. Our data indicate that PCR-RFLP can be used for most diagnostic purposes, whereas the use of allelespecific PCR may be considered with caution under certain circumstances.

Preimplantation genetic diagnosis

Preimplantation genetic diagnosis (PGD) was introduced at the beginning of the 1990s as an alternative to prenatal diagnosis, to prevent termination of pregnancy in couples with a high risk for offspring affected by a sex-linked genetic disease. At that time, embryos obtained in vitro were tested to ascertain their sex, and only female embryos were transferred. Since then, techniques for genetic analysis at the single-cell level, involving assessment of first and second polar bodies from oocytes or blastomeres from cleavage-stage embryos, have evolved. Fluorescence in-situ hybridisation (FISH) has been introduced for the analysis of chromosomes and PCR for the analysis of genes in cases of monogenic diseases. In-vitro culture of embryos has also improved through the use of sequential media. Here, we provide an overview of indications for, and techniques used in, PGD, and discuss results obtained with the technique and outcomes of pregnancies. A brief review of new technologies is also included.

Spinal Muscular Atrophy in the Neonate

Spinal muscular atrophy (SMA) type I is an autosomal recessive disorder characterized by loss of lower motor neurons in the spinal cord. This severe hereditary neurodegenerative disorder is an important cause of morbidity in the neonate and the leading hereditary cause of infant mortality. The characteristic degeneration of anterior horn cells in the spinal cord leads to progressive muscular weakness and atrophy of the skeletal muscles. In SMA type I, the most severe form of SMA, death usually ensues by 2 years of age from respiratory failure or infection. Accurate diagnosis is now available through genetic testing, and progress is being made toward the development of therapy based on understanding of the disease mechanism. The neonatal nurse plays a pivotal role in identifying and caring for these medically fragile infants and in providing support and education for parents and families.

Duplex PCR for preimplantation genetic diagnosis (PGD) of spinal muscular atrophy

The main difficulty in developing a molecular diagnosis of spinal muscular atrophy (SMA) resides in the specific genomic structure of the locus. Indeed, two highly homologous survival motor neurone genes, SMN1 and SMN2, are present at the locus. The detection of the homozygous deletion of exons 7 and 8 of the SMN1 gene, which is present in 90 to 98% of the patients, is based on methods highlighting 1 of the 8 nucleotidic mismatches existing between these 2 genes. In order to offer preimplantation genetic diagnosis (PGD) for SMA, we developed a new allele-specific amplification method. The main disadvantage of our previously described strategy resided in the possibility of diagnosing, in case of amplification failure, an unaffected embryo as affected. We present here a new PGD-SMA method. We established the conditions for three different duplex PCRs, allowing the specific detection of the SMN1 gene and one polymorphic marker, either D5S629, D5S1977, or D5S641. Of the 60 to 90 single cells tested, the PCR efficiency varied from 98 to 100% with a complete genotype obtained in a range between 81 and 87% with a global allele drop-out rate of 9%. Such a test was used to perform 1 PGD cycle for which 7 embryos could be analysed. All the embryos were fully diagnosed, six as unaffected and one as affected. Four embryos were transferred, but no pregnancy ensued.

Toward pre-conceptual genetic analysis of human spermatozoa

Nuclei of mature mammalian spermatozoa are extraordinarily resistant to chemical and thermal injury. Additionally, decondensation of spermatozoa DNA can be accompanied by little or no visual changes of the sperm head. This study tested whether human spermatozoa could be recovered following several cycles of primer extension preamplification (PEP) and used to achieve fertilization and subsequent development of human oocytes. An attempt was also made to amplify PEP buffer after spermatozoon removal. The results demonstrate that the sperm head can be successfully recovered following treatment with KOH or proteinase K followed by one to four cycles of PEP. It is also shown that following this treatment, the spermatozoa can be injected into the oocytes and will transform into a pronucleus if the oocyte is activated by sperm cytosolic fraction. In some cases, it was also possible to obtain polymerase chain reaction signals using a buffer after sperm cells were removed following several cycles of PEP. Although sperm participation in development was confirmed by fluorescence in-situ hybridization, light microscopy revealed some degree of damage to spermatozoal chromosomes. It is concluded that pre-conceptual analysis of sperm cells may be possible, but more research is necessary to determine the optimal conditions that would preserve sperm DNA integrity while allowing accurate diagnoses.

