On the origin of trisomy 21 Down syndrome

Background: Down syndrome, characterized by an extra chromosome 21 is the most common genetic cause for congenital malformations and learning disability. It is well known that the extra chromosome 21 most often originates from the mother, the incidence increases with maternal age, there may be aberrant maternal chromosome 21 recombination and there is a higher recurrence in young women. In spite of intensive efforts to understand the underlying reason(s) for these characteristics, the origin still remains unknown. We hypothesize that maternal trisomy 21 ovarian mosaicism might provide the major causative factor. Results: We used fluorescence in situ hybridization (FISH) with two chromosome 21-specific probes to determine the copy number of chromosome 21 in ovarian cells from eight female foetuses at gestational age 14–22 weeks. All eight phenotypically normal female foetuses were found to be mosaics, containing ovarian cells with an extra chromosome 21. Trisomy 21 occurred with about the same frequency in cells that had entered meiosis as in pre-meiotic and ovarian mesenchymal stroma cells. Conclusion: We suggest that most normal female foetuses are trisomy 21 ovarian mosaics and the maternal age effect is caused by differential selection of these cells during foetal and postnatal development until ovulation. The exceptional occurrence of high-grade ovarian mosaicism may explain why some women have a child with Down syndrome already at young age as well as the associated increased incidence at subsequent conceptions. We also propose that our findings may explain the aberrant maternal recombination patterns previously found by family linkage analysis

Disomy 21 in spermatozoa and the paternal origin of trisomy 21 Down syndrome

BACKGROUND: Trisomy 21 Down syndrome is the most common genetic cause for congenital malformations and intellectual disability. It is well known that in the outstanding majority of cases the extra chromosome 21 originates from the mother but only in less than 10 % from the father. The mechanism underlying this striking difference in parental origin of Trisomy 21 Down syndrome is still unknown. However, it seems likely that the main reason is a much higher stringency in the elimination of any trisomy 21 cells during fetal testicular than ovarian development. We have here focussed attention on the paternal gametic output, i.e. the incidence of disomy 21 in spermatozoa. RESULTS: We have used fluorescence in situ hybridisation (FISH) to determine the copy number of chromosome 21 in spermatozoa from 11 men with normal spermiograms. Due to the well-known risk of false positive and false negative signals using a single FISH probe, we have applied two chromosome 21q probes, and we have added a chromosome 18-specific probe to allow differentiation between disomy 21 and diploidy. Analysing a total number of 2000 spermatozoa per case, we documented an average incidence of disomy 21 at 0.13 %, with a range of 0.00-0.25 % and a SD of 0.08. There was no indication of diploidy in this cohort of 22,000 sperm. CONCLUSION: Numerous previous studies on the incidence of disomy 21 in sperm have been published, using FISH. As far as we are aware, none of these have applied more than a single chromosome 21-specific probe. Accepting our mean of 0.13 % of disomy 21, and providing there is no selective fertilisation capability of disomy 21 sperm in relation to the normal, we conclude that around 1 in 800 conceptions is expected to be trisomic for chromosome 21 of paternal origin. Bearing in mind that the maternal origin likely is at least 10 times more common, we tentatively propose that around 1 in 80 oocytes in the maternal ovarian reserve may be disomy 21. One reason for this discrepancy may be a more stringent selection against aberrant chromosome numbers during spermatogenesis than oogenesis. Further work is required to determine the relevant stages of spermatogenesis at which such a selection may take place

Preimplantation genetic testing legislation and accessibility in the Nordic countries

