PGD - obgynkw-conference.com

Report
Preimplantation
Genetic Diagnosis:
An Overview
Dr. Laila Bastaki, MD
Consultant of Medical Genetics
Director 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 PGD
1.
Chromosomal Disorders
Numerical
Chromosomal aneuploidy
Structural
Inversions
Translocations
Deletions and duplications
2.
Gender determination for severe Xlinked 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 Complex
 Requires special
skills

How is PGD performed?
Ovarian Stimulation
IVF
Blastomere Biopsy on Day 3
Transfer of
Unaffected Embryo
Outcome
Clinically Normal Baby
Genetic Analysis
(FISH or
Molecular)
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 array
Polymerase 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 PGD
Reduction 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 damage
Oocyte 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 Pregnancy
There 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 & Malformations
European Society of Human Reproduction and
Embryology (ESHRE) PGD Consortium, 2003
Major 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 2010
Major 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 PGD
Efforts 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.
Conclusions
For couples at risk for
producing offspring
with either
debilitating
monogenic disorders
or chromosomal
abnormalities
IVF/PGD represents a
major scientific
advance
Conclusions
Complications, both before and after birth,
are no different in type or number from those
found in a comparable ICSI population
Other parameters such as birth weight and
length, are also similar to an ICSI population
PGD 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 the
 risk for having an affected child
 the impact of the disease
 the 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

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