RS46(DXS548) genotyping of reproductive cells: approaching preimplantation testing of the fragile-X syndrome

Development of pre-implantation genetic testing protocol for monogenic disorders (PGT-M) of Hb H disease

Hb H disease is the most severe form of α-thalassemia compatible with post-natal life. Compound heterozygous α0-thalassemia- SEA deletion/α+-thalassemia- 3.7kb deletion is the commonest cause of Hb H disease in Thailand. Preimplantation genetics testing for monogenic disorders (PGT-M) is an alternative for couples at risk of the disorder to begin a pregnancy with a healthy baby. This study aims to develop a novel PCR protocol for PGT-M of Hb H disease- SEA/-3.7kb using multiplex fluorescent PCR. A novel set of primers for α+-thalassemia- 3.7kb deletion was developed and tested. The PCR protocol for α0-thalassemia- SEA deletion was combined for Hb H disease- SEA/-3.7kb genotyping. The PCR protocols were applied to genomic DNA extracted from subjects with different thalassemia genotypes and on whole genome amplification (WGA) products from clinical PGT-M cycles of the families at risk of Hb Bart's. The results were compared and discussed. The results showed three PCR products from α+-thalassemia- 3.7kb primer set, and three from α0thalassemiaSEA primer set. The results were consistent with the known thalassemia genotypes. The novel -α3.7 primers protocol was also tested on 37 WGA products from clinical PGT-M cycles giving accurate genotyping results and a satisfying amplification efficiency with the ADO rates of 2.7%, 0%, and 0% for HBA2, HBA1, and internal control fragments, respectively. This novel PCR protocol can precisely distinguish Hb H disease- SEA/-3.7kb from other genotypes. Additionally, this is the first PCR protocol for Hb H disease- SEA/-3.7kb which is optimal for PGT-M.

Identification of microsatellite markers <1 Mb from the FMR1 CGG repeat and development of a single-tube tetradecaplex PCR panel of highly polymorphic markers for preimplantation genetic diagnosis of fragile X syndrome

PURPOSE

To develop a single-tube polymerase chain reaction (PCR) panel of highly polymorphic markers for preimplantation genetic diagnosis (PGD) of fragile X syndrome (FXS).

METHODS

An in silico search was performed to identify all markers within 1 Mb flanking the FMR1 gene. Selected markers were optimized into a single-tube PCR panel and their polymorphism indices were determined from 272 female samples from three populations. The single-tube assay was also validated on 30 single cells to evaluate its applicability to FXS PGD.

RESULTS

Thirteen markers with potentially high polymorphism information content (PIC) and heterozygosity values were selected and optimized into a single-tube PCR panel together with AMELX/Y for gender determination. Analysis of 272 female samples confirmed the high polymorphism (PIC > 0.5) of most markers, with expected and observed heterozygosities ranging from 0.31 to 0.87. More than 99% of individuals were heterozygous for at least three markers, with 95.8% of individuals heterozygous for at least two markers on either side of the FMR1 CGG repeat.

CONCLUSION

The tetradecaplex marker assay can be performed directly on single cells or after whole-genome amplification, thus supporting its use in FXS PGD either as a standalone linkage-based assay or as a complement to FMR1 mutation detection.Genet Med 18 9, 869-875.

Preimplantation genetic diagnosis for fragile X syndrome using multiplex nested PCR

Fragile X syndrome is caused by a dynamic mutation in the FMR1 gene. Normal individuals have <55 CGG repeats in the 5 untranslated region, premutation carriers have 55-200 repeats and a full mutation has >200 repeats. Female carriers are at risk of having affected offspring. A multiplex nested polymerase chain reaction protocol is described for preimplantation genetic diagnosis (PGD) of fragile X syndrome with simultaneous amplification of the CGG-repeat region, the Sry gene and several flanking polymorphic markers. The amplification efficiency was > or =96% for all loci. The allele dropout rate in heterozygotic females was 9% for the FMR1 CGG-repeat region and 5-10% for the polymorphic markers. Amplification failure for Sry occurred in 5% of single leukocytes isolated from males. PGD was performed in six patients who underwent 15 cycles. Results were confirmed in all cases by amniocentesis or chorionic villous sampling. Five clinical pregnancies were obtained (31% per cycle), four of which resulted in a normal delivery and one miscarried. This technique is associated with high efficiency and accuracy and may be used in carriers of full mutations and unstable high-order premutations.