trisomy 21 ( also referred to as down syndrome ) is the most frequent chromosomal abnormality at birth .
it affects approximately one in every 800 newborns 1 . in france , prenatal diagnosis is always suggested when indicated by ultrasound findings and maternal serum marker assays .
about 92% of cases of down syndrome are due to the presence of an extra , free chromosome 21 in all cells , whereas 23% are due to mosaicism and 56% are due to unbalanced heterologous or homologous acrocentric rearrangements ( of which rob(14;21 ) and rob(21;21 ) are the most common ) . around 70% of rob(14;21 ) rearrangements are de novo , and this value is over 95% for rob(21;21 ) 2,3 .
prenatal diagnosis of recurrent down syndrome due to unbalanced , de novo homologous acrocentric rearrangement is rare . here
, we report on an exceptionally rare case : a recurrent rea(21;21 ) down syndrome that was probably caused by maternal germ - line cell mosaicism of the rearrangement . a 27-year - old woman
( gravida 2 para 1 ) requested a prenatal diagnosis after the detection of increased nuchal translucency in the fetus .
their first pregnancy had been uneventful and the infant ( a girl ) was healthy .
chorionic villus sampling at 13  weeks of gestation ( w.g . ) revealed an abnormal karyotype , with a rearrangement between two chromosomes 21 ( compatible with a rob(21;21 ) event ) on the 16 metaphases analyzed .
karyotyping enabled us to diagnose down syndrome in a female fetus ( 46,xx,+21,rob(21;21)(q10;q10 ) ) . following a request by the couple
given that ( 1 ) the parents ' first child was healthy , ( 2 ) the parental karyotypes ( in blood lymphocytes ) were normal , and ( 3 ) the vast majority of rob(21;21 ) are de novo , the subsequent genetic counseling was reassuring with regard to future pregnancies .
the patient requested a prenatal diagnosis because of increased nuchal translucency and distended jugular lymphatic sacs .
chorionic villus sampling at 13 w.g . revealed rob(21;21 ) down syndrome , with a 46,xy,+21,rob(21;21)(q10;q10 ) karyotype on the 15 metaphases analyzed .
the pregnancy was subsequently terminated . in order to determine the parental origin of this apparently recurrent rob(21;21 ) , we performed a microsatellite marker analysis ( focusing on two shorts tandem repeats : the d21s11 and penta d loci on chromosome 21 ) in the fetus and parents .
this showed that the rearrangement involving chromosomes 21 was an isochromosome 21 of maternal origin , rather than a rob(21;21 ) ( fig.1 ) . with a view to detecting potential low - grade mosaicism in peripheral blood cells from the mother ,
we performed fish analysis ( focusing on dscr1 locus on 21q22 ) of 1000 interphase nuclei ; all were normal .
representative electrophoretograms ( using the microsatellite markers d21s11 and penta d ) for the couple and the aborted male and female fetuses , suggesting an i(21q ) of maternal origin in the first ( female ) fetus and the second ( male ) fetus . considering the area under the curve , the electrophoretograms with the d21s11 marker revealed ( 1 ) a maternal duplication of allele 28 and a paternal allele 33.2 in the female fetus , and ( 2 ) a maternal duplication of allele 28 and a paternal allele 29 in the male fetus .
the area under curve for the penta d marker shows ( 1 ) a maternal duplication of allele 11 and a paternal allele 12 in the female fetus , and ( 2 ) the presence of three alleles in the male fetus ( a paternal allele 13 , a maternal allele 11 and an allele 12 whose origin can not be proven but which is very probably maternal , in view of the results for the d21s11 marker ) .
the results of these microsatellite marker analyses suggest that the i(21q ) is of maternal origin in both fetuses .
the karyotypes of the two aborted fetuses were thus determined to be 46,xx , i(21)(q10 ) and 46,xy , i(21)(q10 ) , respectively .
what mechanism led to the maternally confined mosaicism ? and what genetic counseling should be given to the couple 's healthy daughter ?
as mentioned in the introduction , about 95% of rob(21;21 ) translocations arise de novo 4 .
an isochromosome 21 ( i(21q ) ) is derived from replication of a single chromosome 21 , whereas a homologous rob(21;21 ) involves translocation between two different homologous chromosomes 21 .
conventional cytogenetic analysis is unable to distinguish between rob(21;21 ) and i(21q ) , and so the use of molecular assays of microsatellite markers is required . in the literature ,
most of the initial reports of rob(21;21 ) down syndrome feature an i(21q ) 5 .
