Centromeric association of a microchromosome Y in two male patients.

We characterized two Y-ring microchromosomes (MC) found in an azoospermic patient with Turner stigmata (case A) and a male infant with hypospadias (case B). The karyotypes, as assessed by banding, FISH, and STRs/STSs analyses, were 46,X,r(Y).ish r(Y)(p11.3q11.222)(SRY+,DYZ3+) and 46,X,+r(Y)/45,X.ish r(Y)(p 11.2q11.2)(Xp/Yp-,SRY+,DYZ3+) respectively. In both cases, we evaluated the association of each MC with the centromere of the nearest and second nearest chromosomes in G-banded metaphases by means of measuring the intervening distance according to two criteria: < or =1 time or < or =3 times the size of the MC in each metaphase. The case A's MC was associated 84 times in 98 cells according to the latter or less strict criterion and two times in 98 cells according to the strict criterion; the corresponding values for case B were 84 and two in 95 cells respectively. The centromeric association appears to be related to centromeric attraction mediated by heterochromatin or centromere-specific proteins, the replication time, and the Rabl orientation.

Isodicentric Y chromosomes and secondary microchromosomes.

We report here on two mosaic patients with both an idic(Yp) and a microchromosome. FISH with the DYZ3 alphoid repeat demonstrated that the isodicentrics effectively exhibited two alphoid clusters whereas the small markers had a Y-centromere. These data, along with 4 previous observations, indicate that such microchromosomes effectively result from functional dicentricity of isodicentric Y-chromosomes and represent the excision of one centromere plus various amounts of adjacent chromatin. Other than a real infrequency of such a concurrence or a very low proportion of the cell line(s) containing the microchromosome, the paucity of observations points to a high rate of underdiagnosis as revealed by two idic(Y) instances in which the microchromosome was detected only in samples assessed by FISH.

Del Xq23 in a mosaic Turner female: molecular and cytogenetic studies.

We report a Turner patient aged 22 years with a 45,X/46,X,del(X)(q23) karyotype. Late replication studies showed preferential inactivation of the deleted X chromosome; FISH studies with a probe for total human telomeres showed hybridisation signal in the telomeres on both the normal and the deleted X chromosomes. Microsatellite analysis in the proposita and her family permitted us to conclude to the maternal origin of the deleted X chromosome, and to detect using the marker DXS1106 (Xq22) a probable meiotic recombination event above the breakage point suggesting that the deletion occurred underneath this point. The mild Turner stigmata may be explained by the 45,X cell line, and the gonadal dysgenesis probably by a partial deletion of the gonadal dysgenesis region Xq13-q23 (excluding Xq22).

In search of a 9q13 latent centromere in 9qh polymorphic inversions.

In search of a 9q13 latent centromere in 9qh polymorphic inversions: The presence of alphoid sequences in 9q13 has prompted the suggestion that such a region could harbor a latent centromere which under certain circumstances may appear as a neocentromere. We tested this hypothesis by means of FISH with a centromere 9-specific alphoid probe in lymphocyte metaphases from 13 unrelated individuals with a 9qh polymorphic inversion. Since all inverted chromosomes had the alphoid signal onto the primary constriction, it was not possible to identify any neocentromere . We believe, however, that the number of cases was not enough to conclude that all the polymorphic inversions of chromosome 9 are genuine.

Interchange trisomy 21 by t(1;21)(p22;q22)mat.

Interchange trisomy 21 by t(1:21)(p22:q22)mat: Interchange trisomy 21 by t(1;21)(p22;q22)mat was identified in a sporadic patient with Down syndrome. With a 21q22 specific probe, we observed signals on both normal 21 chromosomes and on the der. We reviewed the 23 published reports of families with reciprocal translocations leading to viable offspring with interchange trisomy 21. The breakpoints in chromosome 21 were mainly located in 21q (19/24 instances, including the present report) and in 19/23 cases the other chromosome involved in the translocation was <> (pairs 1-12). The underlying 3:1 segregation occurred mainly in carrier mothers; only one patient presented a de novo imbalance and in another case the father was the carrier. In addition, there were 4 instances of concurrence with another unbalanced segregation (adjacent-1 or tertiary trisomy) and 3 families with recurrence of interchange trisomy 21. The mean age of 14 female carriers at birth of interchange trisomy 21 offspring (24.8 yr) was lower that the mean (28.3 yr) found in a larger sample of mothers of unbalanced offspring due to 3:1 segregation (mostly tertiary trisomics) and was not increased with respect to the general population average. Overall, these data agree with previous estimates regarding recurrence risk (9-15%) and abortion rate (about 28%) in female carriers ascertained through an interchange trisomic 21 child.

Ring-20-syndrome and loss of telomeric regions.

