Parental origin and mechanisms of formation of triploidy: a study of 25 cases.

Triploidy is one of the most frequently observed chromosome abnormalities in spontaneous abortions in humans. The parental origin of the additional chromosome set is known to have a major impact on the phenotype of the foetuses and to result in differences in size and structure of the placenta. Early studies based on cytogenetic polymorphisms indicated a preponderant diandric origin of the triploidies; such detection method, however, is known to be prone to error. Other studies revealed a predominant digynic origin in cases with longer intrauterine survival. It is now thought that, to some extent, a detection bias in favour of cases with associated partial hydatidiform moles may account for the high incidences of diandric cases reported in some studies. Furthermore, depending on the gestational age of the cases analysed there may indeed be differences in the proportion of diandric and digynic triploidies. We investigated the parental origin and mechanisms of formation of triploidy in a group of 25 probands with gestational ages ranging from 8 to 37 weeks. DNA samples were extracted from foetal material and from blood samples of the parents, and were analysed using microsatellite markers. The parental origin of the triploidies was found to be maternal in 20 cases and paternal in 5. Regarding the digynic cases, an error at meiosis I was inferred in 10 cases, whereas in the other half an error occurred at meiosis II. All five diandric cases included in this study were found to be due to dispermy. No significant differences in the average maternal ages were found amongst the different subgroups of patients.

Isochromosomes 12p and 9p: parental origin and possible mechanisms of formation.

In a recent study Bugge et al and Kotzot et al reported that isochromosomes 18p originate mainly from maternal meiosis II nondisjunction, followed by misdivision. In order to determine if there is a common mechanism for isochromosome formation, three cases with mosaicism for an additional isochromosome 12p and three cases with tetrasomy 9p were studied. Two probands with isochromosomes 12p and the three cases with isochromosome 9p showed 3 alleles (two different maternal alleles and one paternal allele) at several loci mapping to distal 12p and 9p, respectively. Maternal heterozygosity for distal markers was reduced to homozygosity for markers closer to the centromere in both i(12p) cases and in one i(9p) case. For one patient with isochromosome 12p, the maternal band was clearly stronger than the paternal one at some loci, but two distinct maternal alleles were never seen. For one foetus and the patient with tetrasomy 9p, distal markers showed maternal heterozygosity. All proximal markers were not informative in these two i(9p) cases. Our findings indicate common features in different autosomal isochromosomes: the origin of the isochromosomes analysed in predominantly maternal; and a common mechanism appears to underlie their formation, namely due to meiosis II nondisjunction followed by a rearrangements leading to duplication of the short and loss of the long arm.

Recombinant balanced and unbalanced translocations as a consequence of a balanced complex chromosomal rearrangement involving eight breakpoints in four chromosomes.

We report on a family with a balanced complex chromosomal rearrangement (CCR) involving eight breakpoints between chromosomes 6, 7, 18, and 21 in the father. All three sons inherited one derivative chromosome from the father and in addition each inherited a different recombinant chromosome resulting in a partial trisomy 6q in the first, an apparently balanced karyotype in the second, and a partial trisomy 7q in the third son. Fluorescence in situ hybridisation (FISH) and microsatellite analysis were essential for the identification of the breakpoints. In addition, the results were confirmed by a 24-colour FISH experiment using the spectral karyotyping (SKYtrade mark) system. Paternal origin of the de novo CCR in the father was demonstrated for the first time by haplotype analysis. This is the second report of a CCR leading to simpler but unbalanced translocations in offspring as a consequence of recombination during gametogenesis, and the first report of a family case of CCR exhibiting as many as eight breakpoints in the transmitting carrier. The initial prediction that viable offspring would be quite unlikely had to be revised after the birth of three children. Genetic counselling of carriers of balanced complex rearrangements has to consider a higher probability for unbalanced recombinations than has been so far commonly assumed.

Clinical and molecular analysis of five inv dup(15) patients.

Five patients with inv dup(15) chromosomes were investigated with molecular probes on proximal 15q to determine the parental origin and extent of the duplicated segment. Cytogenetic investigation showed that four patients carried one and a fifth patient had two extra chromosomes derived from number 15 in all cells. In situ hybridization with a chromosome 15 library and a centromere 15 probe confirmed that the entire inv dup chromosomes were derived from chromosome 15. Molecular analysis using probes mapping in the region deleted in Prader-Willi syndrome (PWS) and Angelman syndrome (AS) patients implied that in at least two patients the extra chromosomes were asymmetric with one copy of the PWS region on the extra marker chromosome but two copies of the region centromeric to the PWS region. Three other cases had an inv dup(15) with two extra copies of the PWS region, but in one of these, heteromorphisms clearly demonstrated that the two centromeres derived from two different chromosomes. The inv dup(15) presumably resulted from an illegitimate recombination event between two different chromosomes 15 in most or all of these cases. All patients showed a maternal origin of the duplicated chromosome. The clinical severity appears to be associated with dosage of the PWS/AS region rather than with differences in the extent of the duplicated segment.

