Analysis of banding patterns and mosaic configurations in a case of ring chromosome 15

Summary Cytogenetic studies on lymphocytes from a 14-year-old mentally retarded girl with somatic anomalies suggestive of a chromosomal abnormality revealed a ring chromosome 15. The long arm of the defective chromosome is broken at band q24 or q25. The silver staining technique for nucleolus organizer regions showed that the ring had lost the achromatic stalk and the satellite. The chromosomal mosaicism resulting from the structural instability of the ring chromosome was analyzed and compared with 6 cases reported in the literature. It is proposed that the clinical manifestations in the different patients with ring chromosome 15 result from both the deficiency in the long arm and the mosaic configurations.     

A girl with cutaneous hyperpigmentation, café au lait spots and ring chromosome 15 without significant deletion

Ring chromosome 15 [r(15)] syndrome is characterised by specific facial features, café au lait spots, failure to thrive, mental retardation and typically with a terminal deletion of the long arm of chromosome 15. We report a 2.5 year old girl showing normal growth and development, large hyperpigmented skin changes showing hypopigmentated areas inside, multiple café au lait spots and premature graying-like hypopigmentation of scalp hair. She had a karyotype of r(15) in peripheral lymphocytes and fibroblasts. By FISH analysis the breakpoint was located distal to locus D15S936 (15q26.3) and within 300 kb of the end of the chromosome, indicating no deletion of functional genes on 15q. Hyperpigmentation and café au lait spots are rare signs in ring chromosome syndromes, but with r(15) syndrome, café au lait spots have been described in about 30% of patients and have been considered to result from the deletion of gene(s) on distal 15q. Based on the frequent observation of patchy hyperpigmentation with the r(15) syndrome, absent hyperpigmentation in cases of distal 15q deletion without a ring chromosome, and the telomeric breakpoint location in our patient indicating no significant deletion, we propose that the cutaneous hyperpigmentation and café au lait spots in our proband represent effects of the r(15) chromosome but are not caused by the deletion of specific gene(s) on distal 15q. Patchy skin hypopigmentation is a well known nonspecific sign in cytogenetic mosaicism which is commonly seen in ring syndrome.     

Molecular analysis of a ring chromosome X in a family with fragile X syndrome

The phenotypically normal sister of a patient affected by fragile X syndrome was referred for genetic counselling and was found to carry a mosaic karyotype 46,X,r(X)/45,X. Because the probability of the simultaneous chance occurrence of fragile X syndrome and a ring chromosome X in the same family is very low, we postulated that the breakpoint of the ring chromosome X originated in the cytogenetic break in Xq27.3 responsible for fragile X syndrome. In order to determine the relative positions of the breakpoint on the ring chromosome X and the (CGG)n unstable sequence responsible for the fragile X mutation, we used molecular markers to analyse the telomeric regions of chromosome X in this family. The results showed that the ring chromosome X was the maternal fragile X chromosome and that the telomeric deletion on the long arm encompassed the (CGG)n sequence. This suggests that the cytogenetic break in Xq27.3 is distinct from the unstable (CGG)n sequence, or that the break followed by the end-to-end fusion creating the ring chromosome was not completely conservative. Analysis of DNA markers on the short arm of chromosome X evidenced a deletion of a large part of the pseudoautosomal region, allowing us to position the genes involved in stature and in some syndromes associated with telomeric deletions of Xp on the proximal side of the pseudoautosomal region.     

Chromosome 18 replaced by two ring chromosomes of chromosome 18 origin

We here describe the first example of the replacement of an autosome by two ring chromosomes originating from the missing chromosome, presented in a patient with a single chromosome 18 and two additional ring chromosomes. Detailed fluorescence in situ hybridization (FISH) analysis revealed the chromosome 18 origin of both ring chromosomes and characterized the small and the large ring chromosome as derivatives of the short and long arm of chromosome 18, respectively. The loss of subtelomeric regions of the short and the long arm of chromosome 18 in the ring chromosomes was confirmed by FISH studies. Molecular studies showed the exclusive presence of the paternal alleles for microsatellite markers located distal to the short and long arm loci D18S843 and D18S474, respectively. This indicates the maternal origin of both rings and provides evidence for substantial deletions of the distal parts of both arms of chromosome 18 in the ring chromosomes. The dysmorphic features of the patient can be explained by these deletions in both chromosome arms, as the clinical findings partly overlap with observations in 18p- and 18q-syndrome and are similar to some cases of ring chromosome 18. Centromere misdivision is suggested as one mechanism involved in the formation of the ring chromosomes.     

