We have recently described the identification of a second IDS locus (IDS-2) located within 90 kb telomeric of the IDS gene (Bondeson et al. submitted). Here, we show that this region is involved in a recombination event with the IDS gene in about 13% of patients with the Hunter syndrome. Analysis of the resulting rearrangement at the molecular level showed that these patients have suffered a recombination event that results in a disruption of the IDS gene in intron 7 with an inversion of the intervening DNA. Interestingly, all of the six cases with a similar type of rearrangement showed recombination between intron 7 of the IDS gene and sequences close to exon 3 at the IDS-2 locus implying that these regions are hot spots for recombination. Analysis by nucleotide sequencing showed that the inversion is caused by recombination between homologous sequences present in the IDS gene and the IDS-2 locus. No detectable deletions or insertions were observed as a result of the recombination event. The results in this study have practical implications for diagnosis of the Hunter syndrome.

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.

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