Jumping translocations in spontaneous abortions

Chromosome translocations involving one donor chromosome and multiple recipient chromosomes have been referred to as jumping translocations (JTs). Acquired JTs are commonly observed in cancer patients, mainly involving chromosome 1. Constitutional forms of JTs mostly involve the acrocentric chromosomes and their satellites and have been reported in patients with clinical abnormalities. Recognizable phenotypes resulting from these events have included Down, Prader-Willi, and DiGeorge syndromes. The presence of JTs in spontaneous abortions has not been previously described. The breakpoints of all JTs occur in areas rich in repetitive DNA (telomeric, centromeric, and nucleolus organizing regions). We report two different unstable chromosome rearrangements in samples derived from spontaneous abortions. The first case involved a chromosome 15 donor. The recipient chromosomes were 1, 9, 15, and 21, and the respective breakpoints were in either the heterochromatic regions or the centromeres. FISH studies confirmed that the breakpoints of the jumping 15 rearrangement did not involve the Prader-Willi region but originated at the centromere or in the proximal short arm. A second case of instability was observed with a rearrangement resulting from a presumed de novo 8;21 translocation. Three JT cell lines were observed. They consisted of a deleted 8p chromosome, a dicentric 8;21 translocation, and an 8q isochromosome. The instability regions appeared to be at the pericentromeric region of chromosome 8 and the satellite region of chromosome 21. Both cases proved to be de novo events. The unstable nature of the JT resulting in chromosomal imbalance most likely contributed to the fetal loss. It appears that JT events may predispose to chromosomal imbalance via nondisjunction and chromosomal rearrangement and, therefore, may be an unrecognized cause of fetal loss.     

The donor chromosome breakpoint for a jumping translocation is associated with large low-copy repeats in 21q21.3

Jumping translocations (JTs) are very rare chromosome aberrations, usually identified in tumors. We report a constitutional JT between donor chromosome 21q21.3-->qter and recipients 13qter and 18qter, resulting in an approximately 15.5-Mb proximal deletion 21q in a girl with mild developmental delay and minor dysmorphic features. Using fluorescence in situ hybridization (FISH) studies, we identified an approximately 550-kb complex inter- and intra-chromosomal low-copy repeat (LCR) adjacent to the 21q21.3 translocation breakpoint. On the recipient chromosomes 13qter and 18qter, the telomeric sequences TTAGGG were retained. Genotyping revealed that the deletion was of maternal origin. We propose that genome architecture involving LCRs may be a major mechanism responsible for the origin of jumping translocations.     

Chromosome 15 abnormalities and the Prader-Willi syndrome: a follow-up report of 40 cases.

High-resolution chromosome analysis and multiple banding techniques were performed on blood samples from 40 patients with Prader-Willi syndrome (PWS) as a follow-up to our recent report in which we found interstitial deletions of 15q in four of five patients with this syndrome. Of the 40 new patients, 19 had interstitial del(15q), one had an apparently balanced 15;15 translocation, and one was mos46,XX/47,XX+idic(15) (pter leads to q11::q11 leads to pter). These data confirm our previous report and demonstrate that half of all patients with the clinical diagnosis of PWS have chromosome abnormalities involving chromosome 15 detectable by high-resolution methods. Although the majority of these involve a specific deletion of bands 15q11-q12, other alterations of chromosome 15 may be present.     

Deficiency,transposition, and duplication of one 15q region may be alternatively associated with Prader-Willi (or a similar) syndrome. Analysis of seven cases after varying ascertainment

Seven patients are described who have some or all of the symptoms of Prader-Willi syndrome. They were ascertained by varying criteria starting either from the clinical picture or from the identification of a chromosome abnormality involving the proximal portion of the long arm of chromosome 15. The chromosome abnormalities consisted of two balanced translocations (15;18 and 8;15), three unbalanced ones (15;18, 15;19, and 9;15), and one interstitial deletion of bands 15q11 and q12. The seventh case had an unidentified extra chromosome. These data and a review of the literature led to the conclusion that deficiency, transposition, and even duplication of the region(s) 15q11-q13 may all result in a syndrome which is identifiable with or similar to the Prader-Willi syndrome.     

