PGT-M for Premature Ovarian Failure Related to CGG Repeat Expansion of the FMR1 Gene

Primary ovarian failure (POF) is caused by follicle exhaustion and is associated with menstrual irregularities and elevated gonadotropin levels, which lead to infertility before the age of 40 years. The etiology of POI is mostly unknown, but a heterogeneous genetic and familial background can be identified in a subset of cases. Abnormalities in the fragile X mental retardation 1 gene (FMR1) are among the most prevalent monogenic causes of POI. These abnormalities are caused by the expansion of an unstable CGG repeat in the 5' untranslated region of FMR1. Expansions over 200 repeats cause fragile X syndrome (FXS), whereas expansions between 55 and 200 CGG repeats, which are defined as a fragile X premutation, have been associated with premature ovarian failure type 1 (POF1) in heterozygous females. Preimplantation genetic testing for monogenic diseases (PGT-M) can be proposed when the female carries a premutation or a full mutation. In this narrative review, we aim to recapitulate the clinical and molecular features of POF1 and their implications in the context of PGT-M.

Are ovarian response and pregnancy rates similar in selected FMR1 premutated and mutated patients undergoing preimplantation genetic testing?

PURPOSE

To assess if the ovarian response of FMR1 premutated women undergoing preimplantation genetic testing (PGT) for Fragile X syndrome is lower compared with fully mutated patients, due to their frequent premature ovarian failure.

METHODS

In a retrospective cohort study from January 2009 to March 2019, we compared PGT outcomes in 18 FMR1 premutated women and 12 fully mutated women and aimed to identify predictive factors of stimulation outcomes.

RESULTS

Eighty-six IVF/PGT-M cycles for FMR1 PGT were analyzed. Premutation and full mutation patients were comparable in terms of age, body mass index (BMI), basal FSH, antral follicular count, and cycle length. However, premutation carriers had significantly lower AMH (1.9 versus 4.0 ng/mL, p = 0.0167). Premutated patients required higher doses of FSH (2740 versus 1944 IU, p = 0.0069) but had similar numbers of metaphase II oocytes (7.1 versus 6.6, p = 0.871) and embryos (5.6 versus 4.9, p = 0. 554). Pregnancy rates (37.1% versus 13.3%, p = 0.1076) were not statistically different in both groups.

CONCLUSION

In spite of lower ovarian reserve and thanks to an increased total dose of FSH, FMR1 premutated selected patients seem to have similar ovarian response as fully mutated patients. Neither the number of CGG repeats in FMR1 gene nor FMR1 mutation status was good predictors of the number of retrieved oocytes.

Live births following preimplantation genetic testing for dynamic mutation diseases by karyomapping: a report of three cases

PURPOSE

The preimplantation genetic testing for monogenic defects (PGT-M) is a beneficial strategy for the patients suffering from a Mendelian disease, which could protect their offspring from inheriting the disease. The purpose of this study is to report the effectiveness of PGT-M based on karyomapping for three cases of dynamic mutation diseases with trinucleotide repeat expansion.

METHODS

PGT-M was carried out on three couples, whose family members were diagnosed with Huntington's disease or spinocerebellar ataxias 2 or 12. The whole genome amplification was obtained using the multiple displacement amplification (MDA) method. Then, karyomapping was performed to detect the allele that is carrying the trinucleotide repeat expansion using single nucleotide polymorphism (SNP) linkage analyses, and the copy number variations (CNVs) of the embryos were also identified. Prenatal diagnosis was performed to validate the accuracy of PGT-M.

RESULTS

PGT-M was successfully performed on the three couples, and they accepted the transfers of euploid blastocysts without the relevant pathogenic allele. The clinical pregnancies were acquired and the prenatal diagnosis of the three families confirmed the effectiveness of karyomapping. The three born babies were healthy and free of the pathogenic alleles HTT, ATXN2, or PPP2R2B corresponding to Huntington's disease, spinocerebellar ataxias 2 or 12, respectively.

CONCLUSION

This study shows that karyomapping is a highly powerful and efficient approach for dynamic mutation detection in preimplantation embryos. In this work, we first report the birth of healthy babies that are free of the pathogenic gene for dynamic mutation diseases in patients receiving PGT-M by karyomapping.