Abstract Introduction Assisted Reproduction Technologies are in rapid development and implementation of preimplantation genetic testing (PGT) has allowed patients with genetic disorders to initiate pregnancies while minimizing or eliminating the risk of transmitting these disorders to their offspring. Testing for numeric chromosomal anomalies has been proposed as a way to increase efficacy in assisted reproduction, however this remains disputed. Legislation is lagging behind the rapid developments in this field. Material and methods We conducted a structured online survey of legislation and accessibility to Preimplantation Genetic Testing in the Nordic countries to compare the regulation and uptake of this technique. The survey was designed and answered by the authors. Results Key elements in regulation of preimplantation testing for monogenic disorders and structural rearrangements are similar in the Nordic countries although accessibility varies since only Denmark, Finland and Sweden have national clinics offering treatment. In addition, Denmark and Finland have private clinics offering PGT. Regulation is most stringent in Norway where a national board evaluates all couples seeking treatment. Treatment volumes vary between the Nordic countries with Norway and Finland having lowest treatment numbers. Preimplantation genetic testing for aneuploidy in the embryo varies between the Nordic countries where Finland and Iceland are allowed to offer this form of treatment, Denmark and Sweden only in the form of a research protocol while this form of testing is not allowed at all in Norway. Therefore the number of treatment cycles involving testing for embryo aneuploidy are lower in the Nordic countries compared to other countries where this treatment option is more common. Conclusions Science needs to inform politics regarding the rapidly evolving field of reproductive medicine and we recommend harmonization of legislation and accessibility between the Nordic countries.Peer reviewe

Fetal HLA-G mediated immune tolerance and interferon response in preeclampsia

Background: Fetal immune tolerance is crucial for pregnancy success. We studied the link between preeclampsia, a severe pregnancy disorder with uncertain pathogenesis, and fetal human leukocyte antigen G (HLA-G) and other genes regulating maternal immune responses.Methods: We assessed sex ratios and regulatory HLA-G haplotypes in population cohorts and series of preeclampsia and stillbirth. We studied placental mRNA expression of 136 genes by sequencing and HLA-G and interferon alpha (IFN alpha) protein expression by immunohistochemistry.Findings: We found underrepresentation of males in preeclamptic births, especially those delivered preterm or small for gestational age. Balancing selection at HLA-G associated with the sex ratio, stillbirth, and preeclampsia. We observed downregulation of HLA-G, its receptors, and many other tolerogenic genes, and marked upregulation of IFNA1 in preeclamptic placentas.Interpretation: These findings indicate that an evolutionary trade-off between immune tolerance and protection against infections at the maternal-fetal interface promotes genetic diversity in fetal HLA-G, thereby affecting survival, preeclampsia, and sex ratio. We highlight IFNA1 as a potential mediator of preeclampsia and a target for therapeutic trials. (C) 2020 The Authors. Published by Elsevier B.V.</div

Associations between perceived neighbourhood problems and quality of life in older adults with and without osteoarthritis:Results from the Hertfordshire cohort study

This study examined whether the association of quality of life (QoL) with perceived neighbourhood problems is stronger in older adults with osteoarthritis (OA) than in those without OA. Of all 294 participants, 23.8% had OA. More perceived neighbourhood problems were associated with a stronger decrease in QoL over time in participants with OA (B=-0.018; p=0.02) than in those without OA (B=-0.004; p=0.39). Physical activity did not mediate this relationship. Older adults with OA may be less able to deal with more challenging environments

