ANALYSIS OF THE CMT1A-REP REPEAT - MAPPING CROSSOVER BREAKPOINTS IN CMT1A AND HNPP

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 ne uropathy type 1A (CMT1A) and deleted in hereditary neuropathy with lia bility to pressure palsies (HNPP). The CMT1A-REP repeat is proposed to mediate misalignment and unequal crossover resulting in reciprocal ch romosomal rearrangements in CMT1A and HNPP. We have constructed a phys ical map of the proximal and distal CMT1A-REP repeats, Cloned fragment s 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 unrelate d HNPP patients, The CMT1A-REP repeat spans approximately 27 kilobase pairs and appears to be continuous. Locations of restriction enzyme si tes are highly conserved for the proximal and distal CMT1A-REP repeats . All crossovers mapped within the CMT1A-REP repeat sequence and heter ogeneity for breakpoint location was demonstrated. Seventy-seven perce nt (40 of 52) of CMT1A and HNPP chromosomes contained breakpoints whic h mapped within a 7.9 kb interval, suggesting the presence of a possib le 'hotspot' for recombination in CMT1A-REP. DNA sequence analysis for 4 kb of the interval containing the majority of crossovers revealed o ver 98% sequence identity between proximal and distal CMT1A-REP repeat sequences. Probes useful for molecular-based diagnosis of CMT1A and H NPP are described.