Preimplantation Genetic Diagnosis: An Overview

Dr. Laila Bastaki, MDConsultant of Medical GeneticsDirector of KMGC

The development of PGD is one of the most exciting and important milestones in the

history of Assisted Reproductive Technology

Preimplantation Genetic Diagnosis(PGD)PGD is a state-of-the-art procedure used in conjunction with In Vitro Fertilization (IVF)in which the embryo is tested for certain conditions prior to being placed in the womb of the woman.PGD was first reported in 1990. PGD combines the recent advances in molecular genetics and in assisted reproductive technology

Indications for PGD1. Chromosomal Disorders

NumericalChromosomal aneuploidy

StructuralInversionsTranslocationsDeletions and duplications

2. Gender determination for severe X-linked diseases with unknown gene

3. Severe monogenic diseases (cystic fibrosis, ß thalassaemia, sickle cell anemia, fragile X syndrome, myopathies)

4. PGD for HLA-typing (to allow selection of embryos that are histocompatible with live siblings)

HOW IS PREIMPLANTATION GENETIC DIAGNOSIS PERFORMED?

Technically demanding

Very ComplexRequires special skills

How is PGD performed?Ovarian

StimulationIVF

Blastomere Biopsy on Day 3

Genetic Analysis (FISH or

Molecular)

Transfer of Unaffected

EmbryoOutcomeClinically Normal

Baby

The Methods of Preimplantation Genetic Diagnosis

1. Remove a single cell from the 6-8-cell embryo using a fine glass needle to puncture the zona pellucida and aspirate the cell

- In skilled hands, this generally does not harm the developing embryo.

- Each cell is called a blastomere.

Blastomere Biopsy Video

The PGD process provides two categories of analysis

Fluorescence In Situ Hybridization (FISH).Gene Chip arrayPolymerase Chain Reaction (PCR)

Fluorescence In Situ Hybridization (FISH) • Using fluorescent probes specific for each

chromosome. • useful for identifying aneuploidies (incorrect

chromosome numbers) and translocations• procedure destroys the tested cell• limited number of chromosomes can be checked

simultaneously • some abnormalities undetectable

Screening aneuploids with multiple probes

Aneuploidy is the most frequent cause of spontaneous abortions

Gene chip array (Array CGH Analysis)

What is array-CGH analysis? Array-CGH allows the laboratory

to determine if the correct number of each chromosome is present in the egg or embryo

This technology simultaneously tests for all 24 chromosomes (1-22, X and Y)

What is array-CGH analysis?

With array-CGH, the amount of DNA present for each chromosome is compared to that of a normal standard, enabling us to detect monosomies (missing chromosomes), trisomies (extra chromosomes), and other abnormalities

What is array-CGH analysis?

Genetic testing for specific disease loci (PCR)

Polymerase chain reaction (PCR)

-The gene causing the disorder should be confirmed and tested in the couple -Amplification of DNA specific to a gene of interest (family history guides choice of genes)-Second round PCR used for specific exonic sequencing and/or linkage analysis (Fragment analysis)

Fragment analysis for HLA matching

Sequence analysis for a specific familial mutation

Examples of genetic disorders detectable via PCR-based tests:

- Tay Sachs (autosomal recessive)- Cystic fibrosis (autosomal recessive)- Huntington’s disease (autosomal dominant)- Thalassemias (autosomal recessive blood disorder)- Duchenne muscular dystrophy (X-linked recessive)- Spinal muscular atrophy (X-linked recessive)

As more genetic tests are developed as diagnostic tools, more will be used for predictive purposes in PDG.

Limitations of PCR-based tests:

• Both alleles may not amplify equally (allele dropout), leading to misdiagnosis or inconclusive results

• PCR-based tests only detect disorders at target loci; other mutations may exist elsewhere

• To accommodate these limitations, prenatal amniocentesis or chorionic villus sampling is usually recommended as a supplement to PGD.

Benefits of PGDReduction in the

Chance of Having a Child with Aneuploidy

Reduces the possibility of pregnancy termination following a prenatal diagnosis of a genetic disorder.

Risks Embryo damageOocyte and Embryo Biopsy are invasive procedures

Misdiagnosis The accuracy of the PGD for translocation is 90%.

False negative result False positive result

The chance for NO result The chance for mosaicism

IVF Risks

Not Achieving PregnancyThere may not be any normal

embryos available for transfer.The embryos may not implant and

develop even if they do not have the defect.

The workup for PGD is expensive and labor intensive

PGD can only detect a specific genetic disease in an embryo. It cannot detect many genetic disorders at a time and cannot guarantee that the fetus will not have an unrelated birth defect.

Causes of Misdiagnosis

Human Error Mislabeling, misidentification, misinterpretation Wrong embryo transfer Incorrect probes or primers

Technical Probe or primer failure Contamination (maternal, paternal, operator, carry-

over)Intrinsic (embryo) Mosaicism Allele drop out Uniparental Disomy

PGD & MalformationsEuropean Society of Human Reproduction and

Embryology (ESHRE) PGD Consortium, 2003Major malformations: 2.6%

Phocomelia and pulmonary deficiency, chylothorax, congenital hip dislocation, abdominal cystic mass, pes equinivarus, exencephaly

Minor malformations: 1.4% syndactyly, hydrocele testis, ASD, mongolian spot, sacral dimple

Liebaers et al, Belgium 2010Major malformations: 2.1% vs ICSI: 3.4%

chylothorax, VSD, oeasophageal atresia, cataract, umbilical hernia, ichthyosis, cardiopathy

Alternatives to PGD

Conceive naturally and have prenatal diagnosis during pregnancy

Future of PGDEfforts continue to

be focused on improving methods to obtain an accurate diagnosis. PGD holds great promise for the future as techniques and genetic tests are perfected.PGD may become routine in the next few years.

ConclusionsFor couples at risk for producing offspring with either debilitating monogenic disorders or chromosomal abnormalities IVF/PGD represents a major scientific advance

ConclusionsComplications, both before and after birth, are no different in type or number from those found in a comparable ICSI populationOther parameters such as birth weight and length, are also similar to an ICSI populationPGD appears to be a safe method to avoid the birth of children with genetic defects

Conclusions• Before PGD is performed, genetic

counseling must be provided to ensure that patients fully understand therisk for having an affected child the impact of the diseasethe available options the multiple technical limitations

including the possibility of an erroneous result

• Prenatal diagnostic testing is strongly encouraged to confirm the results of PGD

Thank You