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

Novel PGD strategy based on single sperm linkage analysis for carriers of single gene pathogenic variant and chromosome reciprocal translocation

PURPOSE

Preimplantation genetic diagnosis (PGD) analysis can be challenging for couples who carry more than one genetic condition. In this study, we describe a new PGD strategy to select which embryo(s) to transfer for two clinically challenging cases. Both cases lack essential family members for linkage analysis including de novo mutation combined with reciprocal translocation.

METHODS

Diverging from conventional method, we performed direct point mutation detection, quantitative analysis of gene copy number, combined with linkage analysis assisted by SNP information from single sperm (or polar bodies), thus establishing an all-in-one protocol for single embryonic cell preimplantation diagnosis for two co-existing genetic conditions (monogenic disease and chromosomal abnormality) on the NGS-based platform.

RESULTS

Using this newly developed method, 15 embryos from two cases were screened, and two embryos were determined as free of the monogenic disease and specific chromosomal abnormalities created by the prospective father's reciprocal translocations.

CONCLUSION

This novel PGD strategy could effectively select unaffected embryo(s) for couples affected with or carrying a monogenetic disease and a reciprocal chromosome translocation concurrently.

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.

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.

Allele-specific PCR for a cost-effective & time-efficient diagnostic screening of spinal muscular atrophy

Background & objectives:

Genetic diagnosis of spinal muscular atrophy (SMA) is complicated by the presence of SMN2 gene as majority of SMA patients show absence or deletion of SMN1 gene. PCR may amplify both the genes non selectively in presence of high amount of DNA. We evaluated whether allele-specific PCR for diagnostic screening of SMA is reliable in the presence of high amount of genomic DNA, which is commonly used when performing diagnostic screening using restriction enzymes.

Methods:

A total of 126 blood DNA samples were tested in amounts ranging 80-200 ng, referred for the genetic diagnosis of SMA using both conventional PCR-RFLP and allele-specific PCR.

Results:

The results from both methods showed agreement. Further, allele-specific PCR was found to be a time-efficient and cost-effective method.

Interpretation & conclusions:

Our study demonstrated the accuracy of our allele-specific PCR and the results were comparable compatible with that of PCR-RFLP, indicating its practical application in SMA diagnostic screening.

Establishment of a molecular diagnostic system for spinal muscular atrophy experience from a clinical laboratory in china

Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder characterized by degeneration of the anterior horn of the spinal cord. The disease gene survival motor neuron 1 (SMN1) is homozygously absent in approximately 95% of patients, and approximately 5% of patients are believed to have subtle mutations. Although methods for molecular diagnosis of SMA have been reported singly, no diagnostic methodological system to tackle different SMA cases has been reported. Thirty-two families affected by SMA enrolled into this study. Our system comprised PCR-restriction fragment length polymorphism and allele-specific PCR for homozygous deletion analysis of SMN1, multiplex ligation-dependent probe amplification analysis for the determination of the copy number of SMN1, and SMN1 subtle mutation analysis at both the transcript and genomic levels. In 23 families, 21 patients had a homozygous deletion of SMN1. The remaining two patients without a deletion had a single SMN1 copy containing the subtle mutations S230L and L228X, respectively. In nine families in whom samples from the index patients were unavailable, parents from eight families showed one SMN1 copy, and one parent in the remaining family showed two SMN1 copies, one being normal and the other carrying the subtle mutation 22_23insA. To our knowledge, our methodological system for the molecular diagnosis of SMA offers the most complete evaluation of family members affected by SMA at this time.

A duplex allele-specific amplification PCR to detect SMN1 deletion

Spinal muscular atrophy (SMA), the leading genetic cause of death in childhood, is an autosomal recessive neuromuscular disorder characterized by progressive muscle weakness, associated with deletions of the survival motor neuron (SMN) gene identified and mapped to chromosome 5q13. SMN is present in two highly homologous copies (SMN1 and SMN2). In the general population, normal individuals (noncarriers) have at least one telomeric (SMN1) copy, and 5% of them have no copies of SMN2. Approximately 95% of SMA patients carry homologous deletions of SMN1 exon(s) 7 (and 8). SMN1 and SMN2 exons 7 and 8 differ only by 1 bp each, and SMA diagnosis might be performed by single-strand conformational polymorphism, PCR amplification followed by restriction fragment length polymorphism (RFLP), multiple ligation-dependent probe amplification, or realtime PCR of SMNs exons 7 and 8. We developed a simpler and cost-effective method to detect SMN1 exon 7 deletion based on allele-specific amplification PCR.

Preimplantation genetic diagnosis for alpha-thalassaemia in China

PURPOSE

To report the usage of PGD for alpha-thalassaemia with the - -(SEA) genotype.

METHOD

A PGD protocol using fluorescent gap PCR was performed for 51 cycles on 43 couples with the - -(SEA) genotype. Allele drop-out and amplification failure rates were retrospectively analyzed.