Trisomy first, translocation second, uniparental disomy and partial trisomy third: a new mechanism for complex chromosomal aneuploidy.

A 2-year-old, short, microcephalic and developmentally retarded boy revealed a pattern of multiple minor anomalies, hypospadias and a dysplastic right kidney. Maternal age at delivery was 41 years. His karyotype showed two cell lines, one apparently normal, the other with a 1p+ chromosome. FISH examinations showed that the segment attached to 1p was from chromosome 16, and molecular investigations disclosed maternal heterodisomy 16, except for the segment (16)(pter-->p13.1) for which there was mosaicism between trisomy and uniparental disomy (UPD). Most likely, the zygote was trisomic for chromosome 16 due to a maternal meiosis I nondisjunction; a somatic rearrangement would have then occurred at an early postzygotic stage whereby a segment of the paternal chromosome 16 was translocated onto 1p. Subsequently, the paternal chromosomes 16 and 16p- had been lost in the normal and the translocation cell line, respectively. The chromosome aberration was detected secondary to the disclosure of maternal UPD 16 because of the demonstration of a paternal band at several loci on distal 16p. This case shows that chromosome aberrations may be formed in a more complicated manner than primarily assumed. Hence, the phenotype might also be due to underlying factors such as UPD or undetected mosaicism in addition to the more obvious implications of the chromosome rearrangement itself (e.g. partial trisomy).

Biparental inheritance of chromosome 21 polymorphic markers indicates that some Robertsonian translocations t(21;21) occur postzygotically.

Robertsonian translocations between acrocentric chromosomes are the most common structural chromosomal rearrangements in humans and many other organisms, and several mechanisms for their formation have been proposed. We have analyzed highly informative DNA polymorphisms in a family with a non-mosaic de novo Robertsonian translocation 21q;21q, to determine the parental origin of the two 21q arms of the rearranged chromosome. The genotypes indicated a biparental origin, i.e. one 21q was paternal and the other maternal. These results imply that in some cases the formation of the rob(21q;21q) occurs in the zygote or in the first few postzygotic mitotic divisions.

Molecular study of 45,X conceptuses: correlation with clinical findings.

The parental origin of the single X in 45 cases (40 liveborns and 5 fetuses) with a 45,X karyotype was studied using polymorphic DNA probes. The single X was paternal in origin (Xp) in 10 cases (22.2%) and maternal (Xm) in 35 cases (77.8%). Y chromosome material was detected in 1 out of the 35 cases with a 45,Xm constitution. Analysis of parental ages and clinical data of the patients with respect to the origin of the single X revealed no significant differences between the origins.

A 5-year-old girl with interstitial deletion of 3p14: clinical, psychologic, cytogenetic, and molecular studies.

An interstitial deletion of segment 3p14 (breakpoints 3p21.1 and 3p13) was found in a 5-year-old short, microcephalic, and mentally retarded girl with a pattern of anomalies comprising a wide forehead, short up-slanting palpebral fissures, small nose and ears, hypoplasia of larynx, trachea, and bronchi, clino- and camptodactyly of little fingers, and sacral vertebral fusion. Determination of microsatellites mapping to the deleted segment demonstrated that the deletion had occurred in the paternal germ line. This is the seventh patient with a deletion of 3p14, and comparison with the six previously reported cases does not yet allow definition of a specific pattern of minor and major anomalies.

Molecular studies of translocations and trisomy involving chromosome 13.

