BRIEF REPORT |
Prenatal Diagnosis and Molecular Cytogenetic Characterization of an Unusual Complex Structural Rearrangement in a Pregnancy Following Intracytoplasmic Sperm Injection (ICSI)
Institute of Human Genetics, Charité, Campus Virchow Clinic, Universitary Medicine, Berlin, Germany (MT,RV,HN,HT); Institute of Human Genetics and Anthropology, Jena, Germany (TL); and Praxis für Medizinische Genetik, Berlin, Germany (BB,LP)
Correspondence to: M. Trimborn, Institut für Humangenetik, Charité Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. E-mail: marc.trimborn{at}charite.de
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Summary |
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Key Words: prenatal diagnosis ICSI complex chromosomal rearrangement FISH CGH multicolor banding
COMPLEX CHROMOSOMAL REARRANGEMENTS (CCRs) are constitutional structural rearrangements having three or more breakpoints (Gardner and Sutherland 2004). The majority of cases reported in the literature show three-way exchanges, in which three segments from three chromosomes break off, translocate, and unite. There are very few reports on exceptional CCRs involving more than three breakpoints and/or different structural rearrangements than translocations like insertions or inversions (for review see Houge et al. 2003
and Kuechler and Claussen in this issue). A de novo, apparently balanced CCR detected at prenatal diagnosis requires exact molecular cytogenetic analysis and potentially additional molecular genetic analysis to enable proper genetic counseling. We report on the prenatal diagnosis and molecular cytogenetic characterization of an exceptional CCR in a pregnancy following intracytoplasmic sperm injection (ICSI).
A 40-year-old woman was referred for prenatal diagnosis and subsequent karyotyping of amniocytes because of assisted fertilization, maternal age, and a 4-mm choroid plexus cyst. The parents had opted for ICSI after a spermiogram revealed asthenoteratozoospermia with slightly decreased sperm number and explicitly reduced sperm motility, as well as conspicuous sperm morphology. The cause of the abnormal sperm morphology and motility could not be identified, and the karyotypes of both partners were normal at a resolution level of 400 bands per haploid chromosome set in lymphocytes.
Karyotyping of GTG-banded chromosomes from cultured amnion cells at the 18th week of pregnancy revealed a structurally rearranged karyotype with a derivative chromosome 5 consisting mainly of chromosome 5p material and a derivative chromosome 15 apparently composed of nearly the whole chromosome 15 and a rearranged part of the long arm of chromosome 5, presumably an inversion (Figure 1A). This aberrant karyotype was found in all metaphases analyzed from two independent cultures, indicating that the constitutional aberrant karyotype was present in the fetus. There were no hints of mosaicism in 50 cells analyzed. Staining the nuclear organizer regions (NORs) indicated that the der(15) contained interstitial active NORs (Figure 1B). After GTG-banding the karyotype appeared to be balanced, on the basis of 400 bands per haploid genome, and hence has been described as 46,XX,der(5)t(5;15)(q11.2;p12),der(15) t(5;15)(q11.2;p12)inv(5)(q11.2q15).
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To exclude a uniparental disomy (UPD) due to trisomy rescue after a possible 3:1 segregation with interchange trisomy for chromosome 15, we extracted genomic DNA from the amniocytes by standard procedures and performed methylation testing on the imprinting center of the Prader-Willi syndrome (PWS)/Angelman syndrome (AS)critical region, as described by Zeschnigk et al. (1997). Methylation-specific PCR revealed two bands for the fetal DNA and consequently excluded a UPD 15 for the PWS/AS critical region (Figure 2E).
A very rare complex chromosomal aberration was diagnosed after chromosome analysis of cultured amniocytes. Conventional and molecular cytogenetic analysis showed a balanced karyotype [46,XX,der(5)t(5;15) (q11.2;p12),der(15)t(5;15)(q11.2;p12)inv(5)(q11.2q15)]. The risk for phenotypic abnormalities associated with a de novo reciprocal two-breakpoint translocation is estimated to be 6%, 3% above the background risk of about 3% for malformations and/or defects in all pregnancies (Warburton 1991). The risk estimation in Warburton's study is slightly higher for balanced two-breakpoint inversions (9.4%) but probably merely due to a smaller case number. De novo, apparently balanced CCRs have presumably a high risk for mental retardation and malformations. The few postnatal studies suggest that the risk for abnormal phenotypes increases with a greater number of breakpoints (Madan et al. 1997
). In our case, the exact molecular cytogenetic characterization of the rearrangement did not point toward any imbalances. The rearrangement involves only three breakpoints that were defined by cytogenetic analysis: the breakpoint at 5q11.2 is involved in two events, the inversion and the translocation. Furthermore, the breakpoint in chromosome 15 localizes in the short arm of the chromosome, a region containing no relevant single-copy genes, but genes coding for rRNA. Thus, the risk for fetal malformations and developmental or mental retardation should not exceed the 9.4% given for the usual two-breakpoints inversion, although it concerns a complex rearrangement. Non-acrocentric chromosomes with interstitial NORs derived by translocations or insertions from acrocentric chromosomes are rare forms of chromosomal rearrangements without phenotypic effect, if there is no loss of euchromatic material from the non-acrocentric chromosome (Willatt et al. 2001
).
Accurate definition of the chromosomal breakpoints was essential to enable proper genetic counseling. After genetic counseling and additional inconspicuous ultrasound examination, the parents decided to continue with the pregnancy; meanwhile, a healthy girl was born, with a birth weight of 3.005 g and a birth length of 49 cm.
The chromosomal constitution will have considerable implications for the girl in the event of of a possible wish for having her own children in the future. If the rearrangement does not cause a general meiotic arrest, two segregant gametes may be produced by the translocation heterozygote that could lead to viable, severely affected offspring. Tertiary trisomy including the der(5) following 3:1 segregation results in trisomy for the whole short arm of chromosome 5, which is associated with severe malformations and mental retardation. Over 40 cases with partial trisomy of 5p have been reported to date (Grosso et al. 2002). The corresponding tertiary monosomy would result in a constellation with a cri-du-chat syndrome, a well-described partial monosomy resulting from deletion of the short arm of chromosome 5. A trisomy rescue after interchange trisomy for chromosome 15 would cause a PWS due to maternal UPD 15 in prospective offspring of the girl, while monosomy rescue of the paternal chromosome 15 following the corresponding interchange monosomy of chromosome 5 would result in an Angelman syndrome. Recombination products following crossing over in a meiotic inversion loop would lead to dicentric recombinant chromosomes or acentric fragments. These are lethal constellations in early embryonic development and therefore do not enhance the risk for abnormal offspring.
The findings could also have implications for further pregnancies of the couple itself, if one of the partners had a gonadal mosaicism. However, the specified unbalanced segregation products could be easily identified by karyotyping fetal cells in further pregnancies. Exclusion of a UPD 15 would require molecular genetic analysis of fetal DNA.
Although most familial complex rearrangements are transmitted through the mother, paternal transmission has been described in a few cases (Grasshoff et al. 2003; Gardner and Sutherland 2004
). Infertility in male carriers of complex translocations is well known and the abnormal spermiogram could possibly be due to gonadal mosaicism for the rearrangement. A potential gonadal mosaicismif existing in the fathercould be verified by analyzing segregation patterns in sperm analysis using fluorescence in situ hybridization.
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Acknowledgments |
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The authors thank Antje Gerlach and Britta Teubner for excellent technical assistance and Trudy Hocking and Sharon Bain for discussing the ISCN nomenclature.
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Footnotes |
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Received for publication May 18, 2004; accepted August 14, 2004
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