Detecting AGG Interruptions in Females With a FMR1 Premutation by Long-Read Single-Molecule Sequencing: A 1 Year Clinical Experience

The fragile X syndrome arises from the FMR1 CGG expansion of a premutation (55–200 repeats) to a full mutation allele (>200 repeats) and is the most frequent cause of inherited X-linked intellectual disability. The risk for a premutation to expand to a full mutation allele depends on the repeat length and AGG triplets interrupting this repeat. In genetic counseling it is important to have information on both these parameters to provide an accurate risk estimate to women carrying a premutation allele and weighing up having children. For example, in case of a small risk a woman might opt for a natural pregnancy followed up by prenatal diagnosis while she might choose for preimplantation genetic diagnosis (PGD) if the risk is high. Unfortunately, the detection of AGG interruptions was previously hampered by technical difficulties complicating their use in diagnostics. Therefore we recently developed, validated and implemented a new methodology which uses long-read single-molecule sequencing to identify AGG interruptions in females with a FMR1 premutation. Here we report on the assets of AGG interruption detection by sequencing and the impact of implementing the assay on genetic counseling.

Development of ICSI

The first conception outside of the human body that led to the birth of Louise Brown was a tremendous accomplishment, which opened the door to the utilization of assisted reproductive techniques globally. This brought the understanding that accomplishing life in a dish required several steps, the most obvious being the timing and characteristics of fertilization. It soon became obvious in the 1980s that the most disappointing phenomenon was unexpected and complete fertilization failure. Among the approaches that were attempted to treat male factor infertility, ICSI surfaced as the technique that brought the ratio of the gametes to 1:1 and was also able to grant consistent fertilization and a higher pregnancy rate. ICSI has now been implemented for a quarter of a century, proving itself as the ultimate technique utilizing ejaculated spermatozoa independent of the semen parameters and is the sole insemination method to be used with surgically retrieved spermatozoa. There are currently various indications for ICSI that are widely adopted, rendering it the most popular insemination method worldwide. The reliability of ICSI ensures its employment in upcoming techniques involving in vitro spermatogenesis and neogametogenesis.

FMR1 CGG repeat expansion mutation detection and linked haplotype analysis for reliable and accurate preimplantation genetic diagnosis of fragile X syndrome

Fragile X mental retardation 1 (FMR1) full-mutation expansion causes fragile X syndrome. Trans-generational fragile X syndrome transmission can be avoided by preimplantation genetic diagnosis (PGD). We describe a robust PGD strategy that can be applied to virtually any couple at risk of transmitting fragile X syndrome. This novel strategy utilises whole-genome amplification, followed by triplet-primed polymerase chain reaction (TP-PCR) for robust detection of expanded FMR1 alleles, in parallel with linked multi-marker haplotype analysis of 13 highly polymorphic microsatellite markers located within 1 Mb of the FMR1 CGG repeat, and the AMELX/Y dimorphism for gender identification. The assay was optimised and validated on single lymphoblasts isolated from fragile X reference cell lines, and applied to a simulated PGD case and a clinical in vitro fertilisation (IVF)-PGD case. In the simulated PGD case, definitive diagnosis of the expected results was achieved for all ‘embryos’. In the clinical IVF-PGD case, delivery of a healthy baby girl was achieved after transfer of an expansion-negative blastocyst. FMR1 TP-PCR reliably detects presence of expansion mutations and obviates reliance on informative normal alleles for determining expansion status in female embryos. Together with multi-marker haplotyping and gender determination, misdiagnosis and diagnostic ambiguity due to allele dropout is minimised, and couple-specific assay customisation can be avoided.