Occurrence and evolution of homogeneously staining regions may be due to breakage-fusion-bridge cycles following telomere loss

Chromosomes with homogeneously staining regions (HSR) were analysed in a subclone of the H4 rat hepatoma cell line, where they represent amplification of the ribosomal RNA (rRNA) genes. Detailed G-band analysis of the subclone revealed that an HSR on the short arm of chromosome 3 became unstable and changed its position within the chromosome. The evolution of this marker chromosome was associated with the terminal deletion of the normal long arm of the HSR-bearing chromosome 3 and may have involved ring formation as a result of fusion between the HSR on the short arm and the broken end of the long arm. Evidence was obtained for breakage at different sites within the ring, producing chromosomes with HSRs located terminally on either the long arms or both arms. The terminally located HSR underwent elongation in some cells presumably as a result of a breakage-fusion-bridge cycle characteristic of instability due to telomeric loss. It is suggested that terminally located HSRs may generally occur this way.     

A functional centromere lacking CentO sequences in a newly formed ring chromosome in rice

An awned rice(Oryza sativa) plant carrying a tiny extra chromosome was discovered among the progeny of a telotrisomic line 2nt4L. Fluorescence in situ hybridization(FISH) using chromosome specific BAC clones revealed that this extra chromosome was a ring chromosome derived from part of the long arm of chromosome 4. So the aneuploidy plant was accordingly named as 2nt4L ring. We did not detect any Cent O FISH signals on the ring chromosome, and found only the centromeric probe Centromeric Retrotransposon of Rice(CRR) was co-localized with the centromere-specific histone CENH3 as revealed by sequential FISH after immunodetection. The extra ring chromosome exhibited a unique segregation pattern during meiosis, including no pairing between the ring chromosome and normal chromosome 4during prophase I and pre-separation of sister chromatids at anaphase I.     

Malformative syndrome associated with a ring 10 chromosome and a translocated 10q/19 chromosome

Summary A male newborn with a ring 10 chromosome is described. The distal part of the long arm of chromosome 10, deleted during ring formation (10q25), is translocated to the short arm of chromosome 19.     

A new translocation involving three chromosomes in chronic myelocytic leukemia, 46,XY,t(9;11;22).

Bone-marrow metaphases in a 63-year-old male with newly discovered chronic myelocytic leukemia (CML) showed a complex translocation involving chromosomes 9, 11, and 22. About half of the short arm of chromosome 11 was translocated to the terminal part of the long arm of chromosome 9, and the missing fragment on chromosome 22 was translocated to the short arm of the abnormal chromosome 9. The clinical features were typical of CML, and the patient is in good physical condition 10 months after diagnosis on a regimen of busulfan.     

Deletions of the long arm of chromosome 10 in progression of follicular thyroid tumors

Previous studies of follicular thyroid tumors have shown loss of heterozygosity (LOH) on the short arm of chromosome 3 in carcinomas, and on chromosome 10 in atypical adenomas and carcinomas, but not in common adenomas. We studied LOH on these chromosomal arms in 15 follicular thyroid carcinomas, 19 atypical follicular adenomas and 6 anaplastic (undifferentiated) carcinomas. Deletion mapping of chromosome 10 using 15 polymorphic markers showed that 15 (37.5%) of the tumors displayed LOH somewhere along the long arm. Thirteen of these tumors showed deletions involving the telomeric part of chromosome 10q, distal to D1OS 187. LOH on chromosome 3p was found in 8 (20%) cases. Seven of these also showed LOH on chromosome 10q. In eight cases LOH was seen on chromosome 10q but not 3p. In comparison, the retinoblastoma gene locus at chromosome 13q showed LOH in 22% of the tumors. Most of these also had deletions on chromosome 10q. The results indicate that a region at the telomeric part of 10q may be involved in progression of follicular thyroid tumors.     

Chromosome 6q- and associated malformations.

A dysmorphic retarded fourteen-mont-old female with partial deletion of the long arm of chromosome 6 is presented. The breakpoint in 6q was in region 2, probably at band 5. Eight other infants with a deletion involving the long arm of chromosome 6, including five with a ring chromosome 6, have been reported. The affected individuals have in common microcephaly, micrognathia, hypotonia and psychomotor retardation, but do not appear to have a distinctive phenotype.     

