Rapid prenatal diagnosis of common trisomies: discordant results between QF-PCR analysis and karyotype analysis on long-term culture for a case of trisomy 18 detected in CVS.

OBJECTIVES: QF-PCR analysis can be used as a rapid test to diagnose primary trisomy in prenatal samples. Mosaicism in CVS detected by QF-PCR has previously been reported; however, no case has so far been reported in which the QF-PCR result was completely discrepant to that of the karyotype analysis from a long-term culture. METHODS: A CVS, referred because of a high serum screening risk of 1:10 for Down Syndrome and 1:110 for Edwards Syndrome, was tested by QF-PCR analysis and chromosome analysis of cultured cells. Subsequent analyses were carried out on a follow-up amniotic fluid sample and foetal tissue samples. RESULTS: Conflicting results were obtained between QF-PCR analysis on two independent fronds from the chorionic villi and chromosome analysis on cultured CVS. Cytogenetic and molecular analysis on a subsequent amniotic fluid sample indicated trisomy 18 with no evidence of mosaicism. Analysis of follow-up tissue confirmed trisomy in a foetal skin sample and mosaicism for trisomy 18 in four placental sites tested. CONCLUSION: We report here an apparently normal CVS QF-PCR result that was completely discrepant with the trisomy 18 positive karyotype result on long-term culture. This has important implications regarding our current testing protocol.

A new inherited interstitial deletion of the distal long arm of chromosome 4.

A 12-year-old boy showed mild dysmorphic features, late presentation of learning difficulties and behaviour problems, obesity, breast hypertrophy and bilateral slipped capital femoral epiphysis. His mother also has mild dysmorphic features, obesity, and a similar history of late presentation of learning difficulties and behaviour problems. Cytogenetic analysis demonstrated an inherited distal long arm deletion of one chromosome 4. The boy's karyotype was interpreted as 46,XY,del(4)(q32 q33)mat and the mother's karyotype as 46,XX,del(4)(q32 q33). This is the second report of an inherited distal 4q deletion and the first report of interstitial chromosome 4 deletion involving q32 q33 segments.

A DA/DAPI positive human 14p heteromorphism defined by fluorescence in-situ hybridisation using chromosome 15-specific probes D15Z1 (satellite III) and p-TRA-25 (alphoid).

We have investigated the use of the satellite III probe, D15Z1, as an alternative to DA/DAPI staining in the identification of chromosome 15-derived markers. The probe hybridises to the short arm of chromosome 15 under high stringency conditions. We have screened 100 randomly selected patients, by fluorescence in-situ hybridisation (FISH), using co-hybridisation experiments with D15Z1 and a whole chromosome library, pBS-15. 88 individuals showed the expected pattern of two D15Z1 signals on the p-arm of both homologues of chromosome 15, whereas 12 individuals showed an additional signal on a third acrocentric D-group chromosome. Sequential GTC banding and D15Z1 hybridisation revealed that in each case it was one homologue of chromosome 14 that was D15Z1 positive. This pattern always correlated with positive DA/DAPI staining. In contrast, the 15 centromere-specific alphoid probe, pTRA-25, gave the expected two signals on both homologues of chromosome 15 in every case. Thus, D15Z1 and DA/DAPI signals co-localize while pTRA-25 is 15 centromere-specific and should be applied for unequivocal identification of chromosome 15-derived markers in clinical studies. The chromosome 14 heteromorphism, as identified by D15Z1, and defined by pTRA-25, may have arisen by intrachromosomal amplification or interchromosomal exchange.

Meiotic and sperm chromosome analysis in a male carrier of an inverted insertion (3;10)(q13.2;p14p13).

A phenotypically normal male who fathered a son with the karyotype 46,XY,del(10)(p13) was found to be a balanced carrier of an inverted insertion (3;10) (q13.2;p14p13). Karyotyping five later pregnancies showed four to be unbalanced with respect to the insertion, one of which was also trisomic for chromosome 18. The latest pregnancy was balanced with respect to insertion but had the additional complexity of 47,XXY. In the light of six out of six chromosomally abnormal pregnancies, two of which potentially exhibit an interchromosomal effect, it was decided to investigate the gametic output of the father. Testicular biopsy and semen samples were obtained permitting both meiotic and sperm chromosome analysis. Information was thus obtained at three levels of gamete production, that is, prophase I pairing, chiasma frequency distribution at metaphase I, and sperm karyotypes. Electron microscope studies of synaptonemal complexes showed the rearranged chromosomes to pair fully in meiotic prophase I with no indication of the presence of an insertion. This non-homologous pairing of the inserted region was accompanied by an abnormal frequency distribution of pachytene substages. There was also a reduction in chiasma frequency throughout the genome. However, this did not lead to detectable autosomal univalence or abnormally high X/Y univalence. Thus, the trisomy 18 and XXY pregnancies are unlikely to reflect increased non-disjunctional rates either before or during the first meiotic division. Sperm karyotyping showed that the proportion of chromosomally balanced:unbalanced gametes did not differ from the theoretically expected 1:1. There was no evidence of any increase of unrelated abnormalities in the sperm, further indicating that the overall rate of meiotic non-disjunction was not increased above normal.

Chromosome in situ suppression hybridisation in clinical cytogenetics.

The use of chromosome in situ suppression hybridisation with whole chromosome libraries has previously been reported by various research laboratories to be an effective method of identifying specific human chromosomal material. As a clinical cytogenetic service laboratory we have used the technique as a complement to diagnosis by classical chromosome banding. In three examples of structural rearrangements the potential use of the 'chromosome painting' method is assessed for its ability to enhance the routine cytogenetic service currently available.