Chromosome 15 anomalies and the Prader-Willi syndrome: Cytogenetic analysis

Summary The behaviour of chromosome 15 is very different from that of the other acrocentric chromosomes. The cytogenetic characteristics of rearrangements associated with Prader-Willi syndrome (PWS) are analyzed as similar rearrangements irrespective of the associated phenotype (reciprocal translocations of chromosome 15, small bisatellited additional chromosomes, Robertsonian translocations, interstitial deletions, pericentric inversions). This study suggests that: (1) The proximal (15q) region and PWS seem to be indissociable; (2) chromosome 15 has an indisputable cytogenetic originality which could be related to its histochemical properties. Chromosome 15 constitutive heterochromatin usually contains much 5-methylcytosine-rich DNA and a large amount of each of the four satellite DNAs. Furthermore the existence in the proximal (15q) region of one or several palindromic sequences could be postulated to explain the great lability of this region of chromosome 15.     

Investigations with fluorescence in situ hybridization (FISH) demonstrate loss of the telomeres on the reciprocal chromosome in three unbalanced translocations involving chromosome 15 in the Prader-Willi and Angelman syndromes

Two patients with classical features of Angelman syndrome (AS) and one with Prader-Willi syndrome (PWS) had unbalanced reciprocal translocations involving the chromosome 15 proximal long arm and the telomeric region of chromosomes 7, 8 and 10. Fluorescence isitu hybridization (FISH) was used for the detection of chromosome 15(q11-13) deletions (with probes from the PWS/AS region) and to define the involvement of the telomere in the derivative chromosomes (with library probes and telomere-specific probes). The 15(q11-13) region was not deleted in one patient but was deleted in the other two. The telomere on the derivative chromosomes 7, 8 and 10 was deleted in all three cases. Thus, these are true reciprocal translocations in which there has been loss of the small satellited reciprocal chromosome (15) fragment.     

A jumping translocation (5p;15q), (8q;15q), and (12q;15q) (author's transl)]

Three balanced karyotypes (5p;15q), (8q;15q), and (12q;15q) were found simultaneously in a child with the Willi-Prader syndrome. The hypothesis is presented of a "jumping# translocation by affinity of telomeric and interstitial palindromes. The relationship between the Willi-Prader syndrome and a juxtacentric anomaly of the long arm of chromosome 15 is discussed.     

Imprinting mutations suggested by abnormal DNA methylation patterns in familial Angelman and Prader-Willi syndromes.

The D15S9 and D15S63 loci in the Prader-Willi/Angelman syndrome region on chromosome 15 are subject to parent-of-origin-specific DNA methylation. We have found two Prader-Willi syndrome families in which the patients carry a maternal methylation imprint on the paternal chromosome. In one of these families, the patients have a small deletion encompassing the gene for the small nuclear ribonucleoprotein polypeptide N, which maps 130 kb telomeric to D15S63. Furthermore, we have identified a pair of nondeletion Angelman syndrome sibs and two isolated Angelman syndrome patients who carry a paternal methylation imprint on the maternal chromosome. These Angelman and Prader-Willi syndrome patients may have a defect in the imprinting process in 15q11-13. We propose a model in which a cis-acting mutation prevents the resetting of the imprinting signal in the germ line and thus disturbs the expression of imprinted genes in this region.     

Duplication within chromosome region 15q11-q13 in a patient with similarities to Prader-Willi syndrome confirmed by region-specific and band-specific fish.

