Different patterns of X inactivation in MZ twins discordant for red-green color-vision deficiency.

Two female identical twins who were clinically normal were obligatory heterozygotes for X-linked deuteranomaly associated with a green-red fusion gene derived from their deuteranomalous father. On anomaloscopy, one of the twins was phenotypically deuteranomalous while the other had normal color vision. The color vision-defective twin had two sons with normal color vision and one deuteranomalous son. X-inactivation analysis was done with the highly informative probe M27 beta. This probe detects a locus (DXS255) which contains a VNTR and which is somewhat differentially methylated on the active and inactive X chromosomes. In skin cells of the color vision-defective twin, almost all paternal X chromosomes with the abnormal color-vision genes were active, thereby explaining her color-vision defect. In contrast, a different pattern was observed in skin cells from the woman with normal color vision; her maternal X chromosome was mostly active. However, in blood lymphocytes, both twins showed identical patterns with mixtures of inactivated maternal and paternal X chromosomes. Deuteranomaly in one of the twins is explained by extremely skewed X inactivation, as shown in skin cells. Failure to find this skewed pattern in blood cells is explained by the sharing of fetal circulation and exchange of hematopoietic precursor cells between twins. These data give evidence for X inactivation of the color-vision locus and add another MZ twin pair with markedly different X-inactivation patterns for X-linked traits.     

Commitment to X inactivation precedes the twinning event in monochorionic MZ twins.

To gain insight into the timing of twinning, we have examined a closely related event, X-chromosome inactivation, in female MZ twin pairs. X-inactivation patterns in peripheral blood and buccal mucosa were compared between monochorionic MZ (MC-MZ) and dichorionic MZ (DC-MZ) twins. Overall, the MC-MZ twins displayed highly similar X-inactivation patterns, whereas DC-MZ twins frequently differed in their X-inactivation patterns, when both tissues were tested. Previous experimental data suggest that commitment to X inactivation occurs when there are 10-20 cells in the embryo. Simulation of embryo splitting after commitment to X inactivation suggests that MC-MZ twinning occurs three or four rounds of replication after X inactivation, whereas a DC-MZ twinning event occurs earlier, before or around the time of X inactivation. Finally, the overall degree of skewing in the MZ twins was not significantly different from that observed in singletons. This indicates that X inactivation does not play a direct role in the twinning process, and it further suggests that extreme unequal splitting is not a common mechanism of twin formation.     

Studies of X inactivation and isodisomy in twins provide further evidence that the X chromosome is not involved in Rett syndrome.

Rett syndrome (RS), a progressive encephalopathy with onset in infancy, has been attributed to an X-linked mutation, mainly on the basis of its occurrence almost exclusively in females and its concordance in female MZ twins. The underlying mechanisms proposed are an X-linked dominant mutation with male lethality, uniparental disomy of the X chromosome, and/or some disturbance in the process of X inactivation leading to unequal distributions of cells expressing maternal or paternal alleles (referred to as a "nonrandom" or "skewed" pattern of X inactivation). To determine if the X chromosome is in fact involved in RS, we studied a group of affected females including three pairs of MZ twins, two concordant for RS and one uniquely discordant for RS. Analysis of X-inactivation patterns confirms the frequent nonrandom X inactivation previously observed in MZ twins but indicates that this is independent of RS. Analysis of 29 RS females reveals not one instance of uniparental X disomy, extending the observations previously reported. Therefore, our findings contribute no support for the hypothesis that RS is an X-linked disorder. Furthermore, the concordant phenotype in most MZ female twins with RS, which has not been observed in female twins with known X-linked mutations, argues against an X mutation.     

G-11 staining in Turner's syndrome with mos 45,X/46,X,r(?)

Mos 45,X/46,X,r(?) in 4 patients with Turner's syndrome and no signs of virilization, and in one pair of monozygotic twins, one of them with clitoral hypertrophy, was studied using combined cytogenetic techniques and specially G-11 staining for the characterization of the X or Y origin of the rings. In all 6 patients the ring was G-11 positive, attesting its Y origin. Both twins were operated and bilateral streak gonads with a bilateral nodule of testicular tissue were found. Similar small rings were also studied in one patient with mos 46,XX/46,X,r(X) and in one nonvirilized Turner's syndrome patient with a larger ring; in these two cases the ring was G-11 negative. It seems that the small rings occasionally found in Turner's syndrome are more frequently from Y origin and therefore prophylactic gonadectomy should be considered.     