RESULTS

A total of 472 embryos were biopsied. Amplification was achieved in 390 blastomeres, accounting for an amplification rate of 82.6%. In total, 120 wild-type, 94 heterozygotes and 140 homozygous mutant embryos were diagnosed. The successful diagnosis rate was 75.0%. The ADO rate in 49 blastomeres from six donated embryos was 16.4%. One hundred and fifty four embryos were transferred, resulting in 25 clinical pregnancies with an implantation rate of 24.0%.

CONCLUSIONS

Single-round fluorescent gap PCR is a feasible and effective strategy in the PGD for alpha-thalassaemia with the - -(SEA) genotype.

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.

Multiplex nested PCR for preimplantation genetic diagnosis of spinal muscular atrophy

OBJECTIVE

Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder caused in most patients by homozygous deletion of the SMN1 gene. For a carrier couple at a 25% risk of affected offspring, preimplantation genetic diagnosis (PGD) offers an alternative to prenatal diagnosis and termination of affected pregnancies. Our objective was to develop an accurate and reliable single-cell multiplex nested PCR analysis for PGD of SMA.

METHODS

The method was developed on single blood leukocytes, obtained from healthy controls and an adult SMA type III patient with a known homozygous deletion of SMN1 exon 7 and 8. Multiplex nested PCR on single cells was used to co-amplify exons 7 and 8 of SMN. Additional multiplexing was performed with the ZFX/ZFY gene for sexing. Following successful establishment of the multiplex nested PCR protocol in single leukocytes, the technique was employed for PGD in 4 patients for a total of 7 cycles. In 2 patients, sexing was simultaneously performed using ZFX/ZFY.

RESULTS

220 single leukocytes from a normal individual and 220 from an SMA patient were analyzed. Exon 7 of SMN1 was amplified in 99% of normal single leukocytes and in none of the SMA-affected leukocytes. Exon 7 of SMN2 was amplified in 100% of both normal and SMA-affected leukocytes. Exon 8 of SMN1 was amplified in 98% of normal cells and in none of the SMA-affected leukocytes. Exon 8 of SMN2 was amplified in 96% of both normal and SMA-affected leukocytes. Amplification efficiency was 99% for ZFX/ZFY. There were no false-negative results and no contamination was detected in all wash-drop blanks tested. Seven PGD cycles were performed in 4 SMA-carrier couples with successful molecular analysis of 34 embryos and a total of 15 normal embryos transferred in 7 cycles. One clinical pregnancy has resulted in the delivery of a healthy male. Amniocentesis performed at 17 weeks confirmed the correct diagnosis for both SMA and sexing.

CONCLUSIONS

These results suggest that our multiplex nested PCR protocol offers an efficient and accurate method for PGD of SMA while enabling the simultaneous analysis of an additional loci.

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.

Survival Motor Neuron (SMN) Genes for Molecular Allele-Specific Amplification of Exon 7 in the Diagnosis of Spinal Muscular Atrophy

There are two highly homologous survival motor neuron (SMN) genes in humans but molecular defects in the SMN1 gene cause spinal muscular atrophy (SMA). More than 90% of SMA patients are shown to have a homozygous deletion of exon 7 in the SMN1 gene. Therefore, a simple test for exon 7 deletion would be very useful in the molecular diagnosis of SMA. However, limited methods are available, and most of these methods utilize expensive instruments and consumables. Here, we describe a simple allele-specific PCR test, which can be performed using standard equipment in DNA laboratories. The principle of the test is based on a single nucleotide difference (C versus T) between the exon 7 of SMN1 and SMN2 genes. Using allele-specific primers, two PCR amplifications are performed for each sample to amplify a 404-bp diagnostic fragment, and consequent electrophoresis of PCR products on agarose gel provides definitive information concerning the exon 7 deletion To rule out false negatives, a 500-bp fragment from the N-acetyltransferase gene was coamplified as an internal control in each test. We have, so far, analyzed 41 SMA samples with our method, and tested the validity of results using an independent restriction fragment length polymorphism (RFLP) method. Genotyping results obtained by both methods were in complete agreement for all of the samples analyzed. Our method can also be used to detect heterozygous deletion of exon 7 in SMN genes, if the relative intensities of the diagnostic and internal control bands are determined.

Comparison between fluorescence in situ hybridization (FISH) and quantitative-fluorescent polymerase chain reaction (QF-PCR) for the detection of aneuploidies in single blastomeres

OBJECTIVES

The aim of our investigation was to compare the efficiencies of the fluorescence in situ hybridization (FISH) and the quantitative-fluorescent PCR (QF-PCR) methods for the detection of sexing and numerical chromosome disorders in single blastomeres collected from the same preimplantation human embryos.

