Demethylation, reactivation, and destabilization of human fragile X full-mutation alleles in mouse embryocarcinoma cells

Abstract

The major causes of fragile X syndrome are mutational expansion of the CGG repeat in the FMR1 gene, hypermethylation, and transcriptional silencing. Most fragile X embryos develop somatic mosaicism of disease-causing "full" expansions of different lengths. Homogeneity of the mosaic patterns among multiple tissues in the same individual indicates that these previously unstable expansions acquire mitotic stability early in fetal life. Since mitotic stability is found strictly associated with hypermethylation in adult tissues, current theory has fixed the time of instability to developmental stages when fully expanded CGG repeats exist in an unmethylated state. We used murine embryocarcinoma (EC) cells (PC13) as a model system of pluripotent embryonic cells. Hypermethylated and unmethylated full expansions on human fragile X chromosomes were transferred from murine A9 hybrids into EC cells, by means of microcell fusion. As demonstrated in the present study for the first time, even full expansion alleles that were fully methylated and stable in the donors' fibroblasts and in A9 became demethylated, reactivated, and destabilized in undifferentiated EC hybrids. When destabilized expansions were reintroduced from EC cells into A9, instability was reversed to stability. Our results strongly support the idea that fully expanded alleles are initially unstable and unmethylated in the human embryo and gain stability upon genetic or epigenetic change of the embryonic cells.

Figure 1

Southern analysis of fragile X expansions harbored in murine A9 host cells. DNA samples isolated from fibroblasts of donors (ML and GZ) and from A9 hybrid clones were cleaved by restriction enzyme and hybridized to probe Ox1.9. A, Discrete-length alleles of methylated full expansions were isolated into different hybrid clones. DNA samples were cleaved with EcoRI plus EagI. The results for the donor’s (ML) fibroblasts and for six hybrid clones are shown with the clone numbers indicated. Because of methylation of the EagI site in the FMR1 promoter, the expanded fragments are larger than the normal 5.2-kb fragment seen in the control (lane C). Clone 32* was selected for microcell generation. B and C, Unmethylated expansions isolated into A9 hybrid clones (numbers shown above the lanes) and visualized by hybridization to EcoRI-plus-EagI–cleaved DNA. Cleavage of unmethylated EagI sites of expanded alleles results in fragments between 2.8 and 5.2 kb. Unmethylated 2.8-kb and methylated 5.2-kb-fragments are carried on the active and inactive X chromosomes of the female control (lane C). Whereas the donor’s (GZ) fibroblasts presented smears of multiple expansions, the majority of hybrids harbored a single expanded fragment with unmethylated EagI site. Clones 18*, 35*, and 36* were used in further experiments. D and E, Mitotic stability of expansions isolated from GZ into A9 clones 18 and 35 (GZA9.18, GZA9.35). Clonal DNAs were isolated at successive passages and cleaved with HindIII. The passage numbers are given above the lanes in D and E; population doublings are three to four times the passage number. The HindIII fragments of the murine gene, indicated by arrows (←), are seen in D and E above the human fragments carrying 330 and 230 CGGs, respectively.

Figure 2

Morphology and cytogenetics of undifferentiated and differentiated EC hybrids of PC13 harboring human fragile X alleles. A, Undifferentiated morphology of hybrid MLEC30.1. B, Undifferentiated morphology of a subclone isolated from MLEC30.1. C and D, Differentiated morphology of hybrid MLEC30.1. Upon treatment with 1 μM retinoic acid, the cells aggregated to form embryoid bodies (EB) and, subsequently, epitheloid (E) and neuron-like (N) cells. Bars = 100 μm. E and F, Mouse chromosomes harboring a fragment translocated from a human fragile X chromosome, indicated by arrows (→), in MLEC30.1 (E) and GZEC25.2 (F). Metaphase chromosomes were prepared from EC hybrids and analyzed by whole-chromosome painting of the human X chromosome with digoxigenin-labeled P5222-DG.5 (Oncor Appligene).

Figure 3

Southern analysis of EC hybrids and their parental A9 clones. A, Lanes grouped such that each A9 clone is followed by its derivative EC clone(s). Clonal DNAs were cleaved by EcoRI plus EagI. Fragments between 2.8 and 5.2 kb (see control lane C) represent unmethylated alleles. Demethylation of previously methylated EagI sites is evident in GZEC24.16, MLEC30.1, and MLEC30.2. Repeat sizes of all clones are given in table 2. B and C, MLEC30.1, subcloned by dilution plating, and subclonal DNAs, analyzed on EcoRI-plus-EagI (B) and PstI blots (C). Results for 10 subclones are indicated by numbers above the lanes. Blurred and smeared signals consistent with mitotic instability of subcloned expansions occurred as a function of repeat size.

Figure 4

Expression of human and murine FMR1 genes in EC hybrids of PC13 and their parental A9 clones. In the indicated cell lines and in human control fibroblasts (lane C), expression of the human allele was visualized by human-specific RT-PCR (237-bp products). GZA9.18, GZEC21.4, and the control cells carry unmethylated human FMR1 alleles (u). GZA9.35 and MLA9.32 harbor partially methylated (p) and fully methylated (m) alleles that are demethylated (d) in GZEC24.16, which is derived from GZA9.35, and in MLEC30.1 and MLEC30.2, which are derived from GZA9.35. Expression of the endogenous FMR1 alleles was shown by mouse-specific RT-PCR (376-bp product). The reduced intensities of the human RT-PCR signals of GZEC21.4 and GZEC24.16 correspond to the low proportions of cells that retained the human allele in these clones (see fig. 3A).

Figure 5

Morphology and Southern analysis of A9 hybrids harboring demethylated human fragile X expansions that were retransferred from EC cells. Microcells were generated from MLEC30.1 and fused to A9 cells. A, Morphology of a hybrid clone (16*). Bar = 100 μm. B, Isolated clones harboring chromosomes of both murine parents and a human FMR1 gene, analyzed on EcoRI-plus-EagI blots hybridized to Ox1.9. The results for nine clones, indicated by numbers, are shown. Notably sharp and intense signals of full-mutation fragments with unmethylated EagI sites were obtained in many clones; for example, 16* (315 CGGs), 22 (265 CGGs), and 24 (280 CGGs). C, One of the A9 hybrids (16*), subcloned to further prove mitotic stability of the expansion. The unmethylated full-mutation allele could be amplified by cloning (lanes 1–4) and did not experience any significant change of repeat size upon clonal expansion.