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TRISOMY 13 MOSAICISM AT PRENATALDIAGNOSIS: DILEMMAS IN INTERPRETATION

. 1*, . 2 . . 1

1Victorian Clinical Genetics Services, Royal Children’s Hospital, Melbourne, Australia2Department of Cytogenetics, Royal Women’s Hospital, Melbourne, Australia

Received 6 January 1997Revised 16 June 1997Accepted 20 June 1997

SUMMARY

We describe six cases of trisomy 13 mosaicism detected at prenatal diagnosis. Most level I and level II trisomy 13mosaicism detected at prenatal diagnosis is pseudomosaicism or confined placental mosaicism. Rarely, low-levelmosaicism at chorionic villus sampling or amniocentesis reflects a true fetal mosaicism. In this case, a normalphenotype is a possible, but not a certain, outcome. Genetic counselling is not straightforward. ? 1998 John Wiley& Sons, Ltd.

Prenat. Diagn. 18: 45–49, 1998

: trisomy 13; mosiacism; prenatal diagnosis; pseudomosaicism

INTRODUCTION

Trisomy 13 mosaicism is infrequently detectedat prenatal diagnosis. Hsu et al. (1992) recordedtrisomy 13 in 3·3 per cent of single-cell pseudo-mosaicism and 4 per cent of multiple-cell pseudo-mosaicism detected at amniocentesis. The crucialinterpretation to be made is to distinguish betweenpseudomosaicism and true fetal mosaicism. If it isthe latter, what predictions can be made about thephenotype? We report six cases whose study makesa contribution to these questions. The study wasprompted by the presentation of case 2 below atour clinic, and we reviewed the remaining fivecases of trisomy 13 mosaicism identified at theRoyal Women’s Hospital, Melbourne, Australia,over a 5-year period.

CASE REPORTS

We accept a definition of level I mosaicism as asingle abnormal cell found in a prenatal test, while

level II is two or more cells with the same chromo-somal abnormality in a dispersed culture in asingle flask, or a single abnormal colony in anin situ culture. Level III mosaicism is two ormore cells or colonies with the same chromosomeabnormality, distributed over two or moreflasks (Gardner and Sutherland, 1996). ‘Pseudo-mosaicism’ is, by definition, mosaicism that doesnot involve either fetal or extrafetal tissues in vivo,but exists only in cultured cells in vitro. Mosaicisminvolving extrafetal tissues in vivo is ‘confinedplacental mosaicism’.

True mosaicism

Case 1.—A 38-year-old gravida 5, para 2,abortia 2 Malaysian mother had amniocentesis at14 weeks in her fifth pregnancy and level II47,XX,+13/47,XX,+22/46,XX mosaicism wasshown. Of three cultures in three separate flasks,one in situ culture showed 46,XX in all cells; onedispersed culture showed 47,XX,+13 in 7 out of50 cells; and the remaining dispersed cultureshowed 5 out of 50 cells with 47,XX,+22. Sheproceeded to fetal blood sampling at 17 weeksand 100/100 cells were 46,XX. The baby was

*Correspondence to: Dr Martin Delatycki, Victorian Clini-cal Genetics Services, Royal Children’s Hospital, FlemingtonRoad, Parkville, Victoria 3052, Australia.

CCC 0197–3851/98/010045–05 $17.50? 1998 John Wiley & Sons, Ltd.

born at 38 weeks, of weight 3010 g (40th centile),length 50·8 cm (75th centile), and head circum-ference 33·3 cm (20th centile). A cord bloodsample showed 47,XX,+13 in 3 out of 100 cells.Other than jaundice due to ABO blood groupincompatibility which required phototherapy,and an atrial septal defect demonstrated onechocardiography, no clinical problems wereidentified. Cranial and renal ultrasonographywas normal.Upon review at age 5Y months, she presented as

a normal baby with no dysmorphic features andsatisfactory neurodevelopmental progress andgrowth indices (weight 6·5 kg, head circumference40·9 cm, both approximately on the 25th centile).A skin fibroblast study showed 60/60 cells with a46,XX karyotype. Parental karyotypes werenormal. By age 3 years, the atrial septal defecthad spontaneously closed. At the most recentexamination, at age 3 years 6 months, the childwas entirely normal, both morphologically anddevelopmentally. Her growth indices were asfollows: height 97·7 cm (50th centile), weight13·9 kg (25th centile), head circumference 49 cm(50th centile).