Twenty-four cases of trisomy 13 and one case with disomy 13, but a de novo dic(13,13) (p12p12) chromosome, were examined with molecular markers to determine the origin of the extra (or rearranged) chromosome. Twenty-one of 23 informative patients were consistent with a maternal origin of the extra chromosome. Lack of a third allele at any locus in both paternal origin cases indicate a somatic duplication of the paternal chromosome occurred. Five cases had translocation trisomy: one de novo rob(13q14q), one paternally derived rob(13q14q), two de novo t(13q13q), and one mosaic de novo t(13q13q)/r(13). The patient with a paternal rob(13q14q) had a maternal meiotic origin of the trisomy; thus, the paternal inheritance of the translocation chromosome was purely coincidental. Since there is not a significantly increased risk for unbalanced offspring of a t(13q14q) carrier and most trisomies are maternal in origin, this result should not be surprising; however, it illustrates that one cannot infer the origin of translocation trisomy based on parental origin of the translocation. Lack of a third allele at any locus in one of the three t(13q13q) cases indicates that it was most likely an isochromosome of postmeiotic origin, whereas the other two cases showed evidence of recombination. One balanced (nontrisomic) case with a nonmosaic 45, -13, -13, +t(13;13) karyotype was also investigated and was determined to be a somatic Robertsonian translocation between the maternal and paternal homologues, as has been found for all balanced homologous Robertsonian translocations so far investigated. Thus, it is also incorrect to assume in de novo translocation cases that the two involved chromosomes are even from the same parent. Despite a maternal origin of the trisomy, we cannot therefore infer anything about the parental origin of the chromosomes 13 and 14 involved in the translocation in the de novo t(13q14q) case nor for the two t(13;13) chromosomes showing a meiotic origin of the trisomy.

Unbalanced translocation, t(18;21), detected by fluorescence in situ hybridization (FISH) in a child with 18q- syndrome and a ring chromosome 21.

We report on an 8-year-old girl with minor anomalies consistent with 18q- syndrome and mild developmental delay. Initially cytogenetics showed a terminal deletion of chromosome 21 with mosaicism for a small ring chromosome 21 as the only apparent karyotypic abnormality: mos 45,XX,-21/46,XX,+r(21) (48%/52%). Further studies including FISH and DNA analysis demonstrated a de novo unbalanced translocation of chromosomes 18 and 21 with the likely breakpoints in 18q23 and 21q21.1. Most of 21q was translocated to the distal long arm of one chromosome 18, and this derivative 18 appeared to lack 18q23-qter. The small ring chromosome 21 [r(21)], present in only 52% of the patient's blood lymphocytes, did not appear to be associated with the abnormal phenotype since all 13 chromosome 21 markers that were examined in genomic DNA were present in 2 copies, and the phenotype of the patient was consistent with the 18q- syndrome. The karyotype was reinterpreted as mos 45,XX,-18,-21,+der(18) t(18;21) (q23;q21.1)/46,XX,-18,-21,+der(18) t(18;21) (q23;q21.1), +r(21) (p13q21.1) (48%/52%). These results demonstrate the power of FISH in conjunction with DNA analysis for examination of chromosome rearrangements that may be misclassified by traditional cytogenetic studies alone.

Pre-implantation embryos of Chinese hamster. I. Incidence of karyotype anomalies in 226 control embryos.

Karyotyes were determined in 226 pre-implantation embryos (4--8-cell stages) of Chinese hamster. The study was carried out under controlled natural breeding conditions, without superovulation and with the embryos developing in their mothers. A total of 5.3% karyotypically abnormal embryos were found. Over half, 3.1%, were due to ploidy mutations, 5 cases of triploidy and 2 cases of haploidy. Only 0.9% genome mutations were present, consisting of one autosomal trisomy and one autosomal monosomy. Structural aberrations were found in 1.8%, half of these probably due to a balanced maternal aberration and the rest appearing the mosaic condition only. These results are compared with the scarce body of mammalian data from the literature. Compared with the situation in man, the spontaneous aberration rates in the Chinese hamster and other experimental mammals are extremely low. This may be due, in part, to optimal timing of copulation in respect to estrus and ovulation prevailing in these animals but not in man. The low spontaneous aberration rate in the reported system is a valuable asset for purposes of mutagen testing.

Pre-implantation embryos of Chinese hamster. II incidence and type of karyotype anomalies after treatment of the paternal post-meiotic germ cells with an alkylating mutagen.

Ninety-two male Chinese hamsters were treated with a single, sub-lethal dose of the alkylating cytostatic drug Trenimon. After 3--23 days they were mated with untreated females. The great majority of the male germ cells had been exposed to the mutagen while they were in the highly sensitive post-meiotic spermatid stage. The karyotypes of the resulting embryos were studied in the 4--8-cell stage. Out of 221 analysable embryos, 24.4% had aberrant karyotypes. Ploidy and genome mutations were, at 0.9% each, within control limits. Structural aberrations, involving one or several chromosomes, were present in 23.6% of the embryos (control 1.8%). 51% had a single aberrant centric element. The most frequent aberration types were deletions (54%), dicentrics (16%), translocations inversions and complex rearrangements with 22% and rings with 7%. About one-third of the cells, in addition, contained acentric fragments.

An interstitial deletion of proximal 8q (q11-q13) in a girl with Silver-Russell syndrome-like features.