METHODS

FISH analysis was carried out on 145 blastomeres from the 79 surplus embryos with probes specific for chromosomes 13, 18, 21, X, and Y. QF-PCR was performed with each one or two of the primers specific for the same chromosomes on 151 blastomeres from the same embryos obtained from patients undergoing IVF treatment.

RESULTS

Analyses were possible on 135 blastomeres (93%) by FISH and on 117 blastomeres (77%) by QF-PCR. Of 65 embryos, which could be analyzed by both methods, 20 embryos (31%) were diagnosed as abnormal.

CONCLUSION

The present study shows that FISH tests are more accurate than QF-PCR assays for the detection of numerical chromosome disorders when performed on single blastomeres.

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.

Multiplex PCR combining deltaF508 mutation and intragenic microsatellites of the CFTR gene for pre-implantation genetic diagnosis (PGD) of cystic fibrosis

One major limitation of pre-implantation genetic diagnosis (PGD) practice comes from the need to develop single cell PCR protocols. For a disease such as cystic fibrosis (CF), for which almost 1000 mutations have been identified, the development of a mutation based PGD protocol is impracticable. An elegant way to overcome this problem is to set up an indirect diagnosis using polymorphic markers allowing the identification of the pathogenic haplotype instead of the mutation. We present here a new PGD protocol for CF. Our strategy is based on a multiplex fluorescent PCR co-amplifying the DeltaF508 mutation and two CFTR intragenic polymorphic microsatellites (IVS8CA and IVS17bCA). Such an approach is justified since in 91% of the cases at least one partner of the couple carries the DeltaF508 mutation. The use of intragenic markers reduces the risk of misdiagnosis due to meiotic recombination. In 97% of the single lymphoblasts (151/155) tested a PCR signal was obtained. A complete haplotyping was achieved in 137/151 (91%) lymphoblasts and a 6% rate of allele drop out (ADO) was observed. Three cases were performed. Case one was at risk of transmitting mutations DeltaF508 and R1162X, case 2 DeltaF508 and R1066C and case 3 DeltaF508 and 1341+1A. Considering these three cases and the re-analysis of the affected embryos, we have analysed 62 blastomeres from which we had PCR signal for 58 (94%) and a complete haplotype for 49 (84%). With the degree of polymorphism of the markers used in this work (48 and 39%) and the fact that we co-amplified the F508 locus our test should be suitable for nearly 80% of the couples requesting PGD for CF. This fluorescent multiplex PCR indirect diagnosis provides also a safer test since it allows the confirmation of the diagnosis, the detection of contamination and could give an indication on the ploidy of the embryos tested.

Developments in laboratory techniques for prenatal diagnosis

Ongoing trends in prenatal diagnosis aim at early, rapid, and ideally noninvasive diagnosis as well as at the improvement of risk-screening for aneuploidy. Interphase-fluorescence in situ hybridization and quantitative fluorescence polymerase chain reaction are efficient tools for the rapid exclusion of selected aneuploidies in addition to the established direct preparation of chromosomes from chorionic villi. Interphase fluorescence in situ hybridization has also made possible the diagnosis of selected chromosome abnormalities in single cells (e.g. in preimplantation genetic diagnosis) or noninvasive diagnosis. More complex multicolor fluorescence in situ hybridization approaches are currently being evaluated. Single cell polymerase chain reaction is the key technique for the molecular diagnosis of a growing number of monogenic conditions before implantation or, still more experimental, in fetal cells retrieved from the maternal circulation. New sources for noninvasive diagnosis came into play such as fetal DNA or cell nuclei in maternal plasma. The combination of biochemical parameters in the maternal serum, namely free beta-human chorionic gonadotropin with pregnancy associated plasma protein A and sonographic markers, has already dramatically increased the sensitivity of risk screening in the first trimester of pregnancy.

Preimplantation genetic diagnosis for familial amyloidotic polyneuropathy (FAP)

Preimplantation genetic diagnosis (PGD) was developed more than a decade ago to offer an alternative to prenatal diagnosis for couples at risk of transmitting an inherited disease to their offspring. Portuguese-type familial amyloidotic polyneuropathy (FAP type I), is an autosomal dominant disease presenting an inherited mutation in the gene encoding the plasma protein transthyretin (TTR). We here report the first protocol for single-cell detection of the Met30 mutation in FAP type I and its application to PGD. A nested PCR reaction for exon 2 of the TTR gene was developed. The PCR product was then analysed by restriction enzyme analysis and SSCP allowing the detection of the point mutation. Ten clinical cycles were performed in seven couples. From the 93 metaphase II (MII) injected oocytes, 82 were normally fertilized and 78 were biopsied. A positive signal in the nested PCR reaction was obtained in 61 blastomeres, corresponding to a DNA amplification efficiency of 78.2%. No allele dropout (ADO) or contamination were detected. A biochemical pregnancy was obtained in three cases and a clinical pregnancy in one couple is actually in normal evolution.