Dicentric Y-chromosome mosaicism in a girl with clitoral hypertrophy

Summary A 12-year-old girl with small stature and a hypertrophic clitoris was found to be mosaic for 45,X/46,X.dic(Y)(qter»p11:p11»qter)/46,XX/47, XX,dic(Y)(qter»p11:raqter). The dicentric chromosome was identified using Q-banding. These findings indicate mitotic instability of the dicentric Y, as well as the presence of an X chromosome in this patient.     

Indirect immunofluorescence of inactive centromeres as indicator of centromeric function

Summary Two previous single case reports from the literature showed the presence or absence of centromeric antigens at the site of the inactive centromeres in one (X;X) and in one (9;11) dicentric chromosome. We studied nine different dicentric chromosomes using anticentromeric antibodies and immunofluorescence techniques. In the four autosomal dicentrics the inactive centromere was consistently positive while the dicentrics composed of two X chromosomes were either positive or negative; one case of (X;Y) dicentric was negative. The results indicate that the X chromosome mode of replication may be involved in the suppression of immunofluorescence at the site of the inactive centromere and that one centromere of the dicentric chromosome may lose its function but conserve some of its antigenic properties. This indicates that not all these antigens play a rôle in the microtubules-centromere interaction.     

Deletion of specific sequences or modification of centromeric chromatin are responsible for Y chromosome centromere inactivation

Summary Stable dicentric chromosomes behave as monocentrics because one of the centromeres is inactive. The cause of centromere inactivation is unknown; changes in centromere chromatin conformation and loss of centromeric DNA elements have been proposed as possible mechanisms. We studied the phenomenon of inactivation in two Y centromeres, having as a control genetically identical active Y centromeres. The two cases have the following karyotypes: 45,X/46,X,i(Y)(q12) and 46,XY/ 47,XY,+t(X;Y)(p22.3;p11.3). The analysis of the behaviour of the active and inactive Y chromosome centromeres after Da-Dapi staining, CREST immunofluorescence, and in situ hybridization with centromeric probes leads us to conclude that, in the case of the isochromosome, a true deletion of centromeric chromatin is responsible for its stability, whereas in the second case, stability of the dicentric (X;Y) is the result of centromere chromatin modification.     

Dynamic increase of a 45,X cell line in a patient with multicentric ring Y chromosomes

Because ring Y chromosomes are unstable during cell division most reported patients are mosaics, usually including a 45,X cell line. The phenotype varies from normal males or females with streak gonads to sexual ambiguities. We present here the case of a 23-year-old man who was referred at 11 years for growth delay. The GTG-banded karyotypes of lymphocytes revealed two cell lines: 46,X,dic r(Y) seen in 76% of the metaphases analyzed and 45,X (24%). Karyotypes and FISH were performed eight years later with the following probes: DYZ3 (Y centromere), SRY (sex-region of the Y), DYZ1 (Yq heterochromatin), CEPX/Y (X centromere and Yq heterochromatin), TelVysion Xp/Yp, Xq/Yq (X and Y subtelomeres), pan-telomeric, cosmid clones LLycos130G04 and LLycos37C09 (PARII), and BAC clone RP11-5C5 (Yq11.223). The results showed an increase in the 45,X cell line (60%) and a reduction in the 46,X,dic r(Y) cell line (36.4%). The use of Yq probes showed that the ring Y chromosome was dicentric. In addition, other ring Y structures were observed. The breakpoints occurred in proximal Yp11.32 or in Yp11.31 distal to SRY and in Yq12 distal to the PARII region. Therefore, most of the Y remained intact and all genes, with the exception of those in PARI, are present in double dosage in the dic r(Y). The level of mosaicism was important in defining the phenotype.     