Improving preimplantation genetic diagnosis for Fragile X syndrome: two new powerful single-round multiplex indirect and direct tests

Fragile X syndrome (FraX) is caused by the expansion of an unstable CGG repeat located in the Fragile X mental retardation 1 gene (FMR1) gene. Preimplantation genetic diagnosis (PGD) can be proposed to couples at risk of transmitting the disease, that is, when the female carries a premutation or a full mutation. We describe two new single-cell, single-round multiplex PCR for indirect and direct diagnosis of FraX on biopsied embryos. These tests include five unpublished, highly heterozygous simple sequence repeats, and the co-amplification of non-expanded CGG repeats for the direct test. Heterozygosity of the new markers ranged from 69 to 81%. The mean rate of non-informative marker included in the tests was low (26% and 23% for the new indirect and direct tests, respectively). This strategy allows offering a PGD for FraX to 96% of couples requesting it in our centre. A conclusive genotype was obtained in all cells with a rate of cells presenting an allele dropout ranging from 17% for the indirect test to 26% for the direct test. The new indirect test was applied for eight PGD cycles: 32 embryos were analysed, 9 were transferred and 3 healthy babies were born. By multiplexing these highly informative markers, robustness of the diagnosis is improved and the loss of potentially healthy embryos (because they are non-diagnosed or misdiagnosed) is limited. This may increase the chances of success of couples requesting a PGD for FraX, in particular, when premature ovarian insufficiency in premutated women leads to a reduced number of embryos available for analysis.

Multiple displacement amplification for preimplantation genetic diagnosis of fragile X syndrome

Preimplantation genetic diagnosis (PGD) has become an assisted reproductive technique for couples that have genetic risks. Despite the many advantages provided by PGD, there are several problems, including amplification failure, allele drop-out and amplification inefficiency. We evaluated multiple displacement amplification (MDA) for PGD of the fragile X syndrome. Whole genome amplification was performed using MDA. MDA products were subjected to fluorescent PCR of fragile X mental retardation-1 (FMR1) CGG repeats, amelogenin and two polymorphic markers. In the pre-clinical tests, the amplification rates of the FMR1 CGG repeat, DXS1215 and FRAXAC1 were 84.2, 87.5 and 75.0%, respectively, while the allele dropout rates were 31.3, 57.1 and 50.0%, respectively. In two PGD treatment cycles, 20 embryos among 30 embryos were successfully diagnosed as 10 normal embryos, four mutated embryos and six heterozygous carriers. Three healthy embryos were transferred to the uterus; however, no clinical pregnancy was achieved. Our data indicate that MDA and fluorescent PCR with four loci can be successfully applied to PGD for fragile X syndrome. Advanced methods for amplification of minuscule amounts of DNA could improve the sensitivity and reliability of PGD for complicated single gene disorders.

Mechanisms of trinucleotide repeat instability during human development

Trinucleotide expansion underlies several human diseases. Expansion occurs during multiple stages of human development in different cell types, and is sensitive to the gender of the parent who transmits the repeats. Repair and replication models for expansions have been described, but we do not know whether the pathway involved is the same under all conditions and for all repeat tract lengths, which differ among diseases. Currently, researchers rely on bacteria, yeast and mice to study expansion, but these models differ substantially from humans. We need now to connect the dots among human genetics, pathway biochemistry and the appropriate model systems to understand the mechanism of expansion as it occurs in human disease.

PGD for fragile X syndrome: ovarian function is the main determinant of success

BACKGROUND

PGD for fragile X syndrome (FRAX) is inefficient, probably owing to fewer oocytes, poor embryo quality and difficulties in genetic analysis. We investigated IVF-PGD in FRAX mutation carriers compared with controls, looking at the effects of oocyte and embryo number/quality on live birth outcome.

METHODS

We performed IVF-PGD in 27 patients with the FRAX mutation and 33 controls with other genetic diseases. Genetic testing was by multiplex PCR.

RESULTS

Seventy-nine and 108 IVF-PGD cycles were started in FRAX mutation carriers and controls, respectively. Twenty-two patients had a premutation (CGG repeat number 60-200) and five had a full mutation (300-2000 CGG repeats). FRAX patients required higher doses of gonadotrophins (6788 ± 2379 versus 4360 ± 2330, P< 0.001) but had lower peak serum estradiol levels (8166 ± 5880 versus 10 211 ± 4673, P = 0.03) and fewer oocytes retrieved (9.8 ± 6 versus 14 ± 8, P = 0.01). The cancellation rate (unsatisfactory ovarian response) was higher in the FRAX group than in the control group (13 versus 1%, P < 0.001). When embryos were transferred, ongoing pregnancy/live birth rates per transfer were similar (29 versus 36%, P = 0.54).