Pre-implantation genetic diagnosis

Pre-implantation genetic diagnosis is an alternative to prenatal diagnosis for a select group of patients. Patients have to go through in vitro fertilization in order to produce embryos in vitro, from which one or two cells are removed at the 8-cell stage. A fluorescence in situ hybridization or polymerase chain reaction is carried out for the genetic diagnosis. Fluorescence in situ hybridization is used for the analysis of chromosomes for sexing for X-linked disease, chromosome abnormalities and aneuploidy screening. Aneuploidy screening is performed for infertile patients going through in vitro fertilization to try to improve their pregnancy rate. A polymerase chain reaction is used for the diagnosis of single-gene disorders. Since the risk of contamination and allele dropout is high with a polymerase chain reaction, linked or unlinked markers are usually used in a fluorescent multiplex polymerase chain reaction. New techniques, for example comparative genomic hybridization, allow the analysis of all of the chromosomes from one cell at one time. The ethical implications of pre-implantation genetic diagnosis are immense as the technique has already been used for social sexing and human leukocyte antigen matching.

Allele-specific amplification for preimplantation genetic diagnosis (PGD) of spinal muscular atrophy

We have developed a new allele-specific amplification method for the preimplantation genetic diagnosis (PGD) of spinal muscular atrophy (SMA; Werdnig-Hoffmann disease) from a single cell. This method is based on the detection of the deletion of exon 7 of the telomeric copy of the survival motor neurone (SMN(t)) gene. An oligonucleotide was designed to be specific to the SMN(t) nucleotidic sequence with exonic mismatch G (for SMN(t))-->A (for SMN(c)) at its 3' end. This test produces reliable PCR products in 95% of single lymphoblasts (85/88) tested as well as in 16/16 blastomeres from normal controls. Specificity analysis showed that we were able to detect homozygous deletion of the SMN(t) gene in 99% of single lymphoblasts (103/104) from a SMA patient. No contamination was detected in 68 blanks tested. Multiple cell and DNA dilution analysis revealed that the test is accurate and specific up to 100 pg DNA and should thus also be suitable for PGD at the blastocyst stage. This rapid procedure requires a single round of fluorescent PCR and no restriction digestion, while previously described single cell methods include nested PCR followed by restriction enzyme digestion. Two PGD cycles for SMA using this procedure were performed in our centre.

Preimplantation genetic diagnosis: applications for molecular medicine

Preimplantation genetic diagnosis is an alternative to prenatal diagnosis for the detection of genetic disorders. Tests are conducted on single cells biopsied from embryos before they are implanted, allowing the selection of unaffected embryos before a pregnancy has been established. Thus, the issue of pregnancy termination is circumvented. The use of preimplantation genetic diagnosis might have a significant impact on in vitro fertilization success rates as well as allowing the diagnosis of inherited disease.

Controlling misdiagnosis errors in preimplantation genetic diagnosis: a comprehensive model encompassing extrinsic and intrinsic sources of error

We have developed a mathematical model to explore accuracy of preimplantation genetic diagnosis (PGD) using single cell polymerase chain reaction (PCR). The model encompasses both extrinsic technical errors and intrinsic errors related to nuclear and chromosomal abnormalities. Using estimates for these errors, we have calculated the probability of a serious error (affected embryo diagnosed as unaffected) using a variety of strategies designed to increase the accuracy of PGD. Additional information from genotyping a linked marker or a second biopsied cell reduces the probability of replacing an affected embryo, while ensuring that sufficient unaffected embryos can be replaced. For a recessive disease, two genotypes are required to ensure a low probability of replacing an affected embryo (<1%) with a high proportion of unaffected embryos eligible for replacement (68%). These genotypes may be from a single cell with linked marker, or disease genotypes from two cells. PGD of a dominant disease is more difficult, as it relies on the amplification of a single copy of the mutation. Genotypes from two biopsied cells are required to ensure that a high proportion of unaffected embryos are eligible for replacement. This model can be used as a clinical tool to prioritize embryos for transfer in a PGD cycle.

Pre-implantation genetic diagnosis

Pre-implantation genetic diagnosis (PGD) was developed in the UK over 10 years ago. There are now more than 40 centres worldwide carrying out PGD and 150 babies have been born after genetic testing on day 3 of development, at the cleavage stage. This review covers the current status of PGD, the technology used and the types of genetically determined diseases for which testing has been developed.

Preimplantation genetic diagnosis

Embryo biopsy for preimplantation genetic diagnosis can be performed on the oocyte/zygote, cleavage stage embryo, or blastocyst, but the majority of centres perform cleavage stage biopsy. Single-cell diagnosis is undertaken by the polymerase chain reaction or fluorescent in-situ hybridization. Technical difficulties have arisen with preimplantation genetic diagnosis, such as allele dropout and chromosomal mosaicism. However, it is hoped that these difficulties can be overcome in the future with the advent of new techniques.