Rapid in situ harvesting and cytogenetic analysis of perinatal tissue samples

Parental mosaic trisomy 21 detected following maternal cell contamination of an amniotic fluid specimen from a normal male pregnancy

We report a case of maternal mosaic trisomy 21 ascertained at prenatal diagnosis as a result of maternal cell contamination of an amniotic fluid sample. A 34 year old female was referred for karyotyping because of a previous trisomy 21 pregnancy. Chromosome analysis of primary in situ cultures showed a karyotype of 47,XX, + 21[6]/46,XY[32]/46,XX[2]. Molecular testing demonstrated maternal cell contamination of the amniotic fluid sample and G-banded karyotyping of maternal blood showed that 3/200 cells had trisomy 21, consistent with the mother being a Down syndrome mosaic. A normal male baby with a 46,XY chromosome complement was delivered at 30 weeks. This case emphasises the need for close collaboration between cytogenetic and molecular genetics laboratories in resolving unusual cases of mosaicism.

Derivation of human embryonic stem cell lines

Human embryonic stem cells (hESC) are undifferentiated cells derived from an early embryo that can grow in vitro indefinitely, while retaining their capability to differentiate into specialized somatic cell types. Over the last decade there has been great interest in derivation and culture of these cells, as they can potentially provide a supply of readily available differentiated cells and tissues of all types to be used for therapeutic purposes in cell transplantation in humans, as well as for other medical uses such as drug discovery. The source of hESC lines is usually excess human embryos from in vitro fertilization treatments, although novel ways of producing hESCs have been suggested recently. The actual methods of hESC derivation have not changed greatly since the first report by Thomson et al. in 1998 . However, the main emphasis over the last several years has been in finding defined conditions for derivation and culture of hESCs, because to enable the clinical use of hESC for cell transplantation, the use of animal derived biological components is no longer acceptable. For basic research, the aim is to replace even human derived materials with completely defined systems. In this paper we describe methods utilized in our laboratory for hESC derivation and describe two studies conducted in an attempt to improve derivation efficiency and to enable research outcomes to be achieved using fewer embryos.

Mosaic trisomy 6 and maternal uniparental disomy 6 in a 23-week gestation fetus with atrioventricular septal defect

Trisomy 6 is seen in early miscarriages in association with an intact, empty amniotic sac or as a pseudomosaic in amniotic fluid cultures. We report the finding of mosaic trisomy 6 in a 23-week-gestation pregnancy terminated because of intrauterine death. The post-mortem showed a well formed macerated male fetus with an atrioventricular septal defect and an exomphalos. By conventional cytogenetics, trisomy 6 was found in 12 out of 25 (48%) fibroblast colonies from fetal skin and 21 out of 32 (66%) colonies derived from amnion, while the remaining metaphases showed an apparently normal male karyotype. Molecular genetic studies on DNA from uncultured fetal skin and cord samples using polymorphic microsatellite repeat sequences showed no evidence of trisomy 6, but demonstrated that both chromosome 6 homologs were of maternal origin consistent with maternal uniparental disomy (UPD).

Molecular investigation of a dicentric 13;17 chromosome found in a 21-week gestation fetus with multiple congenital abnormalities

We report a 21-week gestation fetus terminated because of multiple congenital abnormalities seen on ultrasound scan, including ventriculomegaly, possible clefting of the hard palate, cervical hemivertebrae, micrognathia, abnormal heart, horseshoe kidney and a 2-vessel umbilical cord. On cytogenetic examination, the fetus was found to have a male karyotype with 45 chromosomes with a dicentric chromosome, which appeared to consist of the long arms of chromosomes 13 and 17. Molecular genetic investigations and fluorescence in situ hybridization (FISH) unexpectedly showed that the derivative chromosome contained two interstitial blocks of chromosome 17 short arm sequences, totalling approximately 7 Mb, between the two centromeres. This effectively made the fetus monosomic for approximately 15 Mb of 17p without the concurrent trisomy for another chromosome normally seen following malsegregation of reciprocal translocations. It also illustrates the complexity involved in the formation of some structurally abnormal chromosomes, which can only be resolved by detailed molecular investigations.

