DNA testing for Down syndrome

8/7/2019 DNA testing for Down syndrome

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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


karyotyping




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


karyotyping




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


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


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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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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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

DNA testing for Down syndrome

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