CONCLUSIONS

Ovarian dysfunction in FRAX carriers is more prevalent and profound than previously appreciated, with a high cancelation rate and reduced efficiency of PGD. The main determinant for successful PGD for FRAX is ovarian dysfunction. When embryo transfer is possible, the results are comparable to PGD for other monogenic diseases.

Preimplantation genetic diagnosis for fragile X syndrome: is there increased transmission of abnormal FMR1 alleles among female heterozygotes?

BACKGROUND

Fragile X syndrome is caused by a CGG triplet-repeat expansion mutation in the FMR1 gene. Previous studies have shown increased transmission of abnormal alleles in the 51-60 repeat range. This study was undertaken to evaluate the performance of preimplantation genetic diagnosis (PGD) for fragile X, and to assess the transmission rate of the abnormal FMR1 alleles in this setting.

METHOD

The study included 18 fragile X carriers who applied for PGD. FMR1 CGG repeats ranged from 70 to 300. PGD was performed using multiplex-nested PCR, with simultaneous amplification of the CGG repeat region and several polymorphic markers, and sex chromosome markers.

RESULTS

Four patients had a poor ovarian response, and could not undergo PGD. The remaining 14 patients underwent 47 PGD cycles. A total of 565 oocytes were aspirated. Of the 386 embryos that were successfully biopsied, 18 (6.4%) could not be analyzed due to amplification failure, and 12 (4.3%) had sex chromosomal abnormalities. Of the remaining 250 embryos, the abnormal allele was transmitted to 124 embryos (49.6%) compared to 126 (50.4%) for the normal allele. This difference was not statistically significant. Only embryos carrying the normal allele were transferred, resulting in 7 clinical pregnancies (18% per embryo transfer).

CONCLUSIONS

Our results demonstrate that PGD for fragile X is feasible, and that carriers transmit the abnormal allele at the same frequency as the normal allele.

Very early premature ovarian failure in two sisters compound heterozygous for the FMR1 premutation

Expansion of the CGG trinucleotide repeat in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene within the premutation range is one of the known genetic factors associated with premature ovarian failure and earlier age at menopause. Studies have shown that approximately 16-26% of female carriers will develop premature ovarian failure, and current research is focussed on the identification of molecular factors that predict its occurrence in female carriers. In this report we present two sisters who are compound heterozygous for a premutation, and who were referred because of very early menopause, occurring at the age of 17 years in the youngest sister. Premature ovarian failure associated with FMR1 premutation at such an early age has not been reported in the literature before.

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.

Birth after pre-implantation genetic diagnosis (PGD) of spinocerebellar ataxia 2 (Sca2)

BACKGROUND

Spinocerebellar ataxia 2 (SCA2) is an autosomal-dominant neurodegenerative disease caused by an extended polyglutamine sequence in the ATXN2 protein. We describe the development of a new single-cell multiplex PCR protocol for pre-implantation genetic diagnosis (PGD) of SCA2 and its successful clinical application.

METHODS

Three duplex tests have been developed, one, which combines the detection of the CAG repeats in addition to the D12S821 microsatellite, another, the amplification of the CAG repeats and the D12S1333 microsatellite and the last, the combination of both microsatellites D12S821 and D12S1333.

RESULTS

PCR conditions were established using 226 single lymphoblasts or patient lymphocysts. Amplification was obtained in an average of 99.6%, a complete genotype in 86%, a conclusive result in 96% and an allelic drop-out (ADO) rate of 10.7% was observed. PGD for SCA2 was performed for a couple with a paternal risk of transmitting the pathology. Two cycles were done from which 18 embryos were biopsied, 8 were diagnosed as unaffected, 9 as affected and 1 gave no results. In both cycles 2 embryos were transferred, with no pregnancy at the first attempt, and a twin pregnancy at the second attempt. The patient delivered one girl and one boy at 36 weeks and 3 days.

Preimplantation genetic diagnosis for Duchenne muscular dystrophy by multiple displacement amplification

OBJECTIVE

To evaluate the use of multiple displacement amplification (MDA) in preimplantation genetic diagnosis (PGD) for female carriers with Duchenne muscular dystrophy (DMD).

