Short report
Myotonic dystrophy: the correlation of (CTG) repeat length in
leucocytes with age at onset is significant only for patients with
small expansions
Marion G Hamsherea b, Helen Harley* c, Peter Harperc, J David Brooka b, John F Y Brookfielda
a Department
of Genetics, Queen's Medical Centre, University of Nottingham,
Nottingham NG7 2UH, UK, b Centre for Medical Genetics, City
Hospital NHS Trust, Nottingham NG6 1PB, UK, c Institute of Medical Genetics,
University of Wales College of Medicine, Cardiff CF4 4XN, UK
Correspondence to: Dr Hamshere.
Received 11 July 1997;
Revised version accepted for publication 18 June 1998
Myotonic dystrophy (DM) was the first of a group of diseases to
be identified for which the genetic basis is the expansion of a triplet
repeat. Myotonic dystrophy also exhibits anticipation, in which the
disease worsens through successive generations. These two features have
led many groups to analyse whether a significant negative correlation
between triplet repeat length and severity of disease exists. However,
the recent molecular finding that two distinct subsets of classically
affected DM patients exist, those who export expansion derived DMPK RNA
and those who do not, led us to question whether combining data from
these two sets of patients is statistically valid. We found that
although patients with small expansions showed a significant
correlation between age at onset and triplet repeat length, those with
larger expansions did not. The theoretical triplet repeat size, which
separated the two groups, was also deduced.
Keywords: myotonic dystrophy; triplet repeats; age at onset
* Present address: Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK.
© 1999 by J Med Genet
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