Analysis of the CMT1A-REP repeat: mapping crossover breakpoints in CMT1A and HNPP

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Abstract
The CMT1A-REP repeat sequence flanks a 1.5 megabase pair (Mb) segment of chromosome 17p11.2–12 which is duplicated in Charcot—Marie—Tooth neuropathy type 1A (CMT1A) and deleted in hereditary neuropathy with liability to pressure palsies (HNPP). The CMT1A-REP repeat is proposed to mediate misalignment and unequal crossover resulting in reciprocal chromosomal rearrangements in CMT1A and HNPP. We have constructed a physical map of the proximal and distal CMT1A-REP repeats. Cloned fragments from CMT1A-REP repeat regions were used to determine the size of the repeats and to assess regions of homology. The crossover breakpoints were mapped in a series of 30 unrelated CMT1A patients and 22 unrelated HNPP patients. The CMT1A-REP repeat spans approximately 27 kilobase pairs and appears to be continuous. Locations of restriction enzyme sites are highly conserved for the proximal and distal CMT1A-REP repeats. All crossovers mapped within the CMT1A-REP repeat sequence and heterogeneity for breakpoint location was demonstrated. Seventy-seven percent (40 of 52) of CMT1A and HNPP chromosomes contained breakpoints which mapped within a 7.9 kb interval, suggesting the presence of a possible ‘hotspot’ for recombination in CMT1A-REP. DNA sequence analysis for 4 kb of the interval containing the majority of crossovers revealed over 98% sequence identity between proximal and distal CMT1A-REP repeat sequences. Probes useful for molecular-based diagnosis of CMT1A and HNPP are described.

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