On the paternal origin of trisomy 21 Down syndrome

Background: Down syndrome (DS), characterized by an extra free chromosome 21 is the most common genetic cause for congenital malformations and learning disability. It is well known that the extra chromosome 21 originates from the mother in more than 90% of cases, the incidence increases with maternal age and there is a high recurrence in young women. In a previous report we have presented data to indicate that maternal trisomy 21 (T21) ovarian mosaicism might provide the major causative factor underlying these patterns of DS inheritance. One important outstanding question concerns the reason why the extra chromosome 21 in DS rarely originates from the father, i.e. in less than 10% of T21 DS cases. We here report data indicating that one reason for this parental sex difference is a very much lower degree of fetal testicular in comparison to ovarian T21 mosaicism. Results: We used fluorescence in situ hybridisation (FISH) with two chromosome 21-specific probes to determine the copy number of chromosome 21 in fetal testicular cell nuclei from four male fetuses, following termination of pregnancy for a non-medical/social reason at gestational age 14-19 weeks. The cells studied were selected on the basis of their morphology alone, pending immunological specification of the relevant cell types. We could not detect any indication of testicular T21 mosaicism in any of these four male fetuses, when analysing at least 2000 cells per case (range 2038-3971, total 11.842). This result is highly statistically significant (p < 0.001) in comparison to the average of 0.54% ovarian T21 mosaicism (range 0.20-0.88%) that we identified in eight female fetuses analysing a total of 12.634 cells, as documented in a previous report in this journal. Conclusion: Based on these observations we suggest that there is a significant sex difference in degrees of fetal germ line T21 mosaicism. Thus, it would appear that most female fetuses are T21 ovarian mosaics, while in sharp contrast most male fetuses may be either very low grade T21 testicular mosaics or they may be non-mosaics. We further propose that this sex difference in germ line T21 mosaicism may explain the much less frequent paternal origin of T21 DS than maternal. The mechanisms underlying the DS cases, where the extra chromosome 21 does originate from the father, remains unknown and further studies in this respect are required

Genetic studies in early embryos with emphasis on preimplantation genetic diagnosis

It has been estimated that as much as 70-80% of all human conceptions do not develop to term. A majority of clinically recognisable pregnancies, which are miscarried, contain chromosome abnormalities. However, data from very early embryo development are difficult to obtain. The recent development of single-cell fluorescent in situ hybridisation (FISH) analysis makes it possible to study chromosome abnormalities and segregation in early embryos. Using preimplantation genetic diagnosis (PGD) for carriers of structural chromosome abnormalities, we were able to perform chromosome studies in embryos as well as evaluate a new diagnostic tool. These experiences were also successfully implemented in a clinical service. A new single needle approach to obtain blastomere biopsies from human preimplantation embryos was tested and subsequently applied to PGD. The method was first evaluated in a mouse system and shown to be compatible with a high degree of in vitro and in vivo development of biopsied mouse embryos. Successful human PGD was performed when this method was applied in our human PGD programme. By using FISH, the presence of aneuploidy and mosaicism in normally fertilised freeze-thawed human embryos of good morphology were studied. The study shows a high degree of numerical chromosome abnormalities and 72% of the embryos displayed one or more abnormal blastomeres for the chromosomes studied. These data show a slightly higher incidence of abnormal embryos compared to those obtained with FISH in non-cryopreserved embryos and confirm that the majority of preimplantation embryos fertilised in vitro contain abnormal blastomeres. The investigation on mosaicism and aneuploidy in preimplantation embryos was continued with the detailed analysis of embryos from translocation carriers undergoing PGD. A high number of mosaic/chaotic embryos (73%) for the chromosomes involved in the translocation were found. A second hybridisation with two additional probes not involved in the translocation showed that the degree of mosaicism in each embryo differed between the two hybridisations. The difference in the average degree of mosaicism for all embryos was 65% regarding the chromosomes involved in the translocation as compared to 35% for two control chromosomes. The chromosome segregation pattern in embryos from carriers of structural chromosome abnormalities was investigated. The distribution of balanced and unbalanced gametes was for the first time studied in a female inversion carrier and the analysis showed that half of the analysed embryos were balanced. Alternate segregation mode was the most common mode of segregation in Robertsonian (85% of the embryos) and reciprocal (50% of the embryos) translocation carriers. Carriers of structural chromosome abnormalities are at high risk of having children with a severe handicap. Assays for the selection of balanced embryos was established using FISH. The first cases of PGD in order to avoid chromosomal imbalance in the progeny of two carriers of chromosome abnormalities, a deletion on chromosome 22q11.2 (DiGeorge syndrome) and an inversion of chromosome 5, respectively, were described. In addition, 11 PGD cycles for carriers of Robertsonian and reciprocal translocations were presented. As these patients are at high risk of having children with a severe handicap, PGD may be an attractive alternative as it allows only unaffected embryos to be selected for transfer to the uterus and the need to terminate the pregnancy is thereby avoided