Hazel E Phillimore1, Rebecca Gould1, Belinda Chong1, Desirée Du Sart1
1 Molecular Genetics Laboratory, Victorian Clinical Genetics Service, Murdoch Children’s Research Institute/Royal Children’s Hospital, 50 Flemington Road, Parkville, Victoria, Australia
Friedreich ataxia (FRDA) is a common autosomal recessive ataxia that is mainly caused by a biallelic hyper-expansion of an intronic GAA trinucleotide repeat in the frataxin (FXN) gene. Most patients (>95%) have two expanded alleles in the affected range (66 -1700 repeats), while the remainder of patients have one expanded allele and an inactivating mutation on the other allele. Conventional PCR will reliably detect normal alleles and smaller expanded alleles. Detecting very large expansions can often prove difficult and carrier status could be missed due to non-amplification of an expansion, sometimes due to preferential amplification of the smaller allele or poor quality DNA.
The introduction of Triplet-primed PCR analysis (TP-PCR) into the analysis of several repeat disorders including FRDA has allowed the presence of an expanded allele to be robustly detected by PCR to confirm a diagnosis or determine carrier status. In particular, when samples appear homozygous with conventional PCR, TP-PCR can confirm the absence of an expansion, or detect the presence of an expanded allele not detected previously.
We present a case where an individual was tested in 2007 and reported not to be a carrier. Recent testing using TP-PCR shows the presence of an expanded allele in the patient, indicating carrier status for FRDA. Further analysis of other family members using conventional PCR has shown the presence of only one expanded allele in the affected family member, a confirmed carrier status in one parent and the absence of an expanded allele in the other parent. However, testing with TP-PCR shows the presence of an expanded allele in both parents as expected. In our presentation, we will present further investigations to determine the reason for the lack of amplification of the one expanded allele in this family.
Hazel Phillimore is a senior medical scientist in the Molecular Genetics Laboratory at VCGS (Victorian Clinical Genetics Service) at the MCRI (Murdoch Childrens Research Institute) within the Royal Children’s Hospital, Melbourne. She has worked in molecular genetics in various roles since 1990 and mostly as a senior scientist in diagnostic genomics as part of a Regional Genetics Service in London, UK. Her skills and knowledge are in a wide range of inherited genetic disorders and different technologies, as well as somatic cancer genetics (from working at the Peter MacCallum Cancer Centre, Melbourne) and in pharmacogenetics (from working at GenesFX, Melbourne).