Preimplantation Genetic Diagnosis (PGD)

Presentation by

Sandesh Kamdi -Medical Advisor

IVF success

Success rate of IVF is only 20%

Something to ponder !

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IVF process

Egg Sperm

Selection of Embryo and Implantation

Causes of low success rate

Chromosomal abnormalities often neglected at the time of

embryo selection

Conventional methods of embryo selection

• Embryoscopy• Physical identification• Morphological characteristics

Types of chromosomal abnormalities

• Aneuploidy: atypical number of chromosomes results from error in cell division

• Translocations: a piece of chromosome go and attached to a wrong chromosome.

• Chromosome Deletions : loss of a chromosome segment resulting in an imbalance in the number of genes present

• Chromosome inversions: when a single chromosome breaks in two places and the material is reconstituted upside down

……leading to embryo death, miscarriage or live birth of an infant with substantial medical problems.

Aneuploidy

• A healthy baby has 46 chromosomes in each cell of the body

• Chromosomal abnormality refers to an atypical number of chromosomes and is generally caused by an error in cell division in either the egg or sperm or in the first few days of embryo development.

Normal cell

Meiosis

Chromosomal abnormality/Aneuploidy

Embryos with chromosomal abnormalities are less likely to implant and more likely to end in miscarriage

Chromosome abnormality with age

Age in years

Normal embryos (%)

Aneuploidy embryos (%)

Other abnormalities (%)

25-35 61 8 3136-37 60 10 3038-39 47 18 3540-41 43 26 3142-44 39 30 31

http://www.fertilityindia.com/indications-for-pgd.php

Impact of chromosome abnormality

• Decreased embryo transfer• Implantation failure, • Recurrent pregnancy loss, or miscarriage• Birth defects • Mental or physical problems in offspring

Genetic disorders

Babies born with chromosome abnormalities have mental retardation and / or birth defects

Ex: Down syndrome

Chromosome abnormality

Genetic disorder

Healthy baby

Who are at the risk?

• Age more than 35 years (chances of embryo transfer (<75%)

• Couples with a family history of X-linked disorders (25% risk of having an affected embryo)

• Carriers of autosomal recessive diseases: means two copies of an abnormal gene (the risk an embryo may be affected is 25%.)

• Carriers of autosomal dominant diseases: abnormal gene in one parent (the risk an embryo may be affected is 50%.)

Sex – linked disorders

Typical scenario

Carrier mother + Normal father

25% chances of Normal boy

25% chances of disease boy

25% chances of Normal girl

25% chances of carrier girl

Disease father + Normal mother

100% chances of Normal boy

100% chances of carrier girl

Carrier: Some one has one abnormal gene (No symptoms) is called a carrier

X- Linked Recessive disorder in womenWomen with X-linked recessive disorder are rare, since women has two X-chromosomes.

Two scenarios:Carrier mother + Disease father

25% chances of Normal boy

25% chances of disease boy

25% chances of carrier girl

25% chances of disease girl

Disease father + Disease mother

100% chances disease boy or girl

http://www.fertilityindia.com/indications-for-pgd.php

What’s the solution ?

Preimplantation Genetic Diagnosis (PGD)

The PGD process

Egg Sperm

Remove one cellOn day 3rd

Embryo 8 cell stage

Test DNA or chromosomes

Test results

Healthy gen conditions

Unhealthy gen conditions

Embryo implanted on day 4

Embryo discarded

Quick and rapid technique

Who should do PGD?

• Couples with advanced maternal age (>35)• Unexplained infertility• Y-chromosome deletion: found in 5-20% men with low sperm

count• Couples who have experienced repeated miscarriages • Couples who have experienced repeated IVF failures • Couples who have previously had a pregnancy with a

chromosomal abnormality • Couples at risk of having children with a particular X-linked

disorder• couples where one partner carries a balanced chromosomal

translocations

Disadvantages of conventional PGD

• In only few centers it is available in India

• Most well known center is Jaslok Hospital, Mumbai

• It takes 3 days

• It is limited to chromosomes 13, 18, 21 only

Advantages of PGD

• PGD is highly accurate across all chromosomes

• It yields quick results which allows for a fresh embryo transfer on day 4 when it is tested on day 3

• PGD prevents transmission of genetic disorders onto future generations.

Couple who never achieved viable pregnancy and used to have spontaneous

miscarriages can have successful conception with PGD with balanced

chromosomal embryos.

Types of PGD

• There are two main types of preimplantation genetic screening

– Aneuploidy screening by fluorescent in situ hybridization (FISH) and

– Aneuploidy screening by Molecular Array Comparative Genomic Hybridization (CGH-24)

FISH

• It screens the most common chromosomes seen in miscarriages and live born abnormality disorders like Down syndrome-chromosomes 13, 16, 18, 21 and 22.

• When one parent is carrier of heritable X-linked disease such as Muscular dystrophy, Hemophilia A, or ectodermal Dysplasia.

• FISH examines chromosomes X and Y plus chromosomes 13, 18, and 21

CGH-24

• This is single cell technology to test aneuploidy, translocations, inversion and chromosomal abnormality

• It is unique and it can make thousands of independent measurements of each chromosome at molecular level

• No freezing and ICISI is required• It is automated and high degree of results accuracy• It gives the results with 20 hours than the standard 36-48

hours

Processes

• On day 3rd: Biopsy of a blastomere from an embryo at IVF center

– The fixation of blastomere on slides for FISH analysis or transfer into PCR tube with lysis buffer for CGH analysis

– Test for FISH or CGH at genetic center : results within 12-20 hours

• Day 4: Embryo transfer on day 4