DESIGN

MDA was used to amplify a whole genome of single cells. Following the setup on single cells, the test was applied in two clinical cases of PGD. One mutant exon, six short tandem repeats (STR) markers within the dystrophin gene, and amelogenin were incorporated into singleplex polymerase chain reaction (PCR) assays on MDA products of single blastomeres.

SETTING

Center for reproductive medicine in First Affiliated Hospital, Sun Yat-sen University, China.

PATIENT(S)

Two female carriers with a duplication of exons 3-11 and a deletion of exons 47-50, respectively.

INTERVENTION(S)

The MDA of single cells and fluorescent PCR assays for PGD.

MAIN OUTCOME MEASURE(S)

The ability to analyze single blastomeres for DMD using MDA.

RESULT(S)

The protocol setup previously allowed for the accurate diagnosis of each embryo. Two clinical cases resulted in a healthy girl, which was the first successful clinical application of MDA in PGD for DMD.

CONCLUSION(S)

We suggest that this protocol is reliable to increase the accuracy of the PGD for DMD.

Preimplantation genetic diagnosis for fragile X syndrome using multiplex nested PCR

Fragile X syndrome is caused by a dynamic mutation in the FMR1 gene. Normal individuals have <55 CGG repeats in the 5 untranslated region, premutation carriers have 55-200 repeats and a full mutation has >200 repeats. Female carriers are at risk of having affected offspring. A multiplex nested polymerase chain reaction protocol is described for preimplantation genetic diagnosis (PGD) of fragile X syndrome with simultaneous amplification of the CGG-repeat region, the Sry gene and several flanking polymorphic markers. The amplification efficiency was > or =96% for all loci. The allele dropout rate in heterozygotic females was 9% for the FMR1 CGG-repeat region and 5-10% for the polymorphic markers. Amplification failure for Sry occurred in 5% of single leukocytes isolated from males. PGD was performed in six patients who underwent 15 cycles. Results were confirmed in all cases by amniocentesis or chorionic villous sampling. Five clinical pregnancies were obtained (31% per cycle), four of which resulted in a normal delivery and one miscarried. This technique is associated with high efficiency and accuracy and may be used in carriers of full mutations and unstable high-order premutations.

Multiple displacement amplification improves PGD for fragile X syndrome

We report an improvement in the PGD test for fragile X syndrome (FXS). Recently, multiple displacement amplification (MDA) has been reported to yield large amounts of DNA from single cells. Taking into account this technique, we developed a new PGD test for FXS, enabling combined analysis of linked polymorphic markers with the study of the non-expanded CGG repeat. Single cell amplification efficiency was first assessed on single lymphocytes. Amplification rate of the different markers ranged from 85 to 95% with an allele drop-out (ADO) rate comprised between 7 and 34%. Using this test, eight PGD cycles were carried out for six couples, and 37 embryos were analysed after preliminary MDA. Amplification rate was increased by this technique from 41 to 66% so that embryos with no results were rarer (14 versus 45% without MDA). Reliability of the test was considerably improved by combining direct with indirect genetic analysis. Furthermore, in cases of fully expanded alleles too large to be amplified by PCR, this test gives an internal amplification control. Embryonic transfers were carried out in all but one PGD cycles. One biochemical and one clinical pregnancy resulted, and a healthy child was born. This single diagnosis procedure could be suitable to most patients carrying FXS.

Real-time PCR analysis of trinucleotide repeat allele expansions in the androgen receptor gene

INTRODUCTION

The expansion of specific trinucleotide repeats results in certain genetic disorders.

METHOD

Real-time PCR analysis was used to rapidly discriminate between normal and expanded (CAG)(n) alleles in the androgen receptor gene.

RESULT

The difference in melting temperature (T(m)) between the most common normal and expanded alleles was approximately 1 degrees C.

CONCLUSION

Real-time PCR analysis seems to be a highly reliable and rapid method, which may facilitate the first molecular approach to human trinucleotide repeat disorders.