Case 2.—Chorionic villus sampling (CVS) foradvanced maternal age in a 42-year-old womanshowed trisomy 13 in 7 of 20 cells in one long-termculture and in 33 of 50 cells in the second culture(level III mosaicism). Mosaic trisomy 13 wassubsequently observed at amniocentesis, with47,XX,+13 in 1/28 colonies of an in situ study, andat cordocentesis 1/400 lymphocytes had the47,XX,+13 karyotype. A decision was made tocontinue the pregnancy. At birth, the child was notdysmorphic, apart from a simple helix unilaterally,and had normal growth indices (weight 75th per-centile, length 50th percentile, head circumference75th percentile). Neurological examination wasunremarkable.An umbilical cord blood sample taken at birth

showed 1/150 cells with 47,XX,+13 and theremaining 149 cells 46,XX. Karyotyping of placen-tal samples revealed 47,XX,+13 in 2/32 cells in theamnion and 0/27 cells in the smooth chorion.Long-term culture of two villus biopsies, one adja-cent to the cord insertion, the other near theplacental margin, showed 47,XX,+13 in 1/30 cellsand 3/30 cells, respectively.The child is currently 17 months of age and

neurological development and growth parametersremain entirely within the normal range.

Possible pseudomosaicism/confined placentalmosaicism

Case 3.—This child was born following a preg-nancy in which amniocentesis had been done foradvanced maternal age, which demonstrated47,XX,+13[4]/46,XX[23] in one dispersed cellculture. Three other independent cultures did notshow any cells with trisomy 13 and the interpret-ation was of a level II mosaicism. Ultrasounds at18 and 31 weeks’ gestation were normal. Fetalblood sampling was declined. Examination of thechild at birth was normal and the karyotype on acord blood was 46,XX.

Case 4.—A 35-year-old woman had an amnio-centesis elsewhere at 14 weeks’ gestation and alevel II trisomy 13 mosaicism was identified. Twoof three independent cultures (one in situ, onesuspension) showed 46,XY. The third, a suspen-sion culture, had 6/20 metaphases with47,XY,+13, with the remaining 14/20 being46,XY. She came to our service for fetal bloodsampling at 18 weeks and 100/100 cells were46,XY; at repeat amniocentesis, 50/50 coloniesfrom an in situ study were 46,XY. The pregnancycontinued and at birth the child was morphologi-cally normal. There was a normal karyotype(46,XY) in umbilical blood (45 cells) and fromthree different placental sites.

Case 5.—A 31-year-old woman had CVS for alinkage study in the setting of a family history ofhaemophilia A. Karyotyping was done as part ofthe study and this showed 47,XX,+13 in all 10cells in short-term culture and in all 15 cells inlong-term culture. The pregnancy was terminatedand fetal skin was karyotyped, revealing 46,XX in50 cells. No other tissues were available for furtherstudy once this result became available. The CVSmaterial was restudied, and 35 further cells fromthe long-term culture were analysed. Five were46,XX and the remaining 30 were 47,XX,+13.

Case 6.—A 38-year-old woman had CVS per-formed at 11 weeks’ gestation for advanced mater-nal age. Trisomy 13 mosaicism was observed intwo independently established long-term cultures,with 5 of 15 cells showing 47,XY,+13 in the firstculture and 4 of 15 cells showing 47,XY,+13 in thesecond. The patient proceeded to amniocentesis at15 weeks’ gestation. Fetal anatomy on ultrasoundat that time was unremarkable. Examination of 32

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colonies from an in situ study revealed a 46,XYkaryotype in all. The child was recently born atterm with a birth weight of 3·1 kg (15th centile),length of 49 cm (25th centile), and a head circum-ference of 33·5 cm (10th centile). The baby was notdysmorphic. Further cytogenetic studies were notundertaken.

DISCUSSION

We report six cases of trisomy 13 mosaicism atprenatal diagnosis, of which two (cases 1 and 2)showed demonstrably true low-grade mosaicismand four (cases 3–6) may represent pseudo-mosaicism or confined placental mosaicism. Incase 1, we assume that there had been a very lowgrade fetal–placental mosaicism, with no discern-ible effect on the phenotype. The existence of thetrisomy 22 cell line at amniocentesis is unexplainedand may represent cultural artefact. The picture incase 2 was similar, with the trisomic cell lineapparently comprising only a very small fractionof the placenta and an extremely small fraction ofat least those fetal and neonatal tissues that weresampled.Trisomy 13 mosaicism detected at prenatal diag-

