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DNA testing for Down
syndrome
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DOWN SYNDROME
Commonest genetic cause of intellectualdisability worldwide
All ethnic groups
Increased risk with advancing maternal
age Age 25 1/1350
Age 35 1/400
Age 40 1/100
Age 45 1/30
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Down syndrome
Birth prevalence 1/700 births worldwide
1/500 in RSA
Cause Non-disjunction / trisomy 95%
Translocation 4% About 50% de novo
Mosaicism 1%
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Photo used with parental consent
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Down syndromeclinical features
Craniofacial Brachycephaly, upslanted eyes, epicanthic folds, flat facial
profile, flat nasal bridge, (protruding tongue), (dysplasticears)
Limbs Brachydactyly, single palmar crease, clinodactyly, sandle-gap
Cardiac (40%) ASD, VSD, AVSD, PDA
CNS Hypotonia, developmental delay, MR Other
GIT abn (~15%), short stature
Features common in normal black neonatesFeatures useful for diagnosis in black patients
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Down syndromecomplications
Atlanto-axial instability
Transient neonatal leukaemia ALL (20 X greater risk)
Hypothyroidism (antibodies) in 30%
Alzheimer disease 8% by 49 yrs
75% by 60yrs
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TESTING
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100 base pairs 105 109
CGH and MLPA
DNA sequencing FISH Chromosome banding
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Live, dividing cells Any tissue
Screen all chromosomes Test specific regions
TAT 2 4 weeks TAT 72 hours
Labour intensive Can be automated
CHROMOSOMES DNA
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Short tandem repeats (STR)
STR = microsatellite markers
10 000s across genome
Stretches of DNA of units of 2-4 nucleotides
TGTGTG
CAACAA.CAA
Different alleles exist for each STR in popn.
Each allele differs in repeat length
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(AAC)10 (AAC)18(AAC)26
(AAC)10 (AAC)15(AAC)26 (AAC)18
(AAC)15
(CC)8 (CC)12
(CC)8 (CC)11 (CC)12 (CC)7
(CC)7(CC)11
Q i i fl PCR
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Quantitative fluorescent PCRaneuploidy screen
(QF-PCR)Principle:
Test STR markers on chromosomes 4-5 on autosomes of choice (13, 18, 21)
Fewer on X and Y
Used to detect numerical chromosome
abnormalities
Can test blood, amniotic fluid, CVS, post-mortem tissue etc.
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QF-PCR
Advantages Rapid result (48 72 hours) 99% accuracy No live cells required
Disadvantages Wont detect mosaicism (
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Past
Pre-and postnatal testing for DS by
karyotyping
Prenatal testing for DS in AMAwomen by QF-PCR
Postnatal testing for DS bykaryotyping
January 2007 May 2008563 requests for DS testing
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Past
Pre-and postnatal testing for DS by
karyotyping
Prenatal testing for DS in AMAwomen by QF-PCR
Postnatal testing for DS bykaryotyping
January 2007 May 2008563 requests for DS testing
307 confirmed DS (54.5%)
+79 confirmed DS
on specimens not requested as DS= 386 confirmed DS
185 not DS
(33%)
67 no result 4 other
chromosomeabnormalities
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Past
Pre-and postnatal testing for DS by
karyotyping
Prenatal testing for DS in AMAwomen by QF-PCR
Postnatal testing for DS bykaryotyping
January 2007 May 2008563 requests for DS testing
307 confirmed DS (54.5%)
+79 confirmed DS
on specimens not requested as DS= 386 confirmed DS
185 not DS
(33%)
67 no result
366 trisomy
95%
6 mosaic
1.5%
14 translocation
3.5%
4 other
chromosomeabnormalities
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present
Indication for DS
testing
Prenatal testing for DS in AMAwomen by QF-PCR
Postnatal testing for DSby QF-PCR
Confirmed DS
98% RRrelates totrisomy
~2% RR relates toinherited
translocation
Offer parentalkaryotype
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present
Indication for DS
testing
Prenatal testing for DS in AMAwomen by QF-PCR
Postnatal testing for DSby QF-PCR
Confirmed DS Negative QF-PCR
Strongly suspect DS?mosaic
Dysmorphic featuresUnknown diagnosis
Request karyotype
Refer to Genetics
98% RRrelates totrisomy
~2% RR relates toinherited
translocation
Offer prenatal testing and/orparental karyotype if recurrence
risk perceived as high
Genetic counselling
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Baby with multiplecongenital
abnormalities
Stillborn
Macerated
Skin snip(saline)
QF-PCRT13, 18, 21, X, Y
Refer to Genetics
Alive
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Baby with multiplecongenital
abnormalities
Stillborn Alive
Fresh SBMacerated
Skin snip(saline)
QF-PCRT13, 18, 21, X, Y
Karyotype
Cardiac blood (heparin)Skin snip (saline)
Refer to Genetics
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Baby with multiplecongenital
abnormalities
Stillborn Alive
Fresh SBMacerated
Skin snip(saline)
MCA - unknown
QF-PCRT13, 18, 21, X, Y
Suspect trisomy
13 or 18
Karyotype
Blood(EDTA)
Blood(heparin)
Karyotype QF-PCR
Negative Positive
Refer to Genetics
Cardiac blood (heparin)Skin snip (saline)
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Queries and Referrals
Division Human Genetics011 489 9223 / 4