Successful PGD cycles for mosaic Robertsonian translocation carriers provide insights into the mechanism of formation of the derivative chromosomes.

Preimplantation genetic diagnosis (PGD) has been carried out for two couples with different mosaic Robertsonian translocations. Two PGD cycles for a mosaic 13;13 homologous Robertsonian translocation carrier resulted in the birth of a healthy child in each cycle, illustrating the importance of scanning G-banded preparations from homologous Robertsonian carriers for the presence of a normal cell line. One couple was referred for PGD because the male partner carried a mosaic 14;15 Robertsonian translocation with a normal cell line. A single PGD cycle resulted in the birth of a healthy child. Follow-up studies and extended FISH analysis of the carrier's lymphocytes detected three cell lines, two carrying different 14;15 Robertsonian chromosomes and one normal cell line. The two 14;15 Robertsonian chromosomes had different breakpoints in the proximal short arm regions. We suggest that the presence of the D15Z1 polymorphism on the short arm of one chromosome 14 mediated the post-zygotic formation of the two different Robertsonian chromosomes.

Meiotic outcomes of three-way translocations ascertained in cleavage-stage embryos: refinement of reproductive risks and implications for PGD.

Our study provides an analysis of the outcome of meiotic segregation of three-way translocations in cleavage-stage embryos and the accuracy and limitations of preimplantation genetic diagnosis (PGD) using the fluorescence in situ hybridization technique. We propose a general model for estimating reproductive risks for carriers of this class of complex chromosome rearrangement. The data presented describe six cycles for four couples where one partner has a three-way translocation. For male heterozygotes, 27.6% of embryos were consistent with 3:3 alternate segregation resulting in a normal or balanced translocation chromosome complement; 41.4% were consistent with 3:3 adjacent segregation of the translocations, comprising 6.9% reflecting adjacent-1 and 34.5% adjacent-2 segregation; 24.1% were consistent with 4:2 nondisjunction; none showed 5:1 or 6:0 segregation; the probable mode could not be ascertained for 6.9% of embryos due to complex mosaicism or nucleus fragmentation. The test accuracy for male heterozygotes was estimated to be 93.1% with 100% sensitivity and 75% specificity. With 72.4% prevalence, the predictive value was estimated to be 91.3% for an abnormal test result and 100% for a normal test result. Two of four couples had a healthy baby following PGD. The proportion of normal/balanced embryo could be significantly less for female heterozygotes, and our model indicates that this could be detrimental to the effectiveness of PGD. A 20% risk of live-born offspring with an unbalanced translocation is generally accepted, largely based on the obstetric history of female heterozygotes; we suggest that a 3% risk may be more appropriate for male carriers.

Multicolor banding remains an important adjunct to array CGH and conventional karyotyping.

BACKGROUND: Array comparative genomic hybridization (CGH) for high resolution detection of chromosome imbalance, and karyotype analysis using G-banded chromosomes for detection of chromosome rearrangements, provide a powerful diagnostic armoury for clinical cytogenetics. However, abnormalities detected by karyotype analysis cannot always be characterised by scrutinising the G-banded pattern alone, and imbalance detected by array CGH cannot always be visualised in the context of metaphase chromosomes. In some cases further techniques are needed for detailed characterisation of chromosomal abnormalities. We investigated seven cases involving structural chromosome rearrangements detected by karyotype analysis, and one case where imbalance was primarily detected by array CGH. Multicolor banding (MCB) was used in all cases and proved invaluable in understanding the detailed structure of the abnormalities. FINDINGS: Karyotype analysis detected structural chromosome rearrangements in 7 cases and MCB was used to help refine the karyotype for each case. Array CGH detected imbalance in an eighth case, where previously, G-banded chromosome analysis had reported a normal karyotype. Karyotype analysis of a second tissue type revealed this abnormality in mosaic form; however, MCB was needed in order to characterise this rearrangement. MCB provided information for the delineation of small deletions, duplications, insertions and inversions and helped to assign breakpoints which were difficult to identify from G-banded preparations due to ambiguous banding patterns. CONCLUSION: Despite the recent advance of array CGH in molecular cytogenetics we conclude that fluorescence in situ hybridization, including MCB, is still required for the elucidation of structural chromosome rearrangements, and remains an essential adjunct in modern diagnostic laboratories.