L20 Genetic Testing 1 Flashcards
Genetic Testing - categories
- before implantation
- prenatally - before birth
- neonatally - new born
- child
- adult
Prenatal
- ultrasound
- maternal blood triple test
- maternal blood: fetal cell sorting, cell free DNA
- fetal blood sampling
- chorionic villus sampling
- amniocentesis
Why test?
Has two associated slides
chromosomal aneuploidies
- extra chromosomes generally inviable
- loss of an autosome inviable
- extra sex chromosome can be viable
- loss of sex chromosome viable but must have an x
aneuploidy frequency increases with maternal age
women of advanced maternal age are at increased risk of producing an embryo affected with a genetic disease
Trisomy 21
see onenote slides
Changes in chromosome number
see onenote
Non-disjunction
see onenote
Non-disjunction at anaphase 2 would produce half normal gametes
Non-disjunction at anaphase 1 - no normal gametes produced
If not 21, 18 or 13 trisomy, would most likely lead to a miscarriage
Aneuploidy
see onenote slides
Older effects - problems
see onenote
more likely to have abnormally functioning spindles - causes an increased rate of chromosomal problems in the mature eggs
Non-disjunction in mitosis - mosaic
Mosaic
- Normal zygote, in one of the cells you get non-disjunction, all cells derived from that will have trisomy
- The other cells dividing normally => mosaic
- Can happen for trisomy 13 and 18, often these children don’t have such severe phenotypes if they are mosaics, has some normal cell lines
Translocation
see onenote
Translocation carrier
- Don’t have phenotypic evidence of being a carrier
- Have a balanced genome
In karyotype we can see in chromosome 13, 18, 21:
- Achrocentric - centromeres are near the end of the chromosome
- Nucleolar organiser region - dna sequence that code for ribosome RNA, these are repeated and so we can lose it without any phenotypic outcome
- Lose nucleolar organiser in these translocation carrier but don’t see any effect as they are repeated
14/21 21
- When fertilised, will get 3 copies of chromosome 21
From translocation carrier
- Can have normal offspring but some of their gametes might not be viable at all => fertility problems
- But can also lead to trisomy 21
Human reproduction - rate
see onenote slides
birth rate is increasing but so is age of parents => genetic testing becomes more relevant
Why test? - other reasons
see onenote
Australian Genetic Testing
see onenote
Preimplantation testing
Preimplantation
- Embryo that develops till 8 cell stage
- Take out single cell
- Has to be done along side IVF, test it before you implant it
PGD requires IVF
see onenote
Preimplantation Genetic Diagnosis
Polar body biopsy
Polar body biopsy
- Sometimes can take a human oocyte (the gamete itself) and do a polar body biopsy
- before you even put the egg and sperm together
- If we know female is het for haemophelia, we know in one cell will have the normal factor 8 gene and the other will have the mutation 8 gene
- Look at polar body to see if they have the mutant or normal factor 8 gene, the oocyte will have the other gene
After biopsy
see onenote
FISH (Fluorescence in situ hybridisation) analysis - sex determination for x-linked diseases
PCR - to amplify region that may contain mutation
To whom is PGD advised?
see onenote
- couples with repeated pregnancy loss due to genetic disorders
- couples who have a child with a genetic disease and are at high risk of having another
- couples who wish to identify a tissue match for a sick sibling who can be cured with transplanted cells - preimplantation tissue typing (PTT)
Intra cytoplasmic sperm injection (ICSI)
for couples with male infertility or to reduce risk of contamination
- used when there is abnormal semen or PCR analysis is required
- to reduce risk of contamination with sperm DNA during PCR
Ultrasound
Ultrasound
- Done 12-20 weeks of pregnancy
- Looking for nuchal (neck translucency) = gap at base of skull, in children with trisomy 21 it’s wider than it should be
- Neural tube defects e.g. spina bifida, anencephaly (Foetus usually dies within minutes of being born)
- Screen, looking for parameters that may indicate the something’s wrong then woman could undergo further testing
Maternal blood triple test
see onenote slides
- screening
three specific substances:
- AFP - protein produced by foetus, high = indicates neural tube defect
- hCG - hormone produced by placenta
- Oestriol - estrogen produced y both fetus and placenta
Estimating - triple test of maternal blood
Age of mother
Ultrasound
Blood test:
- AFP (low in trisomy 21, high with neural tube defects)
- hCG (low in trisomy 21)
- oestriol (high in trisomy 21, trisomy 18 low in all)
Maternal blood - fetal cell sorting
see onenote
70% of pregnancies have fetal cells in maternal circulation
can use Y specific probe
XX fetal cells use fluorescent activated cell sorting - use cell surface differences between mother and fetus
cells from previous pregnancy may remain in circulation - unreliable
NIPD in the UK
Non-invasive pre-natal diagnosis
CVS and aminocentesis carry 1% risk of miscarriae
Maternal blood - cell free DNA
Cell-free fetal DNA (cffDNA) is fetal DNA which circulates freely in the maternal blood. Maternal blood is sampled by venipuncture. Analysis of cffDNA is a method of non-invasive prenatal diagnosis frequently ordered for pregnant women of advanced maternal age.
Cell-free DNA screening is a test that can determine if a woman has a higher chance of having a fetus with Down syndrome (trisomy 21), trisomy 18, trisomy 13 or an abnormality in the sex chromosomes (X and Y chromosomes). With this test, a sample of the woman’s blood is taken after 10 weeks of pregnancy.
Maternal blood: NIPD
see onenote
analyse cell free DNA of fetus in maternal circulation
can test sites to determine aneuploidies and mutations in fetus
NIPT
see onenote
non-invasive prenatal testing
very accurate for detection of down syndrome and edward syndrome
Fetal blood sampling
sample from umbilical cord or fetus
risky
ultrasound may indicate signs of severe anaemia
maybe if fetus if Rh+ve and mother Rh=ve and sensitised check for fetal anaemia
Indications for pre-natal diagnosis in Victoria
see onenote
- abnormal first trimester
- advanced maternal age
- abnormal routine ultrasound
- second trimester serum screen
Chorionic villus sampling
see onenote
- invasive
- performed from 10-13 weeks
- increases spontaneous abortion rate by 0.8%
- 10-30 mg chorion required
- separate maternal and fetal tissue (contamination problem(
- mosaicism
Amniocentesis
see onenote slies
- from 15-20 weeks
- 20ml amniotic fluid
- spontaneous abortion rate increases by 0.3%