Nature and distribution of chromosomal intertwinings in Saccharomyces cerevisiae.

To elucidate yeast chromosome structure and behavior, we examined the breakage of entangled chromosomes in DNA topoisomerase II mutants by hybridization to chromosomal DNA resolved by pulsed-field gel electrophoresis. Our study reveals that large and small chromosomes differ in the nature and distribution of their intertwinings. Probes to large chromosomes (450 kb or larger) detect chromosome breakage, but probes to small chromosomes (380 kb or smaller) reveal no breakage products. Examination of chromosomes with one small arm and one large arm suggests that the two arms behave independently. The acrocentric chromosome XIV breaks only on the long arm, and its preferred region of breakage is approximately 200 kb from the centromere. When the centromere of chromosome XIV is relocated, the preferred region of breakage shifts accordingly. These results suggest that large chromosomes break because they have long arms and small chromosomes do not break because they have small arms. Indeed, a small metacentric chromosome can be made to break if it is rearranged to form a telocentric chromosome with one long arm or a ring with an "infinitely" long arm. These results suggest a model of chromosomal intertwining in which the length of the chromosome arm prevents intertwinings from passively resolving off the end of the arm during chromosome segregation.     

A Y/15 translocation in a 45,X male with Prader-Willi syndrome

We report a male neonate with a 45 X karyotype; the long arm of a chromosome 15 was translocated onto the proximal long arm of the Y chromosome. Breakpoints were identified by in situ fluorescence hybridization (FISH) on the proximal 15q13 and Yq11.2. The derivative chromosome has no primary centromere. Clinical features were compatible with Prader-Willi syndrome. This is the first report case ofmonosomy 15q and Yq deletion with Prader-Willi syndrome.     

Mosaic and non-mosaic trisomy 15q2

Two unrelated patients are presented. In the first mosaicism with normal cells and cells trisomic for the distal long arm (q2) of chromosome 15 was found. The 15q2 trisomy was due to a chromosome 14, to the long arm of which an extra 15q2 region was attached (14pter----14q32::15q22----15qter). In the second trisomy 15q2 was present as a consequence of a balanced t(7;15)(p22;q15) translocation in the mother.     

The impact of imprinting: Prader-Willi syndrome resulting from chromosome translocation, recombination, and nondisjunction.

Prader-Willi syndrome (PWS) is most often the result of a deletion of bands q11.2-q13 of the paternally derived chromosome 15, but it also occurs either because of maternal uniparental disomy (UPD) of this region or, rarely, from a methylation imprinting defect. A significant number of cases are due to structural rearrangements of the pericentromeric region of chromosome 15. We report two cases of PWS with UPD in which there was a meiosis I nondisjunction error involving an altered chromosome 15 produced by both a translocation event between the heteromorphic satellite regions of chromosomes 14 and 15 and recombination. In both cases, high-resolution banding of the long arm was normal, and FISH of probes D15S11, SNRPN, D15S10, and GABRB3 indicated no loss of this material. Chromosome heteromorphism analysis showed that each patient had maternal heterodisomy of the chromosome 15 short arm, whereas PCR of microsatellites demonstrated allele-specific maternal isodisomy and heterodisomy of the long arm. SNRPN gene methylation analysis revealed only a maternal imprint in both patients. We suggest that the chromosome structural rearrangements, combined with recombination in these patients, disrupted normal segregation of an imprinted region, resulting in uniparental disomy and PWS.     

Microdissection and reverse painting reveals a microdeletion 6(q26qter) in a de novo r(6) chromosome

Ring chromosomes 6 are rare constitutional abnormalities with inconsistent phenotypic and clinical features. One of the reasons for this variability is the cytogenetically undetectable loss of chromosomal material from the telomeric segments at 6p or 6q. We have therefore used fluorescence in situ hybridization (FISH) to analyse a ring chromosome 6 that was detected in a newborn boy with dysmorphic features. Reverse painting of the microdissected ring chromosome onto normal metaphase spreads revealed a small deletion of the terminal region of the long arm, 6(q26qter). Moreover, the simple all-telomeric sequence (TTAGG)n was lost, whereas the p-specific subtelomeric sequence was still present. Our findings confirm that microdeletions occur during the formation of r(6) chromosomes and, therefore, are an important determinator of the associated phenotype.     