Silver-Russell syndrome (SRS) is characterized by pre- and postnatal growth retardation, a fine, triangular face, a high frontal hairline and prominent forehead, clinodactyly of the fifth fingers, and sometimes asymmetry of face, trunk and extremities. In a 10-year-old girl referred for SRS, cytogenetic examination disclosed a microdeletion of band 8q12. Dosage analysis of Southern blots hybridized to 8q markers revealed a deletion of three loci: MOS, D8S96 and D8S108, all mapping to 8q11-q12, however the deletion did not include PLAT (8q12-q11). PCR analysis of the D8S166 microsatellite (8q11-q12) showed the lack of paternal inheritance, indicating that the deletion occurred in the paternal chromosome. The patient showed prenatal and postnatal growth retardation, mild developmental delay, microcephaly, a triangular face with high frontal hairline, shallow supraorbital ridges, hypoplastic alae nasi, small and prominent ears, prominent lateral palatine ridges, clinodactyly and brachymesophalangy of the fifth fingers. There were normal female genitalia and no asymmetry or detectable malformations. Screening of 19 other patients with the SRS for a similar cytogenetic and/or molecular deletion at 8q12 and for uniparental disomy 8 was negative. However, 8q12 still remains as one potential locus for a gene whose mutations may cause the clinical findings of SRS and which could be included in a larger deletion in a proband who has additional mild mental retardation.

Trisomy 17p11-pter: unbalanced pericentric inversion, inv(17)(p11q25) in two patients, unbalanced translocations t(4;17)(q27;p11) in a newborn and t(4;17) (p16;p11.2) in a fetus.

Three patients and one fetus with almost complete trisomy 17p due to familial rearrangements are described. Two patients followed unbalanced transmission of a familial pericentric inversion, and one patient and one fetus were due to unbalanced segregation of familial translocations. In the inversion family, another two patients with multiple malformations had died before chromosome examination could be performed. The pattern of congenital anomalies as revealed from eleven cases of trisomy 17p11-pter include as the most prominent features: prenatal growth retardation, microcephaly, downslanting palpebral fissures, small mouth, small mandible, poorly shaped ears, short and webbed neck, genital hypoplasia, clinodactyly of fingers, crowding of toes, a high incidence of congenital heart defects and hernias. Postnatal survival is short mainly in patients with congenital heart defects. From the age of about 6 years onward, clinical findings become more distinct, with some signs of Charcot-Marie-Tooth neuropathy (pes cavus, adducted thumbs, dorsiflexed hallux, camptodactyly and limitation of movements in different joints), and the nose gets narrow and sharp, with hypoplastic alae.

[Prenatal chromosome study: cause, incidence and prevention of erroneous determination of various study samples]. / Pränatale Chromosomenuntersuchung: Ursache, Häufigkeit und Prävention von Fehlbestimmungen an verschiedenen Untersuchungsmaterialien.

False diagnoses in prenatal chromosomal analysis are rare; but the causes are numerous. They can be specific for amniocentesis, chorionic villus samples or independent of the tissue. The sources of mistake and their frequencies are discussed in detail.

Partial trisomy of chromosome 18 (pter----q12) following a familial 18;21 translocation rcp(18;21)(q12;q11).

A 1-year-old boy with trisomy 18 (pter----q12) following a paternal balanced translocation revealed microcephaly, a pattern of minor dysmorphic features including upslanting narrow palpebral fissures, receding forehead, large nose and receding mandible, cryptorchidism, flexion contractures of fingers, a cardiac malformation and moderate mental retardation. While pure trisomy 18p generally goes along with a near-normal phenotype, additional trisomy of only a short segment of the proximal long arm 18 has a distinct negative influence on the phenotype, as seen in our proband.

Ultrasound screening for chromosomal anomalies in the first trimester of pregnancy.

For the last 6 years, sonographic signs for excessive fluid accumulation in the backs of 10- to 12-week-old fetuses have been looked for prior to transabdominal chorionic biopsy. In 1400 pregnancies, subsequent karyotype analyses revealed 28 cases of Down syndrome. In 15 (= 54 per cent), a large fluid cushion over most of the back had been documented at the time of biopsy. Only a few chromosomally normal fetuses with the same peculiarity were observed. The cushion was also present in fetuses with trisomies 18 and 13, and in Turner syndrome. Systematic first-trimester screening for nuchal fluid accumulation seems to be a recommended method for the detection of Down syndrome and other chromosome anomalies in young pregnant women at low risk. It compares favourably with current methods of maternal serum screening performed at 16-18 weeks which require a higher number of invasive procedures.