Two dicentric Y isochromosomes,one without the Yqh heterochromatic segment

Summary Two women with primary amenorrhoea and few other stigmata of Turner's syndrome were found to be chromosome mosaics: 45,X/46,X,idic(Y). In Case 1, the dicentric isochromosome Y was found to have a long-arm breakpoint of formation. This structure was interpreted as containing two Y short arms and centromeres separated by a region derived from the proximal Y long arm. One of the centromeres in the Case 1 —idic(Y) was suppressed in 80% of cells in blood, and in these cells it appeared as a regular Y-shaped chromosome. In Case 2 the idic(Y) was derived by a short-arm breakpoint of formation. In all the dicentrics of this case with one primary constriction (functional monocentrics) there was a single Cd band. In the 10% of dicentrics with two primary constrictions, there were two Cd bands. It is argued that the instability of sex isochromosomes is due to this functional dicentricity in some cells. These cases are compared with 42 other Y isochromosomes with various short- and long-arm breakpoints of formation. It is suggested that some of the nonheterochromatic, nonfluorescent Y chromosomes previously reported may be explained as dicentric i(Y) with proximal long-arm breakpoints of formation and one suppressed centromere.     

A unique dicentric X;Y translocation with Xq and Yp breakpoints: cytogenetic and molecular studies.

A 32-year-old woman presented with secondary amenorrhea and infertility. She was of normal height and her breasts were well developed, but she had streak gonads; there were no signs of virilization, and she showed no somatic stigmata of Turner syndrome. Chromosome analysis revealed a dicentric X;Y translocation with Xq and Yp breakpoints. Centromeric banding demonstrated a Y centromere and a "suppressed" X centromere. The karyotype of the patient was interpreted as 46,X,t(X;Y)(q22;p11). The Yp breakpoint was confirmed by DNA-hybridization studies with six probes detecting Y-specific sequences. These DNA-hybridization studies were consistent with the presence of the long arm, centromere, and much of the proximal short arm of the Y. The Y-DNA studies of this female also revealed the absence of the distal short arm of the Y chromosome, to which the testis-determining factor has previously been localized.     

Interstitial deletions of repetitive DNA blocks in dicentric human Y chromosomes

Cytogenetic analysis of aberrant human Y chromosomes was done by fluorescence in situ hydbridization (FISH) with Y specific repetitive DNA probes. It revealed an interstitial deletion of different DNA blocks in two dicentric chromosome structures. One deletion includes the total alphoid DNA structure of one centromeric region. The second deletion includes the total repetitive DYZ5 DNA structure in the pericentromeric region of one short Y arm. Both dicentric Y chromosomes were iso(Yp) chromosomes with break and fusion point located in Yq11, the euchromatic part of the long Y arm. Their phenotypic appearance was abnormal, resembling small monocentric Yq-chromosomes in metaphase plates. Mosaic cell lines, usually included in karyotypes with dicentric Y chromosomes, were not observed. It is assumed that both deletion events suppress the kinetochore activity in one Y centromeric region and thus stabilize its dicentric structure. Local interstitial deletion events had not been described in dicentric human Y chromosomes, but are common in dicentric yeast chromosomes. This raises the question of whether deletion events in dicentric human chromosomes are rare or restricted to the Y chromosome or also represent a general possibility for stabilization of a dicentric chromosome structure in human.     

True vs. false inv(Y)(p11q11.2): a familial instance concurrent with trisomy 21

A boy with Down syndrome due to a free trisomy 21 also had a metacentric Y chromosome with an arm euchromatic and the other heterochromatic inherited from his phenotypically normal father. This chromosome was mitotically stable and hybridized with the DYZ3 probe precisely at its primary constriction; in addition, a subtelomeric Xp/Yp probe gave the expected signal near the end of the euchromatic arm. So, the proband's karyotype was 47,X,inv(Y)(p11q11.2),+21. Given the high frequency of both chromosome anomalies, we regard its concurrence as a mere coincidence. This observation, along with previous reports, allows us to classify the apparent pericentric inversions of the Y chromosome into two types: "true" inversions characterized by an alphoid single centromere and mitotic stability, and "false" inversions in which a nonalphoid centromere has taken over the usual alphoid centromere; indeed, these chromosomes are dicentric and mitotically unstable. Finally, the inv(Y) polymorphism in man compares with that documented in other mammal species, in which the rearranged Y chromosome neither impairs the fertility nor has other phenotypical consequences.     