Pericentric X inversion in dizygotic twins who differ in X chromosome inactivation and menstrual cycle function

Summary A 21-year-old female dizygotic twin was referred for cytogenetic evaluation because of mild mental retardation. Significant history, clinical, and physical findings included irregular menses, mildly coarse facies, and microcornea. Chromosome analysis revealed a pericentric inversion of the X chromosome, 46,X,inv(X)(p11;q22). Her twin who is phenotypically normal was also found to carry the same inversion. The twins differ significantly in X chromosome inactivation and menstrual cycle function.     

Discordant growth pattern and ovarian function in monozygotic twins with 45,X/46,XX mosaicism

BACKGROUND: We report on phenotypically discordant female monozygotic twins with 45X/46,XX mosaicism in both lymphocytes and fibroblasts. RESULTS: At 11.5 years, twin A was prepubertal, her height was 126.8 cm (-3.15 SD), bone age (BA) 9.7 years (TW2), FSH 47 IU/l and IGF-I 280 ng/ml (-0.89 SD), but twin B was pubertal (P2, B3), her height was 143.4 cm (-0.92 SD), BA 13.6 years (TW2), FSH 3.4 IU/l and IGF-I 380 ng/ml (-0.21 SD). One year later, twin A had grown 11.1 cm due to growth hormone therapy and had IGF-I 1,400 ng/ml (+5.91 SD), whereas the growth velocity of twin B (no therapy) was 5.9 cm, IGF-I 540 ng/ml (+0.57 SD) and she started regular menstruation at 12.1 years. CONCLUSION: To our knowledge, this is the first report on monozygotic twins with Turner mosaicism in both lymphocytes and fibroblasts who developed a discordant phenotype probably due to an unequal distribution of the two cell lines in distinct tissues.     

X inactivation as a source of behavioural differences in monozygotic female twins.

Although members of monozygotic twin pairs are identical in genome sequence, they may differ in patterns of gene expression. One early and irreversible process affecting gene expression, which can create differences within pairs of female monozygotic twins, is X inactivation - one twin can express mainly paternally-received genes on the X chromosome while the other twin expresses mainly maternally-received genes. It follows that non-identical X chromosome expression may cause female monozygotic twins to correlate less strongly than male monozygotic twins on complex behavioural traits affected by X-linked loci. We tested this hypothesis using data from around 4000 same-sex twin pairs on 9 social, behavioural and cognitive measures at ages 2, 3 and 4. Consistent with our hypothesis, monozygotic males were generally more similar than monozygotic females. Three of four significant differences were in traits showing higher correlations in males than females, and these traits - prosocial behaviour, peer problems, and verbal ability - have all been proposed previously in the literature as being influenced by genes on the X chromosome. Interestingly, dizygotic twins showed the reverse pattern of correlations for similar variables, which is also consistent with the X inactivation hypothesis; taken together, then, our monozygotic and dizygotic results suggest the presence of quantitative trait loci on the X chromosome.     

Mosaicism 45,X/46,X, t dic(Xp:Xp) in a girl with short stature

An eight-year-old girl with marked short stature and no apparent stigmata of Turner syndrome was investigated. Clinical features include bilateral epicanthic folds, frontal bossing, prominent ears and normal intelligence. Ultrasound scanning revealed an apparently normal vagina, streak ovaries and no uterus. Bone age was normal. Karyotype analysis of peripheral blood lymphocytes showed mos 45,X/46,X tdic(Xp:Xp) in the ratio 66:34, respectively. In addition, three cells with different abnormal X chromosomes were present which possibly originated from a 46,XX clone. Replication of the duplicated X chromosome was consistently late and symmetrical. Buccal smear confirmation of the karyotype showed Barr body negative in 90% and large or bipartite in 10% of the cells. Karyotypes of the parents were normal. The clinical manifestations in cases of Xp deletion due to terminal rearrangement associated with or without a 45,X cell line are discussed.     