nosis, especially if at a low level, is a dilemma forgenetic counsellors and families. Is this true mosai-cism, involving the fetus; and if so, what would bethe phenotypic outcome? The phenotype of truemosaicism for trisomy 13 mosaicism is very broad,although it is rare that normal intellect is present inreported cases (Delatycki and Gardner, 1997).Albeit follow-up investigations (amniocentesisafter CVS, fetal blood sampling after an amnio-centesis) may show a normal karyotype andultrasonography may indicate a morphologicallynormal fetus, it can never be excluded that there isa true minor cell line in the fetus. If this ‘minorline’ were to involve brain tissue, a significanteffect on the intellect could eventuate. Cytogeneticfindings can be incongruent between tissue types,as case 1 illustrates. The amniotic fluid sample(which includes cells shed from the fetal skin)showed mosaicism and yet this was not confirmedat skin fibroblast culture, and the normal karyo-type from the fetal blood was followed by thefinding of mosaicism on cord blood analysis. Poss-ibly, this may be due to sampling variation inwhich the true fraction of abnormal cells is verylow (for example, in counting 60 cells, a true 3 percent mosaicism would be undetected in 14 per cent

of analyses). Equally, at least in skin, there couldbe a variegated tissue distribution, with the biop-sied region happening to be 46,N while other areasof skin may have included 47,+13 cells.Hsu et al. (1992), in a major review of mosaicism

detected at amniocentesis, documented 15 cases oftrisomy 13 mosaicism. Eleven of these were fol-lowed up and five were recorded as abnormal,although the details of the abnormalities are notgiven. In the U.K. CVS study (Association ofClinical Cytogeneticists Working Party, 1994),there was one case of mosaic trisomy 13 detectedamong 88 mosaic CVS cases. Here, non-mosaictrisomy 13 was found on karyotyping of fetaltissue after a spontaneous abortion. Wang et al.(1994) reported five cases of mosaic trisomy 13 atCVS. In three cases, the trisomic cell line waspresent on direct preparation but not on cultureand a normal outcome was recorded. Another casewas of mosaicism for i(13q) in the direct prep-aration but not on culture and a follow-up amnio-centesis showed 46,XX. The fifth case showed47,XX,+13[12]/48,XX,+7,+13[6]/46,XX[2] ondirect preparation and 48,XX,+7,+13[10]/46,XX[63] on culture, and amniocentesis yielded anormal result. The outcome of the pregnancy isnot recorded. In the U.S. collaborative study onCVS (Ledbetter et al., 1992), there were seven casesof mosaic trisomy 13; five in the direct preparationonly, the other two on culture only. Five wereconsidered confined placental mosaicism andthe other two (one each from the direct prep-aration and culture groups) were therapeuticallyaborted with no follow-up on fetal tissue. Two ofthese cases were mosaic for the presence of aRobertsonian translocation involving two copiesof chromosome 13q (one each from the directpreparation and culture groups), both resultingin a liveborn normal child. Malin et al. (1987)reported three cases of level II mosaic trisomy 13diagnosed on amniocentesis, each of which wasfollowed by the birth of a child with a normalkaryotype and normal 1-year follow-up examina-tion, and these authors made the presumption ofpseudomosaicism. Fejgin et al. (1992) recordeda case in which termination was elected on thefinding of 4/10 trisomy 13 cells in amniotic fluidand 1/160 at fetal blood sampling, but at patho-logical examination 25 cells from kidney, dia-phragm, heart, blood, skin, and placenta wereall 46,XX.Other reports of mosaic trisomy 13 with a

normal outcome include one case from Smidt-

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Table I—Features of the six cases of prenatally diagnosed mosaic trisomy 13

CaseNo.

Initialprenataltest done

Indicationfor

prenataltest

Cytogeneticoutcome of

test

Furtherprenataltest(s)

Cytogeneticoutcome ofthis test(s)

Ultrasoundfindings

Pregnancyoutcome

Postnatalcytogeneticanalysis

Finalinterpretation

of themosaicism

Phenotypeof thechild

1 Amnio AMA Level II47,XX,+13/47,XX,+22/46,XX

FBS 46,XX NAD Liveborninfant

CB47,XX,+13/46,XX (3/100)SF 46,XX

Truemosaicism

ASD,otherwiseNAD

2 CVS AMA Level III47,XX,+13/46,XX

AmnioFBS

Amnio47,XX,+13/46,XX (1/28)FBS47,XX,+13/46,XX(1/400)

NAD Liveborninfant

CB47,XX,+13/46,XX (1/150)Placenta47,XX,+13/46,XX (6/119)

Truemosaicism

Unilateralsimplehelix,otherwiseNAD

3 Amnio AMA Level II47,XX,+13/46,XX

Declined — NAD Liveborninfant

CB 46,XX PS/CPM NAD

4 Amnio AMA Level II47,XX,+13/46,XY

FBS 46,XY NAD Liveborninfant

CB 46,XYPlancenta46,XY

PS/CPM NAD

5 CVS Linkagestudy forhaemophiliaA

Non-mosaic47,XX,+13initially.Later47,XX,+13/46,XX (5/35)