A patient aged 10 years and 8 months with a ring-20-syndrome was studied. Clinically he presented normal psychomotor development until 25 months of age when he began with right simple partial motor seizures. He presented minimal dysmorphism, generalized tonic-clonic seizures refractory to medical therapy and behavioral troubles. He was submitted to a callosotomy when he presented an electric status, subsequently, he was treated with anticonvulsivants and felbamate and the seizures were controlled. The karyotype showed a chromosomal complement 46,XY,r(20)(p13q13.3) with loss of the telomeric regions evidenced by FISH. The mother had normal karyotype. The clinical and cytogenetic features of previous cases described in the literature were compared leading to a better characterization of this syndrome.

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.

Neocentromere at 13q32 in one of two stable markers derived from a 13q21 break.

A 10-month-old girl with psychomotor retardation, microcephaly, bilateral microphthalmia, and postaxial polydactyly of the feet was karyotyped using banding techniques and (single or dual color) fluorescent in situ hybridization (FISH) with four probes: D13Z1/D21Z1, pancentromeric, pantelomeric, and a mix of 13q subtelomeric and 13/21 alphoid repeats. She was found to have a 47-chromosome karyotype in which a normal 13 was replaced by two stable markers derived from a breakpoint at 13q21.1, namely a del(13)(q21.1) and an isofragment(13) (qter-->q21.1::q21.1-->qter). The latter had a single C-negative but Cd-positive primary constriction at 13q32 which, however, was not obvious in about 12% of the cells. FISH studies showed that the small 13q- had the 13-centromere and a 13q telomere (as shown for a specific 13q subtelomeric signal) onto the broken end whereas the isofragment lacked alphoid signals but had 13q subtelomeric sequences on both ends. Parental karyotypes were normal. The patient's rearrangement represents the eighth chromosome-13-derived marker with a nonalphoid neocentromere located at 13q. All in all, such neocentromeres have been described in 29 markers derived from chromosomes 2, 3, 8-11, 13-15, 20, and Y, and plausibly result from the epigenetic activation of a latent centromere, which may even be a telomere with neocentric activity. The 13q telomere found in the del(13q) was probably captured from the homologous chromosome.

Centromere-telomere (12;8p) fusion, telomeric 12q translocation, and i(12p) trisomy.

The concurrence of a short arm isochromosome and a translocation of the entire long arm of the same chromosome to a telomere of another chromosome, implying trisomy for 4p, 5p, 7p, 9p, 10p or 12p, has been described in 13 patients. We have now used fluorescence in situ hybrization (FISH) to better characterize one of these rearrangements in which 12q was translocated to 8pter, whereas 12p was converted into an isochromosome. An alphoid centromere-12 repeat gave a strong signal on the i( 2p) and a weak but distinct signal at the breakpoint junction of the der(8), whereas the pantelomeric probe revealed three clear hybridization sites on the der(8): one at each end and another at the breakpoint junction. These findings suggest that the prime event was a post-fertilization centric fission of chromosome 12 leading to the 12q translocation via a real centromere telomere fusion and the i(12p). Alternatively, the crucial event may have been a centromere telomere recombination. An interstitial telomere has been documented by means of FISH at the breakpoint junction of the sole derivative usually present in 20 constitutional translocations including eight with a jumping behavior. In addition, six other telomeric translocations defined by banding methods, including another case of 12q translocation/i(12p), have also been jumping ones. These telomeric translocations have been de noro events and their proneness to exhibit a jumping behavior appears to be independent of the involved chromosomes, size of the translocated segments, and concomitant abnormalities.

Single cell chromosome rearrangements in indivuduals with reproductive failure

Single cell chromosome rearrangements (SCCR) are incidental findings in cell cultures. Nevertheless, some authors have implicated them in habitual abortion. Ninety individuals classified in four groups were prospectively studied: A) individuals with spontaneous abortions, malformed children or molar pregnancies (N = 36); B) azoospermic males (N = 14); C) individuals with one or more children with either de novo or inherited, constitutional, chromosome abnormalities (N = 26), and D) individuals with healthy children and no reproductive failure (control group, N = 14). Lymphocyte chromosome preparations were stained for GTG bands, and 50-100 metaphases were scored per individual. The rearrangements observed were translocations, deletions, duplications, isochromosomes, rings, fragments and markers. Thirty-four individuals (21 males, 13 females) had a range of 1-5 SCCR. Four had rearrangements only of chromosomes 7 and 14. SCCR frequency (with the exception of rearrangements involving chromosomes 7 and 14) was 0.0063, while that of rearrangements between chromosomes 7 and 14 was 0.0010. Statistical intergroup comparisons c2 with Yates correction) did not show significant differences. Hence, the occurrence of SCCR in our sample was found to be independent of ascertainment mode and sex.

