An Alu-mediated rearrangement as cause of exon skipping in Hunter disease

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Abstract
Hunter syndrome (Mucopolysaccharidosis type II), a rare X-linked lysosomal storage disorder, results from deleterious mutations in the iduronate-2-sulfatase ( IDS) gene located on Xq27.3-q28. Partial or complete deletions and large rearrangements have been extensively reported in the IDS gene as the basis of Hunter disease. The present report, however, is the first report on a Hunter patient in which Alu-mediated recombinations are implicated. Our patient showed the skipping of exon 8 at the cDNA level, without any splice-junction defects at the genomic level, where a new large rearrangement was identified instead. This new mutant allele consisted of an extensive deletion of IDS sequence of about 3 kb, as well as an additional inserted sequence of 157 bp. Two different computer programs were necessary to elucidate the nature of the insert. NCBI-BLAST query detected a single match for 126 bp out of 157 of the fragment that aligned exactly with a specific chromosomal region, Xq25-27.1, where an AluSg sequence is adjacent to an L1. Instead, the Repeat Masker program identified only 83 bp out of 157 of the insert, which was confirmed as an AluS. The observed homology between the AluSc sequence in the IDS intron 8 and the inserted AluS element, as well as the closeness of 26 bp Alu core sequence, considered to be a recombination hotspot, made us hypothesise upon the fact that both an Alu retrotransposition and an Alu-mediated deletion underlie the disease-producing rearrangement. We, therefore, now propose a mechanism that led to the large genomic deletion causing the production of the aberrant mRNA splicing.