Isodicentric Y chromosome: cytogenetic,molecular and clinical studies and review of the literature

Dicentrics are among the most common structural abnormalities of the human Y chromosome. Predicting the phenotypic consequences of different duplications and deletions of dicentric Y chromosomes is usually complicated by varying degrees of mosaicism (45,X cell lines), which may, in some cases, remain undetected. Molecular studies in patients with dicentric Y chromosomes have been few, and only two studies have attempted to determine the presence of SRY (the putative testis-determining factor gene). We report an 18-year-old female with short stature, amenorrhea, hirsutism, hypoplastic labia minora, and clitoromegaly who has a 45,X/46,X,idic(Y)(p11.32)/47,X,idic(Y)(p11.32),idic(Y) (p11.32) karyotype. Southern analysis using Y-specific probes (Y97, 2D6, 1F5, pY3.4) and polymerase chain reaction (PCR) analysis using primers for ZFY and SRY were positive for all loci tested, indicating that almost all of the Y chromosome was present. Our findings and an extensive review of the literature emphasize the importance of molecular analyses of abnormal Y chromosomes before any general conclusions can be reached concerning the relative effects of the Y-chromosome abnormality and mosaicism on sexual differentiation.     

Kinetics of DNA replication in a dicentric X chromosome formed by long arm to long arm fusion

Summary Utilizing the 5-bromo-deoxyuridine (BrdU) incorporation technique, we have recently studied the DNA replication kinetics in a dicentric X chromosome, formed by long arm-to-long arm fusion at band q23, from a 16-year-old black female with primary amenorrhea. The patient has a karyotype 45,X/46,X,dic(X)(q23).In the buccal smear the presence of X chromatin was found in 33% of the cells examined. The Barr bodies are large and 21% of them are bipartite. DNA replication studies were performed on the patient's lymphocytes by the thymidine pulse (T-pulse) method and confirmed comparatively by the BrdU pulse (B-pulse) method. The results indicate that the dicentric X chromosome is always late-replicating. The replication pattern is symmetric on both sides of the breakpoint and the replication sequence is, in order, p11, p22, q1(1–3), q22, q23, p21, and q21. This finding is comparable to those of other investigators and supports the theory that there exist two inactivation centers in the dicentric X chromosome, located on or near the q21 band.     

Structural abnormalities of the Y chromosome. Observations in ten cases]

The authors report ten cases of structural anomalies involving the Y chromosome: five cases of a dicentric Y chromosome, one ring Y chromosome, one case of a Y isochromosome containing the long arms, one deletion of the long arms (Yq-), one case of an abnormally long Y chromosome (Yq+) and one Y-autosome translocation. Analysis of clinical and chromosomal correlations, especially with respect to sexual differentiation, led to discussing the role of the Y chromosome.     

Development and use of metaphase chromosome flow-sorting methodology to obtain recombinant phage libraries enriched for parts of the human X chromosome

Metaphase chromosomes isolated from human lymphoblastoid cell lines containing structurally abnormal X chromosomes have been stained with the bisbenzimidazole dye Hoechst 33258 and analyzed on a FACS II flow system equipped with a 5-W all-lines argon ion laser. The chromosomal fluorescence has been highly resolved at flow rates of 1,000-3,000 chromosomes per second. With the goal of obtaining recombinant DNA libraries from parts of the human X chromosome, fluorescence populations enriched for a dicentric X (Xpter- greater than Xq24::Xq24-greater than Xpter) chromosome and an isochromosome of the long arm of the X [i(Xq)] have been identified. The dicentric X chromosome has been resolved as a discrete peak in the fluorescence flow histogram. In contrast, the fluorescence intensity of the isochromosome is indistinguishable from that of chromosomes 3 and 4. Recombinant DNA libraries from the flow-sorted chromosomes have been constructed in the lambda phage, Charon 21A, and consist of 1.6 X 10(5) and 0.7 X 10(5) plaque-forming units in the case of the dicentric X and the isochromosome, respectively. Ninety percent of the phage in both recombinant libraries contain inserts which hybridize to highly repetitive human DNA sequences. The recombinant phage library from the flow-sorted dicentric X chromosome, which could be assigned to a discrete fluorescence peak, has been further characterized and shows at least a tenfold enrichment for X chromosome-specific DNA sequences as determined by Southern blot hybridization of cloned fragments.     