— — — TOP Fetal skin46,XX

PS/CPM —

6 CVS AMA Level III47,XY,+13/46,XY

Amnio 46,XY NAD Liveborninfant

Not done PS/CPM NAD

AMA=Advanced maternal age; Amnio=amniocentesis; CB=umbilical cord blood sample; CVS=chorionic villus sampling; FBS=fetal blood sampling; NAD=noabnormality detected; ASD=atrial septal defect; PS/CPM=possible pseudomosaicism/confined placental mosaicism; TOP=termination of pregnancy.

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Jensen et al. (1993) and three cases from Pittaliset al. (1994). Roland et al. (1994) had one case ofmosaic trisomy 13 among 26 cases of confinedplacental mosaicism. They found no differencein outcome comparing these 26 cases with 52age- and parity-matched controls but the specificoutcome of the mosaic trisomy 13 case is notstated. Of 39 cases of mosaic aneuploidy reportedby Fryburg et al. (1993), there was one oflevel III trisomy 13 mosaicism which was provento be genuine on a karyotype undertakenpost-termination.In a major review by the European Collabora-

tive Research on Mosaicism in CVS (Hahnemannand Vejerslev, 1997), 192 examples of mosaicismstudied in considerable detail were gleaned from1415 cases of CVS mosaicism or non-mosaic fetal–placental discordance from a total of 92 246 CVSprocedures. Of these 192, 15 had trisomy 13 mosai-cism. In 13 of these, the fetus was normal; thetrisomic line was confined to trophoblast in sevenof the 13, in two it was confined to villus mesen-chyme, and in two both trophoblast and villuscontained the trisomic line. In the remaining twoexamples, all three lineages (fetal, trophoblast,mesenchyme) showed trisomy, either mosaic ornon-mosaic.Thus, it appears that most but not all level II

47,+13/46 mosaicism may be pseudomosaicism,due to cultural artefact. Our case 1 is instructive inmaking the point that even if the fetus actually hasa 47,+13 cell line in the context of a level IImosaicism, it is not necessarily the case that thiswould have a noticeable phenotypic effect. AsMalin et al. (1987) point out, ‘couples need toknow and often rely on the outcome of similarcases in order to make an informed decisionregarding the continuation or termination ofthe pregnancy’, and the cases that we describe addto the limited body of knowledge bearing on theissue.Two other issues warrant mention. Could

mosaicism be the result of a full trisomy self-correcting to disomy, and therefore could uni-parental disomy (UPD) be present? Slater et al.(1994, 1995) have shown that neither maternal norpaternal UPD 13 is associated with an abnormalphenotype, and thus, to the best of our presentunderstanding, the possibility of UPD 13 need notbe a concern. Secondly, since there is a theoreticalpossibility that mosaicism could include gonadaltissue, parents will need to understand that theirchild should, at an appropriate age, learn of the

advisability of prenatal diagnosis for him orherself.Case 5 demonstrates that what can initially

appear as non-mosaic trisomy 13 could never-theless be a cytogenetically normal fetus, althoughwith mosaic trisomy 13 in extrafetal tissues. Thisexperience led us to change our laboratory policyand we now count 30, rather than 15, cells inlong-term CVS culture in cases of apparent non-mosaic trisomy 13 associated with normal ultra-sound findings. If the finding of mosaicism hadbeen known initially, the couple would have beenoffered the option of proceeding to amniocentesis.Case 6 appears to be an example of confined

placental mosaicism with the trisomy 13 cell linenot being detected at amniocentesis.In conclusion, the finding of mosaic trisomy 13

at prenatal diagnosis often represents pseudo-mosaicism or confined placental mosaicism, buteven true fetal mosaicism is not necessarily associ-ated with congenital defects and/or mental abnor-mality, at any rate in the context of low-level(single-digit percentage) mosaicism. If there areno trisomy 13 cells at fetal blood samplingand if ultrasonography is normal, an optimisticapproach can be taken. Nevertheless, low-levelmosaicism of the fetus cannot be completelyexcluded and in that case, neither can thepossibility of some effect on the phenotype andintellectual function.

We thank J. A. Sullivan of the DunedinHospital for details of the cytogenetics in the firstamniocentesis and postnatal studies in case 4.

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Delatycki, M., Gardner, R.J.M. (1997). Three cases oftrisomy 13 mosaicism and a review of the literature,Clin. Genet., 51, 403–407.

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