20.05.07 Rapid Prenatal Screening - QFPCR, FISH

Question 1

Aneuploidy accounts for what percentage of clinically significant chromosomal abnormalities diagnosed prenatally

Answer 1

> 80%

Question 2

What non-invasive methods are used for prenatal screening for chromosomal abnormalities

Answer 2

• Maternal serum sampling
• Nuchal translucency testing
• Lack sensitivity and specificity

Question 3

What test did the UK National Screening Committee recommend in 2004 is offered to all women undergoing an invasive diagnostic procedure

Answer 3

Rapid diagnostic tests by FISH or QF PCR

Question 4

In which cases are QF PCR tests offered without karyotyping

Answer 4

Women with high Down’s risk without an increased NT or any abnormal scan

Question 5

What would the benefits of a QF PCR only strategy be

Answer 5

• Detect 89% of chromosomal abnormalities

- However would miss 1.2% of clinically significant chromosome abnormalities

Question 6

What is QF PCR

Answer 6

• Quantitative Fluorescence polymerase chain reaction
• Amplification of 19 highly polymorphic STRs (Short tandem repeats), which are 2-5 bp repeats.
• 19 markers are found on chromosomes 13, 18, 21 and X and Y chromosomes.
• Fast, easy test, high throughput.
• 5-20% mosaicism detected

Question 7

Disadvantages of QF PCR

Answer 7

• Sensitive to MCC
• Doesn’t test full genome
• Difficulty interpreting sex chromsome abnormalities, e.g. X/XX and XXX or XX and X/XXX (need TAF9)

Question 8

What marker can be used to determine X chromosome aneuploidy

Answer 8

• TAF9 (binds to regions on chromosome 3 and X).

- AMELX/Y, SRY (non-polymorphic)

Question 9

What is interphase FISH

Answer 9

• Interphase Fluorescence in situ hybridisation
• Chromosome specific probes applied to interphase cells - uncultured CVS and amniotic fluid samples. Probes for 13, 18, 21, X, Y.
• Increased resolution to conventional karyotyping
• Can detect microdeletions, subtle duplications, complex structural rearrangements and marker chromosomes.
• Very quick TAT (1-2 days)
• 10% mosaicism detected
• Can distinguish X, XX and XXX

Question 10

Disadvantages of interphase FISH

Answer 10

• Not as cost effective as QF PCR
• Low throughput
• Requires expertise
• Time consuming to analyse
• Higher failure rate than QF PCR
• Limited choice of probes
• Quality affected by gestational age (especially AF from high gestational age)

Question 11

What percentage of chromosomal abnormalities will be missed by interphase FISH and QF PCR

Answer 11

20%