Cytogenetic and molecular investigations of an abnormal Y chromosome: evidence for a pseudo-dicentric (Yq) isochromosome.

A derivative Y chromosome was found in a 55-year-old man with Lambert-Eaton paraneoplasic pseudomyastheniform disease. Small testicles, azoospermia were noticed and hormonal level values were as in the Klinefelter syndrome. A 45,X/46,XYp+ mosa?cism was described on peripheral blood lymphocytes. Cytogenetic investigations with R-G-C- and Q-banding have been performed. In situ hybridization with the GMGY 10 DNA probe showed two copies of proximal Yp sequences. Southern blot analyses were performed using the Y DNA probes 27a, 47z, 64a7, 50f2 disclosing specific Yp and Yq sequences from the pseudoautosomal boundary to the Yq proximal portion. The der(Y) has been defined as a dicentric isochromosome for the long arm with one active and one apparently suppressed centromere. The breakpoint leading to the der(Y), has been located in the pairing segment of the Y short arm (i.e. Yp11.32). So the der(Y) was interpreted as a psu dic(Y) (qter-->cen-->p11.32 ::p11.32-->qter). There was thus an almost complete duplication of the Y chromosome.     

45,X/46,X,dic(Yq) mosaicism and mixed gonadal dysgenesis. Case report and review of the literature.

A 4 year 7 month-old boy with ambiguous genitalia, histological evidence of mixed gonadal dysgenesis, and 45,X/46,X,dic(Yq) mosaicism is reported. The identity of the dicentric Y chromosome was stablished by its typical fluorescent banding patterns and the presence of two centromeres demonstrated by C-band technique. A review of the literature yielded nine additional cases of mosaic 45,X/46,X,dic(Yq). Phenotypical and histological findings among these cases were compared, and the possible localization of the genes responsible for testicle induction and maturation is discussed.     

A high frequency of XO offspring from X(Paf)Y* male mice: evidence that the Paf mutation involves an inversion spanning the X PAR boundary

It has previously been reported that 19% of the daughters of males carrying the X-linked mutation patchy fur (Paf) are XO with a maternally derived X chromosome. We now report that hemizygous Paf males that also carry the variant Y chromosome Y*, show a much increased XO production ( approximately 40% of daughters). We hypothesize that the Paf mutation is associated with an inversion spanning the pseudoautosomal region (PAR) boundary, and that this leads to preferential crossing over between the resulting inverted region of PAR and an equivalent inverted PAR region within the compound Y* PAR. This would lead to the production of dicentric X and acentric Y products and consequent sex chromosome loss. This interpretation is supported by analysis of the sex chromosome complements at the second meiotic metaphase, which revealed a high incidence of dicentrics. Another curious feature of the Paf mutation is that mice that are homozygous Paf have more hair than mice that are hemizygous Paf. This can be explained if the Paf mutation is a hypomorphic mutation that escapes X inactivation because, unlike the wild type allele, it is now located within the PAR.     

Y isochromosome associated with a mosaic karyotype and inactivation of the centromere

Summary A patient with azoospermia and a Y isochromosome is described. The breakpoint producing this i(Y) was within the terminal short arm of the Y chromosome. Lymphocyte cultures from peripheral blood contained a high proportion of 45,X cells and cells with different Y-chromosome rearrangements. The i(Y) had either a monocentric or dicentric appearance. In dicentrics, anti-kinetochore immunofluorescence was present at both centromeres. However, this was also true for most of the functional monocentrics (pseudodicentrics). Kinetochore staining was generally positive at the site of the inactive centromeres; only a minority of the suppressed centromeres had lost their antigenic properties. Permanently growing lymphoblasts consistently showed a monocentric i(Y) with only one fluorescing kinetochore; the immunonegative Y centromere did not recover antigenicity.     

