Myotonic Dystrophy: Accurately Scoring the Boundaries that Define Regions of Triplet Repeat Expansion Mutations.
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Date
2015
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Abstract
Aims: Myotonic Dystrophy type 1 (DM1) is an autosomal dominant neuromuscular multi-systemic
disorder caused by a CTG triplet repeat expansion mutation in the DMPK gene. The clinical
decision points defining the CTG repeat boundaries between normal, premutation and mild disease
ranges are poorly characterised with a lack of commercially available sequenced controls. There
are no US Food and Drug Administration (FDA) approved tests for DM1 so testing protocols are
developed and managed by individual laboratories.Study Design: This paper presents a cross-laboratory exchange scheme between Auckland City
Hospital and Concord Hospital, which took place between October 2013 and January 2014, in
order to validate the scoring of CTG repeats within the DMPK gene and to build comprehensive
allelic libraries.
Methodology: Seven samples ranging from 30-59 repeats, spanning the critical clinical decision
points, were sequenced to confirm the “true” repeat sizes, and 19 samples were tested by both
laboratories using standard and triplet repeat-primed PCR methods.
Results: The results showed a very strong correlation between the sequencing results and the
standard PCR results for the 7 selected samples with a Pearson correlation coefficient of 0.999
and P = 1.20x10-7. The results from the inter-laboratory comparison also showed a very strong
correlation between the diagnostic tests of the two labs with a Pearson correlation coefficient of
0.999 and P = 1x10-29. A paired t-test showed no significant difference between the two
laboratories data with a Mean (SD) = 0.263 (0.828), P = .058.
Conclusion: This study provides two critical outcomes. The first is that the extrapolations that
were used by each of the participating laboratories in determining the number of CTG repeats in
the absence of well-characterised controls in the 35-51 repeat range were within their reported
margins-of-error. The second outcome is that small regional laboratories can gain confidence in
the accuracy of their reported allele calls, specifically around clinically critical decision points, with
inter-laboratory exchange studies and in-house sequencing of relevant control samples.
Description
Keywords
Myotonic dystrophy, trinucleotide repeats, DNA repeat expansion, DNA sequencing
Citation
Dryland Philippa A, Hughes Kimberley, Han Dug Yeo, Aziz Ann, Zhu Danqing, Doherty Elaine, Love Donald R. Myotonic Dystrophy: Accurately Scoring the Boundaries that Define Regions of Triplet Repeat Expansion Mutations. British Journal of Medicine and Medical Research. 2015; 8(8): 724-731.