Double-strand breaks may initiate the inversion mutation causing the Hunter syndrome

Abstract

We have previously shown that patients with the Hunter syndrome frequently have suffered from a recombination event between the IDS gene and its putative pseudogene, IDS-2, resulting in an inversion of the intervening DNA. The inversion, which might be the consequence of an intrachromosomal mispairing, is caused by homologous recombination between sequences located in intron 7 of the IDS gene and sequences located distal of exon 3 in IDS-2. In order to gain insight into the mechanisms causing the inversion, we have isolated both inversion junctions in six unrelated patients. DNA sequence analysis of the junctions showed that all recombinations have taken place within a 1 kb region where the sequence identity is >98%. An interesting finding was the identification of regions with alternating IDS gene and IDS-2 sequences present at one inversion junction, suggesting that the recombination event has been initiated by a double-strand break in intron 7 of the IDS gene. The results from this study suggest that homologous recombination in man could be explained by mechanisms similar to those described for Saccharomyces cerevisiae. The results also have practical implications for diagnosis of patients with the Hunter syndrome.

Homologous nonallelic recombinations between the iduronate-sulfatase gene and pseudogene cause various intragenic deletions and inversions in patients with mucopolysaccharidosis type II

“…The authors found that 94% of the recombination events observed were nonreciprocal, ie conversion events, while 6% led to inversions (were reciprocal) with or without additional conversions. Indeed, IDS and IDS-2 specific polymorphisms are observed frequently at distinct positions of the pseudogene and functional gene, respectively 15 (and Bunge, S, 1997) unpublished observations), which are suggestive of frequent conversion events. As there are no differences in the exonic regions of high homology, such conversions have no clinical consequence and remain unnoticed.…”

“…Interestingly, there is no insertion or deletion of nucleotides at the breakpoints and gene conversion is observed in most cases, which can be explained by the double-strand breakage model of recombination. 15 Remarkably, an experimental model used to investigate mechanisms of mammalian recombination resembles quite well the 'natural' recombination hot spot IDS/IDS-2. In the study by Bollag and Liskay, 17 two invertedly positioned genes bounded by distinguishable markers and defined by two different mutations were stably introduced into the genome of mouse cells.…”

“…Primers IDS62 and IDS63 (similar to 60033-F and 58740-B in ref. 15) were used to amplify a 1.3 kb gene intron 7-fragment, IDS64 and IDS65 (similar to 97690-F and 98855B in ref. 15) for a 1.3 kb IDS-2-specific intron 7-fragment.…”

“…15) were used to amplify a 1.3 kb gene intron 7-fragment, IDS64 and IDS65 (similar to 97690-F and 98855B in ref. 15) for a 1.3 kb IDS-2-specific intron 7-fragment. Localisation of these primers is given in Figures 1 and 3.…”

“…Indeed, this IDS gene-specific polymorphism was found in IDS-2 of control persons. 15 The recombination event in PF can be explained by an initial double-stranded break at any position between nucleotides 1344 and 1823 followed by invasion of single strands into the pseudogene homologous region, branch migration, and conversion of the gene part of the distal inversion area between 1078 and 1344 into pseudogene sequence. 15 If patient P21's ancestor had harboured the gene-specific nucleotide 1090 polymorphism in his pseudogene, a pseudogene to gene conversion would be likely at least for nt 1344 in the distal junction area.…”