FISH and PCR analyses in three patients with 45,X/46,X,idic(Y) karyotype: clinical and pathologic spectrum

OBJECTIVE: To delineate the phenotypic spectrum (clinical and gonadal features) from patients with a 45,X/46,X,mar(Y) karyotype based upon of their clinical, histological, cytogenetic and molecular evaluation. SUBJECTS: Three patients with a 45,X/46,X,mar(Y) karyotype. METHODS: Clinical assessment, karyotyping, endocrine evaluation, FISH and PCR analyses of several Y-chromosome loci and direct sequencing of the SRY gene. RESULTS: The patients, two males and one female had varying degrees of impairment of sexual differentiation, with or without testis formation. One patient (reared as female and aged 17 years) had Turner syndrome with bilateral streak gonads. The second patient (2.4 years old) had ambiguous genitalia and presented a dysgenetic testis with a contralateral streak gonad. A third patient (26 years old) had bilateral dysgenetic testes (dysgenetic male pseudohermaphroditism). The ratio of 45,X vs. 46,X,+mar(Y) cells differed between patients and between different tissues. In each case the marker sexual chromosome was identified as a rearranged Y-chromosome (idic(Y)) using FISH and PCR analyses. In all cases the SRY gene was present in all tissues studied. No mutations were identified in this gene in any of the patients. CONCLUSIONS: The extent of male or female differentiation in these patients depends in part on the prevalence, time occurrence, and distribution of the 45,X cell line.     

Skewed X inactivation in a female MZ twin results in Duchenne muscular dystrophy.

One of female MZ twins presented with muscular dystrophy. Physical examination, creatine phosphokinase levels, and muscle biopsy were consistent with Duchenne muscular dystrophy (DMD). However, because of her sex she was diagnosed as having limb-girdle muscular dystrophy. With cDNA probes to the DMD gene, a gene deletion was detected in the twins and their mother. The de novo mutation which arose in the mother was shown by novel junction fragments generated by HindIII, PstI, or TaqI when probed with cDNA8. Additional evidence of a large gene deletion was given by novel SfiI junction fragments detected by probes p20, J-Bir, and J-66 on pulsed-field gel electrophoresis (PFGE). Immunoblot analysis of muscle from the affected twin showed dystrophin of normal size but of reduced amount. Immunofluorescent visualization of dystrophin revealed foci of dystrophin-positive fibers adjacent to foci of dystrophin-negative fibers. These data indicate that the affected twin is a manifesting carrier of an abnormal DMD gene, her myopathy being a direct result of underexpression of dystrophin. Cytogenetic analysis revealed normal karyotypes, eliminating the possibility of a translocation affecting DMD gene function. Both linkage analysis and DNA fingerprint analysis revealed that each twin has two different X chromosomes, eliminating the possibility of uniparental disomy as a mechanism for DMD expression. On the basis of methylation differences of the paternal and maternal X chromosomes in these MZ twins, we propose uneven lyonization (X chromosome inactivation) as the underlying mechanism for disease expression in the affected female.     

A complex mosaicism 45,X/46,X,del(Xq)/46,X,idic(Xq) in a patient with secondary amenorrhea

A complex mosaicism involving the X chromosome was found in a 35-year-old female affected by secondary amenorrhea and short stature. Her karyotype was: 45,X[20]/46,X,del(X)(pter-->q26::qter)[15]/46,X,idic(X)(pter-->q26::q26-->pter)[9]. No cell contained both abnormal X chromosomes. This observation would suggest a possible mechanism underlying the formation of isodicentric chromosomes.     