Characterization of a small supernumerary marker chromosome as r(8) at prenatal diagnosis by MFISH

OBJECTIVE

To identify the mosaic marker chromosome detected in amniotic fluid cells of a 26-year-old woman, with raised triple test values and an ultrasound scan, which showed a fetus with echogenic bowels.

METHODS

Routine karyotyping with G- and C-banding was carried out for both, amniotic fluid at 18 weeks of gestation as well as fetal blood at 22(+6) weeks. Peripheral blood of both parents was karytoyped. MFISH and the all centromeric human probe were used on fetal lymphocytes to identify the marker chromosome.

RESULTS

Both parents had a normal karyotype. Amniotic fluid culture showed a de novo supernumerary marker chromosome (SMC) in 14 of the 30 colonies from four different cover slip cultures. The marker was confirmed in 50% of the fetal lymphocytes. G- and C-banding provided little information except that the marker had some heterochromatic material. The all centromeric human probe also showed the presence of a centromere along with a rim of euchromatic material. MFISH identified this ring marker to be belonging to chromosome 8.

CONCLUSIONS

SMCs with chromosome 8 have been shown to be variable phenotypes. Presence of only heterochromatic material seems to have no discernable phenotypic effects, but, with the presence of euchromatic material, mental and physical developmental delay has been reported. The parents opted to go ahead with the pregnancy and an apparently normal female baby was born at 40 weeks with no complications.

Functional Xp disomy and hypomelanosis of Ito

BACKGROUND

Hypomelanosis of Ito (HI) is a neurocutaneous phenotype that reflects different mosaicisms, including functional imbalances secondary to chromosome-X inactivation patterns in certain X;autosome translocation carriers.

METHODS

We assessed X inactivation patterns by means of the human androgen receptor (HUMARA) assay and BrdU labeling in affected and unaffected skin of a young female with HI and a de novo t(X;13)(Xp13q;Xq13p). PCR analysis was carried out in DNA extracted from uncultured and cultured skin, whereas the BrdU replication patterns were sought in cultured fibroblasts. Parental DNA was also tested. Fluorescence in situ hybridization (FISH) with X and 13/21 centromere probes (DXZ2 and D13Z1/D21Z1) and a cosmid for the X inactivation center were also performed to refine breakpoint assignments.

RESULTS

An X inactivation pattern implying functional Xpter-->q11 disomy was found in DNA extracted from uncultured hypopigmented skin, whereas preferential inactivation of the normal X was observed in uncultured normal skin as well as in cultured fibroblasts (after one passage) from both affected and unaffected skin areas. PCR analysis also showed paternal origin of the translocation. BrdU labeling of metaphases from hypopigmented and normal skin primary cultures showed der(Xq13p) to be inactive in about 25% of the cells. FISH revealed that der(Xp13q) had a compound centromere, whereas der(Xq13p) retained 13 centromere repeats but lacked X centromere sequences. Hence, breakpoints were assigned to Xq11 and 13q10. The X inactivation center cosmid gave a signal on both normal X and der(Xp13q), indicating that the inactivation center was not disrupted by the translocation.

CONCLUSIONS

These findings confirm that mosaic functional Xp disomy, rather than disruption of X-linked genes, is associated with HI and involvement of the central nervous system (CNS) in some carriers of a structurally balanced X;autosome translocation.

Molecular confirmation of germ line mosaicism for a submicroscopic deletion of chromosome 22q11

Submicroscopic deletions of chromosome 22q11 have been reported in a multiple anomaly syndrome variously labelled as velocardiofacial syndrome, conotruncal anomaly face syndrome, and Di George syndrome. Most 22q11 microdeletions occur sporadically, although in some cases the deletion may be transmitted. We describe two affected sibs with confirmed 22q11 deletions from unaffected parents who are not deleted. Haplotype analysis demonstrates that the deletion in the affected sibs has occurred on the same maternal chromosome 22. Furthermore, an unaffected sib was found to have inherited the same maternal haplotype at 22q11 in an undeleted form. This is the first molecular demonstration of germ line mosaicism for a microdeletion at chromosome 22q11 and highlights the need for caution in estimation of recurrence risks, even when constitutional deletions have been excluded on parental analysis.