Genetic counseling for fragile x syndrome: updated recommendations of the national society of genetic counselors

These recommendations describe the minimum standard criteria for genetic counseling and testing of individuals and families with fragile X syndrome, as well as carriers and potential carriers of a fragile X mutation. The original guidelines (published in 2000) have been revised, replacing a stratified pre- and full mutation model of fragile X syndrome with one based on a continuum of gene effects across the full spectrum of FMR1 CGG trinucleotide repeat expansion. This document reviews the molecular genetics of fragile X syndrome, clinical phenotype (including the spectrum of premature ovarian failure and fragile X-associated tremor-ataxia syndrome), indications for genetic testing and interpretation of results, risks of transmission, family planning options, psychosocial issues, and references for professional and patient resources. These recommendations are the opinions of a multicenter working group of genetic counselors with expertise in fragile X syndrome genetic counseling, and they are based on clinical experience, review of pertinent English language articles, and reports of expert committees. These recommendations should not be construed as dictating an exclusive course of management, nor does use of such recommendations guarantee a particular outcome. The professional judgment of a health care provider, familiar with the facts and circumstances of a specific case, will always supersede these recommendations.

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.

Issues and concerns of couples presenting for preimplantation genetic diagnosis (PGD)

BACKGROUND

The use of preimplantation genetic diagnosis (PGD) to select genetically 'normal' human embryos and to transfer them to the uterus of a woman has generated considerable controversy. Debate has occurred over the implications of PGD, sex selection, safety of embryonic manipulation and eugenics. This study evaluates a range of social and moral concerns of couples towards PGD and assisted reproductive technologies (ART) prior to treatment to obtain unbiased authentic attitudes independent of the treatment cycle and the outcome.

METHODS

A total of 121 subjects were administered a structured questionnaire after each couple's in vitro fertilization (IVF) or genetic counselling session. Group A consisted of 41 subjects presenting for PGD of single gene disorders (PGD-SG) and group B consisted of 48 subjects undertaking PGD for aneuploidy screening (PGD-AS). A control group consisted of 32 subjects that were about to commence their first IVF cycle.

RESULTS AND DISCUSSION

All groups found PGD to be a highly acceptable treatment. They expressed little concern about its extension to testing non-disease states such as sex and they were strongly in favour of a shared decision-making model in which couples have considerable autonomy over decisions about the embryo(s) to transfer. Differences between the groups included issues surrounding the transfer of embryos, restrictions to PGD and the destruction of embryos.

Preimplantation genetic diagnosis for fragile Xa syndrome: difficult but not impossible

BACKGROUND

In this paper, we review our clinical preimplantation genetic diagnosis (PGD) programme for fragile Xa syndrome, analysing if PGD for these couples is still a valuable option, as it is particularly difficult for two reasons. First, the couples have to be informative (the number of triplet repeats on the healthy FMR-1 allele of the mother has to be different from the number of repeats on the healthy FMR-1 allele of the father) and second, women with a premutation are at increased risk of premature ovarian failure.

METHODS

A total of 34 couples attended our genetics department between December 1998 and July 2001, requesting information about PGD for fragile Xa syndrome.

RESULTS

Eight couples decided not to go further with the procedure and of the 26 remaining couples, 16 were informative (61.5%). Four couples have so far not started ovarian stimulation, one patient was totally refractive to stimulation and 11 couples have had a total of 19 oocyte retrievals. From these, there have been 13 embryo transfers with a clinical pregnancy rate per embryo transfer of 23%; the implantation rate was 13.6% and the live birth rate per couple was 27.3%.

CONCLUSIONS

PGD for fragile Xa is feasible for a number of couples. A pre-PGD work-up should include a determination of the premutation or mutation carrier status, the maternal or paternal origin of the premutation and an estimation of the ovarian reserve of the patient. Fragile Xa premutation carriers should be advised not to postpone reproduction for too long.