On telomere replication and fusion in eukaryotes: apropos of a case of 45,X/46,X,ter rea(X;X)(p22.3;p22.3)

A Mexican 181/2-year-old girl with short stature, primary amenorrhea, and mild Turner stigmata was found to have a 45,X/46,X,ter rea(X;X)(p22.3;p22.3) karyotype in her lymphocytes. The rearranged chromosome was twice the size of a normal X, appeared to be attached head-to-head, had no detectable chromatin loss, only one primary constriction, constitutive heterochromatin at both the centromere and pseudocentromere regions, was mitotically stable, and always showed late replication. From the analysis of this and other X;X terminal rearrangements we draw four conclusions: (1) Terminal rearrangements may arise either by telomeric fusion (tel fus) without loss of genetic material or from a conventional telomeric translocation. (2) Telomeric fusions between homologous chromosomes (the commonest type) can be secondary to impaired telomeric replication. (3) Phenotypically, patients with an X;X terminal rearrangement show great variability which depends mainly on whether or not chromatin has been lost in the rejoining process and a 45,X clone. (4) Patients with an X;X telomeric fusion without mosaicism are likely to have an XXX phenotype, whereas turneroid features are expected in mixoploidies that include an X monosomic clone and in cases of translocations involving the short arms.     

X chromosome inactivation patterns correlate with fetal-placental anatomy in monozygotic twin pairs: implications for immune relatedness and concordance for autoimmunity.

BACKGROUND: Monozygotic (MZ) twinning is a poorly understood phenomenon that may result in subtle biologic differences between twins, despite their identical inheritance. These differences may in part account for discordant expression of disease in MZ twin pairs. Due to their stochastic nature, differences in X chromosome inactivation patterns are one source of such variation in female MZ twins. MATERIALS AND METHODS: We investigated X chromosome inactivation patterns in the blood of 41 MZ twin pairs based on methylation of the androgen receptor gene using a Hpa II-PCR assay. Twenty-six female MZ twin pairs with autoimmune disease (rheumatoid arthritis or multiple sclerosis) were studied. In addition, we studied 15 newborn female MZ twin pairs who were characterized at birth with respect to the anatomy of chorionic membranes (dichorionic versus monochorionic). RESULTS: We found a strong correlation between dichorionic fetal anatomy and differences in X chromosome inactivation patterns between members of an MZ twin pair. In contrast, all monochorionic twin pairs had closely correlated patterns of X chromosome inactivation. X chromosome inactivation patterns did not distinguish between MZ twin pairs who were concordant or discordant for autoimmune disease. CONCLUSIONS: The highly similar patterns of X chromosome inactivation among monochorionic twin pairs may result from their shared placental blood supply during intrauterine life. Alternatively, these patterns may indicate that X chromosome inactivation occurs before the twinning event in this anatomic subgroup of MZ twins. The data further suggest that these factors do not make a major contribution to the high discordance rates for autoimmune disease in MZ twin pairs.     

An X1X1X2X2/X1X2Y mechanism of sex determination in a South American rodent, Deltamys kempi (Rodentia, Cricetidae)

Chromosome studies on 14 specimens of Deltamys kempi disclosed six males with 2n = 37, NF = 38, six females with 2n = 38, NF = 38, and two females with 2n = 37, NF = 38. G- and C-band analyses revealed a Y-autosome translocation in the males leading to a multiple chromosome system of sex determination of the type X1X1X2X2/X1X2Y, this being the second case of such a mechanism described in rodents. At meiosis the males presented a trivalent in which C-banding studies showed an alternate orientation of the sex chromosomes due to end-to-end association of the X1 and Y chromosomes, the Y and the X2 being held together by interstitial chiasmata. At metaphase II both n = 17 + Y and n = 18 + X1 are regularly observed. The two females with 2n = 37, NF = 38, are heterozygous for an autosomal centric fusion involving chromosomes 1 and 13. The product of the Y-autosome translocation constitutes the largest element of the karyotype (9.4% of the haploid set); the X1 chromosome amounts to 7.8% of this set, including a large heterochromatic block. When only its euchromatic region is considered, this percentage decreases to 4.6%. From two to seven NORs were observed at the telomeres, with a mean of 4.4 +/- 1.1 per cell.     

