Genetic Screening - Part 3

Question 1

Why do we do FISH tests in prenatal diagnosis?

Answer 1

• Rapid diagnosis after amniocentesis by targeted molecular approaches.
• National Screening Committee said a test would be available within 3 days for a woman who has been given a positive screening result.
• Also can use QF-PCR.

Question 2

What were the driving forces behind the use of rapid screen technology in prenatal diagnosis?

Answer 2

• Women themselves and clinical staff. Stressful time between sampling and availability of final diagnostic result.
• Standardisation of serum screening programmes (NICE):
• National availability of a Down syndrome screening test to all pregnant women.
• Prescribed standards of detection rates and false positive rates.
• National Screening Committee Guidelines.
• “A molecular test result should be available within three working days of diagnostic sampling” - National Screening Committee.
• Aneuploidy testing for trisomy 13, 18 and 21.
• You do not need dividing cells to perform these tests.
• Results available within 24 hours.

Question 3

What is one of the main advantages of FISH?

Answer 3

• You don’t need dividing cells.

Question 4

What problems might we have with the prenatal FISH test?

Answer 4

• MCC - trace of blood contamination in amniotic fluid samples - can;t then offer FISH safely.
• About 2.4% of pregnancies couldn’t offer rapid FISH for.
• In a normal pop will get about 0-15% of cells showing 3 signals for the target chromosome. In a fully trisomic individual would expect to see 75-100% of cells showing 3 signals. Between 15% and 75% is more ambiguous.
• There are a number of possibilities when the % of cells showing 3 signals are in the ambiguous range:

1) . High background - extra signal artefact - in which case you are at risk of a false positive.
2) . MCC - dilutes extra signal - in which case you are at risk of a false negative.
3) . Genuine mosaicism.

• The move over to QF-PCR was partly due to these problems with FISH testing.

Question 5

What is prenatal FISH still used for in centres that primarily use QF-PCR for rapid prenatal tests?

Answer 5

• To confirm an abnormality detected by QF-PCR - pool cells from an existing cytogenetic culture.
• For any query that relates to a possible sex chromosome aneuploidy / micro deletion syndrome - the referral card must clearly indicate that this test is required.
• As a backup service to the QF-PCR test in case of technical problems.
• As part of the CVS service - FISH rapid testing will be used where direct chromosome preps are unsuccessful - FISH rapid testing on direct preps showing trisomy 13 or 18.
• It is important to follow up a rapid test with more detailed chromosome analysis. Rapid tests can give false positives / false negatives.
• Two scenarios show the importance of combining rapid results with chromosome analysis:
  1) . Rapid testing leading to a false result.
  2) . FISH/rapid testing needed to confirm chromosomally abnormal direct preps.

Question 6

Outline future technologies in prenatal diagnosis.

Answer 6

Other molecular techniques for aneuploidy detections:

1) . QF-PCR systems - improved throughput and efficiency - use for sex chromosome aneuploidy testing. Routine in several labs in the UK.
2) . MLPA or other copy number detection systems for aneuploidy screening.
3) . DNA micro array systems for use in prenatal diagnosis adjunct / primary analysis for a specific referral subset (e.g. abnormal scan referrals).
4) . Foetal cell or cell free foetal DNA / RNA in maternal circulation - analysis of foetal sex, paternal mutation / Rhesus phenotype / aneuploidy testing.