Nucleic Acids Research, Vol 26, Issue 10 2353-2358, Copyright © 1998 by Oxford University Press
MC Hirst and PJ White
The normal human FMR1 gene contains a genetically stable (CGG) n
trinucleotide repeat which usually carries interspersed AGG triplets. An
increase in repeat number and the loss of interspersions results in array
instability, predominantly expansion, leading to FMR1 gene silencing.
Instability is directly related to the length of the uninterrupted (CGG) n
repeat and is widely assumed to be related to an increased propensity to
form G-rich secondary structures which lead to expansion through
replication slippage. In order to investigate this we have cloned human
FMR1 arrays with internal structures representing the normal, intermediate
and unstable states. In one replicative orientation, arrays show a
length-dependent instability, deletions occurring in a polar manner. With
longer arrays these extend into the FMR1 5'-flanking DNA, terminating at
either of two short CGG triplet arrays. The orientation-dependent
instability suggests that secondary structure forms in the G-rich lagging
strand template, resolution of which results in intra-array deletion. These
data provide direct in vivo evidence for a G-rich lagging strand secondary
structure which is believed to be involved in the process of triplet
expansion in humans.
ARTICLES
Cloned human FMR1 trinucleotide repeats exhibit a length- and orientation-dependent instability suggestive of in vivo lagging strand secondary structure
Institute of Molecular Medicine, The John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DS, UK. mhirst@worf.molbiol.ox.ac.uk
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