Functional Xp disomy and de novo t(X;13)(q10;q10) in a girl with hypomelanosis of Ito.

We report on a 16 month old girl with hypomelanosis of Ito and a balanced de novo (X;13)(q10;q10) translocation in which the der(Xp13q) had the X centromere (as assessed by FISH with the DXZ3 probe). A functional Xp disomy was shown in a small proportion of cultured lymphocytes by means of a BrdU terminal pulse. This observation supports the notion of a distinct form of hypomelanosis of Ito resulting from a functional Xp disomy.

A further microchromosome with centromeric association.

A 46,XY/47,XY,+mar karyotype was found in a 5-year-old boy with mental retardation and minor dysmorphisms. The marker was present in 68% of lymphocyte metaphases, was about the size of Yp, appeared pale with G- and C-bands and had a single pair of centromeric dots; it was never seen in two or more copies and appeared larger and clearly annular in a few cells. This microchromosome was associated (interchromosomal distance equal to or smaller than 18p) with any other chromosome(s) in 295/344 cells. Among a total of 457 associations, 240 (52%) were centromeric. Such a proportion as well as the mostly random distribution of centromeric associations was similar to 3 previous instances. The paucity of markers with centromeric association may reflect a relative unawareness on the subject but more probable indicates that such a phenomenon is confined to nonsatellited, monocentric and annular microchromosomes.

A familial Xp+ chromosome, dup (Xq26.3-->qter).

A maternally transmitted Xp+ chromosome was associated with an abnormal phenotype, including developmental delay and short stature, in two male cousins and their 12 year old aunt. The respective mothers were not mentally impaired but had short stature. The G banding pattern identified the extra chromosome segment as a repeat of Xq26.3-->qter attached to an apparently intact Xp22.3 sub-band, so the Xp+ chromosome may be described as rea(X)(Xqter-->p22.3::Xq26.3-->Xqter). The rearranged chromosome was late replicating in 97 to 100% of the metaphases in the mothers but it was early replicating in 43% of the lymphocytes in the mentally defective female (n = 100 cells/subject). Fluorescence in situ hybridisation using X and Y chromosome paints, as well as cosmids A and 1A1 specific for loci within Xq28, confirmed both the identity of the extra segment and the entirety of the Xp pseudoautosomal region. Therefore, the phenotypic consequences in this family can be related to the Xq26.3-->qter functional disomy allowing for the effects of X inactivation in the female carriers.

Centromeric association of a microchromosome in a Turner syndrome patient with a pseudodicentric Y.

A 12-year-old patient with Turner syndrome was found to have a complex mosaicism for a microchromosome (MC) and a psu dic(Y)(q11). The MC was smaller than Yp, appeared pale in G, C and late replicating bands, had a pair of small centromeric dots, was associated with other chromosomes in most metaphases, and was rather stable both in size and during mitosis. The psu dic(Y) was Cd-positive only at the active centromere, had two pericentromeric heterochromatic regions, and lacked the Yq12 band. No cells with both abnormal chromosomes were found. To evaluate the association of the MC with all ordinary chromosomes, 857 G-banded cells with the marker were screened. The MC was considered as "associated" whenever the distance between it and other chromosome(s) was equal to, or smaller than, 18p. Out of 848 associations registered, 489 (57.7%) were centromeric, 202 (23.8%) telomeric, and 157 (18.5%) interstitial; i.e., centromeric associations were overrepresented (P < 0.001) and showed a random distribution, except for an excessive involvement of chromosome 8. This association pattern, also exhibited by two similar MCs in human beings, the minute Y of a marsupial and certain B chromosomes in plants, probably reflects the Rabl orientation of chromosomes in interphase.

Opposite imbalances of distal 14q in two unrelated patients.

Two unrelated children were found to have de novo opposite imbalances for distal 14q. One had a 46,XY, del(14)(q24q32) karyotype and exhibited, like three other patients with similar deletions, a distinctive facial appearance including round face, frontal hypertrichosis, thick eyebrows, horizontal narrow palpebral fissures, a short bulbous nose with a flat root, and mild micrognathia. The other had a 46,XX, dir dup(14)(q22----q32) karyotype and stigmata common to patients with comparable duplications, namely high forehead, sparse eyebrows, prominent overlip, gingival hypertrophy, and overriding fingers. Therefore, it is concluded that each of these imbalances originates a distinct syndrome.

Translocation/duplication of 9p onto a duplicated 4q.

A 5-month-old girl with the classical dysmorphism of the 9p trisomy syndrome and a severe heart defect was found to have an unbalanced translocation of 9pter-->p22 onto q35 of a chromosome 4 with an inverted duplication of q32-->q35. This concurrence of two de novo rearrangements suggests that the breakpoint at 4q35 not only participated in the translocation but also predisposed to the segmental duplication of 4q.