We report on a patient presenting with mental retardation and obesity and a proximal duplication of chromosome 15. The patient shared some clinical signs with Prader-Willi syndrome. With a region-specific paint, generated by microdissection, a duplication in region 15q11.2-q13 was shown to be present. Subsequently, FISH with probes localized to chromosome region 15q11.2-q12 and microsatellite analysis was used to characterize this chromosome aberration further and an insertion duplication within the region frequently deleted in Prader-Willi and Angelman syndrome was demonstrated.     

Jumping translocation of 15q24-qter resulting in partial trisomy: a case report

We report on a jumping translocation with five different cell lines detected in four tissues in a 2-year-old patient. This rare type of chromosomal abnormality (not more than 30 cases published so far) proved to be a series of non-reciprocal translocations of the 15q24-qter donor chromosome segment to the telomeric region of chromosomes 5q, 10q, 16q and 19p, respectively. The process, in addition to a few cells without translocation, resulted in partial trisomy of 15q24-qter which was associated with somatic overdevelopment in the patient, with hemihypertrophy and minor anomalies. The phenotype of our patient was different from that of the other two patients found in the literature having the same donor chromosome segment involved in a similar rearrangement. Possibly, the difference in the phenotype lies in the various ratios of somatic mosaicism with five cell lines, in particular the presence of normal one which is extremely rare in patients with jumping translocation. Here we discuss the various ways on how the rearrangement could arise.     

Prader-Willi-Syndrom und Angelman-Syndrom

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are two distinct neurogenetic disorders caused by the loss of function of imprinted genes in the chromosomal region 15q11q13. An approximately 2 Mb region inside 15q11q13 is subject to genomic imprinting. As a consequence the maternal and paternal copies in this region are different in DNA methylation and gene expression. The most frequent genetic lesions in both disorders are an interstitial de novo deletion of the chromosomal region 15q11q13, uniparental disomy 15, an imprinting defect or, in the case of AS, a mutation of the UBE3A gene. Microdeletions in a small number of patients with PWS and AS with an imprinting defect have led to the identification of the chromosome 15 imprinting centre (IC) upstream of the SNURF-SNRPN gene, which acts in cis to regulate imprinting in the whole 15q imprinted domain. The IC consists of two critical elements: one in the more centromeric part which is deleted in patients with AS and which is thought to be responsible for the establishment of imprinting in the female germ line, and one in the more telomeric part which is deleted in patients with PWS and which is required to maintain the paternal imprint.     

Bloom Syndrome and Maternal Uniparental Disomy for Chromosome 15

Bloom syndrome (BS) is an autosomal recessive disorder characterized by increases in the frequency of sister-chromatid exchange and in the incidence of malignancy. Chromosome-transfer studies have shown the BS locus to map to chromosome 15q. This report describes a subject with features of both BS and Prader-Willi syndrome (PWS). Molecular analysis showed maternal uniparental disomy for chromosome 15. Meiotic recombination between the two disomic chromosomes 15 has resulted in heterodisomy for proximal 15q and isodisomy for distal 15q. In this individual BS is probably due to homozygosity for a gene that is telomeric to D15S95 (15q25), rather than to genetic imprinting, the mechanism responsible for the development of PWS. This report represents the first application of disomy analysis to the regional localization of a disease gene. This strategy promises to be useful in the genetic mapping of other uncommon autosomal recessive conditions.     

Localization of the gene encoding the GABAA receptor β3 subunit to the Angelman/Prader-Willi region of human chromosome 15

Deletions of the proximal long arm of chromosome 15 (bands 15q11q13) are found in the majority of patients with two distinct genetic disorders, Angelman syndrome (AS) and Prader-Willi syndrome (PWS). The deleted regions in the two syndromes, defined cytogenetically and by using cloned DNA probes, are similar. However, deletions in AS occur on the maternally inherited chromosome 15, and deletions in PWS occur on the paternally derived chromosome 15. This observation has led to the suggestion that one or more genes in this region show differential expression dependent on parental origin (genetic imprinting). No genes of known function have previously been mapped to this region. We show here that the gene encoding the GABAA (gamma-aminobutyric acid) receptor beta 3 subunit maps to the AS/PWS region. Deletion of this gene (GABRB3) was found in AS and PWS patients with interstitial cytogenetic deletions. Evidence of beta 3 gene deletion was also found in an AS patient with an unbalanced 13;15 translocation but not in a PWS patient with an unbalanced 9;15 translocation. The localization of this receptor gene to the AS/PWS region suggests a possible role of the inhibitory neurotransmitter GABA in the pathogenesis of one or both of these syndromes.     

