Paternal Isodisomy 13 in a Normal Newborn InfantAfter Trisomy Rescue Evidenced byPrenatal Diagnosis

Anna Soler,* Ester Margarit, Rosa Queralt, Ana Carrio, Dolors Costa, David Gomez, andFrancisca BallestaServei de Genetica, Hospital Clınic, Barcelona, Spain

Maternal and paternal uniparental disomyof chromosome 13 have been associatedwith normal phenotypes. We report on anew case of paternal isodisomy 13 in a phe-notypically normal girl. Prenatal diagnosishad shown a 46,XX,-13,der(13;13) karyotypein chorionic villi and a 45,XX,der(13;13)karyotype in amniocytes and fetal blood.Molecular studies demonstrated that the denovo der(13;13) was an isochromosome 13 ofpaternal origin. This observation supportsthe lack of imprinting effects on chromo-some 13 and trisomy rescue as a mechanismleading to uniparental disomy in cases in-volving isochromosomes. Am. J. Med. Genet.90:291–293, 2000. © 2000 Wiley-Liss, Inc.

KEY WORDS: uniparental disomy; imprint-ing; isochromosome 13; tri-somy rescue

INTRODUCTIONUniparental disomy (UPD) is known as the inheri-

tance of a pair of homologous chromosomes from onlyone parent [Engel, 1980]; it may manifest as completeisodisomy, when both members of the pair are identi-cal, as complete heterodisomy when both homologuesfrom the same parent are transmitted, or as segmentalheterodisomy or isodisomy when two homologues fromone parent are transmitted but meiotic recombinationresults in regions of isodisomy.

UPD can result from several mechanisms: gametecomplementation (one nullisomic and one disomic ga-mete), monosomy correction (duplication of the single

chromosome in a monosomic conceptus), trisomy res-cue (loss of the third homologue in a trisomic concep-tus), and other postfertilization errors [Spence et al.,1988]. The most frequent mechanism seems to be tri-somy rescue, due to the occurrence of confined placen-tal mosaicism in 1 to 2% of gestations undergoing pre-natal diagnosis by chorionic villus sampling (CVS)[Ledbetter et al., 1992; Association of Clinical Cytoge-neticists Working Party on Chorionic Villi in PrenatalDiagnosis, 1994; Hahnemann and Verjeslev, 1997].Mosaicism involving a trisomic cell line can have a so-matic (postmeiotic) or a meiotic origin [Wolstenhome,1996; Robinson et al., 1997]. In the latter case, the riskof UPD is one in three.

To our knowledge, two previous cases of maternalUPD 13 and four of paternal UPD 13 have been re-ported [Slater et al., 1994, 1995; Stallard et al., 1995;Jarvela et al., 1998; Berend et al., 1999], all of theminvolving a rob(13;13) or isochromosome 13q. In allcases the phenotype was normal, suggesting lack ofmaternal and paternal imprinting effect on chromo-some 13.

We present here the case of a phenotypically normalgirl born after a prenatal diagnosis of 46,XX,-13,der(13;13) in chorionic villi and 45,XX,der(13;13) inamniocytes and fetal blood, in which molecular analy-sis showed the presence of paternal isodisomy 13.

CLINICAL REPORT AND METHODS

A 32-year-old woman was referred for prenatal diag-nosis because she was a carrier of Becker musculardistrophy. She had a previous affected son and an af-fected male fetus detected in her second pregnancy,which was terminated after the prenatal diagnosis.She underwent chorion biopsy at 10 weeks’ pregnancyfor cytogenetic and molecular diagnosis. The finding ofa translocation trisomy 13 in an apparently normal fe-tus by ultrasound examination suggested the need foran amniocentesis that was performed at 14 weeks’pregnancy. At the same time blood was obtained fromthe parents for karyotyping. Because at amniocentesisthe karyotype was balanced, the pregnancy continued,but a cordocentesis was carried out at 20 weeks in or-der to exclude a fetal mosaic. The pregnancy went on to

Contract grant sponsor: Fondo de Investigaciones Sanitariasdel Ministerio de Sanidad de Espana; Contract grant number:FIS98/0162; Contract grant sponsor: Fundacio Catalana Sın-drome de Down–Marato; Contract grant number: TV3/02.

*Correspondence to: Anna Soler, Servei de Genetica, HospitalClınic, Villarroel 170, 08036 Barcelona, Spain.E-mail: asoler@clinic.ub.es

Received 24 May 1999; Accepted 25 October 1999

American Journal of Medical Genetics 90:291–293 (2000)

© 2000 Wiley-Liss, Inc.

term, and a phenotypically normal girl was born at 39weeks. When the baby was 3 months old, molecularstudies were carried out to search for uniparental di-somy. Part of the CVS sample had been sent to anothercenter for the diagnosis of Becker muscular distrophy,where it was stored for later studies, according to theircurrent protocol for female prenatal samples of reces-sive X-linked genetic diseases.

The karyotype from chorion villi was obtained by thesemidirect technique; 25 G-banded chromosomespreads were examined. Amniotic fluid cells were cul-tured with the in situ method; 16 G-banded chromo-some spreads from a total of 14 clones of six differentcultures were examined.