Quinacrine fluorescence of variant and abnormal human Y chromosomes

Quinacrine fluorescence has been used to examine variant and abnormal human Y chromosomes, at interphase and mitosis. The length difference in variant Y chromosomes has been shown to involve the highly fluorescent segment only. Eight abnormalities of the Y chromosome have been positively identified, two isochromosomes of the long arms of the Y, five pericentric inversions, and a dicentric Y chromosome with two long arms. Contributory evidence for two further abnormalities, a ring Y and a dicentric with two short arms, is also given.     

Sex reversal due to Xp disomy by t(X;Y)(p21;q11).

A 20-month-old infant exhibiting psychomotor retardation, dysmorphisms and ambiguous external genitalia was found to have a 46-chromosome karyotype including a normal X chromosome and a marker Y with most of Yq being replaced by an extra Xp21-->pter segment. The paternal karyotype (G and C bands) was 46,XY. The marker Y composition was verified by means of FISH with a chromosome X painting, an alphoid repeat and a DMD probe. Thus, the final diagnosis was 46,X,der(Y)t(X;Y)(p21;q11)de novo.ish der(Y)(wcpX+,DYZ3+,DMD+). The patient's phenotype is consistent with the spectrum documented in 13 patients with similar Xp duplications in whom sex reversal with female or ambiguous genitalia has occurred in spite of an intact Yp or SRY gene. A review of t(X;Y) identifies five distinct exchanges described two or more times: t(X;Y)(p21;q11), t(X;Y)(p22;p11), t(X;Y)(p22;q11-12), t(X;Y) (q22;q12), and t(X;Y)(q28;q12). These translocations probably result from a recombination secondary to DNA homologies within misaligned sex chromosomes in the paternal germline with the derivatives segregating at anaphase I.     

A male pseudohermaphrodite with a dicentric Y chromosome

Summary A 13-year-old male pseudohermaphrodite (mixed gonadal dysgenesis, unilateral testicular differentiation) was found to be a chromosomal mosaic of the 45, XO/46, XY type, the Y chromosome being a symmetrical dicentric chromosome. Presumed Y chromosomes similar to this one have been observed before, but their identification was not supported by autoradiographic data. In the present study the identification of the dicentric as a Y chromosome was supported by observing a relatively late DNA replication during the S period. The differentiation of testicular tissue on one side may be related to the presence of short arm material of the Y chromosome in the genome of the XY dic cells.This study was supported by a Research Grant from the National Institute of Child Health and Human Development (1RO1-HD0999-03) and by a Public Health Service Training Grant in Pediatric Endocrinology and Metabolic Diseases (2T1-AM-5190).     

46,X,i(Xq)/47,XX,+13 mosaicism

A 10-year-old girl with short stature and other features of Turner's syndrome was found to be a mosaic consisting of 46,X,i(Xq) and 47,XX,+13 cell lines, a hitherto undescribed situation. She had none of the clinical features of trisomy 13 syndrome, with a possible exception of postaxial polydactyly of the left foot. Her PHA-stimulated blood lymphocytes and EB virus-transformed B lymphocytes both revealed the Xi(Xq)/XX,+13 mosaicism, while her skin fibroblasts showed an exclusively 46,X,i(Xq) karyotype. Studies using Q-and R-banding heteromorphisms as markers indicated that the patient started as a 13 trisomic zygote resulting from a maternal meiotic error, followed by the loss of chromosome 13 at an early mitotic division. C-banding analysis revealed two C banding blocks in the iso X chromosome, an indication that the chromosome was dicentric. BrdU-Hoechst-Giemsa analysis revealed that the iso X chromosome was late-replicating with both its arms either synchronously or asynchronously replicating. The iso X chromosome was thus designated as idic (Xq)(p11:p11). In view of the presence of the XX cell line, it was concluded that the patient started as an XX,+13 zygote, followed by two mitotic events, the loss of a chromosome 13 and the formation of the iso X chromosome, occurring either simultaneously or in succession.