ABSTRACT
OBJECTIVES: Constitutional aneuploidy occurs in at least 5% of recognised pregnancies, with apparent preferential involvement of the X chromosome and the smaller autosomes. Molecular cytogenetic investigations of cleavage-stage embryos have revealed anomalies affecting all sizes of chromosomes. The aim was to investigate the variety of anomalies arising during maternal meiosis I by analysis of unfertilised oocytes and polar bodies to gain insight into aneuploidy mechanisms. METHODS: Sequential FISH analysis was carried out with specific probes derived from eight chromosomes, representing all sizes. Only imbalance due to a gain of a whole chromosome or chromatid, represented by extra signals, was counted to avoid artefact. RESULTS: Data were obtained on 236 eggs from 124 patients of average age 32.5 years (range 22-44). Ten patients (average 32.6 years) had abnormal eggs. The abnormality rate for oocytes and for polar bodies was close to 4% for each. Fourteen hyperploidies were found, seven involving additional single chromatids. The abnormalities affected chromosomes 13,16,18, 21 and X but not chromosomes 1, 9 or 12. CONCLUSION: The data provide evidence for several mechanisms leading to aneuploidy, including classical non-disjunction of whole univalents; pre-division of chromatids prior to anaphase I, leading to imbalance detected at metaphase II; gonadal mosaicism for a trisomic cell line and preferential involvement of the smaller chromosomes. Monosomy for the large autosomes is not uncommon in cleavage-stage embryos and may additionally arise from anaphase lag preferentially affecting such chromosomes.
Subject(s)
Aneuploidy , In Situ Hybridization, Fluorescence , Oocytes/ultrastructure , Adult , Chromatids , Chromosomes, Human, Pair 13 , Chromosomes, Human, Pair 16 , Chromosomes, Human, Pair 18 , Chromosomes, Human, Pair 21 , Chromosomes, Human, X , Female , Haploidy , HumansABSTRACT
A chromosomally normal 37-year-old woman was referred for preimplantation genetic diagnosis after having several conceptuses with trisomy 21. Segregation of chromosome 21 was assessed in unfertilised meiosis II oocytes and preimplantation embryos from PGD cycles using fluorescent in situ hybridisation (FISH). Of 7 preimplantation embryos, 5 were chromosomally abnormal with 4 having trisomy 21 and one being tetraploid. Of 4 oocytes, 3 had an abnormal chromosomal constitution with either an extra chromosome 21 or an extra chromatid 21. In one oocyte an extra chromatid 21 was detected in both the metaphase II complement and the first polar body providing the first direct evidence of a maternal trisomic germ cell line. Moreover, this result shows that the extra chromosome 21 can precociously divide into its two chromatids at the first meiotic division.
Subject(s)
Chromatids/genetics , Chromosome Segregation/genetics , Down Syndrome/genetics , Oocytes/metabolism , Adult , Chromatids/chemistry , Down Syndrome/diagnosis , Embryo, Mammalian , Female , Fertilization/genetics , Humans , In Situ Hybridization, Fluorescence , Male , Oocytes/physiology , Pedigree , Polymorphism, Genetic/genetics , Pregnancy , Preimplantation Diagnosis , Recurrence , Sequence Analysis, DNAABSTRACT
The population risk for trisomy 21 is 1 in 700 births but some couples are at a much higher risk owing to parental translocation or mosaicism. We report on the first attempt to carry out preimplantation genetic diagnosis for two such couples using cleavage stage embryo biopsy and dual colour FISH analysis. Each couple underwent two treatment cycles. Couple 1 (suspected gonadal mosaicism for trisomy 21) had two embryos normal for chromosome 21 transferred, but no pregnancy resulted; 64% (7/11) unfertilised oocytes/embryos showed chromosome 21 aneuploidy. Couple 2 (46,XX,t(6;21)(q13;q22.3)) had a single embryo transferred resulting in a biochemical pregnancy; 91% (10/11) oocytes/embryos showed chromosome 21 imbalance, most resulting from 3:1 segregation of this translocation at gametogenesis. The opportunity to test embryos before implantation enables the outcome of female meiosis to be studied for the first time and the recurrence risk for a Down syndrome pregnancy to be assessed.
Subject(s)
Down Syndrome/diagnosis , Down Syndrome/genetics , Mosaicism , Preimplantation Diagnosis , Translocation, Genetic , Adult , Chromosomes, Human, Pair 21/genetics , Embryo, Mammalian/ultrastructure , Female , Fertilization in Vitro , Humans , In Situ Hybridization, Fluorescence , Pregnancy , RiskABSTRACT
Preimplantation genetic diagnosis (PGD) may provide a feasible option for some Robertsonian translocation carriers who experience severe difficulty in achieving a normal pregnancy. We report on five PGD cycles for two such couples, 45,XY,der(13;14)(q10:q10) and 45,XX,der(13;21)(q10;q10), carried out by biopsy of two cells from day 3 post-insemination embryos generated by in vitro fertilisation. Locus-specific YAC probes for chromosomes 13, 14 and 21 were used to detect the chromosomes involved in the translocation using multicolour FISH. Three embryos transfers were carried out (two single embryo transfers and one double transfer) but no clinical pregnancies were established. In two cycles no embryos were transferred as all those biopsied were chromosomally abnormal. Combined results from both couples show 13% (6/45) of embryos analysed were normal for the translocation chromosomes and 87% (39/45) were chromosomally abnormal; these were categorised as 36% aneuploid or aneuploid mosaic and 51% chaotic where the chromosome constitution varied randomly from cell to cell. This suggests two factors may be acting to reduce fertility in these couples; the aneuploid segregation of the parental Robertsonian translocation and also a post-zygotic factor leading to uncontrolled chromosome distribution in early cleavage stages in an exceptionally high proportion of embryos.
