Flashcards in Genetic Testing & Clinical Practise 2 Deck (12):
How would you test for Huntington's Disease?
Use PCR primers to flank the repeating region, and amplify a fragment that contains both flanking non-repetitive sequences, and the CAG repeats. Determine the length of the amplified fragment and subtract the non-repetitive sequence to determine the number of CAG repeats. >40 repeats is indicative of HD
Why is predictive testing different to other kinds of testing?
It's performed in the absence of corroborating evidence and there are potentially extreme life-changing consequences
Give two examples of strict protocol associated with predictive testing
-Two samples are collected and tested independently
- Confirmation of the disease-causing mutation in the proband first
Why would you not use Sanger sequencing to test for mosaicism?
Sanger sequencing uses PCR and then Sanger sequencing techniques to determine the sequence of bases in DNA. However it does not tell you the proportion of DNA at each peak in the resulting electropherogram.
Which test would you use when looking for mosaicism?
Massively parallel sequencing.
This method captures the DNA sequences and records every copy as individual reads, so you can determine the proportion of each allele present.
At which body site would you take a sample from, when testing for overgrowth disorders?
A visibly affected site
Describe the test a lab would carry out to improve confidence of lab results of foetal testing
qfPCR STR analysis. This is a PCR-based tandem repetitive sequencing technique (Short Tandem Repeats), which identifies maternal and paternal alleles, and deduces whether the foetal sample has been contaminated by maternal or paternal DNA. The STRs chosen have a high frequency of heterozygosity
What kind of test would you do on a child with developmental delay and dysmorphic features?
A microarray. This gives information on deletions and duplications across the whole genome, by testing 300,00 markers where there are known single nucleotide polymorphisms
Is there a high risk of mosaicism in a future child, if parents have already produced one child with this genotype?
No - mosaicism occurs post-zygotically
What are some of the inherent risks in prenatal testing?
- Often no corroborating evidence that the foetus is affected
- The foetal sample could be contaminated with maternal cells and the results could reflect maternal genotype rather than foetal genotype
- Risk of sample mix up AKA wrong foetal sample
What would you see in a qf-PCR STR result in which the sample had been contaminated with maternal DNA?
A third peak appearing in the foetal analysis that is identical to the mother's second allele, and/or a skewing of the peaks, indicating that there is more of the maternal allele present than the paternal allele