Trinucelotide repeats

Question 1

What is the front line test for detecting triplet repeat expansions?

Answer 1

F-PCR

Question 2

If there is any ambiguity around the result, what type of PCR is performed on the region of interest?

Answer 2

TP-PCR

Question 3

In chronological order, name the 3 methods used to calculate repeat expansions

Answer 3

1. ABI method
2. Single-fixed point method
3. Two fixed point method

Question 4

Why is the ABI method no longer in use?

Answer 4

Under-estimating repeats

Question 5

Why is the single fixed point method no longer used?*

Answer 5

When the expansion of the allele is much different to the control it becomes inaccurate.

Question 6

SBMA: Why is TP-PCR not necessary to exclude large expansions where a homozygous allele has been detected by f-PCR

Answer 6

The largest expansion detected is 68 repeats

Question 7

FRA X: Two alleles have been detected via f-PCR in a male patient. What should you do?

Answer 7

Repeat f-PCR. If result is consistent, qf-PCR to check for aneuploidy such as XXY (Kleinfelter’s).

Question 8

A repeat expansion in what gene is responsible for Fragile X Syndrome?

Answer 8

FMR gene