X chromosome-inactivation patterns in patients with Rett syndrome

Rett syndrome (RS) is a complex and severely disabling neurologic disorder, restricted to females. As non-random X inactivation could indicate that the X chromosome has a role in the etiology of the syndrome, we performed molecular analysis based on the differential methylation of the active and inactive X chromosomes with probe M27β, taking into account the parental origin of the two Xs, in 24 RS girls (including a pair of concordant monozygote twins), 22 mothers, and a control group of 30 normal women. The results showed a significantly (Fisher’s exact test) increased frequency of skewed X inactivation in lymphocytes from 15/23 RS compared with 4/22 mothers (P = 0.0031) and 6/30 controls (P = 0.0021). Our results, together with those from the literature, showed that as a group, RS patients are apparently more prone to skewed X inactivation than their mothers and normal controls, and this suggests that the X chromosome is somehow involved in RS etiology. Received: 13 February 1997 / Accepted: 5 November 1997     

H-Y antigen-positive male pseudohermaphroditism with 45,X/46,XYq-mosaicism

Summary A 42-year-old male had short stature, microphallus, hypospadias, a bifid scrotum, abdominal undifferentiated testes, a uterus, bilateral fallopian tubes, and 45,X/46,XYq-mosaicism in his blood, skin, and germinal tissue and tissue surrounding the testes as determined by means of G-, Q-, and C-banding. An H-Y antigen assay on skin fibroblasts was positive, indicating that the locus for this antigen is not located in the brightly fluorescent region of the Y chromosome.     

Monozygous female twins discordant for a pattern of X chromosome inactivation as a model for study on X-linked intellectual traits

Ring the last decades, numerous genes for general cognitive ability were identified on human X-chromosome. They were discovered primarily because of X-linked mutations causing nonspecific mental retardation in males. Evidence for imprinted loci on the X chromosome affecting neurodevelopment was found in studies on 45,X females. Investigation of transmission of X-linked traits in normal individuals might further contribute to problem of shaping human being's mind ability. We suggest monozygous female twins discordant for a parent-of-origin of the X chromosome inactivation to be a proper subject for such explorations.     

A longitudinal twin study of skewed X chromosome-inactivation

X-chromosome inactivation (XCI) is a pivotal epigenetic mechanism involved in the dosage compensation of X-linked genes between males and females. In any given cell, the process of XCI in early female development is thought to be random across alleles and clonally maintained once established. Recent studies, however, suggest that XCI might not always be random and that skewed inactivation may become more prevalent with age. The factors influencing such XCI skewing and its changes over time are largely unknown. To elucidate the influence of stochastic, heritable and environmental factors in longitudinal changes in XCI, we examined X inactivation profiles in a sample of monozygotic (MZ) (n = 23) and dizygotic (DZ) (n = 22) female twin-pairs at ages 5 and 10 years. Compared to MZ twins who were highly concordant for allelic XCI ratios, DZ twins showed much lower levels of concordance. Whilst XCI patterns were moderately stable between ages 5 and 10 years, there was some drift over time with an increased prevalence of more extreme XCI skewing at age 10. To our knowledge, this study represents the earliest longitudinal assessment of skewed XCI patterns, and suggests that skewed XCI may already be established in early childhood. Our data also suggest a link between MZ twinning and the establishment of allelic XCI ratios, and demonstrate that acquired skewing in XCI after establishment is primarily mediated by stochastic mechanisms. These data have implications for our understanding about sex differences in complex disease, and the potential causes of phenotypic discordance between MZ female twins.     

The origin of 45,X males.

Maleness in association with the karyotype 45,X is a very rare and hitherto unexplained condition previously described in only four or five patients. This study was carried out to determine whether such males might actually possess Y-chromosomal material. Of the two 45,X males studied, one was found to be a low-grade mosaic with a 46,XY karyotype in less than 3% of fibroblasts; all lymphocytes karyotyped were 45,X. Fibroblast DNA from this individual was found to contain Y-specific repeated sequences in 1%-3% the amount observed in the father, consistent with mosaicism for a 46,XY cell line. No Y-specific repeated sequences were detected in the other patient, in whom all mitoses were 45,X. In neither patient were there detectable amounts of any of the single-copy Y-specific DNA sequences for which we tested. Studies of Xg blood groups and of X-linked restriction fragment length polymorphisms indicated that the single X chromosome was of maternal origin in both 45,X male probands. In contrast to the situation in XX males, we can exclude paternal X-Y interchange as the etiology in the cases described here. Our findings are compatible with mosaicism being the explanation of at least some "45,X" males.