Preimplantation genetic diagnosis for Huntington's disease with exclusion testing

Huntington's disease is an autosomal dominant, late-onset disorder, for which the gene and the causative mutation have been known since 1993. Some at-risk patients choose for presymptomatic testing and can make reproductive choices accordingly. Others however, prefer not to know their carrier status, but may still wish to prevent the birth of a carrier child. For these patients, exclusion testing after prenatal sampling has been an option for many years. A disadvantage of this test is that unaffected pregnancies may be terminated if the parent at risk (50%) has not inherited the grandparental Huntington gene, leading to serious moral and ethical objections. As an alternative, preimplantation genetic diagnosis (PGD) on embryos obtained in vitro may be proposed, after which only embryos free of risk are replaced. Embryos can then be selected, either by the amplification of the CAG repeat in the embryos without communicating results to the patients (ie non-disclosure testing), which brings its own practical and moral problems, or exclusion testing. We describe here the first PGD cycles for exclusion testing for Huntington's disease in five couples. Three couples have had at least one PGD cycle so far. One pregnancy ensued and a healthy female baby was delivered.

Preimplantation genetic diagnosis for single gene disorders: experience with five single gene disorders

We report our experience of 14 preimplantation genetic diagnosis (PGD) cycles in eight couples carrying five different single gene disorders, during the last 18 months. Diagnoses were performed for myotonic dystrophy (DM), cystic fibrosis (CF) [Delta F508 and exon 4 (621+1 G>T)], fragile X and CF simultaneously, and two disorders for which PGD had not been previously attempted, namely neurofibromatosis type 2 (NF2) and Crouzon syndrome. Diagnoses for single gene disorders were carried out on ideally two blastomeres biopsied from Day 3 embryos. A highly polymorphic marker was included in each diagnosis to control against contamination. For the dominant disorders, where possible, linked polymorphisms provided an additional means of determining the genotype of the embryo hence reducing the risk of misdiagnosis due to allele dropout (ADO). Multiplex fluorescent polymerase chain reaction (F-PCR) was used in all cases, followed by fragment analysis and/or single-stranded conformation polymorphism (SSCP) for genotyping. Embryo transfer was performed in 13 cycles resulting in one biochemical pregnancy for CF, three normal deliveries (a twin and a singleton) and one early miscarriage for DM and a singleton for Crouzon syndrome. In each case the untransferred embryos were used to confirm the diagnoses performed on the biopsied cells. The results were concordant in all cases. The inclusion of a polymorphic marker allowed the detection of extraneous DNA contamination in two cells from one case. Knowing the genotype of the contaminating DNA allowed its origin to be traced. All five pregnancies were obtained from embryos in which two blastomeres were biopsied for the diagnosis. Our data demonstrate the successful strategy of using multiplex PCR to simultaneously amplify the mutation site and a polymorphic locus, fluorescent PCR technology to achieve greater sensitivity, and two-cell biopsy to increase the efficiency and success of diagnoses.

Guidelines for the appropriate use of genetic tests in infertile couples

Research on genetic causes of male and female infertility rapidly expanded in the last years, following the development of in vitro fertilising techniques. Genetic tests are now available to explore the cause of the infertility and assess the risk of a given couple to transmit its genetic characteristics. This allows at-risk couples to take an informed decision when electing for a medically assisted reproduction. It also allows the professionals to offer a prenatal diagnosis when appropriate. Thus, the genetic work-up of the infertile couple has become good practice for an appropriate diagnosis, treatment and prognostic assessment. The lack of national or international rules for the genetic approach to the infertile couple, prompted the Italian community of professionals in the field of reproductive medicine to join and set up guidelines for the genetic diagnosis of male and female infertility. The group of clinical and research experts is representative of 12 national scientific societies and was supported by external experts from four international societies. We examine the clinically relevant genetic causes of male and female infertility and suggest the category of patients for which each genetic test is recommended or optional, both for an accurate diagnosis and prior to ART.

Preimplantation genetic diagnosis of the fragile X syndrome by use of linked polymorphic markers

Fragile X syndrome is the most common cause of familial mental retardation. The most common mutation is expansion of a triplet (CGG)(n) repeat in the 5' untranslated region of the FMR1 gene on Xq27.3. The expansion is refractory to PCR due to preferential amplification of the smaller allele in heterozygous cells and the high GC content of the repeat and surrounding sequences. Direct detection of the normal parental alleles in preimplantation embryos has been used for preimplantation genetic diagnosis (PGD) of this disorder. However, this approach is only suitable for approximately 63% of couples due to the heterozygosity of the repeat in the normal population. As an alternative we investigated the use of polymorphic markers flanking the mutation to track the normal and premutation carrying maternal chromosomes in preimplantation embryos. Using a panel of 11 polymorphisms, six (CA)(n) repeats and five single nucleotide polymorphisms, diagnosis was developed for 90% of referred couples. Multiplex amplification of informative markers was tested in 300 single buccal cells from interested couples with efficiency and allele drop out (ADO) rates ranging from 69% to 96% and 6% to 18%, respectively. Use of this approach is accurate and applicable to a larger number of patients at risk of transmitting fragile X to their offspring.