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.     

Fragile site and interstitial telomere repeat sequences at the fusion point of a de novo (Y;13) translocation

We describe a novel fragile site in a rearranged chromosome, associated with the presence of telomeric repeat sequences at the fusion point of a translocation between chromosomes 13 and Y. The case reported in this study shows a de novo (Y;13) translocation, which appears to represent fusion of an apparently intact chromosome Y with a chromosome 13 that has lost only part of its short arm. Ten percent of the cells show a normal karyotype without the (Y;13) translocation. Molecular cytogenetic studies of the derived Y;13 chromosome revealed three hybridization sites of the telomeric probes – one at each end and one at the breakpoint junction. A fragile site is also observed in the intrachromosomic telomeric region. This coincidence suggests that the telomere repeat sequences (TTAGGG)n, when present at an interstitial chromosomal location, can promote the formation of a novel fragile site. Received: 15 November 1995 / Revised: 6 March 1996     

A jumping Robertsonian translocation: a molecular and cytogenetic study

We report a patient with mosaicism for two different Robertsonian translocations, both involving chromosome 21. She carries an unbalanced cell line with an i(21q) and a balanced cell line with a rob(21q22q). She is phenotypically normal but has two children who inherited the i(21q) and have Down syndrome. We demonstrate that both abnormal chromosomes are dicentric and that the proband’s 21/21 rearrangement is an isochromosome formed from a maternally derived chromosome 21. We propose a model in which the i(21q) is the progenitor rearrangement in the proband, which subsequently participated in a nonreciprocal rearrangement characteristic of a jumping translocation. In addition, we review other cases of constitutional mosaicism involving jumping translocations. Received: 4 October 1995 / Revised: 14 February 1996     

Cytogenetic studies of familial Prader-Willi syndrome

Summary A family in which two first cousins were found to have the Prader-Willi syndrome was investigated cytogenetically. Although G-banding analysis of metaphase chromosomes failed to demonstrate abnormality, close analyses on the fine prometaphase bands by G-banding and the DA-DAPI bands by double stainings revealed a distinct chromosome abnormality in this family. A reciprocal translocation, rep(14:15)(q11.2;q13), was detected in three family members: the mother, the maternal grandmother, and a maternal uncle of the proband. And, the proband and one of the first cousins had an unbalanced translocation that was derived from their carrier parents. The karyotypes of the affected cousins were determined as 46,XY or XX,-15,+der(14),rcp(14;15)(q11.2;q13). Therefore, they were considered to have an identical cytogenetic abnormality: a partial trisomy of the 14pterq11.2 segment and a partial monosomy of the 15pterq13 segment. Detailed clinical features of the proband and his affected cousin are described, main features associated with the Prader-Willi syndrome having been observed in both cousins. These observations support a definite relationship between the Prader-Willi syndrome and chromosome 15.     

