Preimplantation genetic diagnosis (PGD) of chromosomal microdeletion/duplication by array comparative genomic hybridization (aCGH)
Fiorentino F., Biricik A., Bono S., Spizzichino L., Dellorusso C., Michiorri S., Nuccitelli A., Baldi M.
“GENOMA”- Molecular Genetics Laboratory, Via Po, 102 00198 Rome – Italy
Microdeletions and microduplications are found in patients with a wide variety of phenotypes, including Mendelian diseases as well as common complex traits, such as developmental delay/intellectual disability or behaviour problems. In several cases, these genomic copy number variants (CNVs) may be characterized by an incomplete penetrance and a highly variable phenotype, ranging from apparently normal phenotype to mental retardation/learning disability, challenging the ability to predict the exact phenotypic outcome and making prenatal diagnosis an ethically debatable option.
In this study we describe the clinical application of preimplantation genetic diagnosis (PGD) by array comparative genomic hybridization (aCGH) technique for detection of submicroscopic chromosomal imbalances as small as ~1 Mb on embryos, as well as aneuploidy screening of all 24 chromosomes.
MATERIAL & METHODS
The microdeletion/duplication included in the study were: Leri-Weill dyschondrosteosis deletion (3 Mb), Charcot-Marie-Tooth disease type 1A duplication (0.6 Mb), 16p13.11 duplication (1.5 Mb); 22q11.2 deletion/duplication (0.6 Mb). The validation of the PGD protocol for each chromosomal abnormality involved aCGH testing of groups of 5-6 lymphocytes derived from patients carrying the CNV. Embryo biopsy was carried out at blastocyst stage (day 5). Cells were first lysed and DNA amplified by whole genome amplification (WGA). WGA products were then processed in 24h by aCGH using Focus Constitutional microarrays, BlueGnome. Normal euploid embryos were then selected for transfer on day 6.
RESULTS: Nine PGD cycles were carried out for 9 couples. Overall, 43/44 (97.7%) embryos were successfully diagnosed, 25.6% (11/43) were normal, 20.9% (9/43) were carrying the submicroscopic chromosomal imbalances and normal for aneuploidy, 23.3% (10/43) had aneuploidy and were unbalanced, 30.2% (13/43) were normal but showed aneuploidies. The latter embryos would have been selected for transfer if only testing for microdeletion/duplication would have been performed, highlighting the importance of screening embryos also for aneuploidy. Eleven embryos were transferred, resulting in 5 currently ongoing clinical pregnancy.
CONCLUSIONS: The results obtained demonstrates the reliability of aCGH for detection of submicroscopic chromosomal imbalances as small as 0.6 Mb. This assay provides a valuable alternative and an improvement on the currently available fluorescence in-situ hybridisation (FISH) or polymerase chain reaction (PCR)-based PGD approaches. Unlike alternative techniques, aCGH does not require preclinical validation before each PGD cycle, allowing a rapid starting of the IVF treatments. It also provides the added benefit of simultaneous aneuploidy screening of all 24 chromosomes.