Preimplantation genetic diagnosis for sickle-cell anemia and for beta-thalassemia

We developed single-cell polymerase chain reaction (PCR) assays for preimplantation genetic diagnosis (PGD) in couples carrying mutations in the beta-globin gene. With PGD the genetic status of an embryo obtained after intracytoplasmic sperm injection (ICSI) is determined by PCR analysis in single blastomeres, allowing only healthy embryos to be transferred to the uterus. We carried out nine PGD cycles using fluorescent PCR for two couples in whom the partners carried sickle-cell trait. Both couples achieved pregnancies, one of which was spontaneously aborted. We have developed two beta-thalassemia PGD protocols: one for the analysis of the 25-26delAA and the IVS2+1G>A mutation, and the other for the simultaneous detection of the IVS1+6T>C and the IVS1+110G>A mutations. For the second protocol, both non-labelled PCR and later fluorescent PCR were used. Both protocols were applied in clinical cycles (two non-labelled PCR cycles and one fluorescent PCR cycle) for two couples. The patient with the fluorescent PCR-PGD cycle became pregnant. Overall, the three fluorescent PCR assays were accurate and reliable with amplification efficiencies of minimum 93% and allele dropout (ADO) rates between 0 and 12%.

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.

First specific preimplantation genetic diagnosis for ornithine transcarbamylase deficiency

Ornithine transcarbamylase (OTC) deficiency is an X-linked dominant metabolic disorder with partial penetrance in heterozygous females. Affected boys usually die from hyperammonemia in the first few days of life, while clinical expression in carrier females ranges from no symptoms to neonatal death. A young couple whose boy had died of OTC deficiency in the neonatal period was referred to our genetic department for their subsequent pregnancy. The fetus was found to be affected, and after genetic counseling the pregnancy was terminated. Prenatal diagnosis of the third pregnancy identified a heterozygous female, who died after a normal birth at age 11 days from hyperammonemia. After this, the couple asked for preimplantation genetic diagnosis (PGD). We have developed a duplex nested PCR assay allowing the amplification of both the mutation and an informative restriction fragment length polymorphism (RFLP) located in the 3' end of the OTC gene. After nested amplification, allele identification was carried out for both loci by double restriction digestion and electrophoresis gel analysis. The co-amplification of both loci provided a means of detecting potential allele dropout or incomplete digestion. Two PGD cycles were carried out, a total of 14 embryos were analysed and a diagnosis could be obtained in 13/14 embryos. There were four unaffected male embryos, four heterozygous females and four unaffected females; the final embryo was an affected one of undetermined gender. In both cycles, three unaffected embryos could be transferred early on Day 4 post-insemination. The second cycle resulted in the birth of a baby boy devoid of the OTC mutation. This constitutes the first birth following PGD carried out by a French team.

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.

Recent advances and future developments in PGD

PGD has now been practised for a decade. The basic techniques currently used involve embryo biopsy, polymerase chain reaction (PCR) and fluorescent in situ hybridization (FISH). Recent advances in molecular diagnostic techniques have included the use of fluorescent PCR, multiplex PCR and whole genome amplification. For cytogenetic analysis, many centres are now using five or more chromosome probes to examine for chromosome abnormalities, sexing and aneuploidy. Future improvements in molecular diagnosis include the use of quantitative PCR, DNA fingerprinting and microarray technology. Developments in methods to analyse chromosomes from a single cell have included interphase chromosome conversion, which has already been clinically applied, and the use of comparative genomic hybridization, which is still being developed. These methods will hopefully enable more accurate and a greater number of diseases to be diagnosed at the single cell level.