De novo unbalanced translocations in Prader-Willi and Angelman syndrome might be the reciprocal product of inv dup(15)s

The 15q11-q13 region is characterized by high instability, caused by the presence of several paralogous segmental duplications. Although most mechanisms dealing with cryptic deletions and amplifications have been at least partly characterized, little is known about the rare translocations involving this region. We characterized at the molecular level five unbalanced translocations, including a jumping one, having most of 15q transposed to the end of another chromosome, whereas the der(15)(pter->q11-q13) was missing. Imbalances were associated either with Prader-Willi or Angelman syndrome. Array-CGH demonstrated the absence of any copy number changes in the recipient chromosome in three cases, while one carried a cryptic terminal deletion and another a large terminal deletion, already diagnosed by classical cytogenetics. We cloned the breakpoint junctions in two cases, whereas cloning was impaired by complex regional genomic architecture and mosaicism in the others. Our results strongly indicate that some of our translocations originated through a prezygotic/postzygotic two-hit mechanism starting with the formation of an acentric 15qter->q1::q1->qter representing the reciprocal product of the inv dup(15) supernumerary marker chromosome. An embryo with such an acentric chromosome plus a normal chromosome 15 inherited from the other parent could survive only if partial trisomy 15 rescue would occur through elimination of part of the acentric chromosome, stabilization of the remaining portion with telomere capture, and formation of a derivative chromosome. All these events likely do not happen concurrently in a single cell but are rather the result of successive stabilization attempts occurring in different cells of which only the fittest will finally survive. Accordingly, jumping translocations might represent successful rescue attempts in different cells rather than transfer of the same 15q portion to different chromosomes. We also hypothesize that neocentromerization of the original acentric chromosome during early embryogenesis may be required to avoid its loss before cell survival is finally assured.     

Prader-Willi syndrome with an unusually large 15q deletion due to an unbalanced translocation t(4;15)

Prader-Willi syndrome (PWS) is a neurobehavioral disorder caused by deletions in the 15q11-q13 region, by maternal uniparental disomy of chromosome 15 or by imprinting defects. Structural rearrangements of chromosome 15 have been described in about 5% of the patients with typical or atypical PWS phenotype. An 8-year-old boy with a clinical diagnosis of PWS, severe neurodevelopmental delay, absence of speech and mental retardation was studied by cytogenetic and molecular techniques, and an unbalanced de novo karyotype 45,XY,der(4)t(4;15)(q35;q14),-15 was detected after GTG-banding. The patient was diagnosed by SNURF-SNRPN exon 1 methylation assay, and the extent of the deletions on chromosomes 4 and 15 was investigated by microsatellite analysis of markers located in 4qter and 15q13-q14 regions. The deletion of chromosome 4q was distal to D4S1652, and that of chromosome 15 was located between D15S1043 and D15S1010. Our patient's severely affected phenotype could be due to the extent of the deletion, larger than usually seen in PWS patients, although the unbalance of the derivative chromosome 4 cannot be ruled out as another possible cause. The breakpoint was located in the subtelomeric region, very close to the telomere, a region that has been described as having the lowest gene concentrations in the human genome.     

Interstitial telomeric repeats are not preferentially involved in radiation-induced chromosome aberrations in human cells

Telomeric repeat sequences, located at the end of eukaryotic chromosomes, have been detected at intrachromosomal locations in many species. Large blocks of telomeric sequences are located near the centromeres in hamster cells, and have been reported to break spontaneously or after exposure to ionizing radiation, leading to chromosome aberrations. In human cells, interstitial telomeric sequences (ITS) can be composed of short tracts of telomeric repeats (less than twenty), or of longer stretches of exact and degenerated hexanucleotides, mainly localized at subtelomeres. In this paper, we analyzed the radiation sensitivity of a naturally occurring short ITS localized in 2q31 and we found that this region is not a hot spot of radiation-induced chromosome breaks. We then selected a human cell line in which approximately 800 bp of telomeric DNA had been introduced by transfection into an internal euchromatic chromosomal region in chromosome 4q. In parallel, a cell line containing the plasmid without telomeric sequences was also analyzed. Both regions containing the transfected plasmids showed a higher frequency of radiation-induced breaks than expected, indicating that the instability of the regions containing the transfected sequences is not due to the presence of telomeric sequences. Taken together, our data show that ITS themselves do not enhance the formation of radiation-induced chromosome rearrangements in these human cell lines.