Lymphocytes from peripheral blood of the parents,and from fetal cord blood, were cultured with standardtechniques. Fifty G-banded metaphases from eachadult and 100 from the fetus were examined.

Genomic DNA was isolated from the peripheral bloodof both parents and the proposita, using standard tech-niques [Miller el al., 1988]; DNA was also obtainedfrom frozen fixed chorionic villi. DNA analysis was per-formed using chromosome 13 microsatellite markers:D13S115, D13S118, D13S162, D13S122, D13S71, andD13S158. Primers were synthesized using primer se-quences available through the Genome Data Base(GDB). Each polymerase chain reaction (PCR) amplifi-cation was performed in a total volume of 12.5 ml con-taining 100 ng of genomic DNA, 10 mM Tris-HCl (pH8.4), 50 mM KCl, 1.5 mM MgCl2, 0.2 mM of each dNTP,10 pmol of each primer, and 0.5 units of Taq I DNApolymerase. PCR reactions were performed in a 9600Turbo Perkin Elmer-Cetus Thermocycler using the fol-lowing parameters: denaturation for 4 min at 94°C, 30cycles for 30 sec at 94°C, 30 sec at 55–60°C, 30 sec at72°C, and a final extension for 4 min at 72°C. PCRproducts were separated on 5–7% denaturing poly-acrylamide gels, which were silver stained and dried.

RESULTS

In all metaphases obtained from chorionic villi thekaryotype was 46,XX,-13, der(13;13)(q10;q10), result-ing in trisomy 13. The amniotic fluid cells had lost thefree chromosome 13, since the karyotype obtained was45,XX,der(13;13)(q10;q10) in all metaphases exam-ined. The karyotypes of both parents were normal, sothe der(13;13) was considered a de novo event. Thefetal cord cells confirmed the balanced karyotype foundin amniocytes.

Table I shows the DNA analysis results. Of six mi-

crosatellites tested, three were informative and showedin the proposita a unique band corresponding to one ofthe paternal alleles. The sample from chorionic villishowed biparental bands, but the intensity of the pa-ternal allele band was clearly greater than that of thematernal one (Fig. 1). Because all the proposita’s lociwere homozygous, we concluded that she had an iso-chromosome 13 of paternal origin, which demonstratedpaternal isodisomy 13, although the possibility of par-tial heterodisomy caused by two crossovers betweentwo informative markers could not be excluded.

The proposita was born at 39 weeks of gestation. Hermeasurements were: weight 2,950 g (50th centile),length 47 cm (<10th centile), and head circumference34 cm (50th centile). She appeared healthy and free ofbirth defects.

At 3 months she weighed 5,520 g (50th centile), shemeasured 57 cm (10th centile), and her head circum-ference was 40 cm (50th centile). Brain and abdominalultrasonography, and echocardiography were all nor-mal. Her psychomotor development was also normal.

DISCUSSION

Carriers of isochromosomes originated from acrocen-tric chromosomes are likely to have UPD [James et al.,1994; Engel, 1995]. Both maternal and paternal UPDhave been reported for all the acrocentric chromo-somes, but only UPD for chromosomes 14 and 15 showsan imprinting effect on the phenotype of the carrier. AllUPD 13 cases previously reported have been pheno-typically normal, suggesting that there are neither ma-ternal nor paternal imprinted genes on chromosome13. Our case, which presents a paternal isodisomy 13,supports this suggestion.

In our case, the most probable explanation of theabnormality is that a normal ovum was fertilized by a

TABLE I. Microsatellite Analysis*

Locus Mother CVS Proposita Father

D13S115a 1,1 1,2,2 2,2 2,3D13S118 1,1 1,1,1 1,1 1,1D13S162 1,2 1,1,1 1,1 1,3D13S122a 2,3 1,1,3 1,1 1,3D13S71 1,3 1,1,1 1,1 1,2D13S158a 1,2 2,3,3 3,3 3,3

* Familial genotypes are coded by numbers. Allele size increases from 1(smaller) to 3 (larger).a Indicates the informative microsatellites.

Fig. 1. Segregation of the informative microsatellite markers D13S115,D13S122, and D13S158, with their approximate position along the chro-mosome 13 idiogram. For each marker, samples from mother, CVS,proposita (NB), and father are shown from left to right. The paternal allelein the CVS shows more intensity than the maternal one, which is absent inthe proposita sample.

292 Soler et al.

sperm carrying the isochromosome, resulting in a tri-somic zygote, just as it was detected in chorion villi.The formation of the isochromosome 13 does not seemto be postzygotic, because it was present in all cell linesexamined. Then, in the cell lineage leading to the em-bryo there occured the loss of the maternal chromo-some 13, restoring a balanced karyotype, just as it wasdetected in amniocytes and fetal blood cells.

In conclusion, this study adds more evidence of thelack of imprinting effects on chromosome 13 and illus-trates how trisomy rescue is a mechanism leading touniparental disomy in cases involving isochromosomes.

ACKNOWLEDGMENTS

We thank family L.M. for their kind cooperation.This study was supported by a grant from Fondo deInvestigaciones Sanitarias del Ministerio de Sanidadde Espana (FIS98/0162) to Dr. Anna Soler and a grantfrom FCSD-Marato TV3/02 to Dr. Francisca Ballesta.

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