Mapping of a locus correcting lack of phosphoribosylaminoimidazole carboxylase activity in Chinese hamster ovary cell Ade-D mutants to human chromosome 4

The human phosphoribosylaminoimidazole (AIR) carboxylase locus has been until this report one of the genes encoding purine biosynthetic enzymes that had not been assigned to an individual human chromosome. Characterization of Chinese hamster ovary (CHO) cell mutant Ade-D showed that the cell line was unable to produce IMP and accumulated AIR. CHO Ade-D cells were fused with normal human lymphocytes utilizing inactivated Sendai virus and the resulting hybrid cell lines were selected for purine prototrophy. Cytogenetic analysis showed a 100% concordance value for chromosome 4. Two of the isolated subclones contained only the long arm of chromosome 4 translocated onto a CHO chromosome, providing evidence for a regional assignment of the Ade-D gene to the long arm of chromosome 4. Two of the subclones containing chromosome 4 were subjected to the BrdU visible light segregation. All of the isolated purine auxotrophic cell lines showed a loss of the q arm of chromosome 4. The localization of the Ade-D locus to the long arm of chromosome 4 may reveal further clustering of the mammalian purine genes since the Ade-A locus has previously been regionally assigned to 4pter-q21.     

Parental origin of a ring 13 chromosome in a female with multiple anomalies

Summary A ring chromosome No. 13 was found in a 21-year-old female with multiple anomalies suggestive of 13q-syndrome. Chromosomes of the girl and her parents, studied by quinacrine staining, revealed the ring to be of paternal origin. Detailed study of the quinacrine banding pattern of the ring indicated loss of the most distal band of the long arm (13q34) and possible partial loss of the next adjacent long arm band (13q33). The short arm (13q11) was present but the stalk (13p12) and satellite (13p13) regions appeared to be missing.This work was supported in part by NIH grants HD 07997; Maternal and Child Health Services 970; HD 08236; CA 16747; by grants from the Medical Research Foundation of Oregon and by a Basil O'Connor Starter Research Grant.     

Trisomy of medial 15q as result of an analphoid supernumerary ring chromosome detected by CGH and FISH

We report a 21-year-old patient with a de novo mosaic, analphoid ring of chromosome 15q22.2-->q24.1. The clinical features of this patient are mild and include tall stature, obesity, striae distensae in the hypogastrium, malocclusion and bilateral gynecomastia with scarce glandular tissue. M-FISH and FISH using a chromosome 15 painting probe indicated that the ring is of chromosome 15 origin. Further CGH analysis and FISH with the PML locus-specific probe demonstrated that the extra material derived from the medial part of the long arm of chromosome 15, including two bands, q22 and q23. Additionally, FISH with BAC probes specific for 15q allowed for a localization of the breakpoints at 15q22.2 and 15q24.1, distal to clones RP11-30M4 and RP11-500O23 respectively. We discuss the relationship between the patient's genotype and phenotype comparing it to reported cases of trisomy of medial 15q.     

Molecular detection of a Yp/18 translocation in a 45,X holoprosencephalic male

Summary Prenatal diagnosis in a fetus with holoprosencephaly showed a 45,X karyotype and a suspected 18p abnormality. At birth, the fetus presented with normal male genitalia. Y chromatin was not cytogenetically detectable by Q-, G-, or G11-banding. Mosaicism for a cell line containing a Y chromosome was not observed in amniocytes, lymphocytes, or skin fibroblasts. Southern blot analysis for 11 different Y-DNA loci demonstrated the presence in the patient's genome of sequences derived from the short arm, centromeric region, and proximal long arm of the Y chromosome (intervals 1–5). The distal long arm of the Y (intervals 6 and 7) was absent. In situ hybridization with the Y-derived probe pDP105 showed silver grains over the short arm of the del(18) chromosome, suggesting a Y/18 translocation with loss of 18p and distal Yq material.     

Interstitial deletion and ring chromosome derived from 19q. Proximal 19q trisomy phenotype.

A small supernumerary ring chromosome has been found in a boy with overweight, dysmorphic facies and mental retardation. His mother had an interstitial deletion of the long arm of chromosome 19 and the same ring chromosome. By means of fluorescence in situ hybridization the ring chromosome was shown to be derived from the deleted chromosome, after the occurrence of two breaks: one in the centromere region, the other in the q-arm of chromosome 19.