found that 17 of 19 cases were i(21q ) , with roughly equivalent maternal ( n  =  10 ) and paternal ( n  =  9 ) inheritance .
when additionally considering four previous studies , 32 of 36 cases were found to be i(21q ) 57 .
again , the frequencies of maternally inheritance ( n  =  17 ) and paternal inheritance ( 15 ) were equivalent .
kovaleva and shaffer 8 later analyzed published cases of rea(21;21 ) down syndrome and found that the prevalence of mosaicism among the parents was unusually high .
however , the recurrence rate for de novo rea(21;21 ) ( rob(21;21 ) or i(21q ) ) down syndrome is reportedly low .
9 studied 112 families in which the child 's down syndrome was caused by a de novo t(gq;gq ) event .
this event was a 21q21q translocation in 77 families , and none of the couples had a second pregnancy affected by down syndrome .
however , steinberg et  al . identified three couples ( one mother and two fathers ) with low - grade mosaicism for rea(21;21 )
translocation down syndrome in blood samples and an additional couple in which the mother carried a pericentric inversion of chromosome 21 .
given that four of the 112 couples ( 3.6% ) had an abnormal karyotype , the researchers estimated the risk recurrence to be between 2% and 3.6% .
caution should be exercised when counseling that the recurrence risk of this event is remote  . a few reports on couples experiencing recurrent de novo rea(21;21 ) down syndrome have demonstrated that mosaicism for rea(21;21 ) in the skin or ovaries of one parent was combined with normal or low - level mosaicism in blood lymphocytes 9 ( table1 ) .
reports of apparent de novo der(21;21 ) , parental mosaicism and the presence or absence of recurrence with regard to our present case ( with a healthy child born 2  years before the first i(21q ) event , the recurrence of a maternally inherited i(21q ) , and normal peripheral lymphocyte karyotypes in both parents ) , what advice should be given ?
since the occurrence of another fetus with the same karyotype can not be ruled out , preimplantation genetic screening may provide this couple with the certitude that a baby will be free of down syndrome .
even though we did not study the woman 's ovarian tissue , the features of the present case suggest the existence of germ - line cell mosaicism with at least three different oocyte populations : those containing a chromosome 21 ( giving rise to normal offspring ) , those containing an i(21q ) ( responsible for recurrent i(21q ) down syndrome ) , and those lacking a chromosome 21 ( responsible for the four miscarriages due to monosomy 21 ) .
however , the couple 's healthy daughter may have resulted from the rescue of an initial i(21q ) down syndrome zygote ( and thus will require careful genetic counseling ) or the rescue of a monosomy 21 .
one possible explanation for the occurrence of the maternal i(21q ) is the reduplication of one of the chromosomes 21 , forming a second trisomic cell line with an isochromosome ( 46,xx , i(21q),+21 ) .
subsequent rescue of the trisomy would have generated a third , balanced cell line with the isochromosome ( 45,xx , i(21q ) ) . in view of the present case and
the literature data , the vast majority of apparently de novo cases of rea(21;21 ) feature an i(21q ) with maternal or paternal inheritance .
recurrence of rea(21;21 ) down syndrome does not therefore appear to be such a rare event , with at least 14 recurrences in 105 reported 9 and documented cases ( table1 ) , that is to say an incidence of 13.3% .
we consider that ( 1 ) the previously reported recurrence rate of 2% is a marked underestimation and ( 2 ) that clinical practice and genetic counseling should be revised accordingly .
when parental mosaicism is evidenced by high - quality cytogenetic analyses , one can even question whether the first - reported cases of rea(21;21 ) in parents with an apparently normal karyotype were truly de novo . in line with kovaleva and shaffer 's report 8
, we recommend performing a dna microsatellite marker analysis as soon as rea(21;21 ) down syndrome is detected ; this should enable one to distinguish between a true rob(21;21 ) and an i(21q ) .
when an i(21q ) is detected , extensive cytogenetic analysis of at least 500 cells ( from a variety of tissues , if possible ) might enable the detection of low - grade parental mosaicism and thus the provision of appropriate genetic counseling for an inherited rearrangement .
 key clinical messagewe report a rare case of recurrent trisomy 21 caused by an isochromosome 21q and what is very likely to be maternal germ - line cell mosaicism . over 90% of cases of rob(21;21 ) reported in the literature 
 are due to an isochromosome 21q , with a risk of recurrence of more than 